@article{kim_bryant_2024, title={Electrohydraulic System Analysis of Variable Recruitment Fluidic Artificial Muscle Bundles With Interaction Effects}, volume={146}, ISSN={["1528-9028"]}, DOI={10.1115/1.4064092}, abstractNote={Abstract This study investigates the system-level performance of variable recruitment (VR) fluidic artificial muscle (FAM) actuator bundles using a model that incorporates FAM interaction effects. A VR bundle combines multiple FAMs to act as one actuator in which the FAMs are sequentially recruited to increase overall efficiency. In a VR bundle, inactive/low-pressure FAMs are compressed beyond their free strains, exerting resistive forces opposing that of active FAMs. A recent model that captures this behavior is used to simulate sinusoidal contraction of a VR bundle with a hanging mass load. The implications of inter-FAM effects on the force-strain space of a VR bundle are discussed and a method of recruitment state transition required to track a sinusoid is proposed. The dynamics of the electrohydraulic subsystems are presented and used to evaluate its system efficiency and bandwidth limits. Three different electrohydraulic configurations are considered: 1) continuous motor operation with constant pump displacement, 2) intermittent motor operation with constant pump displacement and 3) continuous motor operation with variable pump displacement. Simulation results show the superior bandwidth capabilities of VR bundles by demonstrating its ability to track sinusoids with amplitudes up to 16% strain at frequencies greater than 0.5 Hz, compared to that of a single equivalent cross-section area motor unit (SEMU). In addition to increased bandwidth limit, system efficiencies averaged over a range of amplitudes show up to 170% increase when comparing a VR bundle using variable pump displacement to a SEMU using constant pump displacement.}, number={3}, journal={JOURNAL OF DYNAMIC SYSTEMS MEASUREMENT AND CONTROL-TRANSACTIONS OF THE ASME}, author={Kim, Jeong Yong and Bryant, Matthew}, year={2024}, month={May} }
 @article{elfering_metoyer_chatterjee_mazzoleni_bryant_granlund_2023, title={Blade element momentum theory for a skewed coaxial turbine}, volume={269}, ISSN={["1873-5258"]}, url={https://doi.org/10.1016/j.oceaneng.2022.113555}, DOI={10.1016/j.oceaneng.2022.113555}, abstractNote={A coaxial turbine under skew with significant rotor spacing has the potential for increased power output compared to a flow-aligned turbine due to a portion of the downstream rotor experiencing freestream velocity, referred to as a fresh flow region. A lab-scale prototype was designed and built to investigate the skew-to-power relationship of a coaxial turbine system as it compared to a blade element momentum theory model with multiple, sheared streamtubes representing the downstream rotor fresh flow region. The inclusion of the downstream rotor fresh flow region in the theoretical analysis is compared to the experimental data. The results support that the torque and power performance of the downstream rotor and overall skewed coaxial turbine system are predicted more accurately.}, journal={OCEAN ENGINEERING}, author={Elfering, Kelsey and Metoyer, Rodney and Chatterjee, Punnag and Mazzoleni, Andre and Bryant, Matthew and Granlund, Kenneth}, year={2023}, month={Feb} }
 @article{duan_bryant_2023, title={Implications of Spatially Constrained Bipennate Topology on Fluidic Artificial Muscle Bundle Actuation (vol 11, 82, 2022)}, volume={12}, ISSN={["2076-0825"]}, DOI={10.3390/act12020079}, abstractNote={The authors wish to make the following corrections in Section 3 [...]}, number={2}, journal={ACTUATORS}, author={Duan, Emily and Bryant, Matthew}, year={2023}, month={Feb} }
 @article{hughes_gopalarathnam_bryant_2023, title={Modulation and Annihilation of Aeroelastic Limit-Cycle Oscillations Using a Variable-Frequency Disturbance Generator}, ISSN={["1533-385X"]}, DOI={10.2514/1.J062295}, abstractNote={Nonlinear aeroelastic limit-cycle oscillations (LCOs) have become an area of interest due to both detrimental effects on flying vehicles and use in renewable energy harvesting. Initial studies on the interaction between aeroelastic systems and incoming flow disturbances have shown that disturbances can have significant effects on LCO amplitude, with some cases resulting in spontaneous annihilation of the LCO. This paper explores this interaction through wind-tunnel experiments using a variable-frequency disturbance generator to produce flow disturbances at frequencies near the inherent LCO frequency of an aeroelastic system with pitching and heaving degrees of freedom. The results show that incoming disturbances produced at frequencies approaching the LCO frequency from below produce a cyclic growth-decay in LCO amplitude that resembles interference between multiple sine waves with slightly varying frequencies. An aeroelastic inverse technique is applied to the results to study the transfer of energy between the pitching and heaving degrees of freedom as well as the aerodynamic power moving into and out of the system. Finally, the growth-decay cycles are shown to both excite LCOs in an initially stationary wing and annihilate preexisting LCOs in the same wing by appropriately timing the initiation and termination of disturbance generator motion.}, journal={AIAA JOURNAL}, author={Hughes, Michael T. and Gopalarathnam, Ashok and Bryant, Matthew}, year={2023}, month={Feb} }
 @article{jenkins_babu_bryant_gopalarathnam_2023, title={Numerical Study of Circular-Cylinder Disturbance Generators with Rigid Splitter Plates}, ISSN={["1533-385X"]}, DOI={10.2514/1.J062729}, abstractNote={This paper describes the numerical study of oscillating circular cylinders with rigid splitter plates of different lengths. These geometries may be used as disturbance generators for the study of unsteady airfoils and wings operating in highly vortical flowfields. It has been shown that cylinders undergoing forced rotational oscillations at their natural shedding frequency can produce wakes with minimal deviation in cycle-to-cycle vortex strength and position. Adding a splitter plate allows these deviations to be reduced even further. We present cases for oscillating cylinders having splitter-plate lengths up to [Formula: see text] at a Reynolds number of 7600. Frequencies are maintained at the natural shedding frequency, and a rotational amplitude of 45 deg is used. Numerical simulations are performed using a two-dimensional unsteady Reynolds-averaged Navier–Stokes (RANS) code. Results are presented in the form of vorticity contours and cycle-averaged velocity profiles, as well as the dominant frequencies of cylinder lift force and downstream velocity angles. The results show that splitter-plate lengths shorter than [Formula: see text] adversely affect the ability to generate a coherent vortex wake due to shear layer roll-up near the trailing edge of the plate. Splitter plates longer than [Formula: see text] produced a reverse von Kármán wake with consistent cycle-to-cycle vortex shedding.}, journal={AIAA JOURNAL}, author={Jenkins, Michael and Babu, Arun Vishnu Suresh and Bryant, Matthew and Gopalarathnam, Ashok}, year={2023}, month={Oct} }
 @article{fine_mcguire_reed_bryant_vermillion_2023, title={Optimal Cyclic Control of a Structurally Constrained Span-Morphing Underwater Kite in a Spatiotemporally Varying Flow}, ISSN={["2378-5861"]}, DOI={10.23919/ACC55779.2023.10155864}, abstractNote={This work presents a control methodology for maximizing the net power generated by an underwater kite capable of adjusting wingspan in real time. Underwater kite systems generate energy by performing cross-current figure-eight flight maneuvers while tethered to a winch system. These systems generate net positive power through cyclic spooling: spooling out under high-tension cross-current flight and spooling in radially under low tension. In the presence of structural constraints, simultaneous variation in the kite’s angle of attack and span is superior to simply reducing the angle of attack in order to stay within permissible structural loading. Furthermore, the optimal combination of these variables depends on the amount of tether spooled out and the spatiotemporally-varying flow field. Leveraging a multi-degree-of-freedom model previously developed by the authors, the performance of three kites– two with fixed span and one with variable span – was compared. To maximize the performance of the modeled kites, an optimal control framework was developed. For the fixed-span case, spool-out speeds and mean elevation angles for the kite were optimized to maximize energy generation over a spool-out cycle. For the morphing span case, spool-out speeds, mean elevation angle, and wingspan were optimized to maximize energy generation over a spool-out cycle while considering the energetic cost of morphing. Simulation results show that the kite capable of span-morphing generated 38.7% more energy than a fixed-span kite of maximum allowable span and 13.2% more energy than a kite of the optimal fixed-span.}, journal={2023 AMERICAN CONTROL CONFERENCE, ACC}, author={Fine, Jacob B. and McGuire, Carson M. and Reed, James and Bryant, Matthew and Vermillion, Chris}, year={2023}, pages={2084–2090} }
 @article{hassan_bryant_mazzoleni_ramaprabhu_granlund_2022, title={Analytical wake model for coaxial dual-rotor turbines}, ISBN={["978-1-6654-6809-1"]}, ISSN={["0197-7385"]}, DOI={10.1109/OCEANS47191.2022.9977241}, abstractNote={This work develops and validates a novel analytical wake model for coaxial dual-rotor turbines. With the diameters, and axial induction factors of the upstream and downstream rotors, and the freestream velocity, the proposed model estimates the wake velocity deficit in the near- and far-wake of the coaxial turbine. It is developed by utilizing the Bernoulli principle along the streamlines that pass through the near- and far-wake control volumes and the conservation laws for mass and momentum. This simple model can be used to calculate the velocity distribution in the wake using just one parameter. The wake prediction is contrasted with CFD results for various flow conditions to find good agreements between them. The novel wake model can be useful for solving the turbine farm layout optimization problem that involve dual-rotor configurations for the power generators.}, journal={2022 OCEANS HAMPTON ROADS}, author={Hassan, Mehedi and Bryant, Matthew and Mazzoleni, Andre and Ramaprabhu, Praveen and Granlund, Kenneth}, year={2022} }
 @article{abney_reed_naik_bryant_herbert_leonard_vadlamannati_mook_beknalkar_alvarez_et al._2022, title={Autonomous Closed-Loop Experimental Characterization and Dynamic Model Validation of a Scaled Underwater Kite}, volume={144}, ISSN={["1528-9028"]}, DOI={10.1115/1.4054141}, abstractNote={
 This paper presents the closed-loop experimental framework and dynamic model validation for a 1/12-scale underwater kite design. The pool-based tow testing framework described herein, which involves a fully actuated, closed-loop controlled kite and flexible tether, significantly expands upon the capabilities of any previously developed open-source framework for experimental underwater kite characterization. Specifically, the framework has allowed for the validation of three closed-loop flight control strategies, along with a critical comparison between dynamic model predictions and experimental results. In this paper, we provide a detailed presentation of the experimental tow system and kite setup, describe the control algorithms implemented and tested, and quantify the level of agreement between our multi-degree-of-freedom kite dynamic model and experimental data. We also present a sensitivity analysis that helps to identify the most influential parameters to kite performance and further explain remaining mismatches between the model and data.}, number={7}, journal={JOURNAL OF DYNAMIC SYSTEMS MEASUREMENT AND CONTROL-TRANSACTIONS OF THE ASME}, author={Abney, Andrew and Reed, James and Naik, Kartik and Bryant, Samuel and Herbert, Dillon and Leonard, Zak and Vadlamannati, Ashwin and Mook, Mariah and Beknalkar, Sumedh and Alvarez, Miguel and et al.}, year={2022}, month={Jul} }
 @article{williams_bryant_agrawal_mazzoleni_granlund_ramaprabhu_bryant_2022, title={Characterization of the Steady-State Operating Conditions of Tethered Coaxial Turbines}, ISBN={["978-1-6654-6809-1"]}, ISSN={["0197-7385"]}, url={http://dx.doi.org/10.1109/oceans47191.2022.9977052}, DOI={10.1109/OCEANS47191.2022.9977052}, abstractNote={Tethered coaxial turbines (TCTs) may be a feasible configuration to extract hydrokinetic energy from the Gulf Stream’s flow. A TCT consists of two rotors attached to the halves of a rotary generator, which is moored to a mounting point via a tether. Flow causes the rotors to counter-rotate which induce power within the generator. The TCT’s steady-state operating domain and power extraction is determined by the intersection of the hydrodynamic operating domain of the rotors and electromechanic operating domain of the generator. As a result, the TCT’s operating point can be selected with an electrical load resistance, skew angle, and flow speed. Previous analytical methods for evaluating dual rotor devices have assumed ideal rotor, flow, and generator characteristics to simplify the quantification of power extraction. The proposed hydrodynamic analysis modifies traditional blade-element momentum theory (BEMT) to accept nonuniform inflow into the rotor, via a radially and azimuthally discretized BEMT method (RAD-BEMT). RAD-BEMT is leveraged alongside a momentum theory wake development factor to determine the response of the back rotor within the nonuniform wake of the front rotor. The back rotor response is determined by minimizing the difference in mass continuity and rotor torques. Our electromechanical analysis considers an AC generator, and the effects of voltage rectification, system resistance, and capacitance on the TCT’s power extraction capabilities. A case study was performed to demonstrate the ability of torque and mass continuity minimization to locate a hydrodynamic operating point, for axial and skew flow conditions. Additionally, power extraction capabilities, load resistance selection, and the qualitative effects of skew on the minimization domain are discussed.}, journal={2022 OCEANS HAMPTON ROADS}, publisher={IEEE}, author={Williams, Vinson Oliver and Bryant, Samuel and Agrawal, Saurabh and Mazzoleni, Andre P. and Granlund, Kenneth and Ramaprabhu, Praveen and Bryant, Matthew}, year={2022} }
