A
strain gauge (alternatively:
strain gage) is a device used to measure the
strainIn continuum mechanics, the infinitesimal strain theory, sometimes called small deformation theory, small displacement theory, or small displacement-gradient theory, deals with infinitesimal deformations of a continuum body...
of an object. Invented by
Edward E. SimmonsEdward E. Simmons Jr. was an electrical engineer and the inventor of the bonded wire resistance strain gauge....
and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an
insulatingAn insulator, also called a dielectric, is a material that resists the flow of electric current. An insulating material has atoms with tightly bonded valence electrons. These materials are used in parts of electrical equipment, also called insulators or insulation, intended to support or separate...
flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable adhesive, such as
cyanoacrylateCyanoacrylate is the generic name for cyanoacrylate based fast-acting adhesives such as methyl 2-cyanoacrylate, ethyl-2-cyanoacrylate , n-butyl cyanoacrylate...
. As the object is deformed, the foil is deformed, causing its
electrical resistanceThe electrical resistance of an object is a measure of its opposition to the passage of a steady electric current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the...
to change. This resistance change, usually measured using a
Wheatstone bridgeA Wheatstone bridge is a measuring instrument invented by Samuel Hunter Christie in 1833 and improved and popularized by Sir Charles Wheatstone in 1843. It is used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component...
, 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 conductanceElectrical conductance is a measure of how easily electricity flows along a certain path through an electrical element. The SI derived unit of conductance is the siemens...
and its dependence on not merely the
electrical conductivityElectrical conductivity or specific conductance is a measure of a material's ability to conduct an electric current. When an electrical potential difference is placed across a conductor, its movable charges flow, giving rise to an electric current...
of a conductor, which is a property of its material, but also the conductor's geometry. When an
electrical conductorIn science and engineering, an electrical conductor is a material which contains movable electric charges. In metallic conductors, such as copper or aluminum, the movable charged particles are electrons...
is stretched within the limits of its
elasticityIn physics, elasticity is the physical property of a material when it returns to its original shape after the stress under which it deforms is removed. The relative amount of deformation is called the strain.- Modelling elasticity :...
such that it does not break or permanently deform, it will become narrower and longer, changes that increase its electrical resistance end-to-end. Conversely, when a conductor is compressed such that it does not buckle, it will broaden and shorten, changes that decrease its electrical resistance end-to-end. From the measured
electrical resistanceThe electrical resistance of an object is a measure of its opposition to the passage of a steady electric current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the...
of the strain gauge, the amount of applied
stressIn continuum mechanics, the concept of stress, introduced by Cauchy around 1822, is a measure of the average amount of force exerted per unit area of a surface within a deformable body on which internal forces act...
may be inferred. A typical strain gauge arranges a long, thin conductive strip in a zig-zag pattern of parallel lines such that a small amount of stress in the direction of the orientation of the parallel lines results in a multiplicatively larger
strainIn continuum mechanics, the infinitesimal strain theory, sometimes called small deformation theory, small displacement theory, or small displacement-gradient theory, deals with infinitesimal deformations of a continuum body...
over the effective length of the conductor—and hence a multiplicatively larger change in resistance—than would be observed with a single straight-line conductive wire.
Gauge factor
The
gauge factorGauge factor or strain factor of a strain gauge is the ratio of relative change in electrical resistance to the mechanical strain ε, which is the relative change in length. The resistance is also dependent on temperature...
is defined as:
where is the resistance of the undeformed gauge, is the change in resistance caused by strain, and is strain.
For metallic foil gauges, the gauge factor is usually a little over 2. For a single active gauge and three dummy resistors, the output from the bridge is:
where is the bridge excitation voltage.
Foil gauges typically have active areas of about 2-10 mm
2 in size. With careful installation, the correct gauge, and the correct
adhesiveAdhesive or glue is a compound in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. Some modern adhesives are extremely strong, and are becoming increasingly important in modern construction and industry...
, strains up to at least 10% can be measured.
Gauges in practice
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 cellA load cell is an electronic device that is used to convert a force into an electrical signal. This conversion is indirect and happens in two stages. Through a mechanical arrangement, the force being sensed deforms a strain gauge. The strain gauge converts the deformation to electrical signals...
