Gimbaled planar mirrors are used to point and stabilize a camera's or laser's line-ofsight
(LOS). The mirror with its reflection property adds another degree of complexity to
the already complex area of LOS pointing and stabilization modeling and control. For
example, when the optics and detector are located off gimbal and utilizing a 2-axis gimbaled
mirror to point the LOS, the image at the detector rolls one for one with the outer gimbal
rotation. This is difficult to understand unless the equations are developed to show this.
The LOS pointing kinematic equations for a planar gimbaled mirror begin with the
mirror reflectance equation [2,3]. This equation describes the reflected ray or vector in
terms of the incoming ray and the mirror unit normal, and ultimately creates the
mathematical relationship between the reflected ray, the base (or a user-defined reference
frame), and the detector reference frame. This kinematic relationship is differentiated to
form the LOS rate equations from which one can easily see what states are necessary, and
how these states are combined, for inertially stabilizing the LOS about its roll, pitch, and
yaw axes.
KEYWORDS: Sensors, Gyroscopes, Mirrors, Control systems, Digital signal processing, Electronics, Algorithm development, Software development, Line of sight stabilization, Interfaces
The vast majority of gimbaled line-of-sight stabilization systems involve inertial angular rate sensors located on the gimbal assembly. These sensors are mounted to follow the line-of-sight as the gimbals move, thus allowing them to serve as direct feedback elements for the stabilization rate-control loops. For systems in which on-gimbal configurations are not possible, feedforward control configurations using strapdown inertial sensors are required. The gimbal control commands in these systems are calculated in real- time form the inertial sensor outputs and the transformation matrices between the sensor, gimbal, and line-of-sight reference frames. A testbed for the development and test of a feedforward line-of-sight stabilization systems is described. The system includes a three-axis, mirror-stabilized gimbal, a servoamplifier unit, and inertial measurement system, a digital controller, and a personal computer for software development.
Electrical noise in gimbal control loops can directly impact gimbal drift and Line-of-Sight motion in electro-optic systems. When electrical noise saturates the control loop, the loop operates non-linearly, and changes the loop's ability to reject disturbances. Should the requirement for this loop noise stay below the saturation point for adequate gimbal control? This paper provides an avenue to study Line-of-Sight stabilization performance in the presence of saturating loop noise. The rate feedback device is a two-axis piezoelectric gyro that senses gimbal structural flexure and linear vibrations.
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