 @article{mazzoleni_kim_bryant_2022, title={Control of a dynamic load emulator for hardware-in-the-loop testing of fluidic artificial muscle bundles}, volume={12041}, ISBN={["978-1-5106-4957-6"]}, ISSN={["1996-756X"]}, DOI={10.1117/12.2612920}, abstractNote={Fluidic artificial muscles (FAMs) have emerged as a viable and popular robotic actuation technique due to their low cost, compliant nature, and high force-to-weight-ratio. In recent years, the concept of variable recruitment has emerged as a way to improve the efficiency of conventional hydraulic robotic systems. In variable recruitment, groups of FAMs are bundled together and divided into individual motor units. Each motor unit can be activated independently, which is similar to the sequential activation pattern observed in mammalian muscle. Previous researchers have performed quasistatic characterizations of variable recruitment bundles and some simple dynamic analyses and experiments with a simple 1- DOF robot arm. We have developed a linear hydraulic characterization testing platform that will allow for the testing of different types of variable recruitment bundle configurations under different loading conditions. The platform consists of a hydraulic drive cylinder that acts as a cyber-physical hardware-in-the-loop dynamic loading emulator and interfaces with the variable recruitment bundle. The desired inertial, damping and stiffness properties of the emulator can be prescribed and achieved through an admittance controller. In this paper, we test the ability of this admittance controller to emulate different inertial, stiffness, and damping properties in simulation and demonstrate that it can be used in hardware through a proof-of-concept experiment. The primary goal of this work is to develop a unique testing setup that will allow for the testing of different FAM configurations, controllers, or subsystems and their responses to different dynamic loads before they are implemented on more complex robotic systems.}, journal={BIOINSPIRATION, BIOMIMETICS, AND BIOREPLICATION XII}, author={Mazzoleni, Nicholas and Kim, Jeong Yong and Bryant, Matthew}, year={2022} }
 @article{agrawal_williams_tong_hassan_muglia_bryant_granlund_ramaprabhu_mazzoleni_2022, title={Demonstration of a Towed Coaxial Turbine Subscale Prototype for Hydrokinetic Energy Harvesting in Skew}, ISBN={["978-1-6654-6809-1"]}, ISSN={["0197-7385"]}, url={http://dx.doi.org/10.1109/oceans47191.2022.9977395}, DOI={10.1109/OCEANS47191.2022.9977395}, abstractNote={The immense potential for ocean current energy harvesting is being actively explored by researchers, exhibiting the importance of the marine hydrokinetic industry. This paper presents a towed dual rotor coaxial turbine prototype built to demonstrate the ability of tethered, underwater, hydrokinetic devices to harvest energy from ocean currents. A sub-scale test article was developed to measure fluid power conversion and serve as a platform for operational feasibility in open-water testing. Tow testing of this article was done in the freshwaters of Lake Norman in North Carolina at three tow speeds: 1 m/s, 1.25 m/s and 1.5 m/s. Preliminary results demonstrate the ability to extract power, system robustness, waterproofing capabilities, and illuminates the nuances and non-linearities unique to the tethered coaxial turbine system.}, journal={2022 OCEANS HAMPTON ROADS}, publisher={IEEE}, author={Agrawal, Saurabh and Williams, Vinson Oliver and Tong, Xinyang and Hassan, Mehedi and Muglia, Mike and Bryant, Matthew and Granlund, Kenneth and Ramaprabhu, Praveen and Mazzoleni, Andre P.}, year={2022} }
 @article{vemula_kim_mazzoleni_bryant_2022, title={Design, analysis, and validation of an orderly recruitment valve for bio-inspired fluidic artificial muscles}, volume={17}, ISSN={["1748-3190"]}, DOI={10.1088/1748-3190/ac4381}, abstractNote={Biological musculature employs variable recruitment of muscle fibers from smaller to larger units as the load increases. This orderly recruitment strategy has certain physiological advantages like minimizing fatigue and providing finer motor control. Recently fluidic artificial muscles (FAM) are gaining popularity as actuators due to their increased efficiency by employing bio-inspired recruitment strategies such as active variable recruitment (AVR). AVR systems use a multi-valve system (MVS) configuration to selectively recruit individual FAMs depending on the load. However, when using an MVS configuration, an increase in the number of motor units in a bundle corresponds to an increase in the number of valves in the system. This introduces greater complexity and weight. The objective of this paper is to propose, analyze, and demonstrate an orderly recruitment valve (ORV) concept that enables orderly recruitment of multiple FAMs in the system using a single valve. A mathematical model of an ORV-controlled FAM bundle is presented and validated by experiments performed on a proof-of-concept ORV experiment. The modeling is extended to explore a case study of a 1-DOF robot arm system consisting of an electrohydraulic pressurization system, ORV, and a FAM-actuated rotating arm plant and its dynamics are simulated to further demonstrate the capabilities of an ORV-controlled closed-loop system. An orderly recruitment strategy was implemented through a model-based feed forward controller. To benchmark the performance of the ORV, a conventional MVS with equivalent dynamics and controller was also implemented. Trajectory tracking simulations on both the systems revealed lower tracking error for the ORV controlled system compared to the MVS controlled system due to the unique cross-flow effects present in the ORV. However, the MVS, due to its independent and multiple valve setup, proved to be more adaptable for performance. For example, modifications to the recruitment thresholds of the MVS demonstrated improvement in tracking error, albeit with a sacrifice in efficiency. In the ORV, tracking performance remained insensitive to any variation in recruitment threshold. The results show that compared to the MVS, the ORV offers a simpler and more compact valving architecture at the expense of moderate losses in control flexibility and performance.}, number={2}, journal={BIOINSPIRATION & BIOMIMETICS}, author={Vemula, Dheeraj and Kim, Jeong Yong and Mazzoleni, Nicholas and Bryant, Matthew}, year={2022}, month={Mar} }
 @article{hart_duan_bryant_2022, title={Experimental Investigation of Boundary Condition Effects in Bipennate Fluidic Artificial Muscle Bundles}, volume={12041}, ISBN={["978-1-5106-4957-6"]}, ISSN={["1996-756X"]}, DOI={10.1117/12.2615896}, abstractNote={In this study, the implementation and performance of bipennate topology fluidic artificial muscle (FAM) bundles operating under varying boundary conditions is investigated and quantified experimentally. Soft actuators are of great interest to design engineers due to their inherent flexibility and potential to improve safety in human robot interactions. McKibben fluidic artificial muscles are soft actuators which exhibit high force to weight ratios and dynamically replicate natural muscle movement. These features, in addition to their low fabrication cost, set McKibben FAMs apart as attractive components for an actuation system. Previous studies have shown that there are significant advantages in force and contraction outputs when using bipennate topology FAM bundles as compared to the conventional parallel topology1 . In this study, we will experimentally explore the effects of two possible boundary conditions imposed on FAMs within a bipennate topology. One boundary condition is to pin the muscle fiber ends with fixed pin spacings while the other is biologically inspired and constrains the muscle fibers to remain in contact. This paper will outline design considerations for building a test platform for bipennate fluidic artificial muscle bundles with varying boundary conditions and present experimental results quantifying muscle displacement and force output. These metrics are used to analyze the tradespace between the two boundary conditions and the effect of varying pennation angles.}, journal={BIOINSPIRATION, BIOMIMETICS, AND BIOREPLICATION XII}, author={Hart, Rebecca and Duan, Emily and Bryant, Matthew}, year={2022} }
 @article{duan_bryant_2022, title={Implications of Spatially Constrained Bipennate Topology on Fluidic Artificial Muscle Bundle Actuation}, volume={11}, ISSN={["2076-0825"]}, DOI={10.3390/act11030082}, abstractNote={In this paper, we investigate the design of pennate topology fluidic artificial muscle bundles under spatial constraints. Soft fluidic actuators are of great interest to roboticists and engineers, due to their potential for inherent compliance and safe human–robot interaction. McKibben fluidic artificial muscles are an especially attractive type of soft fluidic actuator, due to their high force-to-weight ratio, inherent flexibility, inexpensive construction, and muscle-like force-contraction behavior. The examination of natural muscles has shown that those with pennate fiber topology can achieve higher output force per geometric cross-sectional area. Yet, this is not universally true for fluidic artificial muscle bundles, because the contraction and rotation behavior of individual actuator units (fibers) are both key factors contributing to situations where bipennate muscle topologies are advantageous, as compared to parallel muscle topologies. This paper analytically explores the implications of pennation angle on pennate fluidic artificial muscle bundle performance with spatial bounds. A method for muscle bundle parameterization as a function of desired bundle spatial envelope dimensions has been developed. An analysis of actuation performance metrics for bipennate and parallel topologies shows that bipennate artificial muscle bundles can be designed to amplify the muscle contraction, output force, stiffness, or work output capacity, as compared to a parallel bundle with the same envelope dimensions. In addition to quantifying the performance trade space associated with different pennate topologies, analyzing bundles with different fiber boundary conditions reveals how bipennate fluidic artificial muscle bundles can be designed for extensile motion and negative stiffness behaviors. This study, therefore, enables tailoring the muscle bundle parameters for custom compliant actuation applications.}, number={3}, journal={ACTUATORS}, author={Duan, Emily and Bryant, Matthew}, year={2022}, month={Mar} }
 @article{metoyer_bryant_granlundt_mazzoleni_2022, title={Increased Energy Conversion with a Horizontal Axis Turbine in Translation}, ISBN={["978-1-6654-6809-1"]}, ISSN={["0197-7385"]}, DOI={10.1109/OCEANS47191.2022.9977131}, abstractNote={When fixed to the ground by tower or stanchion, horizontal axis turbines convert hydrokinetic power into electric power by passively exploiting the difference in velocity between the ground and a flowing fluid. This method of converting the available hydrokinetic power is relatively simple, but the maximum amount of power that may be converted to another form by the turbine has a theoretical upper limit, called the Betz limit, which is about 59.25% of the hydrokinetic power in a stream tube of the freestream flow with a cross sectional area equal to the area of the turbine rotor plane. The work presented demonstrates that eschewing the stanchion and making the turbine to translate through the fluid enables conversion of more hydrokinetic power and, when operated in a cyclical mode, more energy over a cycle. It is demonstrated with momentum theory that the maximum energy that may be converted over a cycle is 1.5 times the Betz limit for an equivalent ground-fixed stationary turbine in the same low. Following the theoretical analysis, the concept is proven by simulation for a non-ideal turbine using an engineering design tool developed by the United States National Renewable Energy Laboratory. The results show that a realistic, non-ideal translating turbine can convert over twice as much power as an equivalent stationary turbine. Additionally, a notional tidal current application is presented where the bidirectionality of flow is exploited to achieve energy conversion of more than twice the theoretical limit of an ideal stationary turbine.}, journal={2022 OCEANS HAMPTON ROADS}, author={Metoyer, Rodney and Bryant, Matthew and Granlundt, Kenneth and Mazzoleni, Andre}, year={2022} }
 @article{suresh babu_narsipur_bryant_gopalarathnam_2022, title={Leading-edge-vortex tailoring on unsteady airfoils using an inverse aerodynamic approach}, volume={34}, ISSN={["1089-7666"]}, DOI={10.1063/5.0090328}, abstractNote={In this paper, we present an approach to obtain a desired leading-edge vortex (LEV) shedding pattern from unsteady airfoils through the execution of suitable motion kinematics. Previous research revealed that LEV shedding is associated with the leading-edge suction parameter (LESP) exceeding a maximum threshold. A low-order method called LESP-modulated discrete vortex method (LDVM) was also developed to predict the onset and termination of LEV shedding from an airfoil undergoing prescribed motion kinematics. In the current work, we present an inverse-aerodynamic formulation based on the LDVM to generate the appropriate motion kinematics to achieve a prescribed LESP variation, and thus, the desired LEV shedding characteristics from the airfoil. The algorithm identifies the kinematic state of the airfoil required to attain the target LESP value through an iterative procedure performed inside the LDVM simulation at each time step. Several case studies are presented to demonstrate design scenarios such as tailoring the duration and intensity of LEV shedding, inducing LEV shedding from the chosen surface of the airfoil, promoting or suppressing LEV shedding during an unsteady motion on demand, and achieving similar LEV shedding patterns using different maneuvers. The kinematic profiles generated by the low-order formulation are also simulated using a high-fidelity unsteady Reynolds-averaged Navier–Stokes method to confirm the accuracy of the low-order model.}, number={5}, journal={PHYSICS OF FLUIDS}, author={Suresh Babu, Arun Vishnu and Narsipur, Shreyas and Bryant, Matthew and Gopalarathnam, Ashok}, year={2022}, month={May} }
 @article{mazzoleni_kim_bryant_2022, title={Motor unit buckling in variable recruitment fluidic artificial muscle bundles: implications and mitigations}, volume={31}, ISSN={["1361-665X"]}, DOI={10.1088/1361-665X/ac49d9}, abstractNote={Fluidic artificial muscles (FAMs) are a popular actuation choice due to their compliant nature and high force-to-weight ratio. Variable recruitment is a bio-inspired actuation strategy in which multiple FAMs are combined into motor units that can be pressurized sequentially according to load demand. In a traditional ‘fixed-end’ variable recruitment FAM bundle, inactive units and activated units that are past free strain will compress and buckle outward, resulting in resistive forces that reduce overall bundle force output, increase spatial envelope, and reduce operational life. This paper investigates the use of inextensible tendons as a mitigation strategy for preventing resistive forces and outward buckling of inactive and submaximally activated motor units in a variable recruitment FAM bundle. A traditional analytical fixed-end variable recruitment FAM bundle model is modified to account for tendons, and the force–strain spaces of the two configurations are compared while keeping the overall bundle length constant. Actuation efficiency for the two configurations is compared for two different cases: one case in which the radii of all FAMs within the bundle are equivalent, and one case in which the bundles are sized to consume the same amount of working fluid volume at maximum contraction. Efficiency benefits can be found for either configuration for different locations within their shared force–strain space, so depending on the loading requirements, one configuration may be more efficient than the other. Additionally, a study is performed to quantify the increase in spatial envelope caused by the outward buckling of inactive or low-pressure motor units. It was found that at full activation of recruitment states 1, 2, and 3, the tendoned configuration has a significantly higher volumetric energy density than the fixed-end configuration, indicating that the tendoned configuration has more actuation potential for a given spatial envelope. Overall, the results show that using a resistive force mitigation strategy such as tendons can completely eliminate resistive forces, increase volumetric energy density, and increase system efficiency for certain loading cases. Thus, there is a compelling case to be made for the use of tendoned FAMs in variable recruitment bundles.}, number={3}, journal={SMART MATERIALS AND STRUCTURES}, author={Mazzoleni, Nicholas and Kim, Jeong Yong and Bryant, Matthew}, year={2022}, month={Mar} }
