Linear motor and image reading apparatus

What is claimed is:

1. A linear motor comprising:

a rod-like stator formed of a magnetizable rod-like member provided by magnetization with both a drive field magnet and a plurality of magnet portions providing multiple kinds of position information; and

a movable piece being movable along said stator and having an armature coil opposed to said field magnet and a sensor reading the position information from said plurality of magnet portions.

2. A linear motor according to claim 1, wherein said plurality of magnet portions for providing said position information includes a fine magnet portion used for drive control of said movable piece, and a magnet portion indicating a home position of said movable piece.

3. A linear motor according to claim 1, wherein said field magnet exhibits a magnetic force distribution producing a weak or no magnetic force at a portion provided with said plurality of magnet portions for providing said position information in said stator.

4. A linear motor according to claim 3, wherein said field magnet exhibits the magnetic force distribution producing a maximum magnetic force at a portion opposite to the portion provided with said plurality of magnet portions for providing said position information in said stator.

5. An image reading apparatus comprising:

a rod-like stator including a magnetizable rod-like member provided by magnetization with both a drive field magnet and a plurality of magnet portions providing multiple kinds of position information;

first and second movable pieces each being movable along said stator, and each having an armature coil opposed to said field magnet and a sensor for reading the position information from said plurality of magnet portions;

a first slider for scanning a document image, which is coupled to said first movable piece; and

a second slider coupled to said second movable piece for cooperating with said first slider for scanning the document image.

6. An image reading apparatus according to claim 5, wherein

said first slider carries a lamp for lighting the document on a document table and a mirror reflecting, in a predetermined direction, image light coming from said document, and

said second slider carries a mirror for reflecting, in a predetermined direction, the image light coming from the mirror on said first slider.

7. An image reading apparatus according to claim 5, wherein said plurality of magnet portions at said stator for providing said position information includes:

a fine magnet portion used for drive control of said first slider;

a fine magnet portion used for drive control of said second slider;

a magnet portion indicating a home position of said first slider; and

a magnet portion indicating a home position of said second slider.

8. An image reading apparatus according to claim 7, wherein said plurality of magnet portions at said stator for providing said position information further includes a magnet portion indicating a return position of said first slider, and a magnet portion indicating a return position of said second slider.

9. An image reading apparatus according to claim 5, wherein said plurality of magnet portions formed at said stator for providing said position information include:

a fine magnet portion used for drive control of said first slider;

a fine magnet portion used for drive control of said second slider;

a magnet portion indicating a return position of said first slider; and

a magnet portion indicating a return position of said second slider.

10. A linear motor according to claim 5, wherein said field magnet exhibits a magnetic force distribution producing a weak or no magnetic force at a portion provided with said plurality of magnet portions for providing said position information in said strator.

11. A linear motor according to claim 10, wherein said field magnet exhibits the magnetic force distribution producing a maximum magnetic force at a portion opposite to the portion provided with said plurality of magnet portions for providing said position information in said stator.

12. A linear motor comprising:

a rod-like stator having a drive field magnet; and

a movable piece including an armature coil fitted around said stator, wherein

said stator has a stepped portion at the surface of its portion provided with said field magnet, and

said movable piece has a sensor disposed near said stepped portion for reading a signal from said stator.

13. A linear motor according to claim 12, wherein said stepped portion has a form of a groove, and at least a portion of said sensor on said movable piece is fitted into said groove.

14. A linear motor according to claim 12, wherein said stepped portion has a fine magnet portion forming a portion of a magnetic encoder, and said sensor on said movable piece is provided for reading a magnetic signal from said fine magnet portion.

15. A linear motor according to claim 14, wherein said fine magnet portion at said stepped portion is formed at said field magnet in an overlapped manner.

16. A linear motor according to claim 12, wherein a portion of said stator other than said stepped portion is provided with a fine magnet portion forming a portion of a magnetic encoder.

17. A linear motor according to claim 16, wherein said sensor on said movable piece is operable to read a magnetic signal from said field magnet.

18. A linear motor according to claim 12, further comprising a carrying member carrying said stator, wherein

said carrying member is provided with a hole of a section similar in configuration to the section of said stator, and said stator is inserted into said hole.

