AME 20213: Fundamentals of Measurements and Data Analysis Laboratory Exercise 2 Load Cell Static Cal

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AME 20213 Fundamentals of Measurements and Data
AnalysisLaboratory Exercise 2 Load Cell
Static Calibration Force Measurement

• TAs Ben Mertz
• Nathaniel Hollingsworth

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Introduction

• Objectives
• To statically calibrate a load cell instrumented
  with a four-arm strain gage bridge

• To use this calibration to determine unknown
  weights, mass flow rates, and beam natural
  frequencies.
• Equipment
• Strain gage
• Wheatstone bridge
• Operational amplifier system
• Digital oscilloscope

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Cantilever Beam
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Strain Gage

• One of the most widely used strain measurement
  sensors
• It is a resistive elastic unit whose change in
  resistance is a function of applied strain

http//www.dot.ca.gov/hq/esc/ttsb/instrumentation/
data_acquisition.htm
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Wheatstone Bridge

• A basic Wheatstone bridge circuit contains four
  resistances, a constant voltage input, and a
  voltage gage
• A small change in resistance transformed to
  voltage signal using an electrical circuit

Dont write this equation downjust notice
that a change in voltage is related to a change
in resistance
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Implementation of Wheatstone Bridge
here
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Static calibration

• Obtain a linear relationship between weight and
  output voltage

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Static calibration (cont)

• Use jcaleyIII.m from class website
• There is some error associated with the
  calibration (see pp. 336-343)

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Unknown object measurements

• Use the linear relationship to correspond output
  voltage with of weight pennies

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Unknown object measurements (cont)

• The uncertainty in the average weight of a penny
  decreases with the number of pennies used
• Using more of the strain gage range
• Uncertainties go as the RMS

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Oscilloscope

• A graph-displaying device that draws a graph of
  an electrical signal, shows how signals change
  over time
• vertical (Y) axis represents voltage
• horizontal (X) axis represents time.

• Key to setup oscilloscope
• Put the right signal on the screen Ch1
• If you dont see the trace, press autoset then
  adjust scaling (?t and ?V)
• Use DC Coupling

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Measurement of mass flow rate

• Measure using oscilloscope
• Correspond to using the linear
  relationship

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Dynamical measurement

• Measure beam vibration frequency using
  oscilloscope
• Having the oscilloscope measure frequency
• Measuring periods
• Fast Fourier Transform (FFT)
• Compare measurements to theoretical natural
  frequency

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Cantilever Beam Vibration

• All solid objects vibrate to some extent when
  they are hit
• Vibration can cause wear and reliability problems
  and can induce unwanted noise
• An objects unforced (natural) frequency can be
  determined experimentally or using theoretical
  approximations for some configuration

can be found in section H.3.5
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Calculating Frequency from Period
Remember the pennies
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Calculating Frequency from FFT

• FFT is a plot of amplitude vs. frequency
• Natural frequency will appear as a peak

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Useful Matlab Commands/Hints

• varload(file_name.CVS)
• save file_name.txt var_name ASCII
• coef,Ipolyfit(x_data,y_data,order)
• errorbar(x_data,y_data,error)
• help function_name ?Very useful, use it
• Oscilloscope data ? floppy disk ? thumb
  drive/email/afs
• jcaleyIII.m will give calibration curve, but
  making your own will give you an explicit form
  (you dont have to read graphs)
• Write yourself notes as you do the lab so when
  you are writing your memos, you can explain it
  better.
• MAKE SURE UNITS ARE CONSISTENT!!!!!!!!

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Deliverables

• Plot the amplifier out (V) versus weight (N). Use
  different symbols for the up calibration
  sequence and for down sequence.
• Plot a linear least-squares regression analysis
  of the data and determine the uncertainty in the
  voltage that is related to the standard error of
  the fit (the y estimate). Find the uncertainty
  in determining weight from voltage measurement
  (the x-from-y estimate) (Use jcaleyIII.m)
• Determine the average weight of one penny and
  compare with the standard weight at
  http//www.usmint.gov/faqs/circulating_coins/index
  .cfm?actionfaq_circulating_coin

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Deliverables

• Determine the mass flow rate of the sand, plot
  the data, and calculate the uncertainty in the
  mass flow rate.
• Determine the natural frequency of the beam using
  the data collected from lab. Calculate a
  theoretical natural frequency for the beam.
  Compare the values and give reasons for any
  possible differences.
• Plot the dynamic response of the beam (the trace
  saved during the lab).

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Miscellaneous

• Lab Policy
• Groups of two people maximum
• For variation in lab, each group will run
  different conditions of bridge excitation
• Lab Matrix
• Use Technical Memo Format
• See sections 3.3.3 and 3.3.4 of the text for
  guidelines
• Memo should be approximately 4 pages total.
• Location of Lab Fitzpatrick B 14
• Be there at 400pm on the day you are scheduled.
• B groups Sep. 25, 26, 27
• A groups Oct. 2, 3, 4

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Grading Policy
1. Proper Format and Content 10 2.
Professional Appearance 5 3. Correct Grammar
and Spelling 5 4. Units for All Appropriate
Numbers 5 5. Properly Drawn and Labeled
Figures 5 6. Uncertainty Estimates
Provided 10 7. Technical Deliverables
10x6 Late Policy one point will be deducted
from the final score for each minute late
(enforced for late beyond 5 minutes) For example
10 minutes late for lab total score based on
1-7 95 final score 85
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Lab Matrix
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Questions?

• Please print out the lab handout
• bring it and textbook to the lab
• Read through if possible