@article{patil_acharya_ro_2020, title={Experimental investigation of water spray injection in liquid piston for near-isothermal compression}, volume={259}, ISSN={["1872-9118"]}, DOI={10.1016/j.apenergy.2019.114182}, abstractNote={Near-isothermal compression is desired to achieve high efficiency in many compressor applications. Low heat transfer characteristic of conventional compressors is a major bottleneck in attaining a near-isothermal compression. A high heat transfer rate is possible with an injection of a large number of water droplets using a spray nozzle inside the compression chamber. In this paper, the effectiveness of spray injection to achieve near-isothermal compression is investigated experimentally in a liquid piston compressor for a compression ratio of about 2.5. Parametric investigations are performed by varying injection pressures of spray from 10 psi (69 kPa) to 70 psi (483 kPa), using different spray nozzle angles (60°, 90°, and 120°), and by changing the stroke time of compression. It is observed that water spray injection is highly effective in abating the air temperature rise during the compression process. The pressure-volume plots indicate a significant reduction in the compression work, and they approach near-isothermal compression with spray at higher injection pressures. The isothermal efficiency of compression consistently increases with an increased injection pressure of spray and reaches up to 95% at the highest injection pressure studied (70 psi). Furthermore, the spray nozzle angle marginally affected the isothermal efficiency with a 1–4% improvement with the use of a 60° nozzle angle over a 120° spray angle at all injection pressures. Also, comparable isothermal efficiencies are observed for compression with different stroke times between 3 and 5 s especially at higher injection pressures which highlight the efficacy of spray injection in attaining a high power-density along with high efficiency. Overall, with an optimized spray design, water spray injection can achieve a highly efficient near-isothermal compression in liquid piston.}, journal={APPLIED ENERGY}, author={Patil, Vikram C. and Acharya, Pinaki and Ro, Paul I.}, year={2020}, month={Feb} }
 @article{patil_ro_2020, title={Experimental study of heat transfer enhancement in liquid piston compressor using aqueous foam}, volume={164}, ISSN={["1359-4311"]}, DOI={10.1016/j.applthermaleng.2019.114441}, abstractNote={Efficiency of gas compression can be significantly improved by achieving isothermal compression. A high heat transfer rate in the compression chamber is desired to achieve the isothermal compression process. A large surface area and a high heat transfer coefficient of aqueous foam can be used to achieve a significantly high heat transfer rate in the compression chamber. In this study, a novel heat transfer enhancement technique using aqueous foam is investigated in a compressor for achieving near-isothermal compression. Experiments are performed with the use of aqueous foam generated inside a liquid piston compressor. The volume of aqueous foam in the compression chamber, the air flow rate for foam generation, and various foam generator designs are considered in this parametric investigation. It is observed that the use of aqueous foam in the compression chamber is highly effective in reducing air temperature during the compression process. A higher volume of aqueous foam in the compression chamber leads to a significant increment in isothermal efficiency, however, with higher variability. The higher variability in efficiency is due to the higher cyclic variation of the temperature profiles during compression. A compression chamber completely filled with aqueous foam shows a 4–8% improvement in the efficiency for a compression ratio of 2.5. Moreover, several foam generator designs were tested to identify if there is any dependency of cyclic variability on foam generator design parameters. The results show some promise on optimizing the design to reduce the variability. Overall, the heat transfer enhancement using aqueous foam is effective in achieving an isothermal efficiency up to 92% compared to 86% for the no-foam case in a liquid piston compressor.}, journal={APPLIED THERMAL ENGINEERING}, author={Patil, Vikram C. and Ro, Paul I.}, year={2020}, month={Jan} }
 @article{patil_acharya_ro_2019, title={Experimental investigation of heat transfer in liquid piston compressor}, volume={146}, DOI={10.1016/j.applthermaleng.2018.09.121}, abstractNote={The use of liquid pistons is a promising approach for attaining efficient near-isothermal compression. One of the key factors affecting the efficiency of a liquid piston compressor is heat transfer. Understanding the heat transfer mechanism during compression is crucial for the design and development of an efficient liquid piston compressor. In this paper, heat transfer in the liquid piston compressor is studied experimentally for air compression. An analytical model is presented based on a thermal resistance circuit. Experiments are performed using compression chambers of different materials for a compression ratio of 2.05–2.35 with various stroke times of compression. It is observed that the rate of heat transfer increases with faster stroke time of compression. However, a faster compression process requires a higher compression work and results in a higher air temperature. The convective heat transfer coefficient of air decreases rapidly as compression proceeds and approaches a steady value towards the end of compression. Thermal resistance analysis for compression with different chamber materials indicates that convective thermal resistance of air has a significant contribution in the total thermal resistance. During the initial phase of compression, the high conductivity of the chamber material helps improve the overall heat transfer coefficient; however, it has a marginal effect during the later phase of compression. An isothermal compression efficiency of 84–86% is observed with the liquid piston.}, journal={APPLIED THERMAL ENGINEERING}, author={Patil, Vikram C. and Acharya, Pinaki and Ro, Paul I.}, year={2019}, pages={169–179} }
