Permanent magnet type stepping motor

A permanent magnet type stepping motor comprising first and second stator assemblies arranged coaxially about a longitudinal axis. Each assembly includes first and second ring-shaped stators made of magnetic material. The first ring-shaped stator has a central portion with an opening therein, an annular yoke surrounding the periphery of the central portion and a plurality of pole pieces distributed about the circumference of the opening and extending toward a plane transverse to the axis. The second ring-shaped stator has a central portion with an opening therein, an annular substantially planar yoke surrounding the periphery of the central portion and a plurality of pole pieces distributed about the circumference of the opening and extending away from the transverse plane so as to interleave with the pole pieces of the first stator. The central portion of the second stator protrudes from the plane of the yoke in a direction away from the transverse plane. A coil is interposed between the first and second ring-shaped stators and surrounds the pole pieces. The first and second stator assemblies are arranged with respect to the transverse plane so that the second ring-shaped stator of the first stator assembly is adjacent the second ring-shaped stator of the second stator assembly at the transverse plane thereby forming a gap between the protruding central portions of the second stators of the first and second stator assemblies. A permanent magnet having N and S poles distributed alternately about the outer circumference thereof is rotatably supported within the openings in the central portions of the first and second stators of the first and second stator assemblies.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a permanent magnet type (PM type) stepping 
motor. 
2. Description of Related Art 
A stepping motor can be used for driving, e.g. a print head or a platen 
roll of a printer. A stepping motor of this type is disclosed in U.S. Pat. 
No. 4,990,806. The PM type stepping motor comprises in general coils wound 
around pole pieces, ring-shaped stators and a rotor composed of a 
permanent magnet disposed at the central portion of the stators. 
In conventional PM type stepping motor a problem exists in that the drive 
efficiency of the motor is decreased because of eddy currents and heat 
generation caused by Joule heating occurred. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a PM type stepping 
motor capable of reducing an eddy current generated in its stators. 
It is another object of the present invention to provide a PM type stepping 
motor capable of performing a high drive efficiency. 
To achieve the above objects, the PM stepping motor according to the 
present invention comprises first and second stators each composed of two 
pairs of ring-shaped magnets in which one pair is put on the other pair, 
each pair having pole pieces arranged along inner circumferences thereof, 
first and second coils disposed to surround the pole pieces of the first 
and second stators, and a permanent magnet having N poles and S poles 
arranged alternately along an outer circumference thereof, said permanent 
magnet being rotatably supported by and disposed in the central portion of 
the first and second stators, characterized in that gaps are defined at a 
part of the portion where one pair of the magnets is put on the other 
pair. 
With the arrangement set forth above, the magnetic reluctance between the 
stators is increased and the eddy current can be reduced by reducing the 
magnetic flux which passes in the axial direction of the pole pieces 
positioned at upper and lower positions.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is an external view of a PM type stepping motor according to a 
preferred embodiment of the present invention. As illustrated in FIG. 1, 
the motor comprises stators 1 and 5 and supporting plates 4 and 8. A motor 
shaft 18 is fixed to the central portion of the stators 1 and 5. 
In FIG. 2, elements denoted at 1, 3, 5 and 7 are stators and formed by 
bending or drawing a magnetic plate, stators 1 and 3 forming a first 
stator assembly and stators 5 and 7 forming a second stator assembly. The 
first and second stator assemblies are arranged coaxially about a 
longitudinal axis 100, and are separated by a plane extending transverse 
to the longitudinal axis having a line 102 therein. As illustrated in FIG. 
3, the stators 1 and 5 have respectively cap-shaped configurations and 
have a plurality of pole pieces 1a and 5a formed by bending them at right 
angles at central openings thereof. Stators 1 and 5 are also provided with 
annular yokes 1b and 5b and central portions 1c and 5c respectively. 
Likewise, the stators 3 and 7 are ring-shaped and have a plurality of pole 
pieces 3a and 7a formed by bending them at right angles at the central 
openings thereof. Stators 3 and 7 have peripheral planar portions or yokes 
3b and 7b, and central portions 3c and 7c protruding from the planes of 
their respective yokes. 
As shown in FIG. 4, when the stators 1 and 3 are assembled, the pole pieces 
1a and 3a are alternately arranged about the circumferences thereof. 
