Abstract:
The present invention provides an ink jet printing apparatus and method that can print a high-grade image without being affected by a variation in moving speed of a printing head. To accomplish this, an encoder is used which outputs a pulse each time a printing head and a printing medium are moved a specified amount relative to each other. Driving timings with which ink is ejected from the printing head are adjusted depending on the time interval between the pulses.

Description:
[0001]    This application is based on Patent Application No. 2001-256624 filed Aug. 27, 2001 in Japan, the content of which is incorporated hereinto by reference.  
         BACKGROUD OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an ink jet printing apparatus and method that uses a printing head capable of ejecting ink to print an image on a printing medium.  
           [0004]    2. Description of the Related Art  
           [0005]    Ink jet printing apparatuses are widely used as means installed in printers, facsimile machines, or copiers to print images (including characters and symbols) on printing medium such as paper or thin plastic sheet (OHP and the like) and the like on the basis of image information.  
           [0006]    [0006]FIG. 6 is a perspective view of an essential part of an ink jet printing apparatus. In FIG. 6, a printing medium  201  is supported by a printing medium feeding roller  202  arranged in a print area. The feeding roller  202  is driven by a sheet feeding motor  203  to transport the printing medium  201  in a sub-scanning direction indicated by an arrow α. The sheet feeding motor  203  is composed of a stepping motor or a DC motor. In recent years, the DC motor is often used owing to its quietness or the like. In this case, a rotary encoder (not shown) is installed in the feeding roller  202  so as to control the sheet feeding motor  203  on the basis of an encoder signal obtained from the rotary encoder. A shaft  204  is provided in front of and parallel with the feeding roller  202 . A carriage  205  is movably guided along the shaft  204 , and is reciprocated in a main-scanning direction indicated by an arrow β, in response to a power from a carriage motor  206 . A lubricant such as grease is provided between the shaft  204  and the carriage  204  to reduce the mechanical loads caused by friction or the like.  
           [0007]    The carriage motor  206  is composed of a stepping motor or a DC motor similarly to the sheet feeding motor  203 . In recent years, the DC motor is often used owing to its quietness or the like. In this case, a rotary encoder (not shown) is arranged on the carriage  205 , and a linear encoder (not shown) is arranged parallel with the shaft  204 . Then, on the basis of a signal obtained from this linear encoder, the carriage motor  206  is controlled. Further, on the basis of a signal obtained from this linear encoder, timings are generated with which ink is ejected from a printing head  208 .  
           [0008]    The carriage  205  as printing head moving means has the printing head  208  and a tank  209  mounted thereon, the tank  209  contains print ink. In this example, the printing head is used for printing color images and has a black ink ejecting head  208 -BK, a cyan ink ejecting head  208 -C, a magenta ink ejecting head  208 -M, and a yellow ink ejecting head  208 -Y arranged along a scanning direction of the carriage  205 . Further, the carriage  205  has a tank  209 -BK for black ink (Bk), a tank  209 -C for cyan ink(C), a tank  209 -M for magenta ink (M), and a tank  209 -Y for yellow ink (Y) mounted thereon as the tank  209 . These tanks supply ink to the heads for the corresponding colors. The printing head  209  is provided with an ink ejecting section on a front surface thereof. The front surface is located opposite a printing surface of the printing medium  201  at a predetermined interval (for example, 0.8 mm). The ink ejecting section has a plurality of (for example, 48 or 64) ink ejecting ports arranged in a longitudinal line along a direction crossing the scanning direction of the carriage  205 .  
           [0009]    Further, a control section (not shown) of the printing apparatus including a control circuit (CPU or ASIC) and a ROM, a RAM, and the like annexed to the control circuit, for example, receives information on print modes and print data from a controller of an external host apparatus via an interface. Then, the control section of the printing apparatus controls the printing head  208  via a head drive circuit together with drive sources of the printing apparatus such as various motors, to cause ink to be ejected through the ink ejecting section of the printing head  208  to print an image on the printing medium  201 . That is, an image is printed on the printing medium  201  by alternately repeating an operation of ejecting ink from the ink ejecting section while moving the printing head  208  in the main-scanning direction and an operation of transporting the printing medium  201  in the sub-scanning direction by a predetermined amount.  
           [0010]    [0010]FIG. 7 is a diagram illustrating the relationship between the speed at which the printing head  208  is moved and the positions at which ink droplets impact the printing medium  201 .  
           [0011]    It is assumed that the printing head  208  is mounted on the carriage  205  and is being moved at an ideal head speed V in the main scanning direction, indicated by β in the figure. In this case, when an ink droplet  303  is ejected from the printing head  208  to the printing medium  201  at an ink ejection speed Vd, it flies at a speed determined by synthesizing the vectors of the ideal head speed V and ink ejection speed Vd in a direction determined in the same manner. Then, the ink droplet  303  moves the distance d between the printing head  208  and the printing medium  201  and then impacts an ideal impacting position  306  on the printing medium  201 .  
