Nonlinear Control Techniques
Every physical system in our real life has nonlinearities and very little can be done to overcome them. Many practical systems are sufficiently nonlinear so that important features of their performance may be completely overlooked if they are analysed and designed through linear techniques. In HDD servo systems, major nonlinearities are frictions, high-frequency mechanical resonances and actuator saturation nonlinearities. Among all these, the actuator saturation could be the most significant nonlinearity in designing an HDD servo system. When the actuator saturates, the performance of the control system designed will seriously deteriorate. Interested readers are referred to a recent monograph by Hu and Lin  for a fairly complete coverage of many newly developed results on control systems with actuator nonlinearities.
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