19. An image reading apparatus comprising:

a first slider being reciprocatable and carrying a first mirror reflecting, in a predetermined direction, image light coming from a document;

a second slider being reciprocatable in the same direction as said first slider and carrying a second mirror leading, in a predetermined direction, the image light coming from said first mirror;

a first motor for reciprocating said first slider;

a second motor for reciprocating said second slider;

a first absolute encoder for sensing a current position of said first slider;

a second absolute encoder for sensing a current position of said second slider; and

control means for controlling said first and second motors to move said first and second sliders to their initial positions without mutual collision based on the current positions of said sliders sensed by said encoders, respectively, upon start of supply of a power from a power source to the apparatus.

20. An image reading apparatus comprising:

a first slider being reciprocatable and carrying a first mirror reflecting, in a predetermined direction, image light coming from a document;

a second slider being reciprocatable in the same direction as said first slider and carrying a second mirror leading, in a predetermined direction, the image light coming from said first mirror;

a first motor for reciprocating said first slider;

a second motor for reciprocating said second slider;

a first scale;

a first encoder for sensing a current position of said first slider using the first scale;

a second scale;

a second encoder for sensing a current position of said second slider using the second scale; and

control means for controlling said first and second motors to move said first and second sliders to their initial positions without mutual collision based on the current positions of said slider sensed by said encoders, respectively, upon start of supply of a power from a power source to the apparatus.

21. An image reading apparatus according to claim 20, wherein the first scale and the second scale are coordinated such that the first slider moves at twice the velocity of the second slider.

22. A linear motor according to claim 1, wherein the plurality of magnet portions is distinct from the drive field magnet.

Description Submit all comments and votes

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a linear motor in which a movable piece having an armature coil is movable along a stator having a drive field-magnet, and also relates to an image reading apparatus employing such a linear motor as a drive source.

2. Description of the Background Art

Linear motors have been widely employed in various kinds of technical fields, and it has been attempted to use the linear motor as a unit for driving, e.g., a document image scanning optical system in an image reading apparatus, e.g., in a copying machine or an image scanner.

An example of an image reading apparatus will be described below. Referring to FIG. 18, a lighting lamp 92 is disposed under a document table glass 91 on which a document is laid. Light emitted from the lamp 92 is reflected by reflection mirrors m1 and m2, and is gathered at an image reading or capturing position 93 on the glass 91. Light reflected by the document is reflected by a mirror m3 and a set of mirrors m4 and m5, and is focused by a lens LN to form an image at a CCD camera 94 which is a linear camera element.

The lamp 92 and mirrors m1, m2 and m3 are mounted on a movable slider C1, and the mirrors m4 and m5 are mounted on a movable slider C2. In the image reading operation, the slider C1 moves to drive the lamp 92 and the mirrors m1, m2 and m3 in a document image sub scanning direction X, and the slider C2 moves to drive the mirrors m4 and m5 in the same direction X. In this operation, the lamp 92 and the mirrors m1, m2 and m3 are driven at a speed, of which ratio to the driving speed of the mirrors m4 and m5 is 2:1. Therefore, no change occurs in a distance from the image reading position 93 to the lens LN, and thus a focused state is maintained. The image formed at the CCD camera 94 is photoelectrically converted to send the same to an unillustrated image processing circuit. In this manner, the reading operation is completed.

The magnification of image reading in the sub scanning direction X can be changed by adjusting the moving speed of the sliders C1 and C2, and the magnification in the main scanning direction perpendicular to the direction x can be changed by electrically processing image data issued from the CCD camera 94.

In the image reading apparatus described above, the sliders C1 and C2 are generally driven by a drive mechanism shown in FIGS. 19(A) and 19(B).

In this drive mechanism, as shown in FIG. 19(A), there is arranged a guide rod 81 extending in the sub scanning direction X, onto which first and second movable pieces 82 and 83 are slidably fitted. An end c11 of the slider C1 is coupled to the first movable piece 82, and an end c14 of the slider C2 is coupled to the second movable piece 83.

A free end c12 of the slider C1 is guided parallel to the guide rod 81 by a roller c13 which is arranged at the free end c12 and rolls on a guide 99. A free end c15 of the slider C2 is guided parallel to the guide rod 81 by a roller c16 which is arranged at the free end c15 and rolls on a guide 99.

