Step motor with circumferential stators on opposite sides of disc-like rotor

An electric step motor has a rotor member and a stator member aligned on a common axis with one of the two members having two pairs of interdigital pole teeth extending perpendicularly to the axis. The two pairs of pole teeth are axially spaced from each other with the inner pole teeth in each pair spaced both radially and circumferentially from the outer pole teeth in that pair. A pair of coils are associated with the two pairs of interdigital pole teeth for magnetizing the inner and outer pole teeth in each pair with opposite polarities when the corresponding coil is energized. The other member has multiple permanent magnets spaced circumferentially from each other and located axially between the two pairs of interdigital pole teeth so that energization of either coil draws the permanent magnets into register with a selected pair of interdigital pole teeth. The permanent magnets are polarized in the axial direction with each adjacent pair of the permanent magnets preferably polarized in opposite directions, and the pole teeth in one of the two interdigital pairs are preferably circumferentially offset from the pole teeth in the other pair.

DESCRIPTION OF THE INVENTION 
The present invention relates generally to electric step motors and, more 
particularly, to step motors which include a circumferential array of 
permanent magnets cooperating with circumferential arrays of 
electromagnetic poles whose polarization is controlled by energization of 
at least two different coils to produce stepping movement of the rotor. 
It is a primary object of the present invention to provide an improved step 
motor which provides a relatively high output torque for any given outside 
diameter or volume. 
It is another object of this invention to provide such an improved step 
motor which can be mass produced at a relatively low cost, and without the 
use of complex tooling. 
A further object of the invention is to provide such an improved step motor 
which provides positive detenting when the motor is turned off, so that 
the motor maintains its output shaft at the same position held by the 
shaft just before the motor is turned off. 
Yet another object of the invention is to provide such an improved step 
motor which has a relatively simple construction and long operating life.

Although the invention will be described in connection with certain 
preferred embodiments, it will be understood that it is not intended to 
limit the invention to these particular embodiments. On the contrary, it 
is intended to cover all alternatives, modifications and equivalents as 
may be included within the spirit and scope of the invention as defined by 
the appended claims. 
Turning now to the drawings, the exemplary step motor shown in FIGS. 1 and 
2 includes as its major components a rotor 10 and a two-part stator 11 
comprising a left-hand section 11a and a right-hand section 11b. The rotor 
10 and the two stator sections 11a and 11b are all concentrically aligned 
with the axis of a shaft 12, with the rotor 10 being carried on a 
non-magnetizable hub 13 affixed to the end of the shaft 12. From the hub 
13, the shaft 12 extends through a non-magnetizable bearing sleeve 14 
within the stator section 11a. A snap ring 15 inserted in a groove in the 
splined outer end of the shaft 12 holds the shaft in a fixed axial 
position with the hub 13 riding on the inner end of the bearing sleeve 14. 
The inside surfaces of the two stator sections 11a and 11b are formed by 
inside cylinders 16 and 17 which are made of a magnetically permeable 
material such as soft iron. Surrounding the cylinders 16 and 17 are 
corresponding shells 18 and 19 which form a pair of outside cylinders 20 
and 21 and integral end walls 22 and 23. The spaces between the two pairs 
of cylinders 16, 20 and 17, 21 are occupied by a pair of coils 24 and 25 
which are energized in a controlled manner to magnetize multiple sets of 
interdigital stator poles extending perpendicularly to the motor axis, as 
will be described in more detail below. The two stator sections 11a and 
11b are separated by a spacer ring 26 made of a non-magnetizable material 
such as aluminum, and which also serves to position the outer pole teeth 
of the two stator sections in the desired angular or circumferential 
positions. 
