U.S. patent application number 12/098148 was filed with the patent office on 2008-08-07 for cylinder-type linear motor and moving part thereof.
This patent application is currently assigned to Oriental Motor Co., Ltd.. Invention is credited to Takao Iwasa, Hirobumi Satomi.
Application Number | 20080185982 12/098148 |
Document ID | / |
Family ID | 35756701 |
Filed Date | 2008-08-07 |
United States Patent
Application |
20080185982 |
Kind Code |
A1 |
Iwasa; Takao ; et
al. |
August 7, 2008 |
Cylinder-Type Linear Motor and Moving Part Thereof
Abstract
There is provided a cylinder-type linear motor capable of
shortening the total motor length with respect to a predetermined
stroke length S and capable of being operated without sensor means
for sensing the position of a moving part being added in the axial
direction. The cylinder-type linear motor includes a fixed part 20
having a coil assembly 28 formed by a plurality of (n number of)
ring-shaped coils 22 . . . 27 arranged in the axial direction and a
yoke member 21 made of a magnetic material, and a moving part 10
having a linear motion shaft 11 provided on the axis line of the
fixed part 20 and a permanent magnet assembly 15 having one or more
permanent magnets magnetized in the axial direction. A stroke S is
equal to or smaller than (n.times.C-M), the axial length Y of the
yoke member is set equal to or larger than (M+S+0.8.times.D), and
the ring-shaped coils 22 . . . 27 are arranged in a predetermined
phase order and the ring-shaped coils of the same phase are
connected to each other to form one phase winding. Also, there is
provided a moving part of a cylinder-type linear motor, which can
improve the magnetic flux density distribution waveform near both
end portions of the moving part assembly and can improve the thrust
characteristic by bringing the magnetic flux density distribution
waveform closer to a cosine waveform and by increasing the
amplitude of cosine waveform. In this moving part, a permanent
magnet assembly 5 has a pair of permanent magnets 2 and 4
magnetized in the axial direction, which are arranged so that the
end faces with the same polarity face to each other; a pair of
ring-shaped fixing members 6 and 7 each having an outside diameter
smaller than the outside diameters of the paired permanent magnets
2 and 4, which are fixed on the linear motion shaft 11 so as to be
in contact with both sides of the paired permanent magnets 2 and 4;
and a pair of ring-shaped permanent magnets 8 and 9 magnetized in
the radial direction so that the polarity on the outer peripheral
surface is different from the polarity on the opposed end faces of
the paired permanent magnets 2 and 4, which are provided on the
outer peripheral surfaces of the paired ring-shaped fixing members
6 and 7.
Inventors: |
Iwasa; Takao; (Kashiwa-shi,
JP) ; Satomi; Hirobumi; (Kashiwa-shi, JP) |
Correspondence
Address: |
MYERS BIGEL SIBLEY & SAJOVEC
PO BOX 37428
RALEIGH
NC
27627
US
|
Assignee: |
Oriental Motor Co., Ltd.
|
Family ID: |
35756701 |
Appl. No.: |
12/098148 |
Filed: |
April 4, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11197014 |
Aug 4, 2005 |
7378765 |
|
|
12098148 |
|
|
|
|
Current U.S.
Class: |
318/135 |
Current CPC
Class: |
H02K 41/03 20130101;
H02K 33/00 20130101 |
Class at
Publication: |
318/135 |
International
Class: |
H02K 41/02 20060101
H02K041/02 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 9, 2004 |
JP |
2004-231993 |
Sep 1, 2004 |
JP |
2004-253765 |
Sep 1, 2004 |
JP |
2004-253766 |
Claims
1. A cylinder-type linear motor comprising: a fixed part including
a coil assembly having a plurality of (n number of) ring-shaped
coils disposed in the axial direction with an equal pitch to form a
cylindrical space, and a yoke member made of a magnetic material,
which is provided on the outer periphery side of the coil assembly;
and a moving part including a linear motion shaft provided on the
axis line of the fixed part so as to be capable of reciprocating in
the axial direction, and a permanent magnet assembly having one or
more permanent magnets magnetized in the axial direction, which is
provided on the linear motion shaft, characterized in that when the
pitch of the ring-shaped coil is taken as C, the axial length of
the permanent magnet assembly as M, and the outside diameter
thereof as D, a stroke S is equal to or smaller than (n.times.C-M),
the axial length Y of the yoke member is set equal to or larger
than (M+S+0.8.times.D), and the ring-shaped coils are arranged in a
predetermined phase order and the ring-shaped coils of the same
phase are connected to each other to form one phase winding.
2. The cylinder-type linear motor of claim 1, characterized in that
the permanent magnet assembly of the moving part is arranged so
that when the number of permanent magnets is two or more, the
polarities of adjacent magnets facing each other is the same.
3. The cylinder-type linear motor of claim 1, characterized in that
the yoke member is formed by a cylindrical member, an opening
extending in the axial direction is provided in the cylindrical
member, and sensor means for sensing the magnetic pole position of
the moving part is provided in the opening.
