Apple Inc. (Cupertino, CA)
What is a Patent for Continuous calibration of the sensitivity of accelerometers through proof-mass dithering
The sensitivity of MEMS based accelerometers is normally determined by the supplier of the accelerometer by using a predetermined sequence of orientations in relation to gravity. But, the sensitivity can be dependent on electromechanical properties which are sensitive to strain and/or change over time. This means that the sensitivity changes after installation into the system of the user or changes over the course of the life of the accelerometer. It is difficult to recalibrate thesensitivity because it is impossible to determine the actual input signals after installation.
A calibration system and method are disclosed for continuous calibration of accelerometer sensitivity through proof-mass dithering. In an embodiment, a method of calibrating accelerometer sensitivity of a micro-electrical-mechanical system (MEMS)accelerometer comprises: driving a proof mass of the MEMS accelerometer into oscillation using a drive signal oscillating at a drive frequency; converting a change in capacitance of the MEMS accelerometer caused by a deflection of the oscillating proofmass into an analog signal; demodulating the analog signal using a reference signal oscillating at the drive frequency, the demodulating generating an analog sensitivity signal that is proportional to the accelerometer sensitivity; converting the analogsensitivity signal into a digital sensitivity signal; and calibrating a gain of the MEMS accelerometer according to the digital sensitivity signal.
In an embodiment, a calibration system for calibrating acceleration sensitivity of a micro-electrical-mechanical system (MEMS) accelerometer, comprises: a substrate; a proof mass coupled to the substrate; a first pair of electrodes coupled tothe substrate and configured to sense capacitance variation in response to a deflection of the proof mass; a second pair of electrodes configured to drive the proof mass into oscillation in response to a drive signal; one of more circuits configured to:generate the drive signal; convert the change in capacitance into an analog signal; demodulate the analog signal using a reference signal oscillating at the drive frequency, the demodulating generating an analog sensitivity signal that is proportional tothe accelerometer sensitivity; convert the analog sensitivity signal into a digital sensitivity signal; and calibrate a gain of the MEMS accelerometer according to the digital sensitivity signal.
In an embodiment, an electronic system comprises: a micro-electrical-mechanical system (MEMS) based accelerometer; a calibration system for calibrating acceleration sensitivity of the MEMS based accelerometer, the calibration system comprising:a substrate; a proof mass coupled to the substrate; a first pair of electrodes coupled to the substrate and configured to sense capacitance variation in response to a deflection of the proof mass; a second pair of electrodes configured to drive the proofmass into oscillation in response to a drive signal; one or more circuits configured to: generate the drive signal; convert the change in capacitance into an analog signal; demodulate the analog signal using a reference signal oscillating at the drivefrequency, the demodulating generating an analog sensitivity signal that is proportional to the accelerometer sensitivity; convert the analog sensitivity signal into a digital sensitivity signal; and calibrate a gain of the MEMS accelerometer accordingto the digital sensitivity signal; one or more processors; memory coupled to the one or more processors and storing instructions that when executed by the one or more processors, cause the one or more processors to perform operations comprising:obtaining acceleration data from the MEMS based accelerometer; calculating a location of the electronic system or a step count using the acceleration data; and providing the location or step count to one or more client applications or electronic systemcomponents.
In an embodiment, a calibration system for calibrating acceleration sensitivity of a micro-electrical-mechanical system (MEMS) based accelerometer, comprises: a positive electrode; a negative electrode; a first capacitance-to-voltage (C2V)converter coupled to the positive electrode and configured to generate a first voltage signal in response to a change of capacitance of the MEMS based accelerometer; a second C2V converter coupled to the negative electrode and configured to generate asecond voltage signal in response to the change of capacitance of the MEMS based accelerometer; a summing amplifier coupled to outputs of the first and second C2V converters and configured to convert the first and second voltage signals into a thirdvoltage signal; a first processing path coupled to the output of the summing amplifier, the first processing path including a first demodulator for demodulating a sense signal from the third voltage signal using a first reference signal having a firstfrequency, a first analog-to-digital converter (ADC) for converting the sense signal to a digital sense signal and a filter configured to attenuate the digital sense signal at a test signal frequency; and a second processing path coupled to the output ofthe summing amplifier, the second processing path including a second demodulator for demodulating a sensitivity signal from the third voltage signal using a second reference signal having a second frequency that is lower than a the first frequency, asecond ADC for converting the sensitivity signal to a digital sensitivity signal, a feed through store configured to store the output of the second ADC and a subtraction node configured to subtract contents of the feed through store from the sensitivitysignal, wherein virtual ground inputs of the first and second C2V converters are coupled to one of two virtual ground voltages by a first switch controlled by a switch signal, and wherein the output of the second ADC is coupled to one of the feed throughstore or the subtraction node in response to the switch signal.
The description below and accompanying drawings provide information about the described implementations. Other characteristics, objects and advantages are apparent from the description, drawings and claims.Click here to view the patent on USPTO website.
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