Download - 800 Range Sensor 120Ohms Strain Gauge
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dataTaker120 strain gauge example Page 1
Strain Gauge 800 range logger example.
1. Materials Required.............................................................................................................................. 22. Programming a strain gauge to measure PPM.................................................................................... 2
3. Changing the scaling of a strain gauge from PPM to strain ............................................................. 74. Calculating the Strain value of a shunt resistor .............................................................................. 11
5. Scaling a strain gauge to a practical measurement. .......................................................................... 11
6. The effect a low precision resistor makes on the measurement........................................................ 19
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1. Materials Required PC with DeLogger loaded.
800 range dataTaker data logger
120 strain gauge 3x120 precision resistor
59880 shunt resistor
120 5% carbon resistor
Set of 50cent coins
Multimeter
2. Programming a strain gauge to measure PPM.
2.1. Objective
When presented with a strain gauge of unknown wiring and unknown resistance we need to be able todetermine how to wire the strain gauge and how to determine the wiring configuration. We then need to
program the logger.
2.2. Prerequisites
PC connected to a data logger.
DeLogger running and a connection established to the logger
2.3. Steps
Determine the number of wires
We need to simply count thenumber of connection wires forthe sensor
In this case because it has only three wires it can ONLY be a quarter or half bridge. A fullbridge MUST have 4 Wires
Measure the resistance between each pair of wires
Red-White=120.4Red-Black=120.4Black-White= 0.3
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Here is a full Bridge. The Vo is afunction of the V and the variousresistances
Full Bridge will use a strain
gauge for each resistor. We cansee that it must use at least 4wires.
If the resistors of the strain gaugeis nominally 120The resistance across the bridgewould be:
=
=+=
++
+=
120
008333..0041667.0041667.
1
120120
1
120120
11
R
R
R
The Resistance across one legwould be:
=
=+=
+++=
90
01111.002777.008333.1
120120120
1
120
11
R
R
R
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A Half bridge will require externalresistors when used in a fullbridge channel (i.e. R1 and R2will be required) alternatively itwill not require external resistors
when used in a dataTaker halfbridge channel. (The resistors areeffectively internal to the logger).The half Bridge will generallyhave 3 wires
If the strain gauge is nominally120. You will measureapproximately 120 between thecenter leg and the outer leg andapproximately 240 between the
outer legsThe supplied strain gauge isobviously not this configuration
A Quarter bridge will require 3external resistors (R1, R2 & R3)when connected to a dataTakerchannel. The quarter bridge willgenerally have 3 wires.If the strain gauge is nominally120. You should measureapproximately 120 between two
of the legs and a short circuitbetween another two of the legs
The strain gauge supplied isobviously this type
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There are a number of wiringoptions available. The 800 rangeloggers support voltage andcurrent excitation, 6 wire and 4wire bridges. When used with this
type of quarter bridge straingauge resistors R1, R2 & R3must be supplied externally. Inthis case we are going to use a 4wire current excitationconfiguration (see figure 123dt800 manual) . There are a 10different wiring and configurationpossibilities. (figures 116~125)
Wire the strain gauge using the 3supplied precision 120
resistors.
We have a 500 range loggerconnected to our PC andDeLogger is currently running.In the Prog1.dl8 window.Click R1-Signal and select Bridge
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Click the wiring Button
In this case the default wiring isappropriate.
Enter the Nominal resistance ofthe gauge as a FactorDoble click R1-Factor and enter120
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N= Number of active gauge elements (in this case 1)
If the gauge factor isunknown it is generallyassumed to be 2.Documentation supplied with
the strain gauge will include astatement of the gauge factoralso usually the temperaturecompensation the needs tobe applied.The factor given in this caseis 2.1000.5% (at 24 C).
The Gauge factor in this caseis therefore
1.904811.2
4
4
=
=
=
NG
The Scaling Tab
Click 1-Type
Select Polynomial
Set Coefficient 0 to 0Set Coefficient 1 to 1.9048
Set Text Units to uStrain
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Select Tab A
Click R1-Scaling
Select Y1-uStrain
Send the program to the
logger
In the text window we cannow see the strain figurebeing returned. We observethe current reading is 323strain. If the test piece is inan unloaded state we maywant to adjust this figure tozero. To do this we adjust thescaling factor accordingly.
