Variable Capacitance Accelerometer Meandering Flexures
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Solution Overview
Problem
MEMS accelerometers with serpentine mounting legs face damage from large out-of-plane accelerations due to high stress concentrations, and increasing leg length to reduce resonant frequency results in increased device dimensions, which is undesirable in many applications.
Innovation Solution
The accelerometer design features serpentine mounting legs with thicker end sections and tapered transitions to distribute stress uniformly, reducing the risk of failure while maintaining sensitivity and avoiding size increases, by ensuring the end sections are at least twice as thick as the central parts and incorporating a connecting cross brace for the proof mass elements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the mounting legs are made longer to reduce resonant frequency, then the sensitivity is enhanced and noise is reduced, but the overall dimensions of the device increase
Solution Approach 1:
The mounting legs are configured in a serpentine pattern that folds back on itself, nesting the leg structure within the existing device footprint. This allows the effective length of the mounting legs to be increased without increasing the overall device dimensions, as the legs follow a meandering path through the available space
Solution Approach 2:
The mounting legs transition from a straight linear configuration to a serpentine two-dimensional pattern, utilizing both length and width dimensions of the device plane. This dimensional transformation allows the legs to achieve greater effective length while maintaining compact overall device dimensions
2Length of stationary object
If the mounting legs are made shorter to reduce device dimensions, then the device size is reduced, but the resonant frequency increases and sensitivity decreases
Solution Approach 1:
The serpentine configuration nests the mounting leg structure within the compact device footprint, allowing short overall device dimensions while maintaining long effective leg length through the meandering path
Solution Approach 2:
By transforming the leg configuration from one-dimensional straight lines to two-dimensional serpentine patterns, the design achieves long leg length within short device dimensions by utilizing multiple spatial directions
3Ease of manufacture
If the serpentine mounting legs are made with uniform thickness, then the manufacturing is simplified, but the legs are susceptible to damage from large out-of-plane accelerations due to stress concentrations
Solution Approach 1:
The mounting legs feature variable thickness along their length, with thicker sections at the ends and thinner sections in the middle. This local variation in geometry optimizes the structural properties at different locations, providing enhanced stress concentration resistance at the ends while maintaining manufacturing feasibility
Solution Approach 2:
The thickness parameter of the mounting legs is changed along the length of the legs, transitioning from uniform thickness to a graded thickness profile. This parameter variation optimizes the mechanical properties to resist out-of-plane accelerations while remaining compatible with standard manufacturing processes
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 reduces the risk of damage from out-of-plane accelerations and maintains sensitivity by distributing stress uniformly across the serpentine legs, allowing the device to withstand significant accelerations without increasing overall dimensions.
Implementation Method 1
the mounting legs flexing and applying a restoring force urging the proof mass back towards its rest position
Implementation Method 2
By taking appropriate capacitance measurements, the position or movement of the proof mass relative to the support member can be determined
Data Source
AI summary
An accelerometer comprises a support (12), a proof mass (14) supported for movement relative to the support (12) by a plurality of mounting legs (16), a plurality of fixed capacitor fingers associated with the support (12) and a plurality of movable capacitor fingers associated with the proof mass (14), the fixed capacitor fingers being interdigitated with the movable capacitor fingers, the mounting legs (16) being of serpentine shape, each mounting leg (16) comprising at least a first generally straight section (16a), a second generally straight section (16a), and an end section (16b) of generally U-shaped form interconnecting the first and second generally straight sections (16a), wherein the thickness Te of the end section (16b) is greater than the thickness Tc of a central part (16c) of both of the first and second generally straight sections (16a).