Capacitive Accelerometer Pull-In Electrode Damping
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current MEMS capacitive accelerometers face limitations in achieving low resonance frequency and high sensitivity due to the aspect ratio of trenches in silicon substrates, which restricts squeeze damping and noise performance.
Innovation Solution
Incorporating a further set of interdigitated capacitive electrode fingers acting as 'pull-in' fingers, reducing the lateral spacing between electrode fingers through applied voltage, thereby increasing damping and allowing for a lower resonance frequency while maintaining stability in closed-loop operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If the lateral spacing between electrode fingers is reduced to increase damping, then the damping factor increases and resonance frequency decreases, but the manufacturing precision required increases due to the smaller gaps
Solution Approach 1:
The patent applies a preliminary electrostatic force using fixed capacitor electrodes to pull the movable proof mass and electrode fingers closer to the fixed electrodes before the device operates. This pre-positioning action reduces the lateral spacing between electrode fingers from the originally larger gap (e.g., 10 microns) to a smaller optimized gap (e.g., 2-5 microns), thereby increasing the damping factor without requiring the manufacturing precision to directly create such small gaps during fabrication. The electrostatic pull-in effect is activated by applying a DC voltage to the fixed capacitor electrodes, which creates an attractive force that draws the movable components into the desired closer position.
2Measurement precision
If the resonance frequency is reduced to improve sensitivity, then the bias stability improves, but the damping factor decreases making it difficult to maintain stability
Solution Approach 1:
The patent employs fixed capacitor electrodes that apply an electrostatic pull-in force to the movable proof mass and electrode fingers, preliminarily reducing the lateral spacing between them. This pre-established smaller gap (e.g., reducing from 10 microns to 2-5 microns) increases the air squeeze film damping effect, providing sufficient damping factor even when the resonance frequency is reduced to 100 Hz or lower. The increased damping compensates for the potential instability that would otherwise occur at low resonance frequencies, enabling the device to achieve both high sensitivity and stability simultaneously.
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 results in a 27 times increase in damping factor, enabling a resonance frequency as low as 100 Hz, significantly improving sensitivity and bias stability, and reducing noise from 20 micro-g/root Hz to 25 nano-g/root Hz.
Implementation Method 1
a gaseous medium trapped inside the device provides squeeze damping for the proof mass when it moves in a sensing direction in response to an acceleration being applied
Implementation Method 2
There is typically provided a set of fixed electrodes and a set of movable electrodes attached to the proof mass, with differential capacitance between the electrodes being measured so as to detect deflection of proof mass
Implementation Method 3
electromagnetic force feedback accelerometers with closed loop electronics use a very low resonance frequency sensor which has a high sensitivity
Data Source
Figure 1a
Figure 1b
Figure 2
AI summary
A capacitive accelerometer including: at least one additional fixed capacitor electrode with a plurality of additional fixed capacitive electrode fingers extending along the sensing direction. The proof mass comprises a plurality of moveable capacitive electrode fingers extending from the proof mass along the sensing direction and arranged to interdigitate with the plurality of additional fixed capacitive electrode fingers of the at least one additional fixed capacitor electrode. A means is provided for applying a voltage to the at least one additional fixed capacitor electrode to apply an electrostatic force to the plurality of moveable capacitive electrode fingers that acts to pull the proof mass towards the at least one further fixed capacitor electrode and thereby reduces the lateral spacings between the movable capacitive electrode fingers of the proof mass and the first and second sets of fixed capacitive electrode fingers that provide electrostatic forces for sensing purposes.