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Practical Aspects of Implementing H-Infinity Controllers on a FBW Research Helicopter

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This paper presents a summary of the design and testing of two H-infinity controllers recently flight-tested on the NRCs Bell 205 experimental fly-by-wire helicopter. Lessons leaned from the implementation and testing are described. Both designs were based on low-order mathematical models and H-infinity optimization. The first controller successfully engaged first time, and is believed to be the first H-infinity controller flight-tested on a rotorcraft. It was subsequently evaluated at hover and lowmoderate speed by a test-pilot and found to achieve level 2 Cooper Harper Handling Qualities on a number of tasks. The controller was re-designed using a different mathematical model and a different H-infinity cost-function. The result was a significant reduction in cross-couplings. better though still Level 2 handling qualities ratings of 4-5 Level 1 pitch and roll bandwidths. This paper presents an analysis of data from these flights. The flight testing provided a number of important practical lessons that could be useful to anyone attempting to implement and test modern controllers in flight. The gap between robustness of the design method and accuracy of the flight mechanic model is one of foe most critical issues in high bandwidth control. Improved aircraft models translate directly into better controller performance. Validation of the aircraft model against open loop helicopter flight test data has shown that both the models used were deficient in a variety of ways. Software implementation should be kept as single as possible a discussion of foe methods used for this project is given. The use of an on-board aircraft model greatly assisted in trouble-shooting the code for errors before flying. Use of automated code generation greatly reduces transfer errors from the Mat lab design environment. To assess new control laws fully, an experienced test pilot is essential.

Subject Categories:

  • Helicopters
  • Flight Control and Instrumentation
  • Computer Programming and Software

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