Integrated Optimal Control of Reusable launch Vehicle and Actuation system using Linear Quadratic Regulator

Abstract

Reusable Launch Vehicle (RLV) is used to carry payload from earth's surface to the outer space for more than once. The digital autopilot (DAP) used to control the RLV has an actuation system as its inner loop. The actuation system deflects the control surfaces (eg. fins) to bring about the necessary control action. The actuator is a complex system with many elements which work in co-ordination to bring about the necessary action. But in the design of autopilot, actuator is usually approximated as a second order system having the same bandwidth as that of the actual one. Due to this approximation, an understanding of the entire plant cannot be achieved. During implementation, the approximation of the order of the actuator may lead to deviation of the performance of the system from its specifications. It may also lead to unnecessary control of subsystems which would have been corrected without any control in an integrated model. In this paper, an integrated design of the RLV and its actuation system is developed. The actuation system consists of an electro hydraulic actuator and the associated elements. It is designed to actuate the booster fin of an RLV for the purpose of pitch control. The Linear Quadratic Regulator (LQR) control technique is used to design DAP for the integrated system. LQR is a modern control technique which ensures optimality in the design of the integrated system. A comparison of the performance of the integrated design of RLV and the design with an approximated second order actuator is made. The advantages of LQR technique over the conventional gain design technique are also mentioned.