Capacitive Accelerometer Damping Fingers for Vibration Rectification
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Solution Overview
Problem
Capacitive accelerometers face challenges with residual motion and vibration rectification error due to non-linear response and limited damping, leading to unwanted bias voltages and reduced operational range, especially under shock accelerations.
Innovation Solution
Incorporating dedicated sets of interdigitated damping fingers that are electrically common and provide enhanced squeeze film damping, separate from the capacitive electrode fingers, allowing for reduced residual motion and increased operational range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the interdigitated capacitive electrode fingers are made with smaller spacing to increase squeeze film damping, then the damping effect is improved, but the fingers become too close and risk touching down under acceleration
Solution Approach 1:
The capacitive sensing function is separated from the damping function. The capacitive electrode fingers maintain their original spacing for sensing, while separate damping fingers are introduced with smaller spacing to provide enhanced squeeze film damping without compromising the capacitive measurement capability.
Solution Approach 2:
Damping fingers are introduced as an intermediary element between the capacitive electrode fingers and the proof mass. These damping fingers provide the necessary damping effect through their closer spacing while the capacitive fingers remain spaced appropriately for electrical sensing, with the damping fingers acting as a mechanical mediator.
2Stability of the object's composition
If the capacitive electrode fingers are made stiffer to prevent deformation under applied voltage, then the structural stability is improved, but the squeeze film damping effectiveness is reduced
Solution Approach 1:
The structural support function is separated from the damping function. The capacitive electrode fingers can be made stiffer for structural stability without compromising damping, as the damping function is transferred to the separate damping fingers that are optimized for flexibility and damping effectiveness.
Solution Approach 2:
Damping fingers serve as an intermediary that provides the flexibility needed for effective squeeze film damping, while the capacitive electrode fingers maintain their stiffness for structural stability. The damping fingers mediate between the rigid capacitive structure and the flexible damping requirement.
3Adaptability or versatility
If the proof mass is allowed to move freely to increase dynamic range, then the operational range is improved, but residual motion increases causing vibration rectification error
Solution Approach 1:
Damping fingers are introduced as an intermediary damping mechanism between the proof mass and the fixed substrate. These fingers provide passive viscous damping that reduces residual motion and vibration rectification error while allowing the proof mass to maintain its full dynamic range of motion.
Solution Approach 2:
The damping fingers utilize squeeze film damping, a pneumatic/hydrodynamic effect where the gaseous or liquid medium between the moving damping fingers and fixed structures provides viscous damping. This passive fluid-based damping reduces residual motion without mechanical contact, maintaining dynamic range while improving measurement precision.
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
The solution significantly reduces residual motion and vibration rectification error, enhancing the operational range and stability of capacitive accelerometers by increasing the damping factor and preventing electrode finger contact, thus improving the accuracy and reliability of acceleration measurements.
Implementation Method 1
an effect known as squeeze film damping takes place, wherein a gaseous medium between the electrode fingers damps the motion of the fingers due to the viscosity of the gaseous medium
Data Source
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AI summary
A capacitive accelerometer (202) comprises: a substantially planar proof mass (204) mounted to a fixed substrate by flexible support legs (250) so as to be linearly moveable in an in-plane sensing direction (200). The proof mass comprises first and second sets of moveable capacitive electrode fingers. First and second sets of fixed capacitive electrode fingers interdigitates with the first and second sets of moveable electrode fingers respectively (221, 222). A set of moveable damping fingers (224) extend from the proof mass substantially perpendicular to the sensing direction, laterally spaced in the sensing direction. A set of fixed damping fingers (222) mounted to the fixed substrate interdigitates with the set of moveable damping fingers and comprises an electrical connection (260) to the proof mass so that the interdigitated damping fingers (228, 230) are electrically common. The damping fingers are mounted in a gaseous medium that provides a damping effect.