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Pressure transducers and accelerometers Lecture 6

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(1) Calibrate pressure transducer. Compare calibration with ... Air speed. Brake pressure. Coolant, fuel and oil pressure. Steering system pressure. Sports ... – PowerPoint PPT presentation

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Title: Pressure transducers and accelerometers Lecture 6

1
Pressure transducers and accelerometersLecture
6
• Oct. 30, 2006

2
Objectives
• (1) Calibrate pressure transducer
• Compare calibration with published data
• (2) Calibrate accelerometer
• Static mode
• (3) Compare spring constant values calculated
from
• Static method
• F kx and
• Dynamic method (using accelerometer)
• ? (k/m)1/2

3
Uses of pressure transducers
• To monitor
• Air bag systems
• Air speed
• Brake pressure
• Coolant, fuel and oil pressure
• Steering system pressure
• Sports

4
Variation of pressure with depth
• Hydrostatic pressure increases with increasing
submersion depth
• Weight of the column W mg rAgh
• r density of fluid
• Weight is supported by the net pressure (P-Po)A
• Then P Po rgh
• Pressure increases with depth
• All points at a given depth are
• at the same pressure

PoA
h
PA
5
Strain gages - introduction
• The resistance of a metal changes with a change
in length (strain)
• Hookes law states
• E s/ e
• Where E elastic modulus (constant for a given
material), s stress and e strain
• Strain e D ? / ? o
• D ? / ? o is the proportional change in length

6
Strain gages - introduction
• Resistance R of a metal given by
• R C(? /A)
• C material constant
• A cross-sectional area
• ? length
• The dR/R d?/ ?
• DR CoD ? / ? o Coe
• Strain gages can be used to measure small
displacements

7
Strain gage pressure transducer
• Pressure increases length of wire
• Change in resistance
• Measured and correlated with pressure

Deflection of diaphragm sensed with strain gages
8
Piezoelectric pressure transducers
• Piezo (pressure) electric materials
• Dielectric material
• Alternating and - charge
• Deformation causes charge to build up on metal
plates

Charge dxF d dielectric constant of the
material
9
Piezoelectric pressure transducers
• The piezoelectric material is diffused onto a
silicon diaphragm
• 100X more sensitive than wire strain gage
transducers
• As diaphragm flexes resistance changes, measured
by a bridge circuit

10
(1) In lab - pressure calibration
• Record voltage for Dh 0 to 100 cm

amp
11
In lab - pressure calibration
Do not include gain when calculating
sensitivity Take data every 5 cm
12
In lab - pressure calibration
GAIN 100 red jumper
pressure transducer
Output (DMM)
Amplifier is used because signal small
13
In lab - pressure calibration - electrical
connections
power to pressure transducer
DMM
GAIN 100
DMM
signal from pressure transducer
14
Calibration curve
• Sensitivity volts/cm-H2O
• Plot volts vs cm H2O
• Compare experimental data with data available on
the web
• You are using an Omega PX36 pressure transducer

volts

Dh, cm
15
In lab - measure hysteresis
• Determine the hysteresis for your set-up
• Plot volts vs. cm-H2O steadily increasing and
then decreasing the height
• Or vice versa

16
Accelerometers
• An accelerometer is a device for measuring the
change in velocity of an object (acceleration).
• An accelerometer inherently measures its own
motion

17
Some measures of acceleration
• Good cars can decelerate at 1 g
• 100 km/hr - 0 in 3s
• Race cars can accelerate at 4 g's
• 0 - 100 km/hr in under a second.
• A truck hitting the gravel of a runaway truck
ramp
• decelerate at 3 g's
• A car hitting a set of sand barrels
• decelerate at 5 g's
• Fighter pilots
• withstand 5 gs without blacking out

1 g 9.8 m/sec2
18
Applications for accelerometers
• Car air bags
• Inertial guidance systems
• Vibration sensing
• Footpods
• Runner feedback pace, distance, etc.
• Recently Ipod nano can give audio feedback
• Detection if a laptop is falling
• Automatically moves read/write head away from
inner drive - protect data
• Measurement of gravitational force
• Anti-theft devices

19
How to measure acceleration - simple harmonic
motion
k spring constant m mass
Acceleration d2y/dt2 -w2y Acceleration a
linear function of displacement
20
How to measure acceleration - pendulum motion
Fnet -mgsinq ma -mgsinq a -gsinq
Two simple motions spring pendulum show
that the displacement of an object can be used to
determine acceleration
21
Types of accelerometers
• Strain gage
• Lowest cost, least sensitive
• Piezoelectric
• MEMs
• Micro electrical mechanical system
• This is the accelerometer you will use in the lab

22
Strain gage accelerometers
• Acceleration detected by bending of the
cantilever beam
• Displacement measured by strain gages
• Wheatstone bridge formed with other gages
• Applications for
• Low cost alternative

23
Piezoelectric accelerometers
• Same principle as with piezoelectric pressure
transducers
• Pressure causes material to deform
• Voltage proportional to charge on surface of
metal plates

24
MEMs accelerometer
• A typical MEMS accelerometer uses a silicon mass
suspended by a silicon beam.
• Mechanically equivalent to a spring mass damper
of a traditional mechanical accelerometer.
• Capacitive sensing is used to measure the motion
of the mass.

hair 80µm
25
Lab accelerometer (MEMs device)
2.5 cm
This accelerometer has a built in amplifier
26
(2) In lab - static calibration of accelerometer
• Accelerometer connected to a plate that can sweep
out 0-90
• The force on the accelerometer depends on the
angle
• Wires are connected to a Wheatstone bridge
circuit

27
In lab - static calibration of accelerometer
to DMM
Take data for increasing and decreasing
angles Plot Volts vs. cosq
volts
q
28
(3) Determination of spring constant
N kgm/s2
• Mass on pan connected to spring
• Use Hookes law
• F kx
• k spring constant (N/m)
• Want k in mks units only

29
In lab - spring constant static method
Use 3 different weights to calculate k
NOTE To convert mass to force in N, multiply
the weight (in kg) by 9.8 m2/sec
30
In lab - spring constant dynamic method
• (k/m)1/2 ? 2?/T
• k ?2m (2?/T)2m
• 50 N/m

w 810 gms
The period, T, measured directly from graph T
changes with different weight
T 0.8 sec
Use 3 different weights to calculate k
31
Next week
• Using strain gages on an aluminum alloy
cantilever beam
• Determine deflection under a load
• Calculate elastic modulus (E)
• Calculate Poissons ratio (n)
• Compare to published values
• Aluminum alloy composition and heat treatment is
2024-T4