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.

It would therefore be desirable to provide arrangements for overcoming, or at least mitigating, the problems of conventional electric motors.

In accordance with a first aspect of the present invention there is provided a brushless electric motor wherein the stator comprises one or more permanent magnets and the rotor comprises one or more coils and a commutator.

In such an arrangement, the advantages of the prior-art motors of the first type described above are achieved without the need for the provision of brushes, and the problems specific to prior-art motors of the second type do not arise.

It would be desirable to provide a suitable means of determining the relative positions of the rotor and stator. Thus, in accordance with a second aspect of the present invention there is provided a displacement encoder comprising a plurality of tracks each comprising a sequence of portions respectively exhibiting different sensible characteristics, the arrangement being such that displacement can be measured by sensing the combination of the characteristics exhibited by the respective portion of each of the tracks at that displacement.

Such an arrangement enables displacement to be determined with higher resolution that in conventional encoders incorporating only a single track.

The encoder preferably further comprises means for sensing the characteristics and for generating an output in the form of an n-ary digital number, where n is the number of different sensible characteristics within each track. n may, for example, be two, and the output-generating means may then be arranged to generate a binary digital output.

The sensible characteristics are preferably optical, but could alternatively be magnetic or electric. In the case of optical characteristics, these may comprise different levels of reflectivity, effected preferably means of a lower reflective surface and an upper layer having different degrees of optical transmissivity. One of the reflective characteristics may advantageously be substantially no reflectivity, since this will maximise the contrast between the two levels.

Alternatively, the optical sensible characteristics may comprise different levels of optical transmissivity, such as substantially no transmission of light and substantially total transmission of light.

The encoder may advantageously be arranged to sense angular displacement, in which case the tracks may be in the form of substantially coplanar, concentric rings. In this case, the

encoder may be arranged such that the tracks are in the form of substantially parallel circular bands arranged at successively different positions along the axis of angular displacement. Such an encoder may comprise three tracks each having portions exhibiting one of two possible sensible characteristics and wherein the angular displacement defined by each of six 60-degree sectors is encoded by a respective different combination of the eight possible different sensible characteristics. Such an arrangement balances the need for high resolution of encoding together with a desirably low number of tracks.

In accordance with a third aspect of the present invention there is provided an angular displacement encoder for use with a motor, the encoder comprising a pattern-bearing element, arranged to be rigidly attached to the stator of the motor, and a sensing arrangement for sensing the pattern on the element and arranged to be rigidly attached to the rotor of the motor, thereby to determine the relative angular displacement of the stator and the rotor. Such an arrangement is particularly advantageous for motors which form an aspect of the present invention, in which the electronic switching control circuitry is arranged on the rotor, thereby eliminating the need for a further rotary electrical connection between the encoder sensing circuitry and the switching circuitry.

The pattern borne by the element can preferably be sensed optically.

In accordance with a fourth aspect of the present invention there is provided a displacement encoder comprising an element bearing a pattern in the form of a sequence of at least three portions respectively exhibiting different sensible characteristics. Such an arrangement presents an alternative way of increasing the resolution of the encoder compared with prior-art arrangements, and additionally provides a convenient way of determining direction of movement without requiring more than one sensing arrangement.

Thus, for only two sensible characteristics, A and B, a prior-art arrangement which senses the sequence ABA cannot, in the absence of plural sensing arrangements, distinguish between a constant movement or one involving a reversal. However, with an arrangement involving three sensible characteristics, A, B and C, a constant movement is encoded by the sequence ABC, whereas a reversal of movement is encoded as ABA.

Such an encoder may be arranged to sense rotary displacement, and may comprise 2" portions having a different one of 2'respective sensible characteristics which vary monotonically, i. e. never decreasing or never increasing, with angular position, where n is an integer. This enables the characteristics to be sensed in an analog sensor and subsequently digitised, since the analog output of the sensor will also vary monotonically with the angular position.

Such an encoder thus preferably further comprises means for sensing the pattern and generating an analog output in response thereto and an n-bit analog-to-digital converter for converting the analog output of the sensing means to an n-bit digital output value. n may advantageously be eight.

In accordance with a fifth aspect of the present invention there is provided a displacement encoder comprising an element bearing a pattern in the form of a continuously-varying sensible characteristic. Such an arrangement enables displacement to be encoded with extremely high precision.

