Switchable permanent magnetic device

Details Switchable permanent magnetic device Switchable permanent magnetic device

2001-09-14

2004-03-16

Donovan, Lincoln (Department: 2832)

Electricity: magnetically operated switches, magnets, and electr

Magnets and electromagnets

Work or object holding type

C335S285000, C269S008000

Reexamination Certificate

active

06707360

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a magnetic device. In particular, the present invention relates to a switchable permanent magnet device.
BACKGROUND OF THE INVENTION
Permanent magnets consist of a ferromagnetic material that has been suitably magnetized. Permanent magnets provide a magnetic field without requiring an electric current. Permanent magnets are attracted to and can be firmly affixed ferromagnetic materials. However, permanent magnets suffer from the drawback that their energy output is fixed and they cannot be employed if a variation in magnetic field strength is required.
Due to the above-mentioned drawback with permanent magnets, it has been customary to control magnetic fields by using electric current or electromagnets. The strength and orientation of the magnetic fields of electromagnets can be controlled successfully by controlling the strength and direction of the electric current. However, this introduces the necessity of having to provide a constantly available source of electric current and associated wiring. This can cause complexity and potential hazards.
A number of attempts have also been made to arrange permanent magnets to enable variations to be achieved in magnetic field strength. A number of different approaches have been tried to produce practical solutions. However, these approaches resulted in bulky constructions, complicated constructions, or devices that were expensive to manufacture. The prior art devices also paid little attention to achieving a good holding force between the devices and substrates under conditions where an incomplete external magnetic circuit existed, for example, where there were gaps between the magnet and the keeper, or where the keeper or substrate had a low permeability, such as thin ferrous sheet metal. To achieve good holding forces in the prior art devices, it was necessary that perfect contact between the magnet and the substrate be achieved and that the substrate be of substantial volume to enable a sufficient magnetic flux to be established. This, of course, could be difficult to achieve in practice.
U.S. Pat. No. 3,121,193 (Engelstead) describes a permanent magnet-type work holding device especially a chuck that is adapted for use in turning operations. The device of Engelstead consists of arrays of permanent magnets, which magnets are of generally rectangular configuration. In order to adjust the field strength an array of pole pieces was moved away from and out of alignment with another array of pole pieces.
U.S. Pat. No. 4,241,791 (Yanagisawa) describes a magnetic base which is switchable between an engaged position and releaseable from a substance by an operation of a single rotating permanent magnet disposed notably in a magnetic circuit block. The rotary magnet in Yanagisawa is rotated by 90° to switch between the exited and the unexcited states. This device, relies upon using passive external magnetic members of substantial mass (
12
and
14
of
FIG. 1
of Yanagisawa) having a rotatable magnet (
20
) positioned therein.
United Kingdom patent application no 2130797 relates to a permanent magnet chuck capable of switching from an excited state to an unexcited state by changing the relative positional relationship between fixed permanent magnets and movable permanent magnets. The movable permanent magnets move laterally with respect to the fixed magnets in order to vary the magnetic field.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a switchable magnetic device having improved properties.
In a first aspect, the present invention provides a switchable magnetic device comprising a housing, a first permanent magnet, a second permanent magnet, said first and second permanent magnets being essentially cylindrically shaped, said first and second permanent magnets being diametrically polarised, said first and second permanent magnets being mounted within the housing such that the first and second permanent magnets are rotatable relative to each other, and actuation means for causing relative rotation of the first and second permanent magnets, wherein said device presents a relatively strong external magnetic field when said first and second permanent magnets are positioned relative to each other such that a north and south poles of the first magnet are in substantial alignment with respective north and south poles of the second magnet, and the device presents a relatively weak external magnetic field when the first and second magnets are positioned relative to each other such that the north pole of the first magnet is in substantial alignment with the south pole of the second magnet and vice versa.
Preferable, the first and second magnets are substantially disc-shaped.
Preferably, the first magnet and the second magnet are mounted in the housing such that a face of the first magnet is opposed to a face of the second magnet. Most preferably, one magnet is mounted above the other magnet.
It is preferred that one magnet is fixedly mounted in the housing and that the other magnet is able to rotate in the housing. However, it will be appreciated that relative rotation between the magnets can also result from both magnets being rotatable. Accordingly, the present invention also encompasses devices in which both magnets can rotate.
The housing may comprise a pair of passive ferro-magnetic elements which are magnetically separated, for example, by an air gap or by magnetically high reluctance material(s). The housing most suitably defined in chamber in which the first and second magnets are positioned. The chamber may have open ends or, more preferably, is a closed chamber. Chamber closing members are suitably used to close the chamber.
In another embodiment, the housing is made as a unitary construction or from a single piece of material. In this embodiment, it is preferred that there be two portions of the housing having reduced cross sectional area such that the housing acts as two passive poles. Alternatively, portions of the housing may be treated such that the portions become non-magnetic to thereby result in the housing acting as two passive poles.
The housing is preferably made from a material having a low magnetic reluctance. Soft steel, iron or permalloys are examples of suitable materials for the housing.
The housing most preferably comprises a pair of passive poles. The strength of the external magnetic field can be maximised by shaping the pair of passive poles such that they reflect the magnetic field strength around the perimeter of the first and second permanent magnets.
The first and second magnets are preferably substantially disc-shaped. Most preferably, the first and second magnets are essentially cylindrical in shape and the height of the cylinder is less than the diameter of the cylinder. It will be appreciated that the first and second magnets need not be exactly cylindrical and that slight variations from a circular cross-section also fall within the scope of the present invention. The height of the magnets may vary over a wide range, and the ratio of diameter to height may also vary over a wide range.
The first and second magnets are also diametrically magnetised. By that, it is meant that the north pole region is separated by a diameter of the cylindrical surface of the magnet from the south pole region. The north pole region and south pole region both exist on the upper and lower substantially circular faces of the magnet and extend through the length or height of the magnet.
The first and second magnets may be essentially identical to each other.
Alternatively, the first and second magnets may have different magnetic properties. The magnets may have the same or different physical dimensions to each other. Varying the magnetic properties or the physical dimensions of the first and second magnets can be used in some embodiments of the invention to vary magnetic switching properties.
As mentioned earlier, it is preferred that one magnet is fixed in the housing and one magnet can rotate in the housing. It is especially preferred that the r

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