Patent Application
20110062716
The present invention relates to magnetic bearings. Especially, the invention relates to an apparatus for rotational motion about a rotational axis of a first member relative to a second member, the first member having an outer surface being rotational symmetric about the rotational axis, and the second member having a corresponding rotational symmetric cavity with an inner surface accommodating at least part of the first member with an interspace between the outer surface of the first member and the inner surface of the second member. A repulsive magnetic field between the first and the sec- and member prevents contact between the outer surface of the first member and the inner surface of the second member.
Magnetic bearings are known for a variety of application. One application is for trains, Magnetic Levitation Trains, where the train floats on a magnetic field that arises due to the permanent magnets in the bottom of the train and electromagnets in the railway track. Another application is for a spherical stepper motor for various sorts of robotics, for example as developed at the John Hopkins University in USA (published in IEEE/ASME Transactions on Mechatronics', Vol. 4/4, 1999) and on the Internet web site http://www.jhu.edu/˜news_info/news/home01/jan01/motor.html.
Other examples of magnetic bearings include a vibration damper with a spherical inner body suspended inside six active electromagnetic bearing elements, as disclosed in U.S. Pat. No. 4,947,067.
Further examples of magnetic bearings include solutions with solely permanent magnets, for example for bearings/journals for shafts in a piece of rotating machinery. But in those examples, the basic assumption is that the bearings are cylindrical, which limits the angular flexibility of the bearings, which is a severe disadvantage in case that the shaft receives a force component in a longitudinal direction.
It seems that there is a need for an industrial ‘heavy-duty’ solution where the journal/bearing is simple and robust.
It is therefore an objective of the invention to provide a rotational magnetic bearing that is simple and robust with the option to create other than cylindrical shapes by simple means.
This objective is achieved by a an apparatus for rotational motion about a rotational axis of a first member relative to a second member, the first member having an outer surface being rotational symmetric about the rotational axis, and the second member having a corresponding rotational symmetric cavity with an inner surface accommodating at least part of the first member with an interspace between the outer surface of the first member and the inner surface of the second member. The first member comprises a plurality of permanent dipole magnets arranged with identical polarity directed towards the interspace and that the second member comprises a plurality of permanent dipole magnets arranged with the same polarity towards the interspace opposing the magnetic field of the first member for repulsion of the first member from the inner surface of the cavity.
Between the first and the second member, a repulsive magnetic force is provided by oppositely directed magnetic fields. By accommodating the first member in the cavity of the second member such that the first member is surrounded by the second member in a cross section when viewing along the rotational axis, the repulsive force prevents the first member from contacting the second member within this cross section.
The versatility of the invention is primarily directed towards smoothly bending surfaces, for example an oval shape or most preferably a spherical shape. Possible shapes will be discussed more in detail below.
By using a plurality of dipole magnets, a magnetic field from only North or only South poles can by relatively easy means be provided on the entire outer surface of an arbitrarily shaped first member. For the invention, preferably, smoothly bending bodies of revolution are envisaged, for example smoothly bending shells of revolution, especially spherical shapes where a high number of rod magnets can be assembled radially to resemble a sphere, or spherical shell, with a field form only North or only South poles on its surface.
By providing dipole magnets for the magnetic field between the first and the second member, the system for creation of the magnetic field is maintenance-free, which is highly desirable for a number of applications, for example for apparatus at locations which are not easily accessible, including applications in connection with wind turbines located offshore, especially in deep sea.
In line with the above-mentioned smoothly bending surfaces, the direction of the magnetic fields from at least a number of the plurality of opposing dipoles of the first and of the second member have a directional component parallel with the rotational axis in order to repel the first member from the inner surface of the cavity in a direction parallel with the rotational axis. For example, if the rotational axis is vertical, those field lines that are repelling in the vertical direction can be used for supporting the weight of one member on the other. For example, if the second member is stationary, the repulsive inner surface of the cavity may be used to support the weight of the first member and, optionally, the weight of any additional means loaded on the first member.
As mentioned above, in a preferred solution, part of the outer surface of the first member has a shape being part of a sphere and part of the inner surface of the cavity has a shape being a corresponding part of a sphere—although with larger radius—for accommodating the spherical part of the first member in the spherical part of the cavity. Spherical solutions are relatively easily produced with magnetic dipoles in the form of magnetic rods arranged in a radial configuration. For example, all dipoles may be directed with their North poles towards the interspace between the inner surface of the second member and the outer surface of the first member.
