Bewegungsgrößenerfassung in magnetgelagerten Antrieben mit berührungslosen magnetischen Sensoren

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Marco Schramm
203 g
212x149x12 mm

These days active magnetic bearings are mostly used in large drives or special fields like vacuum or clean room technology. For the displacement detection of magnetically levitated rotors in high speed drives different types of sensor technologies are currently used. According to the respective operating conditions not each of these measuring systems is suitable. The static and dynamic transmission properties essentially define the performance of the magnetic bearing system. In spite of the variety of sensor principles their costs are very high comparing to all other system components, especially in smaller drive units.
In this thesis a novel magnetic displacement sensor is presented, which fulfils the high requirements of measuring and control technology and is furthermore simple, robust and low-cost. The working principle is based on compensating current transformers and the sensor combines a direct measuring of displacement and whose velocity in one unit. By gathering both of these state space variables it is possible to realize a simply structured and observerless state space controller.
Theoretic preliminary considerations by means of analytical and numerical calcula-tions validate the general functionality for magnetic bearing applications. The result of the simulations and optimizations was realized in a heteropolar arranged prototype for radial displacement measuring. For the determination of the static and dynamic transmission properties several tests and measurements were carried out. To improve the linearity and reduce the temperature dependence as well as interferences each two sensors per measuring axis are shifted in differential mode. At higher signal frequencies the eddy current losses in the measuring track are increasing like it can be observed at the well known inductive measuring principles. The cut-off frequency of the magnetic measuring system is therefore mainly limited by the magnetic properties of the measuring track material. In a second development step a homopolar arranged sensor was built up, which is now scalable for different shaft diameter. In combination with an incremental ferromagnetic measuring trace it is possible to detect the angle position as well as the rotational speed of drives.
Finally, in the end of the work, a displacement control of an active, radial magnetic bearing with magnetic distance sensors was realized. Different kind of controllers were tested and compared with a classic PID controller. The direct measuring of the radial velocity offers the advantage of a noise- and lag-free signal which replaces the D-part of the controller. The state space controller with complete state space variables measurement shows significant improvements concerning the command variable and also the disturbance variable behaviour.
In summary it can be stated, that the magnetic displacement sensor is suitable for active magnetic bearings as well as for mechatronical applications like monitoring and failure diagnostic in electrical machines. Furthermore it is to be expected due to the cost reduction of the measuring system, that the acceptance of magnetic bearings in drive technology can be enhanced.