 @article{bhaskaran_ro_park_ramakrishnan_2017, title={Analysis of a Novel Technique for Temperature Rise Abatement in Liquid Piston Compressors-External Gas Injection}, volume={9}, ISSN={["1948-5093"]}, DOI={10.1115/1.4035969}, abstractNote={This paper analyses a novel heat transfer enhancement technique that can be used in compressors to limit the temperature rise during compression. This technique is based on the injection of external high-pressure gas into the chamber during the compression process. The impact of different factors on the effectiveness of this technique has been studied using experimental and computational methods. In the first set of trials, the location and angle of injection of the external air was varied. It was observed that the heat transfer coefficient governing the heat transfer rate from the chamber varied greatly with change in location and angle of injection. In the second set of experiments, the source pressure of the injected gas was varied from 100.66 kPa to 551.58 kPa. It was observed that the temperature rise of air in the chamber was reduced with an increase in source pressure. Additionally, the increase in chamber pressure was steeper in the higher source pressure cases. In the third set of experiments, the injection profile of the injected gas was varied. This parameter did not greatly impact the effectiveness of external gas injection. In the last set of experiments, the time of initiation of injection was varied. Earlier injection had a positive impact on reducing the temperature rise in the chamber. However, the pressure in the chamber was seen to increase more rapidly in the runs with early injection. Considering that these factors could have a positive/negative impact on the temperature and pressure in the chamber (work required for compression), it may be required to optimize the injection of external high-pressure gas depending on the application.}, number={2}, journal={JOURNAL OF THERMAL SCIENCE AND ENGINEERING APPLICATIONS}, author={Bhaskaran, Hari Subramaniam and Ro, Paul and Park, Joong-Kyoo and Ramakrishnan, Kishore Ranganath}, year={2017}, month={Jun} }
 @inproceedings{patil_ro_2017, title={Comparative assessment of different types of ocean compressed air energy storage systems based on exergy analysis}, DOI={10.1115/power-icope2017-3630}, abstractNote={Ocean compressed air energy storage (OCAES) is a promising large-scale energy storage concept. Different types of OCAES viz. - Diabatic, adiabatic and isothermal are possible based on the handling of heat in the system. In diabatic OCAES, compressed air is cooled in a cooler and heated using external heat source before transport to the expander. In Adiabatic OCAES, heat from the compressed air is stored in a thermal energy storage (TES) and reused to reheat compressed air before sending it to the expander. In Isothermal OCAES, air is compressed and expanded isothermally which results in the least compression work and highest expansion work. These OCAES configurations are assessed using exergy analysis in this paper. The exergy efficiency of individual components, exergy flow and overall efficiencies of diabatic, adiabatic and isothermal OCAES are presented. Results show that adiabatic OCAES shows improved efficiency over diabatic OCAES by storing thermal exergy of compressed air in TES and isothermal OCAES shows significantly higher efficiency over adiabatic and diabatic OCAES.}, booktitle={Proceedings of the ASME Power Conference Joint with ICOPE-17, 2017, vol 2}, author={Patil, V. C. and Ro, P. I.}, year={2017} }
 @article{patil_ro_ranganath_2016, title={End-To-End Efficiency of Liquid Piston based Ocean Compressed Air Energy Storage}, DOI={10.1109/oceans.2016.7761399}, abstractNote={Intermittent ocean energy resources need energy storage system for their optimal utilization. Ocean compressed air energy storage (OCAES) is a promising way for a utility scale energy storage. In this paper, a liquid piston based ocean compressed air energy storage is analyzed for end-to-end efficiency. An analytical model for end-to-end efficiency based on efficiencies of individual components in the liquid piston based OCAES is presented. Numerical analysis based on the analytical model and experimental data is done using Monte Carlo simulations. End-To-End efficiency of the liquid piston based OCAES is observed to be highly dependent on the polytropic index of compression and expansion. End-to-End efficiency increases from 24% to 72% with decrease in polytropic index from 1.4 to 1. Experimentally observed polytropic index shows 45% end-to-end efficiency. Comparison of results with end-to-end efficiency of existing compressed air energy storage systems indicates that the isothermal liquid piston based OCAES shows significantly higher end-to-end efficiency.}, journal={OCEANS 2016 MTS/IEEE MONTEREY}, author={Patil, Vikram C. and Ro, Paul I. and Ranganath, Kishore R.}, year={2016} }
 @article{ramakrishnan_ro_patil_2016, title={Temperature Abatement Using Hollow Spheres in Liquid Piston Compressor for Ocean Compressed Air Energy Storage System}, DOI={10.1109/oceans.2016.7761341}, abstractNote={This paper deals with a novel technique to curb the temperature raise during compression in a liquid piston compressor used in Ocean Compressed Air Energy Storage (OCAES) system. Hollow spheres made of various materials, viz. Silicon Carbide (SiC), High Density Polyethylene (HDPE), and Polypropylene (PP) were made to float on the top surface of the liquid column. It was observed that the temperature abatement in each of the three cases was very evident. The heat transfer does not depend on the material of the sphere, but the fact that there is a solid surface between water and air itself plays an important role along with the size of the sphere. The heat transfer per unit area from the simulation and the analytical model have been compared and the values are found to be very similar. Also, polytropic index of the compression process was evaluated in case without and with SiC spheres, and it was found to be closer to the isothermal index of 1 when the spheres are used.}, journal={OCEANS 2016 MTS/IEEE MONTEREY}, author={Ramakrishnan, Kishore Ranganath and Ro, Paul. I. and Patil, Vikram. C.}, year={2016} }
 @article{kim_ro_2015, title={Feasibility Study on Thermoacoustic Cooling for Low-Power Handheld Electronic Devices}, volume={7}, ISSN={["1948-5093"]}, DOI={10.1115/1.4029351}, abstractNote={A feasibility study on developing a small-scale thermoacoustic cooler based on form and size factors for a typical cell phone is presented. First, an approximate analytical model for the temperature difference was derived using the linear theory of thermoacoustics. Cooling performance could be reasonably predicted with the analytical model proposed in this study. Air and helium as the working gases and the operating frequencies of 3 kHz for air and 9.2 kHz for helium are considered within the scope of typical cell phone configurations. A stack as a core of thermoacoustic cooler is designed to accomplish the most effective performance based on normalized parameters. For the 57 mm thermoacoustic cooler operating at 3 kHz with air, the maximum temperature difference of 23.13 °C across the stack in the resonance cavity is achieved with a drive ratio of 2% with air as the medium and Mylar as a stack material. This temperature difference varies depending on the stack placement