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Claims  |
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What is claimed is:
1. A linear drive device for linearly driving a driven object in a
predetermined direction perpendicular to a direction of a width of said
driven object, comprising:
a shaft extending in said predetermined direction and provided with a field
magnet having N- and S-type magnetic poles arranged alternatively in said
predetermined direction;
a moveable piece having an armature coil fitted around said shaft and
opposed to said field magnet for generating a thrust in said predetermined
direction when energized, being reciprocatable along said shaft, and
connected to one end, in the width direction, of said driven object, and
means for eneraizing said armature coil, wherein
thrusts generated by said armature coil at opposite sides, in the width
direction of said driven object, of said shaft are determined such that
the thrust generated at the side near said driven object is sufficiently
larger than the thrust generated at the side remote from said driven
object to ensure linear driving of the driven object in the predetermined
direction.
2. The linear drive device according to claim 1, wherein
magnetic fields formed by said field magnet on a section perpendicular to
said predetermined direction and formed at the opposite sides, in the
width direction of said driven object, of said shaft form an offset
magnetic field and are determined such that the magnetic field at the side
near said driven object is larger in intensity than the magnetic field at
the side remote from said driven object.
3. The linear drive device according to claim 1, wherein said object driven
and said movable piece are two in number, respectively, and the first and
second driven objects forming said two driven objects are connected to the
first and second movable pieces forming said two movable pieces,
respectively.
4. The linear drive device according to claim 1, wherein
said driven object is a slider employed in an image reading apparatus for
optically scanning and reading an original image and being driven in said
predetermined direction together with an optical part carried thereon.
5. The linear drive device of claim 1 wherein
the field magnet has a pole pitch P, the armature coil having a plurality
of coils, each coil having a length in the predetermined direction of
2P/3.
6. The linear drive device of claim 5 wherein
the armature coil has six coils.
7. The linear drive device of claim 1 further comprising:
a sensor arranged on said movable piece for reading information on said
shaft wherein
said sensor is arranged on said movable piece located substantially at a
center of yawing motion of said movable piece occurring during travel of
said movable piece along said shaft.
8. The linear drive device of claim 7 wherein
the shaft has an optical encoder chart mounted thereon extending linearly
in said predetermined direction,
said sensor arranged on said movable piece for reading information held on
said optical encoder chart.
9. The linear drive device of claim 1 further comprising
a sensor arranged on said movable piece for reading information on said
shaft wherein
said sensor is arranged on said movable piece substantially at a center of
pitching motion of said movable piece during travel of said movable piece
along said shaft.
10. The linear device of claim 9 wherein
said shaft contains said information for reading by the sensor on an
optical encoder chart.
11. A linear drive device for linearly driving a driven object in a
predetermined direction perpendicular to a direction of a width of said
driven object, comprising:
a guide member extending in said predetermined direction;
a moveable piece having an armature coil, being reciprocatable along said
guide member and connected to an end, in the width direction, of said
driven object;
a first stator extending linearly in said pre-determined direction, having
a field magnet provided with N-and S-type magnetic poles arranged
alternately in said predetermined direction, and arranged at one of the
opposite sides, in the width direction of said driven object, of said
guide member neighboring to said driven object; and
a second stator extending linearly in said predetermined direction, having
a field magnet provided with N- and S-type magnetic poles arranged
alternately in said predetermined direction, and arranged at the other
side, in the width direction of said driven object, of said guide member
remote from said driven object, wherein
a thrust in said predetermined direction generated by energizing said
armature coil subjected to a magnetic field formed by said field magnet of
said first stator is sufficiently larger than a thrust in said
predetermined direction generated by energizing said armature coil
subjected to a magnetic field formed by said field magnet of said second
stator to ensure linear driving of the driven object in the predetermined
direction.
12. The linear drive device according to claim 11, wherein
intensity of the magnetic field formed by said field magnet of said first
stator is larger than intensity of the magnetic field formed by said field
magnet of said second stator at each position in said predetermined
direction.
13. The linear drive device according to claim 11, wherein
a distance between said first stator and said movable piece is shorter than
a distance between said second stator and said movable piece.
14. The linear drive device according to claim 11, wherein
said driven object is a slider employed in an image reading apparatus for
optically scanning and reading an original image and being driven in said
predetermined direction together with an optical part carried thereon.
