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Electronic Instrumentation

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.

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Electronic Instrumentation

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  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 ENGR-4300 Electronic Instrumentation

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

  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 ENGR-4300 Electronic Instrumentation

  6. Building the Accelerometer Circuit ENGR-4300 Electronic Instrumentation

  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. ENGR-4300 Electronic Instrumentation

  8. Accelerometer Circuit • The AD chip produces a signal proportional to acceleration • The Op Amp amplifies the signal Op Amp Circuit Accelerometer Chip ENGR-4300 Electronic Instrumentation

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

  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 ENGR-4300 Electronic Instrumentation

  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. ENGR-4300 Electronic Instrumentation

  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. ENGR-4300 Electronic Instrumentation

  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 ENGR-4300 Electronic Instrumentation

  14. Mounting the Accelerometer ENGR-4300 Electronic Instrumentation

  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 ENGR-4300 Electronic Instrumentation

  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 [ ] ENGR-4300 Electronic Instrumentation

  17. Calibrate the Position and Velocity Sensors ENGR-4300 Electronic Instrumentation

  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 ENGR-4300 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 ENGR-4300 Electronic Instrumentation

  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 ENGR-4300 Electronic Instrumentation

  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 ENGR-4300 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 ENGR-4300 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 ENGR-4300 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 ENGR-4300 Electronic Instrumentation

  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. ENGR-4300 Electronic Instrumentation

  26. Real Time Measurement ENGR-4300 Electronic Instrumentation

  27. Analog Differentiator • It is possible to differentiate a signal using either a passive or active differentiator. • Passive Differentiator ENGR-4300 Electronic Instrumentation

  28. Analog Differentiator • Active Differentiator • Note that there is no frequency limit ENGR-4300 Electronic Instrumentation

  29. Project Report • Introduction • Application Goals • Educational Goals • Design • Component data, both measured and researched • Full circuit diagram • Testing plan • Have plan checked out ENGR-4300 Electronic Instrumentation

  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 ENGR-4300 Electronic Instrumentation

  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 ENGR-4300 Electronic Instrumentation

  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 ENGR-4300 Electronic Instrumentation

  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 ENGR-4300 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 ENGR-4300 Electronic Instrumentation

  35. Using a Mathematical Model • v(t) = A e-at sin(wt) w=2pf ENGR-4300 Electronic Instrumentation

  36. Typical Acceleration • Compare your results with typical acceleration values you can experience. ENGR-4300 Electronic Instrumentation

  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? ENGR-4300 Electronic Instrumentation

  38. 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 micro-controller to derive all performance data. • Device to be powered by cigarette lighter in vehicle. ENGR-4300 Electronic Instrumentation

  39. Airbags ENGR-4300 Electronic Instrumentation

  40. Crash Test Data • Head on crash at 56.6 mph Ballpark Calc: 56.6mph = 25.3m/s Stopping in 0.1 s Acceleration is about -253 m/s2 = -25.8 g ENGR-4300 Electronic Instrumentation

  41. Crash Test Data • Head on crash at 112.1 mph Ballpark Calc: 112.1mph = 50.1 m/s Stopping in 0.1 s Acceleration is about -501 m/s2 = -51.1 g ENGR-4300 Electronic Instrumentation

  42. Crash Test Analysis Software • Software can be downloaded from NHTSA website • http://www-nrd.nhtsa.dot.gov/software/load-cell-analysis/index.htm ENGR-4300 Electronic Instrumentation

  43. Crash Videos • http://www.sph.emory.edu/CIC/CLIPS/mvcrash.html • http://www.mazda6.de/de/upclose/overview/safety.asp • http://www.arasvo.com/crown_victoria/cv_movies.htm ENGR-4300 Electronic Instrumentation

  44. Airbags • Several types of accelerometers are used & at least 2 must sense excessive acceleration to trigger the airbag. ENGR-4300 Electronic Instrumentation

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