Torque sensors can be found in many areas of everyday life and in the manufacturing industry. The history of torque measurement is much older than most people would expect. But it is the new and modern technology that makes the torque sensor so flexible and versatile in its application and allows the most diverse processes to be controlled by torque measurement. Let‘s take a look together at the possibilities and application areas of torque measurement and the development from the simple torque sensor to today`s sophisticated and technically valuable models.
It is well worth taking a first look at history. The foundations for torque measurement were laid back in 1678. This year Robert Hooke described Hooke‘s law, which described the proportionality between material strain and the associated material stress. In 1833, however, Wheatone‘s stress bridge was discovered, invented by Wheatstone and Hunter Christie. This is a bridge circuit which was and is able to measure and display the smallest voltage changes. Shortly afterwards, in 1856, Lord Kelvin discovered the connection between the mechanical strain of a resistance wire and the associated change in resistance. These inventions and discoveries still form the basis for modern torque measurement today. However, the first real torque sensor was not developed until 1938. This was a strain gauge developed by Prof. A. C. Ruge. From this point on the development of the torque sensor could no longer be stopped and changed parts of the mechanical production until today.
The first torque sensors used in industry had to struggle with a very high susceptibility to faults. This was mainly due to the analog signal outputs used, which were very susceptible to interference from adjacent components and drives. Since the signals during torque measurement were usually very weak and therefore difficult to determine, the signal level was increased so that a better and more effective interference immunity could be achieved. But only the use of digital sensor electronics made it possible to achieve this immunity and thus raise torque measurement to a new level. Various approaches were used to optimally adapt the torque sensor and its measurement results to the component and the area of application and thus achieve higher efficiency with simultaneously lower interference tolerance.
The first torque sensor was therefore a strain gauge and therefore purely static. But the most important field of application for torque measurement was found primarily in the measurements in the rotating shaft train. Therefore a torque sensor had to be invented and developed, which allows a rotating torque measurement. Shortly after 1945, the first rotating sensors were developed based on an inductive technology for torque measurement. Although the technology was far from mature and often error-prone, the technical developments and the triumphal march of torque sensors could no longer be stopped from that point on. The use of measuring amplifiers and the resulting compensation of the measuring signals made it possible to obtain ever more accurate measurement results and thus effectively control many areas with the torque sensor. The miniaturization of the technology has ensured that nowadays almost every torque sensor is already equipped with an integrated measuring amplifier. But let‘s first take a look at the different areas of application for torque sensors over time.
Torque sensors are not only becoming increasingly important in industry and industrial production, but are also becoming increasingly important due to the new and modern electrification of motors. Thanks to the various electric drives in cars and e-bikes, the torque sensor is becoming increasingly important. For this reason, it is important to know why torque measurement is so important in this area and how it can be most effectively integrated and implemented. The condition and performance of an electric motor can most effectively be determined by torque measurement. Thus, this form of measurement represents a kind of control variable for the many different sensors in such an engine and can therefore have a lasting effect on the performance and safety of the entire system. Even though the technology is still in its infancy and therefore not yet mature for series development, many tests have already shown good to very positive results. It remains to be seen what kind of torque measurement will be used in the future, but Magnetic Sense is currently focusing on the segment of contactless magnetic inductive torque technology, which promises the highest possible accuracy and simple control. Thanks to our various laboratory tests and direct field trials at the customers, we can already see that this technology of torque measurement promises the highest possible efficiency and can be very well integrated into the various systems. This is because such a torque sensor with contactless magnetic inductive technology offers the user various advantages. These would include, among other things:
Even though strain gages are still one of the most common applications in the field of torque measurement, there are many different methods and torque sensors on the market that can be used depending on the application.
Especially with high rotation numbers and the correspondingly acting forces, it is often difficult to transfer the measurements. Let us therefore first look at the various processes which can be convincing not only under laboratory conditions, but also in industrial applications. Some of the processes are so susceptible to failure or complicated to be installed that their industrial use will remain questionable in the future. It is important to always select the process that not only promises the highest possible precision, but which can also be used safely and cost-effectively.
In optoelectronic torque measurement, various sensors are used that not only measure the torque, but also the angle of rotation, the speed and the direction of rotation of a shaft. This form of torque measurement is convincing in many areas of application, but due to its size it cannot be installed in all systems. A torque sensor of this type is particularly effective at low torques, as there is also a very low susceptibility to failure.
Strain gauges are among the oldest methods of torque measurement and offer very accurate and good results in many areas. Due to the necessary high accuracy during installation and the high maintenance intensity of the systems, strain gages are currently only used permanently in a few areas. Much more often it is shown that a modern torque sensor can be realized much easier by other alternative technologies.
A special physical property of ferromagnetic materials is used in magnetoelastic measuring methods. The volume and the macroscopic magnetization of such a material are directly related. Thanks to modern technologies and further development of the passive process into active torque measurement, very accurate results can be achieved at low cost pressure.
Although the piezoelectric effect was discovered many years ago, its use in measurement technology is of much more recent origin. When measuring by means of piezoelectronic effects, crystals under pressure will generate an electrical charge directly proportional to the force. This charge can be converted into a proportional output voltage by an amplifier. This type of torque measurement is extremely effective, but is also very susceptible to interference and generally has a lower linearity. However, such a torque sensor is very small and easy to install.
