Process control system using a two wire remote control system

Abstract

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 to supply alternating current power from a source of single-phase or three-phase 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. The power relay is characterized in its provisions for synchronizing the triggering pulses to the triacs with the time of zero voltage across the triac, precluding such firing of a triac when the voltage across an associated triac is less than a predetermined value, delaying for a predetermined time such firing upon a command for reversal in the direction of operation of said motorized valve, locking-out to prevent further firing of the triacs upon rapid reversal of input commands, and monitoring of the triacs to provide an alarm and/or to effect deenergization of the power relay and motorized valve when the triacs fail to fire in accordance with the command.