We present a method for calculating the probability that a robot successfully performs a task described using Signal Temporal Logic (STL). We focus on cases where the failure probability is very small, hence a traditional Monte-Carlo method becomes inefficient due to the large number of samples required to observe failures. Using elliptical sliced sampling, normalizing flows, and Bayesian optimization, we develop an algorithm that, under mild assumptions, is applicable to black-box systems, and can be applied to uncertainty sources with non-Gaussian probabilities. We demonstrate the application of our method on three different simulated robots.

Autonomous robots typically incorporate complex sensors in their decision-making and control loops. These sensors, such as cameras and lidars, have imperfections in their sensing and are influenced by environmental conditions. In this paper, we present a method for probabilistic verification of linearizable systems with Gaussian and Gaussian mixture noise models (e.g. from perception modules, machine learning components). We compute the probabilities of task satisfaction under Signal Temporal Logic (STL) specifications, using its robustness semantics, with a Markov Chain Monte-Carlo slice sampler. As opposed to other techniques, our method avoids over-approximations and double-counting of failure events. Central to our approach is a method for efficient and rejection-free sampling of signals from a Gaussian distribution that satisfy or violate a given STL formula. We show illustrative examples from applications in robot motion planning.

We develop a method for synthesizing control policies for stochastic, linear, time-varying systems that must perform tasks specified in signal temporal logic. We build upon an efficient, sampling-based framework that computes the probability of the system satisfying its specification. By exploiting the properties of linear systems and robustness score in temporal logic specifications, we obtain sample-efficient gradients of the satisfaction probability with respect to con-troller parameters. Therefore, by applying gradient descent we obtain locally optimized controllers that maximize the chances of satisfying the specification. We demonstrate our approach through examples of a mobile robot and a mobile manipulator in simulation.

We develop an assume-guarantee framework for control of large scale linear (time-varying) systems from finite-time reach and avoid or infinite-time invariance specifications. The contracts describe the admissible set of states and controls for individual subsystems. A set of contracts compose correctly if mutual assumptions and guarantees match in a way that we formalize. We propose a rich parameterization of contracts such that the set of parameters that compose correctly is convex. Moreover, we design a potential function of parameters that describes the distance of contracts from a correct composition. Thus, the verification and synthesis for the aggregate system are broken to solving small convex programs for individual subsystems, where correctness is ultimately achieved in a compositional way. Illustrative examples demonstrate the scalability of our method.

We propose a method to control linear time-varying (LTV) discrete-time systems subject to bounded process disturbances and measurable outputs with bounded noise, and polyhedral constraints over system inputs and states. We search over control policies that map the history of measurable outputs to the current control input. We solve the problem in two stages. First, using the original system, we build a linear system that predicts future observations using the past observations. The bounded errors are characterized using zonotopes. Next, we propose control laws based on affine maps of such output prediction errors, and show that controllers can be synthesized using convex linear/quadratic programs. Furthermore, we can add constraints on trajectories and guarantee their satisfaction for all allowable sequences of observation noise and process disturbances. Our method does not require any assumptions about system controllability and observability. The controller design does not directly take into account the state-space dynamics, and its implementation does not require an observer. Instead, partial observability is often sufficient to design a correct controller. We provide the polytopic representation of observability errors and reachable sets in the form of zonotopes. Illustrative examples are included.

We introduce R3T, a reachability-based variant of the rapidly-exploring random tree (RRT) algorithm that is suitable for (optimal) kinodynamic planning in nonlinear and hybrid systems. We developed tools to approximate reachable sets using polytopes and perform sampling-based planning with them. This method has a unique advantage in hybrid systems: different dynamic modes in the reachable set can be explicitly represented using multiple polytopes. We prove that under mild assumptions, R3T is probabilistically complete in kinodynamic systems, and asymptotically optimal through rewiring. Moreover, R3T provides a formal verification method for reachability analysis of nonlinear systems. The advantages of R3T are demonstrated with case studies on nonlinear, hybrid, and contact-rich robotic systems.

Piecewise affine (PWA) systems are widely used to model highly nonlinear behaviors such as contact dynamics in robot locomotion and manipulation. Existing control techniques for PWA systems have computational drawbacks, both in offline design and online implementation. In this paper, we introduce a method to obtain feedback control policies and a corresponding set of admissible initial conditions for discrete-time PWA systems such that all the closed-loop trajectories reach a goal polytope, while a cost function is optimized. The idea is conceptually similar to LQR-trees [1], which consists of 3 steps: (1) open-loop trajectory optimization, (2) feedback control for computation of “funnels” of states around trajectories, and (3) repeating (1) and (2) in a way that the funnels are grown backward from the goal in a tree fashion and fill the state-space as much as possible. We show PWA dynamics can be exploited to combine step (1) and (2) into a single step that is tackled using mixed-integer convex programming, which makes the method suitable for dealing with hard constraints. Illustrative examples on contact-based dynamics are presented.

For many performance-critical control systems, an accurate (simple) model is not available in practice. Thus, designing controllers with formal performance guarantees is challenging. In this paper, we develop a framework to use input-output data from an unknown system to synthesize controllers from signal temporal logic (STL) specifications. First, by imposing mild assumptions on system continuity, we find a set-valued piecewise affine (PWA) model that contains all the possible behaviors of the concrete system. Next, we introduce a novel method for STL control of PWA systems with additive disturbances. By taking advantage of STL quantitative semantics, we provide lower-bound certificates on the degree of STL satisfaction of the closed-loop concrete system. Illustrative examples are presented.