Differential MEMS Accelerometer Self-Test Under Humidity Interference
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Solution Overview
Problem
MEMS accelerometers face challenges in performing accurate self-tests due to moisture penetration, which forms electrolytic cells affecting self-test values and introducing errors.
Innovation Solution
A fully differential MEMS accelerometer design with separate drive signals for each rotor-stator pair, using anti-phase periodic signals and input common mode voltage to eliminate charge sharing and parasitic capacitance effects, allowing self-testing without auto-zeroing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a standard MEMS accelerometer design with shared stators is used, then device complexity is reduced, but self-test accuracy deteriorates due to charge sharing and parasitic capacitance effects from moisture penetration
Solution Approach 1:
The patent divides the stator system into four independent stators (first stator, second stator, third stator, fourth stator), each associated with a specific rotor-stator pair. This segmentation eliminates charge sharing between stators and allows independent drive signals to be applied to each stator, thereby improving self-test accuracy while managing device complexity through modular organization
Solution Approach 2:
The patent introduces separate drive signals (first drive signal, second drive signal, third drive signal, fourth drive signal) as intermediaries between the control circuit and each stator. These intermediary signals enable precise control of each rotor-stator pair, allowing self-test operations without the interference of parasitic capacitance and charge sharing effects
2Measurement precision
If anti-phase periodic drive signals with different voltage swings are applied to rotor-stator pairs, then self-test accuracy is improved by eliminating charge sharing effects, but device complexity increases due to additional drive circuitry
Solution Approach 1:
The patent implements dynamic drive signals with varying voltage swings for different rotor-stator pairs during self-test mode. The first and second drive signals have different voltage swings from the third and fourth drive signals, creating dynamic electrostatic forces that enable independent rotor deflection control. This dynamic signaling approach improves self-test accuracy while the control circuit manages the complexity through coordinated signal generation
Solution Approach 2:
The patent employs periodic anti-phase drive signals to actuate the rotors during self-test operations. The periodic nature of these signals creates controlled oscillatory motion of the rotors, enabling accurate measurement of rotor deflections while eliminating the influence of parasitic capacitance and charge sharing through synchronized periodic actuation
3Ease of manufacture
If moisture penetration occurs in the package, then manufacturing cost is reduced by using non-hermetic molding compound, but measurement precision deteriorates due to electrolytic cell formation and parasitic capacitance
Solution Approach 1:
The patent converts the harmful effect of moisture penetration into a manageable condition by designing a fully differential measurement system with four independent stators. Instead of preventing moisture ingress through expensive hermetic sealing, the invention uses the differential architecture to reject common-mode parasitic capacitance effects, thereby maintaining self-test accuracy despite the presence of moisture and electrolytic cells in the package
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 accurate self-test results by isolating rotor deflections from parasitic capacitance and humidity-induced errors, maintaining precision without requiring auto-zeroing operations.
Implementation Method 1
the drive circuit applies the first stator drive signal to the first stator electrode, the second stator drive signal to the second stator electrode, the third stator drive signal to the third stator electrode, and the fourth stator drive signal to the fourth stator electrode
Implementation Method 2
a first capacitance formed between the first rotor and the first stator electrode; a second capacitance formed between the first rotor and the second stator electrode; a third capacitance formed between the second rotor and the third stator electrode; and a fourth capacitance formed between the second rotor and the fourth stator electrode
Data Source
AI summary
An accelerometer includes first and second stator-electrodes associated with a first-rotor, third and fourth stator-electrodes associated with a second-rotor, a first capacitance formed between the first-rotor and first stator-electrode, a second capacitance formed between the first-rotor and second stator-electrode, a third capacitance formed between the second-rotor and third stator-electrode, and a fourth capacitance formed between the second-rotor and fourth stator-electrode. A drive-circuit generates first and second periodic anti-phase stator drive-signals, and third and fourth periodic anti-phase stator drive-signals. To self-test, the drive-circuit applies the first, second, third, and fourth stator drive-signals to the first, second, third, and fourth stator-electrodes, and drives the first and second stator drive-signals with different voltage-swings but drives the third and fourth stator drive-signals with a same voltage-swing. A capacitance-to-voltage converter has a first-input coupled to the first and second capacitances, and a second-input coupled to the third and fourth capacitances.