"Positioner is none other than a controller but positioner is specifically design for control valve just like the VFD is specifically design for the motor. Cascade control means the control involving one Master Controller and slave controller. The Master Controller output provides the setpoint to the input of the Slave Controller. Inside the positioner itself, there is master and slave controllers. The external controller such as PLC will provide command signal (control signal or setpoint) to the input of postioner which is master controller input (travel position controller). This control valve travel position setpoint will be compared to the process variable (PV) feedback from the travel sensor (potentiometer or LVDT) which is usually located in the positioner nearer to the valve feedback arm. The input error (deviation) will be fed to the PI controller for processing. Then, the PI controller output will provide the pressure setpoint to the slave controller (PID controller). This pressure setpoint will then be compared to the process variable (PV) feedback from the output pressure sensor which is usually located inside the positioner. The input error (deviation) will be fed to the PID controller (pressure controller) for processing. Then, the Pressure controller will output drive signal to I/P transducer. I/P transducer will convert current to pneumatic pressure and this output pneumatic pressure (MV or manipulating variable) will be fed to the actuator of the control valve. This will then cause the valve stem to move and the travel distance will be detected by travel sensor either via the linkage travel arm or via the magnetic array method."
