For many years, external rotor brushless DC motors have been manufactured in accordance with a conventional topography. An example of such a conventional topography of a brushless DC motor is shown in Figure 1 of the accompanying drawings. The basic premise of this topography is that the stator assembly has a stator assembly base with a recess therein. The driving and control circuitry of the motor is attached to the stator assembly base in the recess, the stator windings being located above the circuitry such that the circuitry is sandwiched in the recess between the stator windings and the stator assembly base. Typically, the stator windings and the circuitry are permanently affixed to the stator assembly base.

As is well known, brushless DC motors must be commutated electronically by selectively energising the stator windings according to the angular position of the rotor, the angular position of the rotor conventionally being detected by position sensors usually in the form of Hall effect sensors.

Conventionally, a Hall effect sensor or multiple Hall effect sensors are mounted adjacent the stator winding assembly in the magnetic field generated by the rotor assembly and are therefore usually remove from the control circuitry of the motor.

As set out in co-pending U. K. Patent Application No. 9923900.6 (filed 8th October, 1999), a new external rotor brushless DC motor topography has been devised in which the stator winding assembly and the electronic circuitry of the motor are separated fi-om one another by a base plate of the stator assembly. This arrangement seeks to overcome several drawbacks of the conventional motor topography, such as making it easier to modify or repair the circuitry or components on the printed circuit board without at least partially damaging or destroying the stator windings, or the assembly base. Another drawback with the conventional topography is that it is difficult to dissipate heat, generated by both the motor and the circuitry on the printed circuit board, away from the rotor. Another drawback of the conventional topography is that it is difficult to seal the printed circuit board and the components thereon from the ingress of dust and moisture.

However, when using the motor topography described in UK Patent Application No. 9923900.6 the or each Hall effect sensor of the motor circuitry is separated from the rotor (which carries permanent magnets) by the above- mentioned base plate. Accordingly, the effectiveness of the Hall effect sensors is reduced by virtue of their separation from the magnetic field generated by the rotor. Whilst it would of course be possible still to locate the or each Hall effect sensor above the stator base plate directly within the magnetic field generated by the rotor, this itself has several disadvantages. Firstly, Hall effect sensors are expensive and it is undesirable to subject them to the generally harsh environment which exists in the immediate vicinity of the spinning rotor.

It is therefore desirable to locate the Hall effect sensor with the rest of the motor circuitry such that it is protected from this harsh environment by the stator base plate. Secondly, signals from the Hall effect sensor are sensitive to electrical interference if the Hall effect sensor is remote from the control circuitry. Further, it can be inconvenient and sometimes difficult to seal the area where the Hall sensor would have to pass through the base plate to be in proximity with the magnetic field generated by the rotor.

It is an object of the present invention to seek to provide an external rotor brushless DC motor which does not suffer from the above-mentioned drawbacks.

Accordingly, one aspect of the present invention provides an external rotor brushless DC motor comprising : a winding assembly ; a rotor assembly generating a magnetic field ; the rotor assembly being mounted for rotation extemally of the winding assembly; and a Hall effect sensor for providing a rotor position signal indicative of the angular position of the rotor assembly relative to the winding assembly, wherein a base plate is disposed between the Hall effect sensor and the rotor assembly, the Hall effect sensor having a probe passing through the base plate, the probe magnetically coupling the Hall effect sensor to the magnetic field generated by the rotor assembly to sense the angular position of the rotor assembly relative to the winding assembly.

Preferably, the probe is manufactured from a magnetic material and the base plate or more specifically the area of the base plate around the probe is manufactured from a non magnetic material.

Conveniently, the permeability of the probe is less than the penneability of the area of the base plate area around the probe.

Advantageously, the probe comprises a ferromagnetic material.

Conveniently, the probe comprises any material of low permeability such as low carbon steel.

Advantageously, the base plate is manufactured from any non-magnetic non-ferrous material such as aluminium or zinc or plastic.

Preferably, the motor has a Hall effect sensor associated with the probe.

Alternatively, the motor may have a plurality of said Hall effect sensors each associated with a respective probe.

Advantageously, the or each probe is hermetically sealed to the base plate.

Conveniently, the or each probe is hermetically sealed to the base plate by adhesive, an interference fit with the base plate or preferably a combination of both adhesive and interference fit.

Preferably, the base plate has a side wall, the side wall defining a recess within which the or each Hall effect sensor is located.

Advantageously, the base plate is provided with a cover, the or each Hall effect sensor being located between the base plate and the cover.

Conveniently, the or each Hall effect sensor is housed within an enclosure.

Preferably, the enclosure is hermetically sealed.

