Flexible Magnetic Sensorics
In the rapidly developing market of eMobility there is a strong demand for novel sensor solutions fulfilling the specific requirements imposed by new tasks such as, e.g. the optimization of the eMotor design or the improvement of the dynamical performance (stiffness and damping) as well as a precise positioning accuracy of magnetic bearing systems. Both of these tasks require the measurement of the magnetic flux density in curved and narrow air gaps (400 µm) of electrical machines. In order to accomplish this task, we successfully demonstrated the first prototypes of ultra-thin (150 µm) and flexible magnetic sensorics. This sensor element is prepared on a commercially available 3-layer flexible printed circuit board (Figure 1a). The sensor element was already mounted on the curved stator pole of a brushless electrical motor (Figure 1b) and in a magnetic bearing test stand (Figure 1c). We demonstrated that the sensor can reliably provide measurement of magnetic flux density inside the air gap of 400 µm. This research is carried out in close collaboration with the group of Prof. Hofmann (chair holder “Electrical Machines and Drives” of the TU Dresden).
Figure 1: (a) Ultra-thin and flexible metal-based Hall sensor element on flexible PCB (total thickness: 150 µm). The sensor is attached to the stator of (b) the electrical motor and (c) the magnetic bearing system. The study is carried out with Falk Bahr (chair “Electrical Machines and Drives” of the TU Dresden).
The integration of the flexible sensor on a naturally curved stator pole in a tiny gap between stator and rotor allows a real time measurement of the magnetic flux density. This signal can then be used either as a feedback to control the position of a rotor of magnetic bearings or alternatively to monitor the performance of electrical motors. While the former application is relevant for the improvement of the performance of high-precision machining tools based on active magnetic bearing systems, the later one is crucial to optimize electrical motors for eMobility (eCars and eBikes) and is thus crucial to fabricate more energy efficient and eco-friendly electrical machines. The approach based on the measurement of the magnetic flux density in the air gap between stator and rotor is known. However, up to the present there are no magnetic field sensors available on the market, which can be integrated in a curved gap of less than 500 µm of electrical machines. To meet industry requirements, the total thickness of a magnetic field sensor should not exceed 200 µm. The fabrication of flexible and ultra-thin magnetic Hall sensors allows us to accomplish this task.
The key idea of the technology is the smart combination of ultra-thin polymeric membranes with highly sensitive metal-based magnetic field sensors. This synergy results in intriguing properties of sensor elements which are not only ultra-thin but can be also bent or twisted without sacrificing the sensor’s performance. We already successfully demonstrated that metal-based Hall effect sensing elements can be prepared on flexible printed circuit boards (PCBs) with a thickness of 150 µm including contacts and encapsulation (Figure 1(a)). Our state-of-the-art flexible sensors can be bent down to radii of 5 mm and provide a reliable measurement of a magnetic flux density of up to 2.3 T opening up new fields of application in electrical machines and drives, where conventional rigid semiconductor-based sensors (at least 400 µm thick) cannot be applied.
This sensor element has no alternatives on the market. Therefore, we envision a strong commercialization potential of this technology in the field of (i) eMotors, (ii) pumping systems (vacuum pumps and heart implants) and (iii) high-precision machining tools, e.g. high speed milling machines.
Technological benefits:
Shapeability and small thickness: Flexible sensors can be mounted on a curved surface of the stator pole of electrical machines. Moreover, the flexible Hall sensors are 150 µm thick, which is at least 60% smaller than conventional semiconductor based Hall sensors. This allows our sensors to be integrated in a tiny gap between rotor and stator of conventional electrical machines, which was demanded by the industry but has not been achieved up to the present.
Reduced energy consumption: The measurement of the flux density in the gap allows to position the rotor more precisely, which potentially provides a possibility to reduce the stator-rotor separation distance. This is of great advantage as a lowering of the electrical current can be achieved, which is required to obtain a magnetic field of a certain strength needed for the operation of the machine. The latter is crucial to decrease the energy consumption of big electrical machines and drives, which is an important task in line with the green initiative of the EU.
Green technologies: Ultra-thin and flexible magnetic field sensors can be integrated in the gap between rotor and stator and provide a three-dimensional magnetic flux density profile. This information is crucial to optimize the performance of eMotors. Electrical motors with improved performance are at the heart of the emerging market of eMobility (eCars, eBikes).
Cost reduction (vacuum pumps): The measurement of the magnetic flux density in the gap allows to design magnetic bearings without position measurement systems. In the case of turbo-molecular pumps the cost of the position measurement module is about 30% of the complete bearing, which can be partly saved by using cost efficient flexible sensorics.
Miniaturization (heart implants): Removing the bulky position measurement module has another important aspect: the size of a pump can be reduced, which is crucial for the miniaturization of blood pumps used as heart implants in biomedical technology.
