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Estimating the region of attraction (RoA) for a robot controller is essential for safe application and controller composition. Many existing methods require a closed-form expression that limit applicability to data-driven controllers. Methods that operate only over trajectory rollouts tend to be data-hungry. In prior work, we have demonstrated that topological tools based on Morse Graphs (directed acyclic graphs that combinatorially represent the underlying nonlinear dynamics) offer data-efficient RoA estimation without needing an analytical model. They struggle, however, with high-dimensional systems as they operate over a state-space discretization. This paper presents Morse Graph-aided discovery of Regions of Attraction in a learned Latent Space (MORALS)**. The approach combines auto-encoding neural networks with Morse Graphs. MORALS shows promising predictive capabilities in estimating attractors and their RoAs for data-driven controllers operating over high-dimensional systems, including a 67-dim humanoid robot and a 96-dim 3-fingered manipulator. It first projects the dynamics of the controlled system into a learned latent space. Then, it constructs a reduced form of Morse Graphs representing the bistability of the underlying dynamics, i.e., detecting when the controller results in a desired versus an undesired behavior. The evaluation on high-dimensional robotic datasets indicates data efficiency in RoA estimation.

This paper aims to improve the computational efficiency of motion planning for mobile robots with non-trivial dynamics through the use of learned controllers. Offline, a system-specific controller is first trained in an empty environment. Then, for the target environment, the approach constructs a data structure, a "Roadmap with Gaps," to approximately learn how to solve planning queries using the learned controller. The roadmap nodes correspond to local regions. Edges correspond to applications of the learned controller that approximately connect these regions. Gaps arise as the controller does not perfectly connect pairs of individual states along edges. Online, given a query, a tree sampling-based motion planner uses the roadmap so that the tree’s expansion is informed towards the goal region. The tree expansion selects local subgoals given a wavefront on the roadmap that guides towards the goal. When the controller cannot reach a subgoal region, the planner resorts to random exploration to maintain probabilistic completeness and asymptotic optimality. The accompanying experimental evaluation shows that the approach significantly improves the computational efficiency of motion planning on various benchmarks, including physics-based vehicular models on uneven and varying friction terrains as well as a quadrotor under air pressure effects. Website: https://prx-kinodynamic.github.io/projects/rogue

This paper proposes an integration of surrogate modeling and topology to significantly reduce the amount of data required to describe the underlying global dynamics of robot controllers, including closed-box ones. A Gaussian Process (GP), trained with randomized short trajectories over the state-space, acts as a surrogate model for the underlying dynamical system. Then, a combinatorial representation is built and used to describe the dynamics in the form of a directed acyclic graph, known as Morse graph. The Morse graph is able to describe the system's attractors and their corresponding regions of attraction (RoA). Furthermore, a pointwise confidence level of the global dynamics estimation over the entire state space is provided. In contrast to alternatives, the framework does not require estimation of Lyapunov functions, alleviating the need for high prediction accuracy of the GP. The framework is suit-able for data-driven controllers that do not expose an analytical model as long as Lipschitz-continuity is satisfied. The method is compared against established analytical and recent machine learning alternatives for estimating Roas, outperforming them in data efficiency without sacrificing accuracy. Link to code: https://go.rutgers.edu/49hy35en

We present a new data-driven technique for pre-dicting the motion of a low-cost omnidirectional mobile robot under the influence of motor torques and friction forces. Our method utilizes a novel differentiable physics engine for analytically computing the gradient of the deviation between predicted motion trajectories and real-world trajectories. This allows to automatically learn and fine-tune the unknown friction coefficients on-the-fly, by minimizing a carefully designed loss function using gradient descent. Experiments show that the predicted trajectories are in excellent agreement with their real-world counterparts. Our proposed approach is computationally superior to existing black-box optimization methods, requiring very few real-world samples for accurate trajectory prediction compared to physics-agnostic techniques, such as neural net-works. Experiments also demonstrate that the proposed method allows the robot to quickly adapt to changes in the terrain. Our proposed approach combines the data-efficiency of classical analytical models that are derived from first principles, with the flexibility of data-driven methods, which makes it appropriate for low-cost mobile robots. Project website: https://go.rutgers.edu/mqxn2x6h

This paper explores learning an effective controller for improving the efficiency of kinodynamic planning for vehicular systems navigating uneven terrains. It describes the pipeline for training the corresponding controller and using it for motion planning purposes. The training process uses a soft actor-critic approach with hindsight experience replay to train a model, which is parameterized by the incline of the robot's local terrain. This trained model is then used during the expansion process of an asymptotically optimal kinodynamic planner to generate controls that allow the robot to reach desired local states. It is also used to define a heuristic cost-to-go function for the planner via a wavefront operation that estimates the cost of reaching the global goal. The cost-to-go function is used both for selecting nodes for expansion as well as for generating local goals for the controller to expand towards. The accompanying experimental section applies the integrated planning solution on models of all-terrain robots in a variety of physically simulated terrains. It shows that the proposed terrain-aware controller and the proposed wavefront function based on the cost-to-go model enable motion planners to find solutions in less time and with lower cost than alternatives. An ablation study emphasizes the benefits of a learned controller that is parameterized by the incline of the robot's local terrain as well as of an incremental training process for the controller.

This paper aims to improve the path quality and computational efficiency of sampling-based kinodynamic planners for vehicular navigation. It proposes a learning framework for identifying promising controls during the expansion process of sampling-based planners. Given a dynamics model, a reinforcement learning process is trained offline to return a low-cost control that reaches a local goal state (i.e., a waypoint) in the absence of obstacles. By focusing on the system’s dynamics and not knowing the environment, this process is data-efficient and takes place once for a robotic system. In this way, it can be reused in different environments. The planner generates online local goal states for the learned controller in an informed manner to bias towards the goal and consecutively in an exploratory, random manner. For the informed expansion, local goal states are generated either via (a) medial axis information in environments with obstacles, or (b) wavefront information for setups with traversability costs. The learning process and the resulting planning framework are evaluated for a first and second-order differential drive system, as well as a physically simulated Segway robot. The results show that the proposed integration of learning and planning can produce higher quality paths than sampling-based kinodynamic planning with random controls in fewer iterations and computation time.

We present a novel analysis of AO-RRT: a tree-based planner for motion planning with kinodynamic constraints, originally described by Hauser and Zhou (AO-X, 2016). AO-RRT explores the state-cost space and has been shown to efficiently obtain high-quality solutions in practice without relying on the availability of a computationally-intensive two-point boundary-value solver. Our main contribution is an optimality proof for the single-tree version of the algorithm-a variant that was not analyzed before. Our proof only requires a mild and easily-verifiable set of assumptions on the problem and system: Lipschitz-continuity of the cost function and the dynamics. In particular, we prove that for any system satisfying these assumptions, any trajectory having a piecewise-constant control function and positive clearance from the obstacles can be approximated arbitrarily well by a trajectory found by AORRT. We also discuss practical aspects of AORRT and present experimental comparisons of variants of the algorithm.

In the simulation of quantum circuits the matrices and vectors used to represent unitary operations and qubit states grow exponentially as the number of qubits increase. For instance, the evolution of an n-qubit quantum system in an initial superposition state is described by a 2n x 2n unitary matrix. However, the evolution of an n-qubit quantum system can be described as well as a composition of single-qubit and controlled-not unitary operations which are equivalent to the 2n x 2n unitary matrix. A strategy is suggested for the mapping of onequibit and two-qubit gates onto a three PE systolic array, and then we show how the interconnection of those systolic arrays can be used to implement or describe quantum circuits. As a case study we present the description of the teleportation algorithm.