DIGITAL TRACKER WITH OUTPUT-FEEDBACK - APPLICATION TO AIRCRAFT CONTROLS

A linear-quadratic output-feedback approach is given for designing dig ital servocontrol systems of specified structure. The result is digita l controllers that have a sensible structure based on classical contro l theory, in contrast to standard LQR design using state feedback plus an observer. The correct initial conditions for determining the LQ tr acker output-feedback gains are not uniformly distributed as is tradit ionally assumed, but are shown to be explicitly given in terms of the step command magnitude. Both pole and zero placement are used to optim ize the performance index. Arbitrary systems are treated, not only tho se with integrators in the forward paths, by adding a term to the perf ormance index that weights the steady-state error. The approach works even if the system is non-minimum-phase. Necessary conditions are deri ved that may be used in a gradient-based routine to determine the opti mal digital control gains. The approach does not rely on redesign of a continuous control system using techniques like the bilinear transfor mation, but uses direct discrete-time design. An aircraft digital comm and augmentation system is included as a sample design.