Campbell Scientific 4WFB120, 4WFB350, 4WFB1K User Manual

4WFB120, 4WFB350, 4WFB1K

4 Wire Full Bridge Terminal

Input Modules

Revision: 5/07

Campbell Scientific, Inc.

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4WFB120, 4WFB350, 4WFB1K Table of Contents

PDF viewers note: These page numbers refer to the printed version of this document. Use the Adobe Acrobat® bookmarks tab for links to specific sections.

1. Function........................................................................1

2. Specifications ..............................................................1

3. Measurement Concepts..............................................2

4. Wiring............................................................................3

5. Program Examples......................................................3

5.1 Edlog.........................................................................................................4

5.1.1 CR10(X)..........................................................................................4

5.1.2 21X .................................................................................................7

5.1.3 CR7...............................................................................................10

5.2 CRBasic..................................................................................................13

5.2.1 CR9000(X)....................................................................................14

6. Calculation of Strain..................................................15

Figures

1-1 Terminal Input Module ............................................................................1

2-1 Schematic.................................................................................................2

4-1 Wiring for Example Programs .................................................................3

6-1 Strain Gage in Full Bridge......................................................................15

Table

5-1 Input Locations Used in CR10(X), 21X, and CR7 Examples..................4

i

This is a blank page.

4WFB120, 4WFB350, 4WFB1K 4 Wire Full Bridge Terminal Input Modules

1. Function

Terminal input modules connect directly to the datalogger's input terminals to provide completion resistors for resistive bridge measurements, voltage dividers, and precision current shunts. The 4WFB120, 4WFB350, and 4WFB1K complete a full bridge for a strain gage or other sensor that acts as a single variable resistor. The difference between the three models is in the resistor that matches the nominal resistance of a 120 ohm, 350 ohm, or 1000 ohm quarter bridge strain gage.

H

L

G

H

L

AG

2. Specifications

2:1 Resistive Divider

Resistors Ratio Tolerance @ 25 °C

Ratio Temperature coefficient Power rating 0.25 W

Completion Resistor: 120, 350, or 1000 Ω

Tolerance @ 25 °C Temperature coefficient 0-60 °C

-55-125 °C Power rating 0.25 W

H

L

AG

FIGURE 1-1. Terminal Input Module

1 kΩ/1 kΩ ±0.02%

2 ppm/°C

±0.01%

4 ppm/°C 8 ppm/°C

1

4WFB120, 4WFB350, 4WFB1K 4 Wire Full Bridge Terminal Input Modules

ε

µ

=

∆

µ

x

⋅

x

Vx

H

L

or AG

3. Measurement Concepts

Measuring strain is measuring a change in length. Specifically, the unit strain

()

is the change in length divided by the unstrained length

Strain is typically reported in microstrain length by one millionth of the length.

A metal foil strain gage is a resistive element that changes resistance as it is stretched or compressed. The strain gage is bonded to the object in which strain is measured. The gage factor,

resistance for change in strain:

factor of 2 means that if the length changes by one micrometer per meter of

)

(1

ε

length resistance.

, the resistance will change by two micro-ohms per ohm of

H

1kΩ

FIGURE 2-1. Schematic

H

L

G

()

GF , is the ratio of the relative change in

GF R R l l

120Ω, 350Ω, or 1kΩ

ε

=∆ll/

()

ε

; a microstrain is a change in

//

. For example, a gage

.

Because the actual change in resistance is so small, a full bridge configuration is used to give the maximum resolution. A "quarter bridge" strain gage is so named because the strain gage becomes one of the four resistors that make up a full bridge. The 4WFBxxx module provides the other three resistors (Figure 4-

1). Quarter bridge strain gages are available in nominal unstrained resistances of 120, 350, and 1000 ohms. The 4WFB model must match the resistance of the gage (e.g., the 4WFB120 is used with a 120 ohm strain gage).

The resistance of an installed gage will differ from the nominal value. A zero measurement can be made with the gage installed. This zero measurement can be incorporated into the datalogger program; subsequent measurements can report strain relative to the zero.