4. The cylinder-type linear motor of claim 1, characterized by
being driven by a driving circuit having sensor means for sensing
the magnetic pole position of the moving part, which is provided in
the outer peripheral surface of the coil assembly; a moving part
magnetic pole position detecting section for detecting the magnetic
pole position of the moving part by means of a signal from the
sensor means; a memory section for storing pattern data that are
set so as to correspond to the moving part magnetic pole position
and to correct asymmetry of magnetic flux density distribution
waveform caused by the configuration of the permanent magnet
assembly and asymmetry of mating winding existing in the case where
the number of ring-shaped coils is different according to the
phase; a current control section in which a current command value
of each phase is produced based on the pattern data read from the
memory section so as to correspond to the moving part magnetic pole
position and a current command from a speed control section, the
current command value is compared with an actual current value sent
from a current detecting section for detecting a current flowing in
each phase winding, and a gate signal for carrying out PWM control
is generated so that the difference is zero; and an inverter
section provided with switching means that is ON/OFF controlled by
the gate signal from the current control section.
Description
RELATED APPLICATIONS
[0001] This application is a continuation application of U.S.
application Ser. No. 11/197,014 filed on Aug. 4, 2005, and claims
priority from Japanese Patent Application No. 2004-231993; filed
Aug. 9, 2004; Japanese Patent Application No. 2004-253765; filed
Sep. 1, 2004; and Japanese Patent Application No. 2004-253766;
filed Sep. 1, 2004, the disclosures of which are incorporated by
reference herein in their entirety.
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
[0002] 1. Field of the Invention
[0003] The present invention relates to a cylinder-type linear
motor having a permanent magnet in a moving part section and a
plurality of ring-shaped coils in a fixed part section, and a
moving part thereof.
[0004] 2. Description of Related Art
[0005] FIG. 11 is a sectional view of a two-phase cylinder-type
linear motor that has been known conventionally.
[0006] In FIG. 11, a moving part 100 of the cylinder-type linear
motor includes a linear motion shaft 101 reciprocating in the axial
direction, a cylindrical moving part yoke 102 mounted on the linear
motion shaft 101, and a plurality of ring-shaped permanent magnets
103 arranged on the outer peripheral surface of the moving part
yoke 102 in a magnetized manner such as to adopt an alternate
polarity with a magnetic pole pitch P in the axial direction. Also,
a fixed part core 201 of a fixed part 200 has ring-shaped yoke
portions 202 each having a smaller inside diameter and ring-shaped
yoke portions 203 each having a larger inside diameter, both yoke
portions being laminated alternately in the axial direction. As a
result, on the inner peripheral surface of the fixed part 201, a
large number of ring-shaped tooth portions 204 and ring-shaped
groove portions 205 are formed in the axial direction with an equal
pitch (P/2). In the ring-shaped groove portions 205, ring-shaped
coils 206, 207, . . . , 213 are disposed in the phase order of A
phase and B phase. Therefore, the ring-shaped coils 206, 208, 210
and 212 disposed alternately are connected to each other to form an
A-phase winding, and the remaining coils 207, 209, 211 and 213 are
also connected to each other to form a B-phase winding.
[0007] An axial length M of a moving part core section is longer
than an axial length K of a fixed core section, and therefore is an
axial length in which the moving part and the fixed part face each
other, namely, a thrust contribution length K. Also, a stroke
length S is expressed by (M-K). From FIG. 11, the travel range
length of the moving part is expressed by (K+2S), namely, (thrust
contribution length+2.times.stroke length S). The total motor
length is set so as to satisfy the travel range length of the
moving part. As the related art, Japanese Patent Provisional
Publication No. 7-107732 and Japanese Patent Provisional
Publication No. 5-15139 can be cited.
OBJECT AND SUMMARY OF THE INVENTION
[0008] For the linear motor having the above-described conventional
construction, in order to provide a predetermined stroke S, it is
necessary to set the total length of the motor so as to satisfy the
travel range length (thrust contribution length+2.times.stroke
length S) of the moving part, which results in a problem of greater
total motor length. Also, since the above-described motor having
the conventional construction is of a permanent magnet type, it can
be operated as a brushless DC motor in principle. For this purpose,
however, sensor means for sensing the position of moving part must
be provided separately so as to be adjacent to the fixed part
section in the axial direction. In this case, there arises a
problem in that the total motor length increases further.
[0009] In order to improve the workability in producing the fixed
part, a construction is sometimes employed in which a comb teeth
shaped core having a cross section equivalent to the cross section
of the fixed part core shown in FIG. 11 is fitted from the outside
of the ring-shaped coils. In this case, there arises a problem in
that in order to change the stroke length, it is necessary to
prepare a mold newly, and therefore the stroke length cannot be
changed easily. Further, there also arises a problem in that it is
necessary to change the length of fixed part so that the number of
ring-shaped coils in windings of both phases is equal, and
therefore the degree of freedom in changing the stroke length is
low. Still further, there arises a problem in that when the thrust
contribution length is fixed, the inertia moment of moving part
increases, thereby degrading the response, as the stroke length S
is increased.
