Periodic-Bias Self-Test for Double-Differential MEMS Accelerometers
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Solution Overview
Problem
Existing methods for self-testing double differential MEMS accelerometers are inadequate, particularly in robustly detecting mechanical and electrical failures while avoiding mechanical contact between proof masses and stators, especially under external acceleration conditions.
Innovation Solution
A method and state machine for self-testing capacitive MEMS accelerometers with a double differential configuration, using an intermittent bias voltage pattern above the resonance frequency, allowing independent measurement of proof mass capacitance during readout periods to control deflection magnitude and prevent mechanical contact, and a state machine to manage the self-test process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a continuous bias voltage is applied to deflect the proof mass for self-testing, then the deflection magnitude can be maintained, but mechanical contact between the proof mass and stators may occur, especially under external acceleration conditions
Solution Approach 1:
The patent applies periodic action by using an intermittent bias voltage pattern consisting of repeated cycles of bias periods (when deflection is applied) and readout periods (when bias is removed). This periodic application allows the proof mass to be deflected sufficiently for testing while returning to its original position between cycles, preventing mechanical contact with the stators even when external acceleration is present.
2Productivity
If the bias voltage pattern frequency is increased to improve testing speed, then productivity increases, but the tolerance to external acceleration decreases
Solution Approach 1:
The patent utilizes mechanical vibration principles by applying the bias voltage pattern at a frequency above the resonance frequency and above the -3dB bandwidth of the capacitive MEMS accelerometer. This frequency selection ensures that the proof mass responds primarily to the applied bias voltage rather than to external acceleration, allowing high-speed testing while maintaining tolerance to external acceleration conditions.
3Measurement precision
If independent measurement of each proof mass capacitance is performed during readout periods, then measurement precision is improved, but the device complexity increases
Solution Approach 1:
The patent applies segmentation by dividing the self-test process into distinct phases: bias periods when deflection is applied and readout periods when capacitance is measured. During readout periods, the system performs independent measurement of each proof mass capacitance separately, allowing precise measurement while managing complexity through temporal separation of operations.
4Object-affected harmful factors
If the proof mass is allowed to return to rest position between deflection cycles, then mechanical contact is prevented, but the testing duration increases
Solution Approach 1:
The patent resolves this contradiction through periodic action by implementing rapid cyclic testing where the bias voltage is applied and removed in quick succession. The proof mass returns to its rest position between cycles, preventing mechanical contact, while the repeated cycling allows the system to complete multiple test iterations in a relatively short total duration.
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
Ensures robust self-testing with high tolerance to external acceleration, preventing mechanical contact and ensuring accurate deflection control, thereby maintaining reliability and accuracy of MEMS accelerometer performance.
Implementation Method 1
One or more sensing capacitors are used to measure displacement of the proof mass. As the proof mass moves, the distance and thus the capacitance between the proof mass and fixed electrodes changes, allowing the measurement of the acceleration.
Implementation Method 2
Capacitive electrodes, referred to herein also as sensing capacitors, may be used for biasing by applying a voltage over the gap between the capacitive electrodes between the proof mass and the stator or stators that causes a deflection of the proof mass.
Data Source
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AI summary
The present invention relates to a method for self-testing a capacitive MEMS accelerometer comprising a first proof mass and a second proof mass, the first and second proof masses arranged in double differential configuration, a state machine to implement the self-testing method and a MEMS device comprising at least one capacitive MEMS accelerometer. The method applies a bias pattern comprising alternating bias periods and readout periods. Self-test comprises deflection states and return states, and success or failure of each self-test state is determined.