Strain is calculated in terms of the result of the full bridge measurement. This result is the measured bridge output voltage divided by the bridge excitation

VV

/

voltage millivolts output per volt of excitation, measurement, measurements. Strain is calculated from the change in the bridge

measurement,

outex

. (The actual result of the full bridge instruction is the

1000

VV

/

is stored and used to calculate future strain

0

out e

1000⋅VV

/

out e

) The result of the zero

2

r

x

=

−

4. Wiring

4WFB120, 4WFB350, 4WFB1K 4 Wire Full Bridge Terminal Input Modules

VVV VV

(/)( /0)

out e

ε

=

GF V

out e

V

r

−412

()

r

: 3.1.

3.2.

The calculations are covered in more detail in section 6.

Datalogger

Vx

Figure 4-1 illustrates the wiring of the strain gage to the 4WFB module and the wiring of the module to the datalogger. It is important that the gage be wired as shown with the wire from H connected at the gage, and that the leads to the L and G terminals be the same length, diameter, and wire type. With this configuration, changes in wire resistance due to temperature occur equally in both arms of the bridge with negligible effect on the output from the bridge.

5. Program Examples

The following examples for the CR10(X), 21X, CR7, and CR9000(X) all have a subroutine that measures the unstrained "zero" output of the strain gage. The examples calculate strain using equation 3.2 for a strain gage with a GF=2. These are just examples. Besides adding additional measurement instructions, the programs will probably need to have the scan and d ata storage intervals altered for actual applications. The instructions in the subroutine will also need to be modified for the actual gage factor.

H

L

G

or AG

or G

H

L

Shield

FIGURE 4-1. Wiring for Example Programs

This zeroing subroutine is called automatically when the progr am is first executed. The user can call the subroutine by setting Flag 1 low using the datalogger support software or the *6 mode with the keyboard display. The "zero" reading is then used during normal measurements for the strain calculations.

3

4WFB120, 4WFB350, 4WFB1K 4 Wire Full Bridge Terminal Input Modules

5.1 Edlog

Dataloggers that use Edlog include CR510, CR10(X), 21X, and CR7. The Edlog instruction that measures strain gages is Instruction 6 – Full Bridge.

The Input Locations assignments used in CR10(X), 21X, and CR7 Examples are listed in Table 5-1.

TABLE 5-1. Input

Locations Used in

CR10(X), 21X, and CR7

Examples

Addr Name

1 mVperV 2 mVperV_0 3 Vr 4 uStrain 5 Count 6 GF 7 _4e6 8 Mult 9 1_2Vr 10 Vr_1_2Vr

5.1.1 CR10(X)

;{CR10X} ;

*Table 1 Program 01: 1 Execution Interval (seconds)

;Other measurements could be inserted here or before the Output section

1: If Flag/Port (P91) ;On the first execution (Flag 1 is low) 1: 21 Do if Flag 1 is Low ;or when user sets Flag 1 low 2: 1 Call Subroutine 1 ;call the zeroing subroutine

2: Full Bridge (P6) ;Measure the strain gage 1: 1 Reps 2: 22 ± 7.5 mV 60 Hz Rejection Range 3: 1 DIFF Channel 4: 1 Excite all reps w/Exchan 1 5: 2500 mV Excitation 6: 1 Loc [ mVperV ] 7: 1 Mult 8: 0 Offset

4

4WFB120, 4WFB350, 4WFB1K 4 Wire Full Bridge Terminal Input Modules

3: X-Y (P35) ;Subtract zero reading from the 1: 1 X Loc [ mVperV ] ;measurement 2: 2 Y Loc [ mVperV_0 ] 3: 3 Z Loc [ Vr ]

4: X*F (P37) ;Change Vr from mV/V to V/V 1: 3 Loc [ Vr ] 2: 0.001 3: 3 Loc [ Vr ]

;The following instructions calculate microstrain

5: Z=X*F (P37) 1: 3 X Loc [ Vr ] 2: -2 F 3: 9 Z Loc [ 1_2Vr ]

6: Z=Z+1 (P32) 1: 9 Z Loc [ 1_2Vr ]

7: Z=X/Y (P38) 1: 3 X Loc [ Vr ] 2: 9 Y Loc [ 1_2Vr ] 3: 10 Loc [ Vr_1_2Vr ]

8: Z=X*Y (P36) 1: 10 X Loc [ Vr_1_2Vr ] 2: 8 Y Loc [ Mult ] 3: 4 Z Loc [ uStrain ]

;Output Section ;This example outputs an average of the 1 second readings ;once per minute.