[0010] The present invention has been made to solve the above
problems. Accordingly, it is an object of the present invention to
provide a cylinder-type linear motor in which the total motor
length can be shortened with respect to a predetermined stroke
length S, the linear motor can be operated as a brushless DC motor
without sensor means for sensing the position of a moving part
being added in the axial direction, the stroke length S can be
increased/decreased easily at a low cost, the degree of freedom in
changing the stroke length can be made high, and the inertia moment
of moving part is not affected by the stroke length S.
[0011] Also, it is another object of the present invention to
provide a moving part of a cylinder-type linear motor in which the
total motor length can be shortened with respect to a predetermined
stroke length S, the linear motor can be operated as a brushless DC
motor without sensor means for sensing the position of a moving
part being added in the axial direction, and a magnetic pole
position detecting method, which has been used for the conventional
brushless motor, can be adopted.
[0012] Further, it is still another object of the present invention
to provide a moving part of a cylinder-type linear motor in which
the total motor length can be shortened with respect to a
predetermined stroke length S, the linear motor can be operated as
a brushless DC motor without sensor means for sensing the position
of a moving part being added in the axial direction, the magnetic
flux density distribution waveform near both end portions of the
moving part assembly can be improved, and the thrust characteristic
can be improved by bringing the magnetic flux density distribution
waveform closer to a cosine waveform and by increasing the
amplitude of cosine waveform.
[0013] To solve the above problems, the present invention provides
a cylinder-type linear motor including a fixed part including a
coil assembly having a plurality of (n number of) ring-shaped coils
arranged in the axial direction to form a cylindrical space, and a
yoke member made of a magnetic material, which is provided on the
outer periphery side of the coil assembly; and a moving part
including a linear motion shaft provided on the axis line of the
fixed part so as to be capable of reciprocating in the axial
direction, and a permanent magnet assembly having one or more
permanent magnets magnetized in the axial direction, which is
provided on the linear motion shaft, characterized in that when the
axial length of the ring-shaped coil is taken as C, the axial
length of the permanent magnet assembly as M, and the outside
diameter thereof as D, a stroke S is equal to or smaller than
(n.times.C-M), the axial length Y of the yoke member is set equal
to or larger than (M+S+0.8.times.D), and the ring-shaped coils are
arranged in a predetermined phase order and the ring-shaped coils
of the same phase are connected to each other to form one phase
winding.
[0014] Also, in the cylinder-type linear motor in accordance with
the present invention, the permanent magnet assembly of the moving
part is arranged so that when the number of permanent magnets is
two or more, the end faces with the same polarity face to each
other.
[0015] Further, in the cylinder-type linear motor in accordance
with the present invention, the yoke member is formed by a
cylindrical member, an opening extending in the axial direction is
provided in the cylindrical member, and sensor means for sensing
the magnetic pole position of the moving part is provided in the
opening.
[0016] Still further, in the cylinder-type linear motor in
accordance with the present invention, the yoke member is formed by
a plurality of slender plate-shaped members, these plate-shaped
members are arranged at predetermined intervals in the
circumferential direction so as to cover the outer peripheral
surface of the coil assembly, and the coil assembly is held by the
plate-shaped members.
[0017] Further, in the cylinder-type linear motor in accordance
with the present invention, a plurality of groove portions parallel
with the linear motion shaft are provided in the inner peripheral
surface of an aluminum-made case member for radiating heat from the
coils, and the yoke members are disposed in the groove
portions.
[0018] Still further, in the cylinder-type linear motor in
accordance with the present invention, the linear motor is driven
by a driving circuit having sensor means for sensing the magnetic
pole position of the moving part, which is provided in the outer
peripheral surface of the coil assembly; a moving part magnetic
pole position detecting section for detecting the magnetic pole
position of the moving part by means of a signal from the sensor
means; a memory section for storing pattern data that are set so as
to correspond to the moving part magnetic pole position and to
correct asymmetry of magnetic flux density distribution waveform
caused by the configuration of the permanent magnet assembly and
asymmetry of mating winding existing in the case where the number
of ring-shaped coils is different according to the phase; a current
control section in which a current command value of each phase is
produced based on the pattern data read from the memory section so
as to correspond to the moving part magnetic pole position and a
current command from a speed control section, the current command
value is compared with an actual current value sent from a current
detecting section for detecting a current flowing in each phase
winding, and a gate signal for carrying out PWM control is
generated so that the difference is zero; and an inverter section
provided with switching means that is ON/OFF controlled by the gate
signal from the current control section.
