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1. Effect of Finite-time DS Controllers on Disturbance Rejection for Planar Bipeds

   Williams, Daniel S.; Martin, Anne E. (January 2019, American Controls Conference)

   For many planar bipedal models, each step is divided into a finite time single support period and an instantaneous double support period. During single support, the biped is typically underactuated and thus has limited ability to reject disturbances. The instantaneous nature of the double support period prevents control during this period. However, if the double support period is expanded to finite time, this introduces an overactuated period into the model which may improve disturbance rejection capabilities. This paper derives and compares the performance of two finite-time double support controllers. The first controller uses time to drive the progression of the double support period and controls the joint angles. The second controller uses a time-invariant phase variable to drive the progression of the double support period and controls the joint velocities since it is not possible to control the joint positions. The disturbance rejection capabilities of both controllers are then quantified using simulations. The instantaneous double support model is also simulated for comparison. The instantaneous double support model can recover from the largest disturbances but it requires the greatest number of steps to do. The time-based double support controller can recover from the smallest range of disturbances but requires the fewest number of steps for a given perturbation size. 

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2. Robust Decentralized Secondary Frequency Control in Power Systems: Merits and Trade-Offs

   https://doi.org/10.1109/TAC.2018.2884650  

   Weitenberg, Erik; Jiang, Yan; Zhao, Changhong; Mallada, Enrique; De Persis, Claudio; Dorfler, Florian (December 2018, IEEE Transactions on Automatic Control)

   Frequency restoration in power systems is conventionally performed by broadcasting a centralized signal to local controllers. As a result of the energy transition, technological advances, and the scientific interest in distributed control and optimization methods, a plethora of distributed frequency control strategies have been proposed recently that rely on communication amongst local controllers. In this paper, we propose a fully decentralized leaky integral controller for frequency restoration that is derived from a classic lag element. We study steady-state, asymptotic optimality, nominal stability, input-to-state stability, noise rejection, transient performance, and robustness properties of this controller in closed loop with a nonlinear and multivariable power system model. We demonstrate that the leaky integral controller can strike an acceptable trade-off between performance and robustness as well as between asymptotic disturbance rejection and transient convergence rate by tuning its DC gain and time constant. We compare our findings to conventional decentralized integral control and distributed- averaging-based integral control in theory and simulations. 

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3. MODEL BASED REPETITIVE CONTROL FOR PEELING FRONT GEOMETRY CONTROL IN A ROLL-TO-ROLL PEELING PROCESS

   Zhao, Qishen; Martin, Christopher; Chen, Dongmei; Li, Wei (July 2022, Proceedings of the 2022 International Symposium on Flexible Automation)

   Roll-to-roll(R2R) peeling is an innovative method that transfers flexible electronics and 2D materials from the flexible substrate where they are grown to the end-use substrate. This process enables the full potential of R2R 2D material fabrication methods in a continuous, high-throughput, and environment friendly manner. During the R2R peeling process, the device patterning causes periodic changes in the adhesion energy between the device and substrate. This periodic disturbance can degrade the quality of the final product if not properly controlled. Current control methods used for the R2R peeling process do not explicitly reject the periodic disturbance. It is therefore desirable to develop a controller that is capable of performing periodic disturbance rejection. This paper presents a model-based repetitive controller that integrates a frequency estimation of the disturbance into the R2R peeling control to maintain the optimal peeling process performance. A linear estimator using system identification techniques is employed. The simulation results show that the developed controller achieves better R2R process performance when compared to a conventional model-based controller. 

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4. Energy Shaping Control with Virtual Spring and Damper for Powered Exoskeletons

   https://doi.org/10.1109/CDC40024.2019.9029624  

   Lin, Jianping; Divekar, Nikhil; Lv, Ge; Gregg, Robert D. (December 2019, IEEE Conference on Decision and Control)

   Task-invariant feedback control laws for powered exoskeletons are preferred to assist human users across varying locomotor activities. This goal can be achieved with energy shaping methods, where certain nonlinear partial differential equations, i.e., matching conditions, must be satisfied to find the achievable dynamics. Based on the energy shaping methods, open-loop systems can be mapped to closed-loop systems with a desired analytical expression of energy. In this paper, the desired energy consists of modified potential energy that is well-defined and unified across different contact conditions along with the energy of virtual springs and dampers that improve energy recycling during walking. The human-exoskeleton system achieves the input-output passivity and Lyapunov stability during the whole walking period with the proposed method. The corresponding controller provides assistive torques that closely match the human torques of a simulated biped model and able-bodied human subjects’ data. 

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5. Distributed Model Predictive Control for Autonomous Droop-Controlled Inverter-Based Microgrids

   https://doi.org/10.1109/CDC40024.2019.9028938  

   Anderson, Sean; Hidalgo-Gonzalez, Patricia; Dobbe, Roel; Tomlin, Claire J. (December 2019, 2019 IEEE 58th Conference on Decision and Control (CDC))

   Microgrids must be able to restore voltage and frequency to their reference values during transient events; inverters are used as part of a microgrid's hierarchical control for maintaining power quality. Reviewed methods either do not allow for intuitive trade-off tuning between the objectives of synchronous state restoration, local reference tracking, and disturbance rejection, or do not consider all of these objectives. In this paper, we address all of these objectives for voltage restoration in droop-controlled inverter-based islanded micro-grids. By using distributed model predictive control (DMPC) in series with an unscented Kalman Filter (UKF), we design a secondary voltage controller to restore the voltage to the reference in finite time. The DMPC solves a reference tracking problem while rejecting reactive power disturbances in a noisy system. The method we present accounts for non-zero mean disturbances by design of a random-walk estimator. We validate the method's ability to restore the voltage in finite time via modeling a multi-node microgrid in Simulink. 

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