09: If time is (P92) 1: 0 Minutes (Seconds --) into a 2: 1 Interval (same units as above) 3: 10 Set Output Flag High

10: Set Active Storage Area (P80) 1: 1 Final Storage Area 1 2: 1 Array ID ;Set Array ID = 1 for measurement data

11: Real Time (P77) 1: 1110 Year,Day,Hour/Minute

12: Average (P71) 1: 1 Reps 2: 4 Loc [ uStrain ]

*Table 2 Program 2: 0.0000 Execution Interval (seconds)

5

4WFB120, 4WFB350, 4WFB1K 4 Wire Full Bridge Terminal Input Modules

*Table 3 Subroutines

1: Beginning of Subroutine (P85) ;Subroutine to measure "zero" 1: 1 Subroutine 1

2: Do (P86) ;This prevents calling subroutine 1: 11 Set Flag 1 High ;until user sets flag 1 low again.

3: Z=F (P30) ;Set counter use for average to 0 1: 0 F 2: 0 Exponent of 10 3: 5 Z Loc [ Count ]

4: Z=F (P30) ;load 4 million (4*uS/S) into input location 1: 4 F 2: 6 Exponent of 10 3: 7 Z Loc [ _4e6 ]

5: Z=F (P30) ;Load Gage Factor into input location 1: 2 F ;Enter the actual Gage Factor here 2: 0 Exponent of 10 3: 6 Z Loc [ GF ]

6: Z=X/Y (P38) ;calculate multiplier to use with strain 1: 7 X Loc [ _4e6 ] ;calculation 2: 6 Y Loc [ GF ] 3: 8 Z Loc [ Mult ]

7: Beginning of Loop (P87) ;Loop through 5 times to obtain average 1: 0 Delay ;zero reading 2: 5 Loop Count

8: Z=Z+1 (P32) ;Increment Counter used to determine 1: 5 Z Loc [ Count ] ;when to output

9: Full Bridge (P6) ;Measure Strain Gage 1: 1 Reps 2: 22 ± 7.5 mV 60 Hz Rejection Range 3: 1 DIFF Channel 4: 1 Excite all reps w/Exchan 1 5: 2500 mV Excitation 6: 1 Loc [ mVperV ] 7: 1 Mult 8: 0 Offset

10: IF (X<=>F) (P89) ;Check for last pass through loop 1: 5 X Loc [ Count ] ;to set output flag 2: 3 >= 3: 5 F 4: 10 Set Output Flag High

6

4WFB120, 4WFB350, 4WFB1K 4 Wire Full Bridge Terminal Input Modules

11: Set Active Storage Area (P80) ;Direct averaged "zero" reading 1: 3 Input Storage Area ;to input storage 2: 2 Array ID or Loc [ mVperV_0 ]

12: Average (P71) 1: 1 Reps 2: 1 Loc [ mVperV ]

13: If Flag/Port (P91) ;When average is calculated, 1: 10 Do if Output Flag is High (Flag 0) ;also send it to Final Storage 2: 10 Set Output Flag High

14: Set Active Storage Area (P80) ;Direct Output to Final Storage 1: 1 Final Storage Area 1 2: 11 Array ID ;set Array ID = 11 for zero data

15: Real Time (P77) 1: 110 Day,Hour/Minute

16: Sample (P70) 1: 1 Reps 2: 2 Loc [ mVperV_0 ]

17: End (P95)

18: End (P95)

End Program

5.1.2 21X

;{21X}

*Table 1 Program 01: 1 Execution Interval (seconds)

;Other measurements could be inserted here or before the Output section

1: If Flag/Port (P91) ;On the first execution (Flag 1 is low) 1: 21 Do if Flag 1 is Low ;or when user sets Flag 1 low 2: 1 Call Subroutine 1 ;call the zeroing subroutine