[0019] The present invention provides a moving part of a
cylinder-type linear motor, including a linear motion shaft which
is arranged on an axis line of a cylindrical fixed part so as to
reciprocate on the axis line in the axial direction; and a
permanent magnet assembly which is provided on the linear motion
shaft and is configured so that a plurality of permanent magnets
magnetized in the axial direction are disposed so that the end
faces thereof face to each other, characterized in that a unit is
formed by a first permanent magnet magnetized in a first axial
direction and second and third permanent magnets magnetized in the
direction opposite to the first axial direction, which are arranged
on both sides of the first permanent magnet; one or more units are
arranged in series in the axial direction to form the permanent
magnet assembly; when the first to third permanent magnets are
arranged in the order of the second, first and third permanent
magnets from the left, a distance between the left-hand side end
face of the second permanent magnet and the right-hand side end
face of the third permanent magnet is set at 2.times.L, and a
distance between a first central position between the right-hand
side end face of the second permanent magnet and the left-hand side
end face of the first permanent magnet and a second central
position between the right-hand side end face of the first
permanent magnet and the left-hand side end face of the third
permanent magnet is set at L.
[0020] Also, in the moving part of a cylinder-type linear motor in
accordance with the present invention, the axial length of the
first permanent magnet is L, and the axial lengths of the second
and third permanent magnets each are L/2.
[0021] Further, in the moving part of a cylinder-type linear motor
in accordance with the present invention, the cross-sectional areas
of the second and third permanent magnets are equal to each other,
and each are different from the cross-sectional area of the first
permanent magnet.
[0022] Still further, in the moving part of a cylinder-type linear
motor in accordance with the present invention, the energy products
of the second and third permanent magnets are equal to each other,
and each are different from the energy product of the first
permanent magnet.
[0023] To solve the above problems, the present invention provides
a moving part of a cylinder-type linear motor, including a linear
motion shaft which is arranged on an axis line of a cylindrical
fixed part so as to reciprocate on the axis line in the axial
direction; and a permanent magnet assembly which is provided on the
linear motion shaft and is configured so that a plurality of
permanent magnets magnetized in the axial direction are disposed so
that the end faces thereof face to each other, characterized in
that the permanent magnet assembly has a pair of permanent magnets
magnetized in the axial direction, which are arranged so that the
end faces with the same polarity face to each other; a pair of
ring-shaped fixing members each having an outside diameter smaller
than the outside diameter of the paired permanent magnets, which
are fixed on the linear motion shaft so as to be in contact with
both sides of the paired permanent magnets; and a pair of
ring-shaped permanent magnets magnetized in the radial direction so
that the polarity on the outer peripheral surface is different from
the polarity on the opposed end face of the paired permanent
magnets, which are provided on the outer peripheral surfaces of the
paired ring-shaped fixing members.
[0024] Also, in the moving part of a cylinder-type linear motor in
accordance with the present invention, the ring-shaped fixing
members each are made of a magnetic material.
[0025] Further, in the moving part of a cylinder-type linear motor
in accordance with the present invention, a distance between
central positions in the axial direction of the ring-shaped
permanent magnets is set so as to be two Limes of a magnetic pole
pitch of the moving part.
[0026] According to the present invention, since the cylinder-type
linear motor is configured as described above, the travel range
length of a moving part that determines a necessary total motor
length can be made (thrust contribution length+stroke length S).
Also, since the fixed part has no slot, a mold is not needed, the
increase/decrease in stroke length does not depend on the length of
the moving part, and the number of ring-shaped coils can be
increased or decreased in unit of one coil. Therefore, the stroke
length can be increased or decreased easily at a low cost, and the
degree of freedom in changing the number of ring-shaped coils can
be made high. Also, since the sensor means for sensing the position
of moving part can be disposed on the outside of the ring-shaped
coil, the linear motor can be operated as a brushless DC motor
without increasing the axial length.
[0027] According to the present invention, the distribution
waveform of radial component of magnetic flux density is a
synthesis of distribution waveforms of permanent magnets
constituting the permanent magnet assembly of the moving part.
Therefore, by setting the distribution waveform formed by the
second and third permanent magnets with respect to the distribution
waveform formed by the first permanent magnet, the synthesized
distribution waveform can be improved, so that a magnetic pole
position detecting method, which has been used for conventional
brushless motors, can be adopted.
[0028] Also, according to the present invention, since the
cylinder-type linear motor is configured as described above, the
travel range length of a moving part that determines a necessary
total motor length can be made (thrust contribution length+stroke
length S).
