Torque sensors are not all alike. The market for torque sensors has changed very little in recent decades. This is because in most applications where torque acquisition is of interest, the location to be measured is rotated. The rotation of the shaft presents the measurement of this particular challenges. While static strain gages can be used in static applications and read out with a wire connection, this is not the case with a rotating shaft.
Most sensors today still require a supply line to provide you with energy and an interface for transmitting the measured values. While today‘s data transfer interface can be easily implemented in most applications, transferring the power required to measure is a challenge. „Energy Harvesting“ to use a buzzword that is in many mouth, is still in its infancy. There are many approaches that are still too early for many applications to be a real alternative. This means that in current applications, the sensors still have to be connected via a physical interface, which means they have to be supplied with copper.
Torque sensors, which have to be measured on the rotating shaft and supplied with a cable, are a challenge for the developers. The existing solutions for the measuring task usually aim at the fact that the shaft is interrupted at the measuring point and a torque sensor is used. That At this point, the user gives the sovereignty over the mechanical properties of his system to the manufacturer of the torque sensor. This poses a particular challenge for many OEMs because they know their materials and requirements very well and have been using them for decades. The interruption of the shaft and the introduction of another material are therefore possible only with great effort. Furthermore, there is a big discrepancy between the requirements of the actual shaft and the sensor. While the measuring shaft has been largely dimensioned for its mechanical properties, the torque sensor is designed so that it can detect the best possible and qualitative torque signal. These two requirements are often in conflict because, to measure torque, the sensor wants to see as much strain as possible on the shaft, i. expected as high a signal modulation. Therefore, the users are often anxious to design the shaft so that it can withstand the load even in extreme applications. As a result, the shafts used are often oversized.
Even if the shaft is broken and a torque sensor is used, the manufacturer still has to face the challenge of measuring on the rotating shaft. The manufacturer outsourced this challenge only from the customer application to his own component. Typically, these sensors work by applying conventional strain gauges with electronic intelligence to the measuring shaft. Electronic intelligence is powered by inductive power transfer in most cases and regulates the power management of the sensor unit to capture the measurement data and, if necessary, make dynamic corrections to the sensor signal. These measurement data are then transmitted either inductively or via another wireless transmission to a receiver on the sensor housing, which then takes over the signal modulation to the customer interface. This structure is very complex and leads to high production costs. This manufacturing hurdle and the fact that customers have to break their shaft are the reasons why torque sensors have only prevailed in very unusual series applications.
In many cases, attempts are made to detect the torque via secondary information or to measure the force in bearings in order to conclude the actual torque on the basis of this secondary information. These methods are usually only suitable for rough estimates, but not for exact measurements.
The E-Bike or pedelec, have revolutionized the torque sensor. The absolute requirement for a seriescapable torque measurement is the development of technologies that today are able to detect torques on a rotating shaft without interrupting the shaft. The basis for this technology is a paradigm shift in the observation of the wave. As described above, the shaft, which has been mostly designed as a mechanical component, becomes the sensor. The physical principle of this wave as a sensor is described in the literature as „magnetostriction“ or „inverse magnetostrictive principle“. This effect is based on the fact that the magnetic properties of ferromagnetic materials change when they are loaded. By applying a torque, the magnetic properties change. This change can be detected by a magnetic inductive structure either on or next to the shaft. This is a symbiotic sensor consisting of magneto-inductive signal modulation and measuring wave. Both components work in combination as a sensor. The two sensor units must be well matched / parameterized to achieve maximum measurement capability. Thus, e.g. the measurement frequency and power have a significant influence on the magnetic signal modulation. These parameters depend on the material used and the measuring point dimensioning. For the measuring wave a paradigm shift must take place. In addition to the mechanical properties of the wave, the magnetic properties must also be considered in the future.
The consideration of the magnetic properties is the key to make sensor technology ready for series production. We, the Magnetic Sense company, have greatly expanded our expertise in this field and today we are able to measure almost any existing ferromagnetic material. With suitable coatings, we can also measure torques with our technology on plastics, GRP materials and aluminum shafts.