GB2052175A

GB2052175A - Stepping motor

Info

Publication number: GB2052175A
Application number: GB8019754A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1979-06-20
Filing date: 1980-06-17
Publication date: 1981-01-21
Also published as: DE3022433C2; GB2052175B; FR2459575B1; FR2459575A1; DE3022433A1; NL7904817A; CA1135759A; CH651706A5; JPS5610070A

Abstract

A stepping motor has axially- spaced annular stator sections 3,4, which enclose respective annular energising coils 8, 9 and have at their inner circumferences tooth systems 10, 11 and 12, 13 which cooperate with rotor tooth systems 14, 15 and 16, 17, the motor being characterised in that the stator teeth are aligned axially and the rotor teeth are aligned helically. <IMAGE>

Description

SPECIFICATION Stepping motor The invention relates to a stepping motor having a stator which comprises a first annular stator section with a first annular coil and a first magnetically conductive enclosure surrounding said annular coil, which enclosure terminates in a first and a second annular system of stator teeth, a second annular stator section having a second annular coil and a second magnetically conductive enclosure surrounding said annular coil, which enclosure terminates in a third and a fourth annular system of stator teeth, and a rotor having teeth which co-operates with the first, the second, the third and the fourth system of stator teeth.

Such a stepping motor is known from Patent Application 27648/77 and is suitable for use as a stepping motor with 3 small stepping angle, for example 1.8 geometrical degrees, i.e. 200 steps per rotor revolution. In order to avoid excessive stepping angle errors with such a stepping motor the alignment of the stator tooth systems relative to the rotor teeth should comply with very stringent requirements, which requirements may be costly. In addition, it may be advantageous to introduce specific deviations in the nominal positions of the rotor teeth relative to the stator tooth systems as is described in Application No.

8019755 (Serial No.2052176) (PHN 9491).

It is the object of the invention to provide a stepping motor of the type mentioned in the preamble, which can be aligned accurately in a comparatively easy manner.

To this end the invention is characterised in that the teeth of each of the systems of stator teeth are axially in line with the teeth of the other stator systems and that the teeth of the rotor are formed at the circumference of the rotor between helical grooves in the rotor surface.

The invention is based on the recognition that it is comparatively simple to align the stator teeth in an axial direction and that, because the rotor teeth have a helical structure the angular location around the axis of rotation of the rotor teeth relative to the stator teeth is dictated by axial distances.

The teeth of the rotor toothing are smaller in respect of their axial height than those of the systems of stator teeth cooperating therewith and that the teeth of the system of stator teeth axially overlap the rotor teeth which cooperating therewith in both directions.

In this embodiment the tangential alignment is determined by the location of the axial boundaries of the rotor teeth. This embodiment may further be characterised in that there is provided a first, second, third and fourth system of rotor teeth, which cooperate with the first, the second, the third and the fourth system of stator teeth respectively, which systems of rotor teeth are formed in that in addition to said helical grooves there are also formed circular grooves, which bound the said systems of rotor teeth in the axial direction. As a resuit of this the tangential alignment of the motor is determined by the location of said circular grooves. This location can -simply be changed during series production.

Alternatively, the height, in the axial direction, of the stator teeth may be less than that of the rotor teeth with each rotor tooth axially overlapping both axial boundaries of each stator tooth with which it co-operates. In this arrangement the angular alignment is determined by the location of the axial boundaries of the systems of stator teeth.

Embodiments of the invention will be described in more detail with reference to the drawing, in which Figure 1 shows an axial cross-section of a stepping motor illustrating the invention, Figure 2 is a view of a disassembled stepping motor also illustrating the invention and showing the stator in axial cross-section, Figure 3 schematically shows the location of the stator teeth relative to the rotor teeth in a first embodiment and Figure 4 schematically shows the location of the stator teeth relative to the rotor teeth in a second embodiment.

Referring now to Figures 1 and 2 a stepping motor is substantially rotationally symmetrical about the axis A-A' and comprises a rotor 1 and a stator 2. The stator comprises two coaxial stator sections 3 and 4 with coaxially interposed between said sections an axially magnetized permanent magnetic ring 5. Each of the stator sections 3 and 4 respectively comprises a coaxially disposed annular coil 8 and 9 respectively surrounded by a magnetically conductive enclosure 6 and 7 respectively, which enclosure on the inner side terminates in two annular systems of teeth 10, 11 and 12,13 respectively.The rotor 1 is provided with four annular systems of teeth 1 4, 1 5, 1 6 and 1 7 which co-operate with the annular systems of teeth 10, 11, 12 and 13 respectively. As can be seen in Figure 2 said rotor teeth are bounded by helical grooves, one of said grooves being represented by dashed lines 1 8. The alignment is then such that the teeth of the systems of stator teeth 10, 11, 12 and 13 are axially in line and that if the teeth of the systems of rotor teeth 14 are disposed opposite the teeth of the system of stator teeth 10 the teeth of the systems of stator teeth 11, 12 and 1 3 are shifted relative to the systems of rotor teeth 15,16 and 17 by 1800,900 and 2700 respectively (or alternatively 1 80, 270 and 900 respectively), 3600 corresponding to one tooth pitch.