[0029] According to the present invention, the magnetic flux
density distribution waveform near both end portions of the moving
part assembly can be improved, and the thrust characteristic can be
improved by bringing the magnetic flux density distribution
waveform closer to a cosine waveform and by increasing the
amplitude of cosine waveform.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a longitudinal sectional view showing an
embodiment of a cylinder-type linear motor in accordance with the
present invention;
[0031] FIG. 2 is a longitudinal sectional view in which symbols
designating the lengths of parts shown in FIG. 1 are shown;
[0032] FIG. 3 is a view schematically showing the relationship of a
detent thrust depending on the relationship between a travel range
length of a moving part and a length of a cylindrical yoke;
[0033] FIG. 4 is a view showing an axial distribution waveform of a
radial component of magnetic flux density due to a moving part in
accordance with the present invention and a permanent magnet
assembly corresponding to the moving part;
[0034] FIG. 5 is a control block diagram for illustrating the drive
of a linear motor in accordance with the present invention;
[0035] FIG. 6 is a transverse sectional view showing a modification
of a cylinder-type linear motor in accordance with the present
invention;
[0036] FIG. 7 is a longitudinal sectional view showing an
embodiment of a moving part of a cylinder-type linear motor in
accordance with the present invention;
[0037] FIG. 8 is a schematic view showing an embodiment of a moving
part of a cylinder-type linear motor in accordance with the present
invention and a distribution curve;
[0038] FIG. 9 is a longitudinal sectional view showing another
embodiment of a moving part of a cylinder-type linear motor in
accordance with the present invention;
[0039] FIG. 10 is a schematic view showing another embodiment of a
moving part of a cylinder-type linear motor in accordance with the
present invention and a distribution curve; and
[0040] FIG. 11 is a longitudinal sectional view showing a
conventional cylinder-type linear motor.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0041] Preferred embodiments of the present invention will now be
described exemplarily and in detail with reference to the
accompanying drawings.
[0042] FIGS. 1 and 2 are sectional views showing one embodiment of
a linear motor in accordance with the present invention.
[0043] In FIGS. 1 and 2, a moving part 10 includes a linear motion
shaft 11 reciprocating in the axial direction, and a permanent
magnet assembly 15 in which two permanent magnets 12 and 14
magnetized in the axial direction are disposed with a spacer 13
being held therebetween so that the end surfaces thereof with the
same polarity face to each other. The spacer 13 adjusts a magnetic
pole pitch P (distance between the N and S poles) of the moving
part 10, and also regulates a waveform (magnetic flux density
distribution waveform) in which a radial component of magnetic flux
density changes according to the axial position of moving part. The
spacer 13 may be made of a magnetic material or may be made of a
non-magnetic material. In some cases, the spacer 13 need not be
used.
[0044] The permanent magnet assembly 15 is firmly fixed to the
linear motion shaft 11 by fixing ring members 16 and 17 provided on
both end faces of the permanent magnet assembly 15. The moving part
10 is supported in the center of a pair of brackets 30 and 31
facing to each other with a predetermined distance via bearings 32
and 33 so as to be movable in the axial direction. On the other
hand, a fixed part 20 both ends of which are supported by the
brackets 30 and 31 is composed of a cylindrical yoke 21 made of a
magnetic material, and a coil assembly 28 having a plurality of
ring-shaped coils 22, 23, . . . , 27 disposed in the axial
direction with an equal pitch C. The coil assembly 28 is housed in
the cylindrical yoke 21, and is formed with a cylindrical space 34
for containing the permanent magnet assembly 15 therein. Also, the
cylindrical yoke 21 is provided with an opening 29 extending in the
axial direction, and a circuit board 40 having sensor means 41, not
shown, for sensing the magnetic pole position of the moving part 10
is installed therein. The circuit board 40 is provided with a
terminal for connecting the starting end and trailing end of each
of the coils, and the coils are connected by a printed circuit
provided on the circuit board 40 so that the winding of each phase
is formed.
[0045] FIG. 3 is a view schematically showing the relationship of a
detent thrust depending on the relationship between a travel range
length (M+S) of the moving part 10 and a length Y of the
cylindrical yoke 21. In FIG. 3, the center in the axial direction
of the permanent magnet assembly 15 is taken as an origin of
position. From FIG. 3, it can be seen that a detent thrust that
draws the moving part 10 into the yoke is generated near both ends
of the stroke. Also, it can be seen that if the length Y of the
cylindrical yoke 21 is set so as to be equal to or greater than a
certain value Yo, the detent thrust can be kept at a negligible
value in the travel range length (M+S) of the moving part 10. A
yoke projecting dimension B at the time when Y is equal to Yo
depends on the outside diameter D of the permanent magnet, and is
expressed as B=kD. It is determined analytically that k is proper
when it takes a value of about 0.4. Therefore,
Yo=M+S+2.times.B=M+S+0.8.times.D.
[0046] From the above description, as shown in FIG. 2, when the
axial length of the permanent magnet assembly 15 is taken as M, the
outside diameter thereof as D, and the stroke length as S, the
length Y of the cylindrical yoke 21 is set at a value equal to or
larger than (M+S+0.8.times.D). By doing this, the detent thrust
generated at both ends of stroke at the time of de-energization can
be kept at a negligible value. When the pitch between the
ring-shaped coils 22, 23, . . . , 27 is taken as C, since the
number n of the ring-shaped coils 22, 23, . . . , 27 is six, an
axial length K of the fixed part 20 is 6C. In this case, the thrust
contribution length is M. Also, the stroke length S can take a
value equal to or smaller than (K-M), namely, (6C-M). In FIG. 2,
the number of the ring-shaped coils 22, 23, . . . , 27 is six.