The characteristic may advantageously vary substantially linearly with respect to displacement. This enables the output of an analog sensor also to vary linearly with displacement. The pattern borne by the element is preferably such that it can be sensed optically and could, for example, be defined by a variation in the optical transmissivity of the element, in which case the element may conveniently be embodied by photographic film. Alternatively, the pattern could be defined by a variation in the reflectivity of the element. In this case, the variation in the reflectivity may be effected by means of a lower reflective surface and an upper layer having different degrees of optical transmissivity.

The upper layer may comprise photographic film.

The invention extends to a method of measuring the relative angular displacement of a stator and a rotor within a motor, the method comprising attaching to the stator a pattern- bearing element and attaching to the rotor a sensing arrangement for sensing the pattern, such as an optically sensible pattern, on the element.

It would further be desirable to provide suitable electrical connections between the relatively movable stator and rotor of such a motor.

Thus, in accordance with a sixth aspect of the present invention there is provided an electrical connector comprising first and second pairs of terminals connected respectively to two relatively movable components of a bearing assembly, thereby to provide two independent constant electrical connections between two relatively movable members, the bearing assembly comprising two sub-assemblies for effecting the two respective independent connections between the first and second pairs of terminals, the two sub- assemblies being substantially coplanar.

Such an arrangement enables electric power to be transmitted to a moving load or an electrical signal to be transmitted to a moving electrical circuit in a particularly convenient and compact arrangement, since only one bearing assembly is required, and thus the axial dimension of the space required for the bearing assembly is thus reduced when compared with arrangement involving two separate axially spaced bearing assemblies for effecting the two respective connections.

The bearing assemblies may comprise rotary bearings, and each sub-assembly may comprise a ball bearing assembly and may further comprise contoured seating for each ball, so that each ball defines in conjunction with its associated seating a respective pair of line contacts.

The two relatively movable members are generally disc-shaped, since this further reduces the axial dimension of the space required for the bearing assembly. Furthermore, each of the two disc-shaped members preferably has defined therein two substantially concentric annular channels for containing two respective ball races.

The invention extends to an electric motor comprising such an electrical connector, which preferably further comprises a movable armature to which is electrically and rigidly connected said first pair of terminals.

The invention further extends to a rotary electric motor comprising a rotatable shaft, a rotatable armature and such an electrical connector, wherein the two-sub-assemblies are each arranged coaxially around the shaft.

The motor may further comprise a housing which is mechanically connected to the shaft of the motor by means of two further rotary bearing assemblies positioned on either side of the first-mentioned bearing assembly. Such an arrangement enables the outer bearings to absorb any radial stresses applied to the motor and thereby enhances the electrical contact made through the inner bearings.

Alternatively, a first one of the two relatively movable components of the bearing assembly forms part of a housing which is further mechanically connected to the shaft of the motor by means of an additional rotary bearing assembly positioned on the other side of the first one of the components from the other of the two relatively movable components of the bearing assembly.

The motor preferably further comprises a position encoder for sensing the position of the movable armature.

The invention extends to a method of establishing two independent electrical connections between first and second relatively movable members comprising establishing respective electrical connections between the members and the two relatively movable members of a bearing assembly, the bearing assembly comprising two sub-assemblies for effecting the two respective independent connections, the two sub-assemblies being substantially coplanar.

In some cases, it may be desirable to enhance the electrical connection between the two components of a bearing assembly. This is particularly desirable where small tolerances or movements may result in the temporary loss of electrical connection.

To overcome this problem, in accordance with a seventh aspect of the present invention there is provided an electrical connector comprising a pair of terminals connected respectively to two relatively movable components of a bearing assembly, thereby to

provide a constant electrical connection between two relatively movable members, the bearing assembly further comprising a resilient electrically conductive member located between the two components of the bearing assembly for enhancing an electrical connection therebetween.

The bearing assembly preferably comprises a race of rotary bearing members located between the two relatively movable components of the assembly, and the resilient electrically conductive member is preferably located between one of the two relatively movable components and the rotary bearing members. The connector may additionally comprise a separator element located between each adjacent pair of the rotary bearing members, the separator elements preferably being contoured so as to mate with the adjacent rotary bearing members. The separator elements may be formed as individual resilient components arranged to be fitted into the bearing assembly by snap-fitting, or alternatively may be formed integrally with each other so as to define a cage in which are arranged the rotary bearing members. In either case, the separator elements may be either insulating or conductive.

