Strain gauge

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The strain gauge is the device used to measure the load on the object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of flexible backing insulation that supports metallic foil patterns. The meter is mounted on the object with the appropriate adhesive, such as cyanoacrylate. Because the object is deformed, the foil undergoes deformation, causing the electrical resistance to change. This change in resistance, typically measured using Wheatstone bridges, is related to strains with a quantity known as the measuring factor .

Video Strain gauge

Physical operation

The strain gauge takes advantage of the physical property of electrical conductance and its dependence on conductor geometry. When an electric conductor is stretched within the limits of its elasticity so that it does not break or permanently change, it becomes narrower and longer, a change that increases its electrical end-to-end resistance. Conversely, when a conductor is compressed in such a way that it does not curve, the conductor will widen and shorten, a change that reduces its electrical resistance from end to end. From the electrical resistance measured from the strain gauge, the amount of induced stress can be inferred.

A typical strain gauge regulates long and thin conductive strips in a zigzag pattern of lines from parallel lines. This does not increase the sensitivity, because the percentage change in resistance to the given strain for the entire zig-zag is the same as for every single trace. However, a single linear trace must be very thin and therefore tend to overheat (which will change its resistance and cause it to expand), or it should operate at a much lower voltage, making it more difficult to measure the change in resistance. accurate.

Maps Strain gauge

Measurement factor

Faktor pengukur ${\ displaystyle GF}  ÂÂ$ didefinisikan sebagai:

${\ displaystyle GF = {\ frac {\ Delta R/R_ {G}} {\ epsilon}}}  ÂÂ$

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${\ displaystyle \ Delta R}  ÂÂ$ adalah perubahan resistensi yang disebabkan oleh ketegangan,

${\ displaystyle R_ {G}}  ÂÂ$ adalah hambatan dari pengukur yang tidak terukur, dan

${\ displaystyle \ epsilon}  ÂÂ$ sedang tegang.

For general metal foil gauges, the gauge factor is usually slightly above 2. For a single active gauge and three puppet resistors with the same resistance of the active gauge in the Wheatstone bridge configuration, output ${\ displaystyle v}$ from the bridge over:

${\ displaystyle v = {\ frac {BV \ cdot GF \ cdot \ epsilon} {4} Â Â$

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$>\;             {\displaystyle         B        V      }        \{\backslash \; displaystyle\; BV\}\;  ÂÂ$ adalah tegangan eksitasi jembatan.

The foil gauge usually has an active area of ​​about 2-10 mm ² in size. With careful installation, precise gauges, and correct adhesives, a stretch of up to at least 10% can be measured.

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In practice

The excitation voltage is applied to the input of the measuring network, and the voltage reading is taken from the lead output. The typical input voltage is 5Ã,V or 12Ã,V and the typical output readings are in milivolt.

The foil strain gauge is used in many situations. Different applications place different requirements on the meter. In most cases the orientation of the strain gauge is significant.

The gauges installed in the load cell are usually expected to remain stable for several years, if not for several decades; while those used to measure responses in dynamic experiments may only need to remain attached to the object for several days, energized less than an hour, and operate for less than a second.

The strain gauge is attached to the substrate with a special glue. The type of glue depends on the age required of the measurement system. For short-term measurements (up to several weeks) cyanoacrylate glue is suitable, for durable epoxy glue is required. Usually epoxy glue requires high temperature drying (about 80-100 ° C). Preparation of surfaces in which the strain gauge should be glued is the most important. The surface must be smoothed (for example with very fine sand paper), destroyed with solvent, the solvent trace must be removed and the strain gauge should be glued immediately after this to avoid oxidation or contamination from the prepared area. If these steps are not followed by strain gauges binding to the surface, unacceptable and unexpected measurement errors may be generated.

Technologically based strain gauges are commonly used in the manufacture of pressure sensors. The gauges used in the pressure sensors themselves are generally made of silicon, polysilicon, metallic film, thick film, and bonded foils.

Temperature variation

Temperature variations will have many effects. The object will be resized by thermal expansion, which will be detected as a strain by the gauge. The gauge's resistance will change, and the connecting cable constraints will change.

Most strain gauges are made of constantan alloys. Various alloys of constantan and Karma alloys have been designed so that the temperature effects on resistance of the strain gauge itself cancel the change in resistance of the meter due to the thermal expansion of the object being tested. Since different materials have different amounts of heat expansion, self-temperature compensation (STC) requires the selection of special alloys tailored to the material under test.

Self-compensated tensile strain gauges (such as isoelastic alloys) can be compensated for by temperature using a dummy measuring technique. The dummy gauge (identical to the active strain gauge) is mounted on the non-drained sample of the same material as the test specimen. Samples with a dummy gauge are placed in thermal contact with the specimen, adjacent to the active gauge. The dummy gauge is connected to the Wheatstone bridge in the arm adjacent to the active gauge so that the temperature effects on the active gauge and imitation cancel each other out. (Murphy's law was originally created in response to a set of incorrect measuring instruments transferred to the Wheatstone bridge.)

The temperature effects on the main cable can be canceled by using a "3-wire bridge" or "4-wire ohm circuit" (also called "4-wire Kelvin connection").

