KEYWORDS: Control systems, Scanners, Feedback control, Linear filtering, Control systems design, Mirrors, Laser scanners, 3D scanning, Sensors, Laser systems engineering
We have developed a high-precision scanner control system, which is used on earth observation satellites. This control
system keeps high angular precision with ultra smooth rotation. We designed a feedback controller consists of a rate loop
for rotation-speed control and a position loop for angular control. Adding a feedforward controller using cyclic memory,
1st order rotational synchronizing disturbance caused by eccentric load has been compressed over 50 dB. An online
learning controller sustains long-time robustness by suppressing undesired addition to a cyclic memory. The proposed
control system maintains high pointing accuracy with a standard deviation of 0.003 degree at 80 rpm.
KEYWORDS: Control systems, Scanners, Control systems design, Feedback control, Mirrors, Linear filtering, Sensors, Satellites, Earth observing sensors, Analog electronics
A high-precision whiskbroom scanner control system, which is used on earth observation satellites, is discussed. This
control system is required to keep high angular precision with ultra-smooth rotation. We designed a feedback control
system consisting of a rate loop for rotation-speed control and a position loop for angular control. Adding a feedforward
control system using cyclic memory, 1st order rotational synchronizing disturbance caused by eccentric load has been
compressed over 50 dB. As a result, high pointing accuracy with a standard deviation of 0.003 degree or less has been
achieved in rotating with a constant speed of 79.4 rpm.
A breadboard model of a Wide-range Fine Pointing Mechanism (WFPM) designed for free-space laser communications and composed of electromagnetic actuators and a flexible support system has been developed. The WFPM is compact (45 x 45 x 39 mm3) and light in weight (160 grams). Its actuators, which consist of four moving-coil-type motors with long strokes, ensure a wide optical scan range, +/-4 degrees. The support system, which consists of a center torsion bar and four preformed thin springs, allows the mechanism's 20 mm-diameter mirror to be rotated freely about a vertically constant point; the mirror can be rotated about its x- and y-axes simultaneously. The breadboard model has passed a launch vibration test under conditions of 20 G rms overall. With its coarse and fine quadrant detectors, the WFPM is able to provide stable acquisition and tracking without any need for built-in sensors. The digital control system of the WFPM has a 1 kHz control bandwidth and -90-dB disturbance suppression at 1 Hz. Use of the WFPM by an acquisition and tracking terminal significantly shortens acquisition time and ensures accurate tracking. Acquisition and tracking simulations have shown fast acquisition time (58.7 ms for 3.5-milli-radian initial error) and high tracking precision (+/-1 micro-radian, 3 sigma). To maintain high tracking precision, we employ a tradeoff between noise reduction and disturbance suppression. The WFPM has the potential to play an essential role in helping to provide high-data-rate free-space laser communications.
KEYWORDS: Sensors, Optical communications, Mirrors, Satellites, Actuators, Electromagnetism, Control systems, Metals, Control systems design, Switching
A wide and fine pointing mechanism (WFPM) composed of electromagnetic actuators and flexible supports has been developed for optical inter-satellite communications. It covers the whole of the scan area when it searches for the target satellite. The electromagnetic actuators, which consist of four moving-coil-type motors with long strokes, ensure a wide scan range. The flexible supports, which consist of a metal pivot and four thin springs, have vertical stiffness and rotational flexibility. Balancing the elasticity between the pivot and the thin springs fixes the center of rotation on the mirror surface to keep the beam path length constant. A digital control system is designed to drive the WFPM with gap sensor feedback. It has a two-degrees-of-freedom robust controller that incorporates a disturbance cancellation algorithm that cuts off low-frequency vibrations from the satellite body to achieve highly precise tracking. Experiments show that the WFPM has both a wide scanning range of over 8 degrees and a highly precise pointing capability that is accurate to within 30 microradians, for the case when only gap sensor feedback is used without quadrant detector feedback. These characteristics are suitable for high-speed switching over target satellites.
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