Pulleys 841 and 842 are rotatably arranged at fixed positions near the opposite ends of the guide rod 81. Another pulley (corresponding to a running block) 831 is rotatably arranged at a side surface of the second movable piece 83. As shown in FIG. 19(B), a drive pulley 843 is arranged at a fixed position under and between the pulleys 841 and 842. The drive pulley 843 is rotated by a rotary motor 86 via a belt transmission device 85.

One wire 87 is wound around the drive pulley 843 through an angle larger than 360 degrees. One of the portions of wire 87 extending from the drive pulley 843 is retained around the pulley 841 and the pulley 831 on the second movable piece 83, and is coupled to a spring 88. The spring 88 is coupled to a fixed portion of the apparatus, and is located at a constant position. The other portion of the wire 87 extending from the drive pulley 843 has a portion retained around the other end pulley 842, an intermediate portion coupled to the first movable piece 82, a portion retained around the pulley 831 on the second movable piece 83 and an end coupled to a fixed portion of the apparatus at a position opposed the spring 88.

According to this driving system, when the motor 86 operates to drive and rotate the drive pulley 843, the wire 87 travels, so that the first movable piece 82 coupled thereto moves, and therefore the slider C1 is driven. Also, the second movable piece 83 and therefore the slider C2 are driven at a half speed of the first slider C1.

In the conventional drive system described above, however, many parts such as the belt transmission device, drive pulley, fixed pulleys and wire exist between the motor 86 and the movable pieces 82 and 83, so that the positions of the sliders C1 and C2 are liable to be incorrect and instable due to tolerance, variation in size, expansion/contraction, mounting error and others.

An encoder for sensing a position of the slider is mounted on the rotary motor 86, which operates at a constant speed. In spite of the constant speed of the motor, each of the sliders C1 and C2 may not travel at a constant speed. The position of the slider is determined by counting the number of pulses issued from the encoder associated to the rotary motor 86, which may cause a large error. Home sensors other than the encoder are required for sensing initial positions (home positions) of the sliders C1 and C2.

Meanwhile, linear motors, and particularly linear motors of which movable pieces are movable along a rod-like stator may be employed for driving the sliders C1 and C2 by the movable pieces. In this case, the linear motion of the movable pieces can directly cause the linear motion of the sliders, and the position and others of the sliders can be easily and accurately controlled based on the outputs of the encoders. By the above reason, it has been attempted to utilize the linear motors.

However, in connection with the sliders C1 and C2, it is necessary to drive the slider C1 at double the speed of the slider C2 in the sub scanning direction. It is necessary to prevent collision between the sliders C1 and C2 when they are being set at the home positions during initial setting to be performed upon power-on of the image reading apparatus or the like. Upon completion of the image scanning, the sliders C1 and C2 must start to return from the respective current positions accurately to the predetermined home positions. These operations require complicated control.

By the above reason, a plurality of encoders are required for performing a plurality of kinds of control when the linear motors are utilized for driving the sliders. Therefore, a complicated structure is required by the above and other reasons.

SUMMARY OF THE INVENTION

Accordingly, an object of the invention is to provide a linear motor for driving a driven member, which allows precise control of a position and others of the driven member by a simple structure.

Also, it is an object of the invention to provide an image reading apparatus which has sliders to be driven for scanning a document image, and employs a linear motor as a drive source of the sliders, and in which the operation of the sliders can be controlled precisely with a simple structure, and thereby an image scanning precision is improved.

In order to achieve the above object, the present invention provides a linear motor which includes a rod-like stator formed of a magnetizable rod-like member provided by magnetization with a drive field magnet and a plurality of magnet portions providing multiple kinds of position information; and a movable piece being movable along the stator and having an armature coil opposed to the field magnet and a sensor reading the position information from the plurality of magnet portions.

In order to achieve the above object, the present invention also provides an image reading apparatus which includes a rod-like stator including a magnetizable rod-like member provided by magnetization with a drive field magnet and a plurality of magnet portions providing multiple kinds of position information; first and second movable pieces each being movable along the stator, and each having an armature coil opposed to the field magnet and a sensor for reading the position information from the plurality of magnet portions; a first slider for scanning a document image, which is coupled to the first movable piece; and a second slider coupled to the second movable piece for cooperating with the first slider for scanning the document image.

