Sensors hold considerable importance in the modern world. They are used for the measurement of a number of quantities in a range of applications such as testing, data acquisition, automation and quality assurance. This means that the market is expanding rapidly and new methods are appearing.
This trend towards a wider choice of technologies can lead to increased enthusiasm. However, it is worth remembering that established technologies may have considerable advantages due to experience built over many years.
For example, the foil-type strain gauge is based on well-established scientific principles and have been enhanced by continuing technological advances. Foil-type strain gauges are simple to install and are very low cost even for individual and singe solutions.
Measuring with Foil-type Strain Gauges
Metal foil-type strain gauges are widely used as the main sensing principle in force, torque and pressure measurements. The vast majority of force transducers, load cells, torque transducers and ultra-high pressure transducers are based on this design and are available in a wide variety of measuring bodies.
All foil-type strain gauges are based on a common principle; they use a positive or negative strain to convert mechanical changes into electrical signals. In dedicated spots on the spring body, where high strain occurs under load, four strain gauges – two under positive and two under negative strains – are typically connected in a Wheatstone bridge circuit.
This "double voltage divider“ with changing the resistance opposite to one another, so the output voltage is almost proportional to the deformation of the spring body. Figure 1 shows strain gauge connected to a wheatstone bridge.
Figure 1. Strain gauges are connected to a Wheatstone bridge circuit to give a voltage output that enables any deformation to be easily measured. Image credit: Hottinger Baldwin Messtechnik GmbH
The output signal is given as a ratio between the voltage supply and the output voltage. It is calculated as follows:
Foil-type strain gauge transducers are highly accurate for determining mechanical quantities. The principles of strain gauge operation are well established, meaning that full focus can be maintained on the measuring task. Unlike many other pick-up principles, foil-type strain gauges can be built for nearly unlimited high nominal loads simply by scaling up the measuring body.
Examples are force transducers in MN range, torque transducers in MNm range and ultra-high pressure transducers in GPa range.
In other applications, such as pressure transducers measuring hydrostatic pressure, there are more choices than for other mechanical quantities. Lower pressure applications, which account for the largest segment of this market, usually use capacitive or piezo-resistive MEMS solutions, especially for measurement of low pressures of just a few bars.
Overload resistance is particularly important for measuring high pressures that effectively excludes capacitive and piezo-resistive MEMS solutions – despite some progress with newer designs in recent years.
Figure 2 provides a comparison of the different types of strain gauge technologies and their suitability in pressure measurement from a number of different perspectives.
Figure 2. Comparison of different pressure measurement technologies. Image credit: Hottinger Baldwin Messtechnik GmbH
Examining this table reveals that strain gauge based ultra-high pressure transducers are the primary choice for measurements where very high accuracy and long-term stability are required. This is especially relevant when comparing the results of various national metrology institutes in different counties.
It is possible to undertake a similar analysis of the different principles for every other measurable quantity. This is best when trying to design a measuring chain optimized for a particular measuring task since the selected pick-up principle is an important interface to the process or phenomena being investigated.
Strain gauge transducers provide incredible accuracy, very high long-term stability and good bandwidth suitable for rapid measurements. Passive resistive networks can easily adjust most remaining errors in transducer manufacturing.
Strain gauge-based transducers are the best choice for most large-scale measurements but also individual and single industrial tasks and especially for high precision measurements. The high levels of accuracy facilitate the tracing of mechanical quantities up to national and international levels.
In more mainstream applications the simplicity and low cost of foil-based strain gauges make their use significant in core markets such as load cells of all types ranging from retail up to truck scales.
This information has been sourced, reviewed and adapted from materials provided by HBM, Inc.
For more information on this source, please visit HBM, Inc.