2: Full Bridge (P6) ;Measure the strain gage 1: 1 Reps 2: 2 ± 15 mV Slow Range 3: 1 DIFF Channel 4: 1 Excite all reps w/Exchan 1 5: 5000 mV Excitation 6: 1 Loc [ mVperV ] 7: 1 Mult 8: 0 Offset

7

4WFB120, 4WFB350, 4WFB1K 4 Wire Full Bridge Terminal Input Modules

3: Z=X-Y (P35) ;Subtract zero reading from the 1: 1 X Loc [ mVperV ] ;measurement 2: 2 Y Loc [ mVperV_0 ] 3: 3 Z Loc [ Vr ]

4: Z=X*F (P37) ;Change Vr from mV/V to V/V 1: 3 X Loc [ Vr ] 2: 0.001 F 3: 3 Z Loc [ Vr ]

;The following instructions calculate microstrain

5: Z=X*F (P37) 1: 3 X Loc [ Vr ] 2: -2 F 3: 9 Z Loc [ 1_2Vr ]

6: Z=Z+1 (P32) 1: 9 Z Loc [ 1_2Vr ]

7: Z=X/Y (P38) 1: 3 X Loc [ Vr ] 2: 9 Y Loc [ 1_2Vr ] 3: 10 Z Loc [ Vr_1_2Vr ]

8: Z=X*Y (P36) 1: 10 X Loc [ Vr_1_2Vr ] 2: 8 Y Loc [ Mult ] 3: 4 Z Loc [ uStrain ]

;Output Section ;This example outputs an average of the 1 second readings ;once per minute.

9: If time is (P92) 1: 0 Minutes (Seconds --) into a 2: 1 Interval (same units as above) 3: 10 Set Output Flag High

10: Set Active Storage Area (P80) 1: 1 Final Storage Area 1 2: 1 Array ID ;Set Array ID = 1 for measurement data

11: Real Time (P77) 1: 1110 Year,Day,Hour/Minute

12: Average (P71) 1: 1 Reps 2: 4 Loc [ uStrain ]

*Table 2 Program 01: 0.0000 Execution Interval (seconds)

8

4WFB120, 4WFB350, 4WFB1K 4 Wire Full Bridge Terminal Input Modules

*Table 3 Subroutines

1: Beginning of Subroutine (P85) ;Subroutine to measure "zero" 1: 1 Subroutine 1

2: Do (P86) ;This prevents calling subroutine 1: 11 Set Flag 1 High ;until user sets flag 1 low again.

3: Z=F (P30) ;Set counter use for average to 0 1: 0 F 2: 5 Z Loc [ count ]

4: Z=F (P30) ;load 4000 into 1: 4000 F ;input location 2: 7 Z Loc [ 4e6 ]

5: Z=X*F (P37) ;Multiply by 1000 to get (4*uS/S) 1: 7 X Loc [ 4e6 ] 2: 1000 F 3: 7 Z Loc [ 4e6 ]

6: Z=F (P30) ;Load Gage Factor into input location 1: 2 F ;Enter the actual Gage Factor here 2: 6 Z Loc [ GF ]

7: Z=X/Y (P38) ;calculate multiplier to use with strain 1: 7 X Loc [ 4e6 ] ;calculation 2: 6 Y Loc [ GF ] 3: 8 Z Loc [ Mult ]

8: Beginning of Loop (P87) ;Loop through 5 times to obtain average 1: 0 Delay ;zero reading 2: 5 Loop Count

9: Z=Z+1 (P32) ;Increment Counter used to determine 1: 5 Z Loc [ count ] ;when to output

10: Full Bridge (P6) ;Measure Strain Gage 1: 1 Reps 2: 2 ± 15 mV Slow Range 3: 1 DIFF Channel 4: 1 Excite all reps w/Exchan 1 5: 5000 mV Excitation 6: 1 Loc [ mVperV ] 7: 1 Mult 8: 0 Offset

11: IF (X<=>F) (P89) ;Check for last pass through loop 1: 5 X Loc [ count ] ;to set output flag 2: 3 >= 3: 5 F 4: 10 Set Output Flag High

