Abstract:
A linear position encoding system for a moving a carriage carrying at least two active elements that are spaced apart in the direction (X) of movement of the carriage, comprising a codestrip extending along the path of movement of the carriage and having a plurality of position marks detectable by a sensor mounted on the carriage, wherein the codestrip has the same thermal expansion coefficient as the carriage.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a linear position encoding system for a moving carriage carrying at least two active elements that are spaced apart in the direction of movement of the carriage. The invention includes a codestrip extending along the path of movement of the carriage and having a plurality of position marks detectable by a sensor mounted on the carriage.  
           [0002]    The present invention is applicable, in particular, to printers, plotters or scanners, wherein the carriage carries, as active elements, printing elements, pens and photoelectric sensing elements, respectively.  
           [0003]    An example of an encoding system of this type is disclosed in EP-A-0 807 528, which relates to an ink jet printer. The carriage is movable in a main scanning direction relative to a platen on which a recording sheet, e.g., paper, is fed in a subscanning direction orthogonal to the main scanning direction. The printing elements carried on the carriage are formed by nozzles from which ink droplets are expelled onto the recording sheet under the control of a printer control system. The nozzles are arranged in rows extending in parallel with the subscanning direction, so that, during each scan pass of the carriage, a number of lines corresponding to the number of nozzles in a row can be printed. The encoding system is used for determining the timings at which the individual nozzles are to be fired in accordance with the image information to be printed. The position marks on the codestrip may be formed by dark bars on a light background or vice versa, that are arranged in a very fine regular pattern and may be detected by an optical sensor mounted on the carriage. As an alternative, in case of a transmission-type codestrip, the position marks may be formed by transparent slits in an opaque substrate.  
           [0004]    The accuracy of position detection achieved with the encoding system is subject to errors which may result, among others, from thermal expansion of the codestrip. In the known system, such errors are compensated for by providing two sensors that are mounted on the carriage with a well-defined spacing in the main scanning direction. On the basis of the known distance between the two sensors, the position signals provided by the encoding system can be calibrated electronically. It is a prerequisite however, that the distance between the two sensors is known exactly and is, itself, not subject to any substantial thermal expansion effects.  
           [0005]    In case of an ink jet color printer, the printhead on the carriage comprises a plurality of parallel rows of nozzles, e.g. one for each of the four basic colors cyan, magenta, yellow and black. These rows are spaced apart in the main scanning direction, and the offset between the rows has to be compensated for by appropriately controlling the firing timings of the nozzles on the basis of the position information provided by the encoding system. Composite colors are printed by combining dots of ink in the various basic colors. Then, the hue of a printed pixel, e.g. a green pixel obtained by combining dots in cyan and yellow, will depend on the amount of overlap between the dots in the basic colors. In order to obtain a high quality printed image, it is therefore important that the dots printed with different nozzle rows can be registered with high accuracy. However, thermal expansion or shrinkage of the carriage may lead to changes in the offsets between the different rows of nozzles, which limits the registration accuracy.  
           [0006]    Since the hue of a color pixel depends not only on the amount of overlap between the dots but also on the sequence in which the dots are printed, it has been proposed to use a printhead in which two nozzle rows are provided for each color and the rows dedicated to the various colors are arranged mirror-symmetrically, so that the sequence in which the color dots are printed may be same in the forward stroke and the return stroke of the carriage. Then, however, the length of the carriage in the main scanning direction must be increased further, and the printer will be even more sensitive to thermal expansion of the carriage.  
           [0007]    The problem caused by thermal expansion or shrinkage of the carriage is not only encountered in ink jet printers but, more generally, in any kind of printer in which printing elements are spaced apart in the main scanning direction and it is nevertheless necessary to register the sub-images printed with the different printing elements. A similar problem occurs in plotters, for example when a line that has been drawn with a first pen shall be continued smoothly with the use of another pen. In a scanner, the active elements may be optical sensors that are sensitive to different wavelengths of light. Here, in order to correctly determine the color of a scanned pixel, it is also important that the offset between the two sensors is compensated for correctly by controlling the sample timings.  
           [0008]    In order to obviate the problem caused by thermal expansion of the carriage it is of course possible to use a carriage made of a material that has a thermal expansion coefficient close to zero. This, however, leads to increased costs for the carriage.  
           [0009]    Another approach is to provide a sensor for detecting the temperature of the carriage and to compensate electronically for the effect of thermal expansion. However, this also adds to the complexity and costs of the hardware and requires additional capacity for electronic data processing.  
