Akash Deshpande

SHIFT: A Formalism and a Programming Language for Dynamic Networks of Hybrid Automata

SHIFT is a programming language for the specification and simulation of dynamic networks of hybrid automata. Such systems consist of components which can be created, interconnected and destroyed as the system evolves. Components exhibit hybrid behavior, consisting of continuous-time phases separated by discrete-event transitions. Components may evolve independently, or they may interact through their inputs, outputs and exported events. The interaction network itself may evolve.

The SHIFT programming language for dynamic networks of hybrid automata

SHIFT is a programming language for describing and simulating dynamic networks of hybrid automata. Such systems consist of components which can be created, interconnected, and destroyed as the system evolves. Components exhibit hybrid behavior, e.g. continuous-time phases separated by instantaneous discrete-event transitions. Components may evolve independently, or they may interact through selected state variables and events. The interaction network itself may evolve. The SHIFT model and language were motivated by our need for tools that support dynamically reconfigurable hybrid systems. Our primary application is the specification and analysis of different designs for automatic control of vehicles and highway systems. From our previous experience in modeling, analysis, and implementation, we adopted the hybrid systems approach for modeling the system components. Since spatial relationships between vehicles change as they move, our application is characterized by a dynamically changing network of interactions between system components. SHIFT has also since been used in coordinated autonomous submarines, air traffic control systems, and material handling systems. We examine other work related to the SHIFT approach. In we describe the main features of the SHIFT language-states, inputs, outputs, differential equations, and algebraic definitions, discrete states, and state transitions. We give a simplified version of the SHIFT model. We discuss the models of a type, a component, and the world and give the formal semantics of the model.

Communication protocols for a fault-tolerant automated highway system

We present the design and verification of inter-vehicle communication protocols for the operation of an automated highway system in the presence of faults. The protocols form part of a fault-tolerant control hierarchy proposed in earlier work. Our goal here is to implement discrete-event supervisory controllers to stop the faulty vehicle or take it out of the highway in a safe manner. Because these actions require cooperation among vehicles in the neighborhood of the faulty vehicle, the supervisory controllers are implemented by means of inter-vehicle communication protocols. The logical correctness of the proposed protocols is verified using automatic verification tools. We discuss the safety of the proposed design in terms of the possibility of collisions and highlight the problems associated with carrying out a complete safety analysis.

Design and evaluation tools for automated highway systems

In this paper, the authors present a brief description of the California Partners for Advanced Transit and Highways (PATH) hierarchical control architecture for Automated Highway Systems (AHS). They describe the framework for AHS design and evaluation and give a summary of simulation and analysis tool needs. Some of the tools being developed which are based on the hybrid systems approach are also described.

Design and Verification of Communication Protocols for degraded modes of operation of AHS

In this paper, the authors present the design and verification of inter- vehicle communication protocols for degraded modes of operation on an automated highway system (AHS). Various hardware and sensor faults are considered that can develop on the automated vehicle in an AHS and design discrete event supervisory controllers to stop the faulty vehicle or take it out of the highway in a safe manner. The protocols are verified for logical correctness by using automatic formal verification tools.

Verification of hybrid systems: monotonicity in the AHS control system

Numerous approaches to verifying the safety of vehicles in the AHS architecture of PATH have been proposed (see [1] and [2]). One approach involves finding a boundary between safe and unsafe initial conditions. This task is significantly easier if we can guarantee that the severity of an accident is monotonic in the initial conditions. Simulation results are presented in which it is shown that this condition does not hold for the design of [1], and a hybrid automaton model explaining this situation is presented. Finally, some attempts to modify the control laws to obtain monotonicity are presented.

Information structures for control and verification of hybrid systems

Hybrid systems are continuous variable, continuous time systems with a phased operation. We present the hybrid automaton model and introduce the problems of control and verification. We describe the information structure of a hybrid system in terms of its observation and control spaces, observation map, control strategy, and state evolution. For the important case of symbolic observations and controls-i.e., of finite observation and control spaces-we develop the notions of state space refinement and equivalence partitioning. We review from the literature different information structure setups for control and verification of hybrid systems.

SmartDb: an object-oriented simulation framework for intelligent vehicles and highway systems

SmartDB is a framework for the uniform specification, simulation, optimization, evaluation, and implementation of intelligent vehicle highway system (IVHS) alternatives. The salient concepts in SmartDB are: (1) layered control architecture, (2) coordination of distributed control agents through communication, (3) combined discrete and continuous dynamical systems, known as hybrid systems, and their control and verification, (4) object oriented simulation, and (5) distributed and open architecture. This paper summarizes the first three concepts and describes in detail the simulation constructs and the distributed and open architecture of SmartDB.<<ETX>>

Semantic tableau for control of PLTL formulae

We describe the propositional linear temporal logic (PLTL) formalism and introduce the notions of observations, actions and control in PLTL. We develop the 0-lag and 0-lead possibly blocking and nonblocking control strategies and describe their properties. We present an algorithm to generate finite semantic tableaux for control of PLTL formulae. The semantic tableau is part of the controller state and at each time it classifies the actions available to the controller into three groups: those that would immediately complete the task, those that would postpone task completion and those that would immediately block task completion for ever. The tableau reflects the syntactic structure of the PLTL formula under consideration. The algorithm yields the tableau for the 0-lag possibly blocking control strategy.

The use of SHIFT in system design

SHIFT is a programming language for describing dynamic networks of hybrid automata. Such systems consist of components which can be created, interconnected and destroyed and the system evolves. Components exhibit hybrid behavior, consisting of continuous-time phases separated by discrete-event transitions. Components may evolve independently, or they may interact through their inputs, outputs and exported events. The interaction network itself may evolve. This paper illustrates how SHIFT was used to analyze multiple merge junction automated highways for safety and efficiency by several design teams comprising graduate students, research engineers and faculty.