There are two well-known types of electric motor. In the first type, an electrical power source is connected by means of brushes to coils on an armature which is arranged for rotation about, or within, a stator in the form of one or more permanent magnets. The rotation of the armature relative to the brushes also serves to switch the electric current between the coils. In the second type, the rotor comprises a plurality of permanent magnets, and the stator comprises the coils and the associated electrical circuitry which controls the switching of the electrical power source between the coils.

A problem with the first type is that the use of brushes to connect the power supply to the rotating armature inevitably gives rise to some degree of arcing, and the associated undesirable consequences of wear of the brushes, electromagnetic, typically radio- frequency, interference and audible noise.

The problems associated with brushes do not arise in the second type of motor. However, the rotating magnets of the second type of motor have to be structured so as to withstand the substantial inertial stresses typically encountered in high-speed motors.

Bearing assemblies have been suggested for effecting an electrical connection between two relatively rotatable members. These can take the form of ball bearing assemblies or roller bearing assemblies.

However, the spherical balls used in the first type have the disadvantage that, even when mounted in a contoured track, a true line contact cannot readily be achieved in practice because of the high precision required in the manufacture of the contoured tracks. This, in turn, leads to arcing and the associated undesirable consequences of power loss, the erosion of electrical contacts and radio interference. Even where the spherical balls initially fit perfectly into the contoured tracks, there may still be the tendency for the balls

to jam in use, which precludes free rotation and leads to rapid wear. Furthermore, high rotational speeds can lead to wear of the outermost portion of the contoured tracks as a result of the centripetal force exerted by the tracks on the spherical balls. This effect also gives rise to a deterioration of the line contact between the balls and the tracks.

More importantly, however, even when a line contact is achieved by the use of a contoured seating for spherical ball bearings, it is not possible to achieve a non-slipping line contact in this way, since there will always be a degree of slippage between the sides of the balls and the contoured seating.

The roller bearings used in the second type, when oriented radially, suffer from the disadvantage that the resulting assemblies must also necessarily exhibit a degree of slippage. This results from the fact that the tracks in which the rollers rotate, in moving through a given angle, move a greater linear distance at their radially outermost end than at their radially innermost end, and the roller must therefore slide to some extent either at one end or the other, or at both ends.

It would therefore be desirable to provide arrangements for overcoming, or at least mitigating, some or all of the above problems.

In accordance with the present invention there is provided an electrical connector comprising two relatively rotatable members and a bearing assembly, the bearing assembly comprising a body defining a first substantially conical electrically conductive surface arranged in use to be in simultaneous non-sliding electrical contact with both of the members, the arrangement being such that the body rotates about its cone axis simultaneously with the relative rotation of the two members.

The non-sliding electrical contact arises from the conical surface, since the conical shape compensates for the variation in circumference of the rotating parts of the members with distance from the axis of rotation.

In addition, the line contact which can be achieved between the conical side of the body and the relatively rotatable members will not degrade significantly in use, since there is no

requirement for the surfaces of the relatively rotatable members in contact with the conical body to be contoured, and the rotation need not give rise to any differential wear on any of the surfaces.

The absence of sliding contacts, in turn, enables motors to be designed which are powered using much higher voltages than with conventional motors, since the arcing associated with the sliding contacts of such conventional motors imposes an upper limit on the voltage that can practically be employed. In addition, the absence of arcing facilitates compliance with radio emission legislation.

The body preferably has a second substantially conical electrically conductive surface, electrically insulated from the first surface, arranged in use to be in simultaneous non- sliding electrical contact with both of said members, such that two separate electrical paths are established between the two relatively rotating members.

With such an arrangement, it is possible to connect both a voltage supply and a return conductive path using such a connector. This arrangement is particularly suitable for use in high-quality motors.

The bearing assembly may alternatively have a second body with a substantially conical electrically conductive surface also arranged to be in simultaneous electrical contact with both of said members, the arrangement being such that the second body also rotates about its cone axis simultaneously with the relative rotation of the two members, such that two separate electrical current paths are established between the two members. The first and second bodies may be arranged substantially coaxially.

This provides an alternative arrangement for effecting both a voltage supply and a return conductive path, and this arrangement is particularly suitable for use in cheaper, mass- produced motors.