In accordance with one important aspect of the present invention, the inner 
and outer pole teeth in each stator section are spaced both radially and 
circumferentially from each other so that the inner and outer teeth can be 
magnetized with opposite polarities when the corresponding coil is 
energized, and the rotor is provided with multiple permanent magnets 
spaced circumferentially from each other and located between the two pairs 
of interdigital pole teeth of the stator sections so that energization of 
either coil draws the permanent magnets into register with the stator pole 
teeth associated with the energized coil, the permanent magnets being 
polarized in the axial direction with each adjacent pair of magnets being 
polarized in opposite directions. Thus, in the illustrative embodiment, 
the opposed ends of the two shells 18 and 19 receive two pairs of coplanar 
flat annuli 31, 32, and 33, 34 which form the two axially spaced pairs of 
interdigital pole teeth 31a, 32a and 33a, 34a extending perpendicularly to 
the motor axis. All four annuli 31-34 are made of magnetically permeable 
material such as soft iron. The two outside annuli 31 and 33 form six 
outside teeth 31a, 33a and fit into complementary inside grooves formed in 
the open ends of the two shells 18 and 19 to provide low-reluctance 
circuits for the flow of magnetic flux induced in the outside cylinders 20 
and 21 by the respective coils 24 and 25. The two inside annuli 32 and 34 
form six inside teeth and are formed as integral parts of the two inside 
cylinders 16 and 17 to provide low-reluctance circuits for magnetic flux 
induced in the inside cylinders 16 and 17 by the respective coils 24 and 
25. 
In order to minimize the leakage of magnetic flux between the inner and 
outer pole teeth 31a, 32a or 33a, 34a of either stator section, the 
opposed surfaces of the teeth formed by the two pairs of annuli 31, 32 and 
33, 34 are continuously spaced from each other in both the radial and 
circumferential directions. More specifically, the teeth 31a and 32a are 
separated circmferentially by radially-lengthwise gaps 35 and radially by 
circumferentially-lengthwise gaps 36, and the teeth 33a and 34a are 
separated by circumferentially by radially-lengthwise gaps 37 and radially 
by circumferentially-lengthwise gaps 38 (FIGS. 3 and 4). The air gaps 
produced by this spacing are sufficiently large to cause most of the 
magnetic flux to flow between the stator pole teeth and either the 
associated cylinders 16, 17, 20 and 21 or the permanent magnets in the 
adjacent rotor 10, rather than between adjacent pole teeth. In the 
particular embodiment illustrated, it will be noted that each stator 
section includes a total of 12 interdigital pole teeth equally spaced 
around the circumference, but it will be understood that different numbers 
of pole teeth may be used to achieve different stepping angles. 
Turning next to the rotor 10, 12 permanently magnetized (PM) regions 40 are 
equally spaced around the circumference of a unitary ceramic ring 41 
affixed to the hub 13. The centers of the PM regions 40 are located at 
about the same radial distance from the motor axis as the radial centers 
of the interdigital stator pole teeth 31a-34a on opposite sides of the 
rotor. Thus, a pair of working flux gaps 42 and 43 are formed between the 
opposite axial ends of the PM regions 40 and the adjacent faces of the 
pole teeth 31a-34a. The PM regions 40 are all magnetized in the axial 
direction, with each adjacent pair of PM regions being polarized in 
opposite directions as indicated by the north and south poles "N" and "S" 
indicated in FIG. 3. If desired, the permanent magnets can be preformed 
and mounted in a non-magnetizable carrier secured to the hub 13. 
Surrounding the rotor 10 in the spacer ring 26 which magnetically isolates 
the outside cylinders 20 and 21 of the two stator sections 11a and 11b so 
that magnetic flux passing between the two stator sections must pass 
through the PM regions 40 of the rotor. The spacer ring 26 also forms a 
series of six circumferentially spaced alignment members 51 and 52 
projecting laterally from opposite side surfaces of the ring into the 
spaces between the inside and outside pole teeth in each stator section. 
These alignment members 51 and 52 serve to hold the two pairs of outside 
and inside pole members 31, 32 and 33, 34 in precisely the desired 
circumferential positions relative to each other. The members 51 and 52 
are dimensioned to fit snugly between respective pairs of the outside 
teeth 31a and 33a to hold the outside pole members 31 and 33 in position. 