           [0012]    However, in order to improve print throughput, it may be desirable to perform a printing operation in all movement areas including an acceleration area, a constant speed area, and a deceleration area. Further, even in the constant speed area for the carriage  205 , cockling of the motor  206  or the servo accuracy thereof may vary the moving speed of the carriage  205 . As a result, the ink droplet  303  may be ejected while the moving speed (scanning speed) of the printing head  208  is varying.  
           [0013]    If the moving speed of the printing head  208  varies in this manner, then the direction and speed of the ink droplet  303  vary to cause the impacting position on the printing medium  201  to deviate from the ideal impacting position  306 . In FIG. 7, if the printing head  208  is moved at a speed Vs lower than the ideal head speed V, the ink droplet  303  flies at a speed determined by synthesizing the vectors of the speed Vs and ink ejection speed Vd in a direction determined in the same manner. As a result, in the direction β in which the printing head  208  is moved as shown in the figure, the ink droplet  303  impacts the printing medium at a position located front the ideal impacting position  306 . On the other hand, in FIG. 7, if the printing head  208  is moved at a speed Vf higher than the ideal head speed V, the ink droplet  303  flies at a speed determined by synthesizing the vectors of the speed Vf and ink ejection speed Vd in a direction determined in the same manner. As a result, in the direction β in which the printing head  208  is moved as shown in the figure, the ink droplet  303  impacts the printing medium at a position located beyond the ideal impacting position  306 .  
           [0014]    Thus, when the moving speed of the printing head  208  varies, if the ink droplet  303  is ejected, the impacting position of the ink droplet  303  deviates from the ideal one. Consequently, a print image may be disturbed.  
         SUMMARY OF THE INVENTION  
         [0015]    It is an object of the present invention to provide an ink jet printing apparatus and method that can print a high-grade image without being affected by a variation in moving speed of a printing head.  
           [0016]    In the first aspect of the present invention, there is provided an ink jet printing apparatus using a printing head capable of ejecting ink and printing a printing medium by ejecting ink while moving the printing head and the printing medium relative to each other, the apparatus comprising;  
           [0017]    an encoder that outputs a pulse each time the printing head and the printing medium are moved a specified amount relative to each other;  
           [0018]    detecting means for detecting a time interval between the pulses;  
           [0019]    adjusting means capable of adjusting driving timings with which the ink is ejected from the printing head;  
           [0020]    calculating means for setting the time interval between the pulses obtained when the printing head and the printing medium are moved relative to each other with an expected maximum speed, as a reference time interval, and calculating delay time for a driving timing for the printing head depending on the magnitude of the time interval between the pulses detected by the detecting means; and  
           [0021]    control means for controlling the adjusting means depending on the delay time calculated by the calculating means.  
           [0022]    In the second aspect of the present invention, there is provided an ink jet printing method of using a printing head capable of ejecting ink and printing a printing medium by ejecting ink while moving the printing head and the printing medium relative to each other, the method comprising the steps of:  
           [0023]    using an encoder that outputs a pulse each time the printing head and the printing medium are moved a specified amount relative to each other;  
           [0024]    setting a time interval between the pulses obtained when the printing head and the printing medium are moved relative to each other with an expected maximum speed, as a reference time interval, and calculating delay time for a driving timing for the printing head depending on the magnitude of the time interval between the pulses; and  
           [0025]    adjusting the driving timing with which the ink is elected from the printing head, depending on the delay time.  
           [0026]    The present invention enables ink to always impact a printing medium at an ideal position by adjusting ink ejection timings according to the moving speed of the printing head. As a result, a high-grade image can be printed without being affected by a variation in moving speed of the printing head.  
           [0027]    The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    [0028]FIG. 1 is a block diagram of an essential part of a control system of an ink jet printing apparatus according to the present invention;  
         [0029]    [0029]FIG. 2 is a diagram illustrating output signals from an encoder in FIG. 1;  
         [0030]    [0030]FIG. 3 is a flow chart illustrating a basic calculating operation performed by a delay value calculating section in FIG. 1;  
         [0031]    [0031]FIG. 4 is a flow chart illustrating a more specific calculating operation performed by the delay value calculating section in FIG. 1;  
         [0032]    [0032]FIG. 5 is a diagram illustrating the relationship between print timings for the ink jet printing apparatus in FIG. 1 and an ink droplet impacting position;  
         [0033]    [0033]FIG. 6 is a perspective view of an essential portion of a mechanically constructed part of an ink jet printing apparatus to which the present invention is applicable; and  
         [0034]    [0034]FIG. 7 is a diagram illustrating the relationship between the moving speed of a printing head of the ink jet printing apparatus in FIG. 6 and the ink droplet impacting position. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    An embodiment of the present invention will be described below with reference to the drawings. The mechanical construction of a printing apparatus in this example is similar to that of the conventional example in FIG. 6, described previously.  
