Study On Electromagnetic Actuator Based Stewart Active Vibration Isolation Platform

Micro-vibration on orbit, which are characterized by small amplitude and wide frequency range, is an important issue for spacecrafts. The micro-vibration problem has often been neglected in the past due to the low levels of disturbances. However, it has received a lot of attention by many researchers recently. This is especially true for high-precision spacecraft where micro-vibrations jeopardize the attitude stability of spacecraft and the pointing precision of sensitive payloads. Therefor, there is a urgent need for the suppression of micro-vibrations. Stewart platform is a parallel robot that distributes the payload to several actuators, and thus has advantages in stiffness, payload weight ratio, and accuracy. It has been widely used in the area of vibratio n isolation. In this thesis, a six-DOF stewart platform based on electro- magnetic actuators is studied for active control of micro-vibration, and the main research contents are as follows.The isolation principle and structure design of electromagnetic actuator are discussed as the basis of this thesis. The electromagnetic theory concerning the structure design is introduced. The principles of structure design of the actuator are proposed, and a design scheme is presented. Comparing the FEM simulation results and experimental measurements, the force constant of the electro-magnetic actuator is detemined, and the similar results are used to show the validity of the structure design.A dynamic model of a 6-DOF Stewart platform with the base excitation is formulated in detail. With Newton-Euler method, the kinematics and dynamics of the legs and the upper platform are investigated in task space, and a 6-DOF dynamical equation is established. Then the dynamical equation is linearized.The optimization of the platform’s dynamic isotropy is conducted by Genetic Algorithm to obtain the specific structural parametes. The stiffness matrix and damping matrix for the linearized system are decoupled to guarantee the platform’s kinematical characteristic. Based on the decoupled structure, a natural-frequencybased dynamic isotropy index is proposed. Genetic Algorithm is used to optimize the structural parameters. Based on the optimized result, a 3-D Stewart platform is plotted in Solid Works to verify the assemblage of the structure. Accordingly, the real optimized platform is installed in our laboratory。Finally, a H_∞ robust control law is designed to isolate the micro-vibration of the payload. Simulation results are used to demonstrate that the micro-vibration control is achieved in the target frequency range. Sine sweeping test are carried out to verify the accuracy of the linearized dynamical model and the validity of the H_∞ robust algorithm for active vibration isolation.