What is the function of a controller?

• A. To control the valve and maintain process setpoint
• B. To monitor temperature only
• C. To convert energy from one form to another
• D. To measure flow rate

Answer: (A)

In process control, the controller’s job is to keep a process variable, such as temperature or pressure, at its desired setpoint by adjusting the manipulated variable. It continuously reads the actual process value, compares it to the setpoint, and computes an error. Using a control algorithm (like PID), it converts that error into a corrective signal that moves the valve (the actuator) to increase or decrease flow as needed. This active regulation counters disturbances and maintains the intended conditions. It isn’t merely monitoring, nor converting energy, nor measuring flow rate—those roles belong to sensors or energy converters. The controller’s purpose is to regulate the process by driving the valve to hold the setpoint.