Pressure transducers and accelerometers Lecture 6 - PowerPoint PPT Presentation

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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
  • Steady acceleration
  • 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
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