@article{negi_chakrabortty_2022, title={Optimal co-designs of communication and control in bandwidth-constrained cyber-physical systems}, volume={142}, ISSN={["1873-2836"]}, DOI={10.1016/j.automatica.2022.110288}, abstractNote={We address the problem of sparsity-promoting optimal control of cyber–physical systems (CPSs) in the presence of communication delays. The delays are categorized into two types — namely, an inter-layer delay for passing state and control information between the physical layer and the cyber layer, and an intra-layer delay that operates between the computing agents, referred to here as control nodes (CNs), within the cyber-layer. Our objective is to minimize the closed-loop H2-norm of the physical system by co-designing an optimal combination of these two delays and a sparse state-feedback controller while respecting a given bandwidth cost constraint. We propose a two-loop optimization algorithm for this. Based on the alternating directions method of multipliers (ADMM), the inner loop handles the conflicting directions between the decreasing H2-norm and the increasing sparsity level of the controller. The outer loop comprises a semidefinite program (SDP)-based relaxation of non-convex inequalities necessary for closed-loop stability. Moreover, for CPSs where the state and control information assigned to the CNs are not private, we derive an additional algorithm that further sparsifies the communication topology by modifying the row and column structures of the obtained controller, resulting in a reassignment of the communication map between the cyber and physical layers, and determining which physical agent should send its state information to which CN. Proofs for closed-loop stability and optimality are provided for both algorithms, followed by numerical simulations.}, journal={AUTOMATICA}, author={Negi, Nandini and Chakrabortty, Aranya}, year={2022}, month={Aug} }
@article{negi_chakrabortty_2020, title={Sparsity-promoting optimal control of cyber-physical systems over shared communication networks}, volume={122}, ISSN={["1873-2836"]}, DOI={10.1016/j.automatica.2020.109217}, abstractNote={Recent years have seen several new directions in the design of sparse control of cyber–physical systems (CPSs) driven by the objective of reducing communication costs. One common assumption made in these designs is that the communication happens over a dedicated network. For many practical applications, however, communication must occur over shared networks, leading to two critical design challenges, namely — time-delays in the feedback and fair sharing of bandwidth among users. In this paper, we present a set of sparse H2 control designs under these two design constraints. An essential aspect of our design is that the delay itself can be a function of sparsity, which leads to an interesting pattern of trade-offs in the H2 performance. We present three distinct algorithms. The first algorithm preconditions the assignable bandwidth to the network and produces an initial guess for a stabilizing controller. This is followed by our second algorithm, which sparsifies this controller while simultaneously adapting the feedback delay and optimizing the H2 performance using alternating directions method of multipliers (ADMM). The third algorithm extends this approach to a multiple user scenario where an optimal number of communication links, whose total sum is fixed, is distributed fairly among users by minimizing the variance of their H2 performances. The problem is cast as a difference-of-convex (DC) program with mixed-integer linear program (MILP) constraints. We provide theorems to prove the convergence of these algorithms, followed by validation through numerical simulations.}, journal={AUTOMATICA}, author={Negi, Nandini and Chakrabortty, Aranya}, year={2020}, month={Dec} }
@article{negi_sahoo_chakrabarti_2019, title={Distributed control based power sharing strategy for an islanded AC microgrid}, volume={13}, ISSN={["1751-8695"]}, DOI={10.1049/iet-gtd.2018.5320}, abstractNote={This study presents a novel secondary control strategy for a radial microgrid with end-to-end distributed energy resource (DER) interconnections, operating under islanded conditions. The control strategy utilises sparse communication and precludes the presence of a centralised controller. The distributed algorithm has a twofold objective. The first task is to determine the desired power injection at each DER bus for a load change occurring anywhere in the microgrid. The remaining task includes obtaining the bus angle set points at all the DER units such that the actual power injections converge to the desired power injections at each DER bus. The algorithm enables accurate system wide power injection in proportion to the respective DER base ratings. The control strategy is tested on a three-bus, three-DER system with local loads. The controller is shown to work for two different sets of microgrid ratings. The bounds are evaluated for each design parameter specified according to the system nominal ratings to maintain the stability of the distributed algorithm. Proofs are stated to show the convergence of algorithms to the correct power injection solution for all the nodes. Real-time simulation results are presented to corroborate accurate power sharing through the algorithm.}, number={4}, journal={IET GENERATION TRANSMISSION & DISTRIBUTION}, author={Negi, Nandini and Sahoo, Soumya Ranjan and Chakrabarti, Saikat}, year={2019}, month={Feb}, pages={553–562} }