Inertial navigation systems use a combination of accelerometers and angular rate sensors (gyroscopes) to detect altitude, location, and motion. They may also be capable of detecting attitude, position, velocity, temperature, or magnetic field. Angular rate specifications for inertial navigation systems include angular rate range, bandwidth, transverse sensitivity, and linearity. 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 bandwidth is the frequency range over which a device meets accuracy specifications before rolling off. Because gyros are almost always capable of DC response, only the high-frequency 3-dB rolloff point is included. Angular transverse sensitivity is the maximum output signal due to rotation about an axis orthogonal to the sensitive axis under consideration. It is expressed as a percentage of the orthogonal input angular velocity. Angular linearity or rotary axis linearity is measured over an operating temperature range as a percentage (±) of full scale. Additional specifications for inertial navigation systems include weight, maximum dimension, and operating temperature.
Inertial navigation systems differ in terms of angular rate measurement and linear acceleration measurement technologies. Optical, spinning-mass, or vibrating gyros are used to sense the angular or rotary rate. Optical gyros permit the reflection of a laser ray many times within an enclosure. Spinning mass gyros use a steadily-moving mass with a free-moving axis (gimbal). Vibrating gyros use micro-electro-mechanical system (MEMS) technology and a vibrating, quartz tuning-fork to measure Coriolis force. There are many ways to measure linear acceleration, but most inertial navigation systems measure the displacement of a proof mass. For example, capacitance-based devices measure the variable capacitance between a support structure and proof mass. Null-balance devices keep the mass nearly centered with positional feedback and a servo-mechanism. Other linear acceleration measurement technologies are also available. Inductive position sensors are noncontact devices that determine an object's coordinates (linear or angular) with respect to a reference. Piezoelectric devices compress a piezoelectric material and generate a charge that is measured by a charge amplifier. Piezoresistive devices change resistance when the material is under pressure, stressed, or deflected. Resonant devices provide frequency-shift outputs.
Parameters for inertial navigation systems include electrical outputs, additional outputs, features, and compliance. Choices for electrical output are analog voltage, current loop, pulse or frequency, switch or relay outputs, serial or digital output, and network / fieldbus. Additional outputs provide measurements of magnetic fields, temperature, and linear velocity. Some inertial navigation systems include data recorders or global positioning system (GPS) features. Others are intrinsically safe (IS). Inertial navigation systems that are RoHS compliance meet the requirements of the European Union’s (EU) Restriction of Hazardous Substances directive.
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Gyroscopes are designed to measure angular rate or orientation about a given directional vector.