MAE 3272 - Lecture 4 Notes - Load Cells and LabVIEW

M&AE 3272 - Lecture #4 LabVIEW Intro + Design of Load Cell

24 February 2014

1

WSachse; 2/2014;

Load Measurement System:

Force

Force

M&AE 3272 - Lecture 4

Elastic Member

1

4th-8th Weeks Load Cell Fabrication and Data Aquisition

M&AE 3272: Mechanical Propertyand Performance Laboratory

Excitation

Signal Conditioning and Processing

Display and Analysis via LabVIEW

Strain Gage

Load Cell

WSachse; 2/2014;

Weeks 4-8Module 2:

Apply mechanics and strength of materials concepts to design a force-measuring transducer

Learn how to mount and use strain gages and strain gage instrumentation to produce a force-measuring system

Learn how to write and use data acquisition software (i.e. LabVIEW)

Test, Verify and Calibrate your force sensor (Load cell)

M&AE 3272 - Lecture 4 2

M&AE 3272: Mechanical Propertyand Performance Laboratory


24 February 2014

2

WSachse; 2/2014;

Back to us and M&AE 3272:


Were going to learn how to mount strain gages onto an elastic member in order to

fabricate a Load cell, or Force transducer.

WSachse; 2/2014;

Continue to become familiar with strain gages; characteristics; mounting procedures, etc.

Take a look at the Supplementary Materials on Blackboard related to strain gages.

Learn LabVIEW. Complete the LabVIEW Assignment! This must be uploaded to LabArchives PRIOR to when you will meet in Rhodes 106 to write your vi .

Sections #405, #406, #407, #408, #410, #412 and #414 meet in B30 Upson to learn how to mount strain gages.

Sections #401, #402, #403, #404, #409, and #413 go to Rhodes 106 to write your vi .

Design your Load Cell Todays Lecture!

Continue to become familiar with strain gages; characteristics; mounting procedures, etc.

Take a look at the Supplementary Materials on Blackboard related to strain gages.

Learn LabVIEW. Complete the LabVIEW Assignment! This must be uploaded to LabArchives PRIOR to when you will meet in Rhodes 106 to write your vi .

Sections #405, #406, #407, #408, #410, #412 and #414 meet in B30 Upson to learn how to mount strain gages.

Sections #401, #402, #403, #404, #409, and #413 go to Rhodes 106 to write your vi .

Design your Load Cell Todays Lecture!

Tasks to do THIS Week, February 24th-28th:



24 February 2014

3

WSachse; 2/2014;M&AE 3272 - Lecture 4 5

Introduction to LabVIEW:

A graphical programming environment which is used by many in industry and academia to run experiments, collect and process data.

The computer with ancillary input and output hardware becomes a virtual instrument or vi.

We will use it in Module 2 to collect voltage data, corresponding to small changes in electrical resistance of strain gages attached to elastic members (load cell), while the load cell is being calibrated and or while it is in use.

We will use it in Module 3 to collect data from multiple sensors on a dynamical system (bicycle).


http://www.ni.com/academic/students/learnlabview/

LabVIEW Environment

Graphical ProgrammingExecution Structures


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Running LabVIEW:

WSachse; 2/2014;

LabVIEW Program Example: (by John Buzzi; 3/1/10)


Function: Generate a noisy DC-level Voltage for Simulation

Slider Input: Set Voltage Level Simulate Signal

(Express.vi) Noise 0.6; fs: 1kHz; 100 pts

Input Values: Multiplier/Offset

Convert Signal Stream -> 1D

Array of Scalars

Mean Input of Signal.vi

Display Signal Stream via

Waveform Chart

Slider Level Mean Value

Mean


24 February 2014

5

WSachse; 2/2014;

Strain Gage Sensitivity:


Factional Change of Gage Resistance with Strain:

R

R= (1 + 2)axial

Dimensional

+

piezoresistive

The fractional change of gage resistance per unit strain Strain gage Sensitivity :

R

R 1axial

Gage Factor Sgage = (1 + 2) + 1axial

When 0.3 : the Gage Factor is given bySgage 1 + 0.6 + (0.4 to 2.0) Metallic conductorsSgage 1 + 0.6 + (125 to 175) Semiconductors

P type (e. g. Boron) Sgage > 0N type (e. g. Arsenic) Sgage < 0

Sgage > 0 Rg + > 0 [T] Sgage < 0 Rg < 0 [C]

Numerical Example : Metal foil gage, 120 ; Sgage 2.0 ,then for axial = 1 (i. e. 1 106 in/in) :Rg = Sg Rg axial 2120106 2.4104 [] = 240 []

WSachse; 2/2014;

Wheatstone Bridge Circuit Static Measurements:


Constant voltage (or current) excitation; Resistors R1, R2, R3 andR4 and load resistance RM .

Constant voltage circuit

Output Voltage:

E0 =R1R3 R2R4

(R1 + R2))R3 + R4)Ei

At balance :

E0 = 0 when R1R3 = R2R4

Static Measurements


24 February 2014

6

WSachse; 2/2014;

Wheatstone Bridge Circuit Dynamic Measurements:


Dynamic Measurements: R1 R1 + R1;R2 R2 + R2; R3 R3 + R3 and R4 R4 + R4then . . .

