MINIATURE STEP MOTOR WITH SHOELESS STATOR AND PREWOUND BOBBINS

Abstract

A two-phase stepper motor with a permanent magnet (PM) rotor and a modified hybrid-type stator is provided. The modified hybrid stator can be manufactured even at the smaller motor size because it employs shoeless, straight stator poles without stator teeth and with bobbin coils that are pre-wound outside the motor and easily inserted over each of the stator poles. Each bobbin may be an elongated continuous belt of insulative material with a hollow interior that forms a sleeve that fits around its corresponding stator pole. Conductive wire wound around the sleeve forms the stator coils. Edges of the sleeve may have exterior flanges at radially inner and outer ends of the stator pole to hold windings in place and keep the sleeve from bowing outward. This stator construction allows the motor to be miniaturized so that the PM rotors can be 13 mm diameter or less.

Claims

1. A two-phase permanent magnet step motor, comprising: a permanent magnet rotor having an equal number Nr of magnetic north and magnetic south poles defining a fundamental step angle θ=90°/Nr, the magnetic poles facing radially outward from the rotor and arranged alternately around a circumference of the rotor; and a hybrid stator assembly having a number Ns of straight, shoeless stator poles facing radially inward toward the rotor, wherein Ns is divisible by four and a ratio Nr/Ns=n/4, n being an odd integer, each of the straight, shoeless stator poles having a bobbin pre-wound with conductive stator coils, the bobbins fitting around the respective straight, shoeless stator poles.

2. A step motor as in claim 1, wherein rotor poles are formed by strips of rare-earth magnet material arranged axially on the rotor.

3. A step motor as in claim 1, the rotor has a diameter of at most 13 mm.

4. A step motor as in claim 3, wherein the rotor has a diameter of 8 mm.

5. A step motor as in claim 1, wherein Nr is at most 10.

6. A step motor as in claim 1, wherein each bobbin is an elongated continuous belt of insulative material with a hollow interior that forms a sleeve that fits around its corresponding stator pole, and conductive wire wound around the sleeve to form the conductive stator coils, edges of the sleeve having exterior flanges at radially inner and outer ends of the stator pole to hold windings in place and keep the sleeve from bowing outward.

7. A method of making two-phase permanent magnet step motor, comprising: providing a hybrid stator assembly having a number Ns of straight, shoeless stator poles facing radially inward, wherein Ns is divisible by four; providing a set of Ns bobbins, each bobbin being an elongated continuous belt of insulative material with a hollow interior that forms a sleeve that can fit around a corresponding stator pole, edges of the sleeve having exterior flanges to hold windings in place and keep the sleeve from bowing outward; winding conductive wire wound around the sleeve to form conductive stator coils; fitting the wound bobbins around the respective straight, shoeless stator poles; and rotatably mounting a permanent magnet rotor having an equal number Nr of magnetic north and magnetic south poles defining a fundamental step angle θ=90°/Nr, a ratio Nr/Ns=n/4, n being an odd integer, the magnetic poles facing radially outward from the rotor toward the stator and arranged alternately around a circumference of the rotor.

8. The method as in claim 7, wherein rotor poles are formed by applying strips of rare-earth magnet material arranged axially on the rotor.

9. The method as in claim 7, wherein the rotor has a diameter of at most 13 mm.

10. The method as in claim 9, wherein the rotor has a diameter of 8 mm.

11. The method as in claim 7, wherein Nr is at most 10.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 is a side sectional view of a conventional hybrid stepper of the prior art.

[0015] FIG. 2A is a perspective view of the stator assembly for the hybrid stepper of FIG. 1.

[0016] FIG. 2B is a cutaway perspective view of the stator assembly of FIG. 2A with a hybrid rotor.

[0017] FIG. 3 is a perspective view of a typical hybrid rotor for a 30° low resolution stepper in the prior art.

[0018] FIG. 4 is a perspective view of a permanent magnet rotor, here for a 30° full step angle, for use in the present invention.

[0019] FIG. 5 is a cross-sectional view of a step motor with a permanent magnet rotor and with a conventional stator having pole shoes as in the prior art.

[0020] FIG. 6 is a cross-sectional view of a stator core lamination with straight shoeless stator poles for use in the present invention.

[0021] FIGS. 7A and 7B are side plan and end views of a bobbin coil that when pre-wound with stator windings is fitted over the straight stator poles of FIG. 6 in accord with the present invention.

