In a feedback control loop, what is the purpose of a reference input?

• A. To measure the output
• B. To record fault status
• C. To supply energy to the actuator
• D. To provide the desired setpoint for the controlled variable

Answer: (D)

The reference input provides the desired value for the variable being controlled—the setpoint. In a feedback loop, the controller compares this reference with the actual output measured by sensors and generates an error signal. The controller then drives the actuator to reduce that error, steering the system toward the setpoint defined by the reference input. Without a reference input, the loop has no target to aim for and the output could drift. The reference input is not a measurement, a fault log, or a source of energy for the actuator.