E0 =R1R2

(R1 + R2)2

(R1R1

R2R2

+R3R3

R4R4

)Ei + h. o. t.

The omitted higher-order-terms lead to an error given by

Error : =4

i=1 Ri/Ri4i=1 Ri/Ri + 2

When R1 = R4 and R2 = R3 = 0 or : R2 = R3 and R1 = R4 = 0 Error equals zero.

WSachse; 2/2014;

Common Strain Gage Wheatstone Bridge Circuits:


Dummy Gage: Temperature

Compensation


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Calculation of Bridge

Output Voltages:

Matt Ulinski Notes (2008)

++ --

WSachse; 2/2014;

Load Cell Design (Week of March 3rd Mar 7th):


Gage Dimensions

Lpad 7/16

Wpad 5/16

Lgage

Dedge 1/16

Lpadd

Wpad

Lgage Dedge


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Calculation of Load Cell Voltage Sensitivity:

WSachse; 2/2014;

Load Cell Design:

Load Cell Design Bending Beam Configuration

S-Beam Load CellsS-Beam load cells get their name from their S shape. S-Beam load cells can provide an output if under tension or compression. Applications include tank level, hoppers and truck scales. They provide superior side load rejection. Cost: $250-$400



24 February 2014

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WSachse; 2/2014;

Commercial S-Beam Load Cells:


WSachse; 2/2014;

S-Beam Load Cell Mechanics Analysis:


Load Cell Diagram

Free Body Diagram

Where are you going to glue your four (4) gages?


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Glue gages at points of maximum stresses!The maximum stresses (tensile: T(+), and compressive: C(-)) are expected at:

Beams

Load, P

Load, P

#1

#2

#3

C()

T(+)T(+) C()

C() T(+)T(+)

C()

(d)

Beams

Load, P

Load, P

#1

#2

#3

C()

T(+) C()

T(+)

C()

C()

(e)

Beams

Load, P

Load, P

#1

#2

#3

C()

T(+) C()

C() T(+)

C()

T(+)

T(+)(a)

Beams

Load, P

Load, P

#1

#2

#3

C()

C()

T(+)

T(+)

C()

T(+) C()

T(+)

(b)

Beams

Load, P

Load, P

#1

#2

#3

T(+) C()

C() T(+)

C()

T(+)

C()

T(+)(c)

(d)


Shear/Moment Diagrams; Max Stresses/Strains

Beams

Load, P

Load, P

#1

#2

#3

C()

T(+)T(+) C()

C() T(+)T(+)

C()


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Calculation of Load Cell Sensitivity:


Signal Collection and Processing via LabVIEW:

NI cDAQ-9172

PC

LabVIEW SoftwareLabVIEW Display


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WSachse; 2/2014;

A/D Converter Specifications:


4 Channel, 25 mV/V, 24-Bit Simultaneous Bridge Module NI 9237

4 simultaneously sampled analog inputs24 bit resolution50 kS/s max sampling rate25 mV/V analog inputs

WSachse; 2/2014;

Dynamic Range / Resolution of Load Cell:


A/D Converter Resolution: 24 bits = 16,777,216 voltage levelsA/D Converter Input Signal Range: +/-25 mV/VBridge excitation is 2.5 VMaximum Input Voltage: 25 mV/V2.5 V= 62.5 mV VmaxVoltage Resolution: 262.5 mV/16777216 = 7.451E-9 V

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 = 7.451E-9/Calib_Factor >>> System Load Resolution

System uses NI-9205 Analog Input Module:


24 February 2014

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WSachse; 2/2014;

Re: Load Cell System Range and Resolution:


Load Cell Dynamic Range is likely NOT determined by Vmax of ADC System rather by < Yield/F.S.

Load Cell Resolution may NOT be determined by the Voltage Resolution of the ADC but rather by other factors.

WSachse; 2/2014;

Load Cell: Calibration

Sheet


3.

1.

4.

5.

2.

Due:Week of April 7thto 11th

Two weeksafter you did

the Calibration


24 February 2014

14

WSachse; 2/2014;

Tasks to do during the upcoming three Weeks, March 3rd March 21st:


Sections: 401-404; 409; and 413

Sections 405-408; 410; 412 and 414

Mar 3rd Mar 7th Build Load Cellin Upson B-30

Build Load Cellin Upson B-30

Mar 10th - Mar 14th Finish Building Load Cell

in Upson B-30

Finish Building Load Cell

in Upson B-30

Mar 17th Mar 21st Test and Calibratein Upson B-30

your Load Cell(see Schedule)

WSachse; 2/2014;

You should have familiarity with LabVIEW. Everyone meet in B30 Upson to fabricate your Load Cell. Come

prepared to show your design to your TA so that you can receive parts.

Tasks to do NEXT Week, March 3rd March 7th:


You should have familiarity with LabVIEW. Everyone meet in B30 Upson to fabricate your Load Cell. Come

prepared to show your design to your TA so that you can receive parts.

MAE 3272 - Lecture 4 Notes - Load Cells and LabVIEW

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