Gimbal
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A gimbal is a device using Euler angles to measure the rotation of an object in three dimensions and to control that rotation. The device is usually a set of two or three rings mounted on axes at right angles so as to allow a compass or a ball to remain suspended in a horizontal plane, regardless of the motion of its support.
Gimbals used in spacecraft have three sets of three gyroscopes in the inertial measurement unit (IMU). One for each axis (x, y, and z).
Gimbal lock occurs when all three gyros hit the limits of their ability to move within the sensing mechanism - they hit hard stops and stop moving around.
These gyros provided the "platform" that is referred to. It gives a stable reference to the vehicle's position and attitude in all three dimensions.
In a normal situation, the gyros are aligned with the vehicle's local reference - +x being in the velocity vector, +z being in a radial vector pointing to the earth (for earth orbiting vehicles) and basically pointing downward (using the control panel for reference) in the Apollo CM, and +y being a right-handed complement to the +x vector. Aligning the platform (the IMU's) was done at launch (to give an initial position) and at various times during the mission when the vehicles position is accurately known. During AS-XII, the movement of the vehicle exceeded the normal rates for movement in the respective axis, and was nearly hitting the stops in the IMU.
The "eight-ball" was used for gravity-oriented flight parameters only. It was basically the same as the "eight-ball" flown on airplanes - that is - a gyroscopic platform which aligned to the local gravity field.