Usually it is represented in a unit of 1 x 10 -6 , since the ratio of deformation is often very small. External force applied to a metallic material generates physical deformation and electrical resistance change of the material.
In case that such material is sticked onto test specimen via electrical insulation, the material produces a change of electrical resistance corresponding to the deformation. Strain gauges consist of electrical resistance material and measure strains proportional to the resistance changes. In order to measure change in the resistance value due to extension of metal resistor , how is a strain gauge structured?
A strain gauge is constructed by forming a grid made of fine electric resistance wire or photographically etched metallic resistance foil on an electrical insulation base backing , and attaching gauge leads. We have seen how a strain gauge is structured. Next the principle of how strain is measured based on change in a resistance value due to extension of metal resistor will be explained.
When strain is generated in a test specimen and a strain gauge is atstached, the strain is relayed via the gauge base electrical insulation to the resistance wire or foil in the gauge.
As a result, the fine wire or foil experiences a variation in electrical resistance. This variation is exactly proportional to the strain. Normally, this resistance change is very small and requires a Wheatstone bridge circuit to convert the small resistance change to a more easily measured voltage change.
It supplies the voltage to the strain gage with one pair of leads and measures it with another pair as shown in Figure 7. The six wires are used in pairs for Sense, Excite, and Measure. The Sense lead is a feedback loop to ensure that the Excite voltage is constantly held within specifications. The difference is in the resolution of the system.
That is, the small lb load cell produces 0. Conductors carrying such low level signals are susceptible to noise interference and should be shielded. Low-pass filters, differential voltage measurements, and signal averaging are also effective techniques for suppressing noise interference.
Furthermore, instrumentation amplifiers usually condition the extremely low strain gage signals before feeding them to ADCs. The instrumentation amplifier gain should be adjusted to provide the full-scale output of the strain gage or load cell over the entire range of the ADC. Force and pressure transducers typically generate an offset output signal when no external force is applied. Instrumentation amplifiers usually contain a control to adjust this offset to zero and let the load cell cover the full range of the ADC.
Most instruments also provide adjustable excitation, and gain. A strain-gage signal in a Wheatstone bridge is superimposed on a common-mode voltage equal to half the excitation voltage. CMRR is a measure of how well the amplifier rejects common-mode voltages.
The amplifier can introduce 0. Strain gage signal-conditioning modules usually provide a regulated excitation source with optional Kelvin excitation. Onboard bridge-completion resistors may be connected for quarter and half-bridge strain gages. Instrumentation amplifiers provide input and scaling gain adjustments and an offset adjustment nulls large quiescent loads. This lets input signals use the full range of the data acquisition system and the measurements cover the full resolution of the ADC.
Some strain gage signal conditioners provide fixed gain, offset, and excitation settings, but fixed settings do not take advantage of the maximum dynamic range of the ADC. It decreases the actual available resolution of the measurement. At 10 V, the excitation, offset, and gain trimming are all fixed and no adjustments can be made.
Also, the offset adjustment lets users zero the output offset produced by either a small bridge imbalance or a quiescent deformation of the mechanical member. And the gain adjustment lets users set a gain that provides a full-scale output under maximum load, which optimizes the dynamic range of the ADC. Calibration The signal-conditioning module also typically provides a shunt calibration feature See Figure 7.
It lets users switch their own shunt resistors into either one of the two lower legs of the bridge under software control. For example, a shunt resistor can be calculated to simulate a full load.
Applying a shunt resistor is a convenient way to simulate an imbalance without having to apply a physical load. For any balanced bridge, a specific resistor can be connected in parallel with one of the four bridge elements to obtain a predictable imbalance and output voltage. Many products include calibration software with a Windows-based program that provides several calibration methods, online instruction, and a diagnostic screen for testing the calibrated system.
Transducers and Load Cells Strain gages are commercially available in prefabricated modules such as load cells that measure force, tension, compression, and torque. Load cells typically use a full-bridge configuration and contain four leads for bridge excitation and measurement. The manufacturers provide calibration and accuracy information. Strain Diaphragm Pressure Gages A strained-diaphragm pressure gage consists of two or four strain gages mounted on a thin diaphragm. The gages are wired in a Wheatstone bridge circuit, including bridge completion resistors when needed, so the pressure gage is electrically equivalent to a load cell.
When one side of the diaphragm called the reference pressure side is open to the ambient atmosphere, the gage compares the inlet pressure to the ambient pressure, which is about When the gage measures ambient pressure, the reference chamber must be sealed with either a vacuum reference near zero psi or the sea-level reference. Temperature variations can affect the accuracy of the strain diaphragm pressure gages.
A pressure gage with a sealed non-zero reference pressure exhibits temperature variations consistent with the ideal gas law. Temperature variations can also affect the performance of the strain gages themselves. Transducers must contain temperature compensation circuits to maintain accurate pressure measurements in environments with widely varying temperatures.
All strained-diaphragm pressure gages require a regulated excitation source. Some gages contain internal regulators, so users can connect an unregulated voltage from a power supply.
Some strained-diaphragm pressure gages also employ internal signal conditioning, which amplifies the mV signal output of the Wheatstone bridge to a full-scale voltage from 5 to 10 V.
Gages of this type have low-impedance outputs. In contrast, other pressure gages have no internal signal conditioning so their output impedance equals the Wheatstone bridge resistance several k W for semiconductor types , and their full-scale output is in mV.
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