Likewise, the pole pieces 5a and 7a are alternately arranged about the 
circumferences thereof. The pole pieces 1a and 3a of a first group and 
pole pieces 5a and 7a of a second group are disposed at a displacement of 
P/4 where P is the equal angular interval between the pole pieces. The 
displacement of P/4 corresponds to be .pi./2 angular electrical phase 
difference. The stators 3 and 7 protrude at the central portions 3c aand 
7c thereof while the peripheries thereof do not protrude. The central 
portions 3c of the stator 3 protrudes upward while the central portion 7c 
of the stator 7 protrudes downward. The upward and downward protrusions 
can be formed by subjecting the plate to a drawing process. Inasmuch as 
the central portions of the stators 3 and 5 protrude while the peripheries 
thereof do not protrude, gaps d are defined between the roots of the teeth 
of the pole pieces 3a and 7a. 
Denoted at 2 is a first coil assembly composed of a first bobbin 2b and a 
first coil 2a wound around the bobbin 2b. Denoted at 6 is a second coil 
assembly composed of a second bobbin 6b and a second coil 6a wound around 
the bobbin 6b. Denoted at 9 is a permanent magnet having n pieces of 
N-poles and S-poles alternately polarized and arranged along the 
circumference thereof. A ferrite magnet available at low cost is used as 
the permanent magnet. A rare-earth magnet, such as a SmCo based magnet may 
also be used as the permanent magnet. Denoted at 4 and 8 are supporting 
plates having bearings 4a and 8a at the central portions thereof for 
supporting a shaft 18 of the permanent magnet 9. 
According to the PM type stepping motor, there are defined gaps 15 forming 
an air layer between the roots of the teeth of the pole pieces 3a and 7a 
as shown in FIGS. 2, 3 and 4 for increasing the magnetic reluctance in the 
magnetic circuit formed by the first and second stators 3 and 7 when the 
first and second stators 3 and 7 are assembled. 
The operation of the PM type stepping motor of the present invention will 
be described with reference to FIGS. 2 to 8. 
When alternating first and second currents are applied to the first and 
second coils 2a and 6a, which are staggered by 1/4 period, a rotating 
magnetic field is generated whereby the permanent magnet 9 is rotated in 
synchronization with the rotating magnetic field. When the current is 
applied to the first and second coils 2a and 6a, the route of the magnetic 
flux, i.e., magnetic paths are formed as illustrated in FIGS. 5 and 6. 
Referring to FIGS. 3 and 5, the magnetic flux generated by the coil 2a 
passes through a magnetic path 10 defined by the pole pieces 1a and 3a and 
3b as illustrated by a dotted chain line. Similarly, referring to FIGS. 3 
and 6, the magnetic flux generated by the coil 6a passes through a 
magnetic path 12 defined by the pole pieces 5a and 7a and yokes 5b and 7b. 
The eddy current will be described with reference to FIGS. 3 and 7. The 
eddy current, which follows a path 14 in yokes 3b and 7b, is generated by 
the magnetic flux piercing axially the upper pole pieces 1a and 3a and the 
lower pole pieces 5a and 7a. That is, the eddy current is generated by a 
magnetic flux 13 passing through the magnetic path 12 as illustrated in 
FIG. 6. The magnetic flux 13 generates a circumferential magnetic field in 
yokes 3b and 7b whereby the eddy current is generated in the yokes 3b and 
7b. 
In to the PM stepping motor of the present invention, since an air layer is 
defined between the first and second stators 3 and 7 by the gaps 15 
between the pole pieces 3a and 7a at the roots thereof, the magnetic flux 
passes with difficult to pass through the magnetic path. The reason is 
that the permeability of the air layer is one several hundredth as much as 
that of a conventional magnetic material thereby rendering the magnetic 
reluctance in the air layer extremely high and also making it difficult 
for the magnetic flux to pass through the portion denoted by arrow 16. 
Accordingly, the eddy current is reduced. 
The air layer may be defined as part of the magnetic path 12, as 
illustrated in FIG. 6. Accordingly, the air layer may be defined at an 
outer periphery of the motor instead of at an inner periphery of the motor 
as illustrated in FIG. 3. The air layer may be formed by providing a 
projection 17 at one of the stators as illustrated in FIG. 8. 
As explained above, since the PM stepping motor according to the present 
invention has gaps between the first and second stators, it is possible to 
increase the magnetic reluctance in the magnetic circuit formed by the 
first and second stators. Accordingly, the eddy current loss is reduced 
thereby preventing the drive efficiency of the PM motor from being 
decreased and the PM motor from overheating. 
The PM motor of the present invention enables the print head and the like 
in a printer to be drive stably for a long period of time.