Dynamic Disturbance Decoupling

Abstract

The attenuation of load disturbance is always of a primary concern for any control system design, and is even the ultimate objective for process control, where the set-point may be kept unchanged for years. As such, this and next chapters will deal with decoupling disturbances from the plant output in dynamic and asymptotic senses, respectively. The dynamic disturbance decoupling problem is to find a control scheme such that in the resulting control system the transfer matrix function from the disturbance to the controlled output is zero for all frequencies, i.e. there is no effect of the disturbance on the controlled output. If the disturbance is measurable, the feedforward compensation scheme can be employed to eliminate its effect on the system output, which will be the topic of Section 1 of this chapter. For unmeasurable disturbances which are more often encountered in industry, feedback control has to be adopted. However, there will be inevitably a design tradeoff between the set-point response and disturbance rejection performance. To alleviate this problem, a control scheme, called the disturbance observer, is introduced and it acts as an add-on mechanisms to the conventional feedback system. The disturbance observer estimates the equivalent disturbance as the difference between the actual process output and the output of the nominal model. The estimate is then fed to a process model inverse to produce an extra control effort which can compensate for the disturbance effect on the output.