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1. Learning Reactive and Predictive Differentiable Controllers for Switching Linear Dynamical Models

   Saxena, S.; LaGrassa, A.; Kroemer, O. (April 2021, IEEE International Conference on Robotics and Automation)

   null (Ed.)

   Humans leverage the dynamics of the environment and their own bodies to accomplish challenging tasks such as grasping an object while walking past it or pushing off a wall to turn a corner. Such tasks often involve switching dynamics as the robot makes and breaks contact. Learning these dynamics is a challenging problem and prone to model inaccuracies, especially near contact regions. In this work, we present a framework for learning composite dynamical behaviors from expert demonstrations. We learn a switching linear dynamical model with contacts encoded in switching conditions as a close approximation of our system dynamics. We then use discrete-time LQR as the differentiable policy class for data-efficient learning of control to develop a control strategy that operates over multiple dynamical modes and takes into account discontinuities due to contact. In addition to predicting interactions with the environment, our policy effectively reacts to inaccurate predictions such as unanticipated contacts. Through simulation and real world experiments, we demonstrate generalization of learned behaviors to different scenarios and robustness to model inaccuracies during execution. 

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2. Sample-Efficient Reinforcement Learning with loglog({T}) Switching Cost

   Qiao, Dan; Yin, Ming; Min, Ming; Wang, Yu-Xiang (June 2022, Proceedings of Machine Learning Research)

   Chaudhuri, Kamalika and (Ed.)

   We study the problem of reinforcement learning (RL) with low (policy) switching cost {—} a problem well-motivated by real-life RL applications in which deployments of new policies are costly and the number of policy updates must be low. In this paper, we propose a new algorithm based on stage-wise exploration and adaptive policy elimination that achieves a regret of $$\widetilde{O}(\sqrt{H^4S^2AT})$$ while requiring a switching cost of $$O(HSA \log\log T)$$. This is an exponential improvement over the best-known switching cost $$O(H^2SA\log T)$$ among existing methods with $$\widetilde{O}(\mathrm{poly}(H,S,A)\sqrt{T})$$ regret. In the above, $S,A$ denotes the number of states and actions in an $$H$$-horizon episodic Markov Decision Process model with unknown transitions, and $$T$$ is the number of steps. As a byproduct of our new techniques, we also derive a reward-free exploration algorithm with a switching cost of $O(HSA)$. Furthermore, we prove a pair of information-theoretical lower bounds which say that (1) Any no-regret algorithm must have a switching cost of $$\Omega(HSA)$$; (2) Any $$\widetilde{O}(\sqrt{T})$$ regret algorithm must incur a switching cost of $$\Omega(HSA\log\log T)$$. Both our algorithms are thus optimal in their switching costs. 

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3. Data-Driven Decomposition Control to Output Tracking With Nonperiodic Tracking–Transition Switching Under Input Constraint

   https://doi.org/10.1115/1.4053763  

   Liu, Jiangbo; Zou, Qingze (June 2022, Journal of Dynamic Systems, Measurement, and Control)

   Abstract This paper is concerned with solving, from the learning-based decomposition control viewpoint, the problem of output tracking with nonperiodic tracking–transition switching. Such a nontraditional tracking problem occurs in applications where sessions for tracking a given desired trajectory are alternated with those for transiting the output with given boundary conditions. It is challenging to achieve precision tracking while maintaining smooth tracking–transition switching, as postswitching oscillations can be induced due to the mismatch of the boundary states at the switching instants, and the tracking performance can be limited by the nonminimum-phase (NMP) zeros of the system and effected by factors such as input constraints and external disturbances. Although recently an approach by combining the system-inversion with optimization techniques has been proposed to tackle these challenges, modeling of the system dynamics and complicated online computation are needed, and the controller obtained can be sensitive to model uncertainties. In this work, a learning-based decomposition control technique is developed to overcome these limitations. A dictionary of input–output bases is constructed offline a priori via data-driven iterative learning first. The input–output bases are used online to decompose the desired output in the tracking sessions and design an optimal desired transition trajectory with minimal transition time under input-amplitude constraint. Finally, the control input is synthesized based on the superpositioning principle and further optimized online to account for system variations and external disturbance. The proposed approach is illustrated through a nanopositioning control experiment on a piezoelectric actuator. 

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4. Detecting and adapting to irregular distribution shifts in Bayesian online learning

   Li, Aodong; Boyd, Alex; Smyth, Padhraic; Mandt, Stephan (December 2021, Advances in neural information processing systems)

   We consider the problem of online learning in the presence of distribution shifts that occur at an unknown rate and of unknown intensity. We derive a new Bayesian online inference approach to simultaneously infer these distribution shifts and adapt the model to the detected changes by integrating ideas from change point detection, switching dynamical systems, and Bayesian online learning. Using a binary ‘change variable,’ we construct an informative prior such that--if a change is detected--the model partially erases the information of past model updates by tempering to facilitate adaptation to the new data distribution. Furthermore, the approach uses beam search to track multiple change-point hypotheses and selects the most probable one in hindsight. Our proposed method is model-agnostic, applicable in both supervised and unsupervised learning settings, suitable for an environment of concept drifts or covariate drifts, and yields improvements over state-of-the-art Bayesian online learning approaches. 

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5. Reachability Analysis for Cyber-Physical Systems: Are We There Yet?

   https://doi.org/10.1007/97830310677306  

   Chen, Xin; Sankaranarayanan, Sriram (January 2022, Lecture notes in computer science)

   Deshmukh, Jyotirmoy V.; Havelund, Klaus; Perez, Ivan (Ed.)

   Reachability analysis is a fundamental problem in verification that checks for a given model and set of initial states if the system will reach a given set of unsafe states. Its importance lies in the ability to exhaustively explore the behaviors of a model over a finite or infinite time horizon. The problem of reachability analysis for Cyber-Physical Systems (CPS) is especially challenging because it involves reasoning about the continuous states of the system as well as its switching behavior. Each of these two aspects can by itself cause the reachability analysis problem to be undecidable. In this paper, we survey recent progress in this field beginning with the success of hybrid systems with affine dynamics. We then examine the current state-of-the-art for CPS with nonlinear dynamics and those driven by ``learning-enabled'' components such as neural networks. We conclude with an examination of some promising directions and open challenges. 

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