U.S. patent application number 09/895269 was filed with the patent office on 2002-01-24 for motor for an electric power steering assembly.
This patent application is currently assigned to MITSUBISHI DENKI KABUSHIKI KAISHA. Invention is credited to Daikoku, Akihiro, Ikeda, Ryuichi, Imagi, Akihiko, Sakabe, Shigekazu, Tanaka, Toshinori, Yamamoto, Kyouhei, Yoshikuwa, Yoshio.
Application Number | 20020008430 09/895269 |
Document ID | / |
Family ID | 26347550 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020008430 |
Kind Code |
A1 |
Tanaka, Toshinori ; et
al. |
January 24, 2002 |
Motor for an electric power steering assembly
Abstract
A motor for an electric power steering assembly comprises a
yoke, a multi-polar magnetic field portion composed of at least
four poles secured to the inner wall of the yoke, a shaft disposed
within the yoke so as to be able to rotate freely, an armature
secured to the shaft having a winding constructed by winding wiring
into an even number of slots formed on the outer circumferential
surface of a core so as to extend in the axial direction thereof, a
commutator comprising a plurality of segments secured to an end
portion of the shaft; and a plurality of brushes contacting the
surface of the commutator.
Inventors: |
Tanaka, Toshinori; (Tokyo,
JP) ; Ikeda, Ryuichi; (Tokyo, JP) ; Sakabe,
Shigekazu; (Tokyo, JP) ; Daikoku, Akihiro;
(Tokyo, JP) ; Imagi, Akihiko; (Tokyo, JP) ;
Yoshikuwa, Yoshio; (Tokyo, JP) ; Yamamoto,
Kyouhei; (Tokyo, JP) |
Correspondence
Address: |
SUGHRUE, MION, ZINN,
MACPEAK & SEAS, PLLC
2100 Pennsylvania Avenue, N.W.
Washington
DC
20037-3213
US
|
Assignee: |
MITSUBISHI DENKI KABUSHIKI
KAISHA
|
Family ID: |
26347550 |
Appl. No.: |
09/895269 |
Filed: |
July 2, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09895269 |
Jul 2, 2001 |
|
|
|
09313344 |
May 18, 1999 |
|
|
|
Current U.S.
Class: |
310/68C ;
310/233 |
Current CPC
Class: |
H02K 23/30 20130101;
B62D 5/0403 20130101; H02K 3/28 20130101 |
Class at
Publication: |
310/68.00C ;
310/233 |
International
Class: |
H02K 011/00; H02K
013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 29, 1998 |
JP |
10-182487 |
Jan 20, 1999 |
JP |
11-12017 |
Claims
What is claimed is:
1. A motor for an electric power steering assembly comprising: a
yoke; a multi-polar magnetic field portion composed of at least
four poles secured to the inner wall of said yoke; a shaft disposed
within said yoke so as to be able to rotate freely; an armature
secured to said shaft having a winding constructed by lap winding
wiring into an even number of slots formed on the outer
circumferential surface of a core so as to extend in the axial
direction thereof; a commutator comprising a plurality of segments
secured to an end portion of said shaft; and a plurality of brushes
contacting the surface of said commutator.
2. The motor for an electric power steering assembly according to
claim 1 wherein the number of said slots is even and is not a
multiple of the number of said poles.
3. A motor for an electric power steering assembly comprising: a
yoke; a multi-polar magnetic field portion composed of at least
four poles secured to the inner wall of said yoke; a shaft disposed
within said yoke so as to be able to rotate freely; an armature
secured to said shaft having a winding constructed by lap winding
wiring into a number of slots being a multiple of the number of
pairs of said poles, said slots being formed on the outer
circumferential surface of a core so as to extend in the axial
direction thereof; a commutator comprising a plurality of segments
secured to an end portion of said shaft; and a plurality of brushes
contacting the surface of said commutator.
4. The motor for an electric power steering assembly according to
claim 3 wherein the number of said slots is a multiple of the
number of pairs of said poles and is not a multiple of the number
of said poles.
5. The motor for an electric power steering assembly according to
claim 1 comprising equalizing members for preventing circulating
currents from flowing through said brushes due to differences in
induced electromotive forces arising between circuits within the
circuits of said armature.
6. The motor for an electric power steering assembly according to
claim 5 wherein Ns/(n.times.2).ltoreq.K.ltoreq.Ns, where K is the
number of said equalizing members, Ns is the number of said slots
in said core, and n is the maximum number of segments covered by
said brushes.
7. The motor for an electric power steering assembly according to
claim 1 wherein the current passing through said winding is
controlled by pulse width modulation (PWM) driving.
8. The motor for an electric power steering assembly according to
claim 1 wherein said wiring is enamel-coated round wire.
