MEMS Accelerometer Sensitivity Calibration via Proof-Mass Dithering
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Solution Overview
Problem
MEMS-based accelerometers face challenges in recalibrating sensitivity over their lifespan due to changes in electromechanical properties, making it difficult to adjust for shifts in sensitivity after installation, as the true input stimuli are not observable.
Innovation Solution
A calibration system and method using proof-mass dithering to continuously calibrate accelerometer sensitivity by driving the proof mass into oscillation, converting capacitance changes into analog signals, demodulating them, and converting these signals into digital sensitivity signals to adjust the gain of the MEMS accelerometer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the accelerometer sensitivity is calibrated using known orientations relative to gravity, then the initial sensitivity can be determined, but the sensitivity cannot be recalibrated after installation because the true input stimuli are not observable
Solution Approach 1:
The patent applies mechanical vibration by driving the proof mass into oscillation at a known drive frequency using electrostatic actuation. The proof mass is made to vibrate sinusoidally, creating a known dynamic stimulus that produces measurable capacitance variations. This allows the system to self-calibrate by observing its own response to a controlled mechanical vibration, eliminating the need for external gravity-based calibration after installation.
Solution Approach 2:
The calibration system performs self-service by using the accelerometer's own proof mass and capacitance sensing mechanism to generate and measure the calibration stimulus. The system drives itself into oscillation and measures its own response, enabling autonomous recalibration without requiring external equipment or observable input stimuli. The accelerometer calibrates itself by exploiting its inherent electrostatic actuation and capacitive sensing capabilities.
2Ease of manufacture
If the sensitivity is calibrated at the supplier using gravity-based orientations, then the calibration can be performed with simple equipment, but the sensitivity drifts over the life of the accelerometer due to changes in electromechanical properties
Solution Approach 1:
The system uses mechanical vibration to create a dynamic calibration stimulus that is independent of gravitational orientation. By oscillating the proof mass at a known frequency and measuring the resulting capacitance variations, the system can determine sensitivity without relying on gravity-based static orientations. This dynamic method compensates for drift in electromechanical properties that occur over the accelerometer's operational life.
Solution Approach 2:
The calibration process employs periodic action by driving the proof mass to oscillate sinusoidally at a specific drive frequency. The periodic nature of the vibration creates repeatable, measurable capacitance variations that can be demodulated to extract sensitivity information. This periodic stimulation allows for continuous or periodic recalibration, maintaining reliability over time by compensating for sensitivity drift through repeated measurements at known intervals.
3Adaptability or versatility
If the proof mass is driven into oscillation to enable continuous calibration, then post-installation recalibration becomes possible, but additional circuitry and processing are required
Solution Approach 1:
The patent applies universality by using the same electrostatic actuation electrodes and capacitance sensing circuitry for both normal accelerometer operation and calibration functions. The existing drive electrodes that normally control the proof mass position are reused to drive the proof mass into oscillation for calibration. Similarly, the capacitance sensing circuitry that measures acceleration-induced displacement is also used to measure the oscillation-induced capacitance variations. This multi-functionality reduces the need for separate dedicated calibration components.
Solution Approach 2:
The system implements feedback by measuring the capacitance variations produced by the proof mass oscillation and using this information to calculate and apply a sensitivity correction factor. The measured capacitance signal is demodulated at the drive frequency to extract the sensitivity information, which then feeds back to adjust the accelerometer's gain or calibration coefficients. This closed-loop feedback mechanism enables automatic recalibration that compensates for sensitivity drift without requiring manual intervention or complex external equipment.
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 calibration of accelerometer sensitivity, allowing for accurate recalibration of sensitivity shifts post-installation, improving the accuracy and reliability of MEMS-based accelerometers in various applications.
Implementation Method 1
a second pair of electrodes configured to drive the proof mass into oscillation in response to a drive signal
Implementation Method 2
a first pair of electrodes coupled to the substrate and configured to sense capacitance variation in response to a deflection of the proof mass
Data Source
AI summary
An in-situ test calibration system and method are disclosed where a perpetual out-of-band electrostatic force induced excitation is used to dither the proof-mass of a MEMS based accelerometer where the amount of deflection change is proportional to sensitivity changes. The supplier of the accelerometer would exercise the accelerometer in a calibration station to determine initial sensitivity values. After the calibration and before removing the accelerometer from the calibration station, the supplier would start the dither and calibrate the acceleration equivalent force (FG) to drive voltage transfer function (FG/V). After installation of the accelerometer into a system or sometime later in the field, any changes in the FG/V transfer function due to changes in the sensitivity are observable and can be used for re-calibrating the accelerometer.