Which statement about sensors in closed-loop control is true?

• A. Sensors are not involved in closed-loop control.
• B. Sensors measure only temperature.
• C. Sensors monitor the input signal only.
• D. Sensors provide feedback to detect deviations and enable correction.

Answer: (D)

Sensors in a closed-loop control system provide real-time measurements of the process variable and feed that information back to the controller. This feedback lets the system compare the actual output with the desired setpoint, recognize any deviation, and drive corrective action through the actuator to reduce the error. For example, a thermostat uses a temperature sensor to monitor room temperature; the controller then adjusts heating or cooling to reach the set temperature. Sensors are not limited to temperature and are not just monitoring the input signal; they must measure the output of the process to close the loop.