 @article{atay_bryant_buckner_2021, title={Control and Control Allocation for Bimodal, Rotary Wing, Rolling-Flying Vehicles}, volume={13}, ISSN={["1942-4310"]}, DOI={10.1115/1.4050998}, abstractNote={
 This paper presents a robust method for controlling the terrestrial motion of a bimodal multirotor vehicle that can roll and fly. Factors influencing the mobility and controllability of the vehicle are explored and compared to strictly flying multirotor vehicles; the differences motivate novel control and control allocation strategies that leverage the non-standard configuration of the bimodal design. A fifth-order dynamic model of the vehicle subject to kinematic rolling constraints is the basis for a nonlinear, multi-input, multi-output, sliding mode controller. Constrained optimization techniques are used to develop a novel control allocation strategy that minimizes power consumption while rolling. Simulations of the vehicle under closed-loop control are presented. A functional hardware embodiment of the vehicle is constructed onto which the controllers and control allocation algorithm are deployed. Experimental data of the vehicle under closed-loop control demonstrate good performance and robustness to parameter uncertainty. Data collected also demonstrate that the control allocation algorithm correctly determines a thrust-minimizing solution in real-time.}, number={5}, journal={JOURNAL OF MECHANISMS AND ROBOTICS-TRANSACTIONS OF THE ASME}, author={Atay, Stefan and Bryant, Matthew and Buckner, Gregory}, year={2021}, month={Oct} }
 @article{weisler_waghela_granlund_bryant_2021, title={Finite wing lift during water-to-air transition}, volume={6}, ISSN={["2469-990X"]}, url={https://doi.org/10.1103/PhysRevFluids.6.054002}, DOI={10.1103/PhysRevFluids.6.054002}, abstractNote={We report the experimental investigation of lift generation by an initially submerged aspect ratio 4 wing that translates through the water-air interface. Many animals, such as flying fish and diving seabirds, use wings or fins to produce lift forces as they transit the water-air interface, and cross-domain underwater-aerial vehicles were recently demonstrated, but lift production of a wing egressing from water had not yet been quantified. Our results show that the lift history is markedly different for low egress velocities versus high egress velocities, with low velocities exhibiting a large oscillation in lift coefficient and high velocities exhibiting a more linear lift attenuation.}, number={5}, journal={PHYSICAL REVIEW FLUIDS}, publisher={American Physical Society (APS)}, author={Weisler, W. A. and Waghela, R. and Granlund, K. and Bryant, M.}, year={2021}, month={May} }
 @article{jenkins_atay_buckner_bryant_2021, title={Genetic Algorithm-Based Optimal Design of a Rolling-Flying Vehicle}, volume={13}, ISSN={["1942-4310"]}, DOI={10.1115/1.4050811}, abstractNote={
 This work describes a design optimization framework for a rolling-flying vehicle consisting of a conventional quadrotor configuration with passive wheels. For a baseline comparison, the optimization approach is also applied for a conventional (flight-only) quadrotor. The vehicle range is maximized using a hybrid multi-objective genetic algorithm in conjunction with multi-physics system models. A low Reynolds-number blade element momentum theory aerodynamic model is used with a brushless DC motor model, a terramechanics model, and a vehicle dynamics model to simulate the vehicle range under any operating angle-of-attack and forward velocity. To understand the tradeoff between vehicle size and operating range, variations in Pareto-optimal designs are presented as functions of vehicle size. A sensitivity analysis is used to better understand the impact of deviating from the optimal vehicle design variables. This work builds on current approaches in quadrotor optimization by leveraging a variety of models and formulations from literature and demonstrating the implementation of various design constraints. It also improves upon current ad-hoc rolling-flying vehicle designs created in previous studies. Results show the importance of accounting for oft-neglected component constraints in the design of high range quadrotor vehicles. The optimal vehicle mechanical configuration is shown to be independent of operating point, stressing the importance of a well-matched, optimized propulsion system. By emphasizing key constraints that affect the maximum and nominal vehicle operating points, an optimization framework is constructed that can be used for RFVs and conventional multi-rotors.}, number={5}, journal={JOURNAL OF MECHANISMS AND ROBOTICS-TRANSACTIONS OF THE ASME}, author={Jenkins, Tyler and Atay, Stefan and Buckner, Gregory and Bryant, Matthew}, year={2021}, month={Oct} }
 @article{kim_mazzoleni_bryant_2021, title={Modeling of Resistive Forces and Buckling Behavior in Variable Recruitment Fluidic Artificial Muscle Bundles}, volume={10}, ISSN={["2076-0825"]}, DOI={10.3390/act10030042}, abstractNote={Fluidic artificial muscles (FAMs), also known as McKibben actuators, are a class of fiber-reinforced soft actuators that can be pneumatically or hydraulically pressurized to produce muscle-like contraction and force generation. When multiple FAMs are bundled together in parallel and selectively pressurized, they can act as a multi-chambered actuator with bioinspired variable recruitment capability. The variable recruitment bundle consists of motor units (MUs)—groups of one of more FAMs—that are independently pressurized depending on the force demand, similar to how groups of muscle fibers are sequentially recruited in biological muscles. As the active FAMs contract, the inactive/low-pressure units are compressed, causing them to buckle outward, which increases the spatial envelope of the actuator. Additionally, a FAM compressed past its individual free strain applies a force that opposes the overall force output of active FAMs. In this paper, we propose a model to quantify this resistive force observed in inactive and low-pressure FAMs and study its implications on the performance of a variable recruitment bundle. The resistive force behavior is divided into post-buckling and post-collapse regions and a piecewise model is devised. An empirically-based correction method is proposed to improve the model to fit experimental data. Analysis of a bundle with resistive effects reveals a phenomenon, unique to variable recruitment bundles, defined as free strain gradient reversal.}, number={3}, journal={ACTUATORS}, author={Kim, Jeong Yong and Mazzoleni, Nicholas and Bryant, Matthew}, year={2021}, month={Mar} }
 @article{metoyer_chatterjee_elfering_bryant_granlund_mazzoleni_2021, title={Modeling, simulation, and equilibrium analysis of tethered coaxial dual-rotor ocean current turbines}, volume={243}, ISSN={["1879-2227"]}, DOI={10.1016/j.enconman.2021.113929}, abstractNote={Tethered multirotor axial flow turbines have been proposed to overcome the many challenges associated with extracting ocean current energy where deep waters render seabed mounting strategies infeasible. However, flexible systems are inherently more susceptible to perturbation than fixed systems. The effects of flow misalignment on the hydrokinetic energy conversion of multirotor coaxial turbines have been investigated recently; however, the spatial dynamics and equilibrium behaviors of tethered coaxial turbines have not been well characterized, limiting the ability of designers to explicitly tailor the device behavior. In this work, a computational model of a dual-rotor coaxial turbine is presented, and the model is employed to explore the equilibrium behavior of the turbine with variations in parameters. A complete characterization of the hydrostatic state of the system and a comparative study of representative tethered turbine simulation cases is also presented. Two important findings are presented. First, that a positively buoyant dual-rotor turbine that is anchored to a surface-dwelling platform can operate where the turbine is located at some desired depth below the surface. Second, that more than one turbine system may be anchored to a single point while maintaining the desired orientation and position of each turbine to avoid collision and maximize energy production. The results and methods presented in this paper may be used to inform application-specific coaxial turbine design and to develop additional targeted empirical and simulation studies.}, journal={ENERGY CONVERSION AND MANAGEMENT}, author={Metoyer, Rodney and Chatterjee, Punnag and Elfering, Kelsey and Bryant, Matthew and Granlund, Kenneth and Mazzoleni, Andre}, year={2021}, month={Sep} }
 @article{atay_bryant_buckner_2021, title={The Spherical Rolling-Flying Vehicle: Dynamic Modeling and Control System Design}, volume={13}, ISSN={["1942-4310"]}, DOI={10.1115/1.4050831}, abstractNote={
 This paper presents the dynamic modeling and control of a bi-modal, multirotor vehicle that is capable of omnidirectional terrestrial rolling and multirotor flight. It focuses on the theoretical development of a terrestrial dynamic model and control systems, with experimental validation. The vehicle under consideration may roll along the ground to conserve power and extend endurance but may also fly to provide high mobility and maneuverability when necessary. The vehicle uses a three-axis gimbal system that decouples the rotor orientation from the vehicle’s terrestrial rolling motion. A dynamic model of the vehicle’s terrestrial motion is derived from first principles. The dynamic model becomes the basis for a nonlinear trajectory tracking control system suited to the architecture of the vehicle. The vehicle is over-actuated while rolling, and the additional degrees of actuation can be used to accomplish auxiliary objectives, such as power optimization and gimbal lock avoidance. Experiments with a hardware vehicle demonstrate the efficacy of the trajectory tracking control system.}, number={5}, journal={JOURNAL OF MECHANISMS AND ROBOTICS-TRANSACTIONS OF THE ASME}, author={Atay, Stefan and Bryant, Matthew and Buckner, Gregory}, year={2021}, month={Oct} }
 @article{sureshbabu_medina_rockwood_bryant_gopalarathnam_2021, title={Theoretical and experimental investigation of an unsteady airfoil in the presence of external flow disturbances}, volume={921}, ISSN={["1469-7645"]}, DOI={10.1017/jfm.2021.484}, abstractNote={Abstract While studies on unsteady airfoils in a uniform free stream are abundant, the quest for efficient man-made propulsion and energy harvesting calls for an improved understanding and predictive capability of unsteady airfoils encountering external flow disturbances. In this paper, we conduct experimental and theoretical investigations of the interactions between an airfoil engaged in unsteady motion and external flow disturbances generated by an upstream source. The flow field interactions are experimentally studied using particle-image velocimetry and finite-time Lyapunov exponent techniques. An interesting outcome of the interactions is an interruption of leading-edge vortex (LEV) shedding from the airfoil and a consequent modulation of the lift history, which are dependent on the phase of the disturbances relative to the airfoil kinematics. A low-order model for an unsteady airfoil encountering the disturbances is built upon the leading-edge suction parameter (LESP)-modulated discrete-vortex method (LDVM) developed by Ramesh et al. (J. Fluid Mech., vol. 751, 2014, pp. 500–538). The LDVM distils the determination of the LEV shedding characteristics of unsteady airfoils to a single parameter, the LESP. We show that the LDVM, modified for the current work, is able to predict the effect of the disturbances on the LEV shedding characteristics of the airfoil and the associated lift history in good agreement with experimental observations. In addition to being a predictive tool, the LDVM also augments the experimental study by providing a theoretical framework and various graphical approaches to analyse the flow phenomena from a fundamental perspective and elucidate the role of different factors governing the flow field evolution.}, journal={JOURNAL OF FLUID MECHANICS}, author={SureshBabu, ArunVishnu and Medina, Albert and Rockwood, Matthew and Bryant, Matthew and Gopalarathnam, Ashok}, year={2021}, month={Jul} }
 @article{kirschmeier_pash_gianikos_medina_gopalarathnam_bryant_2020, title={Aeroelastic inverse: Estimation of aerodynamic loads during large amplitude limit cycle oscillations}, volume={98}, ISSN={["0889-9746"]}, DOI={10.1016/j.jfluidstructs.2020.103131}, abstractNote={This paper presents an algorithm to compute the aerodynamic forces and moments of an aeroelastic wing undergoing large amplitude heave and pitch limit cycle oscillations. The technique is based on inverting the equations of motion to solve for the lift and moment experienced by the wing. Bayesian inferencing is used to estimate the structural parameters of the system and generate credible intervals on the lift and moment calculations. The inversion technique is applied to study the affect of mass coupling on limit cycle oscillation amplitude. Examining the force, power, and energy of the system, the reasons for amplitude growth with wind speed can be determined. The results demonstrate that the influence of mass coupling on the pitch–heave difference is the driving factor in amplitude variation. The pitch–heave phase difference not only controls how much aerodynamic energy is transferred into the system but also how the aerodynamic energy is distributed between the degrees of freedom.}, journal={JOURNAL OF FLUIDS AND STRUCTURES}, author={Kirschmeier, Benjamin and Pash, Graham and Gianikos, Zachary and Medina, Albert and Gopalarathnam, Ashok and Bryant, Matthew}, year={2020}, month={Oct} }
 @article{kirschmeier_gianikos_gopalarathnam_bryant_2020, title={Amplitude Annihilation in Wake-Influenced Aeroelastic Limit-Cycle Oscillations}, volume={58}, ISSN={["1533-385X"]}, DOI={10.2514/1.J058942}, abstractNote={This paper investigates the dynamics of a pitching and heaving aeroelastic wing undergoing large-amplitude limit-cycle oscillations influenced by a vortical wake from an upstream rectangular cylind...}, number={9}, journal={AIAA JOURNAL}, author={Kirschmeier, Benjamin A. and Gianikos, Zachary and Gopalarathnam, Ashok and Bryant, Matthew}, year={2020}, month={Sep}, pages={4117–4127} }