To hold the inside pole members 32 and 34, the inside surface of each of 
the alignment members 51 and 52 forms a central recess 51a or 52a which 
receives and holds the tip of one of the inside pole teeth 32a and 34a. 
It will be appreciated that each adjacent pair of pole teeth in each of the 
two circular arrays of interdigital stator pole teeth 31a, 32a and 33a, 
34a will always have opposite polarities because the inner and outer sets 
of teeth are coupled to radially opposite surfaces of the coils 24 and 25. 
The coils are wound circularly about the motor axis so that magnetic flux 
induced in the adjacent cylinders 16, 20 and 17, 21 is in axially opposite 
directions at the radially inner and outer surfaces of the coils. Thus, 
the magnetic flux induced in the inside cylinders 16 and 17 always has a 
polarity opposite that of the flux induced in the outside cylinders 20 and 
21. Consequently, the inside and outside sets of pole teeth in each 
interdigital pair will always have opposite polarities, with the nature of 
those polarities (i.e., north poles on the outside and south poles on the 
inside, or vice versa) depending on the direction of current flow within 
the associated coil 24 or 25. As will be described in more detail below, 
an appropriate switching circuit is provided to reverse the direction of 
current flow each time one of the coils 24 and 25 is energized to control 
the direction of stepping movement of the rotor 10. 
Whenever the coil 24 or 25 in one of the stator sections is energized, that 
section attracts the PM regions 40 of the rotor 10 into register with the 
closest (in the circumferential direction) stator pole teeth of opposite 
polarity. For example, when the stator section 11a is energized to produce 
the polarities indicated on the pole teeth 31a and 32a in FIG. 3, magnetic 
flux induced in the outside cylinder 20 passes into the outside (north) 
stator pole teeth 31a, and then into adjacent south poles of alternate PM 
regions 40 of the rotor 10. At the same time, magnetic flux induced in the 
inside cylinder 16 is joined by flux passing from the north poles of the 
intervening PM regions 40 into the inside (south) stator pole teeth 32a, 
and thence into the inside cylinder 16. From the cylinder 16, the flux 
passes through the end wall 22 of the shell 18. This pattern of magnetic 
flux, which is illustrated schematically in the upper half of FIG. 3, 
holds the rotor 19 in this "detent" position until some change occurs in 
the energization of one or both of the coils 24 and 25. 
In accordance with a further aspect of the present invention, the pole 
teeth in one of the two pairs of interdigital pole teeth are 
circumferentially offset from the pole teeth in the other pair by one-half 
pole pitch. Thus, in the illustrative embodiment each stator unit includes 
a total of 12 pole teeth (six outside teeth and six inside teeth) so that 
the pole pitch is 30.degree.; and the pole teeth 33a and 34a of the 
right-hand stator section are offset from the corresponding pole teeth 31a 
and 32a of the left-hand section by 15.degree.. It will also be noted that 
the pitch of the PM regions 40 in the rotor 10 is the same as that of each 
pair of stator teeth, namely 30.degree.. Consequently, whenever the PM 
regions 40 of the rotor 10 are in register with one pair of interdigital 
pole teeth, the other pair of interdigital pole teeth are in register with 
the spaces between the PM regions 40. For example, in the rotor position 
illustrated in FIG. 3, which is shown more clearly in FIG. 3a, the PM 
regions 40 are in register with the left-hand stator teeth 31a and 32a, so 
the right-hand stator teeth 33a and 34a are in register with the spaces 
between the PM regions 40. In FIGS. 4 and 4a, the PM regions 40 are shown 
in register with the right-hand stator teeth 33a and 34a, so the left-hand 
stator teeth 31a and 32a are in register with the spaces between the PM 
regions. 
