Abstract:
An image reading apparatus comprises an original mounting table, a scanning means for scanning an original mounted on the original mounting table, a carriage for mounting the scanning means thereon, a cable for transmitting a moving force to the carriage, and a guide member for guiding the carriage in the movement direction thereof. The carriage is biased in a rotating direction centering about an axis perpendicular to the original mounting table by the tension of the cable.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to an image reading apparatus for computer input, which reads an original such as a photograph or a document, converts it into digital data and outputs the digital data, and an image reading apparatus corresponding to the original reading portion of an analog copier. 
   2. Related Background Art 
   The construction of a color image reading apparatus according to the prior art is schematically shown in  FIGS. 5A to 5C  of the accompanying drawings. 
   The letter P designates an original to be read, which is placed on an original mounting glass table  100 , and a reading unit  101  is scanned in parallel to the original mounting glass table  100  to thereby read an image on the original. 
   The reading unit  101 , as schematically shown in  FIG. 6  of the accompanying drawings, has incorporated therein LED&#39;s  101 R,  101 G and  101 B of three colors which are light sources for irradiating the original, a rod lens array  101 L for imaging the reflected light from the original on the light receiving element of an image sensor, and the image sensor  101 S. 
   The light sources of three colors are successively changed over and turned on and the image sensor  101 S reads the reflected light of each color from the original, to thereby effect color resolution reading. 
   The reading unit  101  is fixedly supported on a slider  102  slidable on a guide shaft  103  fixed to the main body of the apparatus. Also, a belt  104  for transmitting motive power from a motor  105  which is a scanning drive source is fixed on the slider  102 . 
   By the forward and reverse rotations of the motor  105 , the reading unit  101  can be reciprocally scanned within the range of the original mounting glass table  100 . 
   The constituents of the image reading apparatus further include an electrical equipment portion  106  comprising a control board and a power source, besides what has been described above. 
   These constituents are disposed in a housing comprising a combination of an upper cover  112  for fixedly supporting the original mounting glass table  100  and a lower cover  113 . 
   An original cover  111  for pressing the original against the original mounting glass table  100  is openably and closably mounted on the original mounting glass table  100 . 
     FIG. 7  of the accompanying drawings is a read image data processing block diagram by this image reading apparatus. An image output signal read by the image sensor  101  in synchronism with the LED&#39;s turned on and off in succession is sent to an amplifier  121  and is amplified thereby, and thereafter is converted into a digital image signal by an A/D converter  122 . 
   The A/D converter  122  divides the dynamic range of the image sensor (the reading output difference between the white portion and the black portion of original) into the bit number thereof, and allots the number of gradations in conformity with the brightness of the image on the original. 
   For example, when an A/D converter of resolving power of 8 bits is used, white to black can be discriminated into 256 gradation levels, and when an A/D converter of resolving power of 10 bits is used, white to black can be discriminated into 1,024 gradation levels. Accordingly, in an image reading apparatus using the A/D converter of 8 bits, 24 bits=about 16,700,000 colors can be discriminated in the color reading by light sources of three colors R, G and B, and in the case of 10 bits, 30 bits=about 1,074,000,000 colors can be discriminated. 
   There are several kinds of output forms of the image signal of the image reading apparatus, and depending on the use of an image read, an output form suited therefor can be selected. 
   When a writing is to be read and the content thereof is to be applied to OCR or when a monochromatic line drawing is to be read, a monochromatic binary image is suitable, and use is made of image data obtained by binarizing an image signal obtained with e.g. only G of the above-described light sources of R, G and B turned on, by a certain threshold value in an image processing circuit incorporated in a gate array  123 . 
   When an image is to be read with a view to read an image such as a photograph and output it to a monochromatic printer, use is made of image data binarized by the use of halftone processing such as a dither method or an error diffusing method using the image signal also by the G light source. When the processing of a color image is to be effected, image data of multiple values (24 bits, etc.) are suitable. 
   The image signal passed via the image processing circuit is outputted to a computer  200  which is an apparatus such as a personal computer through an interface circuit  124 . 
   In recent years, however, the image reading apparatus according to the prior art as described above has come to be often used in offices and homes with the spread of personal computers. Along therewith, it has been brought to the fore as an important item in the specification of products to make the installation area occupied on a desk and consumed electric power as small as possible. 
