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TECHNICAL BRIEFS

Optimal Feedforward Prefilter With Frequency Domain Specification for Nonminimum Phase Systems

[+] Author and Article Information
Dirk Torfs, Joris De Schutter

Department of Mechanical Engineering, Division Production Engineering, Machine Design and Automation, Celestijnenlaan 300 B, B-3001 Heverlee, Belgium; Dr. Torfs is currently at Trasys Space, Horizon Center, Leuvensesteenweg 510, B-1930 Zaventem, Belgium

J. Dyn. Sys., Meas., Control 118(4), 791-795 (Dec 01, 1996) (5 pages) doi:10.1115/1.2802359 History: Received February 13, 1995; Online December 03, 2007

Abstract

The paper shows the influence of the location of unstable zeros on the tracking performance of feedforward prefilters. Unstable zeros are divided into a number of classes. It is shown that existing feedforward prefilters (Zero Phase Error Tracking Control (ZPETC), E-filter, Extended Bandwidth ZPETC, ...) perform well for two classes, but fail for a particular class of unstable zeros. For this class, a characteristic frequency, fc , exists such that the induced gain error attenuates all frequencies of the reference trajectory f ≤ fc and amplifies frequencies f > fc . Hence, it is impossible to freely select the tracking bandwidth. Therefore, an optimal feedforward prefilter for discrete time nonminimum phase systems is presented to deal with this class of unstable zeros. As in the ZPETC method, the prefilter compensates for unstable zeros in the inverse system model, retains the zero phase property, and introduces small gain errors. But in addition, the design minimizes a cost function for which a least square solution is found. A frequency and time domain analysis shows the superior performance of the presented optimal prefilter design even for trajectory with high frequency components.

Copyright © 1996 by The American Society of Mechanical Engineers
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