In the above apparatus, the stator and the first movable piece form a linear motor, and the stator and the second movable piece form another linear motor.

The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) shows a schematic structure of an example of an image reading apparatus provided with a linear motor according to the invention;

FIG. 1(B) is a plan of a slider driving portion in the apparatus in FIG. 1(A);

FIG. 2(A) is a cross section of the linear motor shown in FIG. 1(B);

FIG. 2(B) shows a magnetic force distribution of a field magnet in a stator of the linear motor portion and positional state thereof with respect to other magnet portions;

FIG. 2(C) shows magnet portions other than the field magnet in the stator and sensors corresponding to the same;

FIG. 3(A) shows an example of a state of a magnetic signal wave from a fine magnet portion for speed control of a movable piece in a stator of a linear motor;

FIG. 3(B) shows an example of a state of a signal from an absolute magnet portion on the stator;

FIG. 4(A) is a schematic block diagram showing an operation control circuit of the linear motor;

FIG. 4(B) shows a major portion of the operation control circuit including a speed control circuit of a phase-locked-loop servocontrol type;

FIG. 5(A) is a perspective view of another example of a linear motor according to the invention;

FIG. 5(B) is a cross section of the motor taken along a horizontal plane containing an axial line in a longitudinal direction of a stator;

FIG. 6(A) is a cross section of still another example of a linear motor according to the invention taken along a horizontal plane containing an axial line in a longitudinal direction of a stator;

FIG. 6(B) is a cross section of the motor taken along line Y--Y in FIG. 6(A);

FIG. 7 shows a section of a stator of yet another example of a motor according to the invention together with sensors opposed thereto;

FIG. 8 shows a section of a stator of further another example of a motor together with sensors opposed thereto;

FIG. 9 shows a state of arranging a stator carrying member in an instrument for installing the motor stator shown in FIGS. 7 and 8 to the member;

FIGS. 10(A) to 10(D) show examples of stepped portions at a stator in a linear motor according to the invention;

FIG. 11(A) is a schematic plan of an image reading apparatus according to the invention arranged in a digital copying machine;

FIG. 11(B) is a schematic side view of the image reading apparatus shown in FIG. 11(A) with a certain part cut away;

FIG. 12(A) is a side view of linear motors forming first and second slider driving means in the image reading apparatus shown in FIG. 11(A);

FIG. 12(B) is a cross section of the motor shown in FIG. 12(A);

FIG. 12(C) is a plan showing, on an enlarged scale, a magnetic scale on a stator of the linear motor shown in FIG. 12(A);

FIG. 13 shows a magnetic pole pitch and a magnetic force distribution of a field magnet on a stator;

FIG. 14 is a circuit diagram of a controller of a linear motor in the image reading apparatus shown in FIG. 11(A);

FIG. 15 is a flowchart of processing by the controller of the image reading apparatus shown in FIG. 11(A);

FIG. 16 is a flow chart showing contents of processing in the flow chart shown in FIG. 15 for moving first and second sliders to home positions;

FIG. 17(A) is a side view of a linear motor provided with an optical encoder;

FIG. 17(B) is a plan of the same;

FIG. 17(C) shows an example of a principle of an optical encoder;

FIG. 18 shows a schematic structure of an example of an image reading apparatus in the prior art;

FIG. 19(A) shows a plan of a driving portion for an image scanning optical system in the apparatus shown in FIG. 18; and

FIG. 19(B) is a side view of the driving portion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Generally, the first slider carries a lamp for lighting a document on a document table and a mirror for reflecting, in a predetermined direction, light coming from the document, and the second slider carries a mirror for reflecting, in a predetermined direction, the light coming from the mirror on the first slider.

In the linear motor according to the invention as well as the linear motor employed in the image reading apparatus according to the invention, the plurality of magnet portions arranged on the stator for controlling the motor operation are usually used for different kinds of control, respectively. Each of the magnet portions may construct an increment encoder, or the plurality of magnet portions may construct an absolute encoder.

