Answer / carl_ellis

Gain, ti and td are PID proportional control tuning terms/constants. PID controllers use PID tuning constants to calculate an output based on the error signal; final control elements (valves) do not do a PID calculation so valves are not involved with gain, ti and td. The controller is typically in the control room, the valve is somewhere out in the process.

A PID controller calculates the expected valve/damper position from the error signal and the tuning constants, the result of which is an output control signal ranging from 0-100%.

A valve positioner or valve I/P receives that control signal from a controller (4-20mA, Profibus) and supplies air or electric power to the actuator to achieve the correct position demanded by the control signal.

Ti is the term for reset or integration. It is used when the process itself is not an integrating process. Level, pressure, and pH are usually integrating processes. Sometimes temperature is an integrating process, sometimes not. When the process is non-self regulating/integrating process, the ti term is not used in the PID calculation.

Flow and temperature processes typically use the ti term for PID calculation.

An exception to the rule of PID being done in a separate controller is Foundation Fieldbus (FF), which allows function block PID execution in FF devices. In a case where PID control is executed in an FF valve positioner, then a valve would use the PID tuning constants.

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