H-infinity control theory has gained wide acceptance over the past 10 years as a valuable method of controller design. However, for most applications the solution results in a controller of higher dimension than that of the plant, As an alternative to controller order reduction, low-order controllers can be designed by fixing the order of the controller a priori. There also has been a growing interest in the mixed H-2-H-infinity problem. This problem has not been resolved completely, and there are few published solutions for the fixed-order case, A differential game formulation for the H-infinity problem is extended to address the mixed problem. The result is five first-order necessary conditions, and a conjugate gradient algorithm is used to search for solutions. To test the approach, a 10th-order problem describing the lateral dynamics of a high-performance helicopter in forward flight is used. References: 26
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