Relating sampling period and control delay in distributed control systems

The control delay in distributed control systems may be compensated on-line or off-line depending on the required quality of the output, available computational power of the controller node, time stamping and clock synchronizing facilities etc. Irrespective of the compensation technique, there is an upper bound to the constant or variable control delay, which is decided by the plant, controller and the selected sampling period of the distributed computer controlled system. It is shown that in general, if the control delay exceeds the sampling period, the system becomes multi-rate; deteriorate performance as the plant updates do not correspond to the output samples. This paper introduces an adaptive sampling scheme to ensure the control delay is less than the sampling period in steady state and uses the maximum tolerable delay of the system at a particular sampling interval to ensure stable transform from one sampling period to another.