Hemispherical Resonator Gyroscope (HRG) Implemented by Utilizing Quadrature Detection based on Optical Interferometry

Abstract

A gyroscope is a sensor that measures the angular motion of a rotating object. It is a component of inertial navigation systems used in drones and spacecraft. Among them, Hemispherical Resonator Gyroscope (HRG) is a gyroscope that is expected to have high accuracy and low drift. In an ideal situation with no damping and no asymmetry, if the system is rotated after resonating the hemispherical resonator, the vibration pattern will also rotate. At this time, the vibration pattern rotates less than that of the system due to the Coriolis effect. By quantitatively measuring this degree, you can see how much the actual system has rotated. Actual hemispherical resonators have damping and asymmetry that must be solved to implement HRG. There are various operating mechanisms to utilize the damping. When a hemispherical resonator is expressed as a lumped-element model, it can be understood in terms of normal mode model. In this model, the vibration pattern can be separated into two axes with angles of 0° (mode 1) and 45° (mode 2). The mechanism used in this thesis is a method of applying a constant sine wave to the hemispherical resonator. This way, in the ideal case, mode2 divided by mode1 amplitude is proportional to the angular velocity. Asymmetry causes deformation of the vibration pattern, so a post-processing to compensate for this is required. In a lumped-element model of hemispherical resonator, stiffness asymmetry and damping asymmetry exist. Stiffness asymmetry causes quadrature noise due to frequency separation of hemispherical resonators and damping asymmetry causes offset. These asymmetries will be compensated by using the proposed compensation algorithm. The algorithm was created by analyzing the vibration characteristics of the hemispherical resonator. For all HRG driving mechanisms including the one used in this thesis, it is necessary to measure the vibration displacement of mode 1 and mode 2. For this purpose, capacitive sensors have been mainly used in the past, but this has less accurate at non-contact detection used by HRG. this thesis, HRG was implemented utilizing quadrature detection based on optical interferometry with higher resolution with higher resolution than the capacitive sensor. A 3×3 optical interferometer is one of the interferometer utilizing quadrature detection. The 3×3 optical interferometer can be used as a precision displacement measurement sensor which has high resolution of sub-nanometers. Since the 3×3 fiber coupler has an inherent phase difference of 120°, the displacement can be continuously measured regardless of the initial phase. The advantages of utilizing a 3×3 optical interferometer to HRG are non-contact characteristics and high resolution characteristics. The vibration pattern of the hemispherical resonator should not be contacted when the hemispherical resonator resonates so that the system rotation information is not distorted. The high-resolution feature makes the displacement measurement which is needed for calculating the angular velocity of the system more accurate. More precise vibration displacement measuring improves not only the accuracy of the output value of HRG, but also the effect of post-processing work.