1. Electronic Instrumentation
Project 2
Acceleration Measurement

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
Build amplified bridge circuit
Calibrate the sensors
Measure position, velocity and acceleration, 2 channels at a time
Determine the mathematical representations for x, v, and a.
Demonstrate that all 3 signals can be used to find the acceleration of the beam.
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ENGR Electronic Instrumentation

5. Building the Accelerometer Circuit
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ENGR Electronic Instrumentation

6. 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

7. Accelerometer Circuit
MAX473
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

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

9. 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

10. 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

11. 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

12. 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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ENGR Electronic Instrumentation

13. Mounting the Accelerometer
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14. 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

15. ENGR-4300 Electronic Instrumentation
Accelerometer Signal
The output from the accelerometer circuit
is per g, where g is the
acceleration of gravity and -Rf /Ri is the gain of the Op Amp circuit. Gain is negative.
In the equations below, the units are in [ ]
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16. Calibrate the Position and Velocity Sensors
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17. Amplified Strain Gauge Circuit
741
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18. Position Measurement Using the Strain Gauge
Set up the amplified strain gauge circuit
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 (m) in meters/V.
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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 find a ratio that relates the amplitude of the coil to the amplitude of the amplified strain gauge
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ENGR Electronic Instrumentation

22. Comparison of x and v signals
Recall the coil and amplified strain gauge signals should be out of phase by 90 degrees
We want the amplitude of the two signals to be about the same
If RA is not close to 1, change the gain of the differential amplifier
Use a Lissajous pattern to show the ratio of the two amplitudes (circle)
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 using Agilent Intuilink
Determine RA
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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
Using the ratio of the two amplitudes Asg=RAAcoil
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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
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, average the accelerometer 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, average the accelerometer signal and compare the two resulting curves.
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ENGR Electronic Instrumentation

26. ENGR-4300 Electronic Instrumentation
Data Analysis
Take derivatives with the SLOPE function in Excel
The strain gauge signal (converted to meters) must be differentiated twice to find acceleration.
The coil signal (converted to meters/second) must be differentiated once to find acceleration.
Because it uses regression, the SLOPE function will introduce a phase shift proportional to the number of points in the regression.
Use the AVERAGE function on the accelerometer signal
The accelerometer signal must be averaged over the same number of points that you used in the regression for the SLOPE function.
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ENGR Electronic Instrumentation

27. The SLOPE Function (for use on coil and strain gauge signal)
Given an array of points with x in A1:An and y in B1:Bn
Regression over 10 points: In cell C1, put the formula =SLOPE(B1:B9,A1:A9)
Now copy this formula in the rest of C.
Graph the data to compare
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ENGR Electronic Instrumentation

28. The AVERAGE Function (for use on accelerometer signal)
Given an array of points with x in A1:An and y in B1:Bn
For averaging over 10 points: In cell C1, put the formula =AVERAGE(B1:B9)
Now copy this formula in the rest of C.
Graph the data to compare
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ENGR Electronic Instrumentation

29. ENGR-4300 Electronic Instrumentation
Some 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

30. ENGR-4300 Electronic Instrumentation
Project Report
Introduction
Purpose of project
2 topics
Background
Basic theory
How the circuits work
Calibration process
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ENGR Electronic Instrumentation

31. ENGR-4300 Electronic Instrumentation
Project Report
Initial Design
Building and calibrating the accelerometer circuit.
Finding strain gauge and coil amplitude ratio.
Calibrating position and velocity measurements
Taking initial data
Final Design
Compare acc. from strain gauge and accelerometer
Compare acc. from coil and accelerometer
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ENGR Electronic Instrumentation

32. ENGR-4300 Electronic Instrumentation
Project Report
Conclusions
How does your data compare?
Sources of error
Questions
Discuss extra credit
Personal Responsibilities
Appendices
Extra Credit Details
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ENGR Electronic Instrumentation

33. ENGR-4300 Electronic Instrumentation
Extra Credit
Really good results
Strain gauge, coil, and accelerometer signals all on same plot
Velocity comparison in Excel
Build a differentiator
Calibrate the ADXL150
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ENGR Electronic Instrumentation

34. Passive Differentiator
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35. Active Differentiator
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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
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

38. 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

39. Crash Test Analysis Software
Software can be downloaded from NHTSA website
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40. ENGR-4300 Electronic Instrumentation
Crash Videos
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41. 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