Detailed description of Capacitive MEMS accelerometer

1. An Introduction to
Capacitive MEMS
accelerometer

2. Introduction to Accelerometer
● An accelerometer is an electromechanical device that
measures acceleration forces.
● There are many types of accelerometer available in market
but most populer is MEMS(Micro ElectroMechanical
Systems) based, due to its compactness.
● The first micro machined accelerometer was designed in
1979 at Stanford University, but it took over 15 years
before such devices became accepted mainstream products
for large volume applications.

3. Quick view of MEMS
● Micro ElectroMechanical Systems or MEMS is a term coined around 1989 by Prof. R. Howe
and others to describe an emerging research field, where mechanical elements, like cantilevers or
membranes,had been manufactured at a scale more akin to microelectronics circuit than to lathe
machining.
● MEMS include part of Microelectronics but there is a characteristic that electronic circuits do
not share with MEMS.
● Electronic circuits are inherently solid and compact structures, MEMS have holes, cavity,
channels, cantilevers,membranes, etc, and in some way imitate ‘mechanical’ parts.
● MEMS are not only used in accelerometers but also in pressure sensor, Inertia sensor,
Microfluidic bioMEMS, Optical MEMS etc.

4. Working of MEMS capacitive
Accelerometer
● parallel-plate capacitance
● Due to acceleration we face change in d which result
in change in Cap.
● Change in capacitance is measured using proper
electronics circuitary.
● We also use newtons law F=ma.

5. Working of MEMS capacitive
Accelerometer(Cont..)
● Accelerometer structure

6. Working of MEMS capacitive
Accelerometer(Cont..)
● Electric circuit that measures acceleration through
capacitor changes

7. Working of MEMS capacitive
Accelerometer(Cont..)
● According to Hook's law for an ideal spring
● FS
=kS
x, Fs
=Restoring Force and ks
=Spring constant.
● This law with newtons law gives us following
equation.
● ma=md2
x/dt2
=ks
x

8. Criterions to Select an Accelerometer
● Analog output or digital output,
● Number of Axis,
● Sensitivity to g like 1.5g,2g,6g etc,
● Bandwidth,
● Output Impedence,
● Noise cherecteristics etc.

9. Easily available accelerometer available
in market
● ADXL345 from
Analog Devices
● MMA7361l from
FreeScale

10. For more detailed description watch
following videos
● https://www.youtube.com/watch?v=PcsN-FgA3Ds