Compensator design by frequency-weighted approximation.

One of the main attractive features of classical interactive compensator design techniques is that they usually yield low order controllers with acceptable performance. If more sophisticated design techniques as LQ-optimization, H oo-optimization, or simply pole-placement are used, the order of the compensator is usually high (about the order of the model of the plant). In practical applications, such high-order compensators are often not desirable. The standard approach to overcome this problem is to base the design on reduced-order models; however it is difficult to judge on the necessary order and accuracy before the controller design is done. Alternatively, the high-order compensator may be reduced. In this paper, we discuss the choice of an optimal frequency weighting function for this purpose and the effectiveness of a suboptimal approximation technique. The design of the compensator by frequency-weighted approximation can also be used as the key step in the automation of the cla ssical lead-lag design technique based on step-response specifications. We demonstrate, that such an approach is superior to pure interactive design for non-trivial examples. It is argued, that this technique is more effective for semiautomatic controllers design than expert-system based methods. (IITB)