Appendix A. Glossary


A device to convert a control signal (as produced by the controller) into a physical action that directly affects the plant. A heating element is an actuator, as is a stepper motor. Actuators are transducers.

Actuator saturation

Because they are physical devices, actuators have limits in the action they can bring about. (A heating element has a maximum amount of heat it can generate per second, a motor has a maximum velocity, and so on. Notice in particular that a heating element is completely unable to generate any cooling action or negative heat flow.) At the same time, control signals can be arbitrarily large. Whenever an actuator is unable to follow the demands of the control signal, it is “saturated.” Actuator saturation means that the intended control actions are no longer applied and that the control loop is therefore broken. (See also Integrator clamping.)

Bang-bang controller

A colloquial term for an on/off controller (as opposed to a controller that is capable of varying the magnitude in response to its input).

Bumpless transfer

A smooth transition when switching between different controllers—for instance between manual control and closed-loop control or between different control strategies in a gain-scheduling scenario. When using a PID controller, a bumpless transfer requires that the value of the integral terms be synchronized before the transfer. (See also Integral preloading.)

Control problem

Given a system with an input and output, the control ...

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