Gerardo Lafferriere

Discrete abstractions of hybrid systems

A hybrid system is a dynamical system with both discrete and continuous state changes. For analysis purposes, it is often useful to abstract a system in a way that preserves the properties being analysed while hiding the details that are of no interest. We show that interesting classes of hybrid systems can be abstracted to purely discrete systems while preserving all properties that are definable in temporal logic. The classes that permit discrete abstractions fall into two categories. Either the continuous dynamics must be restricted, as is the case for timed and rectangular hybrid systems, or the discrete dynamics must be restricted, as is the case for o-minimal hybrid systems. In this paper, we survey and unify results from both areas.

Decentralized control of vehicle formations

This paper investigates a method for decentralized stabilization of vehicle formations using techniques from algebraic graph theory. The vehicles exchange information according to a pre-specified communication digraph, G. A feedback control is designed using relative information between a vehicle and its in-neighbors in G. We prove that a necessary and sufficient condition for an appropriate decentralized linear stabilizing feedback to exist is that G has a rooted directed spanning tree. We show the direct relationship between the rate of convergence to formation and the eigenvalues of the (directed) Laplacian of G. Various special situations are discussed, including symmetric communication graphs and formations with leaders. Several numerical simulations are used to illustrate the results.

O-Minimal Hybrid Systems

Abstract. An important approach to decidability questions for verification algorithms of hybrid systems has been the construction of a bisimulation. Bisimulations are finite state quotients whose reachability properties are equivalent to those of the original infinite state hybrid system. In this paper we introduce the notion of o-minimal hybrid systems, which are initialized hybrid systems whose relevant sets and flows are definable in an o-minimal theory. We prove that o-minimal hybrid systems always admit finite bisimulations. We then present specific examples of hybrid systems with complex continuous dynamics for which finite bisimulations exist.

Motion planning for controllable systems without drift

A general strategy for solving the motion planning problem for real analytic, controllable systems without drift is proposed. The procedure starts by computing a control that steers the given initial point to the desired target point for an extended system, in which a number of Lie brackets of the system vector fields are added. Using formal calculations with a product expansion relative to P. Hall basis, another control is produced that achieves the desired result on the formal level. This provides an exact solution of the original problem if the given system is nilpotent. For a general system, an iterative algorithm is derived that converges very quickly to a solution. For nonnilpotent systems which are feedback nilpotentizable, the algorithm, in cascade with a precompensator, produces an exact solution. Results of simulations which illustrate the effectiveness of the procedure are presented.<<ETX>>

A New Class of Decidable Hybrid Systems

One of the most important analysis problems of hybrid systems is the reachability problem. State of the art computational tools perform reachability computation for timed automata, multirate automata, and rectangular automata. In this paper, we extend the decidability frontier for classes of linear hybrid systems, which are introduced as hybrid systems with linear vector fields in each discrete location. This result is achieved by showing that any such hybrid system admits a finite bisimulation, and by providing an algorithm that computes it using decision methods from mathematical logic.

Hierarchically consistent control systems

Large-scale control systems typically possess a hierarchical architecture in order to manage complexity. Higher levels of the hierarchy utilize coarser models of the system, resulting from aggregating the detailed lower level models. In this layered control paradigm, the notion of hierarchical consistency is important, as it ensures the implementation of high-level objectives by the lower level system. In this paper, we define a notion of modeling hierarchy for continuous control systems and obtain characterizations for hierarchically consistent linear systems with respect to controllability objectives. As an interesting byproduct, we obtain a hierarchical controllability criterion for linear systems from which we recover the best of the known controllability algorithms from numerical linear algebra.

Symbolic Reachability Computation for Families of Linear Vector Fields

The control paradigm of physical processes being supervised by digital programs has lead to the development of a theory of hybrid systems combining finite state automata with differential equations. One of the most important problems in the verification of hybrid systems is the reachability problem. Even though the computation of reachable spaces for finite state machines is well developed, computing the reachable space of a differential equation is difficult. In this paper, we present the first known families of linear differential equations with a decidable reachability problem. This is achieved by posing the reachability computation as a quantifier elimination problem in the decidable theory of the reals. We illustrate the applicability of our approach by performing computations using the packages Redlog and Qepcad. Such symbolic computations can be incorporated in computer-aided verification tools for purely discrete systems, resulting in verification tools for hybrid systems with linear differential equations.

A Differential Geometric Approach to Motion Planning

We propose a general strategy for solving the motion planning problem for real analytic, controllable systems without drift. The procedure starts by computing a control that steers the given initial point to the desired target point for an extended system, in which a number of Lie brackets of the system vector fields are added to the right-hand side. The main point then is to use formal calculations based on the product expansion relative to a P. Hall basis, to produce another control that achieves the desired result on the formal level. It then turns out that this control provides an exact solution of the original problem if the given system is nilpotent. When the system is not nilpotent, one can still produce an iterative algorithm that converges very fast to a solution. Using the theory of feedback nilpotentization, one can find classes of non-nilpotent systems for which the algorithm, in cascade with a precompensator, produces an exact solution in a finite number of steps. We also include results of simulations which illustrate the effectiveness of the procedure.

Fine manipulation with multifinger hands

The existence of two- and three-finger grasps in the presence of arbitrarily small friction is shown for 2D and 3D smooth objects using a simple technique. No convexity of the objects is assumed. The existence of finger gaits for rotating a planar object using three and four fingers is proved. Additional results for smooth convex objects which describe two different finger gaits using four fingers are presented. The purpose is to move one or more fingers at any one time and at the same time to maintain a grip with the others, and thereby to move the object further than or in other ways than it could be moved with a fixed grip.<<ETX>>

A general strategy for computing steering controls of systems without drift

The author discusses the applicability of a recently proposed strategy for the effective calculation of steering controls for a real analytic, controllable system without drift. The approach uses an approximating nilpotent system to compute the controls. These controls provide an exact solution of the original problem for nilpotent systems and for systems which can be made nilpotent by a feedback transformation. For a general system without drift, an iterative algorithm is used. The procedure can also be applied to systems which are obtained from systems without drift by a dynamic extension.<<ETX>>