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.

Abstract: An anti-reset windup proportional plus integral controller connectable in a proportional control loop of a process control system includes a proportional amplifier circuit and an integrating amplifier circuit responsive to a command signal to provide proportional and integral signals, respectively, which are summed by a summing amplifier to provide a modified command signal having a proportional and an integral component. A signal comparator means compares the modified command with a pair of reference signals which define an output range for said summing amplifier and disables the integrating amplifier circuit, permitting the integrating amplifier circuit to unwind whenever the modified command signal is not within the output range.

Abstract: The disclosure relates to a non-minimum phase linear controller which uses positive servomechanism rate feedback to control hydraulic mechanisms. The disclosure also relates to a controller comprising, in combination, a conventional linear controller and a non-minimum phase linear controller, the outputs of the combined controllers being weighted in a predetermined ratio to provide a single output control signal.