along the length of the resonance cavity, but the maximum difference was achieved when the center of stack is placed at around 7 mm away from the driver end. The drive ratio, which is proportional to the power required to produce the thermoacoustic effect, is shown to be directly related to the cooling performance achieved by thermoacoustic drivers. For example, while a drive ratio of 2% results in a temperature difference of over 20 °C at its maximum, a drive ratio of 0.2% causes a temperature difference less than 1 °C. This will be one of hardware issues to be considered in making commercially viable products. The possibility of omitting heat exchangers in the thermoacoustic cooler is investigated considering their manufacturing cost and the relatively minute improvement they bring to overall cooling for small-scale systems. The numerical result of the thermoacoustic cooling system based on design environment for low-amplitude thermoacoustic energy conversion (DeltaEC) is compared to the theoretical result. Discrepancies between the two results exist in the range of 10–15% mainly due to the limitation imposed by short stack considerations and the linear theory of thermoacoustics.}, number={2}, journal={JOURNAL OF THERMAL SCIENCE AND ENGINEERING APPLICATIONS}, author={Kim, Jangwoo and Ro, Paul I.}, year={2015}, month={Jun} }
 @article{kim_ro_2015, title={Forced Convection Cooling of Low-Power Handheld Devices Using a Vibrating Cantilever Beam}, volume={7}, ISSN={["1948-5093"]}, DOI={10.1115/1.4029677}, abstractNote={In this study, a convection cooling technique for handheld electronic devices is proposed and investigated. The technique uses bulk airflows generated by a vibrating cantilever beam actuated by a rotating imbalance motor. Analytic coupled physics modeling using an approximate integral method within laminar-flow boundary layers was used to analyze the proposed cooling technique. The cantilever beam and enclosure were designed based on the form factors of a typical handheld device. The bulk airflow cooling performances at various probe locations were investigated experimentally for low and high heating loads and numerically verified. The results indicate that a higher heating load of the heat source results in a larger temperature drop at the same convection rate. Also, for the probe locations away from the heat source and closer to the beam, the resulting temperature drops were relatively small despite a stronger velocity field generated by the beam. This is due first to the heat generated by the vibrating beam itself and second to a circulation of the air heated by the heat source to the rest of the regions in the enclosure. In general, a good agreement between experimental and numerical results was attained, even though a slight difference between two results exists. Overall, significant cooling was achieved by the proposed system. With a beam tip deflection of ±4 mm, nearly an 18-fold increase in the cooling performance was achieved compared to a natural convection case. Furthermore, the cooling performance continues to increase as the tip deflection of the cantilever beam increases. Thus, a cooling system using the bulk airflow generated by a vibrating cantilever beam has much potential as a feasible solution for electronic handheld devices.}, number={2}, journal={JOURNAL OF THERMAL SCIENCE AND ENGINEERING APPLICATIONS}, author={Kim, Jangwoo and Ro, Paul I.}, year={2015}, month={Jun} }
 @article{park_ro_he_mazzoleni_2014, title={Analysis, Fabrication, and Testing of a Liquid Piston Compressor Prototype for an Ocean Compressed Air Energy Storage (OCAES) System}, volume={48}, ISSN={["1948-1209"]}, DOI={10.4031/mtsj.48.6.3}, abstractNote={AbstractPrevious work concerning ocean compressed air energy storage (OCAES) systems has revealed the need for an efficient means for compressing air that minimizes the energy lost to heat during the compression process. In this paper, we present analysis, simulation, and
 testing of a tabletop proof-of-concept experiment of a liquid piston compression system coupled with a simulated OCAES system, with special attention given to heat transfer issues. An experimental model of a liquid piston system was built and tested with two different materials, polycarbonate
 and aluminum alloy, used for the compression chamber. This tabletop liquid piston system was tested in conjunction with a simulated OCAES system, which consisted of a hydrostatic tank connected to a compressed-air source from the wall to mimic the constant hydrostatic pressure at ocean depth
 experienced by the air stored in an actual OCAES system. Good agreement was found between the experimental and numerical studies and demonstrated that the heat transfer characteristics of a liquid piston compression process are effective in reducing the increase in air temperature that occurs
 during the compression process. The results also suggest that it may be possible to achieve a near-isothermal process with a fully optimized liquid piston compression system.}, number={6}, journal={Marine Technology Society Journal}, author={Park, J. and Ro, P. and He, X. and Mazzoleni, A.}, year={2014}, pages={86–97} }
 @article{lim_mazzoleni_park_ro_quinlan_2013, title={Conceptual Design of Ocean Compressed Air Energy Storage System}, volume={47}, ISSN={["1948-1209"]}, DOI={10.4031/mtsj.47.2.5}, abstractNote={AbstractIn this paper, an ocean compressed air energy storage (OCAES) system is introduced as a utility-scale energy storage option for electricity generated by wind, ocean currents, tides, and waves off the coast of North Carolina. Geographically, a location from 40 to 70
 km off the coast of Cape Hatteras is shown to be a good location for an OCAES system. Building upon existing compressed air energy storage (CAES) system designs, a conceptual design of an OCAES system with thermal energy storage (TES) is presented. A simple thermodynamic analysis is presented
 for an adiabatic CAES system which shows that the overall efficiency is 66%. In addition, finite element simulations are presented, which show the flow induced loads that will be experienced by OCAES air containers on the ocean floor. We discuss the fact that the combination of the buoyancy
 force and flow-induced lift forces (due to ocean currents) generates a periodic loading on the storage container and seabed, and how this presents engineering challenges related to the development of methods for reliably resisting these loads for decades in a corrosive environment. We also
 present a system, based on hydrolysis, which can be used for storing energy (in the form of oxygen and hydrogen gas) in containers on the ocean floor.}, number={2}, journal={MARINE TECHNOLOGY SOCIETY JOURNAL}, author={Lim, Saniel D. and Mazzoleni, Andre P. and Park, Joong-kyoo and Ro, Paul I. and Quinlan, Brendan}, year={2013}, pages={70–81} }
