Strain Gauge

Overview of the strain gauge

The strain gauge is based on the following phenomena: stretching a strip of conductive metal will not only make it longer and thinner, it will result in an increase in electrical resistance from one end of the strip to another. Inversely, compressing it (without causing it to buckle) will not simply make it shorter and broader, it will also result in a decrease in electrical resistance. Provided these stressive forces do not exceed the strip’s elastic limits (which would cause permanent deformation), the conductive strip can be used repeatedly to measure the degree of mechanical force the object to which it is adhered is being subjected to. It is, in other words, a transducer which converts changes in mechanical force into changes in electrical resistance.

Strain gauges are widely used in mechanical engineering research in order to determine the stresses heavy machinery will generate, for example, and to determine safe limits of the equipment and its environs. It is difficult to overstate how important this is: the testing of aircraft components, for example, is heavily reliant on strain gauges to determine their condition and safety: minute strain gauges are routinely glued to linkages and structural members constituting the airframe in order to measure the degree of mechanical stress they are under. In most applications, strain gauges are very small, not exceeding the size of an ordinary postage stamp. The smallest gauges are no more than 0.38 cm in length and can measure minute strain forces as low as 0.00001 inch per inch.

History

One of the earliest strain gauges, the resistance strain gauge, was invented in 1856 by the British mathematician, engineer and physicist William Thompson, who later became the First Baron Kelvin. He discovered that the electrical resistance in a length of iron or copper wire would increase or decrease if it was stretched or compressed.

Following that discovery, most strain gauges took the somewhat cumbersome form of mirrors and optical levers or compound mechanical lever systems, although they were capable of detecting deformations as small as 0.00005 inch. At ½ to 1 inch in length, they tended to be relatively large and weighty in comparison with modern strain gauges, which rendered them very inaccurate in detecting varying strains in a structure caused by dynamic loading.

The American electrical engineer Edward Simmons and his colleague, mechanical engineer Arthur Ruge, invented the modern bonded wire resistance strain gauge in 1938. The basic design of this transducer is still in use today. Originally, it took the form of a length of exceptionally fine wire which was meticulously looped into a grid pattern, whereupon it sandwiched between two exceedingly thin sheets of paper (cement prevented it from slipping about between the papers). It would then be glued to the surface of the machine or structure in which strain was to be measured.

As the structure undergoes deforming stress forces, any stretching or contraction on its surface will be paralleled by corresponding deforming changes in the strain gauge, which is energized with electricity. The current passing through it is subject to different degrees of resistance depending on whether the gauge undergoes stretching or constricting alterations. The gauge then coverts the resistance change into a measure of strain, which is readable on a display.

Technical aspects

In the unstressed state, most strain gauges show electrical resistance ranging from 3 to 30 Ω. The elastic limits of the gauge inevitably impose force ranges on the device, with resistance altering by only a minute amount when the full force limit is reached. Bigger forces would have the result of permanently deforming the gauge and effectively destroying it. This means than tiny changes in resistance, typically amounting to little more than a fraction of a percentage point, must be measured with an exceptionally high degree of accuracy. While input voltages tend to be either 5V or 12V, output is measured in millivolts.

Bridge measurement circuits tend to be used in modern strain gauges to achieve these exacting requirements for precise measurement. Previously, this was accomplished by means of a Wheatstone bridge, a circuit which required a human operative and a null-balance detector to sustain a balanced state. Today, these are giving way to strain gauge bridge circuits which incorporate a high-precision voltmeter at the centre of the circuit to directly measure the degree of induced imbalance.

Although the “paper and wire sandwich” design described in the section above is still essentially the same in contemporary strain gauges, the materials have changed: they are manufactured as printed circuit grids arranged in ziz-zag patterns, which are etched onto thin, flat constantan foils. Constantan is now the most commonly used material in strain gauges; it is an alloy which is highly sensitive to strain and is ideally suited as a constituent of the gauge’s foil. The foils are adhered to a non-conductive plastic backing known as the carrier, which is usually made from epoxy or glass-fibre reinforced epoxy-phenolic or polyimide. The plastic carrier allows the gauge to be handled safely, although it is typically no more than 0.001 inch in thickness.

Strain gauges will detect deformations in the structures to which they are glued caused by thermally-induced contraction or expansion, which is why it is considered good practice to avoid self-generated heating caused in the device itself by excessive input voltages.

Where the strain gauge is used in manufacturing

Strain gauges are proliferating across the world in a vast array of different industrial applications. They are now routinely used in load cells for weighbridges, hoppers, and scales of various kinds, as well as in educational and medical applications. As mentioned earlier, they are widely used in mechanical engineering research and development to test the condition and safety of automotive, aeronautical and medical equipment, as well as equipment and machinery used in the oil, gas and power generation industries. Building and bridges are also fitted with strain gauges to keep their structural integrity and safety under constant review.

They are exceedingly robust and withstand extreme conditions such as cryogenic equipment, where temperatures can range from -269ºC to 1300ºC, and in high radiation areas. Structures or machines subject to exceedingly high levels of vibration are also routinely fitted with strain gauges as are applications which undergo exceptionally large degrees of elongation.

How the strain gauge differs from other sensors

Although a category of transducer, strain gauges are unique in that they convert changes in mechanical force into changes in electrical resistance, which are then transposed into measurements of strain.