The rotary bearing members may be ball bearings, in which case the resilient electrically conductive member is advantageously contoured so as to provide in conjunction with each of the ball bearings a respective line contact.

Alternatively, the rotary bearing members may comprise roller bearings, in which case the resilient electrically conductive member is advantageously contoured so as to provide in conjunction with each of the roller bearings a respective area of contact.

Preferably the bearing assembly is arranged for relative rotation between the two relatively movable components, and the two relatively movable components may comprise respectively an inner component and an outer component. In this case, the resilient electrically conductive member is preferably located between the outer component and the rotary bearing members.

The resilient electrically conductive member preferably comprises a strip extending around the rotary bearing members, and preferably the two ends of the strip overlap, e. g. such that

the region of overlap extends around approximately half-way around the circumference of the bearing assembly. One or both ends of the strip may be chamfered, enabling the over- all shape of the overlapping strip to be substantially circular.

The resilient electrically conductive member may be made from a beryllium-copper alloy, preferably coated with a layer of ruthenium, palladium, rhodium or an alloy of any two or of all three thereof. Ruthenium possess the advantage of being hard-wearing, palladium exhibits low levels of oxidisation and rhodium is both flexible and durable.

The inner component preferably comprises an insulating material coated with a conductive layer at the region of contact with the rotary bearing members. The conductive layer may be coated using any one of a number of techniques, such as press-fitting or vacuum deposition and preferably comprises crystalline polycarbonate, since this material exhibits low friction, has high strength and low density and is cheap. Alternatively, or in addition, the conductive layer may comprise alumina.

The invention extends to a motor comprising such a connector, in which case one of the two relatively movable members advantageously comprises a housing of the motor, resulting in a compact arrangement.

The motor may be arranged for rotation about a shaft, and comprise two such connectors located at different respective axial positions along the motor shaft.

In accordance with an eighth aspect of the present invention there is provided an electrical connector comprising first and second relatively movable portions structured so as to define a substantially enclosed chamber therebetween for housing a mercury reservoir for providing an electrical conduction path between said two portions.

Such an arrangement enables power to be transmitted to a moving load. Alternatively, or in addition, the arrangement enables an electrical signal to be transmitted to a moving electrical circuit. Furthermore, such an arrangement can be retrofitted on to a conventional electric motor so as to replace the brush assembly.

The connector preferably further comprises two bearing assemblies, such as ball bearing assemblies, arranged to permit the relative movement between said first and second portions, and the reservoir is preferably sealed by two sealing rings mounted on the two respective bearing assemblies.

The relative movement may be relative rotation, such that the connector may be used to effect the electrical connections in an electric motor in which the switching is performed on the rotor.

Thus, the invention extends to an electric motor comprising at least one electrical connector of the above type for supplying electric power to the motor, the armature of the motor preferably being arranged for rotation about the motor shaft.

The motor preferably incorporates two such electrical connectors connected to inner and outer respective electrically conductive sleeves mounted substantially coaxially about the motor shaft for forming two respective conduction paths between the electrical connectors and the armature.

The motor preferably further comprises an inner insulating sleeve mounted between said inner electrically conductive sleeve and said motor shaft and an outer insulating sleeve mounted between said inner and outer electrically conductive sleeves.

The motor preferably further comprises a displacement encoder, such as an optical displacement encoder, for sensing the displacement of the movable armature.

The invention extends to a method of establishing an electrical connection between first and second relatively movable members comprising establishing respective electrical connections between said members via a mercury reservoir.

The invention extends to a solid-state commutator mounted on the armature of a motor and which is arranged to rotate at the speed of the armature.

Furthermore, the invention extends to a solid-state commutator for a motor which is arranged to integrate the switching of current with the speed control and braking system of the motor.

Furthermore, the invention extends to a motor comprising such a commutator.

The invention further extends to a motor comprising a solid-state commutator in which a speed control facility is mounted on the armature.

The invention further extends to a solid-state commutator which can be used in both traction and servo motor applications, and which is arranged to change between the applications during rotation.