However, by increasing the number of the ring-shaped coils 22, 23,
. . . , 27 to seven or decreasing to five, for example, the stroke
length S can be increased or decreased with the coil pitch C being
a unit to (7C-M) or (5C-M). Since the stroke length S does not
depend on the length M of the permanent magnet assembly 15, it can
be seen that even if the stroke length S is increased, the inertia
moment of the moving part 10 does not increase. Also, since the
travel range length of the moving part 10 is (thrust contribution
length+stroke length S), the total motor length can be shortened by
the stroke length S as compared with the conventional example.
[0047] FIG. 4 is a view showing an axial distribution waveform of a
radial component of magnetic flux density due to the moving part 10
in accordance with this embodiment and the permanent magnet
assembly 15 corresponding to the moving part 10. It can be seen
that in the range of the axial length M of the permanent magnet
assembly 15, the distribution waveform of magnetic flux density can
be approximated as a waveform in which a DC component is added to a
cosine component. Specifically, when the axial position with the
axial center position of the permanent magnet assembly 15 being the
origin is taken as (x), the magnetic flux density B(x) is expressed
as B(x)=B1.times.cos(.pi.x/L)+B0, wherein L is a magnetic pole
pitch of moving part. The output voltage of the sensor means 41 is
also a voltage proportional to this, so that the position of the
moving part 10 can be read by utilizing the same relational
expression as described above.
[0048] Also, it can be seen that since the thrust is proportional
to the product of counter electromotive voltage and current, and
the counter electromotive voltage is proportional to a change with
respect to the position of the magnetic flux density, an influence
of the DC component of the magnetic flux density is eliminated.
[0049] FIG. 5 is a control block diagram for illustrating the drive
of the linear motor in accordance with the present invention.
[0050] A signal 42 generated from the sensor means 41 incorporated
in a motor section 60 is sent to a moving part magnetic pole
position detecting section 43 and a moving part speed detecting
section 44, where the magnetic pole position and speed of the
moving part 10 are detected. Detected moving part speed data 45 and
command speed data 46 are input into a speed control section 47,
and a current command 48 corresponding to a difference between a
command speed and an actual moving part speed is sent out of the
speed control section 47. On the other hand, magnetic pole data is
sent out of the moving part magnetic pole position detecting
section 43, and is input into a memory section 49 that stores
three-phase pattern data set corresponding to the moving part
magnetic pole position. Data 50 corresponding to the moving part
position is sent out of the memory section 49, and is input into a
current control section 51 together with the current command 48,
whereby a command value of each phase current is generated. In the
current control section 51, the generated current command is
compared with actual each phase current value 52 detected by a
current detecting section 56, and a gate signal 53 for carrying out
PWM control is generated so that the difference is zero. The gate
signal 53 controls ON/OFF of switching means of an inverter section
54, whereby control is carried out so that each phase current
waveform of the motor section 60 takes a predetermined value. The
memory section 49 stores the aforementioned three-phase pattern
data considering the correction of asymmetry of magnetic flux
density distribution waveform caused by the configuration of the
permanent magnet assembly 15 and asymmetry of phase winding caused
by the occurrence of a difference in the number of coils
constituting the phase winding to obtain a predetermined stroke
length. Therefore, the increase/decrease in stroke can be effected
in unit of one coil, and also smooth speed control can be achieved.
Also, it is a matter of course that positioning control can also be
carried out by adding a position control loop to the control block
diagram shown in FIG. 5.
[0051] The aforementioned yoke member need not necessarily be of a
cylindrical shape. As shown in a transverse sectional view of FIG.
6, slender plate-shaped yoke members 70, 71, 72 and 73 may be
brought into contact with the outer peripheral surface of the coil
assembly 28 at intervals in the circumferential direction. The yoke
members 70, 71, 72 and 73 are installed so as to be inserted in
spaces 91, 92, 93 and 94 formed between groove portions 81, 82, 83
and 84 parallel with the linear motion shaft 11, which are provided
in the inner peripheral surface of an aluminum case 80, and the
coil assembly 28, respectively. The aluminum case 80 is provided
with an opening 85 that is long in the axial direction, and the
circuit board 40 mounted with the sensor means 41 is disposed in
the opening 85. In this case, by adjusting the ratio of the contact
area with the aluminum case 80 to the contact area with the yoke
members 70, 71, 72 and 73, the thrust can be increased while the
heat radiation property is improved.
