The following invention relates to electromagnetic actuators and in particular to electromagnetic actuators as disclosed in my granted patent, EP1,305,807 Bl & foreign equivalents.

Such an actuator comprises an annular field coil and a composite plunger for relative travel threrethrough, the composite plunger comprising two permanent magnets axially in line but spaced one from the other and with like poles facing, wherein the coil experiences a thrust in use, when energised, relative to the plunger by interaction of the magnetic field produced by energization of the coil with the magnetic fields produced by the permanent magnets, and wherein at the centre of a stroke, the thrust is produced by interaction of the magnetic field of the coil with the magnetic fields of both permanent magnets, wherein the length of the annular field coil, the corresponding spacing of the permanent magnets and the said stroke are selected such that at each end of the stroke the thrust is produced by interaction of the magnetic field of the coil with the magnetic field of the permanent magnet closest to said coil such that said thrust is substantially constant over the pre-selected stroke regardless of displacement.

The characteristic of this actuator wherein the force provided at any point over its stroke remains substantially constant differs markedly from that of a convention solenoid. Solenoids exhibit nonlinear force / stroke characteristics, inasmuch that the force provided typically increases considerably with inwards movements of their plungers. A specific use of actuators, solenoids and the like is for applying motive force to a set of grippers, or claws, particularly in an amusement arcade machine. The in/out motion of the actuator is conveyed by a system of levers to operate the arms of the grippers. When used to grip irregularly shaped or especially delicate objects, the permanent magnet actuator of the aforementioned invention, hereinafter referred to as a permanent magnet or pm actuator has a very specific advantage.

On account of the evenness of its force over its stroke, the tongues of the grippers, or claw, operated thereby, exercise the same gripping force regardless of where the object being selected is being gripped. This is a significant advantage for certain applications, for example in the pick & place of delicate electronic components, or the vertically grabbing of toys (eg teddy bears) in amusement arcade machines commonly known as cranes. In the latter case, the amperes fed to the coil of the actuator can be adjusted precisely to pick up, or almost pick up, the desired object regardless of where the claws happen to be grabbing, and thus to enable the owner of the machine to govern the effective payout to the person using the machine.

It is common practice for the coil holding portion of such a permanent actuator to be fabricated from aluminium, or other form of non-ferromagnetic metallic material. This provides a durable housing for the coil, as well as an effective means of providing a "heat-sink" to dissipate heat arising within the coil during operation.

However, one disadvantage pertains to the use of this type of housing material. For both presentational and even psychological reasons, it can be desirable for the grippers both to grip and release an item with a snappy motion. In effect, a fast close and open motion. According to the present invention, an electromagnetic actuator is provided comprising an annular field coil housed in a housing and a composite plunger for relative travel threrethrough, the composite plunger comprising two permanent magnets axially in line but spaced one from the other and with like poles facing, wherein the coil experiences a thrust in use, when energised, relative to the plunger by interaction of the magnetic field produced by energization of the coil with the magnetic fields produced by the permanent magnets, and wherein at the centre of a stroke, the thrust is produced by interaction of the magnetic field of the coil with the magnetic fields of both permanent magnets, characterised by means to prevent or substantially limit the circulation of eddy currents within the housing of the actuator during motion of the plunger.

Thus the present invention ensures a fast open and close motion. This is because without the means to prevent or substantially limit the circulation of eddy currents within the housing of the actuator during motion of the plunger, eddy currents arise and circulate within the short-circuit coil path provided by the housing itself. These eddy currents exercise a parasitical braking effect on the motion of the plunger. This impairs the aforesaid snappy motion often desired and required of this type of actuator. By limiting the eddy currents, the braking force on the plunger is substantially eliminated.

According to an embodiment, the means to prevent the circulation of eddy current comprises one or more slits situated within the housing of the permanent actuator. The slits render impossible the ready circulation of eddy currents around the body of the housing.

In a preferred form the one or more slits are formed longitudinally along all, or substantially all, of the body of the permanent actuator. At least one, preferably two, or even more preferably, three slits are provided. In this case, circulation of eddy currents is substantially eliminated. The plunger of the actuator can thus move with a snappy action, unimpeded by any eddy current braking action as before.

In the case of cranes in amusement arcades, this can enhance both the operational and visual performance of the machines, as upon cutting off current to the coil of the permanent actuator or "grabber" as it is commonly known, the plunger thereof can fall naturally under gravity and release any toy within its clasp instantly.

As will be appreciated, the housing of permanent actuator is typically used for the placing and positioning of internal bearings for guiding the plunger concentrically therein, and/or for holding the coil (e.g. a bobbin upon which the coil has been wound) and which also services as guidance and bearing means for the plunger.

In the case of providing longitudinal slits along the full length of the housing, this may not be able to fulfil the function of holding circular components, such as bearings, within it.

According to an embodiment of the invention, the housing of the actuator is formed of a plurality of longitudinal segments, each formed along their lengths, with interleaving and/or interlocking lips.

Insulation means are provided to prevent electrical connection from one segment to its neighbour. For permanent actuator housings fashioned from extruded aluminium, the insulation means can be provided by the anodising of the segments. Anodising provides a simple and effective means of providing insulation while at the same time a visually appealing finish.

It is common practice to use resin or other glueing means to hold the coil within the housing.

