Line of sight tracking control of an image sensor-based cubic Stewart platform

This paper proposes an image sensor-based tracking control system of the cubic Stewart platform with charge couple device (CCD) measurement for line of sight (LOS) stabilization and orientation. Due to only two-dimensional LOS errors from the CCD, a decoupled control strategy of six-dimensional Stewart actuator based on the transformation of the Jacobi matrix is investigated resulting into two independent tracking loops to control the LOS. High control bandwidth facilitates good closed-loop performance. However, the tracking control performance of the Stewart platforms is inevitably limited due to a low-rate sample of the CCD and low vibration isolation bandwidth of the Stewart platform that requires rejecting high-frequency disturbances. Therefore, a serial proportional integral (PI) controller is proposed to improve the low-frequency tracking performance, resulting in multiple integrators. The serial PI controller is introduced to the tracking loop in the stable tracking stage for preventing integral saturation. Although the error attenuation bandwidth with serial PI controller is a little smaller than that with PI controller, the low-frequency error attenuation performance is enhanced. Both simulations and experiments show the effectiveness of the serial PI controller.