   The pending problem when downsizing the apparatus is that since the span between the bearings of the reading unit  101  in the sub-scanning direction is short, vibration sometimes occurs to the movement of the reading unit  101  due to the fitting backlash between the bearings and the guide shaft  103 . An image reading apparatus connected to a computer needs have the function of interruption/resuming the operation thereof in the course reading in conformity with the processing situation of the computer, and for the image before and after the interruption/resumption to be smoothly connected, there must not be such vibration. 
   Also, a method of making consumed electric power small is to make the electric current supplied to the motor  105  small, and for that purpose, it is necessary to make the driving load of the motor  105  small. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above-noted problem and an object thereof is to provide an image reading apparatus in which the movement of a carriage can be effected smoothly. 
   Another object of the present invention is to provide a compact image reading apparatus. 
   Still another object of the present invention to provide an image reading apparatus comprising:
         an original mounting table;   a scanning means for scanning an original mounted on the original mounting table;   a carriage for mounting the scanning means thereon;   a cable for transmitting a moving force to the carriage; and   a guide member for guiding the carriage in the movement direction thereof;   wherein the carriage is biased in a rotating direction centering about an axis perpendicular to the original mounting table by the tension of the cable.       

   Further objects of the present invention will become apparent from the following detailed description when read with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A ,  1 B and  1 C schematically show the construction of an image reading apparatus according to a first embodiment of the present invention. 
       FIGS. 2A ,  2 B and  2 C are enlarged views showing the image reading apparatus according to the first embodiment. 
       FIGS. 3A ,  3 B and  3 C are enlarged views showing an image reading apparatus according to a second embodiment of the present invention. 
       FIGS. 4A ,  4 B and  4 C are enlarged views showing an image reading apparatus according to a third embodiment of the present invention. 
       FIGS. 5A ,  5 B and  5 C schematically show the construction of an image reading apparatus according to the prior art. 
       FIG. 6  schematically shows the construction of a reading unit. 
       FIG. 7  is a read image data processing block diagram showing the image reading apparatus. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Some preferred embodiments of the present invention will hereinafter be described in detail by way of example with reference to the drawings. However, the dimensions, materials, shapes, relative disposition of constituent parts described in these embodiments, unless otherwise specifically described, are not intended to restrict the scope of this invention thereto. 
   First Embodiment 
   A first embodiment will hereinafter be described with reference to  FIGS. 1A to 1C  and  FIGS. 2A to 2C .  FIGS. 1A to 1C  and  FIGS. 2A to 2C  show an example of the construction of an image reading apparatus according to the first embodiment. 
   The letter P designates an original to be read placed on an original mounting glass table  1 , and a reading unit  2  as scanning means is scanned in parallel to the original mounting glass table  1  to thereby read an image on the original. 
   The reading unit  2  has incorporated therein a light source for irradiating the original, a lens for imaging the reflected light from the original on the light receiving element of an image sensor, and the image sensor. 
   The reference numeral  11  denotes a frame serving also as an outer package cover and having disposed therein the original mounting glass table  1  and the reading unit  2 , and besides these, a rail  12  as a rail member for guiding the running of the reading unit  2 , a control board, a power source, etc. 
   Spacers  21  made of a material of high slidability such as POM are fixed to the opposite ends of the upper surface of the reading unit  2  in the main scanning direction thereof. 
   Also, a support shaft  2   a  in the unit  2  is rotatably supported by an aperture portion  22   a  on a sensor holder (carriage)  22 , and the unit  2  is upwardly biased with the support shaft  2   a  as the center of rotation by a spring  23  fixed onto the holder  22 . 
   As the result, the spacers  21  contact with the back of the original mounting glass table  1 , whereby the reading unit  2  scans the original while keeping the spacing between the surface of the original mounting glass table  1  and the light receiving surface of the image sensor constant. 
   Also, the sensor holder  22  has slider portions  22   b  and  22   c  as sliding portions sliding relative to the rail  12 . These slider portions  22   b  and  22   c , like the spacers  21 , are made of a material such as POM. 
   Further, the sensor holder  22  has rotatably disposed thereon a transmitting mechanism for transmitting a driving force to the reading unit  2  and a pulse motor  31  which is a driving source, a gear train for decelerating the rotation thereof, a driving pulley  32  to which the rotation of the motor  31  is transmitted through the gear train, and an idle pulley  33 . 
   The reading unit  2  is connected to a control board on the main body of the apparatus by a cable, not shown, and the exchange of the electric power, the driving signal and an image signal is executed. 
   A rail  12  for guiding the running of the reading unit  2  is fixedly placed in the frame  11 . 
