Interaction of payload and attitude controller in space robotic systems

摘要

Space manipulators mounted on a free-floating base are structurally flexible mechanical systems. For some applications, it is necessary to control the attitude of the base by the use of on-off thrusters. However, thruster operation produces a rather broad frequency spectrum that can excite sensitive modes of the flexible system. This situation is likely to occur especially when the manipulator is moving a big payload. The excitation of these modes can introduce further disturbances to the attitude control system, and therefore, undesirable fuel replenishing limit cycles may develop. To investigate these dynamic interactions, an approximate two-mass system, where the manipulator is replaced with an equivalent spring-and-dashpot system, is used to reproduce the relative motion of the payload with respect to the spacecraft. A dynamic model of two-flexible-joint planar manipulator was derived to obtain its natural frequencies and then, to determine the corresponding spring stiffness and damping coefficient of the approximate system. SInce the attitude controller assumes the use of on-off thrusters, which are nonlinear devices, the describing function technique, an approximate method for the analysis of nonlinear systems, is used to perform a parametric study investigating the significant parameters of three models studied. This study provides some guidelines for the design of attitude control systems when flexibility is a major concern. As well, this study shows that one of the three models studied is a very good alternative to the actual attitude controllers. Finally, simulations are executed to confirm these results and to study the addition of noise and model uncertainties in the three selected models.