The abbreviation SAW stands for surface acoustic wave and describes a structure-borne sound wave that propagates planar on a surface. The sensor generates a sound wave which propagates over the surface of the measuring medium. Changes in the material or the physical properties also change the signal and thus the result at the torque sensor. This process therefore offers maximum comfort, but is also susceptible to faults and can therefore only be implemented
In total, there are four sensible and target-oriented methods according to which torque measurement can be guaranteed in series applications. Some of the methods mentioned above, for example, are only possible under controlled laboratory conditions, while others have been in use for years. The four methods of torque measurement for series production are torque measurement using strain gauges on the measuring shaft, torque measurement using a strain gauge measuring flange, the passive magnetostrictive torque sensor and the active magnetic inductive torque sensor.
For more than 40 years, torque measurement using strain gauges has been developed and improved. A torque sensor of this kind is particularly convincing due to its simple integration, but it also has disadvantages that can only be eliminated by appropriate know-how. For example, the exact alignment of the strain gauge with respect to the direction of rotation and alignment of the measuring shaft is necessary in order to achieve high accuracy and low error tolerance. A further weak point of this system is the interface between the measuring shaft and the strain gauge, since changes in the adhesive caused by time also change the measurement results. In principle, the systems are nevertheless very powerful and operate with low tolerances, but are very expensive both in installation and maintenance and therefore only interesting for a few industries and areas of application in series applications.
A measuring flange is mounted between two shaft ends and thus lies in the direct force flow of the transmission shaft. This can lead to very accurate and exact results. However, the effort for the integration of such a system is very high, since the shaft has to be interrupted and prepared for installation. In addition, the parallelism between measuring flange and measuring shaft must be very high with this method, since deviations can already massively and significantly falsify the measurement results. In series applications, this technology is hardly used for torque measurement due to the great effort and high costs involved.
Torque measurement using magnetostriction exploits a property of ferromagnetic materials to enable effective torque measurement. There is a correlation between the volume and the macroscopic magnetization of different materials. To enable an effective measurement with different sensors like Flux Gates or Hall sensors, the shaft is pre-magnetized. This makes the measurement signals larger and easier to determine. However, this method also has clear disadvantages. It is extremely susceptible to interference and therefore particularly complicated and expensive to install. The entire sensor field must be protected against external interference. In addition, the wave must always be pre-magnetized, which means additional expenditure in terms of time and money. Nevertheless, passive magnetostrictive torque measurement is also used in various areas.
With this principle of torque measurement, the shaft does not have to be pre-magnetized, but the magnetic field is actively generated during each measurement and coupled into the shaft. Even with this type of measurement, the shaft must be made of ferromagnetic material, but this system offers some advantages. For example, the magnetic field does not age, so that the strength of the magnetic field does not weaken and change over time. In addition, active magnetization makes the magnetic fluxes much larger and thus easier to measure, which significantly increases the error tolerance due to noise. This form of torque measurement can also be implemented in the smallest of spaces and is therefore particularly popular and practicable in customer applications. The use of planar coils and high-quality circuits results in a reliable torque sensor which is convincing in many areas of application.
Modern torque sensors have become an indispensable part of life and industry. Without a suitable torque sensor, many areas can hardly be controlled and certainly not monitored. This becomes particularly clear when you look at just one part of the range of applications for torque measurement. It is impossible to imagine everyday life without the torque sensor. The areas of application for torque measurement include:
As you can see, there are very few areas where torque measurement using a torque sensor is no longer required to achieve the highest possible efficiency. It is only through modern and error-prone torque measurement that many of the current developments on the market have been developed and brought to production maturity. The torque sensor ensures higher safety and a permanent inspection of the system and thus higher efficiency in the many application areas. So it is no wonder that the torque sensor has been able to prevail in so many areas.
Many of the processes presented are not yet suitable for direct use in production or series production. Although such a torque sensor often provides very accurate results, it cannot be installed in an economically viable manner due to the process used. The measurement of torque by means of strain gauges is therefore still extremely widespread, even if the circumstances often lead to particularly expensive and error-prone systems. Currently, the use of conventional strain gages is widespread. In series applications, on the other hand, the use of torque sensors on a magnetic basis is increasingly propagated, as these are both much more robust and easier to integrate into the various customer applications. It remains to be seen whether the SAW process will sooner or later outstrip this technology and can be effectively integrated into series production. Currently, the torque sensor is always selected according to the process used and is often difficult to adapt to the customer‘s areas of application.
Modern torque sensors are now used in the most diverse areas of production. Particularly in the field of predictive maintenance, precise sensors for torque measurement play an important role. Such a torque sensor is so fine and detailed in its measurement results that, for example, the wear of a bearing can be recorded by the torque sensor. This is because changing the bearing also changes the friction and thus the direct efficiency of a system. If this process can be measured by the torque sensor, maintenance and necessary replacement can be detected much earlier, so that it does not have to lead to greater damage or lower production. The torque sensor thus detects problems so early that the component can be replaced at a normal maintenance interval. The data is recorded and processed by the torque sensor. The result of the torque measurement is then used to calculate the failure probability of the components used and thus to determine the optimum point in time at which a replacement of the component appears economically expedient. Thanks to the magnetic inductive technology of modern torque sensors, these can be integrated into the various production processes without a great deal of effort and without excessive costs. Thus it is possible to use such a form of predictive maintenance in many new areas and thus to improve the economic efficiency of the machines and the means of production.