The cone axis of the or each conductive surface is preferably substantially perpendicular to the axis of relative rotation.

The vertex, or projected vertex, of the or each conical conductive surface preferably lies substantially on the axis of relative rotation. Such an arrangement can provide the connector with substantially no slippage between the body and the two relatively rotating members in use.

Preferably the surface of each of the members in contact with the or each conical conductive surface is itself substantially conical and has a respective cone axis substantially coincident with the axis of relative rotation.

Each member preferably comprises an insulating support on which is mounted one or more electrically conductive elements which is in electrical contact with the bearing assembly.

The or each conductive element may be in the form of an annular track whose centre lies substantially on the axis of relative rotation.

Each of the conductive elements is preferably so mounted by one or more projections on either the element or its corresponding support and one or more corresponding recesses in either the support or element respectively.

The connector preferably comprises a plurality of such bodies arranged at different respective angular positions about the axis of relative rotation. Preferably, these are arranged symmetrically about the axis of relative rotation. This provides the benefit of structural integrity.

The connector preferably further comprises one or more dummy bodies having substantially the same shape as said one or more bodies but bearing no conductive surface.

This arrangement is particularly advantageous in cheaper, mass-produced motors, where cost is an important consideration. Such dummy bodies provide the benefit of structural integrity without being used to form conductive paths.

Alternatively, a first one of the bodies may be arranged to effect a first electrical connection between the two relatively rotatable members and a second one of the bodies

may be arranged to effect a second, different electrical connection therebetween. This provides an arrangement wherein the bodies have a simpler structure.

The body or bodies are preferably mounted within a housing, which may be in the form of a substantially cylindrical cage. The housing is preferably arranged to rotate about the axis of relative rotation. Such an arrangement facilitates the assembly of the connector, since the bodies may be mounted with the cage prior to assembly of the cage with the other components of the connector.

The one or more conical surfaces of the connector is preferably substantially smooth, in order to enhance the electrical contacts.

The present invention extends to an electric motor comprising a connector of the above type. Such a motor preferably comprises means mounted on the rotatable armature for effecting electrical switching of an electrical current conducted via the connector between a plurality of armature-mounted coils. In a preferred arrangement, the connector is electrically connected to the armature-mounted coils by means of plurality of snap-fitting electrodes.

Preferred embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates in exploded form a first embodiment of a connector in accordance with the present invention; Figure 2 illustrates the arrangement of Figure 1, viewed from a different angle; Figure 3 is a cross-sectional view of the roller cage of the first embodiment of the present invention ; Figure 4 (a) illustrates the structure of one of the rollers of the first embodiment of the present invention;

Figure 4 (b) is a cross-section of one of the rollers of the first embodiment of the present invention; Figures 5 (a) and 5 (b) illustrate the structure of termination snap electrodes provided on an electric motor in accordance with an embodiment of the present invention; and Figure 6 illustrates in exploded form a second embodiment of the present invention.

With reference to Figures 1 to 4 (b), a connector constituting a first embodiment of the present invention forms part of an electric motor in which three armature-mounted coils are caused to rotate within a housing to which are mounted a sequence of field magnets.

The connector serves to transmit electric power from a source external to the motor to the armature-mounted coils. The connector comprises a generally cylindrical module 1 arranged symmetrically about an axis of rotation. The connector module 1 comprises a cylindrical housing 2 on the outside of which are located two electrically conductive terminals 3,4 for receiving a voltage supply.

Within the housing 2 is arranged an insulating base 5 for supporting inner and outer conductive tracks 6,7 which are electrically connected to the two terminals 3,4 respectively via inner and outer circular sprung electrodes (not shown) made from beryllium-copper alloy, or other suitable alloy. Beryllium-copper alloy exhibits an excellent spring quality, good resistance to corrosion and also an ability to conform to a shape which is stamped out from a sheet for the material. These sprung electrodes are mounted concentrically in a coplanar fashion on a backing disc and separated by a small gap (not shown). The electrodes, together with the associated backing disc, constitute a first assembly in which the manufacturing process is facilitated through the use of"plug and lock"connectors. Mounted on the insulating base 5 is an encoder disc 8, the function of which is described below.

