GB1563031A

GB1563031A - Electromagnetic step-bystep motor

Info

Publication number: GB1563031A
Application number: GB1725477A
Authority: GB
Original assignee: SSIH Management Services SA
Current assignee: SSIH Management Services SA
Priority date: 1976-05-06
Filing date: 1977-04-26
Publication date: 1980-03-19
Also published as: CH567876A4; CH604250B5; DE2720053A1; JPS52135013A; FR2361014A1

Description

(54) ELECTROMAGNETIC STEP-BY-STEP MOTOR (71) We, SOCIETY SUISSE POUR L'INDUSTRIE HORLOGÈRE MAN AGEMENT SERVICES S.A., a Swiss Body Corporate, of 2500 Bienne, Canton of Berne, Switzerland, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to an electromagnetic stepping motor comprising a rotor having magnetized parts, a stator of soft ferro-magnetic material having two parts presenting a coplanar surface towards the rotor, the parts being separated from each other by a sinuous air gap.

Stepping electro-magnetic motors of the above mentioned type are known. One of them, for instance, is disclosed in the Swiss Patent No. 558.043. They comprise a upper stationary portion, constituting the cover, situated above the rotor, supposed to play a double role, that is to say on the one hand to ensure the closing of the magnetic circuit and, on the other hand owing to openings provided therein to produce. with respect to the rotor, periodic variations of reluctance determining the several stable positions of the rotor.

The purpose of the present invention is to furnish a motor of the above mentioned type the height of which is reduced. This invention is based on the ascertainment of the fact that the role of the cover as part of the magnetic circuit is illusory. so that it ca be replaced by means of serving exclusively to ensure the stability of the several angular positions of the rotor.

According to the invention in one aspect, there is provided an electromagnetic stepping motor comprising a rotor and a stator, wherein the rotor includes a flat disc having a plurality of uniformly spaced teeth of equal width to one another and the stator includes two coplanar parts of soft ferromagnetic material parallel to and axially spaced from the disc and separated from each other by a sinuous air gap which is narrower than the width of the teeth of the rotor and follows a closed path round the axis of the motor, the stator further including an annular member of soft ferromagnetic material situated substantially in the plane of and surroundng the disc, the inner edge thereof comprising uniformly spaced teeth of equal width to one another and equal in number to twice the number of teeth on the disc which is magnetised so that all the teeth are of the same polarity, thereby to define a number of angularly stable positions of the rotor equal to twice the number of rotor teeth.

According to the invention in another aspect, there is provided an electromagnetic stepping motor comprising a rotor and a stator, wherein the rotor includes a flat disc having a plurality of uniformly spaced teeth of equal width to one another and the stator includes two coplanar parts of soft ferromagnetic material parallel to and axially spaced from the disc and separated from each other by a sinuous air gap which is narrower than the width of the teeth of the rotor and follows a closed path round the axis of the motor, the stator further including an annular member of soft ferromagnetic material situated substantially in the plane of and surrounding the disc, the inner edge thereof comprising uniformly spaced teeth of equal width to one another and equal in number to the number of teeth on the disc which is magnetised so that adjacent teeth are of opppsite polarity, thereby to define a number of angularly stable positions of the rotor equal to the number of rotor teeth.

The drawing shows, by way of example, one embodiment of the object of the invention. In the drawing: Figure I is an axial sectional view of a known electro-magnetic stepping motor having a cover.

Figure 2 is a plan view of the motor shown in Figure 1, with the cover removed.

Figure 3 is an axial sectional view of a stepping motor in accordance with the invention, for use in a time-piece, and Figure 4 is an exploded perspective view of the motor shown in Figure 3.

The motor represented in Figures 1 and 2 comprises a cylindrical casing 1 provided with a bottom 2 which carries, secured at its centre, a hub 3 around which is arranged a winding 4. The stator of this motor comprises an annular member 5, having the shape of a thin sheet, mounted on the end of the hub 3 opposite to the end which is secured to the bottom 2 of the casing, and which presents an outwardly facing array of six uniformly spaced teeth (Figure 2). The stator comprises moreover a second annular member 6, having also the shape of a thin sheet, situated in the plane of member 5 and which is provided with an inwardly facing array of teeth which is complementary to the teeth of the member 5, interdigitated therein, so as to form a sinuous air gap comprising a number of uniformly spaced segments of equal width to one another. A filling element 7, made of non-magnetic material, is interposed between the members 5 and 6 of the stator, both formed of a soft ferromagnetic material, so as to maintain them assembled to each other. This filling member 7 is not important and could be omitted if desired.