9

4WFB120, 4WFB350, 4WFB1K 4 Wire Full Bridge Terminal Input Modules

12: Set Active Storage Area (P80) ;Direct averaged "zero" reading 1: 3 Input Storage ;to input storage 2: 2 Array ID or Loc [ mVperV_0 ]

13: Average (P71) 1: 1 Reps 2: 1 Loc [ mVperV ]

14: If Flag/Port (P91) ;When average is calculated, 1: 10 Do if Output Flag is High (Flag 0) ;also send it to Final Storage 2: 10 Set Output Flag High

15: Set Active Storage Area (P80) ;Direct Output to Final Storage 1: 1 Final Storage 2: 11 Array ID ;set Array ID = 11 for zero data

16: Real Time (P77) 1: 110 Day,Hour/Minute

17: Sample (P70) 1: 1 Reps 2: 2 Loc [ mVperV_0 ]

18: End (P95)

19: End (P95)

End Program

5.1.3 CR7

;{CR7}

*Table 1 Program 01: 1.0000 Execution Interval (seconds)

;Other measurements could be inserted here or before the Output section

1: If Flag/Port (P91) ;On the first execution (Flag 1 is low) 1: 21 Do if Flag 1 is Low ;or when user sets Flag 1 low 2: 1 Call Subroutine 1 ;call the zeroing subroutine

2: Full Bridge (P6) ;Measure the strain gage 1: 1 Reps 2: 3 ±15 mV Slow Range 3: 1 In Card 4: 1 DIFF Channel 5: 1 Ex Card 6: 1 Ex Channel 7: 1 Meas/Ex 8: 5000 mV Excitation 9: 1 Loc [ mVperV ] 10: 1 Mult 11: 0 Offset

10

4WFB120, 4WFB350, 4WFB1K 4 Wire Full Bridge Terminal Input Modules

3: Z=X-Y (P35) ;Subtract zero reading from the 1: 1 X Loc [ mVperV ] ;measurement 2: 2 Y Loc [ mVperV_0 ] 3: 3 Z LOC [ Vr ]

4: Z=X*F (P37) ;Change Vr from mV/V to V/V 1: 3 X Loc [ Vr ] 2: 0.001 F 3: 3 Z Loc [ Vr ]

;The following instructions calculate microstrain

5: Z=X*F (P37) 1: 3 X Loc [ Vr ] 2: -2 F 3: 9 Z LOC [ 1_2Vr ]

6: Z=Z+1 (P32) 1: 9 Z LOC [ 1_2Vr ]

7: Z=X/Y (P38) 1: 3 X Loc [ Vr ] 2: 9 Y Loc [ 1_2Vr ] 3: 10 Z LOC [ Vr_1_2Vr ]

8: Z=X*Y (P36) 1: 10 X Loc [ Vr_1_2Vr ] 2: 8 Y Loc [ Mult ] 3: 4 Z LOC [ uStrain ]

;Output Section ;This example outputs an average of the 1 second readings ;once per minute.

9: If time is (P92) 1: 0 Minutes (Seconds --) into a 2: 1 Interval (same units as above) 3: 10 Set Output Flag High

10: Set Active Storage Area (P80) 1: 1 Final Storage Area 1 2: 1 Array ID ;Set Array ID = 1 for measurement data

11: Real Time (P77) 1: 1110 Year,Day,Hour/Minute

12: Average (P71) 1: 1 Reps 2: 4 Loc [ uStrain ]

11

4WFB120, 4WFB350, 4WFB1K 4 Wire Full Bridge Terminal Input Modules

*Table 2 Program 01: 0.0000 Execution Interval (seconds)

*Table 3 Subroutines

1: Beginning of Subroutine (P85) ;Subroutine to measure "zero" 1: 1 Subroutine 1

2: Do (P86) ;This prevents calling subroutine 1: 11 Set Flag 1 High ;until user sets flag 1 low again.