Advantageously, the base plate is made from aluminium or other thermally conductive material.

In order that the present invention may be more readily understood, and so that further features thereof may be appreciated, embodiments will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a cross-section through a conventional external rotor brushless DC motor; and Figure 2 is a cross-section through an embodiment of an external rotor brushless DC motor in accordance with the present invention.

Referring initially to Figure 1 of the accompanying drawings, an external brushless DC motor 1 is illustrated having a generally conventional topography. The motor 1 comprises a stator assembly and a rotor assembly.

The stator assembly has a stator assembly base which comprises a substantially circular base 2 having an outer upstanding cylindrical side wall 3. A central cylindrical column 4 is also upstanding from the centre of the circular base 2 and is concentric with the cylindrical side wall 3 so as to define an annular space between the cylindrical side wall 3 and the column 4. It is conventional for the driving and control circuit of the motor to be housed within this annular space, typically on an annular printed circuit board 5 with the main surface of the printed circuit board 5 which carries all of the components 6 being oriented towards the base 2. The printed circuit board 5 is typically permanently connected to and attached to a plurality of multiple stator laminations with windings around each pole forming a stator winding assembly 7 which is attached to the stator assembly base and, more particularly, to the exterior of the column 4 directly above the base 2, the printed circuit board 5 thereby being sandwiched between the stator winding assembly 7 and the base 2. The stator winding assembly 7 is permanently attached to the column 2, preferably by thermoset resin or other permanent adhesive. It will be appreciated that the annular space within which the printed circuit board 5 is to be affixed opens towards the stator winding assembly 7.

The rotor assembly of the conventional topography motor 1 comprises a rotor cup 8 having a series of permanent magnet poles of opposite polarity 9 arranged around the internal periphery thereof. The cup 8 is rotatably engaged in the column 4 by means of a shaft 10 held between two pairs of bearings 11 within the column 4. The rotor assembly is thus journalled to the stator assembly which results in the rotor assembly being mounted for rotation externally of the winding assembly.

In order to provide information regarding the angular position of the rotor assembly relative to the winding assembly, one or more Hall effect sensors 6a are provided on the printed circuit board 5. The or each Hall effect sensor 6a is located in the close vicinity of the magnet poles 9 of the rotor assembly such that the or each Hall effect sensor is located within the magnetic field generated by the magnet poles 9 of the rotor assembly. The Hall effect sensors 6a, as is well known in the art, provide a rotor position signal indicative of the angular position of the rotor assemblv relative to the winding assembly.

This rotor position signal is then used to commutate electronically the stator windings.

As explained in co-pending U. K. Patent Application No. 9923900.6, there are several drawbacks associated with the above-described conventional motor topography, all of which stem from the conventional location of the printed circuit board 5 at a position between the housing base 2 and the stator assembly 7 which are typically both permanently affixed to the other. It is, therefore, extremely difficult to modify or repair the circuitry or components on the printed circuit board without at least partially damaging or destroying the stator winding assembly or the bases. Other drawbacks include the difficulty of sealing the printed circuit board and the components thereon from the ingress of dust and moisture and the difficulty of dissipating heat, generated by both the motor and the circuitry on the printed circuit board, away from the motor and isolating the electronic components from the heat generated within the stator winding assembly 7. Therefore, our above-mentioned co-pending U. K.

Patent Application provides a departure from the conventional topography used in external rotor brushless DC motors.

Figure 2 of the accompanying drawings illustrates the general topography to which the above-mentioned U. K. Patent Application is directed, but also illustrates the present invention applied thereto.

It will be apparent from a study of Figure 2 that the rotor assembly and stator winding assembly 7 are affixed to the column 4 in a conventional manner (as compared to the known arrangement of Figure 1). However, the difference between the motor arrangement to which the above-mentioned U. K. Patent Application is directed and the conventional motor assembly shown in Figure 1 lies in the base portion of the stator assembly.

The stator assembly of base 20 of the motor l illustrated in Figure 2 comprises an effective inversion of the arrangement shown in Figure 1. The stator assembly base comprises a conventional column 4 upstanding from a circular base plate 21. A cylindrical side wall 22 projects from the base plate 21 in the opposite direction to the column 4, the side wall 22 thereby projecting away from the stator winding assembly 7, in the opposite direction to the column 4. Thus, there is no recess between the stator winding assembly 7 and the base plate 21 within which the printed circuit board can be mounted.

In contrast to the conventional topography illustrated in Figure 1, the side wall 22 which depends away from the stator winding assembly 7 defines a space on the opposite side of the base plate 21 to the stator winding assembly 7 within which the circuitry associated with the motor can be mounted. Thus, the recess defined by the side wall 22 and the base plate 21 opens away from the stator winding assembly 7.

