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Overview: For an optic-electro tracking system, image sensors (such as CCD) are usually used for monitoring and positioning as well as tracking a target, but they can only detect line-of-sight (LOS) error and cannot provide target trajectories. Therefore, it brings difficulties to the application of feedforward control which is an effective way to improve tracking performance. As a result, recovering the target trajectory through data fusion is an effective way. However, it needs extra sensors and the effectiveness of the equivalent feedforward control method is based on the accuracy of the synthesized feedforward signal which is affected by the measurement accuracy of the sensor and the prediction accuracy. Inaccurate feedforward signal has no improvement in tracking performance and even leads to instability of the control system. When it comes to tracking system on a moving platform, an inertial measurement unit (IMU) is necessary. The attitude accuracy determined by the IMU always plays an import part in tracking performance. Therefore, the equivalent feedforward control method based on data fusion is not applicable in many cases. For traditional feedback control, high control bandwidth facilitates good closed-loop performance. However, the sampling frequency and time delay of the image sensor are the main factors that restrict tracking bandwidth. Simply using a high-gain feedback controller or improving the order of the control system will decrease the dynamic performance of the system, leading to instability. The error-based observer (EBO) control of an image-based tracking loop is proposed to enhance tracking performance for an optic-electro tracking system on the moving platform. This EBO method combines the LOS error and control output to achieve high gain. The equivalent feedforward control can be plugged into the existing feedback control loop. The closed-loop performance of the image-based control system can be improved by optimizing the feedforward filter Q(s). Since this EBO method does not need extra sensors and it benefits the control system in both disturbance suppression and target tracking, it can be applied to both moving platforms and ground platforms. The control structure decided that Q(s) has to be a low-pass filter. In this paper, an optimal three-order Q31 filter rather than a low-pass filter is improved for this EBO control. Simulations and experiments show that the tracking performance of the EBO method is effectively enhanced in the low frequency compared to traditional control methods and an optimal Q31 filter is more efficient than a simple first-order low-pass filter. This improvement is meaningful because better performance in the low frequency is more important than in the high frequency for many cases.
The basic structure of the control system based on CCD vision tracking
Configuration of classical feedback control on the moving platform
A classical feedforward control scheme
The error-based observer (EBO) control scheme
Bode diagram of 1-Q(jω)e-0.03jω
Bode response of the closed-loop transfer function (a) and sensitivity function (b) from simulations
Configuration of experimental platform
Error comparison of classical feedback control (CFC)(blue) and EBO control with different Q1 filter (red) and with Q31 filter (green) at (a) 0.05 Hz, (b) 0.1 Hz, (c) 0.5 Hz and (d) 1 Hz