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 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
constantanConstantan is a copper-nickel alloy usually consisting of 55% copper and 45% nickel.Its main feature is its resistivity which is constant over a wide range of temperatures...
alloy. Various constantan alloys and Karma alloys have been designed so that the temperature effects on the resistance of the strain gauge itself 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.
Even with strain gauges that are not self-temperature-compensated (such as isoelastic alloy), use of a
Wheatstone bridgeA Wheatstone bridge is a measuring instrument invented by Samuel Hunter Christie in 1833 and improved and popularized by Sir Charles Wheatstone in 1843. It is used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component...
arrangement allows compensating for temperature changes in the specimen under test and the strain gauge. To do this in a Wheatstone bridge made of four gauges, two gauges are attached to the specimen, and two are left unattached, unstrained, and at the same temperature as the specimen and the attached gauges.
Murphy's LawMurphy's law is an adage that broadly states: "Anything that can go wrong will go wrong."- History :The perceived perversity of the universe has long been a subject of comment, and precursors to the modern version of Murphy's law are not hard to find...
was originally coined in response to a set of gauges being incorrectly wired into a Wheatstone bridge.
Temperature effects on the lead wires can be cancelled by using a "3-wire bridge"
http://www.vishay.com/brands/measurements_group/guide/tt/tt612/612c.htm or a "4-wire Ohm circuit" (also called a "4-wire Kelvin connection").
Other gauge types
For measurements of small strain,
semiconductorA semiconductor is a material that has an electrical resistivity between that of a conductor and an insulator, that is, generally in the range 10
3 Siemens/cm to 10
−8 S/cm. Devices made from semiconductor materials are the foundation of modern electronics, including radio,...
strain gauges, so called
piezoresistorsThe piezoresistive effect describes the changing electrical resistance of a material due to applied mechanical stress. The piezoresistive effect differs from the piezoelectric effect...
, 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.
In biological measurements, especially
blood flowBlood flow is the flow of blood in the cardiovascular system.It can be calculated by dividing the vascular resistance into the pressure gradient.-Physics:...
/ 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 sensingA fiber optic sensor is a sensor that uses optical fiber either as the sensing element , or as a means of relaying signals from a remote sensor to the electronics that process the signals . Fibers have many uses in remote sensing...
can be employed to measure strain along an
optical fiberAn optical fiber is a glass or plastic fiber that carries light along its length. Fiber optics is the overlap of applied science and engineering concerned with the design and application of optical fibers...
. Measurements can be distributed along the fiber, or taken at predetermined points on the fiber.
Capacitive strain gauges (Patent 7204162) use a variable capacitor to indicate the level of mechanical deformation.
Mechanical types
Simple mechanical types (such as illustrated to the left) are used in
civil engineeringCivil engineering is a professional engineering discipline that deals with the design, construction and maintenance of the physical and naturally built environment, including works such as bridges, roads, canals, dams and buildings...
to measure movement of buildings, foundations, and other structures. In the illustrated example, the two halves of the device are rigidly attached to the foundation wall on opposite sides of the crack. The red reference lines are on the transparent half and the grid is on the opaque white half. Both vertical and horizontal movement can be monitored over time. In this picture, the crack can be seen to have widened by approximately 0.3 mm (with no vertical movement) since the gauge was installed.
More sophisticated mechanical types incorporate
dial indicatorDial indicators are instruments used to accurately measure small linear distances, and are frequently used in industrial and mechanical processes.A dial indicator may also be known as a dial gauge, probe indicator, or as a clock...
s and mechanisms to compensate for temperature changes. These types can measure movements as small as 0.002 mm.
See also
- Piezoresistive effect
The piezoresistive effect describes the changing electrical resistance of a material due to applied mechanical stress. The piezoresistive effect differs from the piezoelectric effect...
- Resistance thermometer
- Wheatstone bridge
A Wheatstone bridge is a measuring instrument invented by Samuel Hunter Christie in 1833 and improved and popularized by Sir Charles Wheatstone in 1843. It is used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component...
External links