Frequency Modulation Angular Rate Sensor Eliminates Q-Bandwidth Tradeoff
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Solution Overview
Problem
Conventional high-Q angular rate gyroscopes face limitations due to the gain-bandwidth tradeoff and sensitivity to temperature and pressure variations, which restrict their dynamic range and accuracy, and are prone to mechanical and electromagnetic interferences.
Innovation Solution
A micromachined angular rate sensor system based on frequency modulation (FM) with a vibratory gyroscope having two mechanical modes, a control subsystem for periodic or continuous excitation, and a signal processing subsystem for monitoring resonant frequencies, enabling inherently digital output and self-sensing temperature stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional AM-based high-Q angular rate gyroscopes are used, then signal-to-noise ratio is improved, but measurement bandwidth and dynamic range are limited
Solution Approach 1:
The patent replaces the conventional analog amplitude modulation (AM) readout system with a digital frequency modulation (FM) system. Instead of measuring analog displacement signals that require high-Q mode matching, the invention uses digital frequency counters to measure the resonant frequency of the sense mode. This substitution of measurement methodology eliminates the fundamental Q versus bandwidth tradeoff, allowing high-Q structures (Q>100k) to achieve both improved signal-to-noise ratio and extended measurement bandwidth up to 100 Hz or more.
2Temperature
If conventional AM-based gyroscopes operate over temperature variations, then device must be temperature compensated, but accuracy is limited by thermal hysteresis and lag
Solution Approach 1:
The patent replaces temperature compensation methods with a temperature-insensitive measurement approach. By measuring the resonant frequency of the sense mode rather than its amplitude, the system achieves inherent insensitivity to temperature and pressure variations. The resonant frequency measurement using digital frequency counters does not depend on the quality factor Q, which varies with temperature, thereby eliminating thermal hysteresis and lag while maintaining high accuracy.
Solution Approach 2:
The invention changes the measurement parameter from analog amplitude (which is sensitive to Q-factor variations) to digital frequency (which is insensitive to Q-factor variations). This parameter change from amplitude modulation to frequency modulation makes the measurement inherently robust against environmental variations in temperature and pressure, as the resonant frequency remains stable while Q-factor may vary.
3Measurement precision
If analog AM readout with low-noise electronic components is used, then resolution reaches 1e-6, but dynamic range is fundamentally limited
Solution Approach 1:
The patent substitutes analog amplitude measurement with digital frequency measurement. Digital frequency counters can resolve frequency changes with extremely high precision while accommodating a wide dynamic range of input angular rates. The system achieves resolution better than 1e-6 deg/s while extending the dynamic range to exceed 150 dB, far beyond the fundamental limitations of analog AM systems. This is accomplished by counting frequency cycles over a measurement interval, which provides both high resolution and wide dynamic range without the tradeoffs inherent in analog systems.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The FM system achieves improved signal-to-noise ratio without limiting bandwidth, is robust against mechanical and electromagnetic interferences, and provides temperature stability with minimal response variation, enhancing the accuracy and reliability of angular rate measurements.
Implementation Method 1
Sense-mode vibrations are excited by the Coriolis force which is a product of the drive-mode velocity and the input angular rate
Implementation Method 2
maximization of their quality (Q) factors is key to improving performance
Data Source
AI summary
A digital angular rate sensor system based on frequency modulation (FM) of the rotation rate. The new approach relies on tracking of the resonant frequencies of two high-Q mechanical modes of vibration in a MEMS vibratory gyroscope to produce an inherently digital measurement of the input angular rate. The disclosed system is enabled by a combination of a MEMS vibratory high-Q gyroscope and a new signal processing scheme which takes advantage of a previously ignored gyroscope dynamics effect. The FM nature of the system eliminates noise versus bandwidth and resolution versus dynamic range tradeoffs of conventional vibratory rate gyroscopes. The FM approach allows achieving superior signal-to-noise-ratio through the use of ultra-high Q (1 million) mechanical structure without limiting the measurement bandwidth. Stability of 1e-9 can be achieved in the FM system, providing a 1000 times improvement over the state-of-the-art conventional AM gyroscopes with capacitive pick-off.