9. The motor for an electric power steering assembly according to
claim 1 wherein said magnetic field portion comprises a plurality
of permanent magnets disposed so as to be spaced around the inner
wall of said yoke.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a motor for an electric
power steering assembly for assisting the steering force of an
automotive steering wheel.
[0003] 2. Description of the Related Art
[0004] FIG. 15 is a cross-section of a conventional motor for an
electric power steering assembly (hereinafter "electric motor")
100. The electric motor 100 comprises: a cylindrical yoke 1; two
permanent field magnets 2 arranged circumferentially and secured so
as to face each other inside the yoke 1; a shaft 4 disposed inside
the yoke 1 by means of bearings 3 so as to be able to rotate
freely; an armature 5 secured to the shaft 4; a commutator 6
comprising a plurality of copper segments 16 secured to an end
portion of the shaft 4; and brushes 8 placed in contact with the
surface of the commutator 6 by the elastic force of springs 7.
[0005] The armature 5 comprises: a core 9 having a plurality of
slots 11 extending in the axial direction; and a winding 10
constructed by winding wiring into the slots 11 by a lap winding
method.
[0006] In the above 2-pole lap-wound electric motor 100, an
electric current is supplied to the winding 10 from outside by
means of the brushes 8 contacting the segments 16, whereby the
armature 5 rotates together with the shaft 4 due to electromagnetic
action.
[0007] Since the above electric motor 100 is mainly used in
relatively light-weight low-capacity automobiles, the assisting
torque from the electric motor 100 is small and consequently the
operating noise of the electric motor 100 is extremely small so
small that it is practically unnoticeable inside the
automobile.
[0008] However, now that fuel-conservation and weight reduction are
required even in heavy-weight middle- and high-capacity automobiles
due to public demand for fuel efficiency, reduced exhaust
emissions, etc., direct-current motor power steering assemblies are
starting to replace hydraulic power steering assemblies. Electric
motors providing large torque are required in such cases, but since
2-pole lap-wound designs result in large-bodied motors, it is
necessary to increase the number of poles to four or so to reduce
size and produce high torque.
[0009] FIGS. 16 and 17 show comparisons between a 2-pole 14-slot
direct-current motor (hereinafter "2-pole motor") and a 4-pole
21-slot direct-current motor (hereinafter "4-pole motor") given as
an example of a multi-polar machine. These figures show the
differences in magnetic attraction acting on the armatures in
2-pole and 4-pole motors when the armatures are off center and were
obtained by magnetic field analysis by the present inventors. In
FIG. 16, ".circle-solid." represents the center of the stator, that
is, the original center of rotation, and "x " represents the center
of rotation when off center. In FIG. 17, "" represents the force of
eccentricity direction, "" represents the force of right angle
thereof. As can be seen from the figure, vibrations and noise are
generated more easily in a 4-pole motor than in a 2-pole motor.
[0010] That is, when the forces acting on the armatures were
examined with each being placed off center by the same amount (0.1
mm) from the original central position in every angle of
eccentricity from 0 degrees to 360 degrees, the maximum magnetic
attraction acting in the direction of eccentricity in the 4-pole
motor was approximately 2.7 N, or six times the maximum magnetic
attraction acting in the direction of eccentricity in the 2-pole
motor which was approximately 0.45 N. In the 2-pole motor, the
direction of magnetic attraction due to eccentricity can be clearly
seen, and when the force acting is compared to the angle of
eccentricity it is found that when the eccentricity is between the
poles (an angle of eccentricity of 90 degrees or 270 degrees)
approximately twice as much magnetic attraction (0.45/0.21) acts as
when the eccentricity is directed towards the center of a pole (an
angle of eccentricity of 0 degrees or 180 degrees). In the 4-pole
motor, on the other hand, no clear direction can be seen. That is,
the force in the direction of eccentricity is approximately 2.7 N
for every angle of eccentricity from 0 degrees to 360 degrees,
which means that there is a direction of stability with respect to
eccentricity in a 2-pole motor, but no such direction exists in a
4-pole motor, and this difference can be considered to be related
to the differences in vibration and noise.
[0011] Thus, it is necessary to increase the number of poles to
four or so in order to reduce size and produce high torque, but
problems of vibration and noise remain.
[0012] Now, apart from lap winding, wave winding may also be
considered as a winding method for armatures when the number of
poles is increased in order to reduce size and increase torque.
With a lap winding, the number of brushes provided is generally the
same as the number of poles, but with a wave winding two brushes
are generally provided.
[0013] FIGS. 18 and 19 are sets of diagrams and graphs showing the
magnetic attraction acting on a 4-pole 21-slot armature given as an
multi-polar example, FIG. 18 showing a case with a lap winding and
four brushes and FIG. 19 showing a case with a wave winding and two
brushes. In FIGS. 18 and 19, "" represents 100% current flows
perpendicular to the paper in an upward direction, "" represents
100% current flows perpendicular to the paper in an downward
direction, "" represents 50% current flows perpendicular to the
paper in an downward direction, "" represents 50% current flows
perpendicular to the paper in an upward direction, and "{circle
over (O)}" represents current does not flow.