 @article{weisler_miller_jernigan_buckner_bryant_2020, title={Design and testing of a centrifugal fluidic device for populating microarrays of spheroid cancer cell cultures}, volume={14}, ISSN={["1754-1611"]}, DOI={10.1186/s13036-020-0228-6}, abstractNote={In current cancer spheroid culturing methods, the transfer and histological processing of specimens grown in 96-well plates is a time consuming process. A centrifugal fluidic device was developed and tested for rapid extraction of spheroids from a 96-well plate and subsequent deposition into a molded agar receiver block. The deposited spheroids must be compact enough to fit into a standard histology cassette while also maintaining a highly planar arrangement. This size and planarity enable histological processing and sectioning of spheroids in a single section. The device attaches directly to a 96-well plate and uses a standard centrifuge to facilitate spheroid transfer. The agar block is then separated from the device and processed.Testing of the device was conducted using six full 96-well plates of fixed Pa14C pancreatic cancer spheroids. On average, 80% of spheroids were successfully transferred into the agar receiver block. Additionally, the planarity of the deposited spheroids was evaluated using confocal laser scanning microscopy. This revealed that, on average, the optimal section plane bisected individual spheroids within 27% of their mean radius. This shows that spheroids are largely deposited in a planar fashion. For rare cases where spheroids had a normalized distance to the plane greater than 1, the section plane either misses or captures a small cross section of the spheroid volume.These results indicate that the proposed device is capable of a high capture success rate and high sample planarity, thus demonstrating the capabilities of the device to facilitate rapid histological evaluation of spheroids grown in standard 96-well plates. Planarity figures are likely to be improved by adjusting agar block handling prior to imaging to minimize deformation and better preserve the planarity of deposited spheroids. Additionally, investigation into media additives to reduce spheroid adhesion to 96-well plates would greatly increase the capture success rate of this device.}, number={1}, journal={JOURNAL OF BIOLOGICAL ENGINEERING}, author={Weisler, Warren and Miller, Samuel and Jernigan, Shaphan and Buckner, Gregory and Bryant, Matthew}, year={2020}, month={Mar} }
 @article{atay_buckner_bryant_2020, title={Dynamic Modeling for Bi-Modal, Rotary Wing, Rolling-Flying Vehicles}, volume={142}, ISSN={["1528-9028"]}, DOI={10.1115/1.4047693}, abstractNote={
 This paper presents a rigorous analysis of a promising bi-modal multirotor vehicle that can roll and fly. This class of vehicle provides energetic and locomotive advantages over traditional unimodal vehicles. Despite superficial similarities to traditional multirotor vehicles, the dynamics of the vehicle analyzed herein differ substantially. This paper is the first to offer a complete and rigorous derivation, simulation, and validation of the vehicle's terrestrial rolling dynamics. Variational mechanics is used to develop a six degrees-of-freedom dynamic model of the vehicle subject to kinematic rolling constraints and various nonconservative forces. The resulting dynamic system is determined to be differentially flat and the flat outputs of the vehicle are derived. A functional hardware embodiment of the vehicle is constructed, from which empirical motion data are obtained via odometry and inertial sensing. A numerical simulation of the dynamic model is executed, which accurately predicts complex dynamic phenomena observed in the empirical data, such as gravitational and gyroscopic nonlinearities; the comparison of simulation results to empirical data validates the dynamic model.}, number={11}, journal={JOURNAL OF DYNAMIC SYSTEMS MEASUREMENT AND CONTROL-TRANSACTIONS OF THE ASME}, author={Atay, Stefan and Buckner, Gregory and Bryant, Matthew}, year={2020}, month={Nov} }
 @article{stewart_weisler_anderson_bryant_peters_2020, title={Dynamic Modeling of Passively Draining Structures for Aerial-Aquatic Unmanned Vehicles}, volume={45}, ISSN={["1558-1691"]}, DOI={10.1109/JOE.2019.2898069}, abstractNote={In the design of hybrid unmanned aerial and underwater vehicles, buoyancy management and weight are two major factors. Large wing volumes used by unmanned air vehicles to fly efficiently drive vehicle buoyancy up, preventing them from submerging. Heavy active buoyancy control systems can overcome this, but cost weight, energy, and time to transition between underwater operation and flight. An alternative design, consisting of a passively flooding and draining wing, is presented in this paper. Relevant dynamic parameters for a full vehicle dynamic model are identified. A dynamic model of a draining structure is developed and verified experimentally on both a simple cylinder and a full wing structure. With proper tuning, the model captures the salient dynamic behavior of passive draining during vehicle egress. A prototype unmanned aerial and underwater vehicle was built, flown, and used to collect flight test data. The model is used to accurately predict the takeoff performance of the vehicle. As given, the model can be incorporated into a full vehicle dynamic model to aid in the design, simulation, and control of hybrid unmanned aerial and underwater vehicles with passively draining components.}, number={3}, journal={IEEE JOURNAL OF OCEANIC ENGINEERING}, author={Stewart, William and Weisler, Warren and Anderson, Mark and Bryant, Matthew and Peters, Kara}, year={2020}, month={Jul}, pages={840–850} }
 @article{atay_jenkins_buckner_bryant_2020, title={Energetic analysis and optimization of a bi-modal rolling-flying vehicle}, volume={4}, ISSN={["2366-598X"]}, DOI={10.1007/s41315-020-00119-2}, number={1}, journal={INTERNATIONAL JOURNAL OF INTELLIGENT ROBOTICS AND APPLICATIONS}, author={Atay, Stefan and Jenkins, Tyler and Buckner, Gregory and Bryant, Matthew}, year={2020}, month={Mar}, pages={3–20} }
 @article{gianikos_kirschmeier_gopalarathnam_bryant_2020, title={Limit cycle characterization of an aeroelastic wing in a bluff body wake}, volume={95}, ISSN={["1095-8622"]}, DOI={10.1016/j.jfluidstructs.2020.102986}, abstractNote={This paper presents an experimental investigation aimed at characterizing the kinematics of a pitching-heaving aeroelastic wing placed downstream of a rectangular bluff body. The influence of the bluff body wake on the wing is twofold: a viscous wake which produces a velocity deficit downstream and an oscillating induced velocity field due to periodic vortex shedding. The latter effect is the focus of this paper, specifically, the interaction between the wake frequency and the wing limit cycle oscillation (LCO) frequency. Wind tunnel experiments showed that the presence of the upstream bluff body causes modulation of the LCO amplitude. The modulation resembles a beat phenomenon, however the modulation frequency is related to the third harmonic of fLCO rather than the fundamental frequency. The modulation behavior also differs from that of a beat in that the spectral content contains sideband frequencies, characteristic of a multiplication between a carrier wave and a modulation wave rather than a simple sinusoidal superposition. Additionally, the streamwise spacing between the bluff body and the wing significantly influences the wing kinematics, with a closer spacing between the two bodies increasing the intensity of the amplitude modulation. For shedding frequencies sufficiently close to the LCO third harmonic, reducing this streamwise distance was shown to induce an alternation between two distinct modes of amplitude modulation, each with its own intensity and frequency.}, number={0}, journal={JOURNAL OF FLUIDS AND STRUCTURES}, author={Gianikos, Zachary N. and Kirschmeier, Benjamin A. and Gopalarathnam, Ashok and Bryant, Matthew}, year={2020}, month={May} }
 @article{jenkins_bryant_2020, title={Pennate actuators: force, contraction and stiffness}, volume={15}, ISSN={["1748-3190"]}, DOI={10.1088/1748-3190/ab860f}, abstractNote={Hierarchical actuators are comprised of multiple individual actuator elements arranged into a system, resulting in improved and expanded performance. Natural muscle tissue is a complex and multi-level example of hierarchical actuation, with its hierarchy spanning from the micrometer to the centimeter scale. In addition to a hierarchical configuration, muscle tissue exists in varying geometric arrangements. Pennate muscle tissue, denoted by its characteristic fibers extending obliquely away from the muscle tissue line of action, leverages geometric complexity to transform the relationship between fiber inputs and muscle tissue outputs. In this paper, a bioinspired hierarchical pennate actuator is detailed. This work expands on previous pennate actuator studies by deriving constitutive force, contraction, and stiffness models for a general pennate actuator, where the constituent fibers can be constructed from any linear actuator. These models are experimentally validated by studying a pennate actuator with McKibben artificial muscles constituting the actuator fibers. McKibben artificial muscles are used because they have a high force-to-weight ratio and are inexpensive to construct, making them an attractive candidate for hierarchical actuators and mobile robotics. Using the derived constitutive models, general pennate actuator performance is better understood by analyzing the transmission ratio, blocked force, and free contraction. Loaded contractions and stiffness during isotonic and isobaric contractions are also explored. The results allow for informed design decisions and an understanding of the associated tradeoffs when recreating the remarkable properties of pennate musculature. Future work will leverage the results of this paper to create an adaptive pennate actuator that is capable of changing configuration in response to force, contraction and stiffness demands.}, number={4}, journal={BIOINSPIRATION & BIOMIMETICS}, author={Jenkins, Tyler and Bryant, Matthew}, year={2020}, month={Jul} }
 @article{mazzoleni_kim_bryant_2020, title={The Effect of Resistive Forces in Variable Recruitment Fluidic Artificial Muscle Bundles: A Configuration Study}, volume={11374}, ISSN={["1996-756X"]}, DOI={10.1117/12.2557907}, abstractNote={The use of soft, compliant actuators has recently gained research attention as a potential approach to improve human-robot interaction compatibility. Fluidic artificial muscles, or McKibben actuators, are a popular class of soft actuator due to their low cost and high force-to-weight ratio. However, traditional McKibben actuators face efficiency problems, as in most actuation schemes, the actuator is sized for the largest possible load, resulting in energy loss when operating at lower force regimes. To address this issue, our group has developed a bio-inspired actuation strategy called variable recruitment. In variable recruitment, actuators are placed within a bundle and can be sequentially activated depending on the required load. This strategy mimics the hierarchical architecture of mammalian muscle tissue and improves system efficiency and bandwidth while allowing for variable stiffness properties. Previous variable recruitment models and controllers assume that the force output of each actuator is independent and that these forces sum to provide the total bundle force. However, our recent work has shown that there is significant interaction between actuators within a bundle, particularly at lower recruitment states. This is because at these states, inactive or partially activated actuators resist bundle motion and reduce total force production. In this paper, we study these resistive effects at low recruitment states by considering two different variable recruitment configurations: a fixed-end configuration (with resistive forces) and a tendon configuration (designed with tendons to eliminate resistive forces). We then assess the tradeoffs between the two configurations. We found that while using the tendon configuration eliminates resistive forces, if we consider both configurations with the same overall system length, the tendon configuration has less overall system free strain because its FAMs have to be shorter than those of the fixed-end configuration. However, despite this difference in free strain, our results still show that the tendon configuration can have higher maximum load capacity and efficiency than the fixed-end configuration and that the specific application and system requirements will dictate the proper configuration choice.}, journal={BIOINSPIRATION, BIOMIMETICS, AND BIOREPLICATION X}, author={Mazzoleni, Nicholas and Kim, Jeong Yong and Bryant, Matthew}, year={2020} }
 @article{chatterjee_bryant_2019, title={Analysis of tension-tunable clamped-clamped piezoelectric beams for harvesting energy from wind and vibration}, volume={30}, ISSN={["1530-8138"]}, DOI={10.1177/1045389X19862390}, abstractNote={This article presents a nonlinear structural modeling framework to analyze a tensioned doubly clamped energy harvesting device. The device is investigated for energy harvesting under base excitation loading, where the structure undergoes bending motion and aerodynamic loading, where the structure undergoes combined bending–torsion motion. Transfer-matrix method for analyzing torsional motion is modified to include the effects of applied axial preload tension. The tension-modulated mode shapes and natural frequencies of the structure, obtained using transfer-matrix method, are found to be in good agreement with the results obtained using a commercially available Finite Element software. Under base excitation loading, it is shown that the effects of tension on electromechanical coupling and stiffness create competing influences on the power efficiency of the system. For aerodynamic loading, increasing preload tension increases the cut-in wind speed of the device. However, beyond the cut-in wind speed, with proper selection of the preload tension, the tensioned structure can produce an order of magnitude more power than the untensioned case at the same flow speed.}, number={16}, journal={JOURNAL OF INTELLIGENT MATERIAL SYSTEMS AND STRUCTURES}, author={Chatterjee, Punnag and Bryant, Matthew}, year={2019}, month={Sep}, pages={2405–2420} }
 @article{vemula_bryant_2019, title={Bio-inspired orderly recruitment valve for fluidic articial muscles}, volume={10965}, ISBN={["978-1-5106-2585-3"]}, ISSN={["1996-756X"]}, DOI={10.1117/12.2514413}, abstractNote={The objective of this research is to design and analyze a novel hydraulic orderly recruitment valve (ORV) for actuation of fluidic artificial muscle (FAM) systems. More recently artificial muscles are gaining popularity due to their increased efficiency by employing strategies such as variable muscle recruitment. Variable recruitment employs selective recruitment of muscles depending on the load, akin to mammalian muscles. However, existing active variable recruitment systems use as many valves as the muscle actuators in the system. The proposed design of the ORV enables orderly recruitment of multiple FAMs in the system using a single valve. Modeling and analysis of the ORV is carried out to characterize its behavior and understand the dynamics of the system. The analytical model of the ORV is simulated along with an equivalent multi-valve setup to compare the abilities of both the systems to track a prescribed trajectory.}, journal={BIOINSPIRATION, BIOMIMETICS, AND BIOREPLICATION IX}, author={Vemula, Dheeraj and Bryant, Matthew}, year={2019} }
