Gimbal lock
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Gimbal lock, in gyroscopic devices controlled by Euler mechanics or Euler angles, is caused by the alignment of two of the three gimbals together so that one of the rotation references (pitch/yaw/roll, often yaw) is cancelled. This would require a reset of the gimbals using an outside reference.
Or as described in gimbal: 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.
For example, an airplane uses three references, pitch (angle up/down), yaw (angle left/right on a vertical axis) and roll (angle left/right on the horizontal axis). If an airplane heads straight up or down (change of pitch), one other reference (the yaw) is cancelled, one loses a dimension of rotation, because there is always a value for one angle of rotation that yields infinite values of the other two angles (in this case, the yaw).
Compare a similar problem with tangents on triangles -- say one has a right triangle ABC, with angle ACB=90°. Consider angle BAC. tan(BAC) = BC/AC, but BAC is less than 90°. We can make angle BAC closer and closer to 90 degrees by increasing the length of BC, but as we keep doing this, AC stays the same so the ratio BC/AC gets infinitely large. So tan(90°) has no geometric meaning. One leg of the triangle becomes infinitely long and never meets the other leg.
Another real world comparison is latitude and longitude. At the poles (latitude 90° north or south), the definition of longitude becomes meaningless (as all longitude lines meet at a point or singularity).