The first member and the corresponding cavity need not resemble en entire sphere. In a further embodiment, for example, the first member is composed substantially of a first spherical part with a shape being part of a sphere and an axle extending from the spherical part along the rotational axis. In turn, the second member has a cavity with a shape being part of a sphere accommodating the spherical part of the first member. In addition, the second member has a collar in extension of the cavity, the collar accommodates the axle. In order to prevent the axle to contact the inner surface of the collar, the axle and the collar comprises a plurality of dipole magnets with identical polarity facing the interspace between the outer surface of the axle and the inner surface of the collar such that a repulsive force is created. This way, the bearing according to the invention is limited to mainly rotational movement about the rotational axis. However, a certain degree of tolerance is provided in the bearing for motion deviating from the rotational motion. Thus, over a minor extension within the tolerances, the bearing works as an omnidirectional hinge.
Though the spherical shape is the mostly preferred one due to its smooth rotation capabilities, deviations from a sphere are tolerable. Even other alternatives may function in dependence of the application and the necessary tolerances. For example, as an alternative to a sphere, other bodies of revolution with smoothly bending shapes may be used for the invention.
Some examples of these other smooth shapes may be defined in the way that the outer surface of the first member has a shape being part of a surface of revolution with a cross section resembling a Lamé curve and part of the inner surface of the cavity has a shape being a corresponding part of a corresponding surface of revolution resembling a Lamé curve, however, with a larger cross section for accommodating part of the first member inside the cavity. The Lamé curve is defined by (a/x)m+(b/y)n=1 with x and y as variables and where a, b, m, and n are constants, and n>=2 and m>=2. For example, in the case of an ellipse, m=n=2, and for a spherical shape, m=n=2, and a=b. However, other example of smooth shapes are parabolic or hyperbolic bodies of revolution
In an example of such an embodiment, the first member is composed substantially of a body with an outer surface resembling a surface of revolution with such a smooth curve, for example a Lamé curve, as a cross section and an axle extending from the body along the rotational axis, wherein the second member has a corresponding cavity accommodating the body of the first member, and wherein the second member has a collar in extension of the cavity. As in the embodiment with the sphere, the collar accommodates the axle. Both the axle and the collar comprise a plurality of dipole magnets with identical polarity facing the interspace between the inner surface of the collar and the outer surface of the axle for repulsion of the axle from the inner surface of the collar.
In a further embodiment, the collar comprises an induction motor for electromagnetic driving of the axle. Alternatively, in addition or as part of the motor, the collar may comprise an electromagnetic power generator for generating electrical power from rotation of the axle relative to the collar. The latter is relevant in the case that the invention is used for supporting a wind turbine in extension of the axle from the first member.
In a preferred embodiment, the axle has a substantially vertical orientation, and the wind turbine is a Darrieus-type turbine with a plurality of airfoils fastened at their upper and lower ends to an extension of the axle. In the case that the wind turbine is mounted to the axle of the first member, the second member may comprise a vertical floating weight arrangement with cables for fastening of the second member to a sea bed. This floating weight arrangement is provided in the downward direction opposite to the axle of the first member.
In the foregoing, the turbine has been provided in extension of the axle of the first member, and the second member support the weight of the first member. However, the apparatus according to the invention may be used where the second member has a turbine fastened to it, and where the axle of the first member if fixed for supporting the weight of the turbine and the second member.
The invention will be described in more detail with reference to the drawing, where
The ball 2 is connected to a first column/rod 8 extending upward. The first column 8 is contained in a collar 11, where the collar 11 as well as the first column 8 at the height of the collar 11 is provided with a permanent magnetic structure 9, such that the collar is held within the magnetic field 11 between of the column 8 and the collar 11. This way, the Z-ball structure with the ball 2 in the bowl 3 approximately only rotates in a plane normal to the column 8.
The bowl 3, on the other hand, has a second column/rod 12 extending in the opposite direction relative to the first column 8 of the ball 2. Apart from minor movements of the first column 8 relative to the second column 12 due to mutual twisting and movement along the rotational axis 14, the first 8 and the second column 12 are co-linear and may rotate relatively to each other around a common axis 14.
It should be mentioned that the columns/rod 8,12 are only used for illustration and can be substituted by other structures. The column 8 can have en extension in the form of an axle 16 for mount of external devices to the column 8.