 @article{khatri_chatterjee_metoyer_mazzoleni_bryant_granlund_2019, title={Dual-Actuator Disc Theory for Turbines in Yaw}, volume={57}, ISSN={["1533-385X"]}, DOI={10.2514/1.J057740}, abstractNote={No AccessTechnical NotesDual-Actuator Disc Theory for Turbines in YawDheepak N. Khatri, Punnag Chatterjee, Rodney Metoyer, Andre P. Mazzoleni, Matthew Bryant and Kenneth O. GranlundDheepak N. KhatriNorth Carolina State University, Raleigh, North Carolina 27695*Graduate Research Assistant, Department of Mechanical and Aerospace Engineering.Search for more papers by this author, Punnag ChatterjeeNorth Carolina State University, Raleigh, North Carolina 27695*Graduate Research Assistant, Department of Mechanical and Aerospace Engineering.Search for more papers by this author, Rodney MetoyerNorth Carolina State University, Raleigh, North Carolina 27695*Graduate Research Assistant, Department of Mechanical and Aerospace Engineering.Search for more papers by this author, Andre P. MazzoleniNorth Carolina State University, Raleigh, North Carolina 27695†Associate Professor, Department of Mechanical and Aerospace Engineering. Associate Fellow AIAA.Search for more papers by this author, Matthew BryantNorth Carolina State University, Raleigh, North Carolina 27695‡Assistant Professor, Department of Mechanical and Aerospace Engineering.Search for more papers by this author and Kenneth O. GranlundNorth Carolina State University, Raleigh, North Carolina 27695§Assistant Professor, Department of Mechanical and Aerospace Engineering. Senior Member AIAA.Search for more papers by this authorPublished Online:23 Jan 2019https://doi.org/10.2514/1.J057740SectionsRead Now ToolsAdd to favoritesDownload citationTrack citations ShareShare onFacebookTwitterLinked InRedditEmail About References [1] Betz A., “Das Maximum der theoretisch möglichen Ausnützung des Windes durch Windmotoren,” Zeitschrift für das gesamte Turbinenwesen, 1920, pp. 26, 307–309. Google Scholar[2] Newman B. G., “Actuator Disc Theory for Vertical Wind Turbines,” Journal of Wind Engineering and Industrial Aerodynamics, Vol. 15, Nos. 1–3, 1983, pp. 347–355. doi:https://doi.org/10.1016/0167-6105(83)90204-0 JWEAD6 0167-6105 CrossrefGoogle Scholar[3] Rosenberg A., Selvaraj S. and Sharma A., “A Novel Dual-Rotor Turbine for Increased Wind Energy Capture,” Journal of Physics: Conference Series, Vol. 524, 2014, Paper 012078. doi:https://doi.org/10.1088/1742-6596/524/1/012078 JPCSDZ 1742-6588 CrossrefGoogle Scholar[4] Adams Z. and Chen J., “Flux-Line Theory: A Novel Analytical Model for Cycloturbines,” AIAA Journal, Vol. 55, No. 11, 2017, pp. 3851–3867. doi:https://doi.org/10.2514/1.J055804 AIAJAH 0001-1452 LinkGoogle Scholar[5] Anderson M., “Horizontal Axis Wind Turbines in Yaw,” Proceedings of the First British Wind Energy Association (BWEA) Wind Energy Workshop, 1979, pp. 57–67, http://adsabs.harvard.edu/abs/1979wien.work...57A. Google Scholar[6] Grant I., Parkin P. and Wang X., “Optical Vortex Tracking Studies of a Horizontal Axis Wind Turbine in Yaw Using Laser-Sheet, Flow Visualization,” Experiments in Fluids, Vol. 23, No. 6, 1997, pp. 513–519. doi:https://doi.org/10.1007/s003480050142 EXFLDU 0723-4864 CrossrefGoogle Scholar[7] Newman B. G., “Multiple Actuator Disc Theory for Wind Turbines,” Journal of Wind Engineering and Industrial Aerodynamics, Vol. 24, No. 3, 1986, pp. 215–225. doi:https://doi.org/10.1016/0167-6105(86)90023-1 JWEAD6 0167-6105 CrossrefGoogle Scholar[8] Howland M., Bossuyt J., Martinez-Tossas L., Meyers J. and Meneveau C., “Wake Structure in Actuator Disk Models of Wind Turbines in Yaw Under Uniform Inflow Conditions,” Journal of Renewable and Sustainable Energy, Vol. 8, No. 4, 2016, Paper 043301. doi:https://doi.org/10.1063/1.4955091 CrossrefGoogle Scholar Previous article Next article FiguresReferencesRelatedDetailsCited byPool-Based Tow System for Testing Tethered Hydrokinetic Devices Being Developed to Harvest Energy From Ocean CurrentsMarine Technology Society Journal, Vol. 57, No. 1Blade element momentum theory for a skewed coaxial turbineOcean Engineering, Vol. 269Closed-Loop-Flight-Based Combined Geometric and Structural Wing Design optimization Framework for a Marine Hydrokinetic Energy KiteDemonstration of a Towed Coaxial Turbine Subscale Prototype for Hydrokinetic Energy Harvesting in SkewCharacterization of the Steady-State Operating Conditions of Tethered Coaxial TurbinesIncreased Energy Conversion with a Horizontal Axis Turbine in TranslationModeling, simulation, and equilibrium analysis of tethered coaxial dual-rotor ocean current turbinesEnergy Conversion and Management, Vol. 243Experimental analysis of dual coaxial turbines in skewOcean Engineering, Vol. 215 What's Popular Volume 57, Number 5May 2019 CrossmarkInformationCopyright © 2018 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. All requests for copying and permission to reprint should be submitted to CCC at www.copyright.com; employ the eISSN 1533-385X to initiate your request. See also AIAA Rights and Permissions www.aiaa.org/randp. TopicsAerodynamicsAeronautical EngineeringAeronauticsConservation of Momentum EquationsEnergyEnergy FormsEnergy Forms, Production and ConversionEquations of Fluid DynamicsFlow RegimesFluid DynamicsFluid Flow PropertiesTurbinesTurbomachineryWind EngineeringWind Turbine KeywordsTurbinesYawConservation of MassHorizontal Axis TurbineFree Stream VelocityConservation EquationsTwo Dimensional FlowNavier Stokes EquationsKinetic EnergyFluid DensityAcknowledgmentsThis work was funded by a grant from the North Carolina Coastal Studies Institute. The authors would like to thank undergraduate research assistants Tyler Farr and Kyle Weiner for their contributions to these results.PDF Received27 August 2018Accepted2 December 2018Published online23 January 2019}, number={5}, journal={AIAA JOURNAL}, author={Khatri, Dheepak N. and Chatterjee, Punnag and Metoyer, Rodney and Mazzoleni, Andre P. and Bryant, Matthew and Granlund, Kenneth O.}, year={2019}, month={May}, pages={2204–2208} }
 @article{mazzoleni_bryant_2019, title={Toward synergistic performance of integrated wind-solar hybrid energy harvesting structures}, volume={10967}, ISBN={["978-1-5106-2589-1"]}, ISSN={["1996-756X"]}, DOI={10.1117/12.2514123}, abstractNote={This paper considers aerodynamic interactions among an array of tensioned ribbon energy harvesters capable of harvesting both wind and solar energy. Each harvester consists of a thin-film solar cell ribbon supported in tension by a pair of piezoelectric bimorph beams in an inverted-U configuration. These ribbons experience aeroelastic flutter when subjected to crossflow, and the energy from these vibrations can be harvested through the piezoelectric beams. The effect of wind speed on the interaction between two fluttering inverted U-shaped aeroelastic energy harvesters configured in a tandem array was investigated, as previous work suggests that synergistic wake interactions can occur between multiple fluttering energy harvesters. An experimental apparatus was constructed and two thin-film solar ribbons were placed in tandem at a fixed separation distance. Each ribbon was given an applied pre-tension, and wind tunnel testing was performed for a range of wind speeds between 7.5 m/s and 12.5 m/s for each ribbon when fluttering in isolation and when fluttering in tandem. Tandem array efficiency was calculated from the experimental data, and it was determined that there is a wind speed at which peak tandem array efficiency (significantly greater than unity) occurs. It was found that this peak corresponds to the wind speed at which constructive interference due to frequency lock between the two fluttering ribbons begins. Results also show tandem efficiency benefits in both the downstream and upstream harvester, as opposed to previous results that show benefits primarily in the downstream harvester. It is hypothesized that these upstream benefits are due to possible base excitations in the apparatus that have been transmitted by the downstream harvester.}, journal={ACTIVE AND PASSIVE SMART STRUCTURES AND INTEGRATED SYSTEMS XIII}, author={Mazzoleni, Nicholas and Bryant, Matthew}, year={2019} }
 @article{jenkins_bryant_2019, title={Variable stiffness soft robotics using pennate muscle architecture}, volume={10965}, ISBN={["978-1-5106-2585-3"]}, ISSN={["1996-756X"]}, DOI={10.1117/12.2514265}, abstractNote={Biological pennate muscles, denoted by muscle fibers arranged obliquely relative to the line of action, have shown the ability to passively regulate the effective transmission ratio coupling fiber contraction to overall muscle contraction. In this paper, a model for a bio-inspired variable-stiffness pennate actuator is developed. The pennate topology observed in natural musculature is leveraged to create an actuator capable of varying stiffness based on its mutable configuration. Variable Stiffness Actuators (VSA’s) are useful for roboticists and engineers because they enable features atypical of traditional, stiff kinematic linkages, such as energy storage or increased human-interactive safety. Typically, VSA’s are constructed of rigid materials, such as motors and springs. However, by utilizing non-rigid actuators in a pennate configuration, a pliable, soft VSA can be conceived. Previous studies have experimentally utilized McKibben artificial muscles and Twisted Coil Polymer wires in lieu of muscle fibers to recreate the pennate muscle architecture. This paper expands on previous pennate actuator studies by providing a general modeling framework, allowing roboticists to make informed design decisions and understand associated tradeoffs when recreating the remarkable properties of pennate musculature. Theoretical case studies are performed to better understand the design tradeoffs. The Variable Stiffness Pennate Actuator is a promising actuator configuration that can readily integrate with other bio-inspired actuator technologies, such as orderly recruitment.}, journal={BIOINSPIRATION, BIOMIMETICS, AND BIOREPLICATION IX}, author={Jenkins, Tyler and Bryant, Matthew}, year={2019} }
 @article{chatterjee_bryant_2018, title={Aeroelastic-photovoltaic ribbons for integrated wind and solar energy harvesting}, volume={27}, ISSN={["1361-665X"]}, DOI={10.1088/1361-665x/aacbbb}, abstractNote={This letter investigates the energy harvesting capabilities of a novel hybrid wind and solar transduction device. The proposed device is an inverted-U shaped structure with a pair of piezoelectric benders connected by a flexible, tensioned photovoltaic ribbon. The tensioned photovoltaic ribbon member undergoes aeroelastic flutter limit cycle oscillations (LCOs) at low wind speeds, leading to time-varying tension forces applied to the piezoelectric benders, producing cyclic deflections and output voltage. Experimental characterization of the proof of concept energy harvester indicates that it is possible to obtain several milliwatts (mW) of power using the centimeter scale device, with an output power of ∼12.1 mW at a combination of 12 m s−1 wind speed and 100 W m−2 of solar irradiance, for indirect solar applications. It is shown here that an optimal combination of applied tensile preload and preset angle of attack exists which produces the highest wind power output of the system. An explanation of this behavior is also provided through the analysis of data collected on the photovoltaic ribbon velocity, piezoelectric voltage signals, and high-speed imagery, providing further insight to the motion kinematics of the highly nonlinear system response. Interestingly, the solar power output has been observed to remain invariant with increasing vibration velocity. This suggests that the gain in solar power efficiency from vibration-induced convective cooling negates any losses from the deflected incidence angle of the photovoltaic ribbon. These results illustrate that there is a negligible performance penalty in adding wind energy harvesting capability to the solar cells with this device concept.}, number={8}, journal={SMART MATERIALS AND STRUCTURES}, author={Chatterjee, P. and Bryant, M.}, year={2018}, month={Aug} }
 @article{chapman_bryant_2018, title={Bio-inspired passive variable recruitment of fluidic artificial muscles}, volume={10593}, ISSN={["1996-756X"]}, DOI={10.1117/12.2296024}, abstractNote={This paper explores the modeling and analysis of the effect of minimum actuation pressure previously observed in literature. This minimum pressure is similar in kind to the minimum amplitude of the signal for muscle actuation seen in mammalian muscles. This minimum actuation, or “threshold” pressure is used a method of mechanically encoding the control of FAM engagement and the actuation efficiency of a group, or ‘bundle’ of muscles with differing threshold pressures is compared with a single muscle of equivalent force and strain. The results of this analysis indicate the efficacy of using this design and control method; it is advantageous in cases where a range of displacements and forces are necessary.}, journal={BIOINSPIRATION, BIOMIMETICS, AND BIOREPLICATION VIII}, author={Chapman, Edward M. and Bryant, Matthew}, year={2018} }
 @article{chapman_bryant_2018, title={Bioinspired passive variable recruitment of fluidic artificial muscles}, volume={29}, ISSN={["1530-8138"]}, DOI={10.1177/1045389X18783070}, abstractNote={ This article presents a novel, passive approach to creating variable actuator recruitment in bundles of fluidic artificial muscles. The passive recruitment control approach is inspired by the functionality of mammalian muscle tissues, in which a single activation signal from the nervous system sequentially triggers contraction of progressively larger actuation elements until the required force is generated. Biologically, this behavior is encoded by differences in electrical resistance properties between smaller and larger muscle-fiber groups. The approach presented here produces analogous behavior using a uniform applied pressure to all fluidic artificial muscles while creating differential pressure responses and threshold pressures among the fluidic artificial muscles via tailored bladder elasticity parameters. A model for using elastic bladder stiffness to control an artificial muscle bundle with a single valve is explored and used to compare a bundle of fluidic artificial muscles with both low and high threshold pressure units to a single fluidic artificial muscle of equivalent displacement and force capability. The results of this analysis indicate the efficacy of using this control method; it is advantageous in cases where a wide range of displacements and forces are necessary and can increase efficiency when the system primarily operates in a low-force regime but requires occasional bursts of high-force capability. }, number={15}, journal={JOURNAL OF INTELLIGENT MATERIAL SYSTEMS AND STRUCTURES}, author={Chapman, Edward M. and Bryant, Matthew}, year={2018}, month={Sep}, pages={3067–3081} }
