MAE 3272 - Lecture 7 Notes - Load Cell Calibration and Notes

M&AE 3272 - Lecture #8 Load Cell Calibration / Module 3

11 March 2013

1

WSachse; 3/2013;M&AE 3272 - Lecture 8 1

Coming in Module 3:Bicycle Component Modeling and Testing

M&AE 3272: Mechanical Propertyand Performance Laboratory

Bikes of the Future?

WSachse; 3/2013;

But First:Week 8Module 2:

This is it !. . . Load Cell Calibration and Testing Week.

M&AE 3272 - Lecture 8 2



11 March 2013

2

WSachse; 3/2013;

Sections:401-404; 409; and 411

Sections 405-408; 410; and 412

Feb 25th – Mar 1st Build Load Cellin Upson B-30

Construct LabVIEW vi

in 163 RhodesMar 4th - Mar 8th Construct

LabVIEW viin 163 Rhodes

Build Load Cellin Upson B-30

Mar 11th – Mar 15th Test and Calibratein Upson B-30

your Load Cell(see Schedule)

Mar 16th – Mar 24th Spring Break

Tasks to do during the next two Weeks, March 4th- March15th:


WSachse; 3/2013;

Load Cell: Testing Schedule(March 11th - 15th)



11 March 2013

3

WSachse; 3/2013;

Calibration and Testing Measurement Procedure:


• Load your Load Cell in incremental load steps while collecting averaged LabVIEW load cell/Wheatstone Bridge output voltages. Save the calibration to a LabVIEW lvm-file.

• Import lvm-file data into an Excel spreadsheet and process (using either Trendline or LINEST) to determine Slope, K , and zero-offset, V0 . Compare K to what you computed.

• Upload results to LabArchives.

• Use your calibration coefficients to determine the unknown load. Use a number (e.g. 4-9) repeat measurements to be able to determine the measurement Uncertainty.

• Upload results to LabArchives.

WSachse; 3/2013;

Calibration and Testing Measurement Procedure:


Demo . . .. . . with simulated data.


11 March 2013

4

WSachse; 3/2013;

Excel LINEST Output:


Quantity LINEST Output Quantity

Slope out #1 out #2 Intercept

+/- out #3 out #4 +/-

Coeff of Determ, r2 out #5 out #6 σ(y)

F-statistic out #7 out #8 Degs ofFreedom

Regress ss out #9 out #10 Residual ss

WSachse; 3/2013;

Calibration and Testing Measurement Tips:


From a TA . . .. . .We adjusted the DAQ part of the VIs to read in 2000 samples instead of 100 (since this makes the LabVIEW output readings much less volatile).

During calibration and testing, after a load is added or removed (including the bucket), have everyone stand at least a meter away from the set-up and remain still. Wait at least 20 seconds before clicking "Accept Value" button. Unfortunately the readings are very sensitive and do respond to movement on the floor, so this step is necessary.

The table generated from calibration may likely have one or more "bad readings." If it does, don't use the slope calculated by the VI. Open the results file up in Excel, remove the bad point(s), plot it, and calculate the slope using "add trendline."

Use the slope in the 407 output.vi and follow Matt's procedure for testing. Once again, wait 20s - 1min for the load output readings to settle down and have everyone stand far away. . . . amazingly accurate.


11 March 2013

5

WSachse; 3/2013;

Load Cell Static Characterization:

∑=N

0qqPaV(P)


Measure:Load Cell/Bridge Output Voltage, V vs. Input Load, PUnder Standard Conditions:• When P = 0 V = 0 (or V = V0)• Take measurements from 0 to Pmax in steps of 0.1Pmax

(11 Readings)• Quasi-statically• Unload from Pmax to 0 in steps of -0.1Pmax

• Reload and Unload; Repeat (33 Data Points; 44 Total).• Fit Regression:

• We shall assume that: V(P) = K P + V0; K = Sensitivity

WSachse; 3/2013;

Dynamic Range of a Load Cell:


The maximum range of loads that your Load Cell can safely carry is determined by the smallest of:

• Start of inelastic deformation of the “elastic members” comprising your load cell, i.e. εyieldwith a Factor of Safety of 2.0 . For 2024-T4 Al,

εyield = σyield/E = 47ksi/10.6Msi = approx 4400με ; with Factor of Safety, use approx 2200με .

• Failure of the strain gage and/or its attachment adhesive. εgage-fail approx 3% or 30000με .

• Load cell signal exceeds dynamic range of digitizing A/D unit.


11 March 2013

6

WSachse; 3/2013;

Dynamic Range / Resolution of Load Cell:


A/D Converter Resolution: 16 bits = 65,536 voltage levelsA/D Converter Input Signal Range: +/-200 mVMaximum Input Voltage (before “Clipping”): +200 mV VmaxVoltage Resolution: 200 mV/65535 = 3.052 μV

PADmax = Vmax/Calib_Factor >>> System Maximum Load

This will correspond to the Maximum Allowable Load if A/D Converter Controls Dynamic Range.

Load Resolution: Smallest theoretically detectable Load Resolution corresponding to one level or . . .

