"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.

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All of the information in this glossary has been carefully compiled and we believe it to be accurate. However, since any such definitions need to be placed in the context of specific applications, we assume no liability for the usage of information contained in this glossary.

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