Force sensors
There is hardly a sensor that has such a long history as the strain gauge and the very close Wheatstone bridge. The combination of these two principles has made it impossible to imagine the world of sensor without these technologies. The principle of the strain gauge is based on the fact that the resistance of a material changes when it is stretched or compressed, that means experiencing a mechanical force. Strain gauges by itself can not cope with this requirement. Most applications with a strain gauge are based on the resistance changing. The resistor is applied to a substrate that can absorb the force and directly expose the resistor to it. These substrates may be secondary force transducers such as e.g. the measuring membrane at a pressure sensor or primary power transformer i. directly in the application represent the component on which is measured. The former is easier to handle for a sensor manufacturer, as it can be used to build a functional sensor. If the component of the customer makes a contribution to the sensor, it is often more difficult, since the interface becomes significantly more complex.
The material of the strain gauge and its layer structure is very important for the functioning and especially fatigue strength. So the material should not change over the years to cause any signal drift and it must usually withstand the environmental influences that prevail in the application. A very popular material for the strain gauge is therefore a nickel-chromium coating. Even the best material can not be perfect, i. Conversely, every material ages and has corresponding sensitivities to temperature and other environmental factors. In order to minimize these as far as possible through the sensor design, strain gauges are in most cases read out via a so-called Wheatstone half bridge or full bridge. The principle of the Wheatstone bridge is based on the fact that the resistors of the measuring bridge are so applied to the substrate, that under load influence one resistance expands and the other resistance bends. This means that one reading changes in the positive direction and one in the negative direction. If you do not look at the individual measured value, but at the difference, you will get the measurement signal twice as a result. Another side effect besides the double signal yield is that if the resistance of the measuring bridge changes, e.g. With both resistors the resistance increases – the difference of these two resistors is still zero. This method is also called differential measurement. The core message of a sensor is always its signal to noise behavior. A sensor that has a very high sensitivity, but also shows a very large noise or cross-sensitivity to environmental influences is not to use. The development of a sensor therefore always aims to optimize the signal / noise or signal / measurement error as much as possible.
At the heart of any sensor technology that provides only very small measuring signals, it is to find an arrangement that allows to operate the sensor in a differential measurement. This is the basis for the smallest possible measurement errors, due to the influences on the sensor described above. The Wheatstone bridge has established itself as an optimal use for both the differential measurement and the measurement of strain gauges. This means that sensors which can potentially replace the strain gage must be able to operate with a similar signal quality, i. a differential measurement to be operated. When technology has prevailed in an application, that means it is absolutely established and accepted, then it is hard for any new technology to compete against them.
The problems or disadvantages of the strain gauge
The strain gauge has (besides its very good properties regarding accurate measurement) also its disadvantages. As described above, the strain gauge must be applied to the measuring piece, i. it must have a very good mechanical coupling at the measuring point, which not only fulfills these requirements during production, but also after several years of use. To realize this, many manufacturers have developed adhesives that are optimized for this application. However, these must be applied manually in most applications. However, manual application is always a risk factor for the quality of the bond and a corresponding cost factor must be taken into account. Furthermore, it is the case that the smallest angles in the orientation of the strain gage on the measurement object can cause an error, since the force action not only acts in the measuring axis of the strain gauge, but also with a force vector in the other axis. The ratio of the individual force vectors is the potential measurement error seen by the DMS. Finally, it is now necessary to contact the strain gauge which has been attached to the test object in order to apply both the supply voltage and the signal routing. With high-volume serial components, this can happen with a wire bonder, with smaller production volumes wires are often soldered. This wire guide and the soldering point is a weak point in terms of quality requirements.
Magnetic inductive force measurement: a revolution in force measurement
The industry and thus the sensor manufacturers have been satisfied that there is an established technology and it is constantly being optimized. Already in the 60s, however, there were already first efforts, in addition to the pure strain measurement on the resistance change, which is basically a secondary size, to establish a direct force into the mechanical object to be measured. The effect of magnetostriction has become the object of desire. The magnetostriction is based on the fact that the magnetic properties of ferromagnetic metal alloys change under the influence of force. That means that the magnetic permeability or susceptibility of the material changes when a force acts on it. Various names of this direct mechanical force measurement have since been developed, such as e.g. Magnetoelastic force measurement or magnetostriction. But there is also often the concept of inverse magnetostriction. Basically all describe the same behavior. To implement a magnetostrictive force measurement, however, significantly more complex sensor structures are necessary. For this measurement, the sensor must have a possibility to generate a magnetic field to generate a magnetic flux in the measurement object and additionally to measure magnetic field sensors to detect a change in this magnetic flux. Implementing this in pure discrete electronics is nearly impossible and was therefore the “show stopper” for magnetostrictive force measurement in the early 1960’s. There were several other attempts in the 80s and at the turn of the millennium that all got lost in the sand. Today, this picture looks completely different!
The revolution of force measurement by magnetic inductive sensors
Today’s possibilities to produce inductors that are necessary to generate the magnetic fields have changed radically. So it is e.g. possible that in a conventional PCB which is used for the electronic circuit development coils admit. This makes it very reproducible to produce inductors that can be used both for generating an alternating magnetic field, but also to measure back the resulting fields for a sensor. Furthermore, the possibility of integrated circuit development has changed so that today enough computing power is available to realize the most complex signal processing – up to a Fourier analysis of alternating fields. Furthermore, knowledge about the magnetism and behavior of metal allows under the influence of magnetic fields is today – driven by electric machines and e-mobility – at a completely different level, so that e.g. can also perform simulations of magnetostriction.
