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The benefits of using well-proven strain gauges

Sensors are extremely important in the modern world. They are used to measure many different quantities in diverse applications such as testing, data acquisition, automation and quality assurance. This means that the market is expanding rapidly (1) 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(2)(3) 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 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 strain – are typically connected in a Wheatstone bridge circuit. This "double voltage divider“ with changing resistances opposite to one another, so the output voltage is nearly proportional to the deformation of the spring body. Fig. 1. Strain gauges are connected to a Wheatstone bridge circuit to give a voltage output that enables any deformation to be easily measured. 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 the most accurate for determining mechanical quantities. At the same time they are the best choice to provide lowest uncertainties. 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 trasducers in MNm range and ultra-high pressure transducers in GPa range (7). If at the same time dynamics are important, strain zones have to be designed as small as possible ensuring high stiffness (4)(5)(6). 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 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 (8) 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 needed. This is especially relevant when comparing the results of various national metrology institutes in different counties (9). 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.

Conclusion

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 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 level (10)(11)(12). 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.

References

[1] Survey “World Emerging Sensors Markets”, Sensors & Instrumentation, No. M678-01, Frost and Sullivan, 23 Mar 2011, U.S.A.

[2] A. C. Ruge “Strain response apparatus” Patent application no. 2322319 to the United States Patent Office; 16. Sept. 1939, approved 22. June1943

[3] K. Hoffmann “An Introduction to Measurements using Strain Gauges” Publisher Hottinger Baldwin Messtechnik , Darmstadt, Germany

[4] A. Schäfer, “Analogy observation of force transducers compared to strain and pressure transducers based on foil type strain gauges and the piezoelectric principle“, Proceedings of Asia-Pacific Symposium on Measurement of Mass, Force and Torque, Tokyo, Japan, 2009

[5] A. Schäfer, “Force, strain and pressure transducers based on Foil Type strain gauges as well as the piezoelectric principle for the use in industrial applications” Proceedings of “Eurosensors 2008”, Dresden, Germany, 2008

[6] T. Kleckers “Force sensors based on strain gages and piezoelectric crystal-based force transducers in mechatronic systems — a comparison” Proceedings of "Sensor+Test" Conference, Nurnberg, 2011

[7] A. Schäfer, et al. “A new type of transducer for accurate and dynamic pressure measurement up to 15000 bar  using foil type strain gauges”, XVII IMEKO World Congress 2003, Metrology in the 3rd Millennium, Dubrovnik, Croatia

[8] T. Kobata; W. Sabuga et al “Final Report on Supplementary Comparison APMP.M.P-S8 in Hydraulic Gauge Pressure from 100 MPa to 1000 MPa”, The Asia-Pacific Metrology Programme (APMP) and the European Association of National Metrology Institutes (EURAMET) 1000 MPa , Hydraulic pressure inter-laboratory comparation, 2010

[9] A. Schäfer “Answers to the need of higher orders of magnitude for pressure, force and torque measurement explained on the example of wind energy” IEEE I2MTC Conference, Mai 2012, Graz, Austria

[10] A. Schäfer, Examples and proposed solutions regarding the growing importance of calibration of high nominal forces IMEKO 2010 TC3, TC5 and TC22 Conferences, November 22-25, 2010, Pattaya, Chonburi, Thailand

[11] H. Gang, Z. Zhang and Y. Zhang „Internal Large Force Comparison in China”, Mechanics and Acoustics Division, National Institute of Metrology, Beijing, P. R. China, Proceedings of Asia-Pacific Symposium on Measurement of Mass, Force and Torque, Tokyo, Japan, 2009

[12] P.D. Hohmann and A. Schäfer, “Combined Calibration of Torque and Force in a 3 in 1 Calibration unit”, “APMF 2000”, Proceedings of Asia-Pacific Symposium on Measurement of Mass, Force and Torque, pp. 204, Tsukuba, Japan, 2000

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