The reactive component of a motion control architecture for a car-like vehicle intended to move in dynamic and partially known environments is presented in this paper. It is called the execution monitor (EM). The purpose of EM is to generate commands for the servo-systems of the vehicle so as to follow a given nominal trajectory while reacting in real time to unexpected events. EM is designed as a fuzzy controller. i.e. a control system based upon fuzzy logic, whose main component is a set of fuzzy rules encoding the reactive behaviour of the vehicle. A behaviour-based approach is used to set up the fuzzy rule base: the overall behaviour of the vehicle results from the combination of several basic behaviours (trajectory following, obstacle avoidance, etc.), each of which is encoded by a specific set of rules. This approach permits an easy and incremental construction of the fuzzy rule base and also to develop and rest the basic behaviours separately. It is the fuzzy control mechanism that straightforwardly handles the problems of behaviour arbitration and command fusion. The basic behaviour rules are simply obtained through direct encoding of the human expertise about car driving. In addition, weighing coefficients are attached to the rules thus permitting a fine tuning of the influence of each basic behaviour. EM has been implemented and tested on a real computer-controlled car, equipped with sensors of limited precision and reliability. Experimental results obtained with the prototype vehicle are presented. They demonstrate the capability of EM to actually control a real vehicle and to perform trajectory following and obstacle avoidance in real outdoor environments by using simple fuzzy behaviours relying upon low-resolution sensor data. (C) 2001 Elsevier Science B.V. All rights reserved.