Two or more magnet portions may be formed along a longitudinal straight line on the stator. In connection with this, for example, a plurality of magnet portions of different polarities may be arranged along a longitudinal straight line of the stator for controlling the motor operation.

In each of the linear motors in the image forming apparatus described above, the plurality of magnet portions described above on the common stator may be two or more of the following magnet portions, although not restricted to them.

(1) A fine magnet portion for drive control of the first slider.

(2) A fine magnet portion for drive control of the second slider.

(3) A magnet portion for sensing a home position of the first slider.

(4) A magnet portion for sensing a home position of the second slider.

(5) A magnet portion for sensing a return position (e.g., corresponding to each of regular document sizes) of the first slider.

(6) A magnet portion for sensing a return position (e.g., corresponding to each of regular document sizes) of the second slider.

(7) A magnet portion including position information for sensing a document size.

(8) Magnet portions for sensing stop positions of the first and second sliders at the time of shading correction.

(9) Magnet portions for sensing positions of the first and second sliders when the sliders are to be stopped for reading a sheet-through document or a transparent document.

The information reading sensor corresponding to the magnet portion of the above item (1), (3) or (5) is mounted on the first movable piece, and the information reading sensor corresponding to the magnet portion of the above item (2), (4) or (6) is mounted on the second movable piece.

In connection with the magnet portions of the above items (5) and (6), such a structure may be employed that only one of these magnet portions is provided for sensing the return position of one of the first and second sliders, and the return operation of the other slider may be performed in an interlocked manner in accordance with the return operation of the one slider in response to an instruction from a controlling portion which controls the entire operation of the image reading apparatus. In this case, the reading sensor corresponding to one of those of the above items (5) and (6) is not required. The magnet portion for sensing the return position of the first or second slider may be arranged on the same longitudinal straight line of the stator as the magnet portion for sensing the home position of the first or second slider.

In connection with the above item (7), sensing or determination of the document size can be performed, for example, as follows. A sensor for sensing existence/non-existence of the document is mounted on the linear motor for driving the first slider, and only this linear motor is driven. Accordingly, the magnet portion for sensing the document size of the above item (7) may be arranged to indicate the stop position of the movable piece of the linear motor corresponding to each document size. This magnet portion may be arranged on the same straight line as the magnet portions indicating the home position and the return position. In this case, it is preferable that the polarity thereof is opposite to that of the magnet portions indicating the home position and the return position.

With respect to the items (8) and (9), it is necessary to prepare the sensors corresponding to them for both the linear motors for driving the first and second sliders. The magnet portions of the above items (8) and (9) may be arranged on the same straight line, but it is desired that they are arranged on a line different from that for the home position, and/or that the polarity of them is opposite to that of the magnet portion for the home position. These magnet portions may be located at positions opposite to the return position with the home position therebetween.

In the linear motor according to the invention, the movable piece is driven along the stator.

The stator is provided with the plurality of magnet portions formed by magnetization for providing multiple kinds of position information for the control of the motor operation. Also, the movable piece is provided with the sensor for reading the information of the magnet portion. Each magnet portion and the corresponding sensor form the encoder, of which output is used for controlling the operation of the movable piece.

Since the stator is provided with the plurality of magnet portions for the motor operation control, the entire structure of the motor is simplified, and the position information required for the operation of the motor can be easily and accurately derived directly from the magnet portions, which allows precise control of the operation of the movable piece.

According to the image reading apparatus of the invention, the linear motors are utilized for driving the first and second sliders for scanning the document image. Therefore, the entire structure of the apparatus is simplified, and the image scanning can be performed precisely.

As a motor achieving the foregoing object, the invention also provides a linear motor comprising a rod-like stator having a drive field magnet; and a movable piece including an armature coil fitted around the stator, wherein the stator has a stepped portion at the surface of its portion provided with the field magnet, and the movable piece has a sensor disposed near the stepped portion for reading a signal from the stator.

In this linear motor according to the invention, the basic shape of section of the stator may be selected from various shapes such as a triangle, square and ellipse, and is not restricted to a particular shape.

The stepped portion formed at the stator may be considered as a concavity. Therefore, it may have a sectional shape selected from various shapes such as a square, triangular or V-shape, and semicircular or U-shape, or may be a recess having a D-shaped section in a stator of a circular section, as shown in FIGS. 10(A) to 10(D).