15. The linear drive device of claim 11 wherein
said armature coil is formed of a plurality of coils, a first of said coils
being on the side adjacent the first stator and a second coil on the side
adjacent the second stator, current provided to said first coil being
greater than current provided to said second coil.
16. The linear drive device of claim 15 wherein
said armature coil includes three first coils and three second coils, the
magnetic poles of the first stator and second stator having a magnetic
pitch of P, the first coils being shifted relative to each other a
distance of P/3 and the second coils being shifted relative to each other
a distance P/3 along said predetermined direction.
17. A linear drive device comprising:
a guide member extending linearly in a predetermined direction;
a movable piece engaged with said guide member for reciprocation in said
predetermined direction along said guide member; and
a sensor arranged on said movable piece for reading information on said
guide member, wherein
said sensor arranged on said movable piece is located substantially at a
center of a yawing motion of said movable piece occurring during travel of
said movable piece along said guide member.
18. The linear drive device according to claim 17, wherein
said movable piece is connected to a driven object extending in a direction
perpendicular to said predetermined direction.
19. The linear drive device according to claim 18, wherein
said driven object is a slider employed in an image reading apparatus for
optically scanning and reading an original image and being driven in said
predetermined direction together with an optical part carried thereon.
20. A linear drive device comprising:
a guide member extending in a predetermined direction and having an optical
encoder chart extending linearly in said predetermined direction;
a movable piece engaged with said guide member for reciprocation in said
predetermined direction along said guide member; and
a sensor arranged on said movable piece for reading information held on
said optical encoder chart, wherein
said sensor arranged on said movable piece is located substantially at a
center of a yawing motion of said movable piece occurring during travel of
said movable piece along said guide member.
21. The linear drive device according to claim 20, wherein
said guide member has a field magnet having N- and S-type magnetic poles
arranged alternately in said predetermined direction, and said movable
piece has an armature coil opposed to said field magnet for generating a
thrust in said predetermined direction when energized.
22. The linear drive device according to claim 20, wherein
said movable piece is connected to a driven object extending in a direction
perpendicular to said predetermined direction.
23. The linear drive device according to claim 22, wherein
said driven object is a slider employed in an image reading apparatus for
optically scanning and reading an original image and being driven in said
predetermined direction together with an optical part carried thereon.
24. A linear drive device comprising:
a guide member extending linearly in a predetermined direction;
a movable piece engaged with said guide member for reciprocation in said
predetermined direction along said guide member; and
a sensor arranged on said movable piece for reading information on said
guide member, wherein
said sensor arranged on said movable piece is located substantially at a
center of a pitching motion of said movable piece occurring during travel
of said movable piece along said guide member.
25. The linear drive device according to claim 24, wherein
said movable piece is connected to a driven object extending in a direction
perpendicular to said predetermined direction.
26. The linear drive device according to claim 25, wherein
said driven object is a slider employed in an image reading apparatus for
optically scanning and reading an original image and being driven in said
predetermined direction together with an optical part carried thereon.
27. A linear drive device comprising:
a guide member extending in a predetermined direction and having an optical
encoder chart extending linearly in said predetermined direction;
a movable piece engaged with said guide member for reciprocation in said
predetermined direction along said guide member; and
a sensor arranged on said movable piece for reading information held on
said optical encoder chart, wherein
said sensor arranged on said movable piece is located substantially at a
center of a pitching motion of said movable piece occurring during travel
of said movable piece along said guide member.
28. The linear drive device according to claim 27, wherein
said guide member has a field magnet having N- and S-type magnetic poles
arranged alternately in said predetermined direction, and said movable
piece has an armature coil opposed to said field magnet for generating a
thrust in said predetermined direction when energized.
29. The linear drive device according to claim 27, wherein
said movable piece is connected to a driven object extending in a direction
perpendicular to said predetermined direction.
30. The linear drive device according to claim 29, wherein
said driven object is a slider employed in an image reading apparatus for
optically scanning and reading an original image and being driven in said
predetermined direction together with an optical part carried thereon. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a linear drive device which can linearly
drive a driven object (i.e., an object to be driven or a drive target
member), e.g., a slider carrying an optical part in an image reading
apparatus for optically scanning and reading an original document, in a
predetermined direction perpendicular to a direction of a width of the
driven object.
