QFT Digital Controller for an Unmanned Research Vechile (URV)

Quantitative feedback theory (QFT) is used to design the digital flight control system for an unmanned research vehicle (URV). Digital controllers are designed for 3 outputs which are controlled via 7 independent control surfaces. The system is transformed into the w´-domain and a 3×7 plant matrix P of transfer functions is derived relating surface deflections to system outputs incorporating servos. By specifying the form of the output and using the concept of a pseudoinverse, a 7×3 surface weighting matrix ¿ is developed to obtain an effective 3×3 square plant matrix Pe = P¿ whose determinant is minimum-phase (m.p.) for all flight conditions (FC). A single set of fixed controllers and prefilters are obtained via QFT, which apply the appropriate feedback to maintain control over the entire range of uncertainty due to surface failures. Single, double, and triple failures are considered. Failed surfaces are considered locked in the trim condition. Fault detection/isolation and scheduling are not required. Healthy aircraft (A/C) loop bandwidths for roll rate, yaw rate and c* loops are 24, 23.3, and 43.5 rps respectively. For a 6 s pulse input, none of the surfaces are seen to saturate. All A/C states also seem to be reasonable. A modification is made to the controllers to compensate for the addition of more realistic servos and sensors. The resulting design is nearly as robust as the original system.