Geared stepper motor and manufacturing method.

Abstract

A stepper motor comprising a stator (1) provided with a plurality of first teeth (10), a rotor (2) mounted to pivot in the stator (1) and provided with a plurality of second teeth (20), and a plurality of electromagnetic windings (30) for defining stator poles, each tooth (10, 20) having two flanks (11, 21) converging towards each other towards an apex (12, 22) of each tooth (10, 20). Each flank (11, 21) has a gear tooth profile. Method of producing such a motor. FIGURE OF THE ABSTRACT: Fig.2

Description

Title of the invention: Gear-toothed stepper motor and manufacturing method.

[0001] The present invention relates to the field of electric motors and more particularly stepper motors.

[0002] BACKGROUND OF THE INVENTION

[0003] Stepper motors are direct current synchronous motors which comprise (see [Fig.l] schematically illustrating such a motor in section through a diametrical plane):

[0004] - a stator 1 provided with first teeth 10 and a plurality of electro-windings magnetic 30 to define phases,

[0005] - a rotor 2 mounted to pivot in the stator 1 and provided with second teeth 20.

[0006] The operating principle of these motors generally consists of supplying suc The electromagnetic windings are successively arranged to create two stator poles at 90° to each other so as to generate magnetic fields tending to rotate the rotor so that second rotor teeth are aligned with the first teeth at the poles. Unipolar stepper motors also exist.

[0007] Several stepper motor technologies coexist: permanent magnet stepper motors (the rotor carries the permanent magnets positioned to have alternating north and south poles), variable reluctance stepper motors (the rotor is made of soft iron), and hybrid stepper motors combining the two previous technologies.

[0008] One of the main difficulties in producing stepper motors concerns the manufacture of the teeth, particularly for small diameter stepper motors (typically less than 50 millimeters) and with an angular pitch (or tooth pitch) of less than approximately 10°. It is understood that the greater the number of teeth for a given diameter, the more difficult the manufacture.

[0009] Indeed, each tooth 10, 20 normally has two flanks 11, 21 which are plane and inclined relative to the radius of a tooth angle a, a tooth length 1 measured at the apex 12, 22 and is separated from the adjacent teeth by a dental pitch p (see [Fig.3] which shows teeth of a stepping motor of the prior art, in section through a transverse plane).

[0010] These dimensions must be precise because deviations on a few teeth risk short-circuiting the magnetic circuit of the motor and causing a lower and unstable torque during a revolution of the rotor. Manufacturers therefore control these different dimensions on the stepper motors that they manufacture but these dimensions are not standardized, they use different control methods increasing the risk of having relatively significant variations in performance between stepper motors corresponding to the same theoretical dimensioning.

[0011] SUBJECT OF THE INVENTION

[0012] The invention aims in particular to remedy at least in part the aforementioned drawbacks. Summary of the invention

[0013] To this end, the invention provides a stepper motor comprising a stator provided with a plurality of first teeth, a rotor mounted to pivot in the stator and provided with a plurality of second teeth, and a plurality of electromagnetic windings for defining stator poles, each tooth having two flanks converging towards each other towards an apex of each tooth, each flank having a gear tooth profile. The apex may have the shape of a surface or a line forming an edge connecting the two flanks to each other.

[0014] The invention also relates to a method of manufacturing a stepping motor, comprising a step of manufacturing first teeth on a stator of the motor and a step of manufacturing second teeth on a rotor of the motor, the teeth being manufactured by a gear machining method to have a gear tooth profile.

[0015] Thus, forming the stator and rotor teeth into a gear tooth profile facilitates their manufacture and dimensional and geometric control, especially since very precise gear cutting machines exist and most of these profiles are standardized. It is possible to adapt the gear tooth profile so that it exactly matches the ideal profile from a magnetic point of view. In any case, the precision that can be achieved when producing the profile can even compensate for the fact that the chosen gear tooth profile does not exactly match the ideal profile from a magnetic point of view.

[0016] According to optional characteristics, used individually or in whole or in part in combination: - the gear tooth profile is an involute of a circle; - the method comprises the prior step of determining an ideal profile from a magnetic point of view and of determining for the stator and the rotor a number of teeth, a tooth module and a tooth offset such that the teeth have a gear tooth profile as close as possible to the ideal profile from a magnetic point of view; - the method comprises a subsequent step of checking the teeth of the stator and the rotor, the subsequent step of checking the teeth comprising a com comparison of the profile of each tooth with a theoretical gear tooth profile and / or a measurement of a helix of the profile of each tooth and / or a measurement of an angular error between at least two adjacent teeth and / or a comparison of a contact profile of each tooth with a theoretical contact profile.