3: Z=F (P30) ;Set counter use for average to 0 1: 0 F 2: 5 Z LOC [ Count ]

4: Z=F (P30) ;load 4000 into 1: 4000 F ;input location 2: 7 Z LOC [ 4e6 ]

5: Z=X*F (P37) ;Multiply by 1000 to get (4*uS/S) 1: 7 X Loc [ 4e6 ] 2: 1000 F 3: 7 Z LOC [ 4e6 ]

6: Z=F (P30) ;Load Gage Factor into input location 1: 2 F ;Enter the actual Gage Factor here 2: 6 Z LOC [ GF ]

7: Z=X/Y (P38) ;calculate multiplier to use with strain 1: 7 X Loc [ 4e6 ] ;calculation 2: 6 Y Loc [ GF ] 3: 8 Z LOC [ Mult ]

8: Beginning of Loop (P87) ;Loop through 5 times to obtain average 1: 0 Delay ;zero reading 2: 5 Loop Count

9: Z=Z+1 (P32) ;Increment Counter used to determine 1: 5 Z Loc [ Count ] ;when to output

10: Full Bridge (P6) ;Measure Strain Gage 1: 1 Reps 2: 3 ± 15 mV Slow Range 3: 1 In Card 4: 1 DIFF Channel 5: 1 Ex Card 6: 1 Ex Channel 7: 1 Meas/Ex 8: 5000 mV Excitation 9: 1 Loc [ mVperV ] 10: 1 Mult 11: 0 Offset

12

4WFB120, 4WFB350, 4WFB1K 4 Wire Full Bridge Terminal Input Modules

11: IF (X<=>F) (P89) ;Check for last pass through loop 1: 5 X Loc [ Count ] ;to set output flag 2: 3 >= 3: 5 F 4: 10 Set Output Flag High

12: Set Active Storage Area (P80) ;Direct averaged "zero" reading 1: 3 Input Storage ;to input storage 2: 2 Array ID or Loc [ mVperV_0 ]

13: Average (P71) 1: 1 Reps 2: 1 Loc [ mVperV ]

14: If Flag/Port (P91) ;When average is calculated, 1: 10 Do if Output Flag is High (Flag 0) ;also send it to Final Storage 2: 10 Set Output Flag High

15: Set Active Storage Area (P80) ;Direct Output to Final Storage 1: 1 Final Storage 2: 11 Array ID ;set Array ID = 11 for zero data

16: Real Time (P77) 1: 110 Day,Hour/Minute

17: Sample (P70) 1: 1 Reps 2: 2 Loc [ mVperV_0 ]

18: End (P95)

19: End (P95)

End Program

5.2 CRBasic

Dataloggers that use CRBasic include our CR800, CR850, CR1000, CR3000, CR5000, and CR9000(X). CRBasic uses the StrainCalc Instruction for calculating strain from the output of different full bridge configurations:

StrainCalc(Dest,Reps,Source,BrZero,BrConfig,GageFactor,PoissonRatio)

Source is the variable holding the current measurement, BrZero is the zero measurement; this instruction uses the results of the full bridge measurement instruction (multiplier=1, offset=0, mV/V) directly. The code for the Bridge Configuration used with the 4WFB module is -1. Enter the actual gage factor for GageFactor. Enter 0 for the Poisson ratio parameter which is not used with this bridge configuration.

13

4WFB120, 4WFB350, 4WFB1K 4 Wire Full Bridge Terminal Input Modules

5.2.1 CR9000(X)

This example program is slightly different in operation than the examples for the other dataloggers. Data are only output to data table STRAINS when the user sets Flag(1). Every measurement is output (rather than averages like in the other examples) while Flag(1) is high.

