PPx3.Accelerometer

7/25/2019 PPx3.Accelerometer

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Accelerometer


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29/10/2013 HET232 Embedded Microcontrollers 2

Physical Structure

The sensor is a micro-machined structure created in silicon. Ithas a moveable element that is deflected by gravity oracceleration. Its position is measured by having the moveableelement form the centre component in three-element capacitor.The capacitance of the two capacitors formed changes as thecentral element is deflected.

The capacitors are incorporated into a switched capacitormeasurement system that allows these changes to be convertedinto a voltage.

This means that the accelerometer may be used to measureacceleration or the component of gravitational force in one axis

Stationary Plates

Moveable Platedeflected by acceleration

C1

C2

Sensitive Axis

D

AC


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Physical Structure

Micro-machined in silicon very shallow 3-Dstructure

Flexible tethers

Measurement

Acceleration

Suspended Mass

Measurement


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Self-test

Accelerometers are often incorporatedin critical applications e.g. car air-bagsensors.

Need mechanicalself-test function. This can be achieved by

electrostatically deflecting the beam!

This tests the entire system includingthe mechanical elements.


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Output Choices

Internally, the accelerometer produces ananalogue voltage that represents theacceleration.

For some accelerometer (e.g. MMA1260D)

this voltage is amplified and becomes theoutput of the sensor.

Other accelerometers (e.g. AD702A) may

use a pulsed digital output where the periodis related to the acceleration.


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Applications

Car Airbag sensors Pedometers

Measurement of vibration in machines.

Novelty input devices e.g. a mobile phonethat shuts up when you tap or shake it!

Inclinometers (measures gravity component).

Inertial guidance systems.

Computer game controllers (Wii, PS3)


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Measuring Tilt using Gravity

Measures Force in thisdirection

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Documents

PPx3.Accelerometer