[0022] FIG. 8 is a perspective view of an assembled step motor having the stator core of FIG. 6 with pre-wound bobbin coils as in FIGS. 7A-7B and fitted with a PM type rotor like that in FIG. 4.

DETAILED DESCRIPTION

[0023] With reference to FIGS. 1, 2A and 2B, a conventional hybrid step motor 11 of the prior art is shown by way of comparison. This step motor includes a stator assembly 12 and a hybrid-type rotor 13. The rotor 13 is mounted on an axial shaft 14 that is supported for rotation within the stator assembly 12, e.g. via a pair of precision bearings.

[0024] The hybrid-type rotor has two parts with respective rotor teeth 15 and 16 that sandwich a disc magnet 17. The teeth 15 in one of the rotor parts are circumferentially offset by one-half pitch relative to the teeth 16 in the other of the rotor parts, so that teeth 15 define magnetic N rotor poles and teeth 16 define magnetic S rotor poles. Another hybrid-type rotor with fewer rotor teeth 31 is shown in FIG. 3. This kind of hybrid rotor arrangement provides for a long three-dimensional magnetic flux path 18 through the motor 11, resulting in greater reluctance and winding induction, and slower current rise time between successive phases than comparable motors using a PM-type rotor.

[0025] The stator assembly 12 has a stator core with a set of stator poles 20 directed radially inward toward the rotor, each pole 20 terminating in a corresponding stator shoe 21 having a set of stator teeth 22. The stator poles 20 have respective conductive windings that can be driven with applied current to magnetize the poles with successive magnetic polarities for the different phases. Although the conductive wires that form the windings have an insulative coating to prevent electrical shorts, the space between the poles may also have an insulative plastic insert to further separate the stator core material from the windings.

[0026] With reference to FIG. 4, a PM-type rotor may be used in place of a hybrid rotor if the number of rotor poles is relatively small (e.g. Nr=10 or less for 20 or fewer total rotor poles, each 18° or more wide). A PM rotor 41 has N and S rotor poles 42 and 43 that are formed using strips of rare-earth magnet material arranged axially on the rotor 41. The magnetic poles 42 and 43 face radially outward from the rotor and are arranged with N and S facing polarities alternately around the circumference of the rotor 41. Typical magnetic material for PM rotors are sintered neodymium-iron-boron or samarium-cobalt.

[0027] FIG. 5 shows a cross-sectional view for a 30° stepper with 2-phases energized flux paths 57 passing on the face of the stator 51. The rotor 41 is a PM-type rotor with strips of magnetic material forming alternating N rotor poles 42 and S rotor poles 43, three poles of each polarity and six rotor poles in total. The stator 51 has four conventional hybrid stator poles 52 that terminate in corresponding stator shoes 53. The shoes widen the stator poles 52 for maximum magnetic interaction with the corresponding rotor poles 42 and 43, but also help to hold in place the windings (not shown) that pass axially (vertically through the sheet). The flux path 57 for the energized stator poles 52 passes without any axial component, that is in two dimensions only.

[0028] With reference to FIG. 6, a stator core 61 in accord with the present invention has a set of radially inward directed, straight, shoeless stator poles 64, here four in number (but 8 or even 12 poles are possible, as long as the number of stator poles is divisible by 4). The stator poles 64 terminate in a slightly concave toothless surface 65 that collectively follow a circular cylindrical path that is only slightly larger than the rotor surface. Thus, rotor and stator elements face each other across a very small gap of only a few hundred micrometers. For maximum magnetic interaction, the width of the straight poles is made as wide as possible while still allowing pre-wound bobbins to all fit over the poles 64.

