In a charge-coupled device (CCD)-based fast steering mirror (FSM) tracking control system, high control bandwidth is the most effective way to enhance the closed-loop performance. However, the control system usually suffers a great deal from mechanical resonances and time delays induced by the low sampling rate of CCDs. To meet the requirements of high precision and load restriction, fiber-optic gyroscopes (FOGs) are usually used in traditional FSM tracking control systems. In recent years, the MEMS accelerometer and gyroscope are becoming smaller and lighter and their performance have improved gradually, so that they can be used in a fast steering mirror (FSM) to realize the stabilization of the line-of-sight (LOS) of the control system. Therefore, a tentative approach to implement a CCD-based FSM tracking control system, which uses MEMS accelerometers and gyroscopes as feedback components and contains an acceleration loop, a velocity loop and a position loop, is proposed. The disturbance suppression of the proposed method is the product of the error attenuation of the acceleration loop, the velocity loop and the position loop. Extensive experimental results show that the MEMS accelerometers and gyroscopes can act the similar role as the FOG with lower cost for stabilizing the LOS of the FSM tracking control system.

Application of MEMS Accelerometers and Gyroscopes in Fast Steering Mirror Control Systems.

In the charge-coupled device (CCD)-based tracking control system of fast steering mirrors (FSMs), high control bandwidth is the most effective method to enhance closed-loop performance, which, however, usually suffers a great deal from time delay induced by a low CCD sampling rate. Moreover, mechanical resonances also limit high control bandwidth. Therefore, a tentative approach to implementing a CCD-based tracking control system for an FSM with inertial sensor-based cascade feedback is proposed, which is made up of acceleration feedback, velocity feedback, and position feedback. Accelerometers and gyroscopes are all the inertial sensors, sensing vibrations induced by platforms, in turn, which can contribute to disturbance supersession. In theory, the acceleration open-loop frequency response of the FSM includes a quadratic differential, and it is very difficult to compensate a quadratic differential with a double-integral algorithm. A lag controller is used to solve this problem and accomplish acceleration closed-loop control. The disturbance suppression of the proposed method is the product of the error attenuation of the acceleration loop, the velocity loop, and the position loop. Extensive experimental results show that the improved control mode can effectively enhance the error attenuation performance of the line of sight (LOS) for the CCD-based tracking control system.

https://www.spiedigitallibrary.org/journals/Optical-Engineering/volume-55/issue-11/111602/Inertial-sensor-based-multiloop-control-of-fast-steering-mirror-for/10.1117/1.OE.55.11.111602.pdf

Inertial sensor-based multiloop control of fast steering mirror for line of sight stabilization

In this paper, an approach to improve the disturbance suppression performance of a fast steering mirror (FSM) tracking control system based on a charge-coupled device (CCD) and micro-electro-mechanical system (MEMS) inertial sensors is proposed. The disturbance observation and compensation (DOC) control method is recommended to enhance the classical multi-loop feedback control (MFC) for line-of-sight (LOS) stabilization in the FSM system. MEMS accelerometers and gyroscopes have been used in the FSM system tentatively to implement MFC instead of fiber-optic gyroscopes (FOG) because of its smaller, lighter, cheaper features and gradually improved performance. However, the stabilization performance of FSM is still suffering a large number of mechanical resonances and time delay induced by a low CCD sampling rate, which causes insufficient error attenuation when suffering uncertain disturbances. Thus, in order to make further improvements on the stabilization performance, a cascaded MFC enhanced by DOC method is proposed. The sensitivity of this method shows the significant improvement of the conventional MFC system. Simultaneously, the analysis of stabilization accuracy is also presented. A series of comparative experimental results demonstrate the disturbance suppression performance of the FSM control system based on the MEMS inertial sensors can be effectively improved by the proposed approach.

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MEMS Inertial Sensors-Based Multi-Loop Control Enhanced by Disturbance Observation and Compensation for Fast Steering Mirror System

To meet the development demands of the stare imaging system, a single piezoelectric ceramic actuator (PCSA) based fast steering mirror (FSM) with fixed rotation axis and large excursion angle is proposed and designed. The statics model and dynamical model of the FSM are established and analyzed. On this basis, a parameter optimization method is proposed and applied to determine the configuration parameters of the designed FSM. The working principle, excursion angle, and natural frequency of the FSM are validated through theoretical estimating, finite element analyzing by ANSYS, and experimental testing, respectively. The experimental results show that the developed FSM can provide a mechanical excursion angle larger than 3° with natural frequency of 180.4 Hz, and the mechanical excursion angle and the natural frequency can be estimated by the established mathematical models accurately.

Single piezoelectric ceramic stack actuator based fast steering mirror with fixed rotation axis and large excursion angle

The electro-optical tracking system with speedability and maneuverability based on stationary platform is confronted with some problems such as external physical disturbance and detector delay. To obtain better stability, which is the key to achieving high tracking precision, a modified Smith predictor method for inertia stabilization loop control is introduced. This method proposes the Smith predictor control scheme based to compensate delay time caused by the signal filtering of fiber gyroscope sensor in the velocity feedback control, and it utilizes disturbance feedforward control to improve the stability of stabilization platform simultaneously. The design of control structure and analysis of the theory are reported. The experiment results indicate convincingly that the proposed method can effectively improve the bandwidth of velocity closed-loop. Moreover, it can drastically enhance the disturbance suppression performance of tracking platform.

Stabilization Control of Electro-Optical Tracking System With Fiber-Optic Gyroscope Based on Modified Smith Predictor Control Scheme

In the control system of a CCD-based tracking loop for a fast steering mirror (FSM), the most effective method often employed to improve pointing performance is to increase high gain of the control system for a high bandwidth, which, however, usually suffers a great deal from a low CCD sampling rate and the mechanics of the FSM. Moreover, the amount of time delay engendered by sampling and data processing can significantly reduce the performance of a closed-loop system. Therefore, a tentative approach to the implementation of a CCD-based tracking control system with acceleration feedback is proposed. In theory, the position open loop is made of double integrators with a high bandwidth of the acceleration feedback loop; in fact, however, the acceleration open loop of the FSM response includes a quadratic differential, and it is very difficult to compensate a quadratic differential with an integral algorithm. To solve this problem, a novel acceleration closed system such as a bandpass filter is proposed. The position is reconstructed into a simple first-order filter instead of a third-order control system. In addition experimental results show that the acceleration feedback proposed here can effectively enhance the bandwidth of the closed-loop system and its trajectory tracking and pointing performance. (C) 2009 Society of Photo-Optical Instrumentation Engineers. [DOI: 10.1117/1.3065500]

Shunfa Liu

Papers

Acceleration feedback of a CCD-based tracking loop for fast steering mirror