Glossary of Control Engineering Terms - H

H-infinity Control: An optimal control method such that the controller minimises the H-infinity norm of the transfer function from disturbances to the regulated (i.e. process) variable. For a single loop system the H-infinity norm is the peak of the magnitude frequency response of the closed-loop transfer function (often the sensitivity transfer function is used). For multivariable systems the H-infinity norm is the peak of the singular value frequency response. Like LQG control the controller uses state feedback and hence a state estimator, and the controller has as many states as the model used for controller design. When uncertainty is represented as frequency dependent weights can be used to design a robust controller for unstructured model uncertainty.

Hysteresis: In a valve with loose linkages, the air signal to the valve will have to change by an amount equal to the hysteresis before the valve stem will move. Once the valve has begun to move in one direction it will continue to move if the air signal keeps moving in the same direction. When the air signal reverses direction, the valve will not move until the air signal has changed in the new direction by an amount equal to the hysteresis.