This radial offset between the two pairs of rotor teeth provides a 
significant advantage in that one of the interdigital sets of stator pole 
teeth always provides an efficient return path for magnetic flux passing 
between the other interdigital set and the PM regions of the rotor, as 
illustrated by the magnetic flux lines in FIGS. 3 and 4. For example, it 
can be seen that the return path for the upper flux loop in FIG. 3 is 
provided by the inside stator pole teeth 34a, which is a considerably 
lower reluctance return path than that afforded by the "back iron" 
represented by the inside cylinder 17 and the shell 19. Similarly, in the 
lower flux loop shown in FIG. 3, the return path for the flux is provided 
by the inside teeth 32a, which provide a lower reluctance path than the 
"back iron" represented by the inside cylinder 16 and the shell 18. 
It will be noted that the four flux loops shown schematically in FIGS. 3 
and 4 illustrate four different operating conditions which are created 
sequentially to cause the rotor 10 to move in successive 15.degree. steps. 
More specifically, the upper flux loop in FIG. 3 represents a condition in 
which the coil 24 is energized with the current flowing in a first or 
"forward" direction (I.sub.f); the lower flux loop in FIG. 3 represents a 
condition in which the coil 25 is energized with the current flowing in a 
first or "forward" direction (I.sub.f); the upper flux loop in FIG. 4 
represents a condition in which the coil 24 is energized with the current 
flowing in a second or "reverse" direction (I.sub.r); and the lower flux 
loop in FIG. 4 represents a condition in which the coil 25 is energized 
with the current flowing in a second or "reverse" direction (I.sub.r). To 
illustrate these four conditions even more clearly, "linearized" side 
elevations of representative stator pole teeth 31a, 32a and 33a, 34 a and 
PM regions 40 are shown in FIGS. 5 through 8 for each of the four 
conditions. It should be noted that the conditions illustrated by the 
lower flux loop in FIG. 3 and the upper flux loop in FIG. 4 do not 
correspond to the actual physical positions of the rotor 10 in these 
figures, and are included only to show the path of the flux loop when the 
rotor is displaced 15.degree. from the rotor positions actually shown in 
these figures. 
In this preferred embodiment of the invention as illustrated in FIGS. 3-8, 
the rotor 10 is stepped in 15.degree. increments by successively 
de-energizing the previously energized coil, and energizing the other coil 
to advance the rotor in increments of one-half tooth pitch. Furthermore, 
each time a coil is energized, the flow of energizing current is in the 
opposite direction from the previous energization of the same coil. Thus, 
when the rotor 10 is dwelling at the position illustrated in FIGS. 3, 3a 
and 5 due to energization of the coil 24 and it is desired to step the 
rotor in the clockwise direction, the coil 24 is de-energized and the coil 
25 is energized with the current flowing in a direction to induce south 
poles in the outside teeth 33a and north poles in the inside teeth 34a 
(see FIGS. 4, 4a and 6 and the lower flux loop in FIG. 4). This causes the 
rotor to step 15.degree. in the clockwise direction so as to bring the 
north and south poles on the right-hand surfaces of the PM regions 40 into 
register with the south and north poles, respectively, of the interdigital 
pole teeth 33a and 34a of the newly energized stator section. 
Similarly, if it were desired to step the rotor 10 in the counterclockwise 
direction rather than the clockwise direction, the coil 24 would be 
de-energized and coil 25 would be energized with the current flowing in a 
direction to induce the north poles in the outside teeth 33a and south 
poles in the inside teeth 34a. This would cause the rotor to step 
15.degree. in the counterclockwise direction so as to bring the north and 
south poles on the right-hand surfaces of the PM regions 40 into register 
with the south and north poles, respectively, of the interdigital pole 
teeth 33a and 34a. 
The third condition, illustrated by FIG. 7 and the upper flux loop in FIG. 
4, causes the rotor to step another 15.degree. in the clockwise direction. 
