MEMS Accelerometer Variable-Length Damping for High-G Robustness
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Solution Overview
Problem
Existing MEMS accelerometers, particularly those designed for high accelerations, suffer from insufficient mechanical robustness and detection accuracy due to high resonance frequencies, overdamped frequency responses, and in-plane rotation of the sensing mass, which can lead to mechanical breakage.
Innovation Solution
A MEMS accelerometer with an out-of-plane configuration featuring a sensing mass suspended along the out-of-plane direction, coupled to a substrate via torsional springs, and incorporating damping structures with interdigitated movable and fixed fingers of variable length to damp in-plane movements, along with a frame and cap design to minimize area occupation and enhance mechanical robustness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If in-plane mechanical damping structures are used to reduce quality factor Q, then vibration immunity is improved, but mechanical robustness deteriorates due to high area requirements
Solution Approach 1:
The damping structure is configured with fingers extending in the out-of-plane direction (vertical dimension) rather than only in the in-plane direction. This dimensional change allows the damping structure to provide effective vibration damping while occupying minimal in-plane area, thus maintaining mechanical robustness. The movable and fixed fingers are arranged to interdigitate in the vertical space above the substrate.
Solution Approach 2:
The damping structure employs an interdigitated finger configuration that creates a porous-like structure with gaps between fingers. This allows the structure to provide damping through air resistance and viscous effects in the gaps, achieving effective damping with minimal material usage and small in-plane footprint, thereby preserving mechanical strength.
2Measurement precision
If high resonance frequency is used to ensure required full scale, then acceleration detection range is improved, but detection accuracy deteriorates
Solution Approach 1:
The patent optimizes the resonance frequency parameter by carefully designing the mechanical properties of the sensing mass and suspension springs. The damping structure with vertically extending fingers is configured to provide controlled damping that maintains high resonance frequency for broad detection range while preventing excessive damping that would reduce detection accuracy. The variable finger dimensions allow tuning of the damping coefficient to achieve optimal balance.
3Adaptability or versatility
If sensing mass is configured for high-g detection, then full scale range is improved, but mechanical robustness deteriorates due to high in-plane rotation causing breakage
Solution Approach 1:
The damping fingers extend in the out-of-plane direction to provide damping forces that counteract in-plane rotational movements of the sensing mass during high-g events. This vertical configuration allows the damping structure to stabilize the sensing mass against lateral forces without requiring the sensing mass to rotate in-plane, preventing mechanical breakage while maintaining high full-scale range capability.
Solution Approach 2:
The damping structure is pre-configured with movable and fixed fingers positioned to interdigitate before any acceleration event occurs. During high-g events, this pre-positioned structure immediately provides counteracting damping forces to prevent excessive in-plane rotation and potential breakage of the sensing mass, acting in advance to protect the mechanical structure.
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 design optimizes the trade-off between damping effectiveness, area occupation, and mechanical robustness, reducing the risk of breakage and improving reliability under high accelerations.
Implementation Method 1
a damping structure configured to damp an in-plane movement of the sensing mass with respect to the substrate
Implementation Method 2
a sensing mass suspended at a distance from the substrate along an out-of-plane direction, the sensing mass being coupled to the substrate so as to undergo an out-of-plane movement with respect to the substrate, in response to an acceleration along the out-of-plane direction
Data Source
AI summary
A MEMS accelerometer has a substrate and a sensing mass suspended at a distance from the substrate along an out-of-plane direction. The sensing mass is coupled to the substrate so as to undergo an out-of-plane movement with respect to the substrate, in response to an acceleration along the out-of-plane direction. The MEMS accelerometer also has a damping structure configured to damp an in-plane movement of the sensing mass with respect to the substrate. The damping structure has a plurality of movable fingers integral with the sensing mass and a plurality of fixed fingers integral with the substrate and interdigitated with the movable fingers. The movable fingers and/or the fixed fingers have, along a first in-plane direction transversal to the out-of-plane direction, a variable length.