In addition to provision of the stepped portion at the stator, any of the foregoing linear motors according to the invention may have the following structure. The sensor formed at the movable piece for reading a signal from the stator, and specifically, the sensor at the movable piece neighboring to the stepped portion at the stator may be a drive sensor, i.e., sensor for driving, which receives a magnetic signal wave from the field magnet, or a sensor receiving a magnetic signal from position information providing means formed at the stepped portion (e.g., a sensor for encoder receiving a magnetic signal from a magnet portion forming a portion of the encoder and formed at the stepped portion). The drive sensor for reading the magnetic signal wave from the field magnet may be typically a Hall element which is a kind of a magneto-electric conversion element, although not restricted to this.

The magnet portion(s) for the encoder formed at the stepped portion may typically be a fine magnet portion, which is provided for forming a magnetic encoder used for sensing, e.g., a moving direction and a moved distance of the movable piece and is formed by arranging N- and S-poles alternately along a longitudinal direction of the stator at a fine pitch (e.g., 50 .mu.m), and may also be other portions such as magnet portions provided for sensing a home position, return position or the like of the movable piece and arranged partially on the stator along a straight line. The sensor for the encoder on the movable piece corresponding to each of these magnet portions may be typically a magnetic sensor of a galvanomagnetic effect type having a good sensitivity and called an MR element or an MR sensor. The above magnet portion may be prepared by using a rod-like member made of a magnetizable and machinable material as the stator, and magnetizing a portion of the member corresponding to the stepped portion formed at the member. Alternatively, a magnet portion formed of an independent film, belt, sheet belt, belt plate, label or the like may be joined to the portion, which corresponds to the stepped portion and is formed at the rod-like member providing the stator, by appropriate means such as adhesive.

In stead of or in addition to the magnetic encoder described above, the linear motor according to the invention may employ an optical encoder. If the optical encoder is employed, the information providing means for providing the position information, which forms a portion of the encoder and can be provided at the stepped portion of the stator, may typically be an optical scale which includes, in an alternately arranged form, two kinds of portions providing different light reflection states (e.g., concavities and convexities providing different light reflection states, two kinds of surfaces having different surface roughnesses and providing different light reflection states, or two kinds of portions painted in dark and light colors). The information providing means may also employ another structure such as light reflecting portions which are arranged at portions of the stator along a straight line for sensing the home position, return position or the like for the movable piece. The sensor for the encoder on the movable piece corresponding to the above optical information providing means may typically be a sensor including a light source and a photocell. The optical information providing means may be formed directly at the stepped portion of the stator, or may be formed independently in a film-belt form, sheet-belt form, a belt-plate form, a label form or the like and may be joined to the stepped portion by appropriate means such as adhesive.

Regardless of the type of the encoder, the linear motor according to the invention may include such a structure that the stepped portion is formed at the stator surface in the field magnet portion of the stator, the drive sensor is opposed to and arranged near the stepped portion, at least one of stepped portions, which are different from the above stepped portion, are formed at the surface of the stator, the position providing means for providing position information for controlling the motor operation is formed at said at least one stepped portion, and the encoder sensor opposed and neighboring to the information providing means for reading information therefrom is mounted on the movable piece.

In this linear motor, the information providing means for providing positional information for the motor operation control arranged at the stepped portion of the stator as well as the corresponding sensor for the encoder on the movable piece may be of either the magnetic type or the optical type described above.

In any of the linear motors according to the invention including the stator provided with the stepped portion, the stepped portion forms a mark, which allows easy assembly and installation to an instrument compared with a similar kind of motor in the prior art. In connection with this, at least a portion of the stepped portion at the stator may serve as an engagement portion engagable with a member for carrying the stator in order to allow further easy and accurate assembly of the motor and installation thereof to the instrument.

In this case, the engagement portion may be employed for various purposes such as positioning and/or rotation-stop for the stator (e.g., for the stator having a circular section) and others depending on the sectional shape of the stator.

The stepped portion at the stator may serve as a guide groove for the movable piece, and the movable piece may be provided with a member which is movable along the stepped portion. This improves the assembly precision of the motor and the mechanical operation precision of the motor.

