Abstract:
The present invention is related to an apparatus and a method for improving the quality of printing. The reference timings are adjusted by random values to decrease the cyclic unevenness of images associated with the conventional technique. The random values may be generated by the apparatus itself or may be received from an external device, such as a personal computer. The distance between two adjacent dots changes, and the unevenness of images is scattered to provide a higher quality of printing.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a printing apparatus and a method for printing images, and more particularly, to a printing apparatus and a method for forming high-quality images by forming a plurality of dots on a printing medium. 
   2. Description of the Related Art 
   Various types of printing apparatus are widely used in printers, copying machines, fax machines, and other image recording devices. Printing apparatus forms images by forming a plurality of dots on the printing media. The printing media may be paper, plastic film, clothing, etc. The printing apparatuses may use different kinds of method to perform the printing, such as dot-matrix printing, thermal transfer printing, dye-sublimation printing, ink jet printing, and laser printing, . . . etc. Generally, the thermal transfer printing and the ink jet printing methods are widely applied due to their capability of producing high quality images at reasonable cost. 
   Referring to  FIG. 1 , a printing apparatus has a print head  11  that comprises a group of printing elements  12 . When the print head  11  scans over a printing medium, the printing elements  12  form a plurality of dots on the printing medium to form an image. In this example, when the print head  11  scans in the scanning direction, an image of dot patterns  13  of Arabic numeral “2” is formed by the printing elements  12 , as shown in  FIG. 1 . 
   Referring to  FIG. 2 , for most ink jet printing products nowadays, a step motor  22  is used to provide a power so as to drive the print head  21  to move along a scanning direction over the printing medium. As shown in  FIG. 2 , the step motor  22  drives the print head  21  to move in a predetermined scanning direction by using the mechanism such as belt  23 . When the print head  21  scans in the predetermined scanning direction, the printing elements  12  are driven to fire ink droplets during predetermined time interval to form dots on desired positions of the printing medium. 
   The purpose of the step motor is to drive the print head by a power transmission device so that the print head can scan over the printing medium at a constant speed. The power transmission device may be belt. Step motors are generally used because their cost is lower and they&#39;re comparatively easy to control. However, the variation of different components, including step motors, control chips, and belts, may cause a cyclic variation of the print head&#39;s moving speed. Therefore, the print positions of dots would cyclically deviate from the desired positions.  FIG. 3   a  shows a cyclic unevenness phenomenon of images due to the cyclic variation of the scanning speed of the print head. The positions of dots deviate cyclically.  FIG. 3   b  shows distance between two adjacent dots varying cyclically due to the cyclic variation. Such deviations cause noticeable unevenness of the printed image, and thus deteriorate the printing quality. 
   In the U.S. Pat. No. 6,142,598 issued to Iwasaki et al., a method is provided to decrease the cyclic unevenness of printed images by cyclically changing the time intervals between adjacent ink ejections. A predetermined cyclic deviation amount is added to the driving time interval between dots. Thus, a cyclic shift amount appears in the print position of dot. Referring to  FIG. 4   a , it shows shift amounts of dots when a print head moves at a constant speed across the medium. For a printing apparatus with the defect of cyclic unevenness, the Iwasaki&#39;s method shifts the deviated positions of dots. Referring to  FIG. 4   b , it shows the effect on cyclic deviation after Iwasaki&#39;s method is employed.  FIG. 4   c  further shows the effect of the method on the distance between two adjacent dots. As shown is  FIG. 4C , Iwasaki&#39;s method may decrease the original unevenness, however, the improvement is not satisfactory and further improvement is still desired. 
   SUMMARY OF THE INVENTION 
   The present invention provides an apparatus and a method to improve the printing quality by reducing the unevenness of printed images. 