2. Description of the Related Art
In various fields relating to office automation equipment such as copying
machines, image scanners and printers, factory automation equipment such
as X-Y tables and object transporting apparatuses, and optical equipment
such as cameras, it is required to transport or move an object linearly in
a predetermined direction.
The object to be moved linearly is driven linearly, for example, in the
following manner.
As shown in FIG. 9, for example, a driven object 90 can be driven in an X
direction by driving a movable piece 92, which is reciprocatable in the X
direction and is connected to an end, in a direction perpendicular to the
X direction, of the driven object 90. The movable piece 92 is arranged
reciprocatable along a guide member 91 extending in the drive direction,
i.e., X direction. For keeping an attitude of the driven object 90 during
travel and other purposes, a roller r or the like, which can roll on a
plate-like guide member 93 extending in the X direction, is arranged at
the other end of the driven object 90 remote, in the width direction of
the driven object 90, from the end connected to the movable piece 92.
The movable piece 92 can be driven, for example, in the following manner.
For example, a linear motor may be employed. In this linear motor, a field
magnet provided with N- and S-type magnetic poles which are arranged
alternately in the X direction is formed at the guide member 91, and an
armature coil opposed to the field magnet is arranged in the movable piece
92. Thereby, the movable piece 92 can be driven in the X direction by
energizing the armature coil.
Alternatively, by transmitting a drive force, e.g., of a rotary motor
arranged outside the movable piece 92, to the movable piece 92 through a
drive force transmitting mechanism formed of, e.g., wire and pulleys, the
movable piece 92 can be also driven in the X direction.
For driving the driven object 90 in the X direction by the above structure
wherein the end of the driven object 90 is connected to the movable piece
92 driven in the X direction, a linear encoder may be employed for
controlling driving of the movable piece 92 and therefore the driven
object 90. The linear encoder is formed of an encoder chart extending in
the X direction and a sensor which is arranged at a position on the
movable piece 92 opposed to the encoder chart for reading held on the
encoder chart. Encoders of an optical type and of a magnetic type are
known.
An encoder chart 94 is arranged on the guide member 91, for example, as
shown in FIG. 10. As shown in FIG. 11, the chart 94 may be arranged on a
chart member 96 arranged in parallel with the guide member 91 and
therefore extending in the X direction. In FIGS. 10 and 11, 95 indicates
sensors for reading information on the encoder chart 94.
In the above structure having the movable piece 92, which is connected to
one end, in the width direction, of the driven object 90 for driving the
object 90 in the X direction perpendicular to its width direction, the
other end (free end) of the driven object 90 which is not connected to the
movable piece 92 moves with a delay from the movement of the end (driven
end) connected to the movable piece 92 as shown in FIGS. 12(A) and 12(B)
so that the driven object 90 may not be driven precisely with a stable
attitude. It can be considered that the above instability and more
specifically yawing of the driven object 90 is caused by a motion
resistance applied to the free end of the driven object 90 and a pulling
force applied to the driven end thereof by the movable piece 92. The
yawing of the driven object 90 becomes further remarkable when the driven
object 90 does not have a balanced weight distribution in the width
direction, and particularly when the free end of the driven object 90 is
heavier than the driven end. Pitching may also occur at the driven object
90 during driving. On rectangular coordinates shown in FIG. 13, it is
assumed that the x-axis defines the aforementioned X direction and the
y-axis defines the width direction of the driven object 90. In this case,
the yawing is swinging around the z-axis, and the pitching is swinging
around the y-axis.
When the yawing and/or pitching of the driven object 90 and the movable
piece 92 occur in the structure provided with the encoder, the sensor 95
on the movable piece 92 may not occupy a stable position with respect to
the encoder chart 94 as shown in FIGS. 14 and 15, and a distance between
the sensor 95 and the chart 94 varies so that the sensor 95 cannot stably
detect the information. Also, the sensor 95 may be shifted from a position
opposed to the chart 94, and thereby cannot read the information on the
encoder chart. As a result, the driven object 90 cannot be driven
precisely when driving of the movable piece 92 is con trolled based on th
e information detected by the sensor 95.
