Acceleration Sensor Spring Design for Interference Rejection
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Solution Overview
Problem
Acceleration sensors face interference and measurement inaccuracies due to deflections caused by both perpendicular and parallel accelerations, leading to oscillations that falsify measurements, as the stiffness of springs affects sensitivity and natural frequencies.
Innovation Solution
The springs are designed with two interconnected bending bars and crosspieces, providing high stiffness along the substrate surface while maintaining low stiffness perpendicular to it, using polysilicon and formed in one piece with the seismic mass, and connected via flexible projections to reduce force and prevent rupture.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the cross-section of the flexible elements is enlarged to increase stiffness along the substrate surface, then the stiffness perpendicular to the substrate surface increases, but the sensitivity of the acceleration sensor to deflections in the z-direction decreases
Solution Approach 1:
The spring elements are designed with different cross-sectional dimensions in different directions: larger width (b) for high stiffness along the substrate surface, and smaller thickness (h) for low stiffness perpendicular to the substrate. This anisotropic cross-section creates locally optimized mechanical properties that simultaneously achieve high in-plane stiffness and high out-of-plane sensitivity
Solution Approach 2:
The spring cross-section is made asymmetric with width b significantly larger than thickness h (b > h). This asymmetric geometry inherently provides different stiffness characteristics along different axes, allowing the spring to be stiff along the substrate surface while remaining compliant in the measurement direction perpendicular to the substrate
2Reliability
If the stiffness of the springs along the substrate surface is increased to reduce oscillations, then the natural frequency of oscillations increases, but the maximum forces acting on the springs increase, requiring stronger connection to the seismic mass
Solution Approach 1:
The spring elements incorporate curved or bent geometries instead of straight configurations. These curved paths allow the springs to achieve higher stiffness along the substrate surface while distributing and reducing the maximum forces at critical connection points, thereby preventing rupture at the seismic mass interface
3Measurement precision
If the dimensions of the acceleration sensor are increased to achieve sufficiently low stiffness perpendicular to the substrate surface, then the sensitivity improves, but the sensor size increases
Solution Approach 1:
The spring thickness parameter h is optimized to a specific small value that achieves the desired low stiffness perpendicular to the substrate without requiring increased overall sensor dimensions. By precisely controlling this geometric parameter, the sensor maintains compact size while achieving high sensitivity in the measurement direction
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 minimizes interference from oscillations, enhancing the sensor's sensitivity and accuracy by maximizing stiffness along the substrate surface without increasing the sensor's dimensions, thus reducing false measurements from perpendicular accelerations.
Implementation Method 1
each spring (10, 14, 15) includes two bending bars (12, 16) that are interconnected via crosspieces (7, 9)
Implementation Method 2
an acceleration sensor includes a seismic mass (1) which is suspended on springs (2, 10, 14, 15) above a substrate, and is deflectable in a direction perpendicular to a surface of the substrate
Implementation Method 3
The deflection of the seismic mass in a direction perpendicular to the substrate because of an acceleration is detected by an electrode provided below the seismic mass on the substrate
Data Source
AI summary
An acceleration sensor includes a seismic mass which is suspended on springs above a substrate and is deflectable in a direction perpendicular to a surface of the substrate. In order to reduce deflections of the seismic mass along the surface of the substrate because of interference accelerations, which lead to a falsification of the measurements of the deflection of the seismic mass perpendicular to the surface of the substrate, the springs include two bending bars which are interconnected via crosspieces.