   The first embodiment of the present invention discloses that the reference timing sequence is shifted by a random value sequence to generate a driving timing sequence. Then, the driving timing sequence is used to drive the printing elements. The random value sequence induces a shift of dot position to scatter the cyclic unevenness of printed image. Since the shift amount is generated randomly, a better elimination of the cyclic phenomenon is achieved. 
   To illustrate the features of the present invention, some preferred embodiments are recited by referring accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts a print head scanning over a printing medium to form an image; 
       FIG. 2  depicts a print head scanning directionally controlled by a step motor; 
       FIG. 3   a  shows a cyclic deviation of formed dots; 
       FIG. 3   b  shows the distance between two adjacent dots; 
       FIG. 4   a  shows a predetermined cyclic deviation amount; 
       FIG. 4   b  depicts a shift induced by adding a predetermined cyclic deviation amount; 
       FIG. 4   c  shows the distance between two adjacent dots; 
       FIG. 5  shows a logical circuit for controlling the printing elements; 
       FIG. 6  is a pulse diagram of the operation of printing elements; 
       FIG. 7  depicts the movement of the print head; 
       FIG. 8  is a flow diagram showing an exemplary flow of the present invention; 
       FIG. 9  shows an exemplary shift of the reference timing sequence; 
       FIG. 10   a  shows a predetermined random shift amount in one embodiment of the present invention; 
       FIG. 10   b  depicts a shift result induced by adding a predetermined random value sequence; 
       FIG. 10   c  shows the distance between two adjacent dots; 
       FIG. 11   a  shows another predetermined random value sequence in an embodiment; 
       FIG. 11   b  depicts a shift result induced by adding a predetermined random value sequence; 
       FIG. 11   c  shows the distance between two adjacent dots; 
       FIG. 12   a  shows a block diagram for generating a random value sequence in the first embodiment; 
       FIG. 12   b  shows a block diagram for generating a random value sequence in the second embodiment; 
       FIG. 12   c  shows a block diagram for generating a random value sequence in the third embodiment; 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The present invention provides a printing apparatus and a method for improving quality of printed image. The apparatus and method efficiently decrease the cyclic unevenness of printed images. Embodiments of this invention are disclosed as following. 
   It is to be noted that the present invention may also be applied to the printing apparatus illustrated in  FIG. 1 . In the embodiment illustrated in  FIG. 1 , the print head  11  is an ink jet print head including eight printing elements  12 .  FIG. 5  shows an exemplary circuit in this embodiment to control the eight printing elements  12 . Eight heaters # 1  to # 8 , implemented inside the print head  11 , is capable of generating heat so as to jet ink droplets out of the printing elements  12  and form ink dots on the printing medium. Tr 1  to Tr 8  are switches of the corresponding heaters. A, B, and C signals are to control the on or off of the switches Tr 1  to Tr 8 . The A, B, C signals are derived from the reference timing sequence, i. e. clock signal. H signal represents image data. 
   Please refer to  FIG. 6 , a pulse diagram is illustrated to describe the operation of the circuit shown in  FIG. 5 . The firing of the ink droplets from the printing elements  12  are separately controlled by A, B, and C signals. When H is activated, the heater of corresponding printing element  12  is switched on and ink droplets are ejected from the corresponding printing element  12  to form a dot on the printing medium. 
   In the present embodiment, the plurality of printing elements  12  operate in sequence, as shown in  FIG. 6 , to simplify the design. In other words, the printing elements  12  are not driven to eject ink droplets at the same time. Since operating in sequence, the printing elements  12  are arranged in a line slant to the moving direction of the print head, as shown in  FIG. 7 . 
     FIG. 7  shows a printing head  11  with eight printing elements  12 . The arrangement of the printing elements  12 , the moving direction of the print head  11 , and the coordinates of dots are identified in this drawing. To form the dots at predetermined positions, there are some relationships among the inclination angle θ of the printing elements  12 , the distance L between the adjacent printing elements  12 , the moving speed V of print head  11 , and the distance d between the adjacent dots. 
   First, the delay time T 1  between two adjacent printing elements  12 , as shown in  FIG. 6 , is
 
