What is the function of a sensor/transmitter in a closed-loop control system?

• A. To actuate the valve.
• B. To generate a 4-20 mA signal.
• C. To measure the process variable and transmit the signal to the controller.
• D. To store calibration data.

Answer: (C)

In a closed-loop control system, the sensor/transmitter acts as the eyes of the loop. It measures the actual process variable—such as temperature, pressure, or flow—and converts that physical quantity into an electrical signal that the controller can use. The transmitter may condition or scale the signal and often sends it over a standard path (like a 4-20 mA loop) to the controller. The controller then compares this measured value with the setpoint and computes the necessary action to drive the actuator. So, the essential function is to measure the process variable and transmit that information to the controller. The other roles belong to different parts of the system: actuating the valve is the job of the actuator, 4-20 mA is a common signaling method but not the fundamental function, and storing calibration data is a data-management task.