However, among the optical gyroscopes, applications requiring extremely high scale factor stability continue to be achieved only with RLG. The MEMS and optical gyroscopes, in particular Interferometric Fiber-Optic gyroscopes (IFOG), are replacing many of the current systems using Ring Laser Gyros (RLGs) and mechanical gyroscopes. The cost of MEMS gyroscopes is expected to reduce drastically in the next years, leading to an increment in the use of these devices. The increased demand for mobile devices is also responsible for the growth of the MEMS gyroscopes market. Although initially used for expensive military applications, now they are also adopted for low cost commercial applications (low performance at the top-right end of the spectrum in Figure 1) of consumer electronics for Automotive, Defense, Industrial and Medical applications. They measure the rate of rotation of an object around a particular axis: 1-axis, 2-axis, and 3-axis. Micro-Electro-Mechanical System (MEMS) gyroscopes are motion sensors that detect and measure the angular motion of an object. For applications requiring very high performance, the ring laser gyroscope is currently more diffused and has the bigger market share. The main diffused types of optical gyroscopes are IFOG and RLG, which both exploit the physics of the Sagnac effect. Since the 19th century, mechanical gyroscopes (see the left-bottom spectrum in Figure 1), classified as displacement gyroscopes and rate gyroscopes, are the historical ones consisting in a toroid-shaped rotor that rotates around its axis while, since the 20th century, optical gyroscopes (see the spectrum in the middle of Figure 1) operate by sensing the difference in propagation time between counter-propagating laser beams traveling in opposite directions in closed or open optical path. Scale factor stability (i.e., the accuracy of the gyroscope in monitoring the sensed angular velocity), expressed in parts per million (ppm), as a function of the bias stability (intrinsically dependent on the gyroscope technology) for Mechanical Gyroscopes, Ring Laser Gyroscopes (RLG), Interferometric Fiber-Optic gyroscopes (IFOG), Quartz, Dynamically Tuned Gyroscopes (DTG), Rate and Integrating Gyroscopes and MEMS. For more details about performance and applications, see Figure 1. Thus, gyroscope performance and costs are directly related to the application requirements. A minimum scale-factor stability leads to small sensor errors and requires better instruments and improved accuracy, bringing higher cost of the system.
Scale factor represents the sensitivity of the optical gyroscope, while the accuracy of the gyroscopes, which is inversely proportional to the sensitivity and takes into account the measurement errors due to the noise, can be expressed through the resolution, R, or, in the RLGs, by the Angle Random Walk ( ARW), linking R with the bandwidth, B, of the measurement system through ARW = R/. For this reason, one of the more important merit figures is the stability of the scale-factor.
For all classes of gyroscopes, being angular velocity sensors, the major issues are related to the errors in measuring the angular velocity. In particular, mechanical gyroscopes ( Section 2) optical gyroscopes ( Section 3), including Fiber Optic Gyroscopes (FOGs) and Ring Laser Gyroscopes (RLG) and Micro-electromechanical system (MEMS) gyroscopes ( Section 4) have been considered by focusing attention on the operating principles and different improvements in commercial architectures in terms of performance. In this paper, a review of the more commercially diffused classes of gyroscopes is presented. Gyroscopes can be used alone or included in more complex systems, such as Gyrocompass, Inertial Measurement Unit, Inertial Navigation System and Attitude Heading Reference System. Many classes of gyroscopes exist, depending on the operating physical principle and the involved technology. Gyroscopes are devices mounted on a frame and able to sense an angular velocity if the frame is rotating. The term “gyroscope”, conventionally referred to the mechanical class of gyroscopes, derives from the Ancient Greek language, being the Physics of the “precession motion”, a phenomenon also observed in ancient Greek society.
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