   Also, one end  13   a  of a driving wire  13  as a rope-like member is fixed to the reading terminating end side. The driving wire  13  is twined on the driving pulley  32  on the reading unit  13 , and is further passed over the idle pulley  33 , and thereafter is fixed to the reading starting end side of the frame  11  through a spring  14 . 
   The driving wire  13  is passed over as described above, whereby the reading unit  2  receives a moment as indicated by arrow A, and thereby the slider portions  22   b  and  22   c  on the sensor holder  22  come into contact with the rail  12 . 
   The operation of the image reading apparatus constructed as described above will now be described with reference to  FIGS. 1 and 2 . 
   The reading unit  2  usually stands by at its home position on the reading starting end side during the non-operation thereof. When it receives a reading command from a computer connected thereto, the reading unit  2  starts scanning by the rotation of the motor  31 , scans a white reference plate provided between the home position and the original reading starting position of the apparatus and produces shading correction data, whereafter it effects the reading of the image on the original from the reading starting position. 
   Here, the rotation of the motor  31  is decelerated through the gear train and is transmitted to the driving pulley  32 . Usually, the step angle of the motor  31 , the reduction ratio of the gears and the outer diameter of the driving pulley are determined so that the reading unit  2  may be moved by an amount corresponding to one sub-scanning line for a plurality of driving pulses given to the motor  31 . 
   When the motor  31  is rotated in a forward direction, the driving pulley  32  takes up the wire  13  with a result that the reading unit  2  is moved in the scanning direction. 
   Also, when the motor  31  is rotated in a reverse direction, the reading unit  2  is moved toward its home position. 
   As previously described, the reading unit  2  is always biased in the direction of arrow A due to the moment created by the tension applied to the wire  13 , and the slider portions  22   b  and  22   c  contact with the rail  12 , whereby the posture of the reading unit is maintained. 
   That is, such a force that there is no backlash between the rail  12  and the sliding portion of the reading unit  2  and the sliding portion constantly contacts with the rail  12  is working and therefore, it is avoided that during the interruption/resumption of reading, vibration occurs to the movement of the reading unit  2 . 
   The above-mentioned moment can be minimized within a range in which the vibration of the reading unit  2  does not occur, by appropriately choosing the spacing B between the driving pulley  32  and the idle pulley  33  in the lengthwise direction of the reading unit. 
   Also, the space between one end portion  13   a  of the wire  13  and the pulley  32  is parallel to the rail  12  with a first spacing (first portion), the space between the other end  14  of the wire  13  and the pulley  33  is also parallel to the rail  12  with a second spacing (second portion), the space between the first portion and the second portion is non-parallel to the rail  12  (third portion). 
   The basic portion of the driving system in the first embodiment is designated such that with respect to the reading direction in which feeding accuracy is necessary, the reading unit  2  draws in the wire  13  having one end thereof fixed and is moved thereby. 
   Also, in the return direction, there is the possibility of more or less feeding irregularity occurring when the wire  13  is drawn in by the expansion and contraction thereof because the wire  13  is fixed to the apparatus through a tension spring, but feeding accuracy is not much required. Accordingly, the tension applied to the wire  13  may be small as compared with that in driving systems of other types. 
   As described above, in the present invention, the backlash between the bearings and the rail  12  which poses a problem when the bearing span of the reading unit  2  is short can be eliminated by the utilization of the tension applied to the wire  13 . Accordingly, even if there is the interruption/resumption of the operation in the course of reading, a smoothly connected image can be obtained and the downsizing of the apparatus can be expedited. 
   Also, as compared with the prior art system, the necessary driving load can be greatly reduced, and load sources heretofore individually required can be thrown into one and therefore, a reduction in consumed electric power becomes possible. 
   Further, the reading unit  2  and the optical system are arranged very compactly, and this brings about the downsizing of the apparatus. 
   Second Embodiment 
   A second embodiment is one in which the function of the spring for biasing the reading unit  2  toward the original mounting glass table  1  used in the first embodiment is also given to the tension applied to the driving wire  13 , and brings about the effect of further decreasing the driving load. 
   The second embodiment will hereinafter be described with reference to  FIGS. 3A to 3C . In  FIGS. 3A to 3C , portions designated by the same reference numerals as those in the first embodiment are similar in construction and function&#39;to them. 
   A bearing portion  2   b  is provided on the back of the reading unit  2  in the second embodiment, and instead of the spring  23  in the first embodiment, a rotatable roller  24  is mounted thereon. 