This step is effected by de-energizing the coil 25 and energizing coil 24 
with the current flowing in a direction opposite that illustrated in FIG. 
5, thereby inducing south poles in the outside teeth 31a and north poles 
in the inside teeth 32a. In order for the north and south poles on the 
left-hand surfaces of the PM regions 40 to be brought into register with 
this new polar alignment on the stator pole teeth 31a and 32a, the rotor 
10 must step 15.degree. in the clockwise direction. 
The resulting position of the rotor relative to the two sets of 
interdigital pole teeth appears the same as in the first condition (see 
FIG. 3a), but the polarities of the teeth 31a and 32a are reversed. The 
rotor will then remain in this new detent position until some further 
change occurs in the energization of the coils 24 and 25. 
The next clockwise step of the rotor 10 is effected by the polarization 
pattern illustrated in FIG. 8 and the lower flux loop in FIG. 3. In this 
condition, the coil 24 is de-energized and the coil 25 is energized, but 
with the current flowing in a direction opposite that illustrated in FIG. 
6. Consequently, north poles are induced in the outside teeth 33a, and 
south poles are induced in the inside teeth 34a. This causes the rotor to 
step another 15.degree. in the the clockwise direction so as to bring the 
north and south poles on the right-hand surfaces of the PM regions into 
register with the south and north poles, respectively, of the teeth 33a 
and 34a of the newly energized stator section. The rotor is thus brought 
into the same position shown in FIG. 4a, relative to the two sets of 
interdigital pole teeth, but with the polarities of the teeth reversed 
from those produced by the lower flux loop in FIG. 4 in the "second" 
condition. 
This completes one full cycle of the four possible conditions in the 
preferred mode of operation of the illustrative motor, and the next rotor 
step is effected by returning to the first condition illustrated in FIG. 5 
and the upper flux loop in FIG. 3, which has already been described above. 
One of the advantages of the step motor of this invention is that it 
maintains a stable detent position even when the coils 24 and 25 are both 
de-energized. Thus, the minimum-magnetic-reluctance or detent position of 
the rotor when either of the coils 24 or 25 is energized is also the 
detent position when the coils are both de-energized and the only sources 
of magnetic flux are the PM regions 40 of the rotor. This can be seen most 
clearly in FIGS. 5-8 where that PM regions 40 are in full alignment with 
one of the two pairs of stator teeth 31a, 32a or 33a, 34a in each of the 
four possible detent positions produced by energization of one of the 
coils 24 and 25. These are the positions of minimum magnetic reluctance 
for the passage of magnetic flux between the north and south poles of the 
PM regions 40, regardless of whether the coils 24 and 25 are on or off. 
Consequently, whenever the motor is turned off, i.e., both coils 24 and 25 
are de-energized, the PM regions 40 hold the rotor 10 in a stable position 
without shifting in either direction, although the holding torque is 
weaker than the holding torque produced when one or both of the coils are 
energized. 
Another advantage of the preferred embodiment of the invention, in which 
the pole teeth in one of the interdigital pairs are radially offset from 
the pole teeth in the other pair by one-half pole pitch, is that the 
holding torque is maximized in both the energized and de-energized state 
of the motor. This is due to the fact that one of the two sets of 
interdigital pole teeth is always in register with the spaces between the 
PM regions 40, thereby providing a low-reluctance return path for the 
magnetic flux passing between the north and south poles of the PM regions 
40 of the rotor. This low-reluctance path is to be contrasted with the 
path that the flux would follow if the two pairs of stator pole teeth were 
aligned with each other; in this case the flux would always have to pass 
through the longer and higher reluctance path formed by the inside and 
outside bylinders 16, 20 and 17, 21 and the end walls 22, 23 on both sides 
of the rotor. This would be true in both the on and off conditions of the 
motor. However, by radially offsetting the two pairs of stator pole teeth 
from each other, the flux path is always reduced on one side of the rotor, 
as can be seen from the flux patterns illustrated in FIGS. 3-8. This 
maximizes the strength of the magnetic field in the working flux gaps 42 
and 43, which maximizes not only the holding torque but also the dynamic 
output torque of the motor. Moreover, this high output torque is achieved 
with a relatively small outside diameter and overall volume. 
