STRAIN GAUGE MEASUREMENT (uStrain)
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Strain Gauge Measurement with the i100
The i100xx box provides 8 differential analog input voltage channels (14bit) and 8 voltage output channels (i.e. for excitation) that can be used for strain gauge measurement. For specifications, click here.
Strain Gage Measurement, Quarter Bridge (uStrain)
Strain Gage Measurement, Half Bridge, Bending (uStrain)
Strain Gage Measurement, Half Bridge, Axial (uStrain)
Strain Gage Measurement, Full Bridge, Bending (uStrain)
Strain Gage Measurement, Full Bridge, Axial I (uStrain)
Strain Gage Measurement, Full Bridge, Axial II (uStrain)
Strain Gage Measurement, Quarter Bridge (uStrain)
Strain measurement using a 1/4 bridge circuit involves wiring a strain gage as one leg of a full-bridge circuit, applying a voltage across the bridge, and measuring the voltage across the two intermediate bridge nodes via a pair of instruNet Vin+ and Vin- input terminals. The excitation voltage for the bridge is supplied by either the instruNet Vout terminal or by an external voltage source. In figure 3.7, Rgage is a strain gage, Ro is either a fixed resistor of known value or a fixed unstrained strain gage of value Ro, and RLis the lead wire resistance. instruNet calculates the value of the strain, returning "strain" engineering units, using the equations:
Vratio (V/V) = ((Vin+ - Vin-) - Vinit) / Vout
Strain = (-4Vratio / [GF * (1 + 2Vratio)]) * (1 + RL/Ro )
Fig 3.7 - Strain Measurement - Quarter Bridge Circuit
Ro, RL, GF, Vinit and Vout are fixed values that are specified by the user in the Constants Settings area, whereas (Vin+ - Vin-) are measured in realtime by instruNet. Ro and Unstrained-Rgage must be the same value (e.g. 350ohms)in order for the bridge to operate properly.
For more details, please see Maximizing Strain Gauge Accuracy,
Load Cells, and Voltage Ratio Measurements.
To do Strain Gage measurement using a 1/4 Bridge circuit you must:
1. Set the Sensor field in the Hardware settings area to Strain Gage.
2. Set the Range field in the Hardware settings area to +/- 10mV.
3. Set the Ro field in the Constants settings area to the value of one Ro bridge completion resistor, in ohms units.
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4. Set the GF field in the Constants settings area to the gage's gage factor.
5. Set the Vout field in the Constants settings area to specify the voltage that is to be applied to the bridge (1V is typical). If you are applying an external excitation voltage, enter -Ro value in the Ro edit field (e.g. -100 instead of 100 ohms) to tell the software that the excitation is external, and then enter the value of the external excitation voltage into the Vout field (e.g. 4V).
In high current cases (e.g. >2mA), it is often helpful to alternate the polarity of the excitation voltages to evenly burden the +/-12V supplies.
6. Set the delta, Rlead field in the Constants settings area to the resistance, in ohms, of the wires leading to the bridge (0 ohms is typically ok).
7. Set the Vinit field in the Constants settings area to the voltage measured when the bridge is unstrained, in Volts units.
8. Set the Wiring field in the Hardware settings area to Q Bridge.
9. Capacitors across the voltage input terminals are highly recommended for reducing errors caused by RFI. With 350ohm gages, 0.1uF caps create a low pass filter at 4KHz [4K = 1 / (6.28 * 350 * 0.1e-6)], and are ideal at minimizing RFI effects.
10. Wire your voltage source per figure 3.7, click here if you need more guidance setting up the software, and click here if the measured value is not correct.
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Strain Gage Measurement, Half Bridge, Bending (uStrain)
Measuring bending strain using a 1/2 bridge configuration involves wiring two strain gages as shown in figure 3.8, applying a voltage across the bridge, and measuring the voltage across the two intermediate bridge nodes via a pair of instruNet Vin+ and Vin- input terminals. The excitation voltage for the bridge is supplied by either the instruNet Vout terminal or by an external voltage source. In figure 3.8, Rgage is a strain gage, Ro is either a fixed resistor of known value or a fixed unstrained strain gage of value Ro, and RLis the lead wire resistance. instruNet calculates the value of the strain, returning "strain" engineering units, using the equations:
Vratio (V/V) = ((Vin+ - Vin-) - Vinit) / Vout
Strain = (-2Vratio / GF) * (1 + RL/Ro )
Fig 3.8 - Strain Measurement - Half Bridge Circuit
Ro, RL, GF, Vinit and Vout are fixed values that are specified by the user in the Constants Settings area, whereas (Vin+ - Vin-) are measured in realtime by instruNet. Ro and Unstrained-Rgage must be the same value (e.g. 350ohms)in order for the bridge to operate properly.
To do a bending Strain Gage measurement using a 1/2 Bridge circuit you must:
1. Do steps #1 through #7 listed in the previous "Strain Gage Measurement - Quarter Bridge" discussion.
2. Set the Wiring field in the Hardware settings area to H Bridge Bend .
3. Wire your voltage source per figure 3.8, click here if you need more guidance setting up the software, and click here if the measured value is not correct.