The bowl can be expanded with a collar 11 that contains coils 15 for induction. These coils 15 may be used as an induction motor for driving the ball 2 inside the bowl 3 or as a power generator for providing electricity in case that an outer force drives the ball 2 relative to the bowl 3. In addition, the induction motor may be used for braking the ball relative to the bowl 3. Such a braking mechanism may be combined with additional mechanical brakes.
As it will appear in greater detail below, such a device is highly suited for wind turbine of the Darrieus type, for example as disclosed in U.S. Pat. No. 1,835,018 or as illustrated in
The advantage of using permanent magnets 6 for such a construction in contrast to electromagnets is fact that permanents magnets practically do not need maintenance and do not need electric feedback control mechanism. In many instances, for example, in connection with wind turbines a minimum of maintenance and control is desired.
For the permanent magnets, different kinds of construction patterns are possible. Preferably, the pattern for the location of each set of magnets follows a geometrical formula for the most uniform distribution, as illustrated in
A preferred form of magnets is illustrated in
Advantages of the Z-ball according to the invention is that is constitutes a rotational coupling between two parts 2, 3, where the coupling is without friction between the two components eliminating abrasion and minimizing loss of energy. It can keep the rotational, friction-free coupling despite forces acting on the two components, where the forces are in other directions than along the rotational direction. When exposed to forces, the coupling acts resilient by pushing the ball 2 towards the bowl 3. It is possible to incorporate an induction device, for example in the form of a collar, as illustrated in
In the following, examples of field lines are illustrated in connection with the invention. As mentioned above, a large number of dipole magnets of the type as shown in
If two magnet rods are forced with their equal-polarity ends towards each other, a picture is obtained as in
Based on this understanding, a magnetic assembly from a plurality of such dipole magnets can be constructed for the ball 2 and the bowl 3. Field lines for the magnetic field between the ball and the bowl are illustrated in
It can be seen clearly how the flux field is compressed between the magnetic structures 2, 3. This creates pressure between two neighbouring magnets, but the energy/work needed for assembling the magnets is a production question; when all magnets are in place, they are frozen in a matrix to fill-in the gaps, and the matrix provides the necessary shell strength. Thus—from a membrane shell model point of view—the ball 2 and the bowl 3 are born with pre-tension. Preliminary calculations have shown that the resulting stresses should not be crucial if modern composite materials are used for the matrix.
The computer program used for creating
Conceptually, the field acts as a spring-mattress with radially directed springs, where the force increases when the distance between the ball 2 and the bowl 3 is reduced. The characteristic depends upon the type of alloy in the various magnets and the chosen modularity of the geometry. In theory, if the distance between the spherical surfaces goes towards zero, the force between the surfaces goes to infinite. It is not a theoretical constraint that the ball or the bowl should be ideal spherical surfaces or part of such perfect geometrical figures. Deviation from a perfect form is tolerable and often unavoidable, especially in the preferred case, where the magnetic structure is resembled from a plurality of dipole magnets.
According to Coulomb's Law then 2 permanent magnets with charges Q1 and Q2 and distance d exert a force F on each other:
F=[1/(4*π*ε0)]*Q1*Q2*(1/d2)
Here the material is air.
Here, we assume that the charges have the same sign—otherwise we get attraction instead of repulsion. The outer surface of the ball has a permanent magnetism qk [C/m2], and the inner surface of the bowl has a permanent magnetism of qs [C/m2]. The ball has its north pole N directed against the bowl, and the bowl has its North pole N directed against the ball: the ball ‘repulses’ the bowl and vice versa.
The magnetic field between the ball and the bowl is not a static field—it varies with time since the ball oscillates in the bowl. We do no get static equilibrium but dynamic equilibrium, which according to theory can be called stable.
It is impossible to provide a monopole magnetism on the spherical surfaces as assumed above. A realistic solution for a heavy-duty task must be based upon a discretization of the surfaces. In our invention, the ball and the bowl are both assemblies of permanent magnets of the types in
The invention can be used for frictionless rotational coupling between different elements, for example the z-ball principle can be used as an axle bearing for rotational applications including drilling machines, engines and wheel bearing.
