Feedforward Control Based on Error and Disturbance Observation for the CCD and Fiber-Optic Gyroscope-Based Mobile Optoelectronic Tracking System  
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rate [6; 7], which are major causes of instability and performance deterioration [8; 9; 10]. Scholars have adopted many optimization methods to enhance the tracking accuracy and decrease the bad influence of the delay. A PID-I method, with an integration added to the controller, was proposed to reduce the steady-state error of the system [11], which, however, would affect the stability of the system and decrease its dynamic performance. In order to eliminate the effect of delay on system stability, a Smith predictor was introduced to the tracking loop [12; 13]; however, the delay was moved out from the closed loop but still existed in the system, which would restrict the TP. As reported, a multi-loop control structure based on MEMS inertial sensors could increase the bandwidth of the system [14]. Nevertheless, there was little low-frequency improvement of TP. Compared to the difficulties in enhancing the TP, the ADA of the system is relatively easier to improve, for the disturbances usually originate from the base which could be measured by inertial sensors with little delay. Therefore, an inertial sensor, such as a fiber-optic gyroscope (FOG) mounted parallel to the boresight, is commonly used to establish a high-rate inner loop, which would increase the whole ADA of the system [15; 16]. However, the ADA of the closed-loop control is still insufficient for plant uncertainties and large-magnitude disturbances. In summary, the feedback control method is limited in improving either the TP or ADA of the system. In order to get a satisfactory performance, it is necessary to perform feedforward control, including the tracking and disturbance feedforward. Theoretically, the errors could be significantly reduced or even eliminated, and almost all measurable disturbances could be suppressed. Unfortunately, it was difficult to get the trajectory of the target and extract the disturbance signals. A predictive tracking method combining the boresight error with angular sensor for synthesizing the target trajectory was proposed to compensate the errors caused by time delay [17; 18]. However, an additional position sensor was required, which was only suitable for the condition with low measurement noise. Similarly, in order to detect the disturbances, additional sensors should be equipped on the pedestal [19; 20]; the wind disturbance and cogging force cannot be reflected from the pedestal. Hence, the feedforward control based on direct measurement could not be easily implemented, especially for the space and cost limited occasions. In this paper, an unconventional feedforward control method based on the error and disturbance observation was proposed considering the velocity and position double-loop control (VPDC). The CCD-based error observer (EOB) combined the differential of the boresight error and model output with a delay to generate a composite velocity, which simultaneously contained the delayed signals of the target motion and disturbance. Since the delay had little effect on the very low-frequency signal, the low-frequency items of the composite velocity could be fed forward to the velocity closed loop. The EOB was equivalent to a coalition of the low-frequency tracking and disturbance feedforward. Although the TP improvement was in the low frequency, it was satisfactory because the target motion signal mainly distributed there. Unlike the TP, the ADA improvement only in low frequency brought by EOB was not enough because external disturbances nearly distributed in the full frequency band. Therefore, a FOG-based disturbance observer (DOB) was continuously added to the inner loop to increase the ADA.