What is the typical order of main components in a control loop?

What is the typical order of main components in a control loop?

• A. Process, sensor/transmitter, controller, actuator.
• B. Actuator, valve, sensor, controller.
• C. Controller, sensor, process, actuator.
• D. Sensor/transmitter, controller, actuator/valve, and the process; power and communication as needed.

Answer: (D)

The loop starts with sensing the process variable through a sensor/transmitter, which converts the physical condition into a signal the controller can use. The controller compares that signal to the desired setpoint and produces a control signal. That signal drives the actuator or valve, which makes a change in the process. The process responds, changing the process variable, which the sensor again measures, continuing the cycle. Power and communication infrastructure are included as needed to supply and connect the sensors, controller, and actuators. Other sequences place the controller before sensing or omit the process step, which would break the feedback flow and prevent proper control.

The loop starts with sensing the process variable through a sensor/transmitter, which converts the physical condition into a signal the controller can use. The controller compares that signal to the desired setpoint and produces a control signal. That signal drives the actuator or valve, which makes a change in the process. The process responds, changing the process variable, which the sensor again measures, continuing the cycle. Power and communication infrastructure are included as needed to supply and connect the sensors, controller, and actuators. Other sequences place the controller before sensing or omit the process step, which would break the feedback flow and prevent proper control.