Capacitive Accelerometer Self-Test via Electrostatic Deflection
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Solution Overview
Problem
Open loop capacitive accelerometers lack a method to test sensitivity during operation, which can lead to incorrect measurements due to mechanical changes, such as damaged electrode fingers or support legs, especially in low-g applications where sensitivity changes can cause unnecessary activation of safety systems like airbags.
Innovation Solution
A method for capacitive accelerometers that includes a second mode of operation where electrostatic forces are applied to the existing sensing structure to force a predictable deflection, allowing for in-situ sensitivity testing without additional transducers or mechanical parts, using a combination of DC offset voltages and PWM drive signals to measure and compare deflections against expected values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If open loop operation is used to simplify design and reduce cost, then device complexity is reduced, but the ability to detect sensitivity changes during use is lost
Solution Approach 1:
The accelerometer uses its existing electrode fingers and proof mass structure to perform self-testing by applying electrostatic forces to itself. The same sensing structure that detects acceleration also serves to measure sensitivity changes through forced deflection testing, eliminating the need for separate testing mechanisms.
Solution Approach 2:
The electrode fingers serve dual purposes: they function as sensing elements during normal acceleration measurement and as actuating elements during sensitivity testing. The fixed and moveable electrode fingers can be used both to detect proof mass displacement and to apply controlled electrostatic forces for testing.
2Measurement precision
If electrostatic forces are applied to test sensitivity, then measurement precision is improved, but the device operates in a different mode requiring additional control
Solution Approach 1:
The sensitivity testing is performed periodically by switching between normal sensing mode and test mode. During test mode, electrostatic forces are applied to the electrode fingers to force the proof mass to a known position, and the resulting displacement is measured and compared to expected values to detect sensitivity changes.
Solution Approach 2:
The system changes operational parameters by switching the voltage applied to the electrode fingers between a constant sensing voltage and a variable testing voltage. The testing voltage is adjusted to produce a forced deflection that corresponds to a known acceleration level, allowing sensitivity verification.
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
Enables continuous detection of sensitivity errors, allowing for timely repair or replacement of accelerometers, reducing the risk of incorrect measurements and ensuring safety by integrating a built-in test within the existing sensing structure without affecting normal operation.
Implementation Method 1
applying electrostatic forces to said first and second sets of fixed capacitive electrode fingers, in order to cause said proof mass including the first and second sets of moveable capacitive electrode fingers to deflect from the null position
Implementation Method 2
electrically measuring a deflection of the proof mass including the moveable capacitive electrode fingers from the null position under an applied acceleration
Data Source
AI summary
In a method for open loop operation of a capacitive accelerometer, a first mode of operation comprises electrically measuring a deflection of a proof mass (204) from the null position under an applied acceleration using a pickoff amplifier (206) set to a reference voltage Vcm. A second mode of operation comprises applying electrostatic forces in order to cause the proof mass (204) to deflect from the null position, and electrically measuring the forced deflection so caused. In the second mode of operation the pickoff amplifier (206) has its input (211) switched from Vcm to Vss, using a reference control circuit (209), so that drive amplifiers (210) can apply different voltages Vdd to the proof mass (204) and associated fixed electrodes (202).