Control tasks delay reduction under static and dynamic scheduling policies

Industrial application of digital control requires the synergy between well designed control algorithms and carefully implemented control systems. The control performances can be strongly influenced depending on the data acquisition and control action delays. The paper shows how to evaluate these delays (jitter) under static or dynamic scheduling policies. An evaluation of several sets of tasks executed under both scheduling policies is analysed and compared. The results allow us to determine the goodness of both algorithms with respect to the delays due to scheduling. While worst case response time in static scheduling can be easily determined, under EDF scheduling the result is not trivial. A method to determine the worst case response time under EDF scheduling is proposed. The measurement of these delays can be drastically reduced with a task decomposition. This decomposition is studied and evaluated from the delays point of view. The reduction of the data acquisition interval (DAI) and control action interval (CAI) under both scheduling policies can be considered in the control design phase in order to properly adjust the control algorithm