Nonlinear Design of 3-axes Autopilot for Short Range Skid-to-turn Surface-to-surface Homing Missiles

Traditionally, missile autopilots have been designed using linear control approaches with gain scheduling. Autopilot design is typically carried out in the frequency domain and the plant is linearized around various operating points on the trajectory. Moreover, three single axis autopilots are usually designed without considering the interaction among the motion axes, i.e., the autopilots in each of the three axes are designed independently of each other. Such designs can not handle the coupling among pitch-yaw-roll channels, especially under high angles of attack occurring in high maneuver zones. In the last decade, design of missile autopilots has been extensively studied using modern control design paradigms such as, robust control, feedback linearization, sliding mode control, singular perturbation etc. But in most of these studies, realistic factors like fin saturation and limitation of gimbal freedom have not been considered. One therefore cannot really evaluate the performance and relative merits of these methods in practical applications. This work presents a nonlinear multivariable approach to the design of an autopilot for a realistic missile that overcomes these difficulties. At first, exact input-output (IO) feedback linearization and decoupling have been carried out for the dynamic IO characteristics of the inner rate loop of the pitch and yaw channels. In the process, the missile dynamics also becomes largely independent of flight conditions such as missile velocity, air density etc. This enables the design of scalar linear controllers for the inner rate loops. In this work the superiority of the new nonlinear multi-input multi-output (MIMO) autopilot over a traditional autopilot has been demonstrated through realistic simulation results in presence of closed loop guidance and seeker. However performance deteriorates when the plant model is perturbed, due to aerodynamic uncertainties, from the nominal model. A robust IO linearization technique is therefore needed to tackle the aerodynamic uncertainties in the system. In this study H∞ and sliding mode techniques have been applied to design a robust controller for the feedback linearized plant. A nonlinear Luenberger observer for the missile airframe dynamics has also been designed for estimating the unmeasured states that are required for feedback linearization. All the design and simulations have been carried out in a realistic environment and thus the results presented in this thesis may be useful for designing more accurate missile system in future.