MEMS Silicon MEMS Piezoresistive Accelerometer

MEAS Silicon MEMS Piezoresistive

Accelerometer

and its Benefits

May, 2008 /© Copyright 2008, Measurement Specialties, Inc. 2

Piezoresistive

AccelerometersPiezoresistivePiezoresistive

AccelerometersAccelerometers

1.

Bonded Strain Gage type

(Gages bonded to metal seismic mass using epoxy)

•

Undamped

circa 1950’s

•

Fluid (oil) damped

circa 1960’s

2.

Silicon MEMS type

(Gages defused into single piece of silicon)

•

Undamped

circa 1980’s

•

Gas damped

circa 1990’s


Piezoresistive

AccelerometersPiezoresistivePiezoresistive

AccelerometersAccelerometers

Typical Bonded Strain Gage Sub-Assembly

MEAS Silicon MEMSElement

3 mm x 3 mm


Silicon MEMS AccelerometersSilicon MEMS AccelerometersSilicon MEMS Accelerometers

How Silicon MEMS Accelerometers Work


Silicon MEMS Accelerometers Silicon MEMS Accelerometers Silicon MEMS Accelerometers

Works Like a Trampoline

MEMS = Micro Electro Mechanical Systems


Silicon MEMS Construction:

The four springs are wired in a 4-arm Wheatstone Bridge configuration


Piezoresistive

gages sense changes in stress and strain in the hinges (springs in the trampoline) under acceleration


Silicon MEMS Construction, Cross-sectional View

MASS

Mechanical stops to prevent overstress

Bottom Cap

Top Cap

Dir

ecti

on o

f ac

cele

rati

on


Piezoresistive

gages are built into the hinges or springs of the trampoline

Thin air gap: Squeeze Film Damping


Squeeze-Film Gas Damping

Extremely small airgaps

between the moving seismic mass (in the middle) and the top/bottom caps facilitate squeeze film damping.


Damping is obtained from the integrated pressure profile at the surface of the sensor cell. Airgap

thickness controls the effective damping ratio of the device.

cross sectional view top view

Dire

ctio

n of

acc

eler

atio

n



Typical Damping Ratios Using Squeeze-Film TechniqueAMPLITUDE RESPONSE, MODEL 64 SERIES

-30

-25

-20

-15

-10

-5

0

5

10

15

20

25

30

100.00 1000.00 10000.00 100000.00

Frequency [Hz]

Dev

iatio

n in

dB 2000g, Q=0.05

2000g, Q=0.31000g, Q=0.4500g, Q=0.450g, Q=0.5

Q=0.05Q=0.3Q=0.4Q=0.4Q=0.5



What are the Benefits?


Patented double-cantilever webbed design provides low off-axis sensitivity with minimum response to cross and rotation acceleration commonly found in dynamic testing.

MEAS’

MEMS Advantages MEASMEAS’’

MEMS Advantages MEMS Advantages

Benefit: Low Off-Axis Sensitivity

Sensor design with high rotational & translational stiffness

Silicon MEMS sensor element


MEAS’s MEMS accelerometers offers the optimal amount of damping to achieve maximum resonance control when the sensor is exposed shock impact, and still maintains the broadest frequency response required by the various industry regulations, such as SAE-J211 and ISO-6487.

MEAS’



Benefit: Resonance Control & Broad Response


MEAS’



9000

6000

3000

0g

-3000

-6000

-9000

0 5 10 15 20mS

--

Undamped--

Damped

Undamped accelerometer resonated at its natural frequency after exposed to shock impulses

Example of Resonance Control in Damped Accelerometer


Compared to a fluid damped design in which damping characteristics changes dramatically with fluid viscosity at various temperature, frequency response of MEAS’s gas damped accelerometer is not affected by temperature.

Benefit: Stable Response over Temperature

-75%

-50%

-25%

0%

25%

50%

75%

100%

125%

150%

175%

200%1 10 100 1,000

FREQ (HZ)

AM

P D

EV (%

)

+25°C BOTH-40°C GAS-40°C FLUID+100°C GAS+100°C FLUID

MEAS’




Compared to capacitive designs, MEAS’s silicon MEMS piezoresistive accelerometers offer much broader mesurement range and unmatched

amplitude linearity.

MEAS’



Benefit: Excellent Dynamic Range and Linearity

Linearity of a Typical 2000g MEMS Accelerometer


After power is applied

to the sensor, the

zero offset of the accelerometer stabilizes as the heat generated by the gages reaches an equilibrium. In a MEAS design, the thermal imbalance in the full bridge configuration is kept to a minimum due to the uniform heating in all 4 active silicon gages. Competitor’s design using half-bridge configuration (2 active gages and 2 completion resistors) produces thermal imbalance due to uneven the heating characteristics that take much longer to reache an equilibrium.

MEAS’



Benefit: Shorter Warm-Up Time


With higher gage impedance, MEAS’s design

comsumes considerably less power than competitor’s sensor, hence lower heat dissipation. Lower heat dissipation translates into higher thermal stability and faster warm-

up time.

MEAS’



Benefit: Better Thermal Stability

Zero Output Stability, One Hour after Power-On

-6.5

-6.3

-6.1

-5.9

-5.7

-5.5

-5.3

0 500 1000 1500 2000 2500 3000 3500 4000

time, seconds

volta

ge, m

V

Documents

MEMS Silicon MEMS Piezoresistive Accelerometer