Frequency responses to minimize output disturbances caused by parameter variations and noise

A combination of feedforward and feedback control leads to a problem of deciding what to do with each. The fundamental differences between these types of control show up when there are unknown disturbances or unknown parameter variations in the system. Hence, it is knowledge of the expected sizes of these variations which should influence how the feedforward and feedback should be combined. The paper first indicates some of the implications of these relationships in terms of bandwidths. The frequency responses which should optimize the disturbance rejection are then found. These ‘optimal’ results are compared with those from Bode's and Horowitz's ideal frequency characteristics, and with the results expected from minimizing the integral of error-squared on the time responses.