 @article{park_ro_2013, title={Noncontact Manipulation of Light Objects Based on Parameter Modulations of Acoustic Pressure Nodes}, volume={135}, ISSN={["1528-8927"]}, DOI={10.1115/1.4023816}, abstractNote={An investigation of noncontact manipulation techniques based on acoustic levitation was undertaken in air. The standing wave acoustic levitation (SWAL) was observed when standing waves trap small objects at pressure nodes. In this paper, two ultrasonic bolt-clamped Langevin type transducers (BLTs) generating traveling waves by modulating parameters of the two traveling waves were used to manipulate a trapped object. Frequency, amplitude, and phase modulations of the two actuators were exploited. From simulation and experiments, the phase modulation was prominent among other methods due to its long range and smooth operation. It is also found that angles between two actuators affect the trajectory of the trapped object during the parameter modulations. Sinusoidal and elliptic paths of the object were observed experimentally through a combination of parameters at certain tilt angles.}, number={3}, journal={JOURNAL OF VIBRATION AND ACOUSTICS-TRANSACTIONS OF THE ASME}, author={Park, Joong-kyoo and Ro, Paul I.}, year={2013}, month={Jun} }
 @inproceedings{park_ro_lim_mazzoleni_quinlan_2012, title={Analysis and optimization of a quasi-isothermal compression and expansion cycle for Ocean Compressed Air Energy Storage (OCAES)}, DOI={10.1109/oceans.2012.6404964}, abstractNote={A numerical analysis of a quasi-isothermal thermodynamic cycle was undertaken for its application in an underwater energy storage system. The conceptual basis for the quasi-isothermal process is firstly a use of water pistons, as opposed to air or other gas medium, which improve heat transfer rate and minimize the temperature variation on both compression and expansion sides of the cycle and secondly a use of mechanical design that maximizes a surface area of heat transfer. Numerical analysis of the heat transfer cycle confirms the validity of the quasi-isothermal nature of the water pistons. Design factors such as surface area, stroke displacement, and frequency of piston action can be analyzed for optimality. For a case study, a recent commercial design of the quasi-isothermal process is introduced and partially analyzed for its effectiveness. Impact of varying several design factors have been analyzed numerically for further understanding of optimality and for validating the quasi-isothermal nature of the design.}, booktitle={2012 Oceans}, author={Park, J. K. and Ro, P. I. and Lim, S. D. and Mazzoleni, A. P. and Quinlan, B.}, year={2012} }
 @inproceedings{lim_mazzoleni_park_ro_quinlan_2012, title={Conceptual design of ocean compressed air energy storage system}, DOI={10.1109/oceans.2012.6404909}, abstractNote={In this paper, an ocean compressed air energy storage (OCAES) system is introduced as a utility scale energy storage option for electricity generated by wind, ocean currents, tides, and waves off the coast of North Carolina. Geographically, a location from 40km to 70km off the coast of Cape Hatteras is shown to be a good location for an OCAES system. Based on existing compressed air energy storage (CAES) system designs, a conceptual design of an OCAES system with thermal energy storage (TES) is presented. A simple thermodynamic analysis is presented for an adiabatic CAES system which shows that the overall efficiency is 65.9%. In addition, finite element simulations are presented which show the flow induced loads which will be experienced by OCAES air containers on the ocean floor. We discuss the fact that the combination of the buoyancy force and the flow induced lift forces (due to ocean currents) generates a periodic loading on the storage container and seabed, and how this presents engineering challenges related to the development of adequate anchoring systems. We also present a system, based on hydrolysis, which can be used for storing energy (in the form of oxygen and hydrogen gas) in containers on the ocean floor.}, booktitle={2012 Oceans}, author={Lim, S. D. and Mazzoleni, A. P. and Park, J. K. and Ro, P. I. and Quinlan, B.}, year={2012} }
 @article{qin_xu_cao_ro_zhu_2012, title={Measuring True Young's Modulus of a Cantilevered Nanowire: Effect of Clamping on Resonance Frequency}, volume={8}, ISSN={1613-6810}, url={http://dx.doi.org/10.1002/smll.201200314}, DOI={10.1002/smll.201200314}, abstractNote={Abstract}, number={16}, journal={Small}, publisher={Wiley}, author={Qin, Qingquan and Xu, Feng and Cao, Yongqing and Ro, Paul I. and Zhu, Yong}, year={2012}, month={May}, pages={2571–2576} }
 @article{choi_kim_moon_ro_2009, title={Study on a ultra-light dual revolute manipulator with high joint torque}, volume={10}, ISSN={["2005-4602"]}, DOI={10.1007/s12541-009-0070-8}, number={4}, journal={INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING}, author={Choi, Hyeung-Sik and Kim, Tae-Hyeung and Moon, Woong-Ju and Ro, Paul I.}, year={2009}, month={Oct}, pages={49–56} }
 @article{panusittikorn_ro_2008, title={Modeling and control of a magnetostrictive tool servo system}, volume={130}, ISSN={["0022-0434"]}, DOI={10.1115/1.2837432}, abstractNote={A magnetostrictive actuator offers a long mechanical strain output in a broad bandwidth at a cost of a highly nonlinear magnetic hysteresis. Full utilization of this actuator in precision manufacturing requires a feedback loop as well as an advanced control scheme. A robust control scheme using sliding mode control with a variable switching gain was tailored to the nonlinear transducer. Nominal feedforward current controller that drives the magnetostriction was based on the inverse anhysteresis model. An additional switching gain based on the Lyapunov stability condition is implemented to restrain uncertainties. Compared to a traditional closed-loop control design, the proposed algorithm experimentally showed a greatly enhanced performance.}, number={3}, journal={JOURNAL OF DYNAMIC SYSTEMS MEASUREMENT AND CONTROL-TRANSACTIONS OF THE ASME}, author={Panusittikorn, Witoon and Ro, Paul I.}, year={2008}, month={May} }
