Electronic circuitry has been devised to compensate for variations in the resonance quality factor (Q) of a planar vibratory microgyroscope like that described in the first of the two preceding articles. That is, the circuit makes the scale factor of the gyroscope (the factor of proportionality between the rate of rotation, W, and the output signal) independent of Q.
If the Coriolis signal were to constitute the output signal, then the scale factor would be proportional to the amplitude of the input displacement and to Q. The amplitude of the input displacement is maintained constant by the technique described in the immediately preceding article, "Vibration-Regulating Circuit for a Mechanical Resonator," (NPO-20088). However, the rate of damping, and thus Q, does not remain constant; the rate of damping is very sensitive to the density and pressure of the fluid in which the resonator is immersed. Thus, there is need for further processing of the Coriolis signal through circuitry like that described below to make the scale factor remain constant despite variations in Q.
The circuitry in question is part of the overall electronic circuit shown in the figure. To maintain a constant input vibration amplitude, it is necessary to drive the resonator with a voltage proportional to Q. Other parts of the circuit generate such a drive voltage, which is applied not only to the resonator but also to the input terminal of a 90° phase shifter. The output of the phase shifter is fed to one of two input terminals of an analog multiplier. An amplified version of the Coriolis signal is applied to the other input terminal of the analog amplifier. Because the Coriolis signal is approximately 90° out of phase with the drive signal, it is approximately in phase with the output of the phase shifter. Therefore, the time-averaged output of the analog multiplier is proportional to the amplitude of the Coriolis signal (proportional to WQ) and to the amplitude of the drive signal (proportional to Q-1). Thus, the output of the analog multiplier is proportional to W only, as desired, regardless of variations in Q.
This work was done by Christopher Stell, Vatché Vorperian, Roman Gutierrez, and Tony Tang of Caltech for NASA's Jet Propulsion Laboratory.
NPO-20089
This Brief includes a Technical Support Package (TSP).
Q-Compensation Circuit for a Planar Vibratory Microgyroscope
(reference NPO20089) is currently available for download from the TSP library.
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Overview
The document presents a technical support package from NASA detailing a Q-compensation circuit for a planar vibratory microgyroscope, developed by a team of inventors including Roman C. Gutlerrez and Christopher B. Stell. The primary focus of this innovation is to address the challenges posed by variations in the resonance quality factor (Q) of microgyroscopes, which significantly affects their rotational sensitivity or scale factor.
In silicon micro-machined vibratory gyroscopes, the scale factor is highly dependent on the Q of the resonator, which is sensitive to the density and pressure of the surrounding fluid. As these environmental conditions change, the Q value fluctuates, leading to variations in the scale factor and, consequently, the accuracy of the gyroscope's measurements. The document outlines a solution that employs a constant amplitude regulation scheme to drive the resonator into oscillation, thereby stabilizing the output signal.
The circuitry described includes components such as transconductance amplifiers, a summing amplifier, a voltage-controlled amplifier (VCA), and a peak detector circuit. The VCA adjusts the drive voltage amplitude to maintain a constant input vibration amplitude, which is crucial for accurate measurements. The peak detector performs full-wave rectification of the signals, and an integrating error amplifier generates an error signal that helps regulate the system's gain.
To extract rotation information, the drive waveform is phase-shifted by 90 degrees and multiplied by the difference signal from the two transconductance amplifiers. This process ensures that the average voltage of the multiplier output is linearly proportional to the rotation rate, effectively nullifying the effects of scale factor variations due to changes in Q.
The document emphasizes the novelty of this approach, highlighting its automatic compensation for resonator Q variations and the in-situ measurement of Q. Unlike prior methods that relied solely on phase information from the drive voltage, this new scheme utilizes both phase and amplitude information, enhancing the overall performance of the gyroscope.
In summary, the Q-compensation circuit represents a significant advancement in the field of microgyroscopes, providing a robust solution to maintain consistent performance despite environmental fluctuations, thereby improving the reliability and accuracy of rotational measurements in various applications.
