Modeling and Control of a Beam Steering Mechanism for a Deep Space Optical Communication System

Jason J. Gorman, Intelligent Systems Division, Manufacturing Engineering Laboratory
 

Future interstellar explorer spacecraft will require a low power communications system which can transmit signals across our solar system to an Earth-bound receiver. The most feasible approach to this problem is optical communication due to the low power requirements and low signal losses for laser beams in space. However, the large scale over which the communications will take place requires beam steering accuracy on the order of nanoradians. In this research, we have developed models and control approaches for a micropositioner which is targeted for use as a beam steering mechanism. The presented micropositioner is a monolithic flexure hinge mechanism which is driven by piezoelectric actuators. Analytical kinematic and dynamic models of the mechanism have been derived based on the discrete stiffness relations for flexures and the hysteretic behavior of piezoelectric actuators. The physical parameters of the system have been determined using a combination of analytical relations, modal analysis and recursive least-squares system identification. One major factor affecting the beam steering accuracy for the deep space optical communication system is base motion generated by the interstellar explorer which could excite motion in the micropositioner as well as cause deviations in the beam pointing relative to the inertial frame. Therefore, we have developed a disturbance rejection controller which combines a robust tracking controller and an inertial compensator. The robust tracking controller is used to cancel the actuator nonlinearities and the forces resulting from the base excitations, which are measured using an accelerometer. Furthermore, the tracking controller adds damping to limit the effect of the disturbances and provides robustness to the sensor measurement uncertainty by using sliding mode control. The inertial compensator is designed to generate an approximation of the micropositioner base motion relative to an inertial frame based on the accelerometer measurements by using a low pass filter and integrator. The estimated motion is then used to adjust the micropositioner trajectory so that the desired beam path is achieved. The results of controller simulations and initial experiments for a set of typical pointing and acquisition trajectories will be presented.