In addition to servo control and power amplification, motion control systems for optical tracking pedestals feature capabilities such as electro-optical tracking using an integrated Automatic Video Tracker (AVT) card. An electro-optical system tracking loop is comprised of sensors mounted on a pointing pedestal, an AVT that detects a target in the sensor imagery, and a tracking filter algorithm that commands the pedestal to follow the target. The tracking filter algorithm receives the target boresight error from the AVT and calculates motion demands for the pedestal servo controller. This paper presents a tracking algorithm based on target state estimation using a Kalman filter. The servo demands are based on calculating the Kalman filter state estimate from absolute line-of-sight angles to the target. Simulations are used to compare its performance to tracking loops without tracking filters, and to other tracking filter algorithms, such as rate feedback loops closed around boresight error. Issues such as data latency and sensor alignment error are discussed.
Large gimbal systems often demand high accuracy pointing and smooth travel. Feedback control is employed to guarantee this performance in the face of wind, bearing and other disturbances. Both small and large gimbal performance is affected by many common factors. However, in smaller gimbal system, the foundation has little impact on the performance of the gimbal feedback control and can be neglected. For larger gimbal systems, this is not the case. Fortunately, the control system designer can use simplified analytical models to characterize foundation designs assess the impact on the system. This information can be coupled with modern control design techniques to improve the performance under less than ideal foundations. Analytical work is supplemented with test results from two large terrestrial gimbal telescopes with significantly different foundation designs.
The Advanced Electro Optical System 3.67 m Telescope recently installed at the Air Force Maui Optical Station is the government's newest and largest satellite tracking telescope. The primary mirror is a 23:1 meniscus type with an 84 point actively controlled back support system. Each back support contains a microprocessor and electromechanical actuator to control the figure of the mirror based on the primary mirror reflected wavefront. The primary and secondary mirrors are servo positioned to actively control the telescope alignment. The support hardware, the control implementation, and the performance achieved during factory testing and initial integration are discussed.
The accuracies required for modern tracking systems far exceed the capability of most manufacturing processes. In addition, the stability and flexure of real engineering material further limit the as-built accuracy. This paper discusses the various types of static errors which exist in tracking mounts, and the extent to which they can be modeled.
A precision tracking system can function in some applications with very limited absolute accuracy, but all applications require smooth motion. This paper addresses the sources of dynamic motion error which are built into the device itself. The methods of measuring these errors are discussed, and techniques for identifying the various sources. These are illustrated with some examples from precision tracking mounts. An important error source is motor torque ripple, a position-dependant variation in torque gain constant. New techniques in brushless servo motor excitation have resulted in dramatic reductions in torque ripple. An overview of this type of drive system is also presented.
A 1-kHz control system for precision position control of a two-axis gimballed mirror is presented. The control system is unique in that it uses the state estimator's position estimates to excite the Inductosyn position transducers. Also presented is a concise algorithm for proximate time optimum control within available motor torque (or alternatively, maximum gimbal acceleration) and maximum allowable gimbal rate. The newly developed Inductosyn excitation/reduction scheme and proximate time optimum algorithm are successfully demonstrated on hardware designed, constructed, and tested by Contraves USA, where accuracies better than 4 arc seconds were measured.