Basic Principles Of Strain Gauge Technology; Fig. 2 Strain Gauge - Beckhoff EP3356-0022 Documentation

Product overview
2.3

Basic principles of strain gauge technology

Basic information on the technological field of "strain gauges/load cells" as metrological instruments is to be
given below. The information is of general nature; it is up to the user to check the extent to which it applies to
his application.
• Strain gauges serve either to directly measure the static (0 to a few Hz) or dynamic (up to several KHz)
elongations, compressions or torsions of a body by being directly fixed to it, or to measure various
forces or movements as part of a sensor (e.g. load cells/force transducers, displacement sensor,
vibration sensors).
• In the case of the optical strain gauge (e.g. Bragg grating), an application of force causes a
proportional change in the optical characteristics of a fiber used as a sensor. Light with a certain
wavelength is fed into the sensor. Depending upon the deformation of the grating, which is laser-cut
into the sensor, due to the mechanical load, part of the light is reflected and evaluated using a suitable
measuring transducer (interrogator).
The commonest principle in the industrial environment is the electrical strain gauge. There are many
common terms for this type of sensor: load cell, weighbridge, etc.
Structure of electrical strain gauges
A strain gauge consists of a carrier material (e.g. stretchable plastic film) with an applied metal film from
which a lattice of electrically conductive resistive material is worked in very different geometrical forms,
depending on the requirements.
Fig. 2: Strain gauge
This utilizes a behavior whereby, for example in the case of strain, the length of a metallic resistance network
increases and its diameter decreases, as a result of which its electrical resistance increases proportionally.
ΔR/R = k*ε
ε = Δl/l thereby corresponds to the elongation; the strain sensitivity is called the k-factor. This also gives rise
to the typical track layout inside the strain gauge: the resistor track or course is laid in a meandering pattern
in order to expose the longest possible length to the strain.
Example
The elongation ε = 0.1 % of a strain gauge with k-factor 2 causes an increase in the resistance of 0.2 %.
Typical resistive materials are constantan (k~2) or platinum tungsten (92PT, 8W with k ~4). In the case of
semiconductor strain gauges a silicon structure is glued to a carrier material. The conductivity is changed
primarily by deformation of the crystal lattice (piezo-resistive effect); k-factors of up to 200 can be achieved.
Measurement of signals
The change in resistance of an individual strain gauge can be determined in principle by resistance
measurement (current/voltage measurement) using a 2/3/4-conductor measurement technique
10 Version: 1.1.0 EP3356-0022