Process control system using a two wire remote control system

摘要

A process control system including a control station responds to the departure of a process variable from a set point to provide a direct current signal in a two-wire transmission line in a direction dependent upon the direction of such departure and time-proportioned in accordance with the magnitude of the departure. A solid state power relay that accepts the d-c signal at a single pair of input terminals includes high-current rating triacs that are adapted selectively to be fired by triggering pulses generated in response to the d-c signals alternating current power from a source of alternating current to a heavy-duty motorized valve for operation of the latter in one direction or the other and at a speed in accordance with the signals.