What is the principal function of a controller in a control system?

The principal function of a controller in a control system is to maintain the setpoint. This means that the controller continuously compares the current process variable (the actual value of the system, such as temperature, pressure, or flow rate) with the desired setpoint (the target value). When a deviation from the setpoint is detected, the controller generates appropriate control actions to bring the process variable back toward the setpoint, thereby ensuring stability and optimal performance of the system.

For instance, in a temperature control application, if the temperature drops below the setpoint, the controller will signal a heating element to turn on until the desired temperature (setpoint) is reached again. The effectiveness of a control system largely depends on the controller's ability to perform this function accurately and reliably.

Other options, while relevant to various aspects of a control system, do not capture the principal aim of a controller. Displaying measurements and transmitting data are important for monitoring and feedback but do not directly influence the process variable. Manipulating process variables is a broader function that can include actions taken by the control system but is not the primary focus of the controller, which is specifically tasked with maintaining the setpoint.