Combined Optimal/Classical Approach to Robust Missile Autopilot Design

Abstract

The direct application of optimal control techniques to a complete autopilot design leads to a control system which requires excessive instrumentation and is sensitive to variations in the high frequency model. A better approach is presented which utilizes classical control theory to bound the high frequency design and modern control theory to obtain optimal low frequency performance. Results are presented for a separation roll autopilot demonstrating the appropriate tradeoffs and benefits associated w i t h the new design procedure. Air launched missiles require active autopilot control for safe separation from modern launch aircraft because the f low field in the vicinity of the aircraft causes large torque disturbances on the missile and because the aircraft may be m=euvering when it releases its missile. The effect of these disturbances and initial conditions are typically sensed by rate gyros and accelerometers whose out puts are connected through autopilot control circuitry to f in actuators. Initially autopilot design is carried out b 1 decoupling the pitch, yaw and roll channels. ( When this is done,classicd control theory has proven itself to be a valuable and efficient tool for autopilot design. However aerodynamic crosscouplin g exists because the surface features of the missile create asymmetric flow fields over the control surfaces at c??fferent orientations of wind angle and angle of attack. A simplified block diagram showing the cross-coupling appears in Fig. I. Here, if the roll rate response time is close enough to the pitchfyaw response time and the cross-coupling aerodynamic gain is high enough, instability can result. Therefore successful autopilot design must take into consideration aerodynamic cross-couph g . Althou h classical design techniques are well established fi 9 3.4.5) and relatively straight forward for planar autopilot design, trial and error must b? A I ROLL AUTOPILOT 1 ACTUATOR AND a L SENSOR -