Strain gauge | Wikipedia audio article

A strain gauge is a device used to measure
strain on an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most
common type of strain gauge consists of an insulating flexible backing which supports
a metallic foil pattern. The gauge is attached to the object by a suitable adhesive, such
as cyanoacrylate. As the object is deformed, the foil is deformed, causing its electrical
resistance to change. This resistance change, usually measured using a Wheatstone bridge,
is related to the strain by the quantity known as the Gauge factor.==Physical operation==
A strain gauge takes advantage of the physical property of electrical conductance and its
dependence on the conductor’s geometry. When an electrical conductor is stretched within
the limits of its elasticity such that it does not break or permanently deform, it will
become narrower and longer, which increases its electrical resistance end-to-end. Conversely,
when a conductor is compressed such that it does not buckle, it will broaden and shorten,
which decreases its electrical resistance end-to-end. From the measured electrical resistance
of the strain gauge, the amount of induced stress may be inferred.
A typical strain gauge arranges a long, thin conductive strip in a zig-zag pattern of parallel
lines. This does not increase the sensitivity, since the percentage change in resistance
for a given strain for the entire zig-zag is the same as for any single trace. A single
linear trace would have to be extremely thin, hence liable to overheating (which would change
its resistance and cause it to expand), or would need to be operated at a much lower
voltage, making it difficult to measure resistance changes accurately.==Gauge factor==
The gauge factor G
F {\displaystyle GF}
is defined as: G
R / R G ϵ {\displaystyle GF={\frac {\Delta R/R_{G}}{\epsilon
}}} where Δ
R {\displaystyle \Delta R}
is the change in resistance caused by strain, R G {\displaystyle R_{G}}
is the resistance of the undeformed gauge, and ϵ {\displaystyle \epsilon }
is strain.For common metallic foil gauges, the gauge factor is usually a little over
2. For a single active gauge and three dummy resistors of the same resistance about the
active gauge in a balanced Wheatstone bridge configuration, the output sensor voltage S
V {\displaystyle SV}
from the bridge is approximately: S
F ⋅
ϵ 4 {\displaystyle SV=EV{\frac {GF\cdot \epsilon
}{4}}} where E
V {\displaystyle EV}
is the bridge excitation voltage.Foil gauges typically have active areas of about 2–10
mm2 in size. With careful installation, the correct gauge, and the correct adhesive, strains
up to at least 10% can be measured.==In practice==An excitation voltage is applied to input
leads of the gauge network, and a voltage reading is taken from the output leads. Typical
input voltages are 5 V or 12 V and typical output readings are in millivolts.
Foil strain gauges are used in many situations. Different applications place different requirements
on the gauge. In most cases the orientation of the strain gauge is significant.
Gauges attached to a load cell would normally be expected to remain stable over a period
of years, if not decades; while those used to measure response in a dynamic experiment
may only need to remain attached to the object for a few days, be energized for less than
an hour, and operate for less than a second. Strain gauges are attached to the substrate
with a special glue. The type of glue depends on the required lifetime of the measurement
system. For short term measurements (up to some weeks) cyanoacrylate glue is appropriate,
for long lasting installation epoxy glue is required. Usually epoxy glue requires high
temperature curing (at about 80-100 °C). The preparation of the surface where the strain
gauge is to be glued is of the utmost importance. The surface must be smoothed (e.g. with very
fine sand paper), deoiled with solvents, the solvent traces must then be removed and the
strain gauge must be glued immediately after this to avoid oxidation or pollution of the
prepared area. If these steps are not followed the strain gauge binding to the surface may
be unreliable and unpredictable measurement errors may be generated.
Strain gauge based technology is utilized commonly in the manufacture of pressure sensors.
The gauges used in pressure sensors themselves are commonly made from silicon, polysilicon,
metal film, thick film, and bonded foil.===Variations in temperature===
Variations in temperature will cause a multitude of effects. The object will change in size
by thermal expansion, which will be detected as a strain by the gauge. Resistance of the
gauge will change, and resistance of the connecting wires will change.
