Gyroscopes Information

Gyroscopes are designed to measure angular rate or orientation about a given directional vector. Multi-axis gyros provide measurements in two or three orthogonal directions. There are two basic types of gyroscopes: rate and rate-integrating. Rate gyros are single degree-of-freedom (SDOF) devices with a primarily elastic restraint of the spin axis about the output axis. The output signal is produced by precession of the gimbal, which is proportional to the angular rate of the case about the input axis. Most gyroscopes are rate gyros. Rate-integrating gyros are also SDOF devices, but use a primarily viscous restraint of the spin axis about the output axis. The output signal is produced by precession of the gimbal, but is proportional to the integral of the angular rate of the case about the input axis. Physical specifications for both types of gyroscopes include maximum dimension and weight. Output options include analog voltage, current loop, pulse or frequency, and switch or relay outputs. Gyroscopes that are destined for sale in Europe should comply with Restriction of Hazardous Substances (RoHS), a European Union (EU) directive that requires manufactures to demonstrate that their products contain only minimal levels of hazardous substances.

Selecting gyroscopes requires an understanding of angular rate measurement techniques. Optical gyros permit the reflection of a laser ray many times within an enclosure. If the enclosure rotates, the duration between the moment of laser emittance and eventual reception differs. With ring laser gyros (RLF), the laser reflection is achieved with mirrors inside the enclosure. With fiber optic gyros (FOG), the laser reflection is achieved with a coil of optical fiber. Spinning mass gyros use a steadily-moving mass with a free-moving axis (gimbal). These gyroscopes are very fragile and require regular maintenance. When a spinning mass gyro is titled, the gyroscopic effect causes precession – motion orthogonal to the direction tilt sense – along the axis of the rotating mass, indicating that the angle has moved. Because mechanical constraints cause numerous error factors, the axis of a spinning mass gyro is usually fixed with springs. Spring tension is proportional to the precession speed. Vibrating gyros use micro-electro-mechanical system (MEMS) technology and a vibrating, quartz tuning-fork to measure Coriolis force. When rotated, a vibrating element (vibrating resonator) is subjected to the Coriolis Effect, causing secondary vibration orthogonal to the original vibrating direction. By sensing the secondary vibration, the gyro can detect the rate of turn.

Gyroscopes differ in terms of angular or rotary axes and angular rate specifications. There are three choices for angular or rotary axes: uniaxial, biaxial, and triaxial. Uniaxial gyros sense the angular rate around a single axis. Biaxial gyros sense the angular rate around two orthogonal axes. Triaxial gyros sense the angular rate around three orthogonal axes. Angular rate consists of three specifications: angular rate range, linearity, and bandwidth. Angular rate range is the maximum rotary rate for which the gyro is rated. If one product or series can be configured for different rates, then the range of maxima is listed. Angular linearity or rotary axis linearity is measured over an operating temperature range as a percentage (±) of full scale. Angular bandwidth is the frequency range over which the gyroscope meets accuracy specifications before rolling off. Because gyros are almost always capable of DC response, only the high-frequency 3-dB roll off point is included.