The inner and outer conductive tracks 6,7 are each provided with four conductive lugs 9, 10 which serve not only to transmit the voltage from the terminals 3,4 to the tracks 6,7 via the sprung electrodes, but also to maintain the alignment of the tracks 6,7 on the insulating base 5, by fitting into corresponding apertures 11,12 formed in the base 5 and thereby

providing evenly distributed contact force at these four positions symmetrically arranged about the axis of rotation. The tracks 6,7 define the surface of a cone arranged symmetrically about the axis of rotation 2, the apex of which is to the left of the tracks 6, 7 in the arrangement of Figures 1 and 2.

The insulating base 5 is made from a rigid tough plastics material, such as poly (ether ether ketone) (PEEK) or other material having similar characteristics. A major advantage of this material is its low coefficient of expansion, resulting in a component of high dimensional stability both at high and low temperatures, its low level of moisture absorption and high rigidity. The insulating base 5 is formed with the conductive tracks attached thereto as a single component.

Mounted within the housing 2 immediate adjacent the conductive tracks 6,7 is a bearing assembly 13 in the form of a four cone-shaped rollers 14 mounted within a cage housing 15. The cage housing 15 is of a single-piece construction, for reasons of economy, but could alternatively be made from several components, and is made from a slightly flexible but rugged plastics material such as acetal copolymer or poly (ether ether ketone (PEEK).

The assembly 13 is formed with a number of holes for reasons of economy, to minimise the quantity of material required in the manufacturing process. The function of the cage housing 15 is simply to retain the cone-shaped rollers 14 in position.

The structure of the rollers is shown in greater detail in Figures 4 (a) and 4 (b). Each roller 14 comprises an inner conductive cone portion 16 and an outer conductive cone portion 17 separated by an insulating spacer 18. The conductive portions are made from either free- turning brass (machinable brass) having a slightly lower copper content compared with other brasses. Alternatively, aluminium plated with rhodium/palladium alloy, with ruthenium/silver alloy or with any other suitable alloys having appropriate surface, electrical and mechanical properties may be employed. The platings serve to reduce the metal-to-metal contact resistance, in some cases relying on the well-known skin effect.

The spacer extends axially in the interior of the roller and is provided with a respective concave recess 19 at each end to permit the roller to be snap-fitted on to corresponding convex projections 20 within the cage housing 15. The material of the spacer 18 is such as to provide a low friction surface on the two concave recesses 19.

In an alternative embodiment (not shown), each of the rollers is provided with either an inner or an outer conductive cone portion only, which facilitates a simpler arrangement. In this arrangement, rollers bearing an inner conductive cone portion are preferably mounted within the cage housing alternately with those bearing an outer conductive cone portion.

Mounted adjacent the bearing assembly 13 on the side opposite the conductive tracks 6,7 are a further pair of conductive tracks 21,22, also defining a conical surface but one whose apex is to the right of the tracks in the arrangement of Figures 1 and 2. The tracks 21, 22 are structurally similar to the conductive tracks 6,7, except that these further conductive tracks 21,22 are each provided with only three conductive lugs 23,24 which fit within corresponding apertures 25,26 formed in a further insulating base 27. The base 27 is made from the same material as that of the insulating base 5 and is attached, e. g. by moulding, to a ceramic substrate 28 which is provided with a pairs of electrical terminals 29 which are connected to the three pairs of conductive lugs 23,24 attached to the further conductive tracks 21,22.

The ceramic substrate 28 is made from a ceramic material having high rigidity, high thermal conductivity and good dimensional stability at elevated temperatures.

Mounted on the ceramic substrate 28 is switching circuitry 30 which serves to effect sequential connection between the terminals 29 and two of three output terminals 31,32, 33 mounted on the side of the ceramic substrate 28 opposite to the three pairs of terminals 29. The switching circuitry 30 comprises low-loss, fast, high-power MOSFET transistors (or alternatively IGBT's), a MOSFET driver block and a voltage regulator. Such switching circuitry can switch power with no significant arcing. The three output terminals 31,32 33 are connected to three respective coils mounted on the armature of the electric motor via three respective termination snap electrodes 34,35,36 on an armature termination disc 37, as shown in Figures 5 (a) and 5 (b). The coils are connected together in either of the conventional"delta"or"wye"configurations well known in the art of electric motors. Such an arrangement enables a three-coil motor to be powered using only a two pole connector.

