Electronic Instrumentation

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

3
Sensor Signals

• The 3 signals
• Position
• Velocity
• Acceleration

2 of the 3 plots must be scaled to see them on
the same figure.
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.

5
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

6
Building the Accelerometer Circuit
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.

8
Accelerometer Circuit
Op Amp Circuit
Accelerometer Chip

• The AD chip produces a signal proportional to
  acceleration
• The Op Amp amplifies the signal

9
Accelerometer Circuit

• The ADXL150 is surface mounted, so we must use a
  surfboard to connect it to a protoboard

10
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

11
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.

12
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.

13
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

14
Mounting the Accelerometer
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

16
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

17
Calibrate the Position and Velocity Sensors
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

19
Velocity Measurement Using the Magnetic Pickup
Coil

• From Maxwells 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

20
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

21
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

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

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

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, AsgAcoil gives us
• Details here Proj1_Help.PDF

25
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.

26
Real Time Measurement
27
Analog Differentiator

• It is possible to differentiate a signal using
  either a passive or active differentiator.
• Passive Differentiator

28
Analog Differentiator

• Active Differentiator
• Note that there is no frequency limit

29
Project Report

• Introduction
• Application Goals
• Educational Goals
• Design
• Component data, both measured and researched
• Full circuit diagram
• Testing plan
• Have plan checked out

30
Project Report

• Analysis
• PSpice simulation of exact circuit
• Hand calculations where appropriate
• Calibrate position and velocity measurements
• Implementation
• What went wrong?
• Two lessons learned

31
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

32
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

33
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

34
Using the Slope Function

• Given an array of points with x in A1An and y in
  B1Bn
• Find 3 pt In cell C1, put the formula
  slope(B1B3,A1A3) (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

35
Using a Mathematical Model

• v(t) A e-at sin(wt) w2pf

36
Typical Acceleration
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) 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

• Compare your results with typical acceleration
  values you can experience.

37
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?

38
Senior Project from Illinois

• https//courses.ece.uiuc.edu/ece345/projects/sprin
  g2001/project22_presentation.ppt
• 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.

39
Airbags
40
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

41
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

42
Crash Test Analysis Software

• Software can be downloaded from NHTSA website
• http//www-nrd.nhtsa.dot.gov/software/load-cell-an
  alysis/index.htm

43
Crash Videos

• http//www.sph.emory.edu/CIC/CLIPS/mvcrash.html
• http//www.mazda6.de/de/upclose/overview/safety.as
  p
• http//www.arasvo.com/crown_victoria/cv_movies.htm
  

44
Airbags

• Several types of accelerometers are used at
  least 2 must sense excessive acceleration to
  trigger the airbag.