         SUMMARY OF THE INVENTION  
         [0010]    It is an object of the present invention to provide a simple and inexpensive linear position encoding system which is tolerant to thermal expansion or shrinkage of the carriage and does not require a measurement of the temperature of the carriage.  
           [0011]    According to the present invention, this object is achieved by a system wherein the codestrip has the same thermal expansion coefficient as the carriage.  
           [0012]    Since the codestrip is arranged in close proximity to the carriage and, hence, is subject to the same environmental temperature as the carriage, the effect of thermal expansion or shrinkage of the carriage is automatically canceled by a corresponding thermal expansion or shrinkage of the codestrip. Thus, once the timing control for the active elements has been adjusted so that the offsets between the different active elements are compensated for, this compensation will be maintained regardless of any temperature changes.  
           [0013]    It is accordingly a remarkable advantage of the present invention that an inexpensive material with a non-zero thermal expansion coefficient can be used not only for the carriage but also for the codestrip. Of course, the codestrip must be mounted in such a manner that it is free to expand and shrink under temperature changes. This, however, is required for a high quality codestrip, anyway, in order to prevent the codestrip from bulging or becoming distorted in the case of differential thermal expansion of the codestrip and the machine frame. Mounting structures allowing such a free thermal expansion or contraction of the codestrip are well known in the art. For example, the codestrip may be fixed at one end and slidably supported at the other end, or it may be fixed in the center and slidably supported at both ends.  
           [0014]    Since the codestrip should provide a high spatial resolution and, accordingly, must have a pattern of very fine position marks arranged with a very small pitch, the substrate on or in which the position marks are formed must fulfill specific requirements in order for the sensor to produce a well-defined detection signal. In a preferred embodiment, the position marks are formed in a resinous substrate such as a polyester that is fixedly attached (e.g. by gluing or coating) to a carrier strip whose rigidity and tensile strength is significantly larger than that of the substrate. The thermal expansion coefficient will be determined alone by the material of the carrier strip and will not be substantially influenced by the material of the substrate. For example, the carriage and the carrier strip may be made of the same metal material, e.g. stainless steel or aluminum, and the substrate for the position marks may be formed by a polyester film on the carrier strip. In order to provide a transmission type codestrip, the part of the polyester film carrying the position marks may project beyond the edge of the carrier strip.  
           [0015]    Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:  
         [0017]    [0017]FIG. 1 is a schematic, perspective view of an ink jet printhead carriage traveling along the platen of a printer, together with a position encoding system according to the present invention;  
         [0018]    [0018]FIG. 2 is a front view of the printhead;  
         [0019]    [0019]FIG. 3 is a diagram illustrating the effect of thermal shrinkage of the carriage;  
         [0020]    [0020]FIG. 4 is a diagram similar to FIG. 3 illustrating a register error between dots printed with different nozzles of the printhead; and  
         [0021]    [0021]FIG. 5 is a diagram illustrating how to compensate for the registration error in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    In FIG. 1, a carriage  10  carrying an ink jet printhead is moved back and forth in a main scanning direction X along a platen  12  on which a printing medium is transported in a subscanning direction Y normal to the main scanning direction. The carriage  10  is guided on guide rails  14  which are mounted in a fixed position in relation to the platen  12  and a frame body  16 . A codestrip  18  having a plurality of position marks or slits  20  is fixed to the frame body  16  and extends along the guide rails  14 . The codestrip  18  comprises a carrier strip  22  made of the same material as the carriage  10 , e.g. stainless steel, and a substrate of polyester film  24  in which the slits  20  are formed and which is firmly attached to one surface of the carrier strip  22  so as to project beyond one edge of the carrier strip.  
         [0023]    An optical sensor  26  is fixedly mounted on the carriage  10  with its sensitive portion facing the projecting part of the polyester film  24 . Thus, when the carriage  10  moves in the main scanning direction X, the slits  20  can be detected with the sensor  26 , and the sensor delivers a pulse train corresponding to the sequence of slits. By counting these pulses, an electronic control circuit (not shown) of the printer can determine the position of the carriage  10  in X-direction with high accuracy. The positional accuracy depends on the pitch with which the slits  20  are arranged on the codestrip  18 , but may be improved by interpolation techniques on the basis of the known velocity of the carriage  10 .  
         [0024]    As is shown in FIG. 2, the printhead carriage  10  comprises four rows of ink jet nozzles  28 ,  30 ,  32  and  34  for the four basic colors cyan, magenta, yellow and black. The rows extend in the subscanning direction and are spaced apart from each other in the main scanning direction.  
