Electronic Instrumentation - Rensselaer Polytechnic Institute
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Transcript Electronic Instrumentation - Rensselaer Polytechnic Institute
Electronic Instrumentation
Project 1
•1. Configuring an Analog Devices Accelerometer
•2. Finding Acceleration using the Strain Gauge and Coil Outputs
•3. Real Time Measurement
•4. Project Write Up
•5. Practical Questions
Cantilever Beam Sensors
• Position Measurement – obtained from
the strain gauge
• Velocity Measurement – obtained from
the magnetic pickup coil
• Acceleration Measurement – obtained
from the Analog Devices accelerometer
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Sensor Signals
• The 3 signals
x xoe
t
cos t
• Velocity
dx
v
dt
• Acceleration
d 2x
a 2
dt
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100
80
60
40
20
0
-20
-40
-60
-80
-100
-120
0
0.
03
0.
05
0.
08
0.
1
0.
13
0.
15
0.
18
0.
2
0.
23
0.
25
• Position
Comparison of Position, Velocity and Acceleration
Acceleration
200 Times Velocity
15000 Times Position
Time
2 of the 3 plots must be scaled to see them
on the same figure.
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Basic Steps for Project
•
•
•
•
Build the accelerometer circuit
Mount it close to the end of the beam
Calibrate the position and velocity sensors
Measure position, velocity and acceleration, 2
channels at a time
• Determine the mathematical representations
for x, v, and a.
• Demonstrate that the 3 expressions are
consistent.
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Optional
• Build circuits that do the math
• Calibrate the circuits
• Record the 3 signals plus the
appropriate mathematical operations on
the signals
• Demonstrate that all signals are
consistent
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Building the
Accelerometer Circuit
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The Analog Device Accelerometer
• The AD Accelerometer is an excellent
example of a MEMS device in which a large
number of very, very small cantilever beams
are used to measure acceleration. A
simplified view of a beam is shown here.
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Accelerometer Circuit
Op Amp Circuit
Accelerometer Chip
• The AD chip produces a signal
proportional to acceleration
• The Op Amp amplifies the signal
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Accelerometer Circuit
• The ADXL150 is surface mounted, so
we must use a surfboard to connect it to
a protoboard
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Improved Op Amp
• We use a Maxim 473 Op Amp to
improve performance
• This device requires only ground and
5V (rail voltages)
• The output can scan from rail-to-rail
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Part Costs
• Maxim MAX-473 Single Supply 10MHz
Op Amp ($3.58 from Digi-Key)
• Compare with LM741 Dual Supply Op
Amp ($0.22 from Electronix Express)
• Analog ADXL150 Accelerometer $13.70
• Note that these are all single part costs.
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Caution
• Please be very careful with the
accelerometers. While they can stand
quite large g forces, they are electrically
fragile. If you apply the wrong voltages
to them, they will be ruined. AD is
generous with these devices (you can
obtain samples too), but we receive a
limited number each year.
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Extra Protoboard
• You will be given a small protoboard on
which you will construct your
accelerometer circuit.
• Keep your circuit intact until you
complete the project.
• Return the accelerometer surfboard at
the end of each class
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Mounting the Accelerometer
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Mount the Accelerometer Near
the End of the Beam
• Place the small protoboard as close to
the magnetic sensor as possible
• The axis of the accelerometer needs to
be vertical
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Accelerometer Signal
• The output from the accelerometer circuit
is
Rf
Ri
38mV per g, where g is the
acceleration of gravity, Rf is the feedback resistor
and Ri is the input resistor for the Op Amp.
• In the equations below, the units are in [ ]
a(t )[m / s 2 ]
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Va (t )[m V]
38[m V] R f
2
9.8[m / s ] Ri
or a(t )[G ]
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Va (t )[m V]
38[m V] R f
[G ]
Ri
16
Calibrate the Position and
Velocity Sensors
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Position Measurement Using
the Strain Gauge
Vsg (t ) Csg xb (t )
• Set up the strain gauge circuit you used in earlier
experiments
• Place a ruler near the end of the beam
• Make several measurements of bridge output voltage
and beam position
• Find a simple linear relationship between voltage and
beam position
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Velocity Measurement Using
the Magnetic Pickup Coil
• From Maxwell’s Equations, the voltage induced in a coil
due to a moving magnetic field is given by
d BNA
dB
dB dx
dB
V
NA
NA
NAv
dt
dt
dx dt
dx
where v is velocity, B is magnetic field, N is the number
of turns in the coil, and A is the area of the coil.
Simplifying
Vcoil (t ) Ccoilvb (t )
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Velocity Measurement
• For small deflections, the change in the
magnetic field with position is roughly
constant, so the voltage is proportional
to the beam velocity.
• For large deflections, you should notice
that the voltage will not look like a
decaying sinusoid
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Velocity Measurement
• There is no simple direct way to calibrate the
velocity measurement
• However, it can be calibrated by comparing it
to the position measurement
• To facilitate this comparison, we adjust the
amplification of the bridge output until the
strain gauge and pickup coil voltages are
about the same size
• Recall that these signals should be out of
phase
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Comparison of x and v signals
• Use the Lissajou pattern approach to
adjust the strain gauge amplifier output
to be comparable to that of the pickup
coil
• You must use the same voltage scales
for both ‘scope channels for this
comparison to be valid
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Calibrating the Velocity
Measurement
• Note, all measurements of position and
velocity must be taken with the accelerometer
board installed so that the same conditions
hold for all measurements
• Measure the strain gauge and coil outputs
simultaneously
• Capture these signals in Excel using the
Waveform option of Agilent Intuilink
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Calibrating the Velocity Measurement
• The x and v signals are decaying sinusoids.