[0014] Comparing the two figures, we see that whereas in the case
of wave winding the magnetic attraction acting on the armature as
the armature turns by one slot of the core is always directed in a
given radially-outward direction as indicated by the arrow A, in
the case of a lap-wound 21-slot armature, the magnetic attraction
moves circumferentially as indicated by the arrow B, and one
problem with a lap-wound 21-slot armature is that rotational
vibrations arise easily, making the generation of operating noise
that much more likely.
[0015] In the case of a multi-polar odd numbered-slot lap winding,
another problem is that differences arise in the electromotive
forces induced among the circuits of the winding of the armature
due to the influences of imbalances in the electromagnetic circuit
of the yoke, eccentricities in the armature, nonuniform electric
currents flowing through the brushes, engineering errors, etc.,
giving rise to circulating currents within the armature flowing
through the brushes, and as a result the commutating action of the
brushes deteriorates, leading to increases in temperature,
shortened working life, increases in torque ripples in the brushes
and the commutator which accompany an increase in commutation
sparks generated by the brushes, as well as the combined effects
thereof, thereby increasing operating noise.
[0016] At the same time, in the case of a multi-pole odd
numbered-slot wave winding, there are problems such as torque
ripples increasing in magnitude and workability deteriorating due
to increased thickness of the winding in order to reduce the number
of parallel circuits, etc.
SUMMARY OF THE INVENTION
[0017] The present invention aims to solve the above problems and
an object of the present invention is to provide a motor for an
electric power steering assembly enabling reduced operating
noise.
[0018] In order to achieve the above object, according to one
aspect of the present invention, there is provided a motor for an
electric power steering assembly comprising: a yoke; a multi-polar
magnetic field portion composed of at least four poles secured to
the inner wall of the yoke; a shaft disposed within the yoke so as
to be able to rotate freely; an armature secured to the shaft
having a winding constructed by lap winding wiring into an even
number of slots formed on the outer circumferential surface of a
core so as to extend in the axial direction thereof; a commutator
comprising a plurality of segments secured to an end portion of the
shaft; and a plurality of brushes contacting the surface of the
commutator.
[0019] According to one form of the present invention, there is
provided a motor for an electric power steering assembly wherein
the number of slots is even and is not a multiple of the number of
poles.
[0020] According to another aspect of the present invention, there
is provided a motor for an electric power steering assembly
comprising: a yoke; a multi-polar magnetic field portion composed
of at least four poles secured to the inner wall of the yoke; a
shaft disposed within the yoke so as to be able to rotate freely;
an armature secured to the shaft having a winding constructed by
lap winding wiring into a number of slots being a multiple of the
number of pairs of poles, the slots being formed on the outer
circumferential surface of a core so as to extend in the axial
direction thereof; a commutator comprising a plurality of segments
secured to an end portion of the shaft; and a plurality of brushes
contacting the surface of the commutator.
[0021] According to one form of the present invention, there is
provided a motor for an electric power steering assembly wherein
the number of slots is a multiple of the number of pairs of poles
and is not a multiple of the number of poles.
[0022] According to another form of the present invention, there is
provided a motor for an electric power steering assembly comprising
equalizing members for preventing circulating currents from flowing
through the brushes due to differences in induced electromotive
forces arising between circuits within the circuits of the
armature.
[0023] According to still another form of the present invention,
there is provided a motor for an electric power steering assembly
wherein Ns/(n.times.2).ltoreq.K.ltoreq.Ns, where K is the number of
equalizing members, Ns is the number of slots in the core, and n is
the maximum number of segments covered by the brushes.
[0024] According to one form of the present invention, there is
provided a motor for an electric power steering assembly wherein
the current passing through the winding is controlled by pulse
width modulation (PWM) driving.
[0025] According to another form of the present invention, there is
provided a motor for an electric power steering assembly wherein
the wiring is enamel-coated round wire.