These simulations help to predict the behavior of the materials deterministically. The combination of the technologies available today for the production of coils, the necessary processing power in microcontrollers and the knowledge of the magnetism of materials form the basis for the force sensors from Magnetic Sense.
Magnetic inductive force measurement today
The company Magnetic Sense has managed to produce a highly integrated product for series applications by combining the most important core elements for a magnetic inductive force sensor. With a modular system it is now possible to realize a very precise force or torque measurement on almost every ferromagnetic material by parameterizing the sensor. A patented method of realizing the sensor structure makes it possible to carry out a differential measurement at almost any measuring point. For this, both the sensor construction that means the way in which the magnetic fields are generated and detected back is patented as well as the circuit method necessary to digitize the resulting signals and calculate a signal proportional to the force. With these modules, Magnetic Sense has achieved a performance equivalent to that of a strain gauge in a force or torque application. Since the magnetic fields can be coupled into the measurement object over a certain distance, it is not necessary for the sensor to have a mechanical coupling with the measurement location, which has the result that gluing process is not necessary. A completed sensor component can be applied directly to the measurement object to perform measurements there. Magnetic sensors are known to be very robust and stable over time, due to the fact that the magnetic alternating field which is generated is very well known and can be detected and corrected accordingly in case of deviations, but also because the magnetic fields do not penetrate Dust, rust or other contamination of the measuring point can be influenced. The modular construction kit developed by Magnetic Sense can be easily adapted to different customer-specific requirements in terms of installation space, mechanical and electrical interfaces. These technological possibilities find application possibilities in different branches of industry.
Which applications are there for magnetic inductive force sensors?
Force measurement in the agricultural industry
Hydraulic cylinders are used in various agricultural machines to lift loads or to produce constant pressure for soil tillage machines. These cylinders are usually controlled by their hydraulic pressure today. This control is faulty due to friction losses of the piston with the cylinder. A direct force measurement on the hydraulic cylinder can be used to measure these forces much more accurately and thus adjust the application more accurately.
Force measurement on cranes
On construction cranes there are various places where forces are measured today. An important application for force sensors are the support forces in the hydraulic supports. The force in these supports can be used to determine the center of gravity of the crane and to prevent accordingly that this risk of tipping and thus endanger mankind. Furthermore, force sensors can measure the semitrailer weights and thus define the permissible load of the crane outrigger. Another application is direct force measurement in the crane boom to weigh the lifted components from the crane.
Force measurement in the automobile
There are various places in the automobile where force measurements are already being implemented today. On the one hand, load sensors on the trailer hitch can measure the load a trailer has on it, but also the pulling force the car has to apply to tow a trailer. This static information paired with information on the dynamic behavior on the road can be used to detect dangerous driving maneuvers and correspond to prevent control technology.
Force measurement on bridges
Loads and load changes on bridges are important information for assessing long-term stability and its degeneration factors. Force sensors can capture this information and define maintenance actions on the bridge accordingly.
Force measurement in the train
Semitrailer forces of the wagons can be detected with force sensors and thus give a direct information with what weight both the passenger car and the freight wagon is loaded. Traction forces in the drawbar can be used to make it plausible how many cars are attached to the tractor and whether all are still present. Measurements on track sections directly on the track can be used to count the wheels driving over a certain point to determine whether all wagons that have gone into a track section have also left this again.
Force measurement on the truck
EU directives are forcing the truck industry in the future to provide a control authority with direct information about which axle loads they have loaded. This can be realized with force sensors on the axle. This information is intended to prevent the roads in Europe being overloaded by trucks that are heavily loaded but also avoiding dangerous situations on the road. Also on the truck, the forces of the towed trailer can be determined to make it plausible which weights have loaded.
Force measurement in industrial applications
Wherever weight measurements are used, whether it is the weight of a tank or a silo, Magnetic Inductive Force Sensors can be used.
The force sensor in theory
For decades, several major OEMs in the automotive and mechanical engineering industry have been dabbling at technology. Numerous patents have been created and new ideas have been created on how the sensor assembly can unfold its maximum performance. However, no real products have emerged. Only a few years ago, there are occasionally products from various manufacturers in this area. If you take a closer look at the torque sensor of the company Magnetic Sense, the reasons for this can be seen.
The Know-how that contributed to the development of the inductive sensor
The competencies in the development of a magnetic inductive force sensor or torque sensor, are not limited to the electronics and the signal evaluation, but also to the necessary knowledge about the interaction of magnetic fields and matter. The signals that a magneto-inductive torque sensor picks up depend primarily on the permeability of the measuring shaft and its change under load. This change is very material specific and must be specially characterized for each application. However, existing knowledge about the magnetism of metals has only been built up in recent years through the further development of electric motors. This was certainly one of the most important reasons. Read more about the torque sensor revolution here!
How complex computing optimizes the force measurement
Another reason is the necessary computing power of the electronics to convert the important signals from the high-frequency excited inductances into the digital and to carry out the calculations necessary for the linearization. Inductors have their peculiarities with regard to the behavior under temperature and change of the ambient conditions. These changes must be processed and output in complex compensation algorithms with a high measurement bandwidth. This necessary computing power was not available a few years ago. That It was almost impossible to operate the necessary signal processing with the means available 15 years ago to generate a stable sensor signal.
