mems silicon mems piezoresistive accelerometer
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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
May, 2008 /© Copyright 2008, Measurement Specialties, Inc. 3
Piezoresistive
AccelerometersPiezoresistivePiezoresistive
AccelerometersAccelerometers
Typical Bonded Strain Gage Sub-Assembly
MEAS Silicon MEMSElement
3 mm x 3 mm
May, 2008 /© Copyright 2008, Measurement Specialties, Inc. 4
Silicon MEMS AccelerometersSilicon MEMS AccelerometersSilicon MEMS Accelerometers
How Silicon MEMS Accelerometers Work
May, 2008 /© Copyright 2008, Measurement Specialties, Inc. 5
Silicon MEMS Accelerometers Silicon MEMS Accelerometers Silicon MEMS Accelerometers
Works Like a Trampoline
MEMS = Micro Electro Mechanical Systems
May, 2008 /© Copyright 2008, Measurement Specialties, Inc. 6
Silicon MEMS Construction:
The four springs are wired in a 4-arm Wheatstone Bridge configuration
Silicon MEMS Accelerometers Silicon MEMS Accelerometers Silicon MEMS Accelerometers
Piezoresistive
gages sense changes in stress and strain in the hinges (springs in the trampoline) under acceleration
May, 2008 /© Copyright 2008, Measurement Specialties, Inc. 7
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
Silicon MEMS Accelerometers Silicon MEMS Accelerometers Silicon MEMS Accelerometers
Piezoresistive
gages are built into the hinges or springs of the trampoline
Thin air gap: Squeeze Film Damping
May, 2008 /© Copyright 2008, Measurement Specialties, Inc. 8
Squeeze-Film Gas Damping
Extremely small airgaps
between the moving seismic mass (in the middle) and the top/bottom caps facilitate squeeze film damping.
Silicon MEMS Accelerometers Silicon MEMS Accelerometers Silicon MEMS Accelerometers
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
May, 2008 /© Copyright 2008, Measurement Specialties, Inc. 9
Silicon MEMS Accelerometers Silicon MEMS Accelerometers Silicon MEMS Accelerometers
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
May, 2008 /© Copyright 2008, Measurement Specialties, Inc. 10
Silicon MEMS Accelerometers Silicon MEMS Accelerometers Silicon MEMS Accelerometers
What are the Benefits?
May, 2008 /© Copyright 2008, Measurement Specialties, Inc. 11
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
May, 2008 /© Copyright 2008, Measurement Specialties, Inc. 12
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’
MEMS Advantages MEASMEAS’’
MEMS Advantages MEMS Advantages
Benefit: Resonance Control & Broad Response
May, 2008 /© Copyright 2008, Measurement Specialties, Inc. 13
MEAS’
MEMS Advantages MEASMEAS’’
MEMS Advantages MEMS Advantages
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
May, 2008 /© Copyright 2008, Measurement Specialties, Inc. 14
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’
MEMS Advantages MEASMEAS’’
MEMS Advantages MEMS Advantages
May, 2008 /© Copyright 2008, Measurement Specialties, Inc. 15
Compared to capacitive designs, MEAS’s silicon MEMS piezoresistive accelerometers offer much broader mesurement range and unmatched
amplitude linearity.
MEAS’
MEMS Advantages MEASMEAS’’
MEMS Advantages MEMS Advantages
Benefit: Excellent Dynamic Range and Linearity
Linearity of a Typical 2000g MEMS Accelerometer
May, 2008 /© Copyright 2008, Measurement Specialties, Inc. 16
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’
MEMS Advantages MEASMEAS’’
MEMS Advantages MEMS Advantages
Benefit: Shorter Warm-Up Time
May, 2008 /© Copyright 2008, Measurement Specialties, Inc. 17
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’
MEMS Advantages MEASMEAS’’
MEMS Advantages MEMS Advantages
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
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