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Select the Scaling Tab onProg1.dl8
Set Coefficient 0 to -323
Send the Program to thelogger
Program> Send toConnection
Switch to the Text windowand note the approximatelyzero micro strain reading
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4. Calculating the Strain value of a shunt resistor
4.1. Objective
When we are using strain gauges and we are required to calculate strain it can be aconfusing process. The ability to provide an offset of a known strain value can make things
simpler
4.2. Prerequisites
Changing the scaling of a strain gauge from PPM to strain
4.3. Steps
Using the 59880 shunt supplied put it in parallel to the gauge. If the gauge factor isassumed to be 2 this resistor represents 1000 Strain. (Refer to the calculation on theslides.
Note: when the shunt resistor
is place in parallel across thestrain gauge (i.e. between the+ and terminals of channelone) we obtain approximately1000 Strain. The differencebeing between the gaugefactor of 2.0 assumed for thecalculation of the resistancefactor of and the actualgauge factor of 2.1 is thereason for reading 952
instead of the 1000 expected.If we multiply the 1000*2 anddivide by 2.1 we obtain 952.
5. Scaling a strain gauge to a practical measurement.
5.1. Objective
We can use a strain gauge to scale many physical phenomena. If we place the gauge as a cantilever and
progressively place 50-cent pieces on the gauge we can rescale the gauge to measure in units of 50cents.
5.2. Prerequisites
5.3. Steps
Remove the scaling from the logger so the logger reports in PPM and increasethe number of samples.
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Arrange the strip with theload cell as a cantilever andin such a way coins can beadded to the end.
On Tab A in Prog1.dl8
Set the Scaling to None
Select the Parameters TabSet P46 to 200
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Send the Program to thelogger
Program>Send toConnection
Switch to the Text.dltwindow.Record the Current readingsto one PPM.In this case 105ppm
Make a table of the PPM vsCoins
PPM Coins
105 0
110 1
115 2
119 3
124 4
129 5134 6
138 7
143 8
Switch to the Text.dlt window
147 9
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Open Excel and input thedata.
Highlight the data.
Click the Graph WizardButton
Select XY Scatter
Click Next
Click Next
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Click Next
Click Finish
Select the data Points.
Right Click and select AddTrend Line
Select Linear (in this case itis the most appropriate.
Click the Options Tab
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Check Display Equation onchart
Record the equation
In this case
y = 0.2131x - 22.431
Please Note: Sometimesexcel does not display thesenumbers to an appropriatenumber of digits. To change
the number of digitsdisplayed. Right Click on theequation and select FormatData Labels
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Select the Number Tab
Select Category Number
Set the Decimal Places to an
appropriate number.
Click OK
Note the equation isdisplayed to a higherprecision i.e.
y = 0.213059x - 22.430632
In Prog1.dl8Set Coefficient 0 to
22.430632Set Coefficient 1 to 0.213059
Set the units to Coins
Select the A Tab and Set the
scaling to Coins
Send the Program to thelogger.
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Select Form1.dlt
Ensure the Form window is
in Edit Mode
Click Channel 1 Cell
Select the appropriatechannel
Dt500 Com1.JOB1.A.BGIIn this case
Click OK
Click the Value 1 Cell
Click the 0 decimal placessetting
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Click the Edit Mode Button
The Form will now displaythe correct number of coinsplaced on the cantilever.
6. The effect a low precision resistor makes on the measurement.
6.1. Objective
We will replace the precision resistor with a 5% carbon resistor to observe the effect this has upon themeasurement.
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6.2. Prerequisites
6.3. Steps
Remove one of the precisionresistors and replace it with
a 5% carbon 120 resistor.
Remove the scaling from theProg1.dl8Click Scaling and selectNone
Send the program to thelogger
Select the Chart window.Click the Select ChannelsButton
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dataTaker
Select Channel 1
Dt800 Com1.JOB1.A.1BGIfor the Y axis
Click OK
Click the Edit Mode Button
Click The Re-Scale Button
Use the Left or Right Mousekeys to modify the graphappropriately. Hold the 120resistor and note how thereading varies wildly basedupon the currenttemperature. These resistorsare simply not usable as abridge completion resistors