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.
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.
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.
Wavefront aberration, which caused by atmospheric turbulence, needs to be measured in the free space optical communication. The existing sensors of wavefront aberration measurement are mainly divided into two classes, wavefront sensors and image-based sensors. Wavefront sensors , such as Hartmann sensor and shearing interferometry, measure wavefront slope to calculate wavefront aberration. However, wavefront sensors always need most of the laser energy, which means it is hard to use wavefront sensors in free space optical communication in the daytime. Image-based sensors usually requires iteration, which means poor real-time and locally optimal solution. No existing method can measure wavefront aberrations in real time in free space optical communication in the daytime. In this article, a new method of measuring wavefront aberration with CNN is proposed, which can be used in free space optical communication in the daytime and have good real-time performance. We made some modifications in VGG to make it can be used to fitting the Zernike coefficients. The input to the network was the PSF of focal plane and defocus plane and the output was the initial estimate of the Zernike coefficients. 22000 pairs of images were collected in the experiment, which produced by liquid crystal and the wavefront was built by 64 Zernike coefficients when atmospheric coherent length(r0) is 5cm. 20000 pairs of images were used as training sets and the other were used as testing sets. The root-mean-square(RMS) wavefront errors of VGG is on average within 0.0487 waves and the time it needs is 11-12ms. We use RMS wavefront error less than 0.1 waves as the correct standard and the correct rate is 98.75% , while other RMS wavefront errors were properly close to 0.1 waves.
A satellite dedicated for quantum science experiments, has been developed and successfully launched from Jiuquan, China, on August 16, 2016. Two new optical ground stations (OGSs) were built to cooperate with the satellite to complete satellite-ground quantum experiments. OGS corrected its pointing direction by satellite trajectory error to coarse tracking system and uplink beacon sight, therefore fine tracking CCD and uplink beacon optical axis alignment accuracy was to ensure that beacon could cover the quantum satellite in all time when it passed the OGSs. Unfortunately, when we tested specifications of the OGSs, due to the coarse tracking optical system was commercial telescopes, the change of position of the target in the coarse CCD was up to 600μrad along with the change of elevation angle. In this paper, a method of reduce alignment error between beacon beam and fine tracking CCD is proposed. Firstly, OGS fitted the curve of target positions in coarse CCD along with the change of elevation angle. Secondly, OGS fitted the curve of hexapod secondary mirror positions along with the change of elevation angle. Thirdly, when tracking satellite, the fine tracking error unloaded on the real-time zero point position of coarse CCD which computed by the firstly calibration data. Simultaneously the positions of the hexapod secondary mirror were adjusted by the secondly calibration data. Finally the experiment result is proposed. Results show that the alignment error is less than 50μrad.
In this paper, a telescope control method to reject ground-based disturb is proposed to enhance the tracking precision of telescope systems. Telescope systems usually suffer some uncertainouter disturbances, some disturbance come from the torque disturb such as friction orwind loads, some of the others may come from the platform. For astronomical telescope, especially relative large volume telescope, disturb from ground istransferred to the telescope via the pier foundation. And the main mount of disturb is the resonance frequency of pier foundation. The frequency is about 10Hz.A complete vibration test was carried out on a quantum satellite-ground communicationground telescope. Some conclusion is achieved. And a control method based on data fusion to rejecting ground-based disturbance is proposed. The test showed that the method could reduce the track error from 1.5 angular second to 0.28 angular second.
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