         [0025]    [0025]FIG. 3 diagrammatically shows a portion of a recording sheet  36  placed on the platen  12 , and a plan view of the carriage  10  and the encoding system formed by the sensor  26  and the codestrip  18 . The positions of the nozzles  28 - 34  and the position of the sensitive part of the sensor  26  are indicated by short vertical lines.  
         [0026]    When, in FIG. 3, the carriage  10  or, more exactly, the sensitive part of the sensor  26  has reached the slit  20   a , the nozzle  28  is energized to print an ink dot  38  on the recording sheet  36 .  
         [0027]    When the carriage  10  undergoes thermal contraction, the positions of the nozzles  28 - 34  slightly shift, as has been shown exaggeratedly in dotted lines in FIG. 3. Then, the ink dot  38  would be printed in the position  38 ′ which is slightly shifted relative to the intended position. However, since all dots printed with the same nozzle  28  will be shifted in the same way, this does not lead to a distortion of the printed image but only to a minor shift of the image as a whole, which is not perceptible to the human eye.  
         [0028]    Thermal expansion or shrinkage of (only) the carriage  10  does however cause a problem when a predetermined positional relationship (register) is to be maintained between ink dots printed with the different nozzles  28 ,  30 ,  32  and  34 . For simplifying the discussion, it shall now be assumed that the nozzle  34  shall be energized at an appropriate timing so as to print an ink dot right on top of the already printed dot  38 . The carriage moves in the direction X to the right in FIGS. 3 and 4. As is shown in FIG. 4, when the carriage  10  is not subject to thermal shrinkage, the nozzle  34  must be energized when the carriage has reached the slit  20   b  which, in this example, is twenty slits away from the slit  20   a . Now, thermal shrinkage has the effect that the nozzle  34  prints the dot actually in the position  40 ′ which is considerably offset from the position of the dot  38 ′.  
         [0029]    In the discussion of FIGS. 3 and 4 is has been assumed that thermal changes affect only the carriage  10  but not the codestrip  18 .  
         [0030]    In FIG. 5, the carriage in the condition with thermal shrinkage has been shown in continuous bold lines, whereas the original contour of the carriage has been indicated in dotted lines. Since, according to the present invention, the carrier strip  22  which determines the thermal expansion behavior of the codestrip  18  is made of the same material as the carriage  10  and hence has the same thermal expansion coefficient, the codestrip  18  undergoes the same percentage of thermal shrinkage as the carriage  10 . FIG. 5 shows the condition of the codestrip  18  with thermal shrinkage in continuous lines. For comparison, the original state of the codestrip has been shown in dotted lines (with a slight vertical offset). Faint continuous lines indicate the position  10 ′ of the carriage (with thermal shrinkage) at the time when the nozzle  28  prints the dot  38 ′ (the position  10 ′ has also been shown with a slight vertical offset for reasons of clarity).  
         [0031]    As shown in FIG. 5, the nozzle  34  prints the dot  40 ′ when the carriage  10  has traveled twenty slits further to the position  20   b ′ which is shifted from the original position  20   b  due to the thermal shrinkage of the codestrip  18 . Thus, when the sensor detects the position  20   b ′, the nozzle  34  coincides with the position of the dot  38 ′, and the new dot  40 ′ is again printed right on top of the previous dot. This means that the effect of thermal shrinkage of the printhead  10  is completely compensated for by the thermal shrinkage of the codestrip  18 .  
         [0032]    Thermal shrinkage or expansion of the codestrip  18  and the carriage  10  will only have the effect that the printed image, as a whole, undergoes a corresponding shrinkage or expansion in width direction, but this minor distortion is not perceptible to the human eye.  
         [0033]    In FIG. 5, it has been assumed for simplicity that the codestrip  18  is fixed relative to the frame body in the position of the slit  20   a , so that the slit  20   a  does not move due to thermal expansion or shrinkage. However, any change in the position of the slit  20   a  would have affected the positions of both dots  38 ′ and  40 ′ exactly in the same way, so that the effect of thermal expansion is eliminated regardless of where the codestrip is fixed to the frame body. The reason is that, thanks to the identical thermal expansion coefficient of the carriage  10  and the codestrip  18 , the distance between the nozzles  28  and  34  on the carriage is always, for any temperature, identical with the distance between the corresponding slits  20   a  and  20   b  on the codestrip.  
         [0034]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.