   The roller  24  is disposed in the route of the driving wire  13 , and is vertically in such a position that it depresses the driving wire  13  when the original mounting glass table  1  is mounted on the apparatus. Thus, the reading unit  2  is biased toward the original mounting glass table  1  by the tension of the driving wire  13  through the roller  24 . 
   An appropriate biasing force is obtained by suitably choosing the size of the roller  24 . 
   As in the first embodiment, the reading unit  2  is maintained in such a posture that the spacing between the surface of the original mounting glass table  1  and the light receiving surface of the image sensor becomes constant by the spacers  21 . 
   According to the present construction, a similar effect can be obtained by a construction simpler than that of the first embodiment, accordingly in a form more simplified in assembly. 
   Third Embodiment 
   A third embodiment intends to further reduce the driving load by eliminating the frictional load of the slider portions  22   b  and  22   c  with the rail  12 . 
   The third embodiment will hereinafter be described with reference to  FIGS. 4A to 4C . In FIGS.  4 A to  4 C, portions designated by the same reference numerals as those in the first embodiment are similar in construction and function to them. 
   A transmitting mechanism for transmitting a driving force to the reading unit  2 , a pulse motor  31  which is a driving source, a gear train for decelerating the rotation thereof, a driving pulley  34  to which the rotation of the motor  31  is transmitted through the gear train, and an idle pulley  35  are rotatably disposed on the sensor holder  22  in the third embodiment. 
   A U-shaped rail  15  for guiding the running of the reading unit  2  is fixed to and mounted in the frame  11 . 
   Also, one end  13   a  of the driving wire  13  is fixed to the reading terminated end side. The driving wire  13  is twined on the driving pulley  34  on the reading unit  2 , and is further passed over an idle pulley  35 , and thereafter is fixed to the reading starting end side of the apparatus frame  11  through a spring  14 . 
   The driving pulley  34  and the idle pulley  35  have cylindrical surfaces  34   a  and  35   a  as rotary members of the same diameter as the pitch circle diameter thereof (the diameter of the wire at the central position thereof) on the same shaft, and those cylindrical surfaces abut against the inner surface of the rail  15  by a moment created by tension being applied to the wire  13 . 
   In the reading operation, the reading unit  2  is always biased in the direction of arrow A by a moment created by the tension of the wire  13  applied to the driving pulley  34  and the idle pulley  35 , and a posture in which the respective cylindrical surfaces are in contact with the inner surface of the rail  15  is maintained. 
   That is, there is no backlash between the rail  15  and the sliding portion of the reading unit  2  and such a force that the reading unit constantly contacts with the rail  15  is working and therefore, it is avoided that during the interruption/resumption of reading, vibration occurs to the movement of the reading unit  2 . 
   Also, the diameters of the cylindrical portions of the driving pulley  34  and the idle pulley  35  which contact with the rail  15  are equal to the pitch circle diameters of the pulleys  34  and  35 . 
   Accordingly, the peripheral speed given to the pulleys  34  and  35  by the wire  13  is equal to the peripheral speed at which the pulleys  34  and  35  roll on the rail  15 . That is, the pulleys  34  and  35  do not slide but roll on the rail  15 , and no frictional force is produced between the pulleys and the rail  15  and therefore, as compared with the sliding bearing heretofore used, the force necessary for driving can be decreased. 
   The above-mentioned moment can be minimized within a range in which the vibration of the reading unit  2  does not occur, by appropriately choosing the spacing  13  between the driving pulley  34  and the idle pulley  35  in the lengthwise direction of the reading unit. 
   While in the embodiments described hitherto, the driving wire is used as a rope-like member, the present invention can likewise be carried out in a driving system using a timing belt. 
   Also, the biasing utilizing the tension of the rope-like member is not restricted to the elimination of the backlash of the reading unit  2  and the biasing thereof toward the original mounting glass table  1 , and can be applied to the absorption of any backlash and vibration occurring during scanning drive, irrespective of the type of the reading unit  2 . 
   As described above, the present invention can prevent the fitting backlash between the image reading means and the rail member, and can further prevent vibration from occurring to the movement of the image reading means. Accordingly, even if the interruption/resumption of the operation occurs in the course of reading, a smoothly connected image can be obtained and the downsizing of the apparatus can be expedited. 
   Also, the driving load necessary for the driving of the image reading means can be greatly reduced as compared with the prior-art system and therefore, consumed electric power can be made small. 
   The present invention is not restricted to the above-described embodiments, but covers all modifications within the technical idea thereof.