SUMMARY OF THE INVENTION
Accordingly, an object of the invention is to provide a linear drive device
which includes a guide member extending linearly in a predetermined
direction, and a movable piece engaged with the guide member for
reciprocation in the predetermined direction along the guide member and
connected to an end of a driven object for linearly driving the driven
object in the predetermined direction, and more particularly to provide a
linear drive device which can precisely and stably drive the driven
object.
Another object of the invention is to provide a linear drive device which
can suppress yawing and thereby can precisely and stably drive the driven
object. another object of the invention is to provide a linear drive
device provided with a linear encoder, in which encoder chart information
can be precisely and stably read with an encoder sensor arranged on a
movable piece even when the movable piece yaws, and thereby can precisely
and stably drive the driven object.
Yet another object of the invention is to provide a linear drive device
provided with a linear encoder, in which encoder chart information can be
precisely and stably read with an encoder sensor arranged on a movable
piece even when the movable piece pitches, and thereby can precisely and
stably drive the driven object.
In order to achieve the above objects, the invention provides linear drive
devices of the following four types (1)-(4)
(1) A linear drive device (a linear drive device of the first type) for
linearly driving a driven object in a predetermined direction
perpendicular to a direction of a width of the driven object, comprising a
shaft extending in the predetermined direction and provided with a field
magnet having N- and S-type magnetic poles arranged alternately in the
predetermined direction; and a movable piece having an armature coil
fitted around the shaft and opposed to the field magnet for generating a
thrust in the predetermined direction when energized, being reciprocatable
along the shaft, and connected to one end, in the width direction, of the
driven object, wherein thrusts generated by the armature coil at opposite
sides, in the width direction of the driven object, of the shaft are
determined such that the thrust generated at the side near the driven
object is larger than the thrust generated at the side remote from the
driven object.
(2) A linear drive device (a linear drive device of the second type) for
linearly driving a driven object in a predetermined direction
perpendicular to a direction of a width of the driven object, comprising a
guide member extending in the predetermined direction; a movable piece
having an armature coil, being reciprocatable along the guide member and
connected to an end, in the width direction, of the driven object; a first
stator extending linearly in the predetermined direction, having a field
magnet provided with N- and S-type magnetic poles arranged alternately in
the predetermined direction, and arranged at one of the opposite sides, in
the width direction of the driven object, of the guide member neighboring
to the driven object; and a second stator extending linearly in the
predetermined direction, having a field magnet provided with N- and S-type
magnetic poles arranged alternately in the predetermined direction, and
arranged at the other side, in the width direction of the driven object,
of the guide member remote from the driven object, wherein a thrust in the
predetermined direction generated by energizing the armature coil
subjected to a magnetic field formed by the field magnet of the first
stator is larger than a thrust in the predetermined direction generated by
energizing the armature coil subjected to a magnetic field formed by the
field magnet of the second stator.
(3) A linear drive device (a linear drive device of the third type)
comprising a guide member extending linearly in a predetermined direction;
a movable piece engaged with the guide member for reciprocation in the
predetermined direction along the guide member; and a sensor arranged on
the movable piece for reading information on the guide member, wherein the
sensor arranged on the movable piece is located substantially at a center
of a yawing motion of the movable piece occurring during travel of the
movable piece along the guide member.
(4) A linear drive device (a linear drive device of the fourth type)
comprising a guide member extending linearly in a predetermined direction;
a movable piece engaged with the guide member for reciprocation in the
predetermined direction along the guide member; and a sensor arranged on
the movable piece for reading information on the guide member, wherein the
sensor arranged on the movable piece is located substantially at a center
of a pitching motion of the movable piece occurring during travel of the
movable piece along the guide member.
In any one of the linear drive devices of the first to fourth types
described above, when the movable piece is connected to one of the ends,
in the width direction of the driven object, of the driven object, the
driven object can be driven linearly in the predetermined direction
perpendicular to the width direction.
In the linear drive devices of the first and second types described above,
the yawing of the movable piece and the driven object connected thereto
can be suppressed, and thereby the movable piece and the driven object can
be driven precisely and stably.
In the linear drive device of the third type described above, when, for
example, a linear encoder having an encoder chart arranged at the guide
member and the sensor arranged on the movable piece is employed, the
sensor arranged on the movable piece can precisely and stably read the
encoder chart information even when the movable piece and the driven
object yaw, and thereby the driven object can be driven precisely and
stably based on the encoder information.