Most strain gauges are made from a constantan alloy. Various constantan alloys and Karma
alloys have been designed so that the temperature effects on the resistance of the strain gauge
itself largely cancel out the resistance change of the gauge due to the thermal expansion
of the object under test. Because different materials have different amounts of thermal
expansion, self-temperature compensation (STC) requires selecting a particular alloy matched
to the material of the object under test. Strain gauges that are not self-temperature-compensated
(such as isoelastic alloy) can be temperature compensated by use of the dummy gauge technique.
A dummy gauge (identical to the active strain gauge) is installed on an unstrained sample
of the same material as the test specimen. The sample with the dummy gauge is placed
in thermal contact with the test specimen, adjacent to the active gauge. The dummy gauge
is wired into a Wheatstone bridge on an adjacent arm to the active gauge so that the temperature
effects on the active and dummy gauges cancel each other. (Murphy’s Law was originally coined
in response to a set of gauges being incorrectly wired into a Wheatstone bridge.)
Every material reacts when it heats up or when it cools down. This will cause strain
gauges to register a deformation in the material which will make it change signal. To prevent
this from happening strain gauges are made so they will compensate this change due to
temperature. Dependent on the material of the surface where the strain gauge is assembled
on, a different expansion can be measured. Temperature effects on the lead wires can
be cancelled by using a “3-wire bridge” or a “4-wire ohm circuit” (also called a “4-wire
Kelvin connection”). In any case it is a good engineering practice
to keep the Wheatstone bridge voltage drive low enough to avoid the self heating of the
strain gauge. The self heating of the strain gauge depends on its mechanical characteristic
(large strain gauges are less prone to self heating). Low voltage drive levels of the
bridge reduce the sensitivity of the overall system.==Errors and compensations==
Zero Offset – If the impedance of the four gauge arms are not exactly the same after
bonding the gauge to the force collector, there will be a zero offset which can be compensated
by introducing a parallel resistor to one or more of the gauge arms.
Temperature coefficient of gauge factor (TCGF) is the change of sensitivity of the device
to strain with change in temperature. This is generally compensated for by the introduction
of a fixed resistance in the input leg, whereby the effective supplied voltage will decrease
with a temperature increase, compensating for the increase in sensitivity with the temperature
increase. This is known as modulus compensation in transducer circuits. As the temperature
rises the load cell element becomes more elastic and therefore under a constant load will deform
more and lead to an increase in output; but the load is still the same. The clever bit
in all this is that the resistor in the bridge supply must be a temperature sensitive resistor
that is matched to both the material to which the gauge is bonded and also to the gauge
element material. The value of that resistor is dependent on both of those values and can
be calculated. In simple terms if the output increases then the resistor value also increase
thereby reducing the net voltage to the transducer. Get the resistor value right and you will
see no change. Zero shift with temperature – If the TCGF
of each gauge is not the same, there will be a zero shift with temperature. This is
also caused by anomalies in the force collector. This is usually compensated for with one or
more resistors strategically placed in the compensation network.
Linearity is an error whereby the sensitivity changes across the pressure range. This is
commonly a function of the force collection thickness selection for the intended pressure
and the quality of the bonding. Hysteresis is an error of return to zero after
pressure excursion. Repeatability – This error is sometimes tied-in
with hysteresis but is across the pressure range.
EMI induced errors – As strain gauges output voltage is in the mV range, even μV if the
Wheatstone bridge voltage drive is kept low to avoid self heating of the element, special
care must be taken in output signal amplification to avoid amplifying also the superimposed
noise. A solution which is frequently adopted is to use “carrier frequency” amplifiers which
convert the voltage variation into a frequency variation (as in VCOs) and have a narrow bandwidth
thus reducing out of band EMI. Overloading – If a strain gauge is loaded
beyond its design limit (measured in microstrain) its performance degrades and can not be recovered.
Normally good engineering practice suggests not to stress strain gauges beyond ±3000
microstrain. Humidity – If the wires connecting the strain
gauge to the signal conditioner are not protected against humidity, such as bare wire, corrosion
can occur, leading to parasitic resistance. This can allow currents to flow between the
wires and the substrate to which the strain gauge is glued, or between the two wires directly,
introducing an error which competes with the current flowing through the strain gauge.