         [0036]    [0036]FIG. 1 is a block diagram of the printing apparatus in this example.  
         [0037]    Print data transferred from a host apparatus  101  is received by an I/F section  103  in a print control section  102  of the printing apparatus in this example and is then transmitted to a print data generating section  104 . The print data generating section  104  carries out decompression of compressed data, conversion of a data arrangement, and the like to convert the received data into a format that can be printed by a printing head  208 . The printing head  208  may be, for example, an ink jet printing head of a type that ejects ink using thermal energy. The ink jet printing head causes film boiling in ink in an ink channel using thermal energy generated by electrothermal converter provided the in the ink channel. The resulting bubbling energy is used to eject ink droplet through ink ejecting port.  
         [0038]    On the other hand, a carriage  205  driven by a carriage motor  206  has the printing head  208  and an encoder  109  mounted thereon. The encoder  109  outputs a pulse signal each time the carriage  205  is moved a specified distance. A pulse signal generated by the encoder  109  passes through an LPF section  110  in a print control section  102 . In the LPF section  110 , the pulse signal is deprived of noise and then transmitted to an edge trigger generating section  111 . The edge trigger generating section  111  detects predetermined edges (encoder edges) in the received pulse signal to generate trigger pulses. The trigger pulses generated by the edge trigger generating section  111  are transmitted to a speed detecting section  112  and an edge trigger delay section  113 . The speed detecting section  112  measures the interval between the trigger pulses generated by the edge trigger generating section  111 , and transfers the corresponding value to a delay value calculating section  114  as the current speed information. Further, the speed information detected by the speed detecting section  112  is also transmitted to a servo controller (not shown) that servo-controls the carriage motor  206 , as required.  
         [0039]    The delay value calculating section  114  uses the current speed information and the like transmitted by the speed detecting section  112  to calculate an impact correction delay value required to correct the ink droplet impacting position as described later. The edge trigger delay section  113  delays the trigger pulses generated by the edge trigger generating section  111  according to the impact correction delay value calculated by the delay value calculating section  114 . Then, the edge trigger delay section  113  outputs the trigger pulses to a print timing generating section  115 . The print timing generating section  115  generates a print timing signal by converting the trigger pulses transmitted by the edge trigger delay section  113  into print resolutions. Then, the print timing generating section  115  transmits the print timing signal to a print data transferring section  106  and a position detecting section  116 . The position detecting section  116  uses an up/down counter to count the signals transmitted from the edge trigger delay section  113  and print timing generating section  115 , thereby detecting the moving position of the carriage  205 . The position information detected by the position detecting section  116  is transmitted to a print position detecting section  117 . The print position detecting section  117  generates a print start signal when a print start position is detected in the position information, while generating a print end signal when a print end position is detected therein. Then, the print position detecting section  117  transmits this signal to the print data transferring section  106 . The print data transferring section  106  transfers print data generated by the print data generating section  104  to the printing head  208  according to the information from the print timing generating section  115  and print position detecting section  117 . On the basis of the information transmitted from the print data transferring section  106 , the printing head  208  ejects the ink droplet to the printing medium  201 .  
         [0040]    [0040]FIG. 2 shows the waveforms of signals (encoder signals) generated by the encoder  109 .  
         [0041]    Signals generated by the encoder  109  have two waveforms with an A phase  401  and a B phase  402  which deviate from each other through about 90° as in the case with general digital encoder signals. Thus, an advanced phase (normal rotation)  403 , shown in the left of FIG. 2, or a delayed phase (reverse rotation)  404 , shown in the right of FIG. 2, is obtained depending on the movement direction of the carriage  205 . Accordingly, the moving position of the carriage  205  can be detected by, for example, using one-side edges of the A phase as detected points to switch an up/down operation of a position detecting counter at rising and falling edges of the A phase with the B phase fixed at a certain level (in the figure, a low level). More specifically, for example, with the B phase at the low level, the up/down operation of the position detecting counter is switched so that the position detecting counter performs a count up operation each time a rising edge is detected in the A phase, whereas it performs a count down operation each time a falling edge is detected. Then, the moving position of the carriage  205  (the moving position of the printing head  208 ) can be detected from the count value of the position detecting counter.  
         [0042]    The edge trigger generating section  111  detects edges of encoder pulses as shown in FIG. 2 to generate trigger pulses. The speed detecting section  112  measures the interval (time) (also referred to as the “encoder edge interval (time)”) between the trigger pulses to detect the moving speed of the carriage  205 .  