[0052] In the cylinder-type linear motor having the above-described
moving part construction, for example, when the sensor means is a
Hall element, since the output waveform of sensor means is
proportional to the radial component of the magnetic flux density
at a space position of the sensor means, the waveform becomes as
indicated by a curve V in FIG. 4. Therefore, there arises a problem
in that a system such that a zero cross point of the output
waveform is detected to detect the magnetic pole position, which
system having been used in the conventional brushless motor, cannot
be used. For this reason, it is necessary to detect the moving part
position by using storing means that stores the relationship
between the output waveform and the moving part position in
advance, which also presents the problem of a complicated
circuit.
[0053] FIG. 7 shows a cylinder-type linear motor in which the
construction of moving part is changed. In FIG. 7, the same
reference numerals are applied to elements that are the same as
those in FIGS. 1 to 4, and explanation of those elements is
omitted.
[0054] FIG. 8 is a partial sectional view of FIG. 7, showing one
embodiment of the moving part construction of the cylinder-type
linear motor in accordance with the present invention, and a
diagram showing magnetic flux density distribution waveforms of
permanent magnets corresponding to the moving part construction and
a composite waveform thereof. In this case, the central position in
the axial direction of a first permanent magnet is the origin on
the horizontal axis.
[0055] In FIGS. 7 and 8, a moving part 1 includes the linear motion
shaft 11 reciprocating in the axial direction; a permanent magnet
assembly 5 consisting of a first permanent magnet 3 magnetized in
the first axial direction and second and third permanent magnets 2
and 4 magnetized in the direction opposite to the first axial
direction, which are arranged on both sides of the first permanent
magnet 3; and the fixing ring members 16 and 17 provided on both
sides of the permanent magnet assembly 5.
[0056] On the other hand, the fixed part 20 both ends of which are
supported by the brackets 30 and 31 is composed of the cylindrical
yoke 21 made of a magnetic material, and the coil assembly 28
having the plurality of ring-shaped coils 22, 23, . . . , 27
disposed in the axial direction with the equal pitch C. The coil
assembly 28 is housed in the cylindrical yoke 21, and is formed
with the cylindrical space 34 for containing the moving part 1
provided with the permanent magnet assembly 5 therein. The moving
part 1 is arranged on the axis line of the cylindrical space 34,
and is supported so as to reciprocate in the axial direction.
[0057] Also, the cylindrical yoke 21 is provided with the opening
29 extending in the axial direction, and the circuit board 40
having sensor means, not shown, for sensing the magnetic pole
position of the moving part 1 is installed therein.
[0058] When the axial length of the first permanent magnet 3 is
taken as L, the distribution waveform of magnetic flux density
formed by the first permanent magnet 3 is represented by a curve W
shown in FIG. 8. The peaks of magnetic flux density occur at the
axial positions of .+-.L/2, and the magnetic pole pitch (distance
between the N and S poles) P is equal to L. At this time, the
magnetic flux density at the axial positions of .+-.L is not zero,
but has a value of .+-.K as shown in FIG. 8. If the axial lengths
of the second and third permanent magnets 2 and 4 are set at L/2 at
this time, the magnetic flux distribution formed by the second
permanent magnet 2 is represented by a curve X, the peaks thereof
occurring at the axial positions of (-L) and (-L/2).
[0059] Also, the magnetic flux distribution formed by the third
permanent magnet 4 is represented by a curve Y, the peaks thereof
occurring at the axial positions of (L) and (L/2). If the outside
diameters of the second and third permanent magnets 2 and 4 are
made smaller than the outside diameter of the first permanent
magnet 3, the cross-sectional areas of the second and third
permanent magnets 2 and 4 become smaller than the cross-sectional
area of the first permanent magnet 3. Further, the distance between
the second and third permanent magnets 2 and 4 and the sensor means
becomes longer than in the case of the first permanent magnet 3. By
these two facts, the peak values of the second and third permanent
magnets 2 and 4 can be made smaller than the peak values of the
first permanent magnet 3, so that the outside diameters thereof can
be adjusted so that the peak values are substantially close to the
values of .+-.K.
[0060] The magnetic flux density distribution waveform synthesized
by these three permanent magnets 2, 3 and 4 is represented by a
curve Z shown in FIG. 8, namely, a waveform close to a sinusoidal
wave distribution can be obtained. As a result, a magnetic pole
position detecting system, which has been used in the conventional
brushless motor, can be used.
[0061] The method for adjusting the peak values of magnetic flux
density distribution of the second and third permanent magnets 2
and 4 is not merely to decrease the outside diameters of magnets as
in the above-described embodiment, but may be to inversely increase
the outside diameters depending to the space position of the sensor
means. Also, as the method for changing the cross-sectional area,
the inside diameters of the magnets may be changed. Alternatively,
without changing the cross-sectional area, the materials of the
second and third permanent magnets 2 and 4 may be changed to
decrease or increase the energy products thereof as compared with
the energy product of the first permanent magnet 3. Further, these
three methods may be used combinedly.