In the linear drive device of the fourth type described above, when, for
example, a linear encoder having an encoder chart arranged at the guide
member and the sensor arranged on the movable piece is employed, the
sensor arranged on the movable piece can precisely and stably read the
encoder chart information even when the movable piece and the driven
object pitch, and thereby the driven object can be driven precisely and
stably based on the encoder information.
The foregoing and other objects, features, aspects and advantages of the
present invention will become more apparent from the following detailed
description and the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1(A) is a schematic plan showing an example of a linear drive device
according to the invention with a certain portion shown in a sectional
view;
FIG. 1(B) is a schematic cross section of the linear drive device taken
along line I--I in FIG. 1(A);
FIG. 1(C) shows an example of a magnetic field formed by a field magnet
shown in FIG. 1(A) on a section perpendicular to a travel direction of a
driven object;
FIG. 1(D) shows another example of a magnetic field formed by the field
magnet on a section perpendicular to the travel direction of the driven
object;
FIG. 2(A) is a schematic side view of an image reading apparatus provided
with the linear drive device shown in FIG. 1(A);
FIG. 2(B) is a schematic plan view of the image reading apparatus;
FIG. 3(A) is a schematic plan view showing another example of the linear
drive device according to the invention with a certain portion shown in a
sectional view;
FIG. 3(B) is a schematic cross section of the linear drive device taken
along line II--II in FIG. 3(A);
FIG. 3(C) shows magnetic fields formed by field magnets shown in FIG. 3(A)
on a section perpendicular to a travel direction of a driven object;
FIG. 4(A) is a schematic plan view showing still another example of the
linear drive device according to the invention with a certain portion
shown in a sectional view;
FIG. 4(B) shows magnetic fields formed by field magnets shown in FIG. 4(A)
on a section perpendicular to a travel direction of a driven object;
FIG. 5(A) is a schematic plan view showing yet another example of the
linear drive device according to the invention with a certain portion
shown in a sectional view;
FIG. 5(B) is a schematic cross section of the linear drive device taken
along line III--III in FIG. 5(A);
FIG. 5(C) shows magnetic fields formed by field magnets shown in FIG. 5(A)
on a section perpendicular to a travel direction of a driven object;
FIG. 6(A) is a schematic plan view showing further another example of the
linear drive device according to the invention with a certain portion
shown in a sectional view;
FIG. 6(B) shows magnetic fields formed by field magnets shown in FIG. 6(A)
on a section perpendicular to a travel direction of a driven object;
FIG. 7(A) is a schematic plan view showing a further example of the linear
drive device according to the invention;
FIG. 7(B) is a schematic cross section of the linear drive device taken
along line IV--IV in FIG. 7(A);
FIG. 8(A) is a schematic plan view showing a further example of the linear
drive device according to the invention;
FIG. 8(B) is a schematic cross section of the linear drive device taken
along line V--V in FIG. 8(A);
FIG. 9 is a schematic plan view of an example of a linear drive device in
the prior art;
FIG. 10 is a schematic plan view of an example of a linear drive device
provided with a linear encoder in the prior art;
FIG. 11 is a schematic plan view of another example of the linear drive
device provided with the linear encoder in the prior art;
FIGS. 12(A) and 12(B) show yawing of a movable piece in the linear drive
device shown in FIG. 9;
FIG. 13 shows directions of yawing and pitching during travel of the
movable piece;
FIG. 14 shows yawing of the movable piece in the linear drive device shown
in FIG. 10; and
FIG. 15 shows yawing of the movable piece in the linear drive device shown
in FIG. 11.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Linear drive devices of the foregoing first to fourth types will be
successively described below with reference to the drawings.
(A) Linear Drive Device of the First type
As described before, the linear drive device of the first type is provided
for linearly driving a driven object in a predetermined direction
perpendicular to a direction of a width of the driven object (the width
direction of the driven object), and comprises a shaft extending in the
predetermined direction and provided with a field magnet having N- and
S-type magnetic poles arranged alternately in the predetermined direction;
and a movable piece having an armature coil fitted around the shaft and
opposed to the field magnet for generating a thrust in the predetermined
direction when energized, being reciprocatable along the shaft, and
connected to one end, in the width direction, of the driven object. In
this device, thrusts (or propulsion force, driving force) generated by the
armature coil at opposite sides, in the width direction of the driven
object, of the shaft are determined such that the thrust generated at the
side near the driven object is larger than the thrust generated at the
side remote from the driven object.
