Today's Lesson: Gimbals

What is it?

The Universal Joint is also known as a universal coupling, a U-joint, a Cardan joint, or a Hooke's joint. It is a joint in a rigid rod that will allow the rod to bend in any direction. This mechanism is often used in shafts that create rotary motion. The joint is comprised of a pair of hinges located close together, oriented at 90° to each other, connected by a cross shaft.


Animation from http://en.wikipedia.org/

The Universal Joint is based on the gimbal. Gimbals, mechanisms comprised of three concentric rings that are able to rotate in three perpendicular planes, were originally invented in China. The Chinese gimbals used a series of interlocking rings within a device that allowed a candle placed in the center to remain upright regardless of the device's position.

A gimbal is a pivoted support that allows the rotation of an object about a single axis. A set of three gimbals, one mounted on the other with orthogonal pivot axes allows an object mounted on the innermost gimbal to remain independent of the rotation of its support. On a ship, the gyroscopes, shipboard compasses, stoves, and even drink holders, typically use gimbals to keep them upright with respect to the horizon despite the ship's movement.

While a gimbal can be any support that can pivot around an axis, most gimbal systems look like a series of concentric rings. The outermost ring mounts to a larger surface, like a boat's instrument panel. The next largest ring connects to the outermost ring at two points that are perpendicular to the outer ring's surface mount. Then, the third largest ring mounts to the second largest one at two points perpendicular to the connection between the first and second ring, and so on.
gimbal-2.gif
Image from science.howstuffworks.com/gimbal1.htm

By mounting an object to the center of the system, the object can face any particular direction at any time. There is one problem with a gimbal system. This problem is called gimbal lock. Gimbal lock occurs when two axes in a three-gimbal system align. When that happens, the object's movement becomes limited, and an entire range of motion becomes impossible.

There are two ways to avoid gimbal lock. One is to adjust the gimbals, either by adjusting the surface so that the gimbals swing another way or by physically resetting the gimbals themselves. If gimbal lock does occur, you have to reset the gimbals for them to work again. Another solution is to add more gimbals to the system. Adding a fourth gimbal helps eliminate gimbal lock, but the trade off is that the addition makes the system bulkier and more complicated.

Gimbals allow you to create devices that are more flexible than a fixed, and stationary devices. It's also possible to orient the mechanism so that it faces a specific direction independent of its environment. Such an application has many uses, ranging from a cup holder that adjusts so that you don't have to worry about spilling your coffee to an array of satellite antennae that can turn to face incoming signals.

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