Re-engineering cyber-physical control applications for hybrid communication protocols

In this paper, we consider a cyber-physical architecture where multiple control applications are divided into multiple tasks, spatially distributed over various processing units that communicate over a bus implementing a hybrid communication protocol, i.e., a protocol with both time-triggered and event-triggered communication schedules (e.g., FlexRay). In spite of efficient utilization of communication bandwidth (BW), event-triggered protocols suffer from unpredictable temporal behavior, which is exactly the opposite in the case of their time-triggered counterparts. In the context of communication delays experienced by the control-related messages exchanged over the shared communication bus, we observe that a distributed control application is more prone to performance deterioration in transient phases compared to in the steady-state. We exploit this observation to re-engineer control applications to operate in two modes, in order to optimally exploit the bi-modal (time- and event-triggered) characteristics of the underlying communication medium. Depending on the state (transient or steady) of the system, both, the control inputs and the communication schedule are now switched. Using a FlexRay-based case study, we show that such a design provides a good trade-off between control performance and bus utilization.