 @article{wan_wu_chastain_roberts_kuznetsov_ro_2005, title={Forced convective cooling via acoustic streaming in a narrow channel established by a vibrating piezoelectric bimorph}, volume={74}, ISSN={["1573-1987"]}, DOI={10.1007/s10494-005-4132-4}, abstractNote={Forced convection in a narrow channel is investigated both numerically and experimentally. The flow field is established through the mechanism of acoustic streaming. This is accomplished by high frequency vibration of one of the channel walls, which is composed of a piezoelectric bimorph. In the numerical computations, the Navier-Stokes equations are decomposed into the acoustic equations and the streaming equations by the perturbation method. The acoustic field is first numerically obtained, which provides the driving force for the streaming field. The streaming field and the associated temperature field are then obtained numerically. Heat losses from a heat source are measured to determine the efficiency of this as a cooling method. The air-flow patterns in the channel between the heat source and the bimorph actuator are visualized using the particle tracking velocimetry. The visualization clearly shows that vortical streaming (acoustic streaming) can be induced by bimorph vibration, which enhances heat transfer between the heat source and the surrounding air. The temperature decreases obtained computationally and experimentally are in good agreement.}, number={2}, journal={FLOW TURBULENCE AND COMBUSTION}, author={Wan, Q and Wu, T and Chastain, J and Roberts, WL and Kuznetsov, AV and Ro, PI}, year={2005}, month={Mar}, pages={195–206} }
 @article{wu_ro_2005, title={Heat transfer performance of a cooling system using vibrating piezoelectric beams}, volume={15}, ISSN={["1361-6439"]}, DOI={10.1088/0960-1317/15/1/030}, abstractNote={A prototype of a miniature cooling system for microelectronics using vibrating piezoelectric beam was proposed and constructed. The flow patterns and cooling effects of the system were investigated experimentally. The vibration characteristics of the piezoelectric beam were simulated by the finite element method. Cooling effects were measured in terms of the temperature drop of the heat source above the vibrating beam. The electric field applied on the piezoelectric beam and the gap between heat source and actuator were adjusted to find the best cooling result. A temperature drop of 25.9 °C from 92 °C can be observed for the heat source when operating at a certain condition, which indicates the feasibility of using the present miniature cooling system in small devices. The heat transfer performance of the cooling system was analyzed as well. The results show that the enhancement of heat transfer between the heat source and the beam can be up to 210% with the acoustic streaming generated by the beam vibration in the present experimental studies. The enhanced heat transfer can be attributed to the generation of vortical streaming by the vibrating beam which was also captured in the flow visualization experiments.}, number={1}, journal={JOURNAL OF MICROMECHANICS AND MICROENGINEERING}, author={Wu, T and Ro, PI}, year={2005}, month={Jan}, pages={213–220} }
 @article{gutierrez_ro_2005, title={Magnetic servo levitation by sliding-mode control of nonaffine systems with algebraic input invertibility}, volume={52}, ISSN={["1557-9948"]}, DOI={10.1109/TIE.2005.855651}, abstractNote={Magnetic Servo Levitation (MSL) is an important actuation principle with potential applications ranging from ultrahigh-precision positioning to high-speed rail systems. This paper describes a nonlinear controller design technique for MSL that has inherent robustness to both parametric uncertainties and unmodeled dynamics. Most of the currently available literature on sliding mode considers nonlinear systems that are linear (affine) in the input action. The proposed technique allows designing sliding-mode controllers for the family of nonaffine problems that have an input nonlinearity algebraically invertible with respect to the available control action. This differs from the standard approach of input feedback linearization, and is based on a modified sliding condition that can be used to synthesize a switching control law. An equivalent control term can also be included, substantially enhancing the performance of the controller. Experimental results show that the proposed technique can achieve excellent tracking at high speeds in a fast-tool servo system actuated by MSL.}, number={5}, journal={IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS}, author={Gutierrez, HM and Ro, PI}, year={2005}, month={Oct}, pages={1449–1455} }
 @article{wu_ro_2004, title={Dynamic peak amplitude analysis and bonding layer eiiects of piezoelectric bimorph cantilevers}, volume={13}, ISSN={["1361-665X"]}, DOI={10.1088/0964-1726/13/1/024}, abstractNote={An analytical prediction of dynamic performance for piezoelectric bimorph structures was investigated. A damping ratio was assumed and employed to determine dynamic peak amplitudes at resonances by finding the frequency with of the peak amplitude. Finite element simulations were used to validate the proposed improvement strategy. Results show that the peak amplitude determination method is good enough to predict the dynamic performance of piezoelectric bimorphs. The effects of bonding layers were also analysed by both static and dynamic methods. The bonding influence can be minimized by selecting appropriate bonding materials and dimensions of structures.}, number={1}, journal={SMART MATERIALS AND STRUCTURES}, author={Wu, T and Ro, PI}, year={2004}, month={Feb}, pages={203–210} }
 @article{panusittikorn_lee_ro_2004, title={Modeling and sliding-mode control of friction-based object transport using two-mode ultrasonic excitation}, volume={51}, ISSN={["1557-9948"]}, DOI={10.1109/TIE.2004.831761}, abstractNote={This paper introduces a magnetic-free object transport system driven by a friction force on an ultrasonic flexural vibrating beam. This driving mechanism offers unique advantages of silent operation and lubricant-free structure. The characteristics are ideal for a transport system in a clean-room environment. However, the friction drive described by Hertzian contact and Coulomb friction theories is highly nonlinear. The transport system needs a sophisticated control design to deliver a load to a precise location. A traditional feedback controller and sliding-mode control were implemented in the presence of sensor noise and hardware limitation. It is experimentally shown that the tracking performance of the proposed control scheme is superior to the traditional control design.}, number={4}, journal={IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS}, author={Panusittikorn, W and Lee, MC and Ro, PI}, year={2004}, month={Aug}, pages={917–926} }
 @article{wu_ro_kingon_mulling_2003, title={Piezoelectric resonating structures for microelectronic cooling}, volume={12}, DOI={10.1088/0964-1726/12/2/304}, abstractNote={The design of piezoelectric resonating structures was investigated for generating acoustic streaming which may be used for cooling microelectronic components. The vibration characteristics of different piezoelectric structures were simulated by the finite element method and validated with analytical approaches. Considering the product of resonance frequency and dynamic tip deflection as a performance merit, the effects of length and location of the actuators as well as the boundary conditions were analyzed for four different piezoelectric resonator designs. Results show that there exist optimal length and location of actuators on the passive structures.}, number={2}, journal={Smart Materials & Structures}, author={Wu, T. and Ro, P. I. and Kingon, A. I. and Mulling, J. F.}, year={2003}, pages={181–187} }