[0029] FIGS. 7A and 7B show aplastic bobbin 71 for placement over the straight stator poles after having been pre-wound with conductive windings. The bobbin 71 can be seen to comprise an elongated continuous belt 73 of rigid polymer material with a hollow interior that forms, a sleeve 75. The sleeve 75 is slightly longer than the axial dimension of the stator poles and is slightly wider than the circumferential width dimension of the stator poles, so that sleeve 75 can easily fit around a stator pole with minimal play. Exterior flanges 77 and 78 extend circumferentially from the radially inner and outer edges of the sleeve 75. The flanges 77 and 78 function primarily to hold the windings 79 in place, but unlike stator shoes are not a soft magnetic material. The flanges also reinforce the rigidity of the sleeve to keep it from bowing outward when the conductive wires 79 are wound. For each pre-wound bobbin, the windings 79 connect at their two ends to respective terminals 81 located at one axial end of the bobbin 71. Once the bobbins are inserted over the set of stator poles, the terminals 81 can be electrically coupled in any of several well-known configurations to receive applied current from driver to operate the motor. Applying drive current in successive phases through the stator windings cause the motor to step. Using variable drive current amplitude, the motor can pass gradually through the successive phases in a mode known as micro-stepping for smooth operation despite the large step sizes.

[0030] The permanent magnet rotor has an equal number Nr of magnetic north and south poles that defines a fundamental step angle θ=90°/Nr for 360°/Nr steps per revolution. For example, two north poles and two south poles outward facing in a 4-poles rotor will produce a 45°-step motor (Nr=2). Preferably Nr is at most ten (i.e., no more than 20 rotor poles in total), giving the motor good holding torque and capability for high operating speed. The hybrid-type stator for use in the present invention is both shoeless and toothless, with a number Ns of stator poles. Ns should be divisible by 4 for a two-phase motor. Additionally, the ratio Nr/Ns=n/4, where n is an odd integer. Preferably, the motor uses a 4-pole or 8-pole stator.

[0031] The following table illustrates a number of possible rotor-stator pole combinations for two-phase step motors in accord with the present invention.

TABLE-US-00001 Number of Odd Number of Fundamental Rotor Poles Integer Stator Poles Step Angle θ 2 × Nr n Ns  9°   20 5 8 10°   18 9 4  11.25° 16 1 32 ≈12.85° 14 7 4 15°   12 3 8 18°   10 5 4 22.5° 8 1 16 30°   6 3 4 45°   4 1 8 90°   2 1 4
The preferred combinations are Nr=6 and Ns=8 for a 15° stepper; Nr=5 and Ns=4 for a 18° stepper; Nr=3 and Ns=4 for a 30° stepper, and Nr=1 and Ns=4 for a 90° stepper. The larger step angles allow the motor to operate at higher speeds. The motor speed is controlled by the step pulse, rate with no feedback or commutation system (open loop control system). Unlike brushless DC motors, the step motor can run smoothly at low speed via micro-stepping control.

[0032] As previously noted, in a conventional hybrid stepper two rotor sections are offset by of the tooth pitch, and a 3-dimensional magnetic flux path is formed, and magnetic flux passes in axial direction. With the use of a permanent magnet rotor in the present invention, the magnetic flux path is 2-dimensional, without magnetic flux in axial direction, resulting in shorter magnetic flux path and small reluctance. With the lower reluctance, the winding inductance is much smaller to allow for fast current rise in the stator to maintain the torque at the high speed.

[0033] Two-phase step motors in accord with the present invention, when operated at 2-phase ON, provides 100% coil and stator pole utilization. For a 45-degree step motor, all four magnetic rotor poles are 100% utilized to interact with the 8 stator poles. Likewise, for a 15-degree step motor, 8 of the 12 magnetic rotor poles interact directly with the stator effectively, while the remaining 4 magnetic rotor poles always repulse with the energized stator poles to act as a magnetic pusher to minimize the leakage flux from the energized stator. As a result, a highly efficient motor is created.

[0034] Because straight, shoeless stator poles are used with pre-wound bobbins for the coils, the motor can be made much smaller without having to contend with adequate space between stator pole shoes for a winding needle. All winding is conducted while the bobbins are outside of the stator and then slipped over the poles. 2-phase step motors having rotor diameters smaller than 13 mm are possible, including step motors with rotors as small as 8 mm diameter.

[0035] As seen in FIG. 8, an assembled step motor 91 in accord with the present invention can be seen to have a stator core 92 (like stator core 61 in FIG. 6) with pre-wound bobbins (as in FIGS. 7A-7B) which are fitted over each of the stator's shoeless poles. Terminal wires 95 from the windings are seen to come out of one end of the stator (although wires could come out of both ends, if desired). The stator 92 is fitted with a rotor 93 inside. This rotor 93 is a PM type rotor (like the rotor 41 in FIG. 4). The number of rotor and stator poles is per any of the possibilities set forth in the preceding table.