Triaxial Membrane Accelerometer with Dissymmetric Combs
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Solution Overview
Problem
Existing triaxial accelerometers are bulky and costly due to the need for multiple micro-machined components and deep etchings, leading to inaccurate sensitivity axes and spurious signal issues, especially when detecting accelerations in three dimensions.
Innovation Solution
A triaxial accelerometer design featuring a thin-layer membrane suspended above a substrate with a proof mass linked by a central stud and capacitive interdigitated combs, allowing for independent measurement and slaving of the proof mass, with dissymmetric comb configurations to differentiate vertical and horizontal accelerations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If three separate micro-machined accelerometers are used to detect accelerations in three dimensions, then measurement capability is improved, but device complexity and cost increase
Solution Approach 1:
The patent combines three separate accelerometer measurements into a single integrated device. A common proof mass is suspended by three independent elastic beams, each oriented along a different axis (X, Y, Z). Capacitive sensing electrodes are positioned to detect displacements of the proof mass along all three axes simultaneously, eliminating the need for three separate accelerometer components while maintaining full triaxial measurement capability.
Solution Approach 2:
The single proof mass structure serves multiple functions: it detects accelerations along the X-axis, Y-axis, and Z-axis simultaneously. The capacitive sensing system is designed to measure displacements in all three dimensions using the same physical component (the proof mass), making the device universal for triaxial acceleration detection rather than requiring separate specialized sensors for each axis.
2Strength
If deep etchings are used to define the proof mass in thick silicon substrates, then structural integrity is improved, but manufacturing cost and bulkiness increase
Solution Approach 1:
The patent replaces the traditional thick silicon substrate with a thin silicon nitride membrane that serves as the proof mass support structure. This thin film approach (several micrometers thick) provides sufficient structural integrity through the elastic beams while dramatically reducing the need for deep etchings. The membrane can be fabricated using standard thin-film deposition and release techniques, significantly lowering manufacturing complexity and cost compared to deep silicon etching processes.
3Productivity
If substrate transfer techniques are used to produce triaxial accelerometers, then production efficiency is improved, but dimensional accuracy and sensitivity linearity deteriorate
Solution Approach 1:
The patent segments the accelerometer structure into distinct functional layers that can be fabricated separately and then assembled. The thin silicon nitride membrane with embedded elastic beams and capacitive electrodes is fabricated as one segment, while the substrate with remaining sensing electrodes is fabricated as another segment. These segments are then transferred and assembled together, allowing each component to be optimized independently for its specific function while maintaining overall dimensional accuracy.
4Volume of moving object
If the proof mass is suspended in a thin layer above the substrate, then bulkiness is reduced, but the ability to slave the mass position to a central rest position becomes more complex
Solution Approach 1:
The patent uses asymmetric capacitive electrode configurations to provide position-dependent restoring forces that slave the proof mass to its central rest position. The capacitive sensing electrodes are positioned asymmetrically relative to the proof mass, creating electrostatic forces that naturally restore the mass to center when displacement occurs. This asymmetric arrangement enables position control without requiring complex additional mechanical slaving mechanisms, maintaining the benefits of the thin-layer suspended 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
This design reduces bulkiness and cost while improving sensitivity and accuracy by enabling precise measurement of accelerations along three axes with reduced spurious signals and enhanced sensitivity axis independence.
Implementation Method 1
capacitive interdigitated combs distributed about the mass, having movable plates secured to the mass and fixed plates secured to the substrate
Implementation Method 2
an elastically deformable thin-layer membrane suspended above the substrate and secured to the substrate at its periphery
Data Source
AI summary
The invention relates to accelerometer structures micro-machined according to micro-electronics technologies. The accelerometer according to the invention comprises a substrate, an elastically deformable thin-layer membrane suspended above the substrate and secured to the substrate at its periphery, a proof mass suspended above the membrane and linked to the latter by a central stud, and capacitive interdigitated combs distributed about the mass, having movable plates secured to the mass and fixed plates secured to the substrate. The fixed plates and movable plates of the various combs are of differentiated heights to help in the discrimination of the upward and downward vertical accelerations.