Auxiliary channel for direct digital control system

Abstract

An auxiliary channel coupled to a selected loop in a plurality of process control loops included in a direct digital control system, the channel serving to expand the frequency response of the selected loop. The system is provided with a digital computer acting as an electronic controller common to all loops, each loop including a transducer sensing a particular process variable to produce a corresponding analog input signal which is digitized and compared in the computer with a set point signal to yield, after conversion, an analog output signal for operating a final control element in the loop to correct for process variations. The analog input signals from the respective loops are sequentially fed into the computer through a multiplexer at a relatively slow sampling rate whereby the loops are normally responsive only to the low-frequency component in the sampled process variables representing slow changes. The input signal from the selected loop is also fed into the auxiliary channel which includes a filter that passes the high-frequency component in the process variable to an amplifier whose output and the output signal from the selected loop are applied to an adder to produce a composite output signal for operating the final control element in a manner rendering the selected loop responsive to the full range of slow and rapid process variations.