ROBUST TECHNIQUES APPLICATION FOR ATTITUDE CONTROL OF A LAUNCHER DURING ATMOSPHERIC FLIGHT

This communication presents the design of robust digital laws for the attitude control of a launcher, and the singular value analysis of stability effects of the structural couplings between axes. In a first stage control laws on each of the three axes are defined. The retained synthesis method is LQG-LTR (Loop Transfer Recovery). Monoaxis control laws with input robustness properties are obtained, insuring the required performances during whole flight. In a second stage, the effects of the couplings on the performances of those control laws are investigated. The robustness of the monoaxis laws regarding coupling is analyzed through the computation of minimal and maximal singular values of the sensitivity matrix as a function of coupling rate. In order to improve this robustness other LQG criteria are considered. The bending modes act on measures, so their introduction in the criteria can been made systematic by a technique of know robustness qualities : that is to add to the weighting matrix Q a term ρHT H, where H represents the output matrix of the state representation. By adjusting the value of ρ, a compromise between needed robustness and desired performances can be achieved. The interest of this study is to demonstrate, on a rather complex system, the efficiency of the LQG-LTR techniques in the design of digital robust control laws. It leads to the conclusion that the monoaxis synthesis of attitude control for a nonsymmetrical launcher is valid if associated with multidimensional analysis of robustness and stability.