In any case it is a good engineering practice to keep the Wheatstone bridge voltage of the drive low enough to avoid heating its own strain gauge. Self-heating of the strain gauge depends on its mechanical characteristics (large strain gauges are less susceptible to self-heating). The low voltage bridge level reduces overall system sensitivity.

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Errors and compensation

Zero Offset - If the impedance of the four measuring arms is not exactly the same after hooking the gauge to force collector, there will be a zero offset that can be compensated by inserting a parallel resistor to one or more of the gauge sleeve.

• The measuring factor temperature coefficient (TCGF) is a change in the sensitivity of a device to filter with temperature changes. This is generally compensated by the introduction of fixed resistance at the foot of the input, where the applied voltage will effectively decrease with the temperature rise, compensation for the increase in sensitivity with the rise in temperature. This is known as the modulus compensation in the transducer circuit. As the temperature rises, load cell elements become more elastic and therefore under constant load will change more shape and lead to increased output; but the burden is still the same. The smart bit in all of this is that the resistor in the bridge supply must be a temperature sensitive resistor that is matched to both the material whose meter is bound and also to the measuring element material. The value of the resistor depends on both values ​​and can be calculated. In simple terms if the output increases then the resistor value is also increased thereby reducing the net voltage to the transducer. Get the correct resistor value and you will not see any changes.
• Zero shifts with temperature - If the TCGF of each gauge is not equal, there will be zero shift with temperature. This is also caused by anomalies in the style collector. This is usually compensated by one or more resistors strategically placed in the compensation network.
• Linearity is an error where sensitivity changes across the range of pressure. This is usually a function of selecting the thickness of the collection of forces for the intended pressure and the quality of the bond.
• Hysteresis is an error to return to zero after a visit of pressure.
Repeatability - This error is sometimes bound to hysteresis but across the range of pressure.
• EMI induced error - As strain gauges output voltage in mV range, even? V if the drive voltage of the Wheatstone bridge is kept low to avoid self heating of the elements, special care must be taken in the amplification of the output signal to avoid reinforcement as well as the superimposed noise. The most commonly used solution is to use a "carrier frequency" amplifier that converts voltage variations into frequency variations (as in VCO) and has a narrow bandwidth thereby reducing out of the EMI band.
• Overloading - If the strain gauge is loaded outside the design boundary (measured in microstrain), its performance decreases and can not be recovered. Practical techniques that are usually good suggest not to emphasize the strain gauge above Ã, Â ± 3000 microstrain.
• Humidity - If the cable connecting the strain gauge to the signal conditioner is not protected from moisture, such as bare wire, corrosion can occur, leading to parasitic resistance. This may allow the current to flow between the cable and the substrate to which the strain gauge is glued, or between two wires directly, introduces an error competing with the current flowing through the strain gauge. For this reason, the current high resistance strain gauge (120 ohm) is less susceptible to this type of error. To avoid this error, simply protect the strain gauge by isolating the enamel (eg, epoxy or polyurethane type). The strain gauge with an unprotected cable can only be used in a dry laboratory environment but not in an industrial setting.

In some applications, strain gauges add mass and attenuation to the vibration profile of the hardware intended to be measured. In the turbomachinery industry, one of the alternatives used for strain gauge technology in vibration measurements on rotating hardware is the Non-Intrusive Voltage Measurement System, which allows the measurement of blade vibration without a knife or hardware...

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Other types

For small strain measurements, semiconductor strain gauges, called piezoresistors, are often preferred over foil gauges. Semiconductor gauges usually have a larger measuring factor than a foil gauge. Semiconductor gauges tend to be more expensive, more sensitive to temperature changes, and more fragile than foil gauges.

Nanoparticle-based strain gauges appear as promising new technologies. These resistive sensors whose active areas are made by assembling conductive nanoparticles, such as gold or carbon, combine high measuring factors, large deformation ranges and small electrical consumption due to their high impedances.

In biological measurements, especially blood flow and tissue swelling, a variant called mercury-in-rubber gauge strain is used. This type of strain gauge consists of small amounts of liquid mercury flanked in small rubber tubes, used around for example legs or feet. The swelling of the body part produces tube stretching, making it longer and thinner, which increases the electrical resistance.

Fiber optic sensing can be used to measure the tension along the optical fiber. Measurements can be distributed along the fiber, or taken at a predetermined point on the fiber. American Cup 2010 Alinghi 5 and USA-17 both use embedded sensors of this type.

Microscale strain gauges are widely used in microelectromechanical systems (MEMS) to measure strains such as those induced by force, acceleration, pressure or sound. For example, airbags in cars are often triggered with a MEMS accelerometer. As an alternative to piezo resistant strain gauges, integrated optical ring resonators can be used to measure strain in Micro-Opto-Electro-Mechanical Systems (MOEMS).

The capacitive strain gauge uses variable capacitors to indicate the degree of mechanical deformation.

The vibrating wire strain gauge is used in geotechnical applications and civil engineering. The measuring instrument consists of wire vibrated and tightened. The strain is calculated by measuring the resonant frequency of the wire (increasing the tension increases the resonant frequency).

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Mechanical type

A simple mechanical type is used in civil engineering to measure the movement of buildings, foundations, and other structures. More sophisticated mechanical types combine dial indicators and mechanisms to compensate for temperature changes. This type can measure the movement as small as 0.002 mm.

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See also

• Resistance thermometer

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References

Source of the article : Wikipedia