Real-time decision systems are computer-controlled systems that must react to events in the external environment by making decisions based on sensor inputs and state information sufficiently fast to meet environment-imposed timing constraints. They are used in applications that would require human expertise if such decision systems were not available. Human beings tend to be overwhelmed by a transient information overload resulting from an emergency situation, thus expert systems are increasingly used under many circumstances to assist human operators. As the complexity of tools and machineries increases, it is obvious that more intelligent and thus more complex embedded decision systems are expected to be developed and installed to monitor and control the environments in which they are embedded.
Since the solutions to many of these decision problems are often nondeterministic or cannot be easily expressed in algorithmic form, these applications increasingly employ rule-based (or knowledge-based) expert systems. In recent years, such systems are also increasingly used to monitor and control the operations of complex safety-critical real-time systems. This chapter gives an introduction to real-time expert systems by describing a class of these systems in which decisions are computed by propositional-logic rule-based programs implemented in the equational logic language EQL.
We begin by describing EQL and we present several examples. The notion of the state space of an equational rule-based program is then introduced. Next, we demonstrate the use of a set of analysis tools that have been implemented to perform timing and safety analyses of real-time equational rule-based programs. The theoretical formulation and solution strategies of the relevant analysis and synthesis problems are then given. Complexity issues of the various analysis and synthesis problems are also discussed. Next, we present the specification language Estella for customizing 259