For this reason, high-current, low-resistance strain gauges (120 ohm) are less prone to
this type of error. To avoid this error it is sufficient to protect the strain gauges
wires with insulating enamel (e.g., epoxy or polyurethane type). Strain gauges with
unprotected wires may be used only in a dry laboratory environment but not in an industrial
one.In some applications, strain gauges add mass and damping to the vibration profiles
of the hardware they are intended to measure. In the turbomachinery industry, one used alternative
to strain gauge technology in the measurement of vibrations on rotating hardware is the
Non-Intrusive Stress Measurement System, which allows measurement of blade vibrations without
any blade or disc-mounted hardware…==Geometries of straingages==
Different kind of straingages are available in the market. In general we define linear
strain gauges, Membrane Rosette strain gauges, Double linear strain gauges, Full bridge strain
gauges, Shear strain gauges, Half bridge strain gauges, Column strain gauges, 45°-Rosette
(3 measuring directions), 90°-Rosette (2 measuring directions).==Other types==
For measurements of small strain, semiconductor strain gauges, so called piezoresistors, are
often preferred over foil gauges. A semiconductor gauge usually has a larger gauge factor than
a foil gauge. Semiconductor gauges tend to be more expensive, more sensitive to temperature
changes, and are more fragile than foil gauges. Nanoparticle-based strain gauges emerge as
a new promising technology. These resistive sensors whose active area is made by an assembly
of conductive nanoparticles, such as gold or carbon, combine a high gauge factor, a
large deformation range and a small electrical consumption due to their high impedance.
In biological measurements, especially blood flow and tissue swelling, a variant called
mercury-in-rubber strain gauge is used. This kind of strain gauge consists of a small amount
of liquid mercury enclosed in a small rubber tube, which is applied around e.g., a toe
or leg. Swelling of the body part results in stretching of the tube, making it both
longer and thinner, which increases electrical resistance.
Fiber optic sensing can be employed to measure strain along an optical fiber. Measurements
can be distributed along the fiber, or taken at predetermined points on the fiber. The
2010 America’s Cup boats Alinghi 5 and USA-17 both employ embedded sensors of this type. Other optical measuring techniques can be
used to measure strains like ESPI or Digital Image Correlation (DIC).
Microscale strain gauges are widely used in microelectromechanical systems (MEMS) to measure
strains such as those induced by force, acceleration, pressure or sound. As example, airbags in
cars are often triggered with MEMS accelerometers. As alternative to piezo-resistant strain gauges,
integrated optical ring resonators may be used to measure strain in Micro-Opto-Electro-Mechanical
Systems (MOEMS).Capacitive strain gauges use a variable capacitor to indicate the level
of mechanical deformation. Vibrating wire strain gauges are used in geotechnical
and civil engineering applications. The gauge consists of a vibrating, tensioned wire. The
strain is calculated by measuring the resonant frequency of the wire (an increase in tension
increases the resonant frequency).==Mechanical types==
Simple mechanical types are used in civil engineering to measure movement of buildings,
foundations, and other structures. More sophisticated mechanical types incorporate dial indicators
and mechanisms to compensate for temperature changes. These types can measure movements
as small as 0.002 mm.==Non-contact strain measurements==
Strain can also be measured using Digital Image Correlation (DIC). With this technique
one or two cameras are used in conjunction with a DIC software to track features on the
surface of components to detect small motion. The full strain map of the tested sample can
be calculated, providing similar display as a Finite Element Analysis. This technique
is used in many industries to replace traditional strain gauges or other sensors like extensometers,
string pots, LVDT, accelerometers… The accuracy of commercially available DIC software typically
ranges around 1/100th to 1/30th of a pixels for displacements measurements which result
in strain sensitivity between 20 to 100 μm/m. The DIC technique allows to quickly measure
shape, displacements and strain non-contact, avoiding some issues of traditional contacting
methods, especially with impacts, high strain, high-temperature or high cycle fatigue testing.==See also==
Resistance thermometer Digital Image Correlation

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