According to a feature of the invention, the aforesaid interlocking means are profiled in such manner as to prevent, once interlocked with one another, the oozing out externally of resin during the manufacturing process.

The invention will now be described by way of example only with reference to the following diagrams in which:-

Figure 1 shows a typical use of a permanent actuator used to operate gripping arms, with the top figure showing a cross section taken through the longitudinal axis of the actuator and the bottom figure showing a cross section along the longitudinal axis of the whole assembly.

Figure 2 shows in detail the housing thereof according to the invention

Referring to Figure 1, a permanent actuator of the invention is shown at 10. A housing 11 contains a coil 12 and bearings 13. A plunger 14, contains permanent magnets 15 & 16 with like poles facing. The arrangement of the magnets and the sizing and shaping of the coil winding 12, is such as to produce a substantially even force over the full stroke, as disclosed in my granted patent no EP1,305,807 Bl.

A central spacer, which may be fabricated from a non-ferromagnetic material, or partly comprise some ferromagnetic material, depending upon the desired characteristics may be provided between adjacent magnets. The plunger 14 may be completed with a final non-ferromagnetic collar. The collar may be furnished with a central screw thread for connection to mechanisms.

The travelling annular field coil 12 is mounted for slidable movement along the plunger 14. It is guided therealong by bearings 13, preferably at each end, these bearings 13 being contained within the housing 11 which may be in the form of a thin walled tube. The coil 12 is also housed within the housing 11. Preferably the housing 11 is made of a non-magnetic material. Preferably the housing circumferentially surrounds the plunger 14.

The action of the actuator is as follows. When current of the appropriate polarity is fed to the coil 12, the lines of force produced thereby interact with the field pattern emanating from the plunger 14. The lines of force produced by the magnets 15,16 are forced to radiate outwards by virtue of the fact that their poles are in repulsion. As a result of this interaction, the coil 12 experiences a force, in accordance with Fleming's rule. The coil is permitted to move over a pre-selected length -stroke-. The force rendered is largely independent of displacement for the following reasons. When the coil is in its top position, it experiences at its centre, powerful fields emanating from the north pole of the magnet. In its central position, the effective field from the first magnet cutting the turns is weakened, but is doubled overall because of the effect of the bottom magnet. As the coil travels downwards, so the full field of the bottom magnet cuts the turns, as did the top magnet. By judicious spacing of the magnets, and selecting the appropriate length of the field coil 12, so a substantially even thrust profile can be achieved.

Where an enhanced performance is required, end collars may each be fabricated in part or wholly from ferromagnetic material, and similarly the central spacer. The effect of this is to extend and augment the field linkage with the coil 12, and thereby increase the performance of the actuator. To save weight, the end collars and central spacer may be made of tubing, of reasonable wall thickness, without significantly reducing the thrust obtained. A penalty of this arrangement is that the force versus displacement profile may not be as consistent as when non-ferromagnetic components are used, but nevertheless remains within acceptable boundaries for most applications requiring a constant thrust.

In an embodiment, no bobbin is used. Instead the wires of the coil 12 are held in place by resin, optionally with annular bearings 13 on either side of the coil 12. I.e. the coil 12 is effectively not on a bobbin. In practice this may be achieved by winding the coil 12 out of self-bonding wire which holds it together -once cooked- without any bobbin means.

The plunger operates arms 17 & 18 such as to grasp with its claws an object 19 upon energisation of the coil 12. The actuator 10 is held by the housing 11 from above. In the case of vertical operation, upon cessation of current supplied to the coils, the plunger falls under gravity, as shown by the arrow, so releasing the object gripped by the claws.

In the case of a conventional tubular housing fashioned from a metallic material, eddy currents are induced therein by the motion of the permanent magnet plunger. These hamper the free motion of the plunger and limit undesirably its rate of travel.

The housing 11 of the invention however is provided with (longitudinal) slits, as shown at 20 & 21 & 22. These prevent the ready circulation of eddy currents around the housing 11 and thereby allow freer motion of the plunger 14. This is a desirable characteristic, especially for fast motion grippers as may be used in "pick and place" assembly, or even in amusement arcade "cranes" when the visual impact of a snappy motion upon picking up and then (unfortunately) releasing a teddy bear, is desirable.

Referring to Figure 2, the segments of the actuator housing are shown in plan view in greater detail at 23 & 24 & 25.

The longitudinal edges of each segment 23, 24, 25 are designed such as to interlock with their neighbours. When fashioned, eg from extruded aluminium, modern die technology makes possible such accurate profiling that upon assembly, a virtually "solid" housing is created. The interlocking portions, as shown at 26, are effective to prevent the egress of resin or other glues, injected into the housing 11 during manufacture for holding and locating the coil contained therein.

In addition, the segments may be pre-anodised, this effect providing a thorough means of insulation one segment from the other and so making impossible free circumferential circulation of eddy currents. Some very minor circulation can occur within each segment, but this is insignificant compared to the braking effects caused by circumferential circulation. It is common practice to use resin or other glueing means to hold the coil within the housing.

According to a feature of the invention, the aforesaid interlocking means are profiled in such manner as to prevent, once interlocked with one another, the oozing out externally of resin during the manufacturing process.

Numerous variations will be apparent to those skilled in the art.