Although some of the aspects of the present invention relate to electric motors, the invention can clearly be applied to electric generators, and the term"electric motor"as used herein is therefore intended to cover both electric motors and electric generators and, indeed, motors which can be used as generators and vice versa.

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings, wherein: Figure 1 is a radial cross-section of an electric motor in a first embodiment of the present invention; Figure 2 is an oblique view of the bearing assembly used in the embodiment shown in Figure 1; Figure 3 is a cross-sectional view of the connector arrangement of a second embodiment of the present invention; Figures 4 (a) and 4 (b) illustrate cross-sectional views of the embodiments of Figure 3 in planes respectively perpendicular and parallel to the axis of the motor shaft;

Figure 5 is a perspective illustration of part of the connector arrangement of a third embodiment of the present invention; Figures 6 (a) and 6 (b) illustrate cross-sectional views of the embodiments of Figure 5 in planes respectively perpendicular and parallel to the axis of the motor shaft.

Figure 7 is a cross-sectional view of the connector arrangement of a fourth embodiment of the present invention; Figures 8 (a) and 8 (b) illustrate two alternative arrangements of a displacement encoder constituting preferred embodiments of the present invention; Figure 9 is a block diagram illustrates schematically the circuit arrangement for use with the displacement encoders of Figures 8 (a) and 8 (b); Figure 10 illustrates a further alternative arrangement of a displacement encoder constituting a further embodiment of the present invention; Figure 11 is a block diagram illustrates schematically the circuit arrangement for use with the displacement encoders of Figure 10; Figure 12 is a schematic view of the control circuitry on the armature of an electric motor in accordance with preferred embodiments of the present invention; and Figure 13 is a block diagram illustrating the control system for use in an electric motor of the present invention.

A first embodiment of an electric motor is shown in Figure 1. The motor 1 comprises a cylindrical armature 2 mounted for rotation within a cylindrical stator 3 on which is mounted a plurality of permanent magnets. The armature 2 is provided with a plurality of coils, and the motor 1 functions by virtue of electric current supplied to the coils. The armature is further provided with a micro-controller 4 which controls the switching of

electric current between the coils in dependence on the rotational position of the armature relative to the stator.

The armature 2 is rigidly mounted about one end of a shaft 5, which transmits mechanical power to a load (not shown) mounted to the other end of the shaft 5.

The armature 2 and the stator 3 are mounted within a fixed cylindrical housing 6, and the shaft 5 is supported within the housing by means of first and second ball bearing assemblies 7,8.

A source of direct current voltage is applied to two terminals 9,10 located on the outside of the housing 6, but insulated from the housing 6 itself by a plastics insulating material.

Thus terminal 9 could be supplied with a positive dc voltage level and terminal 10 with a negative dc voltage level, or vice versa. An electrical connector module 11 serves to establish an electrical connection between the two terminals 9,10 and the switching circuitry 4 on the armature 2.

The connector module 11 comprises two sub-assemblies in the form of inner and outer annular contact bearings 12,13 mounted coaxially about the motor shaft 5 and substantially within the same plane. Each contact bearing comprises a plurality of balls 14 mounted within a respective race defined by a first, rotatable component 15 and a second, fixed component 16. The rotatable component 15 and the fixed component 16 are each substantially disc-shaped. The surfaces of the first and second components in contact with the balls are curved so as to provide line contacts between each ball. When the motor is running, the first component 15 rotates with respect to the second component 16, and the balls serve to provide not only a low-friction mechanical bearing but also a substantially constant electrical connection between the first and second components of the contact bearings. The balls 14 are located within respective compartments of a cage 17.

The connector module 11 is retained within the housing 6 by means of one or more screws 18.

The inner contact bearing 12 is electrically connected to a first terminal 19 of the electric switching circuitry on the armature 2.

The outer contact bearing 13 is likewise electrically connected to a second terminal 20 of the electric switching circuitry.

In the above-described embodiment, the current path through each of the two contact bearings 12,13 is in a direction substantially parallel to the axis of rotation.

The connector module (without the terminals 9,10 or the screws 18) is shown in an oblique view in Figure 2.

A second embodiment of an electric motor in accordance with the present invention will now be described with reference to Figures 3,4 (a) and 4 (b). The components of the motor 1 other than the connector module are identical to those of the first embodiment.

