What is the relationship between the process variable and the setpoint in a control loop?

• A. The setpoint is the process variable
• B. The process variable is the target value, and the setpoint is the current measurement
• C. The setpoint is the current measurement
• D. The process variable is the current measured value; the setpoint is the target value the controller aims for

Answer: (D)

In a feedback control loop, the setpoint is the target value you want the process to reach, while the process variable is the current value measured from the process. The controller constantly compares these two: error = setpoint minus process variable. This error tells the controller how far off you are and what needs to be adjusted to move the process variable toward the setpoint.

For instance, in a room thermostat, you set the desired temperature (the setpoint). The system continually measures the actual room temperature (the process variable). If the current temperature is below the setpoint, the error is positive, and the controller increases heating to raise the temperature toward the setpoint. If the room is warmer than the setpoint, the error is negative, and cooling or reducing heating lowers the temperature toward the setpoint. This relationship—PV as the current measured value and SP as the target value—best describes how the loop operates.