MEMS Vibrating Beam Accelerometer with Pressure Damping Combs
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Solution Overview
Problem
Existing vibrating beam accelerometers face challenges in achieving underdamped resonators while effectively damping the proof mass, leading to issues with bias errors and vibration rectification errors, particularly in the presence of environmental vibrations.
Innovation Solution
The implementation of damping combs with movable and anchored comb fingers in MEMS vibrating beam accelerometers, which provide air damping to the proof mass while configuring the resonators to avoid air damping, allowing for underdamped resonators with high quality factors and damped proof mass motion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If air damping is applied to the resonator to damp proof mass motion, then proof mass damping is improved, but resonator quality factor deteriorates
Solution Approach 1:
The patent divides the damping function into two separate components: damping combs specifically for proof mass damping, and resonator geometry specifically for maintaining high quality factor. This segmentation allows each component to optimize its function without compromising the other.
Solution Approach 2:
The patent applies different geometric characteristics to different parts of the device: the damping combs have closely spaced fingers optimized for air damping of the proof mass, while the resonator has a geometry optimized for minimal air damping and high quality factor. Each local region has properties tailored to its specific function.
2Measurement precision
If resonator quality factor is increased to reduce bias errors, then measurement precision is improved, but proof mass damping deteriorates
Solution Approach 1:
The patent separates the damping function from the resonator structure by introducing dedicated damping combs. This allows the resonator to be optimized for high quality factor and low bias errors, while the damping combs handle the proof mass damping function independently.
Solution Approach 2:
The damping combs act as an intermediary mechanism between the proof mass and the air environment. They provide controlled air damping to the proof mass without directly affecting the resonator's quality factor, thus mediating between the competing requirements of damping and precision.
3Reliability
If damping structures are integrated into movable elements, then proof mass damping is improved, but device complexity increases
Solution Approach 1:
The patent combines the damping function with the existing comb drive structure of the MEMS accelerometer. The damping combs are integrated into the proof mass and anchored to the substrate, utilizing the same fabrication processes and structural approach as the resonator combs, thereby adding damping functionality without significantly increasing overall device complexity.
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 approach enables navigation-grade accelerometers with reduced cost and size, weight, and power (SWaP) while maintaining bias repeatability and minimizing vibration rectification errors.
Implementation Method 1
These damping comb fingers may provide air damping for the proof mass when the MEMS die is placed into a pressure cavity of a package containing a pressure above a vacuum
Implementation Method 2
A resonator may be designed to change frequency proportional to the load applied to the resonator by the proof mass under acceleration. The resonator may be electrically coupled to signal generation circuitry forming an oscillator, which causes the resonator to vibrate, and in some examples at the resonant frequency of the resonator
Data Source
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Figure 3A
AI summary
The disclosure describes techniques to damp the proof mass motion of an accelerometer while achieving an underdamped resonator. In an example of an in-plane micro-electromechanical systems (MEMS) VBA, the proof mass may contain one or more damping combs that include one or more banks of rotor comb fingers attached to the proof mass. The rotor comb fingers may be interdigitated with stator comb fingers that are attached to fixed geometry. These damping comb fingers may provide air damping for the proof mass when the MEMS die is placed into a package containing a pressure above a vacuum. The geometry of the damping combs with a reduced air gap and large overlap area between the rotor comb fingers and stator comb fingers. The geometry of resonator of the VBA of this disclosure may be configured to avoid air damping.