[0026] According to still another form of the present invention,
there is provided a motor for an electric power steering assembly
wherein the magnetic field portion comprises a plurality of
permanent magnets disposed so as to be spaced around the inner wall
of the yoke.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a partial cross-section of a motor for an electric
power steering assembly according to Embodiment 1 of the present
invention;
[0028] FIG. 2 is an enlargement of part of FIG. 1;
[0029] FIG. 3(a) is a developed front elevation of the equalizer
main body in FIG. 1;
[0030] FIG. 3(b) is a side elevation of FIG. 3(a);
[0031] FIG. 4(a) is a front elevation of the base of the equalizer
main body in FIG. 1;
[0032] FIG. 4(b) is a side elevation of FIG. 4(a);
[0033] FIG. 5 is a front elevation of a terminal of the equalizer
main body in FIG. 1;
[0034] FIG. 6 is a front elevation of an insulating plate of the
equalizer main body in FIG. 1;
[0035] FIG. 7 is a set of diagrams and graphs explaining magnetic
attraction acting on an armature having four poles, a lap winding,
four brushes, and twenty-two slots;
[0036] FIG. 8 is a front elevation showing another example of a
terminal;
[0037] FIG. 9 is a cross-section showing another example of an
armature;
[0038] FIG. 10 is an enlargement of part of FIG. 9;
[0039] FIG. 11 is a graph showing the relationship between the
number of terminals and auditory evaluation;
[0040] FIG. 12 is a graph showing the relationship between motor
output class and motor operating noise for various types of
motor;
[0041] FIG. 13 is a perspective view showing a motor for an
electric power steering assembly mounted on a pinion;
[0042] FIG. 14 is a graph showing the relationships between control
gain, fluctuation in torque, and magnetic attraction in a radial
direction;
[0043] FIG. 15 is a cross-section of a conventional motor for an
electric power steering assembly;
[0044] FIG. 16 is a set of diagrams explaining magnetic attraction
in a 2-pole motor and in a 4-pole motor;
[0045] FIG. 17 is a set of graphs explaining magnetic attraction in
a 2-pole motor and in a 4-pole motor;
[0046] FIG. 18 is a set of diagrams and graphs explaining magnetic
attraction and torque ripples in a 4-pole lap-wound 21-slot 4-brush
motor for an electric power steering assembly;
[0047] FIG. 19 is a set of diagrams and graphs explaining magnetic
attraction and torque ripples in a 4-pole wave-wound 21-slot
2-brush motor for an electric power steering assembly;
[0048] FIG. 20 is a block diagram for a control unit;
[0049] FIG. 21 is a set of diagrams and graphs explaining magnetic
attraction and torque ripples in a 4-pole lap-wound 24-slot 4-brush
motor for an electric power steering assembly;
[0050] FIG. 22 is a set of diagrams and graphs explaining magnetic
attraction and torque ripples in a 4-pole lap-wound 20-slot 4-brush
motor for an electric power steering assembly;
[0051] FIG. 23 is a set of diagrams and graphs explaining magnetic
attraction and torque ripples in a 4-pole lap-wound 26-slot 4-brush
motor for an electric power steering assembly;
[0052] FIG. 24 is a set of diagrams and graphs explaining magnetic
attraction and torque ripples in a 4-pole lap-wound 28-slot 4-brush
motor for an electric power steering assembly;
[0053] FIG. 25 is a table showing the relationship between torque
ripples and magnetic attraction in 4-pole lap-wound 20-, 21-, 22-,
24-, 26-, and 28-slot 4-brush motors for electric power steering
assemblies;
[0054] FIG. 26 is a set of diagrams and graphs explaining magnetic
attraction and torque ripples in a 6-pole lap-wound 25-plot 6-brush
motor for an electric power steering assembly;
[0055] FIG. 27 is a set of diagrams and graphs explaining magnetic
attraction and torque ripples in a 6-pole lap-wound 24-slot 6-brush
motor for an electric power steering assembly;
[0056] FIG. 28 is a set of diagrams and graphs explaining magnetic
attraction and torque ripples in a 6-pole lap-wound 22-slot 6-brush
motor for an electric power steering assembly;
[0057] FIG. 29 is a set of diagrams and graphs explaining magnetic
attraction and torque ripples in a 6-pole lap-wound 26-slot 6-brush
motor for an electric power steering assembly;
[0058] FIG. 30 is a set of diagrams and graphs explaining magnetic
attraction and torque ripples in a 6-pole lap-wound 21-slot 6-brush
motor for an electric power steering assembly;
[0059] FIG. 31 is a set of diagrams and graphs explaining magnetic
attraction and torque ripples in a 6-pole lap-wound 27-slot 6-brush
motor for an electric power steering assembly; and
[0060] FIG. 32 is a table showing the relationship between torque
ripples and magnetic attraction in 6-pole lap-wound 21-, 22-, 24-,
25-, 26-, and 27-slot 6-brush motors for electric power steering
assemblies.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Embodiment 1
[0061] An example of a motor for an electric power steering
assembly (hereinafter "electric motor") according to the present
invention will now be explained, and parts the same as or
corresponding to those of the conventional example will be given
the same numbering.
[0062] FIG. 1 is a cross-section of the internal construction of an
electric motor according to Embodiment 1 of the present invention,
and FIG. 2 is an enlargement of part of FIG. 1. The electric motor
comprises: a cylindrical yoke 1; four permanent magnets 2 composed
of ferrite spaced circumferentially and secured inside the yoke 1;
a shaft 4 disposed inside the yoke 1 by means of bearings 3 so as
to be able to rotate freely; an armature 20 secured to the shaft 4;
a commutator 6 comprising a plurality of segments 16 secured to an
end portion of the shaft 4; four brushes 8 spaced equidistantly and
placed in contact with the surface of the commutator 6 by the
elastic force of springs 7; and an equalizer main body 22 secured
to the shaft 4 between the armature 20 and the commutator 6.
