Hemispherical resonator gyroscope

Abstract: As microelectromechanical system (MEMS) gyros were being developed for automotive safety and military tactical applications, in 1994 Boeing selected a conventionally-machined hemispherical resonator gyroscope (HRG) for high performance, continuous space pointing applications. In that same year research was begun into high performance MEMS gyros for compact, low-cost space pointing applications. Collaboration with several national MEMS research labs and operational experience with the HRG led to an understanding of the benefits of high Q, symmetrical resonator designs in MEMS. Early post resonator designs led to closed loop, tuned, low-noise electronics design and operation with capacitive sensing but required undesirable 3D assembly of the post onto the micro-machined flexures. High dynamic loading and imprecision of the bonded joints led to gyro bias that was not stable over the long run. This led to the conception of the Disc Resonator Gyroscope (DRG) which yielded a compact planar micro-machined design with central support and carrying no critical loads. Successive optimization of the layout, scale, material selection and fabrication design as well as the operational electronics has led to progressively more stable performance. A recent fixed orientation laboratory run demonstrated a stable rate within 0.01 o /h over a week of continual measurement, believed to be a record for a MEMS gyroscope. This research background behind the DRG and its principle of operation will be presented along with the latest test results which promise high performance, as well as compact, low-cost MEMS gyroscopes for space applications.

Abstract: In this paper we report the fabrication of hemispherical polycrystalline diamond resonators fabricated on a novel high-temperature glass substrate. The hemispherical resonator gyroscope is one of the most accurate and rugged of the mechanical gyroscopes, and can be operated in either rate or whole-angle mode due to its high degree of symmetry. A fabrication sequence for creating extremely symmetric 3D MEMS hemispheres is presented. Mode shapes and frequencies obtained with a laser vibrometer are shown, as well as curves of Q versus pressure, and the dependence of frequency on anchor size. Fundamental mode frequency matching to <0.1% in as-fabricated devices has been achieved, which is essential to gyroscope operation in whole-angle mode.

Abstract: We report a scale factor self-calibration in MEMS Coriolis vibratory gyroscopes enabled by real time control of the sense-mode closed loop gain. Similarly to the closed loop scale factor employed in the Hemispherical Resonator Gyroscope (HRG), we measure and remove scale factor changes by injecting a known dither signal (virtual input rate) into the sense-mode dynamics during the normal operation of the gyroscope. The approach was validated using an inhouse developed sub-°/hr silicon MEMS Quad Mass Gyroscope (QMG) with HRG-like structural symmetry and low dissipation. We demonstrated a 350 ppm accuracy (limited by the setup) and a 1 ppm precision by simultaneously measuring the true and self-calibrated scale factors over a 10 °C dynamic temperature range.

Abstract: In this paper a systematic approach to precision electrostatic frequency tuning of the operational modes of a MEMS ring vibratory gyroscope is presented. In both rate and rate integrating gyroscopes the frequency split between the two modes of vibration which detect the Coriolis acceleration is one of the principal factors in determining the sensitivity and noise floor of the sensor. In high precision applications in the defence/aerospace sector a frequency split of the order of 10 mHz or less is highly desirable. In the ground-breaking Hemispherical Resonator Gyroscope (HRG) electrostatic tuning has been employed as a tuning mechanism. However, a description of the procedure is not available in the literature. The tuning scheme described here involves assessing mode mistuning by the ratio of the in-phase and quadrature components of the response to an external force that has similar properties to the gyroscopic force resulting from Coriolis action, and choosing the tuning voltages so that independent modification of the elements of the system stiffness matrix can be achieved. Experiments on a commercially available gyroscope with a natural frequency of 14 kHz show that the frequency split can be reduced from 1.5 Hz to 6 mHz. This represents a frequency precision of better than 1 part in a million.

Abstract: A procedure for balancing the four lower harmonics of the mass distribution defect in a fused quartz hemispherical toothless resonator of a hemispherical resonator gyroscope is considered. Chemical etching of unbalanced mass from the surface of a partially immersed resonator is done in accordance with analytically calculated angle of the resonator rotation about the axis of symmetry, inclination and depth of the resonator immersion into a chemical bath, and the time of chemical etching. It is shown that the proposed method significantly reduces the balancing time and labor input as compared with ion plasma etching.