MEMS Common Mode Rejection via Anti-Phase Drive
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Solution Overview
Problem
MEMS angular rate sensors are susceptible to common mode excitations, leading to inaccuracy or failure due to in-phase motion, which is exacerbated by low common mode frequencies.
Innovation Solution
The implementation of a MEMS device with a common mode rejection structure, including a drive spring system for drive masses and a sense spring system for sense masses, along with coupling masses to decouple drive and sense motions, effectively rejecting in-phase motion and reducing quadrature error and electrical noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional MEMS angular rate sensor is used without common mode rejection structure, then the device complexity is low, but the sensor accuracy deteriorates due to susceptibility to common mode excitations and in-phase motion
Solution Approach 1:
The drive assembly is segmented into a first drive mass and a second drive mass that move in phase opposition. The sense assembly is segmented into a first sense mass and a second sense mass. This segmentation allows differential mode operation where only out-of-phase motions are sensed, inherently rejecting common mode excitations and improving measurement precision while managing device complexity through functional division.
Solution Approach 2:
Coupling masses are introduced as intermediary elements between the drive masses and sense masses. These coupling masses facilitate the transfer of Coriolis acceleration from the drive motion to the sense masses while maintaining the anti-phase relationship. The intermediary coupling masses help isolate the sense masses from direct common mode excitations, improving sensor accuracy without significantly increasing overall device complexity.
2Reliability
If the common mode frequency is low, then the device structure is simpler, but the reliability deteriorates due to increased susceptibility to in-phase motion from external stimuli
Solution Approach 1:
The patent applies preliminary anti-action by designing the drive masses to move in phase opposition (anti-phase) and configuring the coupling masses and sense masses to respond only to differential mode motion. This preliminary anti-phase configuration creates inherent common mode rejection, preventing in-phase motion from affecting measurement accuracy. The anti-phase drive motion generates Coriolis acceleration that couples to the sense masses only when there is rotational motion, while common mode excitations are rejected, thereby improving reliability.
3Measurement precision
If in-phase motion is not rejected, then the device operation is simpler, but the measurement precision deteriorates due to inaccuracy from common mode excitations
Solution Approach 1:
The patent utilizes mechanical vibration principles by driving the first and second drive masses in phase opposition at a specific drive frequency. The coupling masses and sense masses are designed to resonate in response to the Coriolis acceleration generated by the anti-phase drive motion. This mechanical vibration approach enables selective sensing of angular rate through resonant enhancement of the differential mode signal while the anti-phase configuration inherently rejects common mode vibrations, improving measurement precision.
Solution Approach 2:
The patent changes the operational parameters by operating the drive masses at a specific anti-phase frequency and designing the coupling and sense masses with specific stiffness and mass ratios. By adjusting these parameters, the system optimizes the Coriolis coupling while maintaining common mode rejection. The parameter changes enable the sense masses to be highly sensitive to differential mode motion (angular rate) while being insensitive to common mode excitations, thereby improving measurement accuracy.
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 configuration suppresses in-phase motion in both drive and sense modes, enhancing the robustness of the sensor against vibrations and shocks, thereby improving accuracy and reliability by increasing common mode frequencies and reducing noise interference.
Implementation Method 1
In response to an external angular stimulus about an input axis, the pair of sense masses move in phase opposition by exploiting a Coriolis acceleration component.
Implementation Method 2
An embodiment of a MEMS device includes a planar substrate, a drive assembly, a stiff beam, an elastic element... The first elastic element is coupled between a first end of the stiff beam and the first drive mass
Implementation Method 3
The stiff beam and the first and second elastic elements are configured to enable the first and second drive masses to move in phase opposition... suppressing or reducing in-phase motion of the first and second drive masses
Data Source
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AI summary
A MEMS device includes a drive spring system coupling a pair of drive masses and a sense spring system coupling a pair of sense masses. The drive spring system includes a constrained stiff beam and flexures interconnecting the pair of drive masses. In response to drive movement of the drive masses the flexures enable pivotal movement of the constrained stiff beam about its center hinge point to enable anti-phase drive motion of the drive masses and to suppress in-phase motion of the drive masses. The sense spring system includes diagonally oriented stiff beams and a spring system that enable anti-phase sense motion of the sense masses while suppressing in-phase motion of the sense masses. Coupling masses interposed between the drive and sense masses decouple the drive motion of the drive masses from the sense motion of the sense masses.