Flight Control Design Using Observer-based Loop Transfer Recovery

This paper presents an output feedback flight control design method that is focused on designing a high-performance robust control. The architecture is similar to Linear Quadratic Gaussian controllers. When using full order observers, or Kalman filters, the stability robustness properties can typically degrade, and the system can be sensitive to errors in the plant model. The method presented here, called observer-based loop transfer recovery, asymptotically achieves positive real system behavior at certain loop break points during recovery. This paper presents design insight into using this method for flight control design problems, including systems using acceleration feedbacks that are nonminimum phase. During the design process the observer is artificially squared-up. This adds fictitious inputs to make the number of controls equal the number of measurements, and makes the observer design model minimum phase. This step is central to achieving the positive real behavior during recovery. To place the zeros in a desired location during plant squaring, a pole-placement algorithm is used. A design example using the X-45A Joint Unmanned Combat Air System dynamics is utilized to highlight the design approach.