A circuit for controlling energization and de-energization of the coils 24 
and 25 from a voltage source V1 is shown in simplified form in FIG. 9. 
While the switching devices SA1, SA2, SB1, and SB2 in this circuit will 
usually be in the form of transistors or other solid state devices adapted 
to be rendered conductive and non-conductive in response to control 
signals, the switching devices are illustrated in FIG. 9 simply as on-off 
switches adapted to be sequentially closed and opened. Switches SA1 and 
SA2 control the current flow through coil 24, while switches SB1 and SB2 
control the current flow through coil 25. 
It can be seen that there are two possible connections for each of the two 
coils 24 and 25. When switch SA1 is closed and the other three switches 
SA2, SB1, and SB2 are open, the coil 24 is energized with current I.sub.f 
flowing downwardly through the coil as viewed in FIG. 9 to produce the 
"first" flux pattern illustrated in FIG. 5 and by the upper flux loop in 
FIG. 3. To de-energize the coil 24 and energize coil 25 with current 
I.sub.f flowing downwardly through the coil, the switch SA1 is opened, 
switch SB1 is closed, and the other two switches SA2 and SB2 remain open. 
This produces the "second" flux pattern illustrated in FIG. 6 and by the 
lower flux loop in FIG. 4. 
To de-energize coil 25 and energize the coil 24 with current I.sub.r 
flowing in the reverse direction, the switch SB1 is opened and switch 
SA.sub.2 is closed. This produces the "third" flux pattern illustrated in 
FIG. 7 and by the upper flux loop in FIG. 4. For the "fourth" flux pattern 
illustrated in FIG. 8 and by the lower flux loop in FIG. 3, the switch SA2 
is opened and switch SB2 is closed. This energizes the coil 25 with 
current I.sub.r flowing in the reverse direction, while de-energizing coil 
24. 
Thus, it can be seen that by closing the four switches SA1, SA2, SB1, and 
SB2 one at a time, with the other three switches always being open, the 
four flux patterns illustrated in FIGS. 3-8 can be produced to step the 
rotor 10 in 15.degree. increments. The sequence of switch closures 
required to produce this sequential stepping action is illustrated in the 
following table, in which an "x" represents a switch closure: 
______________________________________ 
STEP ACTUATED SWITCH 
NO. SA1 SA2 SB1 SB2 
______________________________________ 
0 X 
1 X 
2 X 
3 X 
4 X 
5 X 
6 X 
______________________________________ 
To drive the rotor in the opposite direction, the sequence of switch 
closures in the above table is simply reversed. 
The pattern of coil excitation produced by the switching sequence described 
above is illustrated by the waveforms in FIG. 10 as a function of time. It 
will be noted that a separate waveform is illustrated for each of the two 
coils 24 and 25, and a "+" region in either waveform represents current 
flow in one direction, e.g., I.sub.f, and a "-" region represents current 
flow in the opposite direction, e.g., I.sub.r. Thus, waveform A indicates 
that coil 24 is first energized with current flow I.sub.f during the time 
interval t.sub.0 to t.sub.1. As indicated by waveform B, coil 25 is 
de-energized during the interval t.sub.0 to t.sub.1. At time t.sub.1, 
coil 24 is de-energized, and coil 25 is energized with current flow 
I.sub.f from time t.sub.1 to t.sub.2. At this point, coil 25 is 
de-energized (waveform B) and coil 24 is re-energized with current flow 
I.sub.r from time t.sub.2 to t.sub.3 (waveform A). Coil 24 is then 
de-energized again at time t.sub.3 (waveform A), and coil 25 is 
re-energized with current flow I.sub.r from time t.sub.3 to t.sub.4 
(waveform B). It will be appreciated that the rotor advances in successive 
steps of 15.degree. each at the beginning of each of the intervals t.sub.0 
-t.sub.1, t.sub.1 -t.sub.2, t.sub.2 -t.sub.3, and t.sub.3 -t.sub.4. 