 @article{waghela_bryant_wu_2018, title={Control design in cyber-physical fluid-structure interaction experiments}, volume={82}, ISSN={["0889-9746"]}, DOI={10.1016/j.jfluidstructs.2018.06.018}, abstractNote={Cyber-physical fluid dynamics is a hybrid experimental–computational approach to study fluid–structure interaction (FSI). It enables on-the-fly changes to structure inertia, damping, stiffness, and even kinematic constraints by replacing traditional elastically-mounted structures with actuators and a controller. The control design plays a central role in matching the closed-loop dynamics of the cyber-physical structure (CPS) to those of the desired structure. Control designs based on traditional proportional–integral–derivative (PID) and post-modern H∞ control are presented. The controllers are synthesized to match the linearized desired structural dynamics (or the input–output response) but no assumption of linearity is levied on the fluid behavior. To quantify the matching of input–output response, a CPS deviation index is defined based on H∞ norms. To evaluate and compare the performance of the control designs, two well-known FSI instabilities are considered, galloping and aeroelastic flutter. These FSI instabilities represent convenient test cases because they can be analyzed with linear aerodynamic models. Comparing the critical instability flow velocity and oscillation frequency of the CPS with different control designs and the desired mechanical structure demonstrates that the internal structure of the controller is crucial to fully matching the response of the desired structure. H∞ model-matching control with admittance causality is found to be the most adept control design for the CPS.}, journal={JOURNAL OF FLUIDS AND STRUCTURES}, author={Waghela, R. and Bryant, M. and Wu, F.}, year={2018}, month={Oct}, pages={86–100} }
 @article{chapman_jenkins_bryant_2018, title={Design and analysis of electrohydraulic pressure systems for variable recruitment in fluidic artificial muscles}, volume={27}, ISSN={["1361-665X"]}, DOI={10.1088/1361-665X/aadbff}, abstractNote={This paper investigates the energetics and performance of an electrohydraulic power system with variable recruitment fluidic artificial muscle (FAM) actuators. A coupled dynamic model of the system is developed and applied to study the implications of hydraulic power system architecture for both variable recruitment actuator bundles and equivalent single-muscle actuators. This analysis extends previous FAM literature by considering both actuator recruitment methodology as well as the complete electromechanical circuit and the interactions of these two subsystems. Specifically, for both single-muscle actuators and variable recruitment muscle bundles, hydraulic architectures with a continuously-operating motor and pump are compared with a system in which the motor is intermittently shut down and restarted based on accumulator pressure. The results reveal that variable recruitment offers bandwidth advantages over the single equivalent actuator regardless of the hydraulic power architecture that is selected. However, use of the intermittently-operating motor and pump system allowed the variable recruitment system to achieve efficiency advantages over the other configurations considered. A steady-state analytic solution for the operating envelopes of the variable recruitment and single-muscle systems, including force limits and flow rate limits, was also developed and used to investigate effects of pump displacement on system bandwidth and stroke. The results of these analyses provide tools for the selection of actuator configuration, system architecture, and component design in FAM-actuated electrohydraulic robots.}, number={10}, journal={SMART MATERIALS AND STRUCTURES}, author={Chapman, Edward M. and Jenkins, Tyler and Bryant, Matthew}, year={2018}, month={Oct} }
 @article{meller_kogan_bryant_garcia_2018, title={Model-based feedforward and cascade control of hydraulic McKibben muscles}, volume={275}, ISSN={["0924-4247"]}, DOI={10.1016/j.sna.2018.03.036}, abstractNote={McKibben artificial muscle actuators are predominantly pneumatically powered. Recently, hydraulic operation has gained interest due to its higher rigidity and efficiency. While there has been extensive control system development for pneumatic artificial muscles, little has been conducted hydraulically. This paper investigates three different controllers developed for a loaded robotic arm actuated with oil-hydraulic McKibben muscles. The goal was to achieve good angular position tracking over a range of frequencies up to 1 Hz. The first scheme, serving as the baseline, is a proportional-integral controller. The second architecture adds a nonlinear model-based feedforward term to the baseline controller; the feedforward includes the expected flowrate demands based on the actuator kinematics as well as the valve flow gain. The last scheme adds an inner pressure feedback loop to the second architecture. All controllers were evaluated with frequency and step response experiments. The results show that a simple proportional-integral controller has significant phase lag and attenuation at the higher frequencies tested; including the feedforward term almost completely eliminates these. The cascaded loop improves rise and settling times.}, journal={SENSORS AND ACTUATORS A-PHYSICAL}, author={Meller, Michael and Kogan, Boris and Bryant, Matthew and Garcia, Ephrahim}, year={2018}, month={Jun}, pages={88–98} }
 @inproceedings{waghela_bryant_2017, title={Control design for high-fidelity cyber-physical systems with applications to experimental fluid-structure interaction studies}, DOI={10.1115/smasis2017-3740}, abstractNote={A cyber-physical system (CPS) combines active actuation, sensing, and a control algorithm to virtually replicate a physical structure with desired inertia, stiffness, and damping properties. The interaction of a CPS with a fluid flow can be used to study complex fluid-structure interaction phenomena. This paper highlights some of the control design challenges associated with the design of CPS and elaborates on issues pertaining to performance and lag. A model for including the interaction force and a potential work-around to inertia compensation are presented. Finally, a case study compares classical PID control with H∞ based model-matching control design.}, booktitle={Proceedings of the asme conference on smart materials adaptive}, author={Waghela, R. and Bryant, M.}, year={2017} }
 @article{macleod_bryant_2017, title={Dynamic Modeling, Analysis, and Testing of a Variable Buoyancy System for Unmanned Multidomain Vehicles}, volume={42}, ISSN={["1558-1691"]}, DOI={10.1109/joe.2016.2586802}, abstractNote={This paper presents the system design and dynamic model of an active variable buoyancy system (VBS) actuator with applications to unmanned multidomain vehicles. Unmanned multidomain vehicles have a unique concept of operations that require nontraditional VBS designs. We present a VBS actuator design that focuses on vehicle design objectives of high endurance, stealth, and loitering while underwater. The design consists of an elastic bladder housed within a rigid ballast tank, hydraulic pump, and proportionally controlled vent valve. Ambient surrounding water is the system working fluid and the elastic bladder serves to separate the gas–water interface, eliminating the risk of the compressed gas escaping when venting the water during extreme pitch maneuvers. A nonlinear analytic model of the VBS is derived and used to examine the parameter design space and the effects on water flow rate, actuation force, and energy efficiency. The VBS actuator design is shown to require a smaller, denser energy storage device than a comparable buoyancy system that uses consumable compressed air. A vehicle model is studied that features forward and aft VBS actuators, which enables vehicle pitch control by shifting the center of gravity along the vehicle's longitudinal axis. The coupling between the VBS actuator dynamics and vehicle dynamics is presented and discussed. A proof-of-concept demonstration is presented and compared to the analytical system model.}, number={3}, journal={IEEE JOURNAL OF OCEANIC ENGINEERING}, author={MacLeod, Marc and Bryant, Matthew}, year={2017}, month={Jul}, pages={511–521} }
 @article{nabavi_chakrabortty_2017, title={Structured Identification of Reduced-Order Models of Power Systems in a Differential-Algebraic Form}, volume={32}, ISSN={0885-8950 1558-0679}, url={http://dx.doi.org/10.1109/tpwrs.2016.2554154}, DOI={10.1109/tpwrs.2016.2554154}, abstractNote={In a recent paper, we proposed a system identification method for constructing reduced-order models for the electro-mechanical dynamics of large power systems, divided into multiple coherent clusters, using Synchrophasors. Every cluster in the actual model was represented as an aggregate generator in the reduced-order model. An aggregate network graph connected one aggregate generator to another. In this paper, we extend this identification approach to differential-algebraic (DAE) models. First, every cluster is associated with a unique terminal bus, referred to as the pilot bus, that couples its internal network to the rest of the system. The proposed algorithm uses Synchrophasor measurements from the pilot buses to identify the dynamic model of the aggregate generator for each cluster using nonlinear least squares while retaining the identity of all the pilot buses. The resulting reduced-order model is in the form of a nonlinear electric circuit described by aggregate differential and algebraic equations. We illustrate our results using two case studies, one for the IEEE 9-bus power system and another for the IEEE 39-bus power system. We also discuss how these reduced-order DAE models may be useful for designing shunt controllers at the pilot buses by using Synchrophasor feedback.}, number={1}, journal={IEEE Transactions on Power Systems}, publisher={Institute of Electrical and Electronics Engineers (IEEE)}, author={Nabavi, Seyedbehzad and Chakrabortty, Aranya}, year={2017}, month={Jan}, pages={198–207} }
 @article{chatterjee_bryant_2016, title={Aeroelastic modeling of a Piezo-Solar tensioned energy harvesting ribbon}, volume={9799}, ISSN={["1996-756X"]}, DOI={10.1117/12.2222109}, abstractNote={A multifunctional compliant structure is proposed that can harvest electrical power from both incident sunlight and ambient mechanical energy including wind flow or vibration. The proposed energy harvesting device consists of a slender, ribbon-like, flexible thin film solar cell that is laminated with piezoelectric patches at either ends and mounted in the cross flow of wind in a clamped-clamped end condition with an adjustable axial preload. Taking this motivation forward a system model of the energy harvester is developed which captures the structural response of the solar ribbon and couples it with Theodorsen unsteady aerodynamics to predict the flutter boundary conditions as a function of applied axial preload tension. The model also accounts for geometric and material discontinuities, by effective use of Transfer Matrix Method (TMM) modeling technique both in bending and torsional degrees of freedom. This paper also derives TMM technique for torsional vibrations with an applied axial load from first principles, verifies the method and presents its applicability for the proposed energy harvester. The paper also points out that the flutter instability arises out of different structural modes at different values applied axial tension, with the help of a sample modal convergence plot. The analysis also presents the possibility to tune the solar ribbon to operate at an optimal reduced frequency by adjusting the applied axial preload.}, journal={ACTIVE AND PASSIVE SMART STRUCTURES AND INTEGRATED SYSTEMS 2016}, author={Chatterjee, Punnag and Bryant, Matthew}, year={2016} }
 @article{jenkins_chapman_bryant_2016, title={Bio-inspired online variable recruitment control of fluidic artificial muscles}, volume={25}, ISSN={["1361-665X"]}, DOI={10.1088/0964-1726/25/12/125016}, abstractNote={This paper details the creation of a hybrid variable recruitment control scheme for fluidic artificial muscle (FAM) actuators with an emphasis on maximizing system efficiency and switching control performance. Variable recruitment is the process of altering a system’s active number of actuators, allowing operation in distinct force regimes. Previously, FAM variable recruitment was only quantified with offline, manual valve switching; this study addresses the creation and characterization of novel, on-line FAM switching control algorithms. The bio-inspired algorithms are implemented in conjunction with a PID and model-based controller, and applied to a simulated plant model. Variable recruitment transition effects and chatter rejection are explored via a sensitivity analysis, allowing a system designer to weigh tradeoffs in actuator modeling, algorithm choice, and necessary hardware. Variable recruitment is further developed through simulation of a robotic arm tracking a variety of spline position inputs, requiring several levels of actuator recruitment. Switching controller performance is quantified and compared with baseline systems lacking variable recruitment. The work extends current variable recruitment knowledge by creating novel online variable recruitment control schemes, and exploring how online actuator recruitment affects system efficiency and control performance. Key topics associated with implementing a variable recruitment scheme, including the effects of modeling inaccuracies, hardware considerations, and switching transition concerns are also addressed.}, number={12}, journal={SMART MATERIALS AND STRUCTURES}, author={Jenkins, Tyler E. and Chapman, Edward M. and Bryant, Matthew}, year={2016}, month={Dec} }
 @article{jenkins_chapman_bryant_2016, title={Control approach development for variable recruitment artificial muscles}, volume={9799}, ISSN={["1996-756X"]}, DOI={10.1117/12.2222201}, abstractNote={This study characterizes hybrid control approaches for the variable recruitment of fluidic artificial muscles with double acting (antagonistic) actuation. Fluidic artificial muscle actuators have been explored by researchers due to their natural compliance, high force-to-weight ratio, and low cost of fabrication. Previous studies have attempted to improve system efficiency of the actuators through variable recruitment, i.e. using discrete changes in the number of active actuators. While current variable recruitment research utilizes manual valve switching, this paper details the current development of an online variable recruitment control scheme. By continuously controlling applied pressure and discretely controlling the number of active actuators, operation in the lowest possible recruitment state is ensured and working fluid consumption is minimized. Results provide insight into switching control scheme effects on working fluids, fabrication material choices, actuator modeling, and controller development decisions.}, journal={ACTIVE AND PASSIVE SMART STRUCTURES AND INTEGRATED SYSTEMS 2016}, author={Jenkins, Tyler E. and Chapman, Edward M. and Bryant, Matthew}, year={2016} }
