For the former settlement in California, see Gimbal, California.

Illustration of a simple two-axis gimbal set.

A gimbal is a pivoted support that allows the rotation of an object about a single axis. A set of two gimbals, one mounted on the other with pivot axes orthogonal, may be used to allow an object mounted on the innermost gimbal to remain immobile (e.g. vertical in the animation) regardless of the motion of its support. For example, gyroscopes, shipboard compasses, stoves and even drink holders typically use gimbals to keep them upright with respect to the horizon despite the ship's pitching and rolling.

The 'Cardan' suspension of a gimbal is named after the Italian inventor Gerolamo Cardano (1501–1576),^[1] who described the device in detail, but did not claim its invention.

[edit] History

Cardan suspension in Villard de Honnecourt's sketchbook (ca. 1230)

Early modern dry compass suspended by gimbals (1570)

The gimbal was first invented by the Greek inventor Philo of Byzantium (280–220 BC).^[2][3][4][5] Philo described an eight-sided ink pot with an opening on each side, which can be turned so that while any face is on top, a pen can be dipped and inked - yet the ink never runs out through the holes of the other sides. This was done by the suspension of the inkwell at the center, which was mounted on a series of concentric metal rings which remained stationary no matter which way the pot is turned.^[2]

Philo's original text of the Pneumatica describing this use of the gimbal exists in a later Arabic translation made roughly the time of Caliph Al-Ma'mun (r. 813–833).^[2] Carra de Vaux, author of the French translation which still provides the basis for modern scholars,^[6] regards the Pneumatics as essentially genuine.^[7] George Sarton (1959) asserts that it is safe to assume the Arabic version is a faithful copying of Philo's original, and credits Philon explicitly with the invention.^[8] So does Michael Lewis (2001).^[9] The sinologist Joseph Needham (1965) suspects Arab interpolation.^[10] Lucio Russo and Silvio Levy (2004) write that the transmitted text of Philo was likely corrupted in the Arab version which may have mixed in materials from other sources, since it is "highly uneven in terms of technical sophistication and subject matter"^[11], but do not specify whether this attributes to the relevant passage containing the gimbal.

Recent research by Michael Lewis (1997), however, has shown that the letter sequences of the Arab copy contain Greek letters which fell out of use after the time of Christ, thereby asserting that the Arabic version is faithful to Philo's original,^[12] a view also shared by Andrew Wilson (2002).^[13]

The ancient author Athenaeus Mechanicus, who flourished during the reign of Augustus (30 BC–14 AD), described the military use of a gimbal mechanism, calling it "little ape" (pithêkion): When preparing to attack coastal towns from the sea-side, military engineers used to yoke merchant-ships together to take the siege machines up to the walls. But to prevent the shipborne machinery from rolling around the deck in heavy seas, Athenaeus advises that "you must fix the pithêkion on the platform attached to the merchant-ships in the middle, so that the machine stays upright in any angle".^[14]

After antiquity, gimbals remained widely known in the Near East. In the Latin West, reference to the device appeared again in the 9th century recipe book called the Little Key of Painting (Latin: Mappae Clavicula).^[15] The French inventor Villard de Honnecourt depicts a set of gimbals in his famous sketchbook (see right). In the early modern period, dry compasses were suspended in gimbals.

In China, the Han Dynasty (202 BC – 220 AD) inventor Ding Huan (丁緩) created a gimbal incense burner around 180 AD.^[16][2] There is a hint in the writing of the earlier Sima Xiangru (179–117 BC) that the gimbal existed in China since the 2nd century BC.^[17] There is mention during the Liang Dynasty (502–557) that gimbals were used for hinges of doors and windows, while an artisan once presented a portable warming stove to Empress Wu Zetian (r. 690–705) which employed gimbals.^[18] Extant specimens of Chinese gimbals used for incense burners date to the early Tang Dynasty (618–907), and were part of the silver-smithing tradition in China.^[19]

[edit] Inertial navigation

In inertial navigation, as applied to ships and submarines, a minimum of three gimbals is needed to allow an Inertial Navigation System platform (stable table) to remain fixed in inertial space, compensating for the ship's Yaw (direction) as well as its Pitch and Roll. In this application, the Inertial Measurement Unit (IMU) is equipped with three orthogonally mounted gyros to sense rotation about all axes in three dimensional space. The gyro outputs drive motors controlling the orientation of the three gimbals as required to maintain the orientation of the IMU. In turn, angular measurement devices called "resolvers" mounted on the three gimbals provide the nine cosine values for the direction cosine matrix needed to orient the ship.

Similar sensing platforms can be used on aircraft.

In inertial navigation systems, gimbal lock may occur when vehicle rotation causes two of the three gimbal rings to align with their pivot axes in a single plane. When this occurs, it is no longer possible to maintain the sensing platform's orientation.

[edit] Rocket engines

In spacecraft propulsion, rocket engines are generally mounted on a pair of gimbals to allow a single engine to vector thrust about both the pitch and yaw axes; or sometimes just one axis is provided per engine. To control roll, twin engines with differential pitch or yaw control signals are used to provide torque about the vehicle's roll axis.

[edit] Fishing

In big-game fishing, a two axis gimbal set may be used as a fixed pivot for the butt of the rod, with the gimbals mounted in a "fighting belt" or a "fighting chair". In either case, this is a considerable advantage to the angler. Inside the gimbal there is usually a horizontal pin that the fishing rod locks into, preventing rotation about the long axis of the rod, making it easier to reel. This is demonstrated in the 1975 film Jaws, where the character Quint uses a "fighting chair" and a "fighting belt/harness" to attempt to catch the shark they are pursuing.

[edit] See also

• Gimbal lock
• Canfield joint
• Heligimbal
• Cardan shaft
• Gyroscope
• Keyhole problem

[edit] References