With the widening of the application scenarios for target observation, the traditional ground-based fixed LAT system has been unable to meet the requirement of high precision tracking ,the LAT need to be installed on different motion platforms. In this case, a series of photoelectric tracking systems based on motion platforms have been derived. The combination of LAT and moving platform brings a new problem--Disturbance such as braking, vibrating, and shaking of the platform will cause the deflection and shaking of the LAT imaging screen, which will have a great impact on the LAT tracking. In this paper, a method to determine the change of the attitude angle θ of the target in the field of view caused by the movement of a moving platform is proposed to suppressed overcome the disturbance introduced caused by the application of LAT in moving platform carrier. According to the angular position data of the target tracking point acquired by the CCD system and the attitude information of the motion platform in the geodetic coordinate system provided by the inertial navigation system, the attitude angle of the target can be calculated by applying the proposed attitude estimation algorithm. This method not only, provides a specific estimation process of target attitude angle, but also offers principle information for tracking state decision, image processing, and fiducial direction calculation.
KEYWORDS: Control systems, Fiber optic gyroscopes, Linear filtering, Servomechanisms, Charge-coupled devices, Line of sight stabilization, Mirrors, Sensor technology, Sensor fusion, Systems modeling
A feedforward control method based on sensor fusing is proposed to improve the bandwidth of disturbance suppression for compound axis servomechanism tracking system (CASTS). This method takes advantage of the fiber-optic gyroscope (FOG) and the fine charge couple device (FCCD). Two sensor signals are decoupled and fused to obtain a precise observation of the disturbance. The FOG signal contains disturbance information and target moving information, yet the target information cannot be used for feedforward because it’ll deteriorate target tracking capability of the system. The high-pass filter is designed to filter out the low-frequency target signal. However, the high-frequency sampling of the fine FCCD sensor is insufficient. To avoid the inappropriate high-frequency signal of the FCCD, the complementary low-pass filter is applied to the FCCD. An accurate disturbance observation can be achieved by decoupling and fusing the FOG signal and the FCCD signal. The fused disturbance is fed forward to the fast- steering mirror (FSM) which possesses a higher control bandwidth to achieve the line-of-sight (LOS) stabilization control. Finally, comparative simulations are conducted to verify the disturbance rejecting performance of the proposed method. The results indicate that the disturbance suppression performance of the system can be significantly improved by applying the proposed method. A higher disturbance rejection band and a higher disturbance rejection ratio (DRR) can be obtained compared to the conventional control method.
Compound axis servomechanism (CAS) is the most popular form of structure used in the large aperture telescope (LAT) system in the last few decades. In the control system of the CAS, the sub-axis which is driven by a fast-steering mirror (FSM) plays a decisive role in observation target tracking. In this paper, the kinetic dynamic of fast observation target (FOT) is analyzed by the time-frequency spectrum method. A sub-axis controller base on the FOT’s kinetic dynamic is proposed to improve the tracking performance of the CAS system. Traditional double-loop control is applied in sub-axis control. The inner loop is designed to provide a large bandwidth to cope with the FOT. As for the outer loop, a new control approach that is PIPI double integrations control is introduced to achieve a better tracking performance for the FOT. Additionally, the relationship between time delay and bandwidth of the LAT system is analyzed to demonstrate that the time delay is the main restriction to further improve the bandwidth of a CCD-based sub-axis controller. The theory and the simulation result indicate the proposed approach can improve the tracking performance of the LAT system to a certain extent, but still not sufficient when it comes to the FOT.
Characterized by its compact structure and fast response, the rotational double prisms system is broadly applicable for high-precision pointing and tracking. In particular, closed-loop tracking technology based on an image detector can overcome beam deflection errors caused by prism parameter and target-guiding errors. However, the rotations of each prism affect beam deflection angles in both the x- and y-directions by different amounts in different areas. Therefore, aimed at this problem of the tracking error being coupled to the rotation angle of the two prisms, we proposed a real-time sector selection closed-loop tracking method by inputting error value feedback from the detector and outputting the adjustment values of the prisms. This method can simultaneously close-loop the error signal in two directions and is not limited by the target distance. We established a test platform to verify the proposed method. The test results showed that the proposed method can continuously track moving targets across different areas of a field of view for an extended period. When the maximum moving speed of the target was 0.32 deg / s, the root mean square tracking error of the noncentral area was <1 arcsec. The simulated and experimental results confirmed the effectiveness of this method.
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