The rotor of the motor, which is rotatably mounted on a journal 8 force-fitted in the hub 3, comprises a member 9, made of plastic material, carrying a thin disc 10 made of a ferro-magnetic material having a high coercivity, such as a platinum-cobalt alloy, for instance, axially magnetised in alternate senses. This disc 10 is provided with twelve outwardly facing teeth situated, axially, opposite the radially outer ends of the segments of the air gap. As a modification, the disc 10 could have only six teeth which will then be axially magnetised in the same sense.

The stator is completed by a cover 11, also formed of a ferro-magnetic material, and which is pierced with twelve elongated openings 12. disposed radially, so as to present, with respect to the disc 10 of the rotor which it covers, periodic variations of reluctance which define twelve stable positions of the rotor, it being understood that the disc of the rotor has twelve teeth.

The motor represented in Figures 3 and 4, constituting one embodiment of the invention, is distinguished from the motor of Figure 1 mainly by the fact that the covershaped member 11 has been omitted. the role it is supposed to play as a closing means of the magnetic circuit being illusory, and has been replaced by an annular member 13 provided with an inwardly facing array 13a, of twelve teeth, situated in the plane of the disc 10 of the rotor, and which produces periodic variations of reluctance, with respect to the rotor, defining the twelve stable positions of this latter.

The omission of the cover 11 of the motor of Figure 1 permits a reduction in the height of the order of about 30%, which is an appreciable reduction.

This omission has moreover a second advantage of rendering the motor much less sensible to possible imprecision so far as the axial position of the rotor is concerned. As a matter of fact, in the known motor of Figure 1, any variation of the axial position of the rotor has for consequence a double effect on the maintaining torque to which the rotor is submitted when it is stopped and which is afforded by the stator on the one hand and by the cover on the other hand. This double effect is due to the fact that any reduction of the distance between the rotor and the stator causes a corresponding increase in the distance between the cover and the rotor and vice versa. Since the cover is omitted, the axial position of the rotor is much less critical: any variation in the distance between the rotor and the stator, being without effect on the distance of the rotor from the annular member 13.

It is to be noted that the elements of the motor of Figures 1 and 2 other than the cover have been maintained in the motor of Figures 3 and 4 where they have been designated by the same reference numerals as in Figures 1 and 2.

It is also to be noted that, in the example represented, the sinuous air gap of the stator has six segments and the rotor and the positioning member each have twelve teeth.

These data are not critical, the most important feature being that the number of poles of the rotor is twice the number of segments of the sinuous air gap of the stator. For example the rotor may have six teeth magnetised in the same sense.

WHAT WE CLAIM IS: 1. An electromagnetic stepping motor comprising a rotor and a stator, wherein the rotor includes a flat disc having a plurality of uniformly spaced teeth of equal width to one another and the stator includes two coplanar parts of soft ferro-magnetic material parallel to and axially spaced from the disc and separated from each other by a sinuous air gap which is narrower than the width of the teeth of the rotor and follows a closed path round the axis of the motor, the stator further including an annular member of soft ferro-magnetic material situated substantially in the plane of and surrounding