' Program name: STRAIN.DLD

Public Count, ZStrain, StMeas, Strain, Flag(8) 'Declare all variables as public

'Data Table STRAINS samples every measurement when user Sets Flag(1) High

DataTable(STRAINS,Flag(1),-1) DataInterval(0,0,0,100) 'Interval = Scan, 100 lapses Sample (1,Strain,Ieee4) EndTable

'DataTable ZERO_1 stores the "zero" measurements

DataTable(ZERO_1,Count>99,100) 'Trigger on Count 100 Average(1,ZStrain,IEEE4,0) EndTable

'Subroutine to measure Zero, Called when user sets Flag(2)low

Sub Zero Count = 0 'Reset Count Scan(10,mSec,0,100) 'Scan 100 times BrFull(ZStrain,1,mV50,5,1,6,7,1,5000,1,0,0,100,1,0) Count = Count + 1 'Increment Counter used By DataTable CallTable ZERO_1 'Zero_1 outputs on last scan (Count=100) Next Scan ZStrain = ZERO_1.ZStrain_Avg(1,1) 'Set ZStrain = averaged value Flag(1) = True End Sub

BeginProg Scan(10,mSec,0,0) 'Scan 10(mSecs) If Not Flag(2) Then Zero BrFull(StMeas,1,mV50,5,1,6,7,1,5000,1,0,0,100,1,0) StrainCalc(Strain,1,StMeas,ZStrain,-1,2,0) CallTable STRAINS 'Strains outputs only when Flag(1)=True Next Scan EndProg

14

4WFB120, 4WFB350, 4WFB1K 4 Wire Full Bridge Terminal Input Modules

⎛

⎞

R

∆

R

+

∆

6. Calculation of Strain

Vx

H

R1

L

R2

or AG

FIGURE 6-1. Strain Gage in Full Bridge

Figure 6-1 is the diagram of the strain gage in the full bridge configuration provided by the terminal input module. The result of the datalogger's full bridge measurement when a multiplier of 1 and an offset of 0 is used is the measured bridge output in millivolts divided by the excitation in volts (1000 mV=1V):

Vout

1000 1000

⋅=⋅+−

Vin

The result is output in the units of millivolts output per volt o f excitation because the output voltage is small relative to the excitation voltage; these units allow the result to be a larger number easier for the datalogger to display and store (see data format discussion in the datalogger manual). The output is a ratio because: 1) the datalogger's ratiometric measurement technique allows this ratio to be more accurate than the measurement of the output voltage (errors in the excitation and measured output cancel). 2) This ratio can be used directly in the calculation of strain.

H

R3

L

Rg

G

R

g

⎜

RRRRR

⎝

3

g

2

+

12

6.3.

⎟ ⎠

When strain is calculated the direct ratio of the voltages (volts per volt not millivolts per volt) will be used:

Vout

=

Vin

If the previous equation is taken as the result when the gage is unstrained, then when the gage is strained it will change resistance by

the bridge output is:

Vout

Vin

=

strained

g

RRRRR

+

3

g

gg

RR RRRR

++

gg

3

−

12

∆

2

6.4.

+

R

. The equation for

g

2

−

12

6.5.

+

15

4WFB120, 4WFB350, 4WFB1K 4 Wire Full Bridge Terminal Input Modules

r

+

∆

=

⋅

∆∆∆∆∆

(

∆

Subtracting the unstrained (zero) result from the strained result gives

Vout

⎛

⎞

⎜

V

=

r

=

The terminal input module is selected so that

:

R

3

V

=

r

Solving for strain:

⎟

⎝

⎠

Vin

strained unstrained

RR R RR

()()

++ ⋅ +

gg g

33

++ ⋅ +

()()424

RR R RR

gg g gg

Vout

⎛ ⎜

− ⎝

Vin

RR

⋅

∆

g

3

∆

RR

gg

42

⎞ ⎟ ⎠

=

+=

∆∆

RRV

ggr

+=

RV RV R

gr gr g

∆∆

RR

gg

=

RR RRRR

++

gggg

3

RR

gg

2

+

RRRRRR

gggggg

)

R

g

−

∆

. Substituting R for

g3

=

+

3

+

2

V

:

6.6.

g

6.7.

∆

42

412

Strain is calculated by dividing equation 6.8 by the gage factor. The units are converted to microstrain by multiplying by 10

µε

=−

RV R RV

RV R V

=

∆∆

gr g gr

=−

∆

gr g r

4

−

12VV

410

GF V

()

R

r

6

⋅

−

12

()

g

=

V

r

6.8.

R

g

6

uS/S.

6

10

=

GF R

r

⋅

R

g

6.9.

g

16

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