Download LECTURE 18 S-G SIGNAL

MECH 373
Instrumentation and Measurements
Lecture 18
Measurement of Solid-Mechanical
Quantities (Chapter 8)
Strain Gauge Signal Conditioning
Lecture 18
Lecture Notes on MECH 373 – Instrumentation and Measurements
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Strain measurements
• Measuring gauge resistance before and
after loading – Not practical - Significant
error– Resistance change in the order of
measuring device resolution.
• Better approach – Device to measure the
resistance change rather than resistance,
i.e., Wheatstone bridge.
Lecture 18
Lecture Notes on MECH 373 – Instrumentation and Measurements
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Wheatstone Bridge
• Four identical resistors.
• Voltages at points B and D
– same – V0 = 0.0, bridge =
balance.
• Active strain gauge R3 ,
Load R3 – bridge is
unbalanced and V0 ≠ 0.0.
• Adjust R2 – equal to R3,
then V0 = 0.0 – bridge is
balanced – Amount of
adjustment = change of
resistance in R3. DAS.
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Lecture Notes on MECH 373 – Instrumentation and Measurements
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Strain measurements
• Modern computer DAS –
– Balance the bridge initially.
– No need to re-balance it when the active
gauge under-strained .
– Output voltage proportional to change in
gauge resistance. (later slides)
Lecture 18
Lecture Notes on MECH 373 – Instrumentation and Measurements
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Wheatstone bridge
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Lecture Notes on MECH 373 – Instrumentation and Measurements
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Strain measurements
Lecture 18
Lecture Notes on MECH 373 – Instrumentation and Measurements
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Strain measurements
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Lecture Notes on MECH 373 – Instrumentation and Measurements
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Strain measurements
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Lecture Notes on MECH 373 – Instrumentation and Measurements
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Strain measurements
• In practice, gauges used are not truly identical.
• Uncertainty in an order of 0.1%.
• Vo – nonzero even before the strain applied –
Offset – Symmetric error.
• Adjust the resistor to bring the bridge into
initial balance.
• If not – subtract it from the actual readings.
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Lecture Notes on MECH 373 – Instrumentation and Measurements
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Strain measurements
• To improve the sensitivity of
circuit for a given small strain.
• Application – Symmetrical
loading situations – Same strain
but opposite sign – tension and
compression – e.g. sensor – load
cells.
• R1 and R3 – Tensile gauges.
• R2 and R4 – Compressive gauges.
• Full bridge = 4 times the quarter
bridge.
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Lecture Notes on MECH 373 – Instrumentation and Measurements
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Temperature compensation
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Lecture Notes on MECH 373 – Instrumentation and Measurements
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Temperature compensation
• Temperature:
– Affect resistivity, hence gauge factor S = dR/R/εa.
– Cause differential thermal expansion = structure and gauge, hence
strain without mech. load.
• Minimize the temp effects:
– Connect R2 and R3 in structure with the same temperature., R3 =
Active gauge, R2 = dummy (compensating) gauge – Temp change on
both R2 and R3 in same direction, hence no strain.
– (Note, R2 can be active gauge if strain is equal and opposite to R3.)
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Lecture Notes on MECH 373 – Instrumentation and Measurements
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Strain gauge in practical set-up
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Lecture Notes on MECH 373 – Instrumentation and Measurements
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Strain gauge in practical set-up
• Required many gauges to measure structure strain/stress.
• Separate bridge for each gauge.
• Connect output terminals of each bridge to input channels of
DAS.
• Use common power supply.
• R1 and R4 = Dummy gauges.
• Each active gauge paired with a fixed gauge R2 (compensating
or active).
• Bridge always complete.
– Continuous recording or switch to screen for monitoring (critical
gauges)
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Lecture Notes on MECH 373 – Instrumentation and Measurements
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Example – Quarter bridge
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Lecture Notes on MECH 373 – Instrumentation and Measurements
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Example Full bridge
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Lecture Notes on MECH 373 – Instrumentation and Measurements
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