[0062] Although an example in which the unit number of permanent
magnet assemblies is one has been described in this embodiment, it
is a matter of course that the unit number may be plural.
[0063] However, in the cylinder-type linear motor having the
above-described moving part construction, as shown in FIG. 4, the
magnetic flux density distribution near both end portions of the
moving part assembly greatly deviates from a cosine waveform, so
that the thrust characteristic is sometimes deteriorated. Also,
there arises a problem in that the range in which the
aforementioned approximate expression of
B(x)=B1.times.cos(.pi.x/L)+B0 can be applied is limited to the
range of 2.times.L shown in the figure.
[0064] FIG. 9 shows a moving part of the cylinder-type linear motor
in accordance with the present invention, which can improve the
magnetic flux density distribution waveform near both end portions
of the moving part assembly by changing the construction of the
moving part, and can improve the thrust characteristic by bringing
the waveform closer to a cosine waveform and by increasing the
amplitude of the cosine waveform. In FIG. 9, the same reference
numerals are applied to elements that are the same as those in
FIGS. 1 to 4, and explanation of those elements is omitted.
[0065] FIG. 10 is a partial sectional view of FIG. 9, showing one
embodiment of the moving part of the cylinder-type linear motor in
accordance with the present invention, and a diagram showing
magnetic flux density distribution waveforms of permanent magnets
corresponding to the moving part construction and a composite
waveform thereof. In this case, the central position in the axial
direction of the second and third permanent magnet is the origin on
the horizontal axis.
[0066] In FIGS. 9 and 10, a moving part 1 includes the linear
motion shaft 11 reciprocating in the axial direction; a pair of
permanent magnets 2 and 4 consisting of the permanent magnet 2
magnetized in the axial direction (left to right) and the permanent
magnet 4 magnetized in the opposite direction (right to left),
which is arranged so as to face to the permanent magnet 2; the
paired fixing ring members 6 and 7 which are brought into contact
with both end faces of the paired permanent magnets 2 and 4 and are
fixed on the linear motion shaft 11; a pair of ring-shaped
permanent magnets 8 and 9 which are provided so as to be fitted on
or brought into contact with the outer peripheral surfaces of the
fixing ring members 6 and 7. The paired ring-shaped permanent
magnets 8 and 9 are magnetized in the radial direction so that the
polarity of outer peripheral surface is different from the polarity
of the opposed surfaces of the paired permanent magnets 2 and 4.
Also, the distance between the central positions of the axial
length of the ring-shaped permanent magnets 8 and 9 is set at
2.times.L, and the outside diameters thereof are set so as to be
equal to the outside diameters of the paired permanent magnets 2
and 4.
[0067] On the other hand, the fixed part 20, both ends of which are
supported by the brackets 30 and 31, is composed of the cylindrical
yoke 21 made of a magnetic material, and the coil assembly 28
having the plurality of ring-shaped coils 22, 23, . . . , 27
disposed in the axial direction with the equal pitch C. The coil
assembly 28 is housed in the cylindrical yoke 21, and is formed
with the cylindrical space 34 for containing the moving part 1
provided with the permanent magnet assembly 5 therein. The moving
part 1 is arranged on the axis line of the cylindrical space 34,
and is supported so as to reciprocate in the axial direction.
[0068] Also, the cylindrical yoke 21 is provided with the opening
29 extending in the axial direction, and the circuit board 40
having sensor means, not shown, for sensing the magnetic pole
position of the moving part 1 is installed therein.
[0069] Since the moving part 1 is configured as described above,
the distribution waveform of magnetic flux density formed by the
paired permanent magnets 2 and 4 is represented by a curve W in
FIG. 10, and the magnetic flux density distribution waveform formed
by the paired ring-shaped permanent magnets 8 and 9 is represented
by a curve X. If the values of magnetic density at point A of the
curve W and at point B of the curve X are set so as to cancel each
other, the synthesized magnetic flux density distribution waveform
can be made a curve Y, and the magnetic flux density distribution
waveform near both end portions of the permanent magnet assembly 5
can be brought closer to a cosine waveform.
[0070] The ring-shaped permanent magnets 8 and 9 are not limited to
those formed into a cylindrical shape. Arc-shaped permanent magnets
may be affixed to each other to form a ring shape. Also, an
appropriate spacer may be interposed between the ring-shaped
permanent magnet 8 and the permanent magnet 2, between the
permanent magnet 2 and the permanent magnet 4, and between the
permanent magnet 4 and the ring-shaped permanent magnet 9.
[0071] As the result of the above configuration, the construction
shown in FIG. 9 can be achieved without changing the length of the
moving part 1 including the fixing ring members 6 and 7 shown in
FIG. 10, so that the characteristic can be improved without
changing the length of the moving part.
[0072] The present invention is not limited to the above-described
embodiments, and it is a matter of course that changes and
modifications can be made appropriately without departing from the
spirit and scope of the present invention.
* * * * *