MEMS Resonator Amplitude Control Using Frequency Offset Drive
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Solution Overview
Problem
Conventional MEMS devices face challenges in stabilizing the amplitude of mechanical resonators due to varying Q-values and capacitive coupling issues, which affect the accuracy of angular rate detection in gyroscopes and magnetic field measurement in magnetometers, especially when using piezoelectric transducers where DC bias voltage stabilization is not feasible.
Innovation Solution
A method and apparatus that utilize an essentially constant amplitude input AC voltage with a frequency offset from the resonant frequency of the mechanical resonator, employing a drive loop circuitry to maintain stable oscillation amplitude, independent of Q-value variations, and adjust the frequency difference to control the oscillation amplitude, thereby minimizing capacitive coupling and ensuring quadrature phase alignment for effective signal separation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If resonance drive is used to excite the mechanical resonator, then the oscillation amplitude can be achieved with relatively low input force, but the amplitude becomes highly sensitive to Q-value variations and requires long rise time to reach steady state
Solution Approach 1:
The patent applies preliminary action by pre-setting the input AC voltage frequency to be offset from the resonant frequency before oscillation begins. This frequency offset is maintained throughout operation to prevent the resonator from reaching steady state resonance, thereby avoiding amplitude sensitivity to Q-value changes while still achieving effective oscillation excitation
Solution Approach 2:
The patent changes the frequency parameter of the input AC voltage from the resonant frequency to a frequency offset from resonance. This parameter change fundamentally alters the oscillation characteristics, making the amplitude less sensitive to Q-value variations and eliminating the need for automatic gain control while maintaining reliable operation
2Reliability
If automatic gain control is implemented to stabilize oscillation amplitude, then amplitude sensitivity to Q-value changes is reduced, but device complexity increases due to additional control circuitry
Solution Approach 1:
The patent extracts and eliminates the automatic gain control circuitry from the system by using frequency offset drive instead. The amplitude stability is achieved passively through the frequency offset method, removing the need for complex active control loops including detectors, controllers, and variable gain amplifiers
Solution Approach 2:
The patent replaces the mechanical/control system approach (automatic gain control with feedback loops) with an electrical frequency control approach. By controlling the input frequency offset, the system achieves amplitude stability without mechanical or electronic feedback control circuitry
3Reliability
If DC bias voltage stabilization is used for piezoelectric transducers, then transducer performance can be optimized, but this approach is not feasible for piezoelectric transducers
Solution Approach 1:
The patent creates a universal drive method that works with piezoelectric transducers without requiring DC bias voltage stabilization. The frequency offset AC drive approach is universally applicable to piezoelectric materials, eliminating the need for complex DC bias circuits while maintaining effective transducer operation across different piezoelectric device types
4Measurement precision
If resonator oscillation amplitude is increased to improve signal strength, then detection sensitivity improves, but capacitive coupling effects increase causing measurement errors
Solution Approach 1:
The patent converts the harmful capacitive coupling effect into a beneficial signal characteristic by using frequency offset drive. The capacitive coupling produces a signal at the input frequency, which is distinct from the resonant frequency where the actual measurement signal appears. This frequency separation allows easy filtering and compensation of capacitive coupling effects, transforming what was previously a harmful interference into a manageable signal component
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
This approach reduces the dependence of the input AC voltage on Q-value variations, maintaining stable oscillation amplitude and minimizing capacitive coupling, leading to improved accuracy and reliability in MEMS sensor devices by keeping the input AC voltage essentially constant and ensuring easy compensation of unwanted capacitive coupling signals.
Implementation Method 1
A primary resonator of a conventional MEMS gyroscope or a coil resonator of an induction type magnetometer is excited with a force oscillating at the resonant frequency of the resonator
Implementation Method 2
In a first embodiment, the input transducer is a piezoelectric transducer
Implementation Method 3
the driving with the input AC voltage occurs at a frequency that deviates from the resonant frequency of the mechanical resonator by a first frequency difference. The first frequency difference is configured to stabilize the amplitude of the mechanical vibration
Implementation Method 4
the motion of the resonator is sensed by an amplitude detector and stabilized to the desired value
Implementation Method 5
In induction type magnetometers, a suspended coil is placed in a magnetic field and made to periodically oscillate around a selected axis. Due to the electromagnetic induction a voltage is generated in the coil
Implementation Method 6
When an oscillating gyroscope is subjected to an angular motion orthogonal to the direction of the primary motion, an undulating Coriolis force results that is proportional to the angular velocity of the angular motion and to the velocity of the primary motion
Data Source
AI summary
The present invention relates to a method and a device for stabilization of amplitude of a mechanical vibration of a mechanical resonator in a microelectromechanical sensor device. The method comprises exciting the mechanical resonator with an oscillating excitation force by an input transducer. The input transducer is driven with an input AC voltage having essentially constant amplitude at a frequency that deviates from the resonant frequency of the mechanical resonator by a first frequency difference. The first frequency difference is configured to stabilize the amplitude of the mechanical vibration.