 @article{loh_hyun_ro_kleinstreuer_2002, title={Acoustic streaming induced by ultrasonic flexural vibrations and associated enhancement of convective heat transfer}, volume={111}, ISSN={["0001-4966"]}, DOI={10.1121/1.1433811}, abstractNote={Acoustic streaming induced by ultrasonic flexural vibrations and the associated convection enhancement are investigated. Acoustic streaming pattern, streaming velocity, and associated heat transfer characteristics are experimentally observed. Moreover, analytical analysis based on Nyborg’s formulation is performed along with computational fluid dynamics (CFD) simulation using a numerical solver CFX 4.3. Two distinctive acoustic streaming patterns in half-wavelength of the flexural vibrations are observed, which agree well with the theory. However, acoustic streaming velocities obtained from CFD simulation, based on the incompressible flow assumption, exceed the theoretically estimated velocity by a factor ranging from 10 to 100, depending upon the location along the beam. Both CFD simulation and analytical analysis reveal that the acoustic streaming velocity is proportional to the square of the vibration amplitude and the wavelength of the vibrating beam that decreases with the excitation frequency. It is observed that the streaming velocity decreases with the excitation frequency. Also, with an open-ended channel, a substantial increase in streaming velocity is observed from CFD simulations. Using acoustic streaming, a temperature drop of 40 °C with a vibration amplitude of 25 μm at 28.4 kHz is experimentally achieved.}, number={2}, journal={JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA}, author={Loh, BG and Hyun, S and Ro, PI and Kleinstreuer, C}, year={2002}, month={Feb}, pages={875–883} }
 @article{kim_ro_2002, title={An accurate full car ride model using model reducing techniques}, volume={124}, ISSN={["1528-9001"]}, DOI={10.1115/1.1503065}, abstractNote={In this study, an approach to obtain an accurate yet simple model for full-vehicle ride analysis is proposed. The approach involves linearization of a full car MBD (multibody dynamics) model to obtain a large-order vehicle model. The states of the model are divided into two groups depending on their effects on the ride quality and handling performance. Singular perturbation method is then applied to reduce the model size. Comparing the responses of the proposed model and the original MBD model shows an accurate matching between the two systems. A set of identified parameters that makes the well-known seven degree-of-freedom model very close to the full car MBD model is obtained. Finally, the benefits of the approach are illustrated through design of an active suspension system. The identified model exhibits improved performance over the nominal models in the sense that the accurate model leads to the appropriate selection of control gains. This study also provides an analytical method to investigate the effects of model complexity on model accuracy for vehicle suspension systems.}, number={4}, journal={JOURNAL OF MECHANICAL DESIGN}, author={Kim, C and Ro, PI}, year={2002}, month={Dec}, pages={697–705} }
 @article{ro_loh_2001, title={Feasibility of using ultrasonic flexural waves as a cooling mechanism}, volume={48}, ISSN={["1557-9948"]}, DOI={10.1109/41.904574}, abstractNote={The potential convective heat transfer capability of an ultrasonic flexural wave (UFW) is experimentally investigated. The UFW includes an ultrasonic flexural standing wave (USW) and an ultrasonic flexural traveling wave (UTW). The factors that might affect the cooling performance of the UFW are investigated. Those include the vibration amplitude of the UFW, the gap between the cooling source and the object above it, and the temperature of the object being cooled. It was observed that the temperature drop increased with the vibration amplitude. At gaps below 100 /spl mu/m, a temperature drop was not observed. As the gap was increased to more than 100 /spl mu/m, the temperature drop increased until it reached an optimum gap producing maximum temperature drop. Beyond the optimum gap, the temperature drop began to decrease. Also, it was observed that the temperature drop increased as the temperature difference between the object and ambient air increased. The cooling performance of the USW and UTW was investigated and compared. The differences in cooling performance were found to be insignificant. This indicates that acoustic streaming is the dominant factor in the convective heat transfer using the UFW. However, using resonance, the UTW creates a temperature drop six times greater than the UTW for a given power supply. With the USW having a vibration amplitude of 25 /spl mu/m, an object at 98/spl deg/C was cooled down to 58/spl deg/C in 5 min. The temperature drop obtained by using the USW was approximately 80% of a conventional fan oriented with respect to the heated object such that the maximum heat transfer occurs. The UFW-based fan offers advantages over the conventional fan, such as silent operation, minimal heat dissipation, lack of wearing parts, and slim profile. These benefits make the fan an ideal candidate for cooling miniature parts in an enclosed workspace. Finally, a possible design option for minimizing the fan using thin-film PZT is presented.}, number={1}, journal={IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS}, author={Ro, PI and Loh, BG}, year={2001}, month={Feb}, pages={143–150} }
 @article{loh_ro_2000, title={An object transport system using flexural ultrasonic progressive waves generated by two-mode excitation}, volume={47}, ISSN={["0885-3010"]}, DOI={10.1109/58.852083}, abstractNote={An object transport system using low amplitude and high frequency progressive waves generated by two-mode excitation is presented. A theoretical model for the system was developed using normal mode expansion and the modal participation factor. To identify the factors that affect the transport speed, the changes with the mass of objects on the beam, the input power, the phase difference, and the excitation frequency were experimentally investigated. With a power input of 40 W, a transport speed of 10 cm/s was obtained for an object weighing 30 g. The tests indicate that, not only the phase difference but also the excitation frequency, were the dominant factors in determining the transport speed and direction. Specifically, when the excitation frequency was chosen to be at the exact midpoint of the two modes, the object stopped moving. A slight change of frequency in either direction resulted in change of object transport direction. For actual factory application, a simple stop-go and tracking control using the General Purpose Interface Bus (GPIB) were implemented.}, number={4}, journal={IEEE TRANSACTIONS ON ULTRASONICS FERROELECTRICS AND FREQUENCY CONTROL}, author={Loh, BG and Ro, PI}, year={2000}, month={Jul}, pages={994–999} }
