Gimbal
Encyclopedia : G : GI : GIM : Gimbal
A gimbal is a mechanical device that allows the rotation of an object in three dimensions. It is typically made up of a pair of pivots, mounted on axes at right angles, that allow an object mounted on it to remain in a horizontal plane regardless of the motion of its support. Shipboard compasses, chronometers, and even drink holders are typically mounted on gimbals to keep them level to the horizon at all times.
In aerospace navigation, a gimbal is a device using Euler angles to measure the rotation of an object in three dimensions and to control that rotation. Gimbals used in spacecraft have three sets of three gyroscopes in the inertial measurement unit (IMU), one for each axis (x, y, and z). These gyros provide a stable reference to the vehicle's position and attitude in all three dimensions.
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.
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 vehicle's 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.
Gimbal mechanisms are also used in some mirror mounts, for precision rotation of optical elements without translation.
In spacecraft propulsion, "gimbaled" refers to a pivoting or swiveling mount, often for a rocket engine. For example the space shuttle main engines are gimbaled -- they can point in different directions to control the vehicle flight path.
The world's largest gimbal was constructed to simulate the capsizing of the USS Oklahoma in the movie Pearl Harbor.
See also
From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.