A Frequency Domain Approach To Anti-Windup Compensator Design

The effect of input saturation in linear time invariant control systems is addressed. We utilize a controller in polynomial form, with a three degrees of freedom structure. This makes it possible to add a saturation compensator (an anti--windup filter) separately, after tuning of the controller properties for unsaturated signals. Our aim is that the effect of a saturation event should decay as quickly as possible after desaturation, while the tendency for repeated re--saturations, due to nonlinear oscillations, should be supressed. This is achieved by modifying the relevant loop gain by a safety margin. The modified loop gain is then adjusted, so that it touches the describing function of the saturation nonlinearity. The construction of the safety margin takes the magnitude of exogenous signals into account. The method requires only simple calculations. It can be automated, using a one--dimensional numerical search. Furthermore, the effect of D/A--quantization in digital feedback syste..