In this embodiment, the connector module 11'comprises an integral arrangement of two sub-assemblies in the form of annular contact bearings 12', 13'mounted about the motor shaft 5 at adjacent axial positions therealong. Each contact bearing comprises an inner disc 21 made from a plastics insulating material, such as crystalline polycarbonate, and a plurality of balls 14'. The outer surface of the disc 21 is coated with a layer of conductive material 22 and is contoured to as to form a respective electrical line contact with each ball 14'.

Within, and mounted to, the housing 6 of the motor 1 there are provided two resilient electrical conductors in the form of strips 23 wound around the balls 14'of the respective contact bearing 12', 13", which serve to maintain a constant electrical contact between the balls 14'and the terminals 9', 10'on the motor housing. The resilient electrical conductors are contoured so as to effect a line contact with each of the balls 14'and are arranged so as to effect an inward bias against the balls 14'. Each ball 14'is mounted within a respective compartment of one of two cages 17'. An internal conductor within each disc 21 connects the electrically conductive coating 22 to a respective terminal on the side of the disc 21 of the bearing 12'nearer to the armature 2 of the motor 1, and electrical contact with the

switching control circuitry on the armature is effected in the same way as in the first embodiment.

A third embodiment will now be described with reference to Figures 5,6 (a) and 6 (b), in which primed or double-primed reference numerals indicate those components of the first and second embodiments corresponding to the components referenced in the drawings of those embodiments by the same, respectively unprimed or single-primed, reference numerals. In this embodiment, the spherical ball bearings 14'of the second embodiment are replaced with roller bearings 14". It will be appreciated that, with this arrangement, areal contacts are established between the rollers 14"and the contoured seating of the discs 21".

A fourth embodiment of the present invention will now be described with reference to Figure 7, wherein the same reference numerals as in the previously-described drawings are used to indicate the same components. In this arrangement, there are first and second connector modules 12"', 13"', each in the form of an outer casing 24 mounted for relative rotation about an inner casing 25 by means of two ball bearing assemblies 26. With the motor running, the inner casings 25 rotate with respect to the outer casings 24, and the balls 14"'of the bearing assemblies 26 serve to provide a low-friction mechanical bearing.

The outer and inner and casing 24,25 of each of the two connector modules 12"', 13"'are shaped so as to define respective reservoirs 27 filled with liquid mercury and permanently sealed with a plurality of neoprene knife-edge sealing rings 28. The mercury in the two reservoirs 27 is electrically connected to a respective one of two terminals 9', 10'by means of conductive screws 29.

The inner casing 25 of the first connector module 13"'is electrically connected to a first conductive sleeve 30 which is mounted on a first insulating sleeve 31 on the shaft 5 of the motor. The first conductive sleeve 30 forms a conductive path between the mercury in the reservoir 27 of the first connector module 12'and the first terminal 19 of the micro- controller 4 via a first conductor 34. Likewise, the inner casing 25 of the second connector module 12"'is electrically connected to a second conductive sleeve 32 which is mounted on a second insulating sleeve 24', which, in turn, is mounted on the first conductive sleeve

18'. The second conductive sleeve 18'forms a conductive path between the mercury in the reservoir 31 of the second connector module 13'and the second terminal 20 of the micro- controller 4 via a second conductor 35.

Embodiments of the displacement encoder for sensing the relative position of the rotor and the stator of the motor will now be described with reference to Figure 8 (a), 8 (b), 9,10 and 11.

Figure 8 (a) illustrates an encoder strip 36 which, when used in a motor 1 of the type described above, is located on the inner surface of the housing 6 such that it completely surrounds the shaft 5 of the motor 1. As can be seen from the drawing, the strip 36 bears a pattern in the form of three parallel tracks 37 each having a sequence of two different optical characteristics, such as reflectivity or transmissivity, but illustrated simply as black and white. In the case where the strip 36 bears portions of different transmissivity, a reflective surface is positioned between the strip 36 and the inner surface of the housing 6, such that the over-all effect is still one of differing reflectivity.

