Low-noise Multi-axis MEMS Accelerometer with Symmetric Proof Masses
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Solution Overview
Problem
MEMS accelerometers face challenges in achieving high accuracy and low noise, particularly due to size constraints and susceptibility to cross-axis interference and parasitic modes of movement, which are exacerbated by the use of individual proof masses for each sense axis in high-accuracy applications and compromised by shared proof masses in cost-effective designs.
Innovation Solution
The use of at least two symmetric out-of-plane see-saw type proof masses that rotate in opposite directions, minimizing cross-axis errors and allowing larger proof masses for improved accuracy and noise reduction, with comb capacitors and gap detection capacitors for differential measurements to sense movement and rotation, maintaining the combined center of mass at the accelerometer's center and resisting external torque.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If individual proof masses are used for each sense axis to reduce cross-axis interference, then measurement precision is improved, but device complexity increases and proof mass size is reduced
Solution Approach 1:
The patent merges the functions of multiple proof masses into a single integrated proof mass structure that senses acceleration along multiple axes simultaneously. This single proof mass is designed with specific geometric features and suspension mechanisms that enable it to detect both in-plane and out-of-plane acceleration components without requiring separate proof masses for each axis, thereby reducing device complexity while maintaining measurement precision.
Solution Approach 2:
The single proof mass is designed to perform multiple sensing functions simultaneously - it can detect acceleration along different axes and in different planes (in-plane and out-of-plane). This multi-functional design allows one proof mass to replace what would traditionally require multiple separate proof masses, reducing overall system complexity while maintaining the ability to accurately measure acceleration in multiple directions.
2Measurement precision
If individual proof masses are used for each sense axis, then cross-axis interference is reduced, but proof mass size is reduced leading to increased noise susceptibility
Solution Approach 1:
The single proof mass incorporates localized geometric features and asymmetric mass distribution that are specifically designed to minimize cross-axis interference in different regions of the proof mass. By optimizing the local mass distribution and geometric characteristics in different areas, the design achieves both cross-axis accuracy and sufficient overall proof mass size to reduce noise susceptibility.
Solution Approach 2:
The proof mass employs dynamic suspension mechanisms and elastic elements that allow the proof mass to move independently along different axes while maintaining proper decoupling. The dynamic characteristics of the suspension system are optimized to ensure that motion in one axis does not significantly affect sensing in another axis, achieving cross-axis accuracy while maintaining sufficient proof mass size.
3Device complexity
If a single proof mass is used for multiple axes to reduce system complexity, then device complexity is reduced, but cross-axis error and parasitic modes increase significantly
Solution Approach 1:
The single proof mass features asymmetric mass distribution and geometric asymmetry that are deliberately designed to decouple motion along different axes. The asymmetric design creates different inertial properties and suspension characteristics for different axes, which helps to minimize cross-axis coupling errors while maintaining a single integrated proof mass structure for low device complexity.
Solution Approach 2:
While using a single integrated proof mass, the design effectively segments the sensing functions by creating distinct suspension mechanisms and measurement regions for different axes. The proof mass is designed with separate suspension points and measurement zones that allow independent sensing along different axes, reducing cross-axis coupling while maintaining overall structural integration.
4Device complexity
If a single proof mass is used for multiple axes, then system complexity and cost are reduced, but parasitic modes of movement significantly reduce accuracy
Solution Approach 1:
The patent employs capacitive sensing mechanisms to detect proof mass displacement, replacing direct mechanical coupling mechanisms that would be prone to parasitic modes. The capacitive sensors measure changes in electrical capacitance caused by proof mass movement, providing a non-contact or minimal-contact measurement method that reduces mechanical parasitic modes while maintaining a single integrated proof mass structure.
Solution Approach 2:
The suspension system is designed with specific dynamic characteristics that allow the proof mass to move freely in the desired sensing directions while constraining unwanted parasitic modes of movement. The elastic elements and suspension geometry are optimized to create stiff constraints against parasitic motions while maintaining compliance in the intended sensing directions, thereby improving accuracy while using a single proof mass.
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 configuration enhances the accuracy and noise performance of MEMS accelerometers by allowing larger out-of-plane proof masses, reducing parasitic modes, and minimizing cross-axis errors, enabling either a smaller accelerometer with equivalent performance or a similarly sized one with improved accuracy and noise reduction.
Implementation Method 1
capacitors are formed between each fixed sensing block and the first proof mass for sensing acceleration along two in-plane directions
Implementation Method 2
two differential capacitors with the second proof mass and the third proof mass for sensing the out-of-plane acceleration
Data Source
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AI summary
The present invention provides a high-accuracy low-noise MEMS accelerometer by using at least two symmetric out-of-plane proof masses for both out-of-plane and in-plane axes. Movement of the proof masses in one or more in-plane sense axes is measured by comb capacitors with mirrored comb electrodes that minimise cross-axis error from in-plane movement of the proof mass out of the sense axis of the capacitor. The two out-of-plane proof masses rotate in opposite directions, thus maintaining their combined centre of mass at the centre of the accelerometer even as they rotate out of plane.