1. 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

2. 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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ENGR Electronic Instrumentation

3. ENGR-4300 Electronic Instrumentation
Sensor Signals
The 3 signals
Position
Velocity
Acceleration
2 of the 3 plots must be scaled to see them on the same figure.
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ENGR Electronic Instrumentation

4. 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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ENGR Electronic Instrumentation

5. ENGR-4300 Electronic Instrumentation
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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ENGR Electronic Instrumentation

6. Building the Accelerometer Circuit
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7. 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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ENGR-4300 Electronic Instrumentation

8. 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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ENGR Electronic Instrumentation

9. Accelerometer Circuit
The ADXL150 is surface mounted, so we must use a surfboard to connect it to a protoboard
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10. ENGR-4300 Electronic Instrumentation
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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ENGR Electronic Instrumentation

11. ENGR-4300 Electronic Instrumentation
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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ENGR Electronic Instrumentation

12. ENGR-4300 Electronic Instrumentation
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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ENGR Electronic Instrumentation

13. ENGR-4300 Electronic Instrumentation
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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14. Mounting the Accelerometer
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15. 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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ENGR Electronic Instrumentation

16. ENGR-4300 Electronic Instrumentation
Accelerometer Signal
The output from the accelerometer circuit
is 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 [ ]
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17. Calibrate the Position and Velocity Sensors
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18. Position Measurement Using the Strain Gauge
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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ENGR Electronic Instrumentation

19. 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
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
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ENGR Electronic Instrumentation

20. ENGR-4300 Electronic Instrumentation
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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ENGR Electronic Instrumentation

21. ENGR-4300 Electronic Instrumentation
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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ENGR Electronic Instrumentation

22. 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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ENGR Electronic Instrumentation

23. 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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ENGR Electronic Instrumentation

24. Calibrating the Velocity Measurement
The x and v signals are decaying sinusoids.
We know the calibration for x
And we know that
The matched amplitudes, Asg=Acoil gives us
Details here: Proj1_Help.PDF
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ENGR Electronic Instrumentation

25. ENGR-4300 Electronic Instrumentation
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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ENGR Electronic Instrumentation

26. ENGR-4300 Electronic Instrumentation
Real Time Measurement
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27. Analog Differentiator
It is possible to differentiate a signal using either a passive or active differentiator.
Passive Differentiator
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28. Analog Differentiator
Active Differentiator
Note that there is no frequency limit
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29. ENGR-4300 Electronic Instrumentation
Project Report
Introduction
Application Goals
Educational Goals
Design
Component data, both measured and researched
Full circuit diagram
Testing plan
Have plan checked out
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ENGR Electronic Instrumentation

30. ENGR-4300 Electronic Instrumentation
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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ENGR Electronic Instrumentation

31. ENGR-4300 Electronic Instrumentation
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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ENGR Electronic Instrumentation

32. ENGR-4300 Electronic Instrumentation
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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ENGR Electronic Instrumentation

33. ENGR-4300 Electronic Instrumentation
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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ENGR Electronic Instrumentation

34. 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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35. Using a Mathematical Model
v(t) = A e-at sin(wt) w=2pf
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36. ENGR-4300 Electronic Instrumentation
Elevator (fast service)
0.3 g
Automobile (take off) g
Automobile (brake or corner)
0.6-1 g
Automobile (racing)
1-2.5 g
aircraft take off
0.5 g
Earth (free-fall)
1 g
Space Shuttle (take off)
3 g
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
Typical Acceleration
Compare your results with typical acceleration values you can experience.
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ENGR Electronic Instrumentation

37. ENGR-4300 Electronic Instrumentation
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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ENGR Electronic Instrumentation

38. Senior Project from Illinois
Objective
To create a portable device that monitors the performance of an automobile.
Device to use only acceleration and a micro-controller to derive all performance data.
Device to be powered by cigarette lighter in vehicle.
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ENGR Electronic Instrumentation

39. ENGR-4300 Electronic Instrumentation
Airbags
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40. ENGR-4300 Electronic Instrumentation
Crash Test Data
Ballpark Calc:
56.6mph = 25.3m/s
Stopping in 0.1 s
Acceleration is about
-253 m/s2 = g
Head on crash at 56.6 mph
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ENGR Electronic Instrumentation

41. ENGR-4300 Electronic Instrumentation
Crash Test Data
Ballpark Calc:
112.1mph = 50.1 m/s
Stopping in 0.1 s
Acceleration is about
-501 m/s2 = g
Head on crash at mph
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ENGR Electronic Instrumentation

42. Crash Test Analysis Software
Software can be downloaded from NHTSA website
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43. ENGR-4300 Electronic Instrumentation
Crash Videos
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44. ENGR-4300 Electronic Instrumentation
Airbags
Several types of accelerometers are used & at least 2 must sense excessive acceleration to trigger the airbag.
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ENGR Electronic Instrumentation