 @inproceedings{chapman_macleod_bryant_2016, title={Electrohydraulic modeling of a fluidic artificial muscle actuation system for robot locomotion}, DOI={10.1115/smasis2015-8834}, abstractNote={Fluidic artificial muscles have the potential for a wide range of uses; from injury rehabilitation to high-powered hydraulic systems. Their modeling to date has largely been quasi-static and relied on the operator to adjust pressure so as to control force output and utilization while little work has been done to date to analyze the kinematics of the driving-systems involved in their operation. This paper establishes a combined electro-hydraulic model of a fluidic artificial muscle actuated climbing robot to establish a method for studying the relationships between muscle size, robot size and function, and system design. The study indicates a strong relationship between appropriate system component selection and not only system efficiency but individual component effectiveness. The results of the study show that robot mass, operating pressure, muscle size, and motor-pump selection have noteworthy impacts on the efficiency and thereby longevity of the robot for performing its task.}, booktitle={ASME Conference on Smart Materials, Adaptive Structures and Intelligent Systems, 2015, vol 1}, author={Chapman, E. and Macleod, M. and Bryant, M.}, year={2016} }
 @article{meller_chipka_volkov_bryant_garcia_2016, title={Improving actuation efficiency through variable recruitment hydraulic McKibben muscles: modeling, orderly recruitment control, and experiments}, volume={11}, ISSN={["1748-3190"]}, DOI={10.1088/1748-3190/11/6/065004}, abstractNote={Hydraulic control systems have become increasingly popular as the means of actuation for human-scale legged robots and assistive devices. One of the biggest limitations to these systems is their run time untethered from a power source. One way to increase endurance is by improving actuation efficiency. We investigate reducing servovalve throttling losses by using a selective recruitment artificial muscle bundle comprised of three motor units. Each motor unit is made up of a pair of hydraulic McKibben muscles connected to one servovalve. The pressure and recruitment state of the artificial muscle bundle can be adjusted to match the load in an efficient manner, much like the firing rate and total number of recruited motor units is adjusted in skeletal muscle. A volume-based effective initial braid angle is used in the model of each recruitment level. This semi-empirical model is utilized to predict the efficiency gains of the proposed variable recruitment actuation scheme versus a throttling-only approach. A real-time orderly recruitment controller with pressure-based thresholds is developed. This controller is used to experimentally validate the model-predicted efficiency gains of recruitment on a robot arm. The results show that utilizing variable recruitment allows for much higher efficiencies over a broader operating envelope.}, number={6}, journal={BIOINSPIRATION & BIOMIMETICS}, author={Meller, Michael and Chipka, Jordan and Volkov, Alexander and Bryant, Matthew and Garcia, Ephrahim}, year={2016}, month={Oct} }
 @inproceedings{chapman_jenkins_bryant_2016, title={Parametric study of a fluidic artificial muscle actuated electrohydraulic system}, DOI={10.1115/smasis2016-9044}, abstractNote={Fluidic artificial muscles have the potential for a wide range of uses; from injury rehabilitation to high-powered hydraulic systems. Their modeling to date has largely been quasi-static and relied on the operator to adjust pressure so as to control force output and utilization while little work has been done to analyze the kinematics of the driving-systems involved in their operation. This paper utilizes an established electro-hydraulic model to perform a study of the components of a fluidic artificial muscle actuated climbing robot. Its purpose is to determine the effect of the robotic subsystems on function and efficiency for a small-scale system in order to extrapolate more general design and analysis schemes for future use. Its results indicate that important aspects to consider in design of the hydraulic system are system payload, operating pressure, pump selection, and FAM construction.}, booktitle={Proceedings of the asme conference on smart materials adaptive}, author={Chapman, E. and Jenkins, T. and Bryant, M.}, year={2016} }
 @article{kirschmeier_bryant_2016, title={Toward efficient aeroelastic energy harvesting through limit cycle shaping}, volume={9799}, ISSN={["1996-756X"]}, DOI={10.1117/12.2218437}, abstractNote={Increasing demand to harvest energy from renewable resources has caused significant research interest in unsteady aerodynamic and hydrodynamic phenomena. Apart from the traditional horizontal axis wind turbines, there has been significant growth in the study of bio-inspired oscillating wings for energy harvesting. These systems are being built to harvest electricity for wireless devices, as well as for large scale mega-watt power generation. Such systems can be driven by aeroelastic flutter phenomena which, beyond a critical wind speed, will cause the system to enter into limitcycle oscillations. When the airfoil enters large amplitude, high frequency motion, leading and trailing edge vortices form and, when properly synchronized with the airfoil kinematics, enhance the energy extraction efficiency of the device. A reduced order dynamic stall model is employed on a nonlinear aeroelastic structural model to investigate whether the parameters of a fully passive aeroelastic device can be tuned to produce limit cycle oscillations at desired kinematics. This process is done through an optimization technique to find the necessary structural parameters to achieve desired structural forces and moments corresponding to a target limit cycle. Structural nonlinearities are explored to determine the essential nonlinearities such that the system’s limit cycle closely matches the desired kinematic trajectory. The results from this process demonstrate that it is possible to tune system parameters such that a desired limit cycle trajectory can be achieved. The simulations also demonstrate that the high efficiencies predicted by previous computational aerodynamics studies can be achieved in fully passive aeroelastic devices.}, journal={ACTIVE AND PASSIVE SMART STRUCTURES AND INTEGRATED SYSTEMS 2016}, author={Kirschmeier, Benjamin and Bryant, Matthew}, year={2016} }
 @article{macleod_bryant_2016, title={Variable Buoyancy System for Unmanned Multi-Domain Vehicles}, volume={9799}, ISSN={["1996-756X"]}, DOI={10.1117/12.2219289}, abstractNote={This paper presents the system design, construction, and testing of an active variable buoyancy system (VBS) actuator with applications to unmanned multi-domain vehicles. Unmanned multi-domain vehicles require nontraditional VBS designs because of their unique operation requirements. We present a VBS actuator design that targets multi-domain vehicle design objectives of high endurance, stealth, and underwater loitering. The design features a rigid ballast tank with an inner elastic bladder connected to a hydraulic pump and a proportionally controlled vent valve. The system working fluid is obtained from the ambient surrounding water and the elastic bladder separates the water from pressurized gas, thus preventing any gas from escaping during a venting operation. An analytic model of the VBS characterizing the system dynamics is derived. Ballast tank prototype design and construction is discussed. A VBS test platform vehicle is presented, featuring two ballast tanks, motor, pump, and RF receiver for control.}, journal={ACTIVE AND PASSIVE SMART STRUCTURES AND INTEGRATED SYSTEMS 2016}, author={MacLeod, Marc and Bryant, Matthew}, year={2016} }
 @article{gomez_bryant_garcia_2015, title={Low-Order Modeling of the Unsteady Aerodynamics in Flapping Wings}, volume={52}, ISSN={["1533-3868"]}, DOI={10.2514/1.c032962}, abstractNote={Reduced-order modeling of the unsteady aerodynamics generated by three-dimensional flapping wings undergoing three-dimensional motion is investigated. A low-order quasi-steady model based on rotational lift and a revised version incorporating dynamic stall are compared with experimental data generated with matching kinematics. The dynamic-stall-based model in general showed the ability to produce force curves with scale and shape similar to the experimental data, capturing most salient features but not properly resolving some of the peaks and troughs that appear post-half-cycle. The rotational lift-based model in general closely resembled the static force curves at the midcycle and overestimated the salient features around the half-cycle. This varying behavior between models is due to the primary differences in the influencing factors of the rotational lift and dynamic stall terms. These differences are exploited in a combined model that shows greater agreement with the experimental data.}, number={5}, journal={JOURNAL OF AIRCRAFT}, author={Gomez, Juan C. and Bryant, Matthew and Garcia, Ephrahim}, year={2015}, pages={1586–1595} }
 @article{meller_chipka_bryant_garcia_2015, title={Modeling of the energy savings of variable recruitment McKibben muscle bundles}, volume={9429}, ISSN={["1996-756X"]}, DOI={10.1117/12.2084444}, abstractNote={McKibben artificial muscles are often utilized in mobile robotic applications that require compliant and light weight actuation capable of producing large forces. In order to increase the endurance of these mobile robotic platforms, actuation efficiency must be addressed. Since pneumatic systems are rarely more than 30% efficient due to the compressibility of the working fluid, the McKibben muscles are hydraulically powered. Additionally, these McKibben artificial muscles utilize an inelastic bladder to reduce the energy losses associated with elastic energy storage in the usual rubber tube bladders. The largest energy losses in traditional valve-controlled hydraulic systems are found in the valving implementation to match the required loads. This is performed by throttling, which results in large pressure drops over the control valves and significant fluid power being wasted as heat. This paper discusses how these throttling losses are reduced by grouping multiple artificial muscles to form a muscle bundle where, like in skeletal muscle, more elements that make up the muscle bundle are recruited to match the load. This greatly lessens the pressure drops by effectively changing the actuator area, leading to much higher efficiencies over a broader operation envelope. Simulations of several different loading scenarios are discussed that reveal the benefits of such an actuation scheme.}, journal={BIOINSPIRATION, BIOMIMETICS, AND BIOREPLICATION 2015}, author={Meller, Michael A. and Chipka, Jordan B. and Bryant, Matthew J. and Garcia, Ephrahim}, year={2015} }
 @article{kirschmeier_bryant_2015, title={Soap film flow visualization investigation of oscillating wing energy harvesters}, volume={9429}, ISSN={["1996-756X"]}, DOI={10.1117/12.2086523}, abstractNote={With increasing population and proliferation of wireless electronics, significant research attention has turned to harvesting energy from ambient sources such as wind and water flows at scales ranging from micro-watt to mega-watt levels. One technique that has recently attracted attention is the application of bio-inspired flapping wings for energy harvesting. This type of system uses a heaving and pitching airfoil to extract flow energy and generate electricity. Such a device can be realized using passive devices excited by aeroelastic flutter phenomena, kinematic mechanisms driven by mechanical linkages, or semi-active devices that are actively controlled in one degree of freedom and passively driven in another. For these types of systems, numerical simulations have showed strong dependence on efficiency and vortex interaction. In this paper we propose a new apparatus for reproducing arbitrary pitch-heave waveforms to perform flow visualization experiments in a soap film tunnel. The vertically falling, gravity driven soap film tunnel is used to replicate flows with a chord Reynolds number on the order of 4x104. The soap film tunnel is used to investigate leading edge vortex (LEV) and trailing edge vortex (TEV) interactions for sinusoidal and non-sinusoidal waveforms. From a qualitative analysis of the fluid structure interaction, we have been able to demonstrate that the LEVs for non-sinusoidal motion convect faster over the airfoil compared with sinusoidal motion. Signifying that optimal flapping frequency is dependent on the motion profile.}, journal={BIOINSPIRATION, BIOMIMETICS, AND BIOREPLICATION 2015}, author={Kirschmeier, Benjamin and Bryant, Matthew}, year={2015} }
 @article{chatterjee_bryant_2015, title={Structural modelling of a compliant flexure flow energy harvester}, volume={24}, ISSN={["1361-665X"]}, DOI={10.1088/0964-1726/24/9/094007}, abstractNote={This paper presents the concept of a flow-induced vibration energy harvester based on a one-piece compliant flexure structure. This energy harvester utilizes the aeroelastic flutter phenomenon to convert flow energy to structural vibrational energy and to electrical power output through piezoelectric transducers. This flexure creates a discontinuity in the structural stiffness and geometry that can be used to tailor the mode shapes and natural frequencies of the device to the desired operating flow regime while eliminating the need for discrete hinges that are subject to fouling and friction. An approximate representation of the flexure rigidity is developed from the flexure link geometry, and a model of the complete discontinuous structure and integrated flexure is formulated based on the transfer matrix method. The natural frequencies and mode shapes predicted by the model are validated using finite element simulations and are shown to be in close agreement. A proof-of-concept energy harvester incorporating the proposed flexure design has been fabricated and investigated in wind tunnel testing. The aeroelastic modal convergence, critical flutter wind speed, power output and limit cycle behavior of this device is experimentally determined and discussed.}, number={9}, journal={SMART MATERIALS AND STRUCTURES}, author={Chatterjee, Punnag and Bryant, Matthew}, year={2015}, month={Sep} }
 @inproceedings{chatterjee_bryant_2015, title={Transfer Matrix Modeling of a Tensioned Piezo-Solar Hybrid Energy Harvesting Ribbon}, volume={9431}, DOI={10.1117/12.2086138}, abstractNote={This paper proposes a multifunctional compliant structure that can harvest electrical power from both incident sunlight and ambient mechanical energy including wind flow or vibration. The energy harvesting device consists of a slender, ribbon-like, flexible thin film solar cell that is laminated with piezoelectric patches. The harvester is mounted in longitudinal tension and subjected to a transverse wind flow to excite flow-induced aeroelastic vibrations. This paper formulates an analytic model of the bending dynamics of the device. We present a Transfer Matrix formulation that also accounts for the changes in natural frequencies and mode shapes of the system when subjected to axial loads in a beam. It also observed that mode shape obtained using TMM formulation shows numerical stability even for very high tensile loads providing results consistent with the geometric boundary conditions applied at the ends of a beam. This article also discusses about structurally modeling a piezo - solar energy harvester using TMM methodology, where a thin clampedclamped solar film is bonded with piezo patches having a much higher bending stiffness. Additionally, the effect of axial tension on the mode shape of the thin host structure of the piezo-solar ribbon is presented and it is shown how this tension can be used advantageously to affect the strain distribution of the entire structure and introduce higher strains at the piezo patches.}, booktitle={Active and passive smart structures and integrated systems 2015}, author={Chatterjee, P. and Bryant, M.}, year={2015} }
