Error-Recovering Supervisory Control of Automated Manufacturing Systems

Abstract

Error detection and recovery are essential aspects of the continuous and safe operation of automated manufacturing systems. In the past these were sufficiently simple to allow informal approaches to supervisory control and therefore to error recovery. The increasing level of automation, integration, and flexibility encountered in automated manufacturing systems renders formal approaches to the supervisory control system development a necessity. The controlled-automata based approach to supervisory control development considered in this work is one such approach, and provides a solution to the error recovery problem. It offers important advantages over other approaches. It guarantees that: (i) The resulting controlled behaviors do not contradict the behavioral specifications and are nonblocking (Ramadge and Wonham, 1989; Wonham and Ramadge, 1988), and (ii) the controlled behaviors are maximally permissive within the behavioral specifications (Ramadge and Wonham, 1989). Typically, these include safety and error recovery specifications, and production specifications such as routing and sequencing: In this paper, errors or their symptoms are interpreted as any event occurring in the plant behavior, and error recovery is approached as any specification on the behavior of the plant. This paper introduces supervisory-control-system development and illustrates the key concepts through a small manufacturing cell example. Subsequently, the development of an error-recovering supervisory control system for an automated assembly cell, in which the robots of the various assembly stations may break down and perform faulty assemblies, is described. “Off the shelf” industrial programmable controllers are used in the corresponding implementation.