A modified excitation pattern is illustrated in FIG. 11 for a variation of 
the invention in which the two sets of axially spaced pole teeth are in 
circumferential alignment with each other, rather than being offset by 
one-half tooth pitch. In this case, the two sets of pole teeth are both 
energized at the same time but with staggered polarities, and the stable 
"detent" position of the rotor 10 is always a position where the PM 
regions 40 are located midway between (in the circumferential direction) 
the pole teeth on opposite sides thereof. Thus, referring to FIG. 11, from 
time t.sub.0 to t.sub.2, coil 24 is energized with current flow I.sub.r to 
produce south poles on the outside teeth 31a and north poles on the inside 
teeth 31a. During the interval from time t.sub.0 to t.sub.1, coil 25 is 
energized with current flow I.sub.f to produce north poles on the outside 
teeth 33a and south poles on the inside teeth 34a. Thus the rotor will be 
positioned with the PM regions 40 in circumferential alignment with the 
stator pole teeth because each axially opposed pair of teeth have opposite 
polarities. 
At time t.sub.1, the excitation of coil 24 remains unchanged, but the 
current flow through coil 25 is reversed from I.sub.f to I.sub.r, thereby 
producing south poles on the outside teeth 33a and north poles on the 
inside teeth 34a. This causes the rotor to step 15.degree. to a new stable 
position where the PM regions 40 are in circumferential alignment with the 
spaces between the stator pole teeth because each axially opposed pair of 
teeth have the same polarity. With this polarization pattern, the opposed 
magnetic forces on the rotor 10 are in equilibrium when the PM regions are 
aligned with the spaces between the pole teeth. 
At time t.sub.2, the energization of coil 25 remains unchanged, but the 
current flow is reversed from I.sub.r to I.sub.f in coil 24, thereby 
producing north poles on the outside teeth 31a and south poles on the 
inside teeth 32a. Thus, the axially opposed pairs of stator teeth once 
again have opposite polarities, thereby causing the rotor to step another 
15.degree. to a new stable position where the PM regions 40 are again in 
circumferential alignment with the stator pole teeth. 
At time t.sub.3, coil 24 continues to be energized with current flow 
I.sub.f, and the current flow through coil 25 is reversed from I.sub.r to 
I.sub.f, thereby changing the polarity of the teeth 33a, 34a from S-N to 
N-S and stepping the rotor another 15.degree.. In this new stable position 
of the rotor, the PM regions 40 are again in circumferential alignment 
with the spaces between the stator pole teeth because the axially opposed 
pairs of teeth again have the same polarity. The stepping movement 
continues in this manner as long as the staggered energization pattern of 
FIG. 11 continues. 
While the invention has been described with specific reference to the use 
of the ring carrying the permanent magnets as a rotor, and the use of the 
two pairs of interdigital pole teeth as parts of the stator, it should be 
noted that the roles of these elements can be reversed. Thus, the ring 41 
carrying the permanent magnets can be journaled on the shaft 12 and fixed 
to the non-magnetic spacer 26 so that it functions as a stator, with the 
two pairs of interdigital pole teeth being keyed to the shaft 12 to 
function as the rotor. 
As can be seen from the foregoing detailed description, this invention 
provides an improved step motor which offers a relatively high output 
torque for any given outside diameter or volume. This improved motor can 
be mass produced at a relatively low cost, and without the use of complex 
tooling because of the relatively simple design of the various components. 
The improved motor also provides positive detenting when the motor is 
turned off, so that the motor maintains its output shaft at the same 
position held by the shaft just before the motor is turned off. Together 
with its simple construction, the motor has a long operating life.