Moreover, the yoke 1, permanent magnets 2, bearings 3, springs 7,
and brush 8 are not shown in these figures.
[0063] The armature 20 comprises: a core 9 having twenty-four slots
11 extending in the axial direction; and a winding 21 constructed
by winding wiring 19 into the slots 11 by a lap winding method.
[0064] FIG. 3(a) is a developed front elevation of the equalizer
main body 22, and FIG. 3(b) is a side elevation of the equalizer
main body 22 in FIG. 3(a). The equalizer main body 22 comprises:
twelve terminals 24 composed of copper plate, etc., which are
equalizing members; and twelve insulating plates 25, alternately
stacked in layers on a base 23.
[0065] FIGS. 4(a) and 4(b) are a front elevation and a side
elevation, respectively, of the base 23. Twelve pins 26 are
arranged so as to stand equidistantly around the circumference of a
toric base main body 27.
[0066] FIG. 5 is a front elevation of a terminal 24 being an
equalizing member. Apertures 29 are formed at 24 points spaced
equidistantly around the circumference of the annular terminal main
body 28 of each of the terminals 24 being equalizing members.
Furthermore, equalizer lead portions 30a, 30b extending radially
outwards in opposite directions are disposed on the terminal main
body 28.
[0067] FIG. 6 is a front elevation of an insulating plate 25.
Apertures 32 are formed at 24 points spaced equidistantly around
the circumference of the annular insulating plate main body 31 of
each of the insulating plates 25.
[0068] In the above electric motor, the equalizer main body 22 is
assembled by alternately stacking the twelve terminals 24 and the
twelve insulating plates 25 on the base 23. During this process,
each successive terminal 24 is rotated by 15 degrees and the
terminals 24 are secured to the base 23 by passing the pins 26 of
the base 23 through the apertures 29 in the terminals 24.
Furthermore, the insulating plates 25 are secured to the base 23 by
passing the pins 26 of the base 23 through the apertures 32 in the
insulating plates 25. Then, the equalizer main body 22 is
integrated by crimping the ends of the pins 26.
[0069] Next, the equalizer main body 22 and the commutator 6 are
fitted onto the shaft 4 in that order. Protrusions 14 extending in
the axial direction are formed on the shaft 4 in order to position
the equalizer main body 22 and the commutator 6 relative to the
direction of rotation, and the base 23 and the commutator main body
15, which are both composed of phenol resin, are secured to the
protrusions 14 by elastic deformation.
[0070] Next, the armature 20 is formed by bending the equalizer
lead portions 30a, 30b to align with hooks 34, and forming the
winding 21 by winding the wiring 19 onto the core 9 by a lap
winding method, then the equalizer lead portions 30a, 30b and the
hooks 34 are electrically connected at twenty-four points by
simultaneous fusion or the like.
[0071] An electric motor of the above construction has four
magnetic poles, twenty-four slots 11, a lap winding, and a 4-brush
system. FIG. 21 is a set of diagrams and graphs of magnetic
attraction and torque ripples acting on the armature 20 in the
above motor which the present inventors obtained by magnetic field
analysis. Whereas in the case of the 4-pole lap-wound 4-brush,
21-slot armature of FIG. 18 described above, the magnetic
attraction acting on the armature moves circumferentially and
rotational vibrations arise easily, making the generation of
operating noise that much more likely, it is clear that in the case
of a 24-slot lap-wound armature, the total magnetic attraction
acting on the armature is zero and that operating noise therefore
does not arise due to rotational vibrations.
[0072] FIG. 7 is a set of diagrams and graphs of magnetic
attraction and torque ripples acting on an armature having four
poles, a lap winding, and an even numbered twenty-two slots which
the present inventors obtained by magnetic field analysis.
[0073] As can be seen from the figure, in the case of a 22-slot
lap-wound armature, the total magnetic attraction acting on the
armature is also zero and operating noise therefore does not arise
due to rotational vibrations.
[0074] Furthermore, whereas in the case of the wave-wound armature
of FIG. 19 described above, the torque ripples (p-p) represented by
the ratio of vertical variance in the torque wave to the total
torque are 1.37 percent, in the case of the 22-slot lap-wound
armature the torque ripples (p-p) are smaller than the wave-wound
case at 0.876 percent. For that reason, in an electric motor 18
driven by pulse width modulation (PWM) by means of a motor drive
signal from a control unit 13 as shown in FIG. 20, the torque
ripples are reduced, improving the feel of the steering wheel 12 to
the driver compared with a wave-wound electric motor.
