Glossary of Control Engineering Terms - A

"A to D" or A/D Converter: A to D stands for Analog to Digital. This electronic hardware converts an analog voltage signal, typically representing a plant output such as temperature or pressure in a control loop, into a digital number that a computer can process and interpret.

Actuator: In a closed-loop control system, that part of the final control element that translates the controller output into an action by the control device.

Adaptive Control: A class of controllers that adapts its gains to a changing environment to maintain performance and stability. This term encompasses simple Gain Scheduling control and Self-tuning control.

Anti-Windup: With a PID controller, integral action will continue to change the controller output value beyond a physical limit on the actual output. This is called reset (integral) windup. For example, if the controller is connected to a valve which is 100% open, the valve cannot open farther. However, the controller's calculation of its output can go past 100%, asking for more and more output even though the hardware cannot go past 100%. This can inject a delay in controller response, and destabilise the control loop in some cases. Most controllers use an "anti-reset windup" feature that disables integral action when the controller output reaches a limit.