The shaft provided with the field magnet and the movable piece, which has
the armature coil opposed to the field magnet and can reciprocate along
the shaft, form a so-called shaft-type linear motor. The shaft provided
with the field magnet functions as a stator of the linear motor. The shaft
serves as a guide member for guiding the movable piece in the
predetermined direction.
The movable piece is connected to one of the opposite ends, in the width
direction of the driven object, of the driven object. When the armature
coil in the movable piece is energized, in other words, when a current is
supplied to the armature coil, a thrust or driving force in the
predetermined direction is applied to the movable piece owing to an
interaction between the current flowing through the armature coil and a
magnetic field formed by the field magnet so that the movable piece can be
driven in the predetermined direction. Thereby, the driven object
connected to the movable piece is driven at its one end to move in the
predetermined direction.
The thrust generated by the armature coil and driving the movable piece in
the predetermined direction, and more particularly the thrusts which are
generated at the opposite sides, in the width direction (i.e., the
direction perpendicular to the predetermined direction) of the driven
object, of the shaft, are determined such that the thrust generated at the
side neighboring to the driven object is larger than the thrust generated
at the other side remote from driven object. A difference between these
thrusts, which are generated by the armature coil at the opposite sides in
the width direction of the driven object, acts as a force for driving the
end (free end) of the driven object not connected to the movable piece to
precede the other end (driven end) connected to the movable piece. This
force acts to cancel a force which acts to retard the free end of the
driven object so that a delay in movement or travel of the free end of the
driven object can be reduced as compared with the prior art. Owing to
this, the driven object can be moved in the travel direction
(predetermined direction) while keeping a regular attitude.
The thrusts generated by the armature coil at the opposite sides, in the
width direction of the driven object, of the shaft are determined such
that the thrust generated at the side near the driven object is larger
than the thrust generated at the side remote from the driven object. This
relationship can be achieved, for example, by the following manner.
The magnetic fields formed by the field magnet on the section perpendicular
to the predetermined direction, and particularly the magnetic fields at
the opposite sides, in the width direction of the driven object, of the
shaft may form an offset or eccentric magnetic field, and are determined
such that the magnetic field at the side near the driven object is larger
in intensity than the magnetic field at the side remote from the driven
object.
When two driven objects are to be reciprocated in the same predetermined
direction, two movable pieces are employed for reciprocation along the
common shaft, and one of them, i.e., the first movable piece is connected
to the first driven object. The other, i.e., the second movable piece is
connected to the second driven object.
The driven object may be a carriage or a slider which is employed in an
image reading apparatus for optically scanning and reading, e.g., an
original image and is driven in the above predetermined direction together
with an optical part carried thereon. In the image reading apparatus, it
may be required to reciprocate two sliders in the same predetermined
direction.
An embodiment of the linear drive device of the first type will now be
described below with reference to the drawings.
FIG. 1(A) is a schematic plan view showing an example of the linear drive
device of the first type with a certain portion shown in a sectional view.
FIG. 1(B) is a schematic cross section of the linear drive device taken
along line A--A in FIG. 1(A).
This linear drive device D1 is employed for reciprocating a driven object 3
in an X direction perpendicular to the width direction (lateral direction
in the figure) of the object 3.
The linear drive device D1 is provided with a shaft 11 extending in the X
direction and having a circular section, and a movable piece 21 which is
fitted around the shaft 11 for reciprocation along the shaft 11 and is
connected to one end 3a, in the width direction, of the driven object 3.
For stably driving the driven object 3 in the X direction, the driven
object 3 is provided at the other end 3b with a roller r which rolls on a
guide rail G arranged in the X direction.
The shaft 11 is made of a machinable and magnetizable material, and has a
smooth surface. The shaft 11 has been magnetized to form a field magnet
111 which is provided with N- and S-type magnetic poles arranged
alternately in the X direction with an equal pitch P.
