strain gauge is one of the most useful tools for precisely measuring expansion
or contraction of a material as forces are applied. Strain gauges are also
useful for measuring applied forces indirectly if they are aligned
approximately linearly with the deformation of the material.
gauges are sensors whose electrical resistance varies in proportion to the
amount of strain (deformation of a material). An ideal strain gauge would
change its resistance in proportion to the longitudinal strain on the surface
to which the sensor is attached. However, there are other factors that can
affect resistance, such as temperature, material properties, and the adhesive
that bonds the gauge to the material.
gauge consists of a parallel grid of very fine metallic wire or foil bonded to
the strained surface by a thin insulated layer of epoxy. When the bonded
material is strained, the strain is transmitted through the adhesive. The grid
shape is designed in a pattern that provides maximum resistance change per unit
selecting a strain gauge for an application, the three main considerations are
operating temperature, the nature of the strain to be detected, and stability
requirements. As a strain gauge is mounted to a strained surface, it is
important that the gauge is strained equally with the surface. The adhesive
material should be selected carefully to transmit the strain to the sensor
reliably over a wide temperature range and other conditions.
resistance value varies as a function of the applied strain according to:
R/R = S e
is the resistance, e
is the strain, and S is the strain sensitivity
factor. For metallic foil gauges, the strain sensitivity factor is about 2. The
increments of strain are usually less than 0.005 inch/inch and are often
expressed in micro-strain units. From the formula, it is seen that the strain
gauge's resistance will change in very small amounts with the given strain, in
the order of 0.1%. A voltage reading can then be taken off this resistor
in terms of milli-volts per volt (mV/V) to provide the measurement value for
Poisson Ratio is a measure of the thinning and elongation that occurs in
material as it is strained. If a tensile force is applied to a resistive wire
for example, the wire would become slightly longer, and the same time become
thinner. This ratio of these two strains is the Poisson Ratio. This is the
basic principle behind strain gauge measurements, as the wire resistance would
proportionally increase due to the Poisson effect.
accurately measure a small change in resistance, strain gauges are almost
always found in a bridge configuration with a voltage excitation source. The
Wheatstone bridge is commonly used as shown in the diagram. The bridge is
balanced when the resistor ratios are equal on both sides, or R1/R2 = R4/R3.
Evidently, the output voltage is zero under this condition.
strain gauge resistance (Rg) changes, the output voltage (Vout) changes by a
few milliVolts, and this voltage is then amplified by a differential amplifier
to return a readable value.
Wheatstone circuit is also well suited for temperature compensation - it can
almost eliminate the effects of temperature.
Sometimes the gauge material is designed to compensate for thermal
expansion, but this does not totally remove the thermal sensitivity. To achieve
better thermal compensation, a resistor such as R3 could be replaced by a
similar strain gauge. This would tend to nullify temperature effects. In fact,
all four resistors could be replaced by strain gauge sensors for maximum
temperature stability. Two of them (R1 and R3)
could be set up to measure compression, while the other two (R2
and R4) are set up to measure tension. Not only will this
compensate for temperature, but it also increases the sensitivity by a factor
gauges with electric resistance elements are by far the most common type of
sensor for measuring strain, as they possess the advantages of lower cost, as
well as being well-established. They are available in small sizes and are only
moderately affected by temperature changes, simultaneously achieving error of
less than +/-0.10%. Bonded resistance strain gauges are also highly sensitive,
and can be used to measure both static and dynamic strain. However, there are
other types available for certain applications, such as piezo-resistive,
carbon-resistive, semi-conductive, acoustic, optical, and inductive. There are
even strain gauge sensors based on
a capacitor circuit.