[0017] Other characteristics and advantages of the invention will emerge from reading the following description of a particular and non-limiting embodiment of the invention. Brief description of the drawings

[0018] Reference will be made to the accompanying drawings, among which:

[0019] [Fig-1] [Fig.l] schematically illustrates a stepper motor of the prior art, in section through a transverse plane;

[0020] [Fig.2] [Fig.2] schematically illustrates a stepper motor according to the invention, in section through a transverse plane;

[0021] [Fig.3] [Fig.3] is an enlarged detail view of the teeth of a stepper motor of the prior art, in section through a transverse plane;

[0022] [Fig.4] [Fig.4] is an enlarged detail view of a tooth of a stepper motor of the invention, in section through a transverse plane;

[0023] [Fig.5] [Fig.5] is a partial view of the rotor of a stepper motor of the invention, in section through a transverse plane;

[0024] [Fig.6] [Fig.6] shows tooth profile measurement curves according to the method control implemented in the manufacturing process of the invention;

[0025] [Fig.7] [Fig.7] shows curves of tooth helix measurement according to the method control implemented in the manufacturing process of the invention;

[0026] [Fig.8] [Fig.8] shows measurement curves of an individual division error and a cumulative division error of the teeth according to the control method implemented in the manufacturing method of the invention;

[0027] [Fig.9] [Fig.9] shows a control curve of the contact profile of a toothing according to the control method implemented in the manufacturing process of the invention. DETAILED DESCRIPTION OF THE INVENTION

[0028] With reference to Figures 2, 4, 5, the stepper motor according to the invention comprises a stator 1 provided with a plurality of first teeth 10, a rotor 2 mounted to pivot in the stator 1 and provided with a plurality of second teeth 20, and a plurality of electromagnetic windings 30 to define stator poles. This structure, the materials used to manufacture the stator 1 and the rotor 2, and the operation of the stepper motor are known per se and will not be described further here. The stator is for example formed of magnetic sheets of cobalt iron, iron nickel or silicon iron. The rotor is, for example, made of a solid block of cobalt iron, nickel iron or silicon iron.

[0029] Each tooth 10, 20 comprising two flanks 11, 21 converging towards each other towards a vertex 12, 22 of each tooth 10, 20.

[0030] According to the invention, each flank 11, 21 has a gear tooth profile. Preferably, the gear tooth profile is an involute of a circle. It is recalled that gear tooth profiles are known in themselves in the field of motion transmission: when two straight-toothed wheels rotate while meshing with each other, two teeth are in contact each by a flank along a line, parallel to the axis of rotation, moving parallel to itself along the profile of the flank of each tooth: the profiles of the two teeth in contact have at each instant a common normal on the contact line.

[0031] In the invention, it is understood that the teeth 10, 20 never come into contact with each other. The gear tooth profile is used solely for its ease of production with existing machines for manufacturing gears and its ease of control with existing control methods for the dimensional and geometric control of gears.

[0032] For example, the stepper motor has a diameter less than 100 mm and an angular precision of between 100 and 200 steps per revolution, i.e. between 3.6° and 1.8°.

[0033] The invention also relates to a method of manufacturing a stepper motor comprising: - steps, known in themselves, of manufacturing a stator blank and a rotor blank, - a step of manufacturing first teeth 10 on the stator blank to form the stator 1, - a step of manufacturing second teeth 20 on the rotor blank to form the rotor 2.

[0034] According to the invention, the teeth 10, 20 are here manufactured by a gear cutting method to have a gear tooth profile. The gear cutting method used here is a generation cutting method and more particularly a hob cutting method.

[0035] The method comprises the prior step of determining an ideal profile from a magnetic point of view (see profile P in [Fig.4]) and of determining for the stator 1 and the rotor 2 a number of teeth 10 and 20, a tooth module and a tooth offset such that the teeth 10, 20 have a gear tooth profile as close as possible to the ideal profile from a magnetic point of view P.

[0036] The ideal tooth profile from a magnetic point of view minimizes friction (generated by magnetism) while maximizing the useful torque (in static and dynamic).

[0037] The method according to the invention comprises, after the manufacturing steps, a subsequent step of toothing control.

[0038] The control is implemented here using AGMA (American Gear Manufacturing Association) standards. The control step makes the manufacturing process more robust and reliable. Of course, other standards can be used, such as ISO (International Standard Organization) and API (American Petrol Institute) standards.

[0039] The subsequent step of tooth control comprises a comparison of the profile of each tooth 10, 20 with a theoretical gear tooth profile (see FIG. 6). Figure 5 shows: in a solid line the actual profile of the teeth 20 of the rotor 2 and in a dotted line the theoretical gear tooth profile referenced P'. The theoretical gear tooth profile P' has a theoretical tooth pitch pr and a cumulative pitch over k teeth fcp while the actual profile of the teeth 20 of the rotor 2 has an individual deviation fpt with respect to the theoretical tooth pitch fi and a cumulative deviation over k teeth Fpk.