The axially magnetized permanent-magnetic ring 5 magnetizes the systems of teeth 10 and 11 with a specific polarity and the systems of teeth 1 2 and 1 3 with an opposite polarity. For a given direction of the current through the annular coil 8 or 9 the field, produced by the coil, in the air gaps between stator and rotor associated with the stator tooth systems 10 and 1 2 then has the same direction as the field produced in said air gaps by the permanent magnetic ring and is opposed thereto in the other air gaps associated with the systems 11 and 13.For the opposite direction of the current through the annular coil 8 or 9 the field, produced by the coil, in the air gaps associated with the systems 11 and 1 3 has the same direction as the field produced by the permanent magnetic ring and is opposed thereto in the other air gaps associated with the systems 10 and 12. By the choice of the energizing direction of the current in one or both coils 8 and 9, one or two, as the case may be, rotor-stator teeth systems can produce a torque, so that it is possible to drive the rotor in steps of 1/4 tooth pitch.

The location of the permanent magnet may be different from that illustrated: for example it may be incorporated in the rotor at a location designated 5' in Figure 1 or may for example surround the two stator sections 3 and 4 as a cylindrical sleeve. Alternatively, the permanent magnet may be repiaced by a d.c. energized coil.

Figure 3a schematically represents a development of a part of the rotor tooth systems showing the position of two teeth of each stator teeth system 10,11, 12 and 13. When the distance between these stator teeth systems in combination with the angle relative to the axis at which the rotor teeth are located, which in the development are situated on an oblique row, has been selected correctly, then, if the teeth of stator teeth system 10 are situated opposite the teeth of the rotor teeth system 14 (defined as-an angle of 0 between the two systems), the teeth of stator system 11 are situated exactly between the teeth of rotor system 15 (defined as an angle of 1800 between the two systems), the teeth of stator system 12 are situated halfway opposite the teeth of the rotor system 16 (defined as an angle of 900 between the two systems of teeth), and the teeth of the stator systems 1 3 are situated halfway opposite the teeth of the rotor system 17 (defined as an angle of 2700 between the two systems of teeth).

In the situation as shown in Figure 3a, the rotor teeth extend in an axial direction beyond the stator teeth. In principle this may be extended so far that the teeth of the various rotor systems 14, 1 5, 1 6 and 1 7 adjoin each other and form helical ridges which extend between the two end faces of the rotor. However, in order to reduce the moment of inertia of the rotor it is generally advantageous to minimize the rotor teeth height hr as shown in see Figure 3b.

In order to illustrate the effect of a rotor-stator tooth construction as shown in Figure 3a on the alignment of the systems of teeth relative to each other, Figure 3b shows a rotor tooth t, with a stator tooth opposite to it and Figure 3c the same configuration in which, relative to the situation in Figure 3b, a stator tooth has been shifted over a distance x in an axial direction with the same position of the rotor tooth tr.In Figure 3b the centre aS of the stator tooth ts is situated at a distance y/2 to the right of the centre line br of the rotor tooth tr After shifting the stator tooth ts over a distance x in an axial direction as shown in Figure 3c the centre aS of stator tooth ts is located at a distance y/2 to the left of the centre line b, of the rotor tooth true Thus, a shift of the stator tooth ts over a distance x in an axial direction has the effect of a shift y of the stator tooth relative to the rotor tooth in a circumferential direction.In this way the circumferential alignment of the stator systems relative to the rotor teeth is determined by the axial distances between the systems stator tooth, which axial alignment can be realized far more simply than a direct circumferential alignment. An additional advantage may be that the ratio y/x is equal to the tangent of the angle a which the centre line of the rotor tooth makes with that of the stator tooth. If this angle is smaller than 450, a displacement x in an axial direction results in a smaller displacement y in a circumferential direction, so that a specific tolerance in y corresponds to a greater tolerance in x.

