Gimbal

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.

Related Topics:
Euler angles - Compass

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Gimbals used in spacecraft have three sets of three gyroscopes in the inertial measurement unit (IMU). One for each axis (x, y, and z).

Related Topics:
Spacecraft - Gyroscope

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Gimbal lock occurs when all three gyros hit the limits of their

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ability to move within the sensing mechanism - they hit hard

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stops and stop moving around.

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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.

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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.

Related Topics:
Vector - Apollo

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The "eight-ball" was used for gravity-oriented flight parameters

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only. It was basically the same as the "eight-ball" flown on

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airplanes - that is - a gyroscopic platform which aligned to the

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local gravity field.

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