 @misc{loh_ro_2000, title={Changing the propagation direction of flexural ultrasonic progressive waves by modulating excitation frequency}, volume={238}, ISSN={["0022-460X"]}, DOI={10.1006/jsvi.2000.3013}, number={1}, journal={JOURNAL OF SOUND AND VIBRATION}, author={Loh, BG and Ro, PI}, year={2000}, month={Nov}, pages={171–178} }
 @article{ro_loh_2000, title={Feasibility of contact and non-contact approaches to material handling using traveling waves and transition characteristics}, volume={47}, number={6}, journal={IEEE Transactions on Industrial Electronics}, author={Ro, P. I. and Loh, B.G.}, year={2000}, pages={1134–1135} }
 @article{ro_2000, title={Reduced order modeling and parameter estimation for quarter car suspension system}, volume={214}, DOI={10.1177/095440700021400804}, abstractNote={ This paper presents a new approach to obtaining an accurate simple model for complex mechanical systems. The methodology is applied to a quarter-car suspension system with complex linkage structures. Firstly, a multi-body dynamic model which includes kinematic characteristics is developed. Using a linearization technique, a 32-state linear model for a quarter-car system is obtained. Secondly, model reduction techniques are applied to find a reasonable reduced-order model. The result of the model reduction shows the validity of the two-mass model given that the parameters are correctly identified. The paper presents both an analytical and an experimental way of identifying the parameters of the two-mass system based on the reduced-order model. The identified parameters are shown to vary significantly from component data typically used for the two-mass system depending on kinematic structures of the suspension system. The modelling procedures outlined in this paper provide a precise and efficient way of designing active suspension systems that minimizes a necessary tuning process. }, number={D}, journal={Journal of Automobile Engineering}, author={Ro, P. I.}, year={2000}, pages={851–864} }
 @article{kim_ro_2000, title={Reduced-order modelling and parameter estimation for a quarter-car suspension system}, volume={214}, ISSN={["2041-2991"]}, DOI={10.1243/0954407001527907}, abstractNote={This paper presents a new approach to obtaining an accurate simple model for complex mechanical systems. The methodology is applied to a quarter-car suspension system with complex linkage structures. Firstly, a multi-body dynamic model which includes kinematic characteristics is developed. Using a linearization technique, a 32-state linear model for a quarter-car system is obtained. Secondly, model reduction techniques are applied to find a reasonable reduced-order model. The result of the model reduction shows the validity of the two-mass model given that the parameters are correctly identified. The paper presents both an analytical and an experimental way of identifying the parameters of the two-mass system based on the reduced-order model. The identified parameters are shown to vary significantly from component data typically used for the two-mass system depending on kinematic structures of the suspension system. The modelling procedures outlined in this paper provide a precise and efficient way of designing active suspension systems that minimizes a necessary tuning process.}, number={D8}, journal={PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART D-JOURNAL OF AUTOMOBILE ENGINEERING}, author={Kim, C and Ro, PI}, year={2000}, pages={851–864} }
 @article{ro_shim_jeong_2000, title={Robust friction compensation for submicrometer positioning and tracking for a ball-screw-driven slide system}, volume={24}, ISSN={["1873-2372"]}, DOI={10.1016/S0141-6359(00)00030-1}, abstractNote={Ball-screw-driven slide systems are largely used in industry for motion control applications. Their performance using standard proportional-integral-derivative (PID) control algorithm is unsatisfactory in submicrometer motion control because of nonlinear friction effects. In this article, controllers based on a bristle-type nonlinear contact model are developed and implemented for submicrometer motion. For submicrometer positioning, a proportional-derivative (PD) control scheme with a nonlinear friction estimate algorithm is developed, and its performance is compared with that of a PID controller. For tracking, a disturbance observer was added to reject external disturbances and to improve robustness. The experimental results indicate that the proposed controller has consistent performance in positioning with under 1.5% of steady-state error in the submicrometer range. For tracking performance, the proposed controller shows good and robust tracking with respect to parameter variation.}, number={2}, journal={PRECISION ENGINEERING-JOURNAL OF THE INTERNATIONAL SOCIETIES FOR PRECISION ENGINEERING AND NANOTECHNOLOGY}, author={Ro, PI and Shim, W and Jeong, S}, year={2000}, month={Apr}, pages={160–173} }
 @article{hong_ro_2000, title={The law of large numbers for fuzzy numbers with unbounded supports}, volume={116}, ISSN={["0165-0114"]}, DOI={10.1016/S0165-0114(98)00188-2}, abstractNote={We study laws of large numbers for mutually T-related fuzzy numbers with unbounded supports where T is an Archimedean t-norm and generalize earlier result of Badard.}, number={2}, journal={FUZZY SETS AND SYSTEMS}, author={Hong, DH and Ro, PI}, year={2000}, month={Dec}, pages={269–274} }
 @article{kim_ro_kim_1999, title={Effect of the suspension structure on equivalent suspension parameters}, volume={213}, ISSN={["0954-4070"]}, DOI={10.1243/0954407991527026}, abstractNote={ This paper examines the uncertainties in modelling a real suspension system that are due to the effect of suspension linkage layout (or structure) on the equivalent suspension parameters of a corresponding mathematical model. In most research on active suspension systems, a quarter-car model of two masses is very often used. However, without considering the influence of the suspension kinematic structure, the simple model may not be as effective as might be expected because of the uncertainties in the suspension parameters. Two sets of identified parameters for different suspension systems are compared to show the effect of suspension structure on the equivalent parameters. The relationships between specific parameters and changes in certain suspension linkage layouts are also investigated. The benefits of the parameter identification are demonstrated in the process of designing two active systems (one using a sky-hook control law and the other using a sliding mode control technique). The results show that suspension structure has a strong effect on the equivalent suspension parameters and this relationship becomes more important as the structure of suspension increases in complexity. The advantage of the identification process is crucial in designing both linear and non-linear active suspension systems. }, number={D5}, journal={PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART D-JOURNAL OF AUTOMOBILE ENGINEERING}, author={Kim, C and Ro, PI and Kim, H}, year={1999}, pages={457–470} }