A preferred application is in connection with a wind turbine 20 of the Darrieus type with a vertical rotation axis 14, as illustrated in
Especially, the invention is suitable for a new type of deep-water offshore wind turbine for generation of electrical power. The term deep-water is preferably meant for a water-depth D>50 m. It should be stressed, however, that our invention is not limited to D>50 m—in principle it also works ‘near-shore’ and onshore, as the principle of the Z-ball is universal. However, the Z-ball is highly advantageous in the case of offshore turbines, if the turbine is part of a floating structure.
In one embodiment, the total construction of wind turbine 20, fastening wires 23, 25 and Z-ball 1 are designed to be a unit in an offshore wind turbine park, which is illustrated schematically in
For example, the sea bed foundation may be fixed seabed installations, suction anchors, or arrow anchors. There can be one anchoring point for two cables 23, 25 going to the SPAR 21 as well as to the spinning top 24, but according to the environment, including sea bed 22 conditions, wind, waves, and the resulting forces in the cables 2325, it might be necessary to apply one group of anchoring points for the SPAR 21 and another group of anchoring points for the cables 23, 25 to the spinning top.
Onshore, these cables are installed with a high pre-tension to prevent collision with the airfoils 17, and to ensure that the natural frequencies of the cables do not interfere with the frequencies of the rotor (including the frequencies for the airfoils) under normal operational conditions. These considerations are naturally also valid offshore, and particularly—and more seriously—in view of the fact that the length of the cables is increased considerably. The upper limit for the water-depth D is thus determined by the necessary pre-tension and the existing technology for mooring/anchoring systems.
Our invention belongs to the group of ‘compliant offshore structures’. Since the Z-ball will oscillate due to the cable oscillations and the SPAR oscillations (due to wave excitation forces), then the ‘envelope’ for the whole structure can only be determined for a given environment: water-depth, wind & wave spectra, seabed, etc.
Two of the largest commercialized Darrieus-type turbines are shown in
However, the invention is in no way depending upon these types with two or three airfoils. Other types with other number of airfoils have also been available on the market. We based our preliminary calculations on a 3-bladed rotor of the ‘FloWind 19-meter’ type in
With reference to
The strength and production for polymeric materials have now advanced to a degree where polymer cables can be used offshore instead of the former heavy anchoring chains or steel ropes/wires. The light weight density of these advanced synthetic materials implies that they are virtually ‘weightless’ when submerged under water.
Especially, a Darrieus Wind Turbine with its characteristic vertical axis has 3 advantageous characteristic features:
The following advantages are considered in connection with the wind turbine according to the invention.
1. The construction takes into account and minimizes the effect of bending moments
2. The Z-ball minimizes/eliminates the tear & wear and subsequent loss of power known from onshore Darrieus turbines. Thus the life-time is simply prolonged.
3. The induction device in the collar of the Z-ball acts as gear, generator, and control/brake; these components represent a substantial mass in a traditional turbine. This simplification, in connection with the elimination of the yaw mechanism, represents a considerable reduction in mass.
4. The number of mechanical components is reduced. That means simpler production methods, less mechanical components to transport and install, and fewer maintenance problems.
5. We estimate that our invention will have a ratio (MW/kg-installed-mass) which is 5 times higher than the ratio found in the various projects for deep-water offshore based installations with wind turbines having horizontal turbine axis.
6. If the chosen site for the wind turbine park should turn out to be a wrong choice due to different reasons, for example change of climate or politics, then the turbine, the Z-ball, and the SPAR can be split-up and moved independently with existing offshore vessels. This implies a high degree of mobility for the wind turbine park as such, and a high degree of freedom for the positioning of each unit in the park.
7. The invention can be adapted to almost any environment. Under the assumption that there are used two types of rotors, changes due to other parameters are diameter and length of the SPAR and cable lengths. The number of cables for the SPAR and for the top spinner should be at least six. Systems with three cables are possible in theory, but not to be recommended, as the redundancy of more cables implies a higher safety. The mooring/anchoring system will depend upon the site.
8. A short series of experiments has indicated that a Darrieus turbine is less sensitive to airfoil icing problems than most wind turbines with horizontal axis.
The Z-ball is the most preferred embodiment. However, other shapes, including slight deviations from a sphere, are also possible. Preferred solutions include rounded forms, for example, a shell/body of revolution with a cross section having an elliptical shape with the longest axis in the horizontal or vertical direction, respectively.
| Number | Date | Country | Kind |
|---|---|---|---|
| PA200700171 | Feb 2007 | DK | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/DK2008/000046 | 2/1/2008 | WO | 00 | 3/2/2010 |