As shown in Figures 5 (a) and 5 (b), each of the termination snap electrodes 34,35,36 is in the form of a folded spring 38 bearing a hemispherical projection 39 which is arranged to mate with a corresponding hemispherical recess in the associated output terminal 31,32 or 33. The output terminals 31,32,33 have a chamfered profile 40, as can be seen in Figure 5 (b), facilitating the location of the snap electrodes 34,35,36. During assembly, the connector is connected to the termination disc 37 under compression, and the snap electrodes are 34,35,36 are thereby snap-fitted into the associated respective output terminals 31,32,33.

Also mounted on the ceramic substrate 28 is an angular displacement detector 41 arranged to sense the angular position of the semiconductor substrate 28 relative to the field magnets of the motor by sensing patterns on the encoder disc 8 mounted on the base 5 which is stationary relative to the field magnets. The angular displacement encoder is a three-bit reflective gray scale encoder, the pattern being sensed using the radiation from infrared light-emitting diodes and detected by phototransistors, the light-emitting diodes and the phototransistors constituting part of the angular displacement detector 41. Alternatively, a Hall-effect, magnetic sensor may be employed.

The encoder is structured to resolve 60-degree sectors, which is sufficient for normal motor applications involving traction. However, if the motor is intended to be used for servo purposes, then an encoder having a much higher resolution would be required. This could take the form either of an additional encoder, which would be fitted externally to the motor casing, in which case the internal encoder would still be required for effecting the sequential energising of the armature coils, or the internal encoder could itself be structured for high-resolution detection.

The switching effected by the circuitry 30 is controlled in dependence on the sensed angular position in accordance with a pre-stored program and/or by a program transmitted to the circuitry 30 using a wireless, e. g. infra-red, communications link. With both a wireless communications link and internal closed-loop positional feedback, such motors can be programmed to act both in servo and in traction modes.

It will be appreciated that the connector module as a whole not only effects an electrical connection between the two terminals 3,4 on the housing 1 and the three terminals 34,35, 36 on the armature termination disc 37, but also effects the commutation switching between the three terminals 34,35,36 by virtue of the switching circuitry 30. Thus, no additional switching by the use of brushes or the like is required.

The connector module 1 is designed to pass over a motor output shaft and to be snap-fitted into position in which it mates with the armature termination disc via the termination snap electrodes. Once fitted on the motor shaft, it constitutes a unit which is sealed against ingress of contaminants, greatly enhancing the reliability and performance of the motor system.

A second embodiment of the present invention is illustrated in Figure 6. In this arrangement, the cage housing 15 comprises a pair of semicircular ribs 42, and two of the four cone-shaped rollers 14 of the first embodiment have been replaced by two pairs of cone-shaped rollers 43, each roller being snap-fitted within the cage between one of the semicircular ribs 42 and either the innermost wall or the outermost wall of the cage housing 15. The other two of the four cone-shaped rollers 14 of the first embodiment have been replaced by two non-conductive dummy rollers 44. In addition, the sprung electrodes, together with the associated backing disc are omitted, their function being performed by the cylindrical housing 2. The armature termination disc is also omitted, but a spring sleeve 45 is instead provided at the armature end of the connector module. These variations result from the fact that lower-grade, mass-produced motors are not normally required to be serviceable.

Although preferred embodiments of the present invention have been described above, it will be clear to those skilled in the art that many modifications may be made to these without departing from the scope of the invention as defined in the following claims. For example, although the motor of the preferred embodiments is provided with three armature-mounted coils, it would of course be possible to provide a different number of coils, with consequential changes to the number of electrodes and the switching circuitry.

In addition, although a two-pole connector has been described, it would be possible to provide one of more further sets of conductive portions on the cone-shaped rollers, or one or more further sets of cone-shaped rollers to create a multi-polar connector. Such an arrangement could be used to supply three-phase electric power to the coils of the motor.

The drive arrangements may be full-bridge, half-bridge or unipolar.