Control system

Abstract

A control system for controlling the variables of a process, comprising at least two proportional-integral-differential controllers each comprising a linear signal path, a differentiating signal path and an integrating signal path, all connected to an input junction. The outputs of the linear signal path and the differentiating signal path are added at an intermediate junction and the resulting sum signal is added to the output signal of the integrating signal path at an output junction. Each proportional-integral-differential controller supplies an output suitable for controlling one of the process variables such that this process variable is maintained as close as possible to a predetermined maximum value without any of the other process variables exceeding its respective predetermined maximum value. The difference between the value of each of the process variables and the maximum value applying to each of the respective process variables is supplied to a separate proportional-integral-differential controller assigned to the associated process variable. The outputs of all controllers are supplied to a comparator circuit which delivers the smallest of these signals through feedback signal paths to a difference junction in each controller. The difference is formed as a difference signal between the comparator output and sum signal at the intermediate junction in the associated controller, and this difference signal is periodically sampled and used for periodically resetting the integrating circuit in the integrating signal path in each controller.