Control-based programming of electro-mechanical controllers

We are developing techniques for model-based constructionof software for computationallycontrolled electromechanical systems, such as photocopiers. These systems are examples of real-time, reactive systems — those that react continuously with their environment at a rate controlled by the environment. Execution in a reactive system proceeds in bursts of activity. In each phase, the environment stimulates the system with an input (e.g. a sensor is tripped by a passing sheet), which starts a computation (e.g., a timer to activate a downstream gate) and responds with the output. The system then stays inactive until the next input comes in from the environment. In such a setting, real-time response is as important as standard liveness and safety requirements. Our approach to constructing such software is based on the development of compositional and declarative models of the components that make up the electromechanical device. The intuition is that these models can be combined with software systems corresponding to specific tasks (such as simulation, control code generation etc.), via customized reasoning engines. These engines will take as input the specification of a system configuration, the software architecture and the model of the components to semiautomatically produce the target system. This general approach can be used for producing a variety of systems including controllers, simulators, testers, productivityanalysers and diagnosis tools. In this paper, we consider the nature of the language that should be used to describe the output control code. Even though the proper handling of time is an integral aspect of such software, control software has traditionally been programmed in conventional languages that do not support adequate constructs for handling time. Traditionally, realtime software has been written in untimed languages such as C augmented with some new primitive constructs for real-time operations. This has obvious drawbacks — it does not help in analysis, and anyway it is difficult to get real-time software correct. More recently, attempts have been made to provide an application and platform independent Real-Time Application Programmer’s Interface (API), within a language such as C++. Typically, in a setting like this, real-time programs are written as C++ programs that make calls to run-time routines implementing timers, event handlers, event queues etc. Hardware I/O channels are associated with memory locations. Call-back procedures are registered with an event handler, to be called when an event is raised. Priority levels