[0075] Moreover, annular terminal main bodies 28 are used in an
equalizer main body 22 of the above construction, but arc-shaped
terminal main bodies 50 may be used in terminals 52, as shown in
FIG. 8, in order to conserve the amount of copper material
used.
[0076] Furthermore, as shown in FIGS. 9 and 10, six terminals 24
and six insulating plates 25 of an equalizer main body 60 may be
alternately stacked on the base 23 and a terminal 24 electrically
connected to every second hook 34, or a terminal may be
electrically connected to every third hook 34.
[0077] In order to prevent circulating currents from flowing
through the brushes due to differences in the induced electromotive
force arising between the circuits, the greater the number of
terminals being equalizing members the greater the effect, but as
explained above, the number may be reduced to allow for easier
production and lower costs for the equalizer main body.
[0078] It was found that operating noise was smallest when the
number of terminals satisfied the equation
Ns/(n.times.2).ltoreq.K.ltoreq.Ns, where K is the number of
terminals, Ns is the number of slots in the core, and n is the
maximum number of segments covered by the brushes. FIG. 11 is an
evaluation for the case where Ns=22 and n=3, and the above formula
satisfies the evaluation criteria, where six or more out of ten is
passable.
[0079] Furthermore, in an electric motor of the above construction,
machine winding of the wiring 19 of the winding 21 using
enamel-coated round wire is possible in order to reduce production
costs and enable mass production, but even a wiring machine cannot
wind in perfect rows and there is a risk that irregularities in the
resistance and inductance between circuits of the winding will
increase. However, because circulating currents are prevented from
flowing through the brushes due to differences in the induced
electromotive force arising between the circuits by the provision
of the equalizer main body 22, problems arising from irregularities
in the resistance and inductance between circuits of the winding do
not occur.
[0080] Furthermore, in an electric motor of the above construction,
permanent field magnets 2 composed of ferrite are used in order to
reduce the torque ripples most associated with steering. When the
field is generated by electromagnets, the magnetic flux density is
generally higher than that of permanent magnets, intensifying the
changes in flux density in the gap as the slots and the teeth of
the core alternately face the poles due to changes in position in
the direction of rotation of the armature, thereby increasing
torque ripples. Whereas the average flux density in the gap in the
case of permanent ferrite field magnets is normally approximately
0.3 to 0.4 Tesla, it is approximately double in the case of
electromagnets at 0.7 to 0.8 Tesla, and in the case of
electromagnets, torque ripples increase, fluctuations in magnetic
attraction also increase at the teeth of the core, and
electromagnetic noise also increases. Furthermore, when permanent
ferrite field magnets are used, it becomes possible to reduce the
size of the motor, simplify the assembly operation, and reducing
costs.
[0081] Thus, it is effective to use permanent ferrite field magnets
in an electric motor, but in that case, since the magnetic flux
density of the field is low, it is necessary to increase the number
of winds of the wiring in the armature to ensure torque quality.
For that reason, the field magnets are greatly affected by the
reaction from the armature, and the magnetic center of the flux
distribution of the magnetic field poles is shifted greatly in the
opposite direction to the rotational direction of the armature. In
an ordinary motor, this shift in magnetic center is compensated for
by offsetting the brushes from the geometric center of the magnetic
poles in the opposite direction to the rotational direction of the
armature to obtain a good flux distribution. However, because this
electric motor rotates in both directions, it is not possible to
compensate for shifts in magnetic center by offsetting the brushes
in the opposite direction to the rotational direction of the
armature in order to obtain a good flux distribution.
[0082] Consequently, in this electric motor, good flux distribution
is ensured by improving the balance of the induced voltage in each
of the circuits of the winding by providing an equalizer main body
22 on the armature 20 in order to compensate for poor flux
distribution, and the special effects described below are
obtained.
[0083] (1) Because the operating noise of this electric motor is
reduced as shown in FIG. 12, the driver does not notice any
unpleasant operating noise while steering, even if this electric
motor is mounted on the steering column. Moreover, since this
electric motor can be mounted on the column within the automobile
cabin, it is placed in a more advantageous environment with respect
to heat and water than a conventional electric motor 100 which is
mounted, for example, on a rack 40 in the engine compartment as
shown in FIG. 13, enabling this electric motor to be manufactured
more cheaply.
[0084] (2) Because this electric motor adopts a lap-wound 4-brush
method, torque ripples can be reduced, and even if this motor is
driven by pulse width modulation (PWM) by means of a motor drive
signal from a control unit 13, vibrations transmitted to the
steering wheel 12 during activation of this electric motor are
practically nonexistent, preventing deterioration of the feel of
the steering wheel to the driver.
