Torque and speed as input Introduction
A good supervisor is aware that the performance of his group is crucial to success. Performance is work per time and thus the most important parameter. This not only applies to companies, but also to many mechanical applications. In many application specifications in which drive shafts move, it is necessary to determine the torque and use it as a control variable. The torque alone, however, is often not meaningful, since the associated speed has a significant share of the performance of the system.
P = W/t
Speed measurement
There are many different possibilities for speed measurements that have been established in various applications for years. The most conventional methods are:
• Eddy current measurement of a texture on the shaft or a sprocket
• Hall sensor measurement of a texture ona magnetic tape
These methods are based on an incremental method. Another method of measuring speed is via angular velocity, which means the angle change per time. However, absolute angle sensors are not used in most cases, since the implementation is considered to be very costly. Therefore, the angle measurement is often implemented with incremental angle measurements. This implementation is sufficient when it only comes to determine the speed.
Torque measurement in E-Bike
A good example of the need for power measurement is the e-bike or pedelec. The regulation of the assistance by the electric motor is based on the performance and not on the work. For the e-bike, this requirement is partly comfort for the driver’s sense of control, but also a safety requirement. For example, the e-bike does not go off when it is at the traffic lights and the driver steps on the pedals applying torque to the shaft.
The integrated torque sensor
The company Magnetic Sense has developed a technology that makes it possible to combine a torque sensor with an absolute angle sensor and offers it as an integrated sensor solution. This sensor solution not only allows to determine the torque, the speed and the power, but can also determine the absolute angular position. The absolute angular position may e.g. be used in an e-bike application to determine the pedal position of the user. Moreover, it may be implemented in the scheme and thus increase ride comfort.
Advantages of the torque sensor
- Contactless measuring principle
- Insensitive to mechanical overload
- Digital literarized output signal
- Robust against interference fields
- No mechanical / magnetic processing of the measuring point necessary
- No specific requirements for the material of the shaft
- No aging effects
Torque detection in series applications
Many different sensors have manifested themselves in our applications and are now indispensable. However, there are still unresolved tasks in various areas. Thus, in medical technology, the non-invasive detection of blood pressure data is a previously unsatisfactory solved problem. In the field of industrial applications, torque measurement is the supreme discipline for sensor technology. There are different solutions with different approaches to measure the torque. In the following we will discuss the different possibilities.
Strain gauge on the measuring shaft
The strain gauge has been continuously developed for 40 years and has reached maturity in various applications. The torque measurement by means of strain gauges is established in many areas. To do this, the strain gauge is glued to the measuring shaft in order to measure the force exerted on the measuring shaft by a change in the resistance. In order to achieve good measurement results with this measuring method, a very accurate alignment of the strain gauge with respect to the direction of rotation of the measuring shaft is necessary. In addition, the interface between the strain gauge and the measuring shaft is considered a critical point because the adhesive transfers
the physical rotation of the shaft to the strain gauge. If this mechanical force transmission changes due to aging effects of the adhesive used, this will affect the quality of the measuring signal. In addition, the voltage supply for the strain gauge and the signal acquisition must be transmitted as an electrical interface via the rotating shaft. For this purpose, either sliding contacts or so-called telemetry systems can be used to transmit the acquired measurement data by means of wireless transmission. At the same time, they provide the necessary energy for the DMS. These torque systems have a high precision, but are very expensive to buy and maintain, which in turn is an obstacle to the high volume use in the series.
Strain gauge measuring flange
In addition to the direct application of the strain gauge to the rotating shaft, there are systems in which a strain gauge flange is used. The strain gauge flange is fixed between two shaft ends and thus lies in the power flow of the transmission shaft. In order to integrate this DMS measuring flange, it is necessary that the used shaft is interrupted and that the measuring shaft is provided with connecting grooves. This means a great deal of effort for the integration of the torque sensor. Furthermore, this measurement method places high demands on the parallelism of measuring flange and measuring shaft, since even small deviations can lead to transverse loads that already significantly distort the torque signal. Despite the established technology and the precise measurements that are made possible here, the integration wall and the necessary space requirements pose a challenge for any application. In addition, the systems are manufactured very complex, which makes the use in a series uninteresting due to the high unit prices.
Passive magnetostrictive torque sensor
A technology that has existed for some 15 years, but whose measuring effect has been known for several centuries, is the torque measurement using magnetostriction. This technology exploits the effect of ferromagnetic materials, which leads to a relationship between the volume and the macroscopic magnetization of a material. The effect of torques on a ferromagnetic shaft changes their volume and thus the macroscopic magnetization. The change in magnetization can be measured by the use of GMR (Giant Magneto Resistance), flux gates or Hall sensors. In order to convert this very small measuring effect into a robust application, the waves are premagnetized. This bias leads to the measurement signals becoming larger and thus an improved signal to noise behavior can be expected. With this measuring method, there is the possibility of measuring torques without contact on the shaft. Due to the use of magnetic field sensors and the very low signal levels, these torque sensors are unfortunately very susceptible to external interference. Thus, even the smallest changes in the earth’s magnetic field or magnetic fields generated by an electric motor can change the measurement signal. This means that the effort which has to be made to protect the sensor signal from disturbing influences is very large. An additional challenge is the magnetization of the shaft. This is based on a cumbersome method, since the wave must be divided into different magnetic patterns making the process very complex. Another challenge is the aging or temporal change of the magnetic field. A temporal weakening of the magnetic field leads to a change in the sensitivity and thus to an error in the signal.
Active magnetic inductive torque sensors
Based on the principle of passive magnetic torque sensors, we find the active magnetic inductive torque sensors. This technology is based on the fact that
the measurement of the magnetostriction does not happen with an already biased wave, but, on the contrary, it actively generates the magnetic field with each measurement, couples it into the wave and measures the resulting magnetic fields. Even with this active magnetization of the shaft, the material used must correspond to the ferromagnetic base. The advantage of this permanent magnetization of the shaft is that the magnetic field does not age, and the sensor has information about the magnetic field strength. Due to this active magnetization, the magnetic fluxes are significantly larger and thus the resulting signal to noise behavior much better. This contributes significantly to a noise immunity. Furthermore, with this active magnetization and e.g. with a ferrite, a spatially and temporally very defined magnetic field can be impressed into the shaft. This form of torque measurement, like the passive technology, is non-contact and can be integrated in a very small space in a customer application. The use of a highly integrated circuit and planar coils as inductance creates a very robust and reliable sensor.
Conclusion
There are other methods to measure torques on rotating shafts, which are still in their technological premature phase and are not mentioned here. In summary, it can be said that the conventional strain gauge torque sensors are to be used when it comes to measuring very precisely, such as, for instance, in test bench applications. For series applications, the use of magnetic-based torque sensors is certainly the right approach. Here, the use of active magnetic inductive torque sensors has significant advantages in terms of robustness and integration in customer applications.