PADmin = 3.052E-6/Calib_Factor >>> System Load Resolution

System uses NI-9205 Analog Input Module:

WSachse; 3/2013;

Using a Calibrated Load Cell:


V(P) = K P + V0

Pest = {Vmeas - V0}/K

σ2 = (1/N) Σ[{Vout - V0}/K – Pin]2

•Thus if a measured load cell voltage is Vmeas then the estimate of the true load is: Pest +/- 3σ with a probability of 99.7%.

•The bias in the measurement is given by: Pest – Ptrue

•The quantity 3σ corresponds to the Imprecision or Uncertainty of the load determination.

From Calibration Measurements:

Measurement of an Unknown Weight:


11 March 2013

7

WSachse; 3/2013;

Module 2 Deliverable:

Load Cell Calibration

Data and Curve:


0 20 40 60 80

Applied Load [lbs]

0

5

10

15

20

Bri

dge

Out

put

Vol

tage

[mV

]

Measured Calibration DataLinear Fit: 0.215 [mV/lb]

Load Cell Calibration

4 Calibration Data Points

2 Linear Fit Line

3a Slope:Calibration Constantor 1/Sensitivity

3b Zero

6

WSachse; 3/2013;

Load Cell: Calibration

Sheet


3.

1.

4.

5.

2.

Due:Week of

March 25th

to 29th

Two weeksafter you did

the Calibration


11 March 2013

8


NOTICE: Load Cell Calibration and Application:

During your testing period your TA will ask each member of your group questions about your load cell and measurement system:• Aspects of your load cell design• Elements and properties of strain gages• Your measurement system and especially

your LabVIEW vi routine• How you are/will be processing your data• Modifying and Interfering effects• Errors and Uncertainties and how you will

deal with them


Load Cell CalibrationHand-in Schedule

(March 25th – 29th):Please . . .Hand-in your Group’s Calibration

Sheet and Calibration curve:• To your TA during Office hours

-- AND --• Each person hands-in his/her

Abstract to your TA during Office hours. . . – OR-certainly by the date/time indicated on the chart at right(exactly two (2) weeks after you calibrated your load cell!)


11 March 2013

9

WSachse; 3/2013;

Sections:401-404; 409; and 411

Sections 405-408; 410; and 412

Feb 25th – Mar 1st Build Load Cellin Upson B-30

Construct LabVIEW vi

in 163 RhodesMar 4th - Mar 8th Construct

LabVIEW viin 163 Rhodes

Build Load Cellin Upson B-30

Mar 11th – Mar 15th Test and Calibratein Upson B-30

your Load Cell(see Schedule)

Mar 16th – Mar 24th Spring Break

Tasks to do during the next two Weeks, March 4th- March15th:





A/D Conversion; Signal Processing;

and Display


11 March 2013

10

WSachse; 3/2013;

Module 3: Modeling - Four Bicycle Crank Designs


Crank #1:

Crank #2:

Crank #3: Crank #4:

Parasolid files of these geometries are available to you for direct input to ANSYS


Measurement Systems:

Stage 1 Stage n

Signal Conditioning/

Processing Stages

… Output Stage

Materialor Systemunder Test

Control Stage

ActuatorSystemActuator

Sensing Element

TransductionSystem


11 March 2013

11

WSachse; 3/2013;

-4000

-3000

-2000

-1000

0

1000

2000

3000

4000

5000

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2

-30

-20

-10

0

10

20

30

40

50

60

Module 3: Bicycle Component Modeling/Testing

M&AE 3272 - Lecture 8

etc. . . etc. . . etc. . .

21



Crank Geometry Crank designs based on past years’ M&AE 3250 designs

ANSYS Modeling Model with ANSYS to find two locations for strain gage rosettes:

(1) High stress concentration;(2) Uniform stress location

Gaging and static testing

Pre-machined crank arms provided; Gage with strain gage rosettes; Static loading; comparison with ANSYS modeling

In-situ Testing Crank installed on bicycle: Pedal Force components; Crank Position; LabVIEW vi provided! (22 data sets)2 Strain gage rosettes (2 times 3 strain components)

Data Comparison Compare principal strains/directions with ANSYS model predictions

Presentations Group makes 5-Slide Presentation of results


11 March 2013

12


Summary for Module 2:Tasks to complete this Week (March 11th–15th):

• Testing of Load Cells on your Group’s scheduled testing period! e.g. Monday: Groups #401(A-D) followed by Groups #405(A-D); Tuesday: Groups #402(A-D) and #406(A-D), etc. All Groups must finish by 4 pm Friday!

• OPEN Lab Periods (8 am to 2 pm) for Last-Minute Help with your LabVIEW program or Load Cell Construction and Final Testing.

• If you wish: Download Module #3 “Bicycle Component Modeling and Testing” Handout and carry it with you next week. Impress your mom and friends!

• Catch ride/bus/plane head off for Spring Break.Have fun! And re-charge your batteries for Module #3.

MAE 3272 - Lecture 7 Notes - Load Cell Calibration and Notes

Documents

Transcript of MAE 3272 - Lecture 7 Notes - Load Cell Calibration and Notes