FIG. 1(C) shows a magnetic field formed by the field magnet 111 on a
section perpendicular to the X direction. The field magnet 111 forms an
offset or eccentric magnetic field on the section perpendicular to the X
direction as described below. In FIG. 1(C), lines with arrows represent
magnetic lines of force. FIG. 1(C) shows the magnetic field around the
N-pole of the field magnet 111. The magnetic field around the S-pole is
similar to that shown in FIG. 1(C) except that the directions of the
magnetic field are opposite to those shown in FIG. 1(C).
The magnetic fields formed by the field magnet 111 on the section
perpendicular to the X direction and formed at the opposite sides, in the
width direction of the driven object 3, of the shaft 11 are determined as
follows. The magnetic field at the side near the driven object 3 (i.e., at
the right side in FIG. 1(C)) is larger in intensity than the magnetic
field at the other side remote from the driven object 3 (i.e., at the left
side in FIG. 1(C)). In this embodiment, the magnetic field in the X
direction formed by the field magnet 111 has a sinusoidal waveform having
a period defined by the N- and S-poles, and provides the offset magnetic
field as described above.
The movable piece 21 has a ring-shaped armature coil 211 which is fitted
around the shaft 11 provided with the field magnet 111 with a space. In
this embodiment, the armature coil 211 is formed of six coils L.sub.U1,
L.sub.V1, L.sub.W1, L.sub.U2, L.sub.V2 and L.sub.W2. Each coil has a width
(i.e., length in the X direction) equal to 2/3 of the magnetic pole pitch
P of the field magnet 111. These coils are shifted by 2P/3 from each
other. The armature coil 211 is supported at an inner side by a
cylindrical yoke 213 made of a magnetic material. The yoke 213 is provided
at its opposite ends in the X direction with bearings 212. The movable
piece 21 is guided by the shaft 11 through the bearings 212 for smooth
movement. The shaft 11 functions as a guide for linear motion of the
movable piece 21.
In this linear drive device D1, the shaft 11 provided with the field magnet
111 and the movable piece 21 having the armature coil 211 form the linear
motor of a so-called shaft type. The shaft 11 provided with the field
magnet 111 functions also as a stator of the linear motor.
In the linear drive device D1, a thrust (or propulsion force, driving
force) in the X direction is generated by energizing the armature coil 211
of the movable piece 21 so that the movable piece 21 is driven along the
shaft 11. Thereby, the driven object 3 coupled to the movable piece 21 is
driven at its one side so that the driven object 3 moves in the X
direction.
Since the field magnet 111 produces the offset magnetic field shown in FIG.
1(C) on the section perpendicular to the X direction, the thrust generated
by the armature coil 211 for driving the movable piece 21 in the X
direction, and more specifically the thrusts at the opposite sides of the
shaft 11 in the width direction of the object 3 have such a relationship
that a thrust fb generated at the side near the driven object 3 is larger
than a thrust fa generated at the side remote from the driven object 3.
This difference (fb-fa) between the thrusts acts as a force, by which the
free end 3b in the width direction of the driven object 3 not connected to
the movable piece 21 is driven to precede the driven end 3a connected to
the movable piece 21 in the advancing direction (parallel to the X
direction specifically including this advancing direction and a returning
direction). This force, which acts on the free end 3b of the driven object
3 to precede the other end 3a, cancels a force which acts to retard the
free end 3b, which can suppress yawing of the driven object 3 which may be
caused in the prior art by the force acting to retard the free end of the
driven object 3. The force, which acts to retard the free end 3b of the
driven object 3, may be produced due to a pulling force acting on the
driven end 3a of the driven object 3, a motion resistance at the free end
3b, imbalance in a weight distribution in the width direction of the
driven object 3 and other factors. Thereby, the driven object 3 can be
driven more precisely with a more stable attitude.
In the offset magnetic field shown in FIG. 1(C), the specific intensities
of the magnetic fields at the left and right sides in the figure can be
determined based on the weight balance of the driven object 3 itself and
parts carried thereon, the motion resistance at the end 3b, results of
experiments and/or other factors. Instead of the offset magnetic field
shown in FIG. 1(C), an offset magnetic field shown in FIG. 1(D) may be
employed.
An example of an image reading apparatus provided with the linear drive
device according to the invention described above will be briefly
described below with reference to FIGS. 2(A) and 2(B).
FIG. 2(A) is a schematic side view of the image reading apparatus, and FIG.