 T   1 =( L× sin θ)/ V.  
 
   T 1  must be set complied with the above equation so that the ink dots can be formed at predetermined positions on the printing medium. T 1  is used to control the formation of a group of vertical dots having identical x coordinate. 
   T2 is the time interval needed to form next adjacent dots along the x-coordinate. The relation among T2, V, and d is
 
 T   2   =d/V.  
 
   The present invention generates a driving timing sequence by shifting the reference timing sequence with a random value sequence. Due to the shift, the unevenness of printed image is scattered and a better quality of printed images can be obtained. 
   The reference timing is shifted by adding or multiplying an amount of a random value. The random value may be obtained by different approaches. For example, the random value may have a value ranges from 0 to 1. The actual driving timing is determined by adding or multiplying the random value to the reference timing. 
   Alternatively, the shift amount may also be determined by using a function to generate the random value. For example, if the random value has a value (V i ) ranges from 0 to 1, the following function may be applied to determine the shift amount (S i ).
 
 Shift amount   i (S i )=0 , if  0&lt; V   i &lt;0.5
 
 Shift amount   i (S i )=−0.2 , if  0.5&lt; V   i &lt;0.75
 
 Shift amount   i (S i )=0.2 , if  0.75 &lt;V   i &lt;1
 
wherein i is a reference index of the random value. The implement of function simplifies the determination of random values.
 
     FIG. 8  illustrates a flow chart to explain the operation of this embodiment. A reference timing generation unit  82  generates a reference timing by conventional methods. The present invention includes a random value generation unit  81  for generating a random value sequence as described above. A driving timing generation unit  83  generates a driving timing sequence by shifting the reference timing, responsive to the reference timing and the random value. The unit  83  shifts the reference timing either by using addition or multiplication method. A driving pulse generation unit  84  generates a pulse signal to the print driving unit  85 , responsive to the driving timing sequence. The print driving unit  85 , receiving the pulse signal and the image data, drives a printing element  86  to form image at desired positions. 
   Please refer to  FIG. 9(   a ), ( b ), ( c ). Numerals  91  and  92  are examples of shift amount in this embodiment. The T 2  of clock  1  signal is shifted by a random value  91  and the T 2  of clock  2  signal is shifted by a random value  92 . 
     FIG. 10   a  shows an example of a random value sequence. The positions of the printed dots are determined by the reference timing sequence shifted by the random value sequence.  FIG. 10   b  depicts the shifted positions of dots. The positions are shifted by adding the shift amounts in  FIG. 10   a  to the original reference timing.  FIG. 10   c  shows the distance between two adjacent dots after operation of the invention. It is clear that the cyclic deviation of dots is scattered. Hence the unevenness of printed image is decreased to obtain an image of higher quality. 
     FIG. 11   a  shows another example of random value sequence.  FIG. 11   b  shows the result of the operation of the invention.  FIG. 11   c  shows the distance between two adjacent dots after operation of the invention. 
   Please refer to  FIG. 12   a . In one embodiment of the present invention, a circuit may be used as a random value generation unit  1203 . The random value generation unit  1203  generates the random value sequence. Then the random value sequence is transmitted to a control unit  1201 . The control unit  1201  then drives the printing elements  1202  to form dots, in accordance with the present invention. 
   Please refer to  FIG. 12   b . In another embodiment, a random value sequence is stored in a memory unit  1206 . The printing elements  1205  are driven by the control unit  1204  according to the random value sequence previously stored in the memory unit  1206 . 
   Please refer to  FIG. 12   c . The random value sequence may also be generated by an external generation unit  1207 , such as personal computer. The random value sequence is transmitted to a control unit  1209  via a transmission protocol. Control unit  1209  then drives the printing elements  1210  by the reference timing sequence and the random value sequence. 
   The above description sets forth various exemplary embodiments of the invention only, and is not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, various changes may be made in the function and arrangement of the elements described in these embodiments without departing from the spirit and scope of the invention. Thus, the protected scope of the present invention is as set forth in the appended claims.