Characteristics of active magnetic bearings
Linear magnetic bearings feature several advantages in comparison to contacting ball bearings, which can be useful in miscellaneous applications. The most prominent application is the magnetic levitation train Transrapid, which mainly employs the potential for high speeds, good damping and the complete lack of wear.
Apart from this, other applications may take advantage of the complete lack of friction of the contactless bearing, the absence of abrasion particles and sparks, the almost silent operation and the controllability of the bearing’s air gap.
In contrast to these a magnetic bearing also has some distinct disadvantages, which provide very interesting and manifold research opportunities. It requires noticeably more space than a passive bearing, an energy supply and most of all an active control unit. Furthermore, the load capacity and the dynamic stiffness against disturbances are significantly restricted. In the area of machine tools this may be the most significant handicap. At last, one has to consider the thermal losses in the flux carrying bearing parts and the fact, that a magnet only exerts forces in one axis, and then only attracting forces. All these are starting points for further research and optimizations, which will make magnetic bearings suitable for applications in machine tools, extending their current uses as transport systems and rotatory bearings for high-speed shafts and spindles.
Newly developed magnet modules
Building upon the results of a concluded project, WZL is constructing a second prototype of a machine tool table with magnetic bearings. Its main characteristics will be high velocities and accelerations as well as a significantly improved dynamic stiffness against disturbances.
To this end, new magnetic modules have been developed, which surpass existing bearings consisting of several U-shaped electromagnets. The modules are superior in terms of load capacity and power consumption and consist of a combination of electro- and permanent magnets, whose forces are superposed and result in a controllable bearing force. The geometric shape of the magnetic core avoids the common disadvantages of a hybrid magnet, namely a permeation of the permanent magnets by the flux of the electromagnet and thus a force-reducing, virtual enlargement of the air gap.
Every module can exert a force of 2,5 kN at maximum current, at higher current levels the iron core starts to saturate and reduces the controllability of the module due to its nonlinear behaviour.
The single-degree-of-freedom test bench as first stage
The characteristics of the modules have been investigated on a single-degree-of-freedom test bench, which allows movement in just one axis for one prototype module. Held in a solid case and guided with ball bearings the magnet module can move vertically and levitate above his backyoke. This is a stationary application, a horizontal feed motion is not included.
At this test bench the state control for the module is tested and optimized, it controls the stable levitation and is the decisive source for the stiffness of the bearing. Its speed while processing sensor signals and generating reference voltages for the power converter in addition to the current rise time in the magnet’s coils directs the amplitude and the length of the disturbance in the air gap width. Decisive factors also are the quality of the sensor signals and the magnitude of the disturbance.
Possible are tests with step- and periodical disturbances, as well as force and frequency response measurements. Enhancements to the control in a later stage, e.g. additional sensors and algorithms, will increase the stiffness further. In addition, other control principles can be tested and optimized on this test bench.
The levitating machine tool table
The machine tool table, which is just being built, features four magnet modules to generate the necessary vertical load capacity and two more modules to stabilize the table transversely to the feed direction. These six modules control the necessary five degrees of freedom, existing systems require twelve electromagnets for this task.
The feed force for the table is generated by two linear direct drives, which are facing each other, in order to compensate the enormous normal attraction forces. They will accelerate the table with approximately 20 m/s² to a final speed of more than 3 m/s.
If the motors are slightly inclined, the shift in the normal forces causes a vertical component which is no longer compensated by the double comb and is directed against the static gravitational force of the table slide. Thus the bearings are partially unloaded, which improves the power consumption and the thermal losses.
For manufacturing reasons the basic structure of the moving slide is made of steel. However, an aluminium construction, as it had originally been planned, would be very advantageous in terms of moved mass. In one of the last stages of the project, the achieved results will be analyzed and scaled with regard to the respective lower mass of an aluminium slide. This would mainly improve the maximum velocity and the acceleration of the table.
The high velocity can e.g. be used for a grinding process with a stationary grinding wheel, with the table moving beneath the wheel with frequent repetitions. This application also would have the advantage of limited process forces, which can be handled by the magnetic bearing without difficulty. Other applications, which might profit from high velocities can be found in the field of material handling.