[0085] Furthermore, because torque ripples are reduced in this
electric motor, the degree of freedom in designing the PWM driving
method of the control unit 13 is increased, allowing improvements
in responsiveness and microcurrent control to be introduced,
further improving the feel of the steering wheel.
[0086] Furthermore, holding noise (the noise generated by
vibrations caused by an electric motor caused by changes in torque
due to changes in the current flowing through the armature
resulting from minute changes in the contact between the brushes 8
and the segments 16 when the steering wheel 12 is held in a given
position; or the vibrating noise generated in the period of minute
displacement due to backlash from the system when an electric motor
is not active) can be reduced. In a conventional wave-wound 2-brush
method, torque ripples are large and holding noise is easily
generated, but when attempts are made to suppress the generation of
this holding noise by means of the control unit 13 by increasing
control gain, torque fluctuations indicating the degree of holding
noise are reduced as shown in FIG. 14, while operating noise
(magnetic attraction in the radial direction) is increased, and it
is not possible to suppress both holding noise and operating noise
simultaneously. On the other hand, in this electric motor employing
a lap-wound 4-brush method, it is possible to suppress both holding
noise and operating noise simultaneously.
[0087] (3) Because this electric motor adopts a lap-wound 4-brush
method, the current density in the brushes 8 can be reduced,
enabling the allowable current-bearing time of this electric motor
to be lengthened. During reverse parking, U-turns, etc., the
steering wheel 12 is frequently turned to its maximum angle and
used in a so-called "stationary steering" or "end locked" state,
but at that time the armature of an electric motor hardly rotates
at all while torque is still being generated, and the electric
motor is used in a constrained state. This electric motor allows
the current density in the brushes 8 to be reduced at that time,
when temperature increases are harshest, enabling the allowable
period of use in a "stationary steering" or "end locked" state to
be lengthened, thereby increasing the utility of the electric
motor.
[0088] Furthermore, the working life of the brushes 8 is lengthened
thereby, improving the reliability and durability of the electric
motor.
[0089] (4) Because this electric motor adopts a lap-wound 4-brush
method, the cross-sectional area of the wiring in the winding 21
can be approximately half that of a wave winding under identical
conditions, facilitating shaping of the wire and improving winding,
and because the diameter of the wire is small, there are fewer gaps
between portions of the wiring within the slots 11 of the core 9,
improving the wire-to-space ratio and enabling the size of the
electric motor to be reduced. Consequently, the moment of inertia
and torque loss of the armature 20 which are important factors in
steering can be reduced.
[0090] (5) By improving the balance of the induced voltage between
each of the circuits of the winding, an overall reduction in torque
ripples can be achieved, reducing the torque ripples transmitted to
the steering wheel, and enabling an overall improvement in the feel
of the steering wheel to the driver.
[0091] (6) Because this electric motor provides a good commutating
action, in addition to enabling effects such as the lengthening of
the working life of the brushes 8, the suppression of temperature
increases in the brushes 8, and the reduction of commutator noise
(spark noise) in the brushes 8, it is advantageous with respect to
radio noise, etc., because the generation of sparks is reduced. In
particular, when mounted on the steering column where use in close
proximity to radio power circuitry, etc., cannot be avoided, the
effects on radio noise, etc., are small.
[0092] Furthermore, because the generation of sparks is reduced,
the load of the springs 7 pressing the brushes 8 against the
commutator 6 can be reduced, enabling the reduction of torque loss
due to brush pressure, and also enabling the reduction of
frictional heat due to the pressure of the brushes 8. Consequently,
even though this electric motor adopts a lap-wound 4-brush method,
torque loss can be maintained at the same level as that of a
wave-wound 2-brush method.
[0093] Moreover, the above embodiment was explained for 4-pole 24-
and 22-slot lap-wound motors for electric power steering
assemblies, but the number of slots is not limited to these
numbers, and provided that the number of slots is an even number
which does not give rise to magnetic attraction in the radial
direction relative to the armature, the noise reduction effect will
be realized.
[0094] Additionally, provided that the number of slots is not a
multiple of the number of poles, torque ripples can also be
reduced.
[0095] FIGS. 22 to 24 show the magnetic attraction and torque
ripples acting on an armature in the cases of 4-pole 20-slot,
4-pole 26-slot, and 4-pole 28-slot lap windings, and it is clear
that magnetic attraction does not act in the radial direction in
any of these cases. FIG. 25 summarizes these results, and it can be
seen that magnetic attraction does not occur in the radial
direction when the number of slots chosen is an even number or a
multiple of the number of pairs of poles, and that torque ripples
can be reduced if the number of slots is not a multiple of the
number of poles.
[0096] Furthermore, the number of poles is not limited to four, and
may be any number from four upwards, such as six, eight, etc. FIGS.
26 to 31 show examples of 6-pole 25-plot, 6-pole 24-slot, 6-pole
22-slot, 6-pole 26-slot, 6-pole 21-slot, and 6-pole 27-slot lap
windings.
