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Hardware
Published in Hanky Sjafrie, Introduction to Self-Driving Vehicle Technology, 2019
IMUs make use of inertial motion sensors, particularly gyroscopes. They identify the orientation of the vehicle based on a fixed reference frame. There are several types of gyroscopes. Mechanical gyroscopes use a spinning wheel or a fast-spinning rotor mounted on two gimbals and a support frame. Due to the angular momentum of the wheel, the original orientation of the wheel is constantly preserved regardless of changes in orientation of the support frame. Thus, the change of orientation can be deduced by measuring the angular displacements between the two gimbals with respect to an inertial reference frame. Optical gyroscopes are typically laser-based, and utilize a physical phenomenon called the Sagnac effect [14]. When two optical beams propagate in opposite directions in a rotating ring path, their propagation time, i.e., the time for the beam to return to its starting point, will differ fractionally; the beam that travels in the same direction as the rotation will take longer to return to the starting point than its counterpart, as shown in Figure 2.13. The angular velocity applied to the ring can be deduced by measuring the phase difference of the beams.
Inertial navigation systems
Published in Mike Tooley, David Wyatt, Aircraft Communications and Navigation Systems, 2017
Ring laser gyros are very expensive to manufacture; they require very high quality glass, cavities machined to close tolerances and precision mirrors. There are also life issues associated with the technology. A variation of this laser gyro technology is the fibre optic gyroscope (FOG), where the transmission paths are through coiled fibre optic cables packaged into a canister arrangement to sense pitch, roll and yaw, see Figure 17.9. The fibre optic gyro also uses the interference of light through several kilometers of coiled fibre optic cable to detect angular rotation. Two light beams travel along the fibre in opposite directions and produce a phase shift due to the Sagnac effect. Fibre optic gyros have a life expectancy in excess of 3.5 million hours.
Fiber Optic Sensors for Oil and Gas Applications
Published in Hisham K. Hisham, Fiber Bragg Grating Sensors, 2019
In practice, many known techniques are used to detect changes in circulation, but those that rely on the principle of Sagnac effect, which lack inertia and which have a major advantage, detect the rotation itself. Two types of gyroscope systems can be distinguished: ring lasers [11] and optical fiber seismic scales [12, 13], both of which depend on the technical implementation of the Sagnac interference [14].
Light propagation and local speed in the linear Sagnac effect
Published in Journal of Modern Optics, 2019
Gianfranco Spavieri, George T. Gillies, Espen Gaarder Haug, Arturo Sanchez
In the standard (circular) Sagnac effect (Figure 2(a)), two counter-propagating light signals (photons) are emitted by the source (interferometer, or clock C) co-moving with a rotating disk along the circumference . If is the velocity of the clock (or interferometer) fixed on the rotating disk circumference relative to the laboratory frame, to the first order in v/c the time interval recorded by clock C for round-trip light counter-propagation is given by , and for light co-propagation. Then, the time delay between the time of arrival of the two light signals back to clock C observed by Sagnac, is , valid to the first order in v/c (non-relativistic approximation where the Newtonian and Einsteinian interpretations coincide). There are countless descriptions and interpretations of the Sagnac effect in the literature. The most common interpretation (32) is done from the laboratory frame, where the centre of the disk is stationary and the light speed c is assumed to be isotropic and constant. In this case, the result that is different from is due to the fact that the two light signals are seen as traversing paths of different lengths, as measured from the lab frame, in their motion relative to clock C.
A 2.71 fJ/conversion-step 10-bit 50 MSPS split-capacitor array SAR ADC for FOG systems
Published in International Journal of Electronics, 2022
Chua-Chin Wang, Ralph Gerard B. Sangalang, Meng-Jie Wu, Tzung-Je Lee, Yi-Jen Chiu, Lean Karlo S. Tolentino, Oliver Lexter July A. Jose
A fibre optic gyroscope system makes use of optical fibre to measure angular movement based upon the process called the Sagnac Effect. When the gyro makes a rotation, the optical signal passing through the fibre optic cable experiences phase shift sensed by a photo detector. This then goes through digital conversion for further processing to get the rotational angle as depicted in Figure 1. The required sampling rate for a system with 0.5/hr specification is 50 MS/s at 10 bits resolution for military applications.
The Sagnac effect and the role of simultaneity in relativity theory
Published in Journal of Modern Optics, 2021
Gianfranco Spavieri, George T. Gillies, Espen Gaarder Haug
The experimental result was interpreted by Sagnac as showing that the two light rays, moving on the disk circumference, have different local speeds (c + v and c−v) relative to the clock. Then, the time of flights, (of first order in v/c), for the two light rays covering the same round trip path are different and consistent with the time delay detected at their arrival back to the clock. Among the subsequent many papers on the Sagnac effect, there are several supporting Sagnac's point of view. Most influential is the position of Selleri [3] who elaborated a paradox showing that the result of Sagnac's experiment is not compatible with Einstein's second postulate on the invariance of the speed of light c. Since the Sagnac effect is of first order in v/c, if the experiment is described in the laboratory or any other relatively moving inertial frame where the speed of light is assumed to be constant and equal to c, the result (2) of the Sagnac experiment can be predicted equally well, to first order, by both Newtonian physics and Einstein's special relativity. Because of this, disregarding the arguments of Einstein's detractors, most physicists adhere to the current time paradigm arguing that, by predicting the correct result, relativity theory coherently interprets the Sagnac effect. However, the controversial point is not to foresee the result of the Sagnac effect, a result that nobody questions because it is easily foreseen by both Newtonian and relativistic physics. Indeed, according to Sagnac and several others [1,3,6,8,9,14,16–18], the real problem is to interpret the result of the experiment when seen by an observer co-moving with the measuring apparatus. In this case, as also pointed out particularly by Landau and Lifshiz [20], there are problems with Einstein synchronization applied to the closed contour and, therefore, the claim is that a constant local light speed c is not consistent with Sagnac's result.