[0040] A profile error has a direct impact on engine performance.

[0041] The subsequent step of tooth control also includes a measurement of a helix of the profile of each tooth 10, 20 (see [Fig.7]). The teeth 10, 20 are cut to be straight, that is to say that their flanks have generators parallel to the axis of rotation of the rotor 2 (in other words, the generators have a zero angle relative to the axis of rotation). If the generators have a non-zero angle relative to the axis of rotation, the maximum torque generated by the motor undergoes a reduction whose value depends on said angle.

[0042] The subsequent step of tooth control also comprises a measurement of an angular error between at least two adjacent teeth 10, 20 (error also called division error - see [Fig.8]). The measured angular error is the individual deviation fpt and the cumulative deviation over k teeth Fpk. These deviations can generate undesirable torque harmonics and it is therefore appropriate to keep them to a minimum determined according to the expected performance of the motor.

[0043] The subsequent step of tooth control may comprise a comparison of the profile of each tooth 10, 20 with the theoretical gear tooth profile P' over a complete revolution (see [Fig.9]). This makes it possible to control both the shape of the teeth 10, 20 and the circularity of the stator 1 and the rotor 2. The tooth deviations and the circularity deviations have an impact on the magnetic flux passing between the stator 1 and the rotor 2 and therefore on the performance of the motor. This comparison makes it possible to reduce the number of checks and provides: - the tooth gaps fpt and Fpk; a tolerance between a minimum and a maximum of circularity; - a tolerance on variations in profile oscillation which are linked to the shape of the teeth.

[0044] The manufacturing method according to the invention allows rapid manufacturing, using proven tools and methods for manufacturing and controlling the manufactured parts, thus limiting rejects and re-machining aimed at correcting errors.

[0045] Of course, the invention is not limited to the embodiment described but encompasses any variant falling within the scope of the invention as defined by the claims.

[0046] In particular, the stator and the rotor may have different shapes and dimensions, particularly with regard to the number of teeth and the dimensions.

[0047] Alternatively, the cutting of the teeth can be carried out by another method and for example by a rack tool or a pinion tool or by cutting without generation. It is also possible to produce the teeth by punching, laser cutting, cutting by electroerosion.

[0048] The stators and rotors can have a solid structure or be made by stacking sheets (the stacked sheets are fixed to each other, for example, by gluing). In the latter case, the teeth are preferably cut from the sheets and the precision control of the teeth is carried out after stacking / fixing the sheets to each other. Alternatively, the teeth can also be checked on each of the sheets individually.

[0049] All or part of the controls may be omitted.

[0050] The above-mentioned controls are optional and may be omitted in whole or in part.

Claims

Claims

1. A stepper motor comprising a stator (1) provided with a plurality of first teeth (10), a rotor (2) mounted to pivot in the stator (1) and provided with a plurality of second teeth (20), and a plurality of electromagnetic windings (30) for defining stator poles, each tooth (10, 20) having two flanks (11, 21) converging towards each other towards an apex (12, 22) of each tooth (10, 20), characterized in that each flank (11, 21) has a gear tooth profile.

2. A stepper motor according to claim 1, wherein the gear tooth profile is an involute of a circle.

3. A method of producing a stepper motor, comprising a step of manufacturing first teeth (10) on a stator (1) of the motor and a step of manufacturing second teeth (20) on a rotor (2) of the motor, characterized in that the teeth (10, 20) are manufactured by a gear machining method to have a gear tooth profile.

4. Method according to claim 3, comprising the prior step of determining an ideal profile from a magnetic point of view (P) and of determining for the stator (1) and the rotor (2) a number of teeth (10, 20), a tooth module and a tooth offset such that the teeth (10, 20) have a gear tooth profile as close as possible to the ideal profile from a magnetic point of view (P).

5. Method according to claim 3 or 4, comprising a subsequent step of checking the teeth of the stator (1) and the rotor (2).

6. A method according to claim 5, wherein the subsequent step of tooth control comprises comparing the profile of each tooth (10, 20) with a theoretical gear tooth profile (P').

7. A method according to claim 5, wherein the subsequent step of tooth control comprises measuring a helix of the profile of each tooth (10, 20).

8. A method according to claim 5, wherein the subsequent step of tooth control comprises measuring an angular error between at least two adjacent teeth (10, 20).

9. A method according to claim 5, wherein the subsequent step of tooth control comprises comparing a contact profile of each tooth (10, 20) with a theoretical contact profile.

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