Formal Design Models for Distributed Embedded Control Systems

The paper presents a formal specification of the software design models used in COMDES-II - a component-based framework for distributed control systems, featuring open architecture and predictable operation under hard real-time constraints. In this framework, an application is modelled as a network of distributed embedded actors that communicate transparently by exchanging labeled messages (signals), independent of their allocation on network nodes. Actors are configured from prefabricated executable components such as modal function blocks controlled by a master state machine, whereby actor structure is specified by a data flow model (function block network). Accordingly, actor behaviour is specified by composite functions representing signal transformations - from input to output signals, and system behaviour - by actor-level composite functions representing the overall sequence of computation - from system input to system output signals. Input and output signals are exchanged with the controlled plant at precisely specified time instants in accordance with the concept of Distributed Timed Multitasking, resulting in the elimination of transaction I/O jitter. System operation is ultimately described by a clocked synchronous model of computation featuring communicating actors, atomic (zero-time) execution of input and output actions and constant, non-zero execution time of system reactions.