 @article{kim_ro_1998, title={A sliding mode controller for vehicle active suspension systems with non-linearities}, volume={212}, ISSN={["0954-4070"]}, DOI={10.1243/0954407981525812}, abstractNote={ In this paper, the control of an active suspension system using a quarter car model has been investigated. Due to the presence of non-linearities such as a hardening spring, a quadratic damping force and the ‘tyre lift-off’ phenomenon in a real suspension system, it is very difficult to achieve desired performance using linear control techniques. To ensure robustness for a wide range of operating conditions, a sliding mode controller has been designed and compared with an existing nonlinear adaptive control scheme in the literature. The sliding mode scheme utilizes a variant of a sky-hook damper system as a reference model which does not require real-time measurement of road input. The robustness of the scheme is investigated through computer simulation, and the efficacy of the scheme is shown both in time and frequency domains. In particular, when the vertical load to the sprung mass is changed, the sliding mode control resumes normal operation faster than the nonlinear self-tuning control and the passive system by factors of 3 and 6, respectively, and suspension deflection is kept to a minimum. Other results showed advantages of the sliding mode control scheme in a quarter car system with realistic non-linearities. }, number={D2}, journal={PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART D-JOURNAL OF AUTOMOBILE ENGINEERING}, author={Kim, C and Ro, PI}, year={1998}, pages={79–92} }
 @article{yih_ro_1998, title={Nonlinear optimization-based motion planning of nonholonomic systems using an iterative algorithm}, volume={4}, DOI={10.1080/10798587.1998.10750733}, abstractNote={ABSTRACTA nonlinear optimization-based scheme is developed for the motion planning of nonholonomic systems. By specifying the final states constraints and using the performance criteria involving the control energy, the motion planning of nonholonomic systems can be recast as a nonlinear optimization problem which is to find suitable control inputs for steering the system along a feasible path from an initial state to a final state. An iterative algorithm is proposed to solve for a feasible path satisfying nonholonomic constraints and necessary optimality conditions. First, multi-point shooting is used to convert the motion planning problem into the problem of finding the solution of nonlinear equations. Modified Newton's method with line search is then used to ensure the global convergence of the numerical algorithm. The proposed scheme is applied to an one-leg hopping robot and a two-wheeled mobile robot. The results of numerical simulation clearly demonstrate the effectiveness of the proposed motion pl...}, number={3}, journal={Intelligent Automation and Soft Computing}, author={Yih, C. C. and Ro, P. I.}, year={1998}, pages={227–239} }
 @article{gutierrez_ro_1998, title={Parametric modeling and control of a long-range actuator using magnetic servo levitation}, volume={34}, ISSN={["0018-9464"]}, DOI={10.1109/20.718530}, abstractNote={A mechatronic device based on magnetic servo levitation (MSL) is proposed for long range and wide bandwidth actuation. The force capability of this type of actuator can be significantly larger than a linear motor or voice-coil actuator of similar dimensions. A novel parametric model of the electromagneto-mechanical coupled system has been developed to describe its behavior over a large range of motion and frequency, as opposed to more conventional attractive force small-perturbation models. It also provides a convenient way of devising a feedback linearizing control scheme that eliminates the need for high biasing currents present in most magnetic bearing systems. This paper presents a feedback-linearized controller coupled with a Kalman filter as a first approach to solve the tracking problem for such an actuator. Good tracking performance have been found both in simulation and experiments.}, number={5}, journal={IEEE TRANSACTIONS ON MAGNETICS}, author={Gutierrez, HM and Ro, PI}, year={1998}, month={Sep}, pages={3689–3695} }
 @article{gutierrez_po_1998, title={Sliding-mode control of a nonlinear-input system: Application to a magnetically levitated fast-tool servo}, volume={45}, ISSN={["0278-0046"]}, DOI={10.1109/41.735336}, abstractNote={Magnetic servo levitation (MSL) is currently being investigated as an alternative to drive fast-tool servo systems that could overcome the range limitations inherent to piezoelectric driven devices while operating over a wide bandwidth. To control such systems, a feedback-linearized controller coupled with a Kalman filter has been previously described. Performance limitations that degrade tracking accuracy suggest the use of a more robust controller design approach, such as sliding-mode control. Current literature on sliding mode deals almost exclusively with systems that are affine on the input, while the magnetic fast-tool servo is nonlinear on it when the control action is current command. This paper discusses a sliding mode-based controller that overcomes the aforementioned problem by defining a modified sliding condition to calculate control action. Experimental results demonstrate the feasibility of achieving long-range fast tracking with magnetically levitated devices by using sliding-mode control.}, number={6}, journal={IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS}, author={Gutierrez, HM and Po, PI}, year={1998}, month={Dec}, pages={921–927} }
 @article{lee_ro_1994, title={Path finding and grasp planning for robotic assembly}, volume={12}, ISSN={0263-5747 1469-8668}, url={http://dx.doi.org/10.1017/S0263574700017379}, DOI={10.1017/S0263574700017379}, abstractNote={SUMMARY}, number={4}, journal={Robotica}, publisher={Cambridge University Press (CUP)}, author={Lee, Byung R. and Ro, Paul I.}, year={1994}, month={Jul}, pages={353–360} }