A cover plate 23 is seated on an internal step formed around the free edge of the side wall 22. An enclosure 24 is thereby defined between the base plate 21 and the cover plate 23 which is bounded by the side wall 22.

Preferably, further bosses 25 or partition walls (not shown) are formed within the side wall 22 so as to provide, respectively, means by which the cover plate 23 can be secured to the base plate 21 and a means to divide the main enclosure 24 into various sub-enclosures within the boundary of the side wall 22.

In the embodiment of the motor illustrated in Figure 2, the printed circuit board 5 is attached to the base plate 21. The components 6 on the printed circuit board 5 are positioned on the opposite surface of the printed circuit board 5 to that adjacent to the base plate 21. Therefore, the components 6 are readily accessible upon removal of the cover plate 23. Thus, the components 6 and circuitry of the printed circuit board 5 can be easily repaire or maintained without any need to cause damage to the winding assembly 7 or any other part of the stator assembly. Suitable wiring 7a connects the winding assembly 7 to a printed circuit board 5 through the base plate 21. The opening through which the wiring 7a passes through the base plate 21 is sealed by a rubber seal or the like.

In the embodiment illustrated in Figure 2, the enclosure 24 is hermetically sealable by a rubber O-ring 33 or other similar sealing arrangement so as to protect the circuitry and components within the enclosure 24 from the ingress of dust or moisture. Additionally, the thermal transfer properties of the motor are enhanced in that heat from the motor can be more readily dissipated through the stator assembly, along the base plate 21, down the side wall 22 and from the cover plate 23. External heat sinks or the like may be connected to the cover plate 23 to facilitate further heat sinking.

Having separated the electronic circuitry of the motor arrangement of Figure 2 from the winding assembly 7 (and hence also the motor assembly with its associated permanent magnets 9), it is advantageous also to locate the Hall effect sensor or sensors 6a on the opposite side of the base plate 21 so as also to protect the Hall effect sensor or sensors 6a which, as noted above, are sensitive and easy to damage. However, locating the Hall effect sensors 6a on the opposite side of the base plate 21 to the permanent magnets 9 of the rotor assembly reduces the effectiveness of the Hall effect sensor because at the very least it is no longer located in the strongest part of the magnetic field generated by the magnets 9, and at worse, is not affected at all by the magnetic field generated by the permanent magnets 9.

In order to solve the above-mentioned problem of the reduced effectiveness of the Hall effect sensors when located on the opposite side of the base plate 21 to the permanent magnets 9. it is proposed to provide each Hall effect sensor 6a with a probe 35 passing through the base plate 21 and terminating at a position indicated generally at 36 which is located well within the magnetic field generated by the permanent magnets 9 of the rotor. The probes 35 extend through the base plate 21 to a point generally corresponding to the point at which the Hall effect sensors of the conventional motor topography illustrated in Figure 1 are located.

The or each probe 35 is adapted to couple magnetically its respective Hall effect sensor to the magnetic field generated by the rotor assembly, such that the Hall effect sensor is still effectively influenced by the magnetic field in the usual way so as to provide a rotor position signal indicative of the angular position of the rotor assembly relative to the winding assembly, which signal can then be used for electronically commutating the motor. The probe 35 shown on the right hand side of Figure 2 is proud at the base plate 21.

Preferably, the probe 35 as shown on the left hand side of Figure 2 is flush with the base plate 21. Also, it will be noted that the Hall effect sensor 6A on the left hand side is located within a through hole 37 in the printed circuit board 5 thereby shortening the length of the probe 35.

A preferred embodiment of the present invention uses probes 35 made of a magnetic material mounted in a base plate of a non magnetic material. More preferably, the probes 35 comprise a low remnance magnetic material such as, for example, a low carbon steel or any other magnetically conductive material.

The material of the plate 21 is a non-magnetic thermally conductive material such as aluminium or zinc, but could also be made of plastic for lower power motors.

As explained previously, the cover plate 23 is preferably hermetically sealed to the downwardly depending side wall 22 of the base plate 21 so as to ensure good protection of the electonic circuitry therebetween. Therefore to ensure that the enclosure as a whole is hermetically sealed, the or each probe 35 is preferably hermetically sealed to the base plate 21 by way of, for example, adhesive, an interference fit in the base plate 21 or a combination of adhesive and interference fit.

In the present specification"comprise"means"includes or consists of' and"comprising"means"including or consisting of'.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilise for realising the invention in diverse forms thereof.