In the arrangement shown in Figure 8 (b), the encoder is in the form of a disc 36', again bearing a pattern in the form of three parallel tracks 37'but arranged concentrically about the axis of rotation, as opposed to the Figure 8 (a) arrangement, wherein the tracks are arranged at axially-offset positions. Again, the optical characteristics can be either reflective or transmissive, but in the case of a transmissive characteristic, the provision of an auxiliary reflecting surface is optional, depending on the sensing arrangement employed.

For each of the two encoder embodiments illustrated in Figures 8 (a) and 8 (b), a corresponding sensing arrangement 38 is employed for sensing the optical characteristic at the particular rotational position of the sensing arrangement relative to the encoder element. The sensing arrangement 38 is in the form of three light sources 39 and three corresponding photodetectors 40 arranged to detect the light from its corresponding light source 39 after reflection at the surface of the encoder element 36'. In the case of the arrangement shown in Figure 8 (b), and where the optically varying characteristic of the

disc is its transmissivity, the light sources are arranged on one side of the disc 36', and the three photodetectors 40 are arranged on the other side.

The output of the displacement encoder sensing arrangement is supplied to circuitry, as illustrated in Figure 9. The sensing arrangement 38 senses its angular position with respect to the stationary encoder element 34 and supplies an analog output signal to a combined signal-conditioning circuit and amplifier 41, the output of which is supplied to an analog- to-digital converter 42, which may be either a serial or parallel converter, and which converts the conditioned analog signal into a digital signal which, in turn, is supplied to the micro-controller 4.

A third form of displacement encoder is shown in Figure 10. As with the arrangement shown in Figure 8 (a), the encoder element 36"is located on the inner surface of the motor housing 6. However, in this case, the pattern on the encoder element is in the form of a single track 36", but the number of optical characteristics is greater than two such that displacement of the sensing arrangement relative to the stationary encoder 36"can be determined with high precision without the need for a plurality of tracks and a corresponding number of light sources and detectors. In this case, there are 256 portions which extend over 360 degrees, and this enables an 8-bit analog-to-digital converter to be employed to convert the analog output of the photodetector 40'to an 8-bit digital output.

The encoder element 36"is in the form of a strip of reflective film covered with a film having 256 portions exhibiting an optical transmissivity varying linearly from 0% to 100%.

It has been found that the optimal distance between the sensing arrangement 38'and the encoder strip 36"is about 4 mm.

The output of the displacement encoder sensing arrangement is supplied to circuitry, as illustrated in Figure 11. The sensing arrangement 38'senses its angular position with respect to the stationary encoder element 34 and supplies an analog output signal to a Schmitt trigger 43 which conditions the analog signal by means of a double-threshold arrangement well known in the art of signal processing. The output of the Schmitt trigger 43 is supplied to a latch circuit 40', the output of which, in turn, is supplied to the micro- controller 4.

For each of the embodiments described above, the control circuitry on the armature of the electric motor is as illustrated in Figure 12.

The electric current is supplied to the first and second terminals 19,20 on the armature 2, which are connected as a power input to the micro-controller 4, which controls switching circuitry in the form of three H-bridge networks 44 comprising semiconductor switches, such as IGFETs or MOSFETs. The micro-controller 4 could be, for example, one of the PIC series of micro-controllers available from Microchip Technology Inc. which has the particular advantage of its very small size.

The control arrangement of the electric motor is as illustrated in Figure 13. A rotary position encoder 45 attached to the motor housing 6 senses the angular position of the motor shaft 5 relative to the housing 6, using an optical, e. g. infrared, radiation detection scheme as described above, and supplies the micro-controller 4 with a position signal, which is used by the micro-controller 4 to control the timing of the switching of the electric current, and the polarity thereof, applied to the motor coils 46 on the armature 2.

The control circuitry comprises, in addition, a semiconductor temperature sensor 47 and a piezoelectric vibration sensor 48.

The motor can be controlled by the use of two-way infrared communications link 49 between a control module 50 outside the motor housing 6 and the micro-controller 4.

Although the present invention has been described with reference to preferred embodiments, it will be appreciated that many variation and modifications may be made without departing from the scope of the invention which is defined by the claims appended hereto. For example, the micro-controller could be replaced by any programmable logic chip, and specific embodiments of the displacement encoder which is used to sense the relative angular position of the armature and stator could be replaced by a Hall effect sensor or any other suitable angular position encoder.