Vsg (t ) Asg sin( t )
Vcoil (t ) Acoil cos( t )
• We know the calibration for x Vsg (t ) Csg xb (t )
• And we know that
dxb (t )
d sin( t )
vb (t )
vb (t ) Csg Asg
dt
dt
vb (t ) Csg Asg cos( t )
• The matched amplitudes, Asg=Acoil gives us
vb (t ) Csg Acoil cos( t ) Csg Vcoil (t )
• Details here: Proj1_Help.PDF
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Acceleration
• When both x and v are calibrated, it is possible to find
the acceleration a(t) by taking derivatives of these
expressions
• You can either fit a decaying sinusoid to the signals
and take the derivatives mathematically or take the
derivatives of the data directly with Excel
• Record the accelerometer signal at the same time as
the coil, use the calibration factors you have found to
adjust both signals, take the derivative of the coil
signal and compare the two resulting curves.
• Record the accelerometer signal at the same time as
the strain gauge, use the calibration factors you have
found to adjust both signals, take the second
derivative of the strain gauge signal and compare the
two resulting curves.
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Real Time Measurement
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Analog Differentiator
• It is possible to differentiate a signal
using either a passive or active
differentiator.
• Passive Differentiator
Vin
VR
I
R
Vout
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Vout
C
dVC
IC
dt
dVC
dVin
V R RC
RC
dt
dt
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R
0
1
RC
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Analog Differentiator
• Active Differentiator
R1
2
-
V-
C1
Vin
OS1
1n
3
+
V+
OUT
OS2
1
6
Vout
5
7
Vout
dVin
RC
dt
4
1k
0
• Note that there is no frequency limit
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Project Report
• Introduction
• Application Goals
• Educational Goals
• Design
•
•
•
•
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Component data, both measured and researched
Full circuit diagram
Testing plan
Have plan checked out
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Project Report
• Analysis
• PSpice simulation of exact circuit
• Hand calculations where appropriate
• Calibrate position and velocity
measurements
• Implementation
• What went wrong?
• Two lessons learned
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Project Report
• Final Design and Testing
• Demonstrate that x and v signals are comparable
in amplitude
• Complete description of final design
• Demonstrate that two methods for finding
acceleration are consistent using your testing plan
• Get data checked off
• Discussion
• How good are your results?
• Sources of error
• What types of accelerations could the “cantilever
beam accelerometer” be used to measure?
• Answer random questions in slide 39
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Project Report
• Personal Responsibilities
• Make a list of all tasks to be completed as
part of this project
• Testing plan
• Keeping everyone on task
• Assign responsibility for each task to one
person (tasks cannot be shared)
• Have task assignment list checked out
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Appendices
• Useful data or results from experiments
• Information resources
• From the web
• From the library or other sources
• Only attach useful information
• Useless information will result is a loss of
points
• Explain the purpose of each piece of info
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Using the Slope Function
• Given an array of points with x in A1:An
and y in B1:Bn
• Find 3 pt: In cell C1, put the formula
slope(B1:B3,A1:A3) (May need more
than 3.)
• Now copy this formula in the rest of C.
• Graph the data to compare
• An example is posted on the web page
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Using a Mathematical Model
• v(t) = A e-at sin(t) =2pf
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Typical
Acceleration
• Compare your
results with
typical
acceleration
values you
can
experience.
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Elevator (fast service)
0.3 g
Automobile (take off)
0.1-0.5g
Automobile (brake or corner) 0.6-1 g
Automobile (racing)
1-2.5 g
aircraft take off
0.5 g
Earth (free-fall)
1g
Space Shuttle (take off)
3g
parachute landing
3.5 g
Plop down in chair
10 g
30 mph car crash w airbag
60 g
football tackle
40 g
seat ejection (jet)
100 g
jumping flea
200 g
high speed car crash
700 g
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Some Random Questions
• How would you use some of the
accelerometer signals in your car to
enhance your driving experience?
• If there are so many accelerometers in
present day cars, why is acceleration
not displayed for the driver? (If you find
a car with one, let us know.)
• If you had a portable accelerometer,
what would you do with it?
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Senior Project from Illinois
• https://courses.ece.uiuc.edu/ece345/projects/
spring2001/project22_presentation.ppt
• Objective
• To create a portable device that monitors the
performance of an automobile.
• Device to use only acceleration and a microcontroller to derive all performance data.
• Device to be powered by cigarette lighter in
vehicle.
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Airbags
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Crash Test Data
Ballpark Calc:
56.6mph = 25.3m/s
Stopping in 0.1 s
Acceleration is about
-253 m/s2 = -25.8 g
• Head on crash at 56.6 mph
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Crash Test Data
Ballpark Calc:
112.1mph = 50.1 m/s
Stopping in 0.1 s
Acceleration is about
-501 m/s2 = -51.1 g
• Head on crash at 112.1 mph
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Crash Test Analysis Software
• Software can be downloaded from
NHTSA website
• http://wwwnrd.nhtsa.dot.gov/software/load-cellanalysis/index.htm
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Crash Videos
• http://www.sph.emory.edu/CIC/CLIPS/mvcras
h.html
• http://www.mazda6.de/de/upclose/overview/s
afety.asp
• http://www.arasvo.com/crown_victoria/cv_mo
vies.htm
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Airbags
• Several types of accelerometers are used &
at least 2 must sense excessive acceleration
to trigger the airbag.
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