Figure 4a shows a similar situation to that of Figures 3a, but with stator teeth which extend axially beyond the rotor teeth. Figure 4b shows a rotor tooth tr of the configuration of Figure 4a for a specific position with superposed on it a stator tooth ts, and Figure 4c shows the same situation but with a shift over a distance x in an axial direction of the helix along which it is situated; the effect is that of shifting, over a distance x in an axial direction, the two planes which extend transverse to the axis of rotation and which bound the system or rotor teeth to which the tooth tr belongs.In the situation of Figure 4b the centre ar or rotor tooth tr is located at a distance y/2 to the left of the centre line bs of the stator tooth ts A displacement of the rotor tooth along the helix on which it is located over an axial distance x results in the situation of Figure 4c, where the center ar of the rotor tooth is situated at a distance y/2 to the right of the centre line hs of stator tooth ts. As the smaller tooth determines the effective relative position of the two teeth, the relative circumferential position of two teeth is defined by the circumferential distance between the centre line of the longer tooth and the centre of the shorter tooth.

A shift of the axial bounding surfaces of the rotor tooth systems over a distance x thus results in a shift y of the stator tooth relative to the rotor tooth in a circumferential direction. In this way the circumferential alignment is determined by the axial distances between the system of rotor teeth.

In the situation shown in Figure 3 the height hr of the rotor teeth is greater than the height h6 of the stator teeth, the stator teeth being within the height boundaries represented by the top and bottom limits, as drawn in Figure 3, of the rotor teeth. With this arrangement the axial alignment of the stator sections, i.e. the axial distances between the stator tooth systems 10 to 13, determines the circumferential alignment and the location of the surfaces which bound the rotor tooth systems 14 to 17 in an axial direction is non-critical. In the situation shown in Figure 4 this is the other way round.In this case the height hr of the teeth of the rotor systems 14 to 1 7 is less than the height hs of the stator teeth, so that the surfaces which bound the rotor teeth systems in an axial direction determine the axial alignment, whereas the axial distances between the stator tooth systems are non-critical.

Alternatively it is possible for the rotor and stator teeth to be the same height, in-which case the axial location of the stator and the rotor systems relative to each other then determines the circumferential alignment. However, an incorrect location then not only influences the circumferential alignment but also the torque which is dependent upon the extent of this alignment that is to say is dependent on what may be referred to as the "overlapping" area of the opposing tooth faces as illustrated in the diagrams of Figures 3 and 4. Therefore, it is preferable to make the axial heights of the rotor and the stator teeth different with the smallerheight teeth lying axially within the boundaries set by the height of the layer-height teeth.

A rotor of a motor embodying the invention can be manufactured by forming helical grooves in a cylindrical body, for example by moving a milling tool over the surface of the cylinder in an axial direction whilst rotating said cylinder. If separate systems of teeth are required on the rotor the material between the systems of teeth may be removed previously or subsequently for example by means of a suitable turning operation. In the case shown in Figure 4 the location where said material is removed determines the circumferential alignment of the rotor relative to the stator. This method then has the advantage that it is comparatively simple to change this alignment during series production by selecting a different setting of the machine tool used, for example a lathe. An alternative is to start from a rotor construction as shown in Figure 2, but without the helical groove defining the teeth, and to form these grooves in all four systems of teeth in one operation.

The invention is not limited to the embodiment shown. In addition to the hybrid stepping motor shown, the invention is also applicable to a reluctance stepping motor of similar construction to the hybrid stepping motor shown, i.e. to any motor with axially spaced annular and rotationsymmetrical systems of teeth.

Claims

1. A stepping motor having a stator which comprises a first annular stator section with a first annular coil and a first magnetically conductive enclosure surrounding said annular coil, which enclosure terminates in a first and a second annular system of stator teeth, a second annular stator section with a second annular coil and a second magnetically conductive enclosure surrounding said annular coil, which enclosure terminates in a third and a fourth annular system of stator teeth, and a rotor having teeth which co-.

operate with the first, second, third and fourth stator tooth systems characterised in that the teeth of each of the systems of stator teeth are disposed axially in line with those of the other stator systems and that the teeth of the rotor are formed at the circumference of the rotor between helical grooves in the rotor surface.

2. A stepping motor as claimed in Claim 1, characterised in that there is provided a first, a second, a third and a fourth rotor tooth system, which cooperates with the first, the second, the third and the fourth stator tooth systems respectively, which rotor teeth systems are formed between circular grooves which axially bound the said systems of rotor teeth.

3. A stepping motor as claimed in Claim 2, characterised in that the height, in the axial direction of the rotor teeth is less than that of the stator teeth with which they co-operate and that the teeth of each system of stator teeth axially overlap both axial boundaries of the rotor teeth with which they co-operate.

4. A stepping motor as claimed in Claim 1 or Claim 2 characterised in that the height, in the axial direction, of the stator teeth is less than that of the rotor teeth and that each rotor tooth axially overlaps both axial boundaries of the stator tooth with which it co-operates.

5. A stepping motor as claimed in Claim 1, substantially as herein described with reference to Figures 1,2 and 3 or Figures 1,2 and 4 of the drawings.