Robust multivariable control design : an application to a bank-to-turn missile.
Multi-input multi-output (MIMO) control system design is much more difficult than single-input single output (SISO) design due to the combination of cross-coupling and uncertainty. An investigation is undertaken into both the classical Quantitative Feedback Theory (QFT) and modern H-infinity frequency domain design methods. These design tools are applied to a bank-to-turn (BTT) missile plant at multiple operating points for a gain scheduled implementation. A new method is presented that exploits both QFT and H-infinity design methods. It is shown that this method gives insight into the H-infinity design and provides a classical approach to tuning the final H-infinity controller. The use of “true” inversionfree design equations, unlike the theory that appears in current literature, is shown to provide less conservative bounds at frequencies near and beyond the gain cross-over frequency. All of the techniques investigated and presented are applied to the BTT missile to show their application to a practical problem. It was found that the H-infinity design method was able to produce satisfactory controllers at high angles of attack where there were no QFT solutions found. Although an H-infinity controller was produced for all operating points except the last, the controllers were found to be of very high-order, contain very poorly damped second order terms and generally more conservative, as opposed to the QFT designs. An investigation into simultaneous stabilization of multiple plants using Hinfinity is also presented. Although a solution to this was not found, a strongly justified case to entice further investigation is presented.