[0097] From FIG. 26, it can be seen that magnetic attraction acts
in the radial direction because the number of slots is neither an
even number nor a multiple of the number of pairs of poles. In FIG.
27, the torque ripples are large because the number of slots is a
multiple of the number of poles. In FIGS. 28 and 29, the number of
slots is an even number but not a multiple of the number of poles,
and in FIGS. 30 and 31, the number of slots is a multiple of the
number of pairs of poles but not a multiple of the number of poles,
and so magnetic attraction does not act in the radial direction in
any of these cases, and torque ripples are minimized. FIG. 32
summarizes these results, and as with the 4-pole cases, it can be
seen that magnetic attraction does not occur in the radial
direction if the number of slots is an even number or a multiple of
the number of pairs of poles, and that torque ripples can be
reduced if the number of slots is not a multiple of the number of
poles. The same applies to cases with eight poles or more. When the
number of slots is a multiple of the number of pairs of poles, the
equalizing members described above can be provided, enabling
circulating current to be prevented and the commutating action to
be improved.
[0098] As explained above, according to one aspect of the present
invention, there is provided a motor for an electric power steering
assembly comprising: a yoke; a multi-polar magnetic field portion
composed of at least four poles secured to the inner wall of the
yoke; a shaft disposed within the yoke so as to be able to rotate
freely; an armature secured to the shaft having a winding
constructed by lap winding wiring into an even number of slots
formed on the outer circumferential surface of a core so as to
extend in the axial direction thereof; a commutator comprising a
plurality of segments secured to an end portion of the shaft; and a
plurality of brushes contacting the surface of the commutator,
whereby the total magnetic attraction acting on the armature is
zero and rotational vibrations which cause operating noise do not
arise, thereby enabling operating noise to be reduced.
[0099] According to one form of the present invention, there is
provided a motor for an electric power steering assembly wherein
the number of slots is even and is not a multiple of the number of
poles, enabling operating noise to be reduced, as well as reducing
torque ripples and improving the feel of the steering wheel to the
driver.
[0100] According to another aspect of the present invention, there
is provided a motor for an electric power steering assembly
comprising: a yoke; a multi-polar magnetic field portion composed
of at least four poles secured to the inner wall of the yoke; a
shaft disposed within the yoke so as to be able to rotate freely;
an armature secured to the shaft having a winding constructed by
lap winding wiring into a number of slots being a multiple of the
number of pairs of poles, the slots being formed on the outer
circumferential surface of a core so as to extend in the axial
direction thereof; a commutator comprising a plurality of segments
secured to an end portion of the shaft; and a plurality of brushes
contacting the surface of the commutator, whereby the total
magnetic attraction acting on the armature is zero and rotational
vibrations which cause operating noise do not arise, thereby
enabling operating noise to be reduced.
[0101] According to one form of the present invention, there is
provided a motor for an electric power steering assembly wherein
the number of slots is a multiple of the number of pairs of poles
and is not a multiple of the number of poles, enabling operating
noise to be reduced, as well as reducing torque ripples and
improving the feel of the steering wheel to the driver.
[0102] According to one another form of the present invention, the
circuits of the armature are electrically connected to each other
using equalizing members, enabling the prevention of circulating
currents from flowing through the brushes due to differences in
induced electromotive forces arising between the circuits of the
armature, thereby enabling the commutating action of the brushes to
be improved, and also enabling the suppression of commutator sparks
generated by the brushes. Furthermore, the magnitudes of both
operating noise and torque ripples can be reduced thereby.
[0103] According to still another form of the present invention,
there is provided a motor for an electric power steering assembly
wherein the number of equalizing members is determined by
Ns/(n.times.2).ltoreq.K.lto- req.Ns, where K is the number of
equalizing members, Ns is the number of slots in the core, and n is
the maximum number of segments covered by the brushes, enabling the
appropriate number to be determined, thereby enabling the
elimination of excess.
[0104] According to one form of the present invention, there is
provided a motor for an electric power steering assembly wherein
the current passing through the winding is controlled by pulse
width modulation (PWM) driving, whereby the desired voltage can be
applied with reduced output loss, and the size of the control unit
can be reduced.
[0105] According to another form of the present invention, there is
provided a motor for an electric power steering assembly wherein
the wiring is enamel-coated round wire, facilitating the
mechanization of the step of winding the wiring onto the core,
thereby enabling mass production of the armature and reducing
production costs.
[0106] According to still another form of the present invention,
there is provided a motor for an electric power steering assembly
wherein the magnetic field portion comprises a plurality of
permanent magnets disposed so as to be spaced around the inner wall
of the yoke, enabling the magnitude of torque ripples to be
reduced. Reductions in size, improvements in the assembly
operation, and cost reductions are also enabled.
* * * * *