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Strain Gage Measurement, Half Bridge, Axial (uStrain)
Measuring axial strain using a 1/2 bridge configuration involves wiring two strain gages as shown in figure 3.9, applying a voltage across the bridge, and measuring the voltage across the two intermediate bridge nodes via a pair of instruNet Vin+ and Vin- input terminals. The excitation voltage for the bridge is supplied by either the instruNet Vout terminal or by an external voltage source. In figure 3.9, Rgage is a strain gage, Ro is either a fixed resistor of known value or a fixed unstrained strain gage of value Ro, and RLis the lead wire resistance. instruNet calculates the value of the strain, returning "strain" engineering units, using the equations:
Vratio (V/V) = ((Vin+ - Vin-) - Vinit) / Vout
Strain = (-4Vratio / [GF * [(1 + u) - 2Vratio (u - 1)]]) * (1 + RL / Ro)
Fig 3.9 - Strain Measurement - Half Bridge Circuit (Axial)
u, Ro, RL, GF, Vinit and Vout are fixed values that are specified by the user in the Constants Settings area, whereas (Vin+ - Vin-) are measured in realtime by instruNet. Ro and Unstrained-Rgage must be the same value (e.g. 350ohms)in order for the bridge to operate properly4.
To do axial Strain Gage measurement using a 1/2 Bridge circuit you must:
1. Do steps #1 through #7 listed in the previous "Strain Gage Measurement - Quarter Bridge" discussion.
2. Set the Wiring field in the Hardware settings area to H Bridge Axial .
3. Set the v_Poisson field in the Constants settings area to the upoissson value of the material that you are twisting (e.g. aluminum is .32).
4. Wire your voltage source per figure 3.9, click here if you need more guidance setting up the software, and click here if the measured value is not correct.
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Strain Gage Measurement, Full Bridge, Bending (uStrain)
Measuring bending strain using a full bridge configuration involves wiring four strain gages as shown in figure 3.10, applying a voltage across the bridge, and measuring the voltage across the two intermediate bridge nodes via a pair of instruNet Vin+ and Vin- input terminals. The excitation voltage for the bridge is supplied by either the instruNet Vout terminal or by an external voltage source. instruNet calculates the value of the strain, returning "strain" engineering units, using the equations:
Vratio (V/V) = ((Vin+ - Vin-) - Vinit) / Vout
Strain = -Vratio / GF
Fig 3.10 - Strain Measurement - Full Bridge Circuit (Bending)
GF, Vinit and Vout are fixed values that are specified by the user in the Constants Settings area, whereas (Vin+ - Vin-) are measured in realtime by instruNet.
To do bending Strain Gage measurement using a Full Bridge circuit you must:
1. Do steps #1 through #7 listed in the previous "Strain Gage Measurement - Quarter Bridge" discussion.
2. Set the Wiring field in the Hardware settings area to F Bridge Bend.
3. Wire your voltage source per figure 3.10, click here if you need more guidance setting up the software, and click here if the measured value is not correct.
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Strain Gage Measurement, Full Bridge, Axial I (uStrain)
Measuring axial strain using a full bridge configuration involves wiring four strain gages as shown in either figure 3.11 or figure 3.12, applying a voltage across the bridge, and measuring the voltage across the two intermediate bridge nodes via a pair of instruNet Vin+ and Vin- input terminals. The excitation voltage for the bridge is supplied by either the instruNet Vout terminal or by an external voltage source. instruNet calculates the value of the strain, returning "strain" engineering units, using the equations:
Vratio (V/V) = ((Vin+ - Vin-) - Vinit) / Vout
Strain = -2Vratio / GF ( u+ 1)
Fig 3.11 - Strain Measurement - Full Bridge Circuit (Axial I)
u, GF, Vinit and Vout are fixed values that are specified by the user in the Constants Settings area, whereas (Vin+ - Vin-) are measured in realtime by instruNet.
To do Axial Strain Gage measurement using a Full Bridge circuit you must:
1. Do steps #1 through #7 listed in the previous "Strain Gage Measurement - Quarter Bridge" discussion.
2. Set the Wiring field in the Hardware settings area to F Bridge Axl I.
3. Set the v_Poisson field in the Constants settings area to the upoison value of the material that your are twisting (e.g. aluminum is .32).
4. Wire your voltage source per figure 3.11, click here if you need more guidance setting up the software, and click here if the measured value is not correct.
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Strain Gage Measurement, Full Bridge, Axial II (uStrain)
Measuring axial strain using a full bridge configuration involves wiring four strain gages as shown in figure 3.12, applying a voltage across the bridge, and measuring the voltage across the two intermediate bridge nodes via a pair of instruNet Vin+ and Vin- input terminals. The excitation voltage for the bridge is supplied by either the instruNet Vout terminal or by an external voltage source. instruNet calculates the value of the strain, returning "strain" engineering units, using the equations:
Vratio (V/V) = ((Vin+ - Vin-) - Vinit) / Vout
Strain = -2Vratio / [GF [( u+ 1) - Vratio (u- 1)]]
Fig 3.12 - Strain Measurement - Full Bridge Circuit (Axial II)
u, GF, Vinit and Vout are fixed values that are specified by the user in the Constants Settings area, whereas (Vin+ - Vin-) are measured in realtime by instruNet.
To do Axial Strain Gage measurement using a Bridge circuit you must:
1. Do steps #1 through #6 listed in the previous "Strain Gage Measurement - Quarter Bridge" discussion.
2. Set the Wiring field in the Hardware settings area to F Bridge II .
3. Set the v_Poisson field in the Constants settings area to the upoisson value of the material that your are twisting (e.g. aluminum is .32).
4. Wire your voltage source per figure 3.12, click here if you need more guidance setting up the software, and click here if the measured value is not correct.
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