Coherence of Distributed Applications Under Critical Time Constraints

Abstract

The increasing complexity of control systems, as well as the large dimensions of the controlled processes, lead to the development of distributed control. New control architectures are mainly based on communication and on intelligent instrumentation concepts. Thus, field buses, as well as smart sensors and actuators, constitute a new field for investigation, in terms of their own definition and working mechanisms, as well as in terms of their use within the framework of distributed applications design. Field bus protocols like FIP (Factory Instrumentation Protocol), are based on a producer-consumers diffusion process, and introduce critical time concepts, which should be very usefull when designing automatic control systems, whose algorithms arc most frequently based on discrete-time state-space representations of the process to control or to supervise. However, the communication protocol gives some tools in order to handle the critical time constraints in real time operation, but does not provide any means for the designers to settle the numerical values of these constraints. This paper describes a real time application as a collection of software and hardware modules, each of them being characterized by the variables it produces and consumes. The consumed variables are associated to critical time constraints, since they have to represent the state of the process at times which arc coherent with the execution instants of the modules which consume them. Logical and temporal coherence arc defined on such an application : logical coherence ensures that each consumed variable has been produced, and that its producer is unique ; temporal coherence checks whether all the consumers of a produced variable are able to fulfill their critical time requirement.