Symmetrical Mass Body with Inclined Cantilever Beams for 3-Axis Accelerometer
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Solution Overview
Problem
Existing 3-axis accelerometers suffer from cross-talk between axes and inefficient use of wafer surface area, leading to reduced accuracy and increased size.
Innovation Solution
A compact 3-axis accelerometer design featuring a symmetrical mass body with inclined cantilever beams, where the mass body's major surfaces are parallel to the wafer plane, minimizing dead space and allowing for symmetric electrode placement, thereby reducing torque generation and enhancing frequency separation of mechanical modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If asymmetric mass body with slanted side-walls is used (WO 94/12886), then cross-axis sensitivity is reduced, but dead space increases and electrode area is reduced
Solution Approach 1:
The patent applies asymmetry in the beam configuration (slanted beams vs symmetric beams) to reduce cross-axis sensitivity, while maintaining symmetry in the mass body to maximize electrode area. This selective application of asymmetry resolves the contradiction by achieving cross-axis immunity without sacrificing electrode area.
Solution Approach 2:
The patent segments the sensing element into distinct functional parts: the symmetric mass body (for maximizing electrode area) and the slanted beams (for reducing cross-axis sensitivity). This segmentation allows each part to optimize its specific function without compromising the other.
2Use of energy by moving object
If more electrode area is provided, then applied voltage can be reduced, but device size increases
Solution Approach 1:
The patent changes the geometric parameters of the mass body (making it symmetric and extending in the wafer thickness direction) to maximize electrode area within the constrained device footprint. This parameter optimization allows larger electrode area without proportionally increasing device size.
Solution Approach 2:
The patent utilizes the wafer thickness direction (z-dimension) to extend the mass body, creating additional electrode area in the vertical dimension rather than only expanding in the planar dimensions. This dimensional approach increases electrode area while minimizing footprint expansion.
3Device complexity
If symmetric mass body is used, then electrode placement is simplified and torque is minimized, but cross-axis sensitivity may increase
Solution Approach 1:
The patent segments the sensing element into distinct functional parts: the symmetric mass body (for maximizing electrode area) and the slanted beams (for reducing cross-axis sensitivity). This segmentation allows each part to optimize its specific function without compromising the other.
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 achieves better utilization of wafer surface area for electrodes, reducing the applied voltage required and minimizing unwanted mechanical resonance, resulting in a more accurate and compact device with improved frequency separation of modes.
Implementation Method 1
The device is produced by anisotropic wet etching, which works from two opposite crystal planes of the single crystal material
Implementation Method 2
In basic capacitive detection, the displacement of the seismic mass is measured by measuring the capacitance between a fixed plate and the mass body
Implementation Method 3
A piezoresistor changes its resistance depending on mechanical stress
Implementation Method 4
In a capacitive closed-loop feedback system, electrostatic forces are applied to the mass by the electrodes to counter acceleration forces
Data Source
Figure 1
Figure 2a~2b
Figure 3a~3b
AI summary
A device for measuring force components formed from a single crystal material, wherein the device comprises at least one cantilever beam inclined to a wafer plane normal and formed in one piece with a mass body, which mass body provides a mass of inertia. The mass body has a first and a second major surface which are substantially parallel with a wafer plane. A mass body cross section presents a portion which is substantially symmetrical along a centrally (in the thickness direction) located plane parallel with the wafer plane. Disclosed is also a method for its production and an accelerometer comprising at least one such device. The device allow for a more compact 3-axis accelerometer.