 @article{bryant_fitzgerald_miller_saltzman_kim_lin_garcia_2014, title={Climbing Robot Actuated by Meso-Hydraulic Artificial Muscles}, volume={9057}, ISSN={["1996-756X"]}, DOI={10.1117/12.2046368}, abstractNote={This paper presents the design, construction, experimental characterization, and system testing of a legged, wall-climbing robot actuated by meso-scale hydraulic artificial muscles. While small wall-climbing robots have seen increased research attention in recent years, most authors have primarily focused on designs for the gripping and adhesion of the robot to the wall, while using only standard DC servo-motors for actuation. This project seeks to explore and demonstrate a different actuation mechanism that utilizes hydraulic artificial muscles. A four-limb climbing robot platform that includes a full closed-loop hydraulic power and control system, custom hydraulic artificial muscles for actuation, an on-board microcontroller and RF receiver for control, and compliant claws with integrated sensing for gripping a variety of wall surfaces has been constructed and is currently being tested to investigate this actuation method. On-board power consumption data-logging during climbing operation, analysis of the robot kinematics and climbing behavior, and artificial muscle force-displacement characterization are presented to investigate and this actuation method.}, journal={ACTIVE AND PASSIVE SMART STRUCTURES AND INTEGRATED SYSTEMS 2014}, author={Bryant, Matthew and Fitzgerald, Jason and Miller, Samuel and Saltzman, Jonah and Kim, Sangkyu and Lin, Yong and Garcia, Ephrahim}, year={2014} }
 @inproceedings{chatterjee_bryant_2014, title={Design of a compliant flexure joint for use in a flow energy harvester}, DOI={10.1115/smasis2014-7503}, abstractNote={This paper presents an initial experimental and computational investigation of a flow-induced vibration energy harvester with a compliant flexure mechanism. This energy harvester utilizes the aeroelastic flutter phenomenon to convert the flow energy to vibrational energy which can be converted into useful electrical power using piezoelectric transducers. However, unlike previous flutter-based flow energy harvesters [1] which require assembling multiple components to create the necessary aeroelastic arrangement, the device described here utilizes a monolithic, compact design to achieve the same. In this paper, we propose a flexure design for this device and model it using analytic methods and finite element simulations. A proof of concept energy harvester incorporating this flexure design has been fabricated and experimentally investigated in wind tunnel testing.}, booktitle={Proceedings of the ASME Conference on Smart Materials, Adaptive Structures and Intelligent Systems, 2014, vol 2}, author={Chatterjee, P. and Bryant, M.}, year={2014} }
 @article{bryant_pizzonia_mehallow_garcia_2014, title={Energy Harvesting for Self-Powered Aerostructure Actuation}, volume={9057}, ISSN={["1996-756X"]}, DOI={10.1117/12.2046408}, abstractNote={This paper proposes and experimentally investigates applying piezoelectric energy harvesting devices driven by flow induced vibrations to create self-powered actuation of aerostructure surfaces such as tabs, flaps, spoilers, or morphing devices. Recently, we have investigated flow-induced vibrations and limit cycle oscillations due to aeroelastic flutter phenomena in piezoelectric structures as a mechanism to harvest energy from an ambient fluid flow. We will describe how our experimental investigations in a wind tunnel have demonstrated that this harvested energy can be stored and used on-demand to actuate a control surface such as a trailing edge flap in the airflow. This actuated control surface could take the form of a separate and discrete actuated flap, or could constitute rotating or deflecting the oscillating energy harvester itself to produce a non-zero mean angle of attack. Such a rotation of the energy harvester and the associated change in aerodynamic force is shown to influence the operating wind speed range of the device, its limit cycle oscillation (LCO) amplitude, and its harvested power output; hence creating a coupling between the device’s performance as an energy harvester and as a control surface. Finally, the induced changes in the lift, pitching moment, and drag acting on a wing model are quantified and compared for a control surface equipped with an oscillating energy harvester and a traditional, static control surface of the same geometry. The results show that when operated in small amplitude LCO the energy harvester adds negligible aerodynamic drag.}, journal={ACTIVE AND PASSIVE SMART STRUCTURES AND INTEGRATED SYSTEMS 2014}, author={Bryant, Matthew and Pizzonia, Matthew and Mehallow, Michael and Garcia, Ephrahim}, year={2014} }
 @article{bryant_meller_garcia_2014, title={Variable recruitment fluidic artificial muscles: modeling and experiments}, volume={23}, ISSN={["1361-665X"]}, DOI={10.1088/0964-1726/23/7/074009}, abstractNote={We investigate taking advantage of the lightweight, compliant nature of fluidic artificial muscles to create variable recruitment actuators in the form of artificial muscle bundles. Several actuator elements at different diameter scales are packaged to act as a single actuator device. The actuator elements of the bundle can be connected to the fluidic control circuit so that different groups of actuator elements, much like individual muscle fibers, can be activated independently depending on the required force output and motion. This novel actuation concept allows us to save energy by effectively impedance matching the active size of the actuators on the fly based on the instantaneous required load. This design also allows a single bundled actuator to operate in substantially different force regimes, which could be valuable for robots that need to perform a wide variety of tasks and interact safely with humans. This paper proposes, models and analyzes the actuation efficiency of this actuator concept. The analysis shows that variable recruitment operation can create an actuator that reduces throttling valve losses to operate more efficiently over a broader range of its force–strain operating space. We also present preliminary results of the design, fabrication and experimental characterization of three such bioinspired variable recruitment actuator prototypes.}, number={7}, journal={SMART MATERIALS AND STRUCTURES}, author={Bryant, Matthew and Meller, Michael A. and Garcia, Ephrahim}, year={2014}, month={Jul} }
 @article{bryant_gomez_garcia_2013, title={Reduced-Order Aerodynamic Modeling of Flapping Wing Energy Harvesting at Low Reynolds Number}, volume={51}, ISSN={["1533-385X"]}, DOI={10.2514/1.j052364}, abstractNote={Energy harvesting from flowing fluids using flapping wings and fluttering aeroelastic structures has recently gained significant research attention as a possible alternative to traditional rotary turbines, especially at and below the centimeter scale. One promising approach uses an aeroelastic flutter instability to drive limit cycle oscillations of a flexible piezoelectric energy harvesting structure. Such a system is well suited to miniaturization and could be used to create self-powered wireless sensors wherever ambient flows are available. In this paper, we examine modeling of the aerodynamic forces, power extraction, and efficiency of such a flapping wing energy harvester at a low Reynolds number on the order of 1000. Two modeling approaches are considered: a quasi-steady method generalized from existing models of insect flight and a modified model that includes terms to account for the effects of dynamic stall. These two modeling approaches are applied to predicting the instantaneous aerodynamic for...}, number={12}, journal={AIAA JOURNAL}, author={Bryant, Matthew and Gomez, Juan Carlos and Garcia, Ephrahim}, year={2013}, month={Dec}, pages={2771–2782} }
 @article{shafer_bryant_garcia_2012, title={Designing maximum power output into piezoelectric energy harvesters}, volume={21}, ISSN={0964-1726 1361-665X}, url={http://dx.doi.org/10.1088/0964-1726/21/8/085008}, DOI={10.1088/0964-1726/21/8/085008}, abstractNote={Energy harvesting from vibrational sources has been the focus of extensive research in the last decade, but fundamental questions remain concerning the design of these harvesters. We consider a piezoelectric bimorph energy harvester and seek to translate design requirements, such as mass and target natural frequency, into beam dimensions that maximize power output. Our method centers around optimizing the thickness of the piezoelectric layers of a beam relative to the total beam thickness, otherwise known as the thickness ratio. This method uses approximations for the fundamental frequency and mode shape. This allows for the development of algebraic expressions for the modal parameters required for the prediction of power output. The resulting expression for power is fully defined by the fixed system level requirements and the only unknown parameters, the piezoelectric thickness ratio and the damping ratio. We show in an example case that, for typical damping ratio values, the ideal thickness ratio is not significantly affected by changes in the damping ratio. As such, the method requires a simple sweep of the thickness ratio in order to determine the beam design which maximizes the power. We develop the design method for both systems where the piezoelectric material is continuous and where the thickness is selected from a discrete set of values. Because our method produces a single algebraic expression for the power, the resulting beam design can be developed extremely quickly from a set of design requirements, and thus does not require optimization algorithms. We also show that our design method achieves more power output and requires less piezoelectric material than an approach which maximizes the coupling coefficient.}, number={8}, journal={Smart Materials and Structures}, publisher={IOP Publishing}, author={Shafer, Michael W and Bryant, Matthew and Garcia, Ephrahim}, year={2012}, month={Jul}, pages={085008} }
 @article{shafer_bryant_garcia_2012, title={Erratum: Designing maximum power output into piezoelectric energy harvesters}, volume={21}, ISSN={0964-1726 1361-665X}, url={http://dx.doi.org/10.1088/0964-1726/21/10/109601}, DOI={10.1088/0964-1726/21/10/109601}, abstractNote={The full text of this article is available in the PDF provided.}, number={10}, journal={Smart Materials and Structures}, publisher={IOP Publishing}, author={Shafer, Michael W and Bryant, Matthew and Garcia, Ephrahim}, year={2012}, month={Aug}, pages={109601} }
 @article{bryant_mahtani_garcia_2012, title={Wake synergies enhance performance in aeroelastic vibration energy harvesting}, volume={23}, ISSN={["1530-8138"]}, DOI={10.1177/1045389x12443599}, abstractNote={ This study experimentally demonstrates that a closely spaced array of aeroelastic flutter energy harvesters can exploit synergistic wake interactions to outperform the same number of harvesters operating in isolation. The fluttering motion of each energy harvester imparts an oscillating vortex wake into the flow downstream of the device. Wind tunnel experiments with arrays of two and four flutter energy harvesters show that this wake structure has significant effects on the vibration amplitude, frequency, and power output of the trailing devices. These wake interaction effects are shown to vary with the stream-wise and cross-stream separation distance between the harvesters. Over a defined range of separations, an advantageous frequency lock-in between the devices arises. When this occurs, the trailing harvesters can extract additional energy from the wake of upstream harvesters, causing larger oscillation amplitudes and higher power output in the trailing devices. Experiments to characterize this variation in power output due to these wake interaction effects and to determine the optimal spacing of the energy harvesters are presented and discussed. Smoke-wire flow visualization is used to examine the wake structure and investigate the mechanism of the array interactions. }, number={10}, journal={JOURNAL OF INTELLIGENT MATERIAL SYSTEMS AND STRUCTURES}, author={Bryant, Matthew and Mahtani, Ranjeev L. and Garcia, Ephrahim}, year={2012}, month={Jul}, pages={1131–1141} }
 @article{bryant_wolff_garcia_2011, title={Aeroelastic flutter energy harvester design: the sensitivity of the driving instability to system parameters}, volume={20}, ISSN={["1361-665X"]}, DOI={10.1088/0964-1726/20/12/125017}, abstractNote={This study examines the design parameters affecting the stability characteristics of a novel fluid flow energy harvesting device powered by aeroelastic flutter vibrations. The energy harvester makes use of a modal convergence flutter instability to generate limit cycle bending oscillations of a cantilevered piezoelectric beam with a small flap connected to its free end by a revolute joint. The critical flow speed at which destabilizing aerodynamic effects cause self-excited vibrations of the structure to emerge is essential to the design of the energy harvester because it sets the lower bound on the operating wind speed and frequency range of the system. A linearized analytic model of the device that accounts for the three-way coupling between the structural, unsteady aerodynamic, and electrical aspects of the system is used to examine tuning several design parameters while the size of the system is held fixed. The effects on the aeroelastic system dynamics and relative sensitivity of the flutter stability boundary are presented and discussed. A wind tunnel experiment is performed to validate the model predictions for the most significant system parameters.}, number={12}, journal={SMART MATERIALS AND STRUCTURES}, author={Bryant, Matthew and Wolff, Eric and Garcia, Ephrahim}, year={2011}, month={Dec} }
 @article{bryant_garcia_2011, title={Modeling and testing of a novel aeroelastic flutter energy harvester}, volume={133}, DOI={10.1115/1.4002788}, abstractNote={This paper proposes a novel piezoelectric energy harvesting device driven by aeroelastic flutter vibrations of a simple pin connected flap and beam. The system is subject to a modal convergence flutter response above a critical wind speed and then oscillates in a limit cycle at higher wind speeds. A linearized analytical model of the device is derived to include the effects of the three-way coupling between the structural, unsteady aerodynamic, and electrical aspects of the system. A stability analysis of this model is presented to determine the frequency and wind speed at the onset of the flutter instability, which dictates the cut-in conditions for energy harvesting. In order to estimate the electrical output of the energy harvester, the amplitude and frequency of the flutter limit cycle are also investigated. The limit cycle behavior is simulated in the time domain with a semi-empirical nonlinear model that accounts for the effects of the dynamic stall over the flap at large deflections. Wind tunnel test results are presented to determine the empirical aerodynamic model coefficients and to characterize the power output and flutter frequency of the energy harvester as functions of incident wind speed.}, journal={Journal of Vibration and Acoustics}, author={Bryant, Matthew and Garcia, Ephrahim}, year={2011} }