2(B) is a schematic plan view of the image reading apparatus. This image
reading apparatus can be utilized in a digital copying machine, an image
scanner or the like.
This image reading apparatus employs linear drive devices D1a and D1b
according to the invention for driving two carriages (sliders) 31 and 32
carrying optical parts for optical image scanning.
The linear drive devices D1a and D1b are substantially the same as the
linear drive device D1 shown in FIG. 1, and include movable pieces 21a and
21b. FIG. 2(A) shows only the movable pieces 21a and 21b with respect to
the devices D1a, D1b. Each of the movable pieces 21a and 21b has an
armature coil (not shown). These two movable pieces 21a and 21b are fitted
around the common shaft 11 provided with the field magnet 111 for
reciprocation along the shaft 11. The field magnet 111 forms the offset
magnetic field shown in FIG. 1(C). The shaft 11 is arranged along the X
direction in which the carriages 31 and 32 are to be driven. The movable
piece 21a is connected to one end of the carriage 31, and the movable
piece 21b is connected to one end of the carriage 32.
Although not shown, each of the linear drive devices D1a and D1b has a
linear encoder for detecting positions in the X direction of the movable
pieces 21a and 21b, respectively. The linear encoders are utilized for
drive control of the corresponding movable pieces, respectively.
This image reading apparatus is provided with a platen PL made of a flat
transparent glass plate. The carriages 31 and 32, and the linear drive
devices D1a and D1b are arranged under the platen PL.
The carriage 31 carries an illumination lamp LP for illuminating an
original document laid on the platen PL, reflection mirrors m1 and m2 for
directing the illumination light beams emitted from the illumination lamp
LP toward the original document, and a reflection mirror m3 for leading
the light beams reflected by the original document toward the carriage 32.
The carriage 31 is provided at its free end with a roller r1 which rolls
on a guide rail G arranged parallel to the platen PL and the shaft 11.
The carriage 32 carries reflection mirrors m4 and m5 for leading image
light beams led by the reflection mirror m3 toward a read unit 5. The
carriage 32 is provided at its free end with a roller r2 which rolls on
the guide rail G.
The read unit 5 has a lens 51 and an imaging element, i.e., a CCD 52. The
lens 51 focuses the image light beams led by the reflection mirrors m4 and
m5 on the carriage 32 onto the CCD 52. This read unit 5 is fixed to the
image reading apparatus by an unillustrated support.
When an image of the original document laid on the platen PL is to be read,
the lamp LP is turned on, a nd the linear drive devices D1a and D1b drive
the carriages 31 and 32 in the same X direction for optically scanning the
original document. These carriages 31 and 32 are driven with a speed ratio
of 2:1. During this operation, the image light beams reflected by the
original document are led to the lens 51 by the mirrors m3, m4 and m5, are
focused by the lens 51 onto the CCD 52, and are read by the CCD 52. In
this image reading apparatus, each carriage is driven by the linear drive
device according to the invention, and therefore yawing of each carriage
can be suppressed during driving. Therefore, this image reading apparatus
can perform a good image reading operation.
(B) Linear Drive Device of the Second type
The linear drive device of the second type will now be described below.
As already described, the linear drive device of the second type is
provided for linearly driving a driven object in a predetermined direction
perpendicular to a direction of a width of the driven object, and
comprises a guide member extending in the predetermined direction; a
movable piece having an armature coil, being reciprocatable along the
guide member and connected to one of the ends, in the width of the driven
object; a first stator extending linearly in the predetermined direction,
having a field magnet provided with N- and S-type magnetic poles arranged
alternately in the predetermined direction, and arranged at one of the
opposite sides, in the width direction of the driven object, of the guide
member neighboring to the driven object; and a second stator extending
linearly in the predetermined direction, having a field magnet provided
with N- and S-type magnetic poles arranged alternately in the
predetermined direction, and arranged at the other side, in the width
direction of the driven object, of the guide member remote from the driven
object. In this device, a thrust in the predetermined direction generated
by energizing the armature coil subjected to a magnetic field formed by
the field magnet of the first stator is larger than a thrust in the
predetermined direction generated by energizing the armature coil
subjected to a magnetic field formed by the field magnet of said second
stator.
The movable piece having the armature coil is engaged with the guide member
and reciprocatable in | | |