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Figure I is an axial sectional view of a known electro-magnetic stepping motor having a cover. Figure 2 is a plan view of the motor shown in Figure 1, with the cover removed. Figure 3 is an axial sectional view of a stepping motor in accordance with the invention, for use in a time-piece, and Figure 4 is an exploded perspective view of the motor shown in Figure 3. The motor represented in Figures 1 and 2 comprises a cylindrical casing 1 provided with a bottom 2 which carries, secured at its centre, a hub 3 around which is arranged a winding 4. The stator of this motor comprises an annular member 5, having the shape of a thin sheet, mounted on the end of the hub 3 opposite to the end which is secured to the bottom 2 of the casing, and which presents an outwardly facing array of six uniformly spaced teeth (Figure 2). The stator comprises moreover a second annular member 6, having also the shape of a thin sheet, situated in the plane of member 5 and which is provided with an inwardly facing array of teeth which is complementary to the teeth of the member 5, interdigitated therein, so as to form a sinuous air gap comprising a number of uniformly spaced segments of equal width to one another. A filling element 7, made of non-magnetic material, is interposed between the members 5 and 6 of the stator, both formed of a soft ferromagnetic material, so as to maintain them assembled to each other. This filling member 7 is not important and could be omitted if desired. The rotor of the motor, which is rotatably mounted on a journal 8 force-fitted in the hub 3, comprises a member 9, made of plastic material, carrying a thin disc 10 made of a ferro-magnetic material having a high coercivity, such as a platinum-cobalt alloy, for instance, axially magnetised in alternate senses. This disc 10 is provided with twelve outwardly facing teeth situated, axially, opposite the radially outer ends of the segments of the air gap. As a modification, the disc 10 could have only six teeth which will then be axially magnetised in the same sense. The stator is completed by a cover 11, also formed of a ferro-magnetic material, and which is pierced with twelve elongated openings 12. disposed radially, so as to present, with respect to the disc 10 of the rotor which it covers, periodic variations of reluctance which define twelve stable positions of the rotor, it being understood that the disc of the rotor has twelve teeth. The motor represented in Figures 3 and 4, constituting one embodiment of the invention, is distinguished from the motor of Figure 1 mainly by the fact that the covershaped member 11 has been omitted. the role it is supposed to play as a closing means of the magnetic circuit being illusory, and has been replaced by an annular member 13 provided with an inwardly facing array 13a, of twelve teeth, situated in the plane of the disc 10 of the rotor, and which produces periodic variations of reluctance, with respect to the rotor, defining the twelve stable positions of this latter. The omission of the cover 11 of the motor of Figure 1 permits a reduction in the height of the order of about 30%, which is an appreciable reduction. This omission has moreover a second advantage of rendering the motor much less sensible to possible imprecision so far as the axial position of the rotor is concerned. As a matter of fact, in the known motor of Figure 1, any variation of the axial position of the rotor has for consequence a double effect on the maintaining torque to which the rotor is submitted when it is stopped and which is afforded by the stator on the one hand and by the cover on the other hand. This double effect is due to the fact that any reduction of the distance between the rotor and the stator causes a corresponding increase in the distance between the cover and the rotor and vice versa. Since the cover is omitted, the axial position of the rotor is much less critical: any variation in the distance between the rotor and the stator, being without effect on the distance of the rotor from the annular member 13. It is to be noted that the elements of the motor of Figures 1 and 2 other than the cover have been maintained in the motor of Figures 3 and 4 where they have been designated by the same reference numerals as in Figures 1 and 2. It is also to be noted that, in the example represented, the sinuous air gap of the stator has six segments and the rotor and the positioning member each have twelve teeth. These data are not critical, the most important feature being that the number of poles of the rotor is twice the number of segments of the sinuous air gap of the stator. For example the rotor may have six teeth magnetised in the same sense. WHAT WE CLAIM IS:

1. An electromagnetic stepping motor comprising a rotor and a stator, wherein the rotor includes a flat disc having a plurality of uniformly spaced teeth of equal width to one another and the stator includes two coplanar parts of soft ferro-magnetic material parallel to and axially spaced from the disc and separated from each other by a sinuous air gap which is narrower than the width of the teeth of the rotor and follows a closed path round the axis of the motor, the stator further including an annular member of soft ferro-magnetic material situated substantially in the plane of and surrounding

the disc, the inner edge thereof comprising uniformly spaced teeth of equal width to one another and equal in number to twice the number of teeth on the disc which is magnetised so that all the teeth are of the same polarity, thereby to define a number of angularly stable positions of the rotor equal to twice the number of rotor teeth.

2. An electromagnetic stepping motor comprising a rotor and a stator, wherein the rotor includes a flat disc having a plurality of uniformly spaced teeth of equal width to one another and the stator includes two coplanar parts of soft ferro-magnetic material parallel to and axially spaced from the disc and separated from each other by a sinuous air gap which is narrower than the width of the teeth of the rotor and follows a closed path round the axis of the motor, the stator further including an annular member of soft ferro-magnetic material situated substantially in the plane of and surrounding the disc, the inner edge thereof comprising uniformly spaced teeth of equal width to one another and equal in number to the number of teeth on the disc which is magnetised so that adjacent teeth are of opposite polarity, thereby to define a number of angularly stable positions of the rotor equal to the number of rotor teeth.

3. An electromagnetic stepping motor substantially as hereinbefore described with reference to Figures 3 and 4 of the accompanying drawings.