The stepped portion may usually be in a concave groove form, for example, having a section shown in FIGS. 10(A) and 10(B) or FIG. 10(C). A ball-spline mechanism may be formed by the groove and a ball on the movable piece fitted thereto.

In any of the linear motors according to the invention provided with the stepped portion at the stator, the movable piece can be driven along the stator by supplying a current to the armature coil of the movable piece under the control based on the output of the drive sensor which reads a magnetic signal wave from the field magnet.

In the motor of this type, the drive sensor for reading the magnetic signal wave from the field magnet may be opposed closely to the stepped portion at the stator. Alternatively, the information providing means for providing the position information for the motor operation control may be arranged at the stepped portion on the stator, and the sensor for the encoder on the movable piece may be opposed closely to the information providing means. Further alternatively, the drive sensor may be opposed closely to the stepped portion at the stator, and the information providing means for providing the position information for the motor operation control may be arranged at the stepped portion. In these motors, therefore, at least one kind of information used for the motor operation on the stator can be easily and precisely sensed, and further the whole structure can be compact and can have a small size. Further, by using the stepped portion at the stator as a mark, the assembly of motor and the installation thereof to an instrument can be performed easily and precisely.

In the case where at least a portion of the stepped portion at the stator serves as the engagement portion engagable with the carrying member for carrying the stator, the assembly of motor and the installation thereof to an instrument can be performed easily and precisely by using the stepped portion.

In the case where the stepped portion at the stator serves also as the guide groove for the movable piece, and the movable piece is provided with the member which is movable along the stepped portion, this structure improves the motor assembling precision and the mechanical operation precision of the motor.

The linear motor of the type provided with the stepped portion at the stator may be utilized for driving the first and second sliders in the image reading apparatus already described.

In order to achieve the foregoing object, the present invention also provides an image reading apparatus including:

a first slider being reciprocatable and carrying a first mirror reflecting, in a predetermined direction, image light coming from a document;

a second slider being reciprocatable in the same direction as the first slider and carrying a second mirror leading, in a predetermined direction, the image light coming from the first mirror;

a first motor for reciprocating the first slider;

a second motor for reciprocating the second slider;

a first absolute encoder for sensing a current position of the first slider;

a second absolute encoder for sensing a current position of the second slider; and

control means for controlling the first and second motors to move the first and second sliders to their initial positions without mutual collision based on the current positions of the sliders sensed by the encoders, respectively, upon start of supply of a power from a power source to the apparatus.

According to this image reading apparatus, when the power is supplied from the power source, each absolute encoder is powered on so that the current absolute position of the slider corresponding to each encoder is sensed prior to the power-on of each motor. Then, the control means operates based on the position information sensed by the encoders, and specifically allows the power-on of the respective motor to move both the sliders to their initial positions while suppressing collision of the sliders with each other.

The number, position and direction of the mirror(s) mounted on the first slider as well as the number, position and direction of the mirror(s) mounted on the second slider can be determined appropriately.

The power-on of the respective encoders prior to the power-on of the respective motors may be executed, for example, simultaneously with the supply of the power to the image reading apparatus from the power source or in response to an instruction from the control means.

The first and second motors may be either the linear motors or the rotary motors.

Each of the absolute encoders may be a magnetic encoder or an optical encoder.

The control means may operate as follows for returning the respective sliders to the initial positions without causing collision with each other. For example, prior to the power-on of the respective motors, the direction and distance which each slider should move are calculated based on the current positions of the sliders sensed by the encoders, and the operation of the respective motors is controlled based on the results of calculation.

In any case, the control means may control the operation of the respective motors in the following manner. When each encoder senses the position of the corresponding slider prior to return of the slider to the initial position, a distance between these sliders is derived from the sensed position information. When the distance is smaller than a predetermined distance (in the case where there is a possibility that both the sliders are in contact with each other, or are excessively close to each other and thus may cause collision), both the sliders are moved apart to set a predetermined distance therebetween, and then both the sliders are moved to the respective initial positions without collision based on the current position information of the sliders supplied from the respective encoders.

In any case, the control means may control the operation of the respective motors as