Capacitive Accelerometer Proof Mass Central Anchor Design
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Capacitive accelerometers face bias stability issues due to thermal expansion mismatches between silicon and glass substrates, leading to scale factor shifts and bias shifts under temperature changes, which affect sensitivity and accuracy.
Innovation Solution
The proof mass is anchored centrally with flexible legs extending inwardly to a proof mass anchor, and fixed capacitor electrodes are arranged with cantilevered arms to reduce thermal stress, allowing anchor points to be grouped closer to the center, thereby minimizing thermal gradient effects and improving bias stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the fixed capacitor electrodes are mounted separately on the substrate with their own anchor points, then each electrode can be electrically isolated and mounted independently, but the anchor points are distributed far apart causing differential thermal expansion and bias shifts
Solution Approach 1:
Multiple fixed capacitor electrodes are merged onto a single common substrate anchor point rather than having separate anchor points. This consolidation brings all electrode mounting locations close to the center line, reducing differential thermal expansion between the glass support and silicon substrate, thereby improving bias stability while maintaining electrical isolation through the substrate material
2Ease of manufacture
If the proof mass anchor points are distributed away from the center, then the flexible legs can be mounted separately, but thermal gradients cause asymmetric electrode movement and bias shifts
Solution Approach 1:
Multiple flexible legs are merged to connect to a single common proof mass anchor point located on or near the center line. This centralization ensures that all flexible legs and electrode structures experience similar thermal conditions, preventing asymmetric movement due to thermal gradients while still allowing separate flexible leg fabrication and assembly
3Adaptability or versatility
If the anchor points are spread out on the substrate, then each electrode can be independently positioned, but the coefficient of thermal expansion mismatch between glass and silicon causes scale factor shifts
Solution Approach 1:
Multiple electrode structures are merged to share common anchor points on the substrate. This reduces the spatial distribution of anchor points from multiple locations to a centralized location near the center line, minimizing the effect of thermal expansion mismatch between glass and silicon materials on scale factor stability
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 design achieves a tenfold improvement in bias stability from 1 mg to 0.1 mg, reducing the impact of thermal stresses and die bond effects, enhancing the accuracy of accelerometer output.
Implementation Method 1
a proof mass mounted to a support by flexible legs for movement in response to an applied acceleration
Implementation Method 2
differential capacitance between the electrode fingers being measurable so as to detect deflection of the proof mass
Implementation Method 3
The atmospheric pressure gas (typically argon) provides critical squeeze film damping for the proof mass when it moves
Implementation Method 4
The silicon substrate is typically anodically bonded to a glass support with pre-cavitation of the glass where the elements move
Data Source
Figure 1
Figure 2
Figure 3
AI summary
A sensing structure for an accelerometer, comprising: a support and a proof mass mounted thereto by flexible legs; the proof mass comprising moveable electrode fingers perpendicular to the sensing direction; at least four fixed capacitor electrodes, with fixed capacitor electrode fingers perpendicular to the sensing direction; the fixed capacitor electrode fingers interdigitating with the movable electrode fingers; the proof mass mounted to the support by an anchor on a centre line of the proof mass. The proof mass has an outer frame surrounding the fixed capacitor electrodes. The flexible legs extend laterally inwardly from the proof mass to the anchor. The fixed capacitor electrodes comprise two inner electrodes, one on each side of the proof mass centre line, and two outer electrodes, one on each side of the proof mass centre line. Each inner electrode comprises an anchored part anchored to the substrate and a cantilevered arm extending from the anchored part away from the centre line and having electrode fingers mounted thereto. Each outer electrode comprises an anchored part anchored to the substrate at a location that overlaps in the sensing direction with the cantilevered arm of an inner fixed capacitor electrode and a cantilevered arm extending from the anchored part away from the centre line and having fixed capacitor electrode fingers mounted thereto. Thus the anchored part is reduced in extent so that it does not take up the same axial extent as its fingers. The anchored part of the adjacent outer fixed electrode is moved closer to the centre line and the proof mass anchor, thereby reducing the effects of a thermal gradient across the sensing structure.