         [0043]    [0043]FIG. 3 is a flow chart illustrating a basic calculating operation performed by the delay value calculating section  114 .  
         [0044]    In FIG. 3, t1 denotes the current encoder edge interval (time), which corresponds to the current speed of the carriage  205  (the current speed of the printing head  208 ). t2 denotes the encoder edge interval (time) at the expected maximum speed, which corresponds to the maximum speed of the carriage  205  (the maximum speed of the printing head  208 ). Y denotes the result of the calculation (t1−t2), and A denotes the constant of the value (t3/t2). t3 denotes the time required for the ink droplet  303  ejected from the printing head  208  to impact the printing medium  201 . t denotes an impact correction delay value as the result of a calculation executed by the delay value calculating section  114 .  
         [0045]    In this case, the expected maximum speed is a virtual speed which is higher than a speed achieved by scanning of the carriage. It is advantageous to set the maximum speed to be higher than the speed achieved by scanning of the carriage. However, the delay value increases as the maximum speed is set to be higher. Therefore, if the set maximum speed is too high, the delay value exceeds one period of the encoder signal, thus the requiring circuit and the like which holds the delay value until the next period of the encoder signal. Accordingly, it is desirable that the maximum speed be higher than the speed achieved by scanning of the carriage, and be set within the range such that the delay value does not exceed such one period of the encoder signal.  
         [0046]    First, after a calculation has been started, it is checked whether a delay calculation mode is ON or OFF (S 1 ). If this mode is OFF, the calculation is ended. If the mode is ON, the calculation is continued When the delay calculation mode is ON, the calculation (t1−t2) is executed to determine the value Y (S 2 ). Then, the calculation (Y×A) is executed to determine the impact correction delay value t (S 3 ). Accordingly, an operational expression for the impact correction delay value t in FIG. 3 is shown below.  
           t= ( t 1 −t 2)× A    (1)  
         [0047]    The value t decreases with increasing current speed of the carriage  205  and consistently with the encoder edge interval t1. Conversely, the value t increases with decreasing current speed of the carriage  205  and consistently with the encoder edge interval t1.  
         [0048]    [0048]FIG. 4 is a flow chart illustrating a more specific calculating operation performed by the delay value calculating section  114 .  
         [0049]    In FIG. 4, a constant C containing B (the power of 2) is used in place of the constant A (=t3/t2). That is, the constant C is set as a fixed value by calculating (A×B). Consequently, C=A×B=(t3/t2)×B. An operational expression for the impart compression delay value t is shown below.  
           t ={( t 1 −t 2)×C}/ B    (2)  
         [0050]    Further, Y(n) in FIG. 4 is the value of the n-th bit of the value Y as expressed by binary notation.  
         [0051]    First, after a calculation has been started (S 501 ), it is checked whether the delay calculation mode is ON or OFF (S 502 ). If this mode is OFF, the calculation is ended. If the mode is ON, the calculation is continued. When the delay calculation mode is ON, the calculation (t1−t2) is executed to determine the value Y (S 503 ). Then, the n-th bit of the value Y (at the first time, n−0) is recognized as bx (S 504 ). Further, n “0”s (at the first time, n=0) are added to the value C as expressed by binary notation, at the least significant positions. That is, the value C as expressed by binary notation is shifted by n bits to obtain a value C′ (C 505 ). At the first time, n=0, so that C=C′. Subsequently, it is determined whether or not the value bx is 1 (S 506 ), the value Y is corrected to the value (Y+C′) when bx=1 (S 507 ). The value Y remains unchanged when bx=0. At the first time, bx=0, so that the value Y remains unchanged.  
         [0052]    Then, it is determined whether or not the value n exceeds {(the number of bits of the value Y)−1} (S 508 ). If the result of the determination is negative, the value n is increased to (n+1) (S 509 ), and the process returns to step S 504 . Thus, steps S 504  to S 507  are repeated until the value n reaches {(the number of bits of the value Y)−1}. The number of the repetitions equals the number of bits of the value Y.  
         [0053]    During the second repetition of steps S 504  to S 507 , the value n changes from 0 to 1, and the first bit of the value Y is recognized as bx (S 504 ). Further, one “0” is added to the value C as expressed by binary notation, at the least significant position, i.e. the value C is shifted by one bit to obtain a value C′ (S 505 ). Accordingly, the value C′ is obtained by doubling the value C (×2). Subsequently, it is determined whether or not the value bx is 1 (S 506 ). When bx=1, the value Y is corrected to (Y+C′) (S 507 ). When bx=0, the value Y remains unchanged.  
         [0054]    Similarly, steps S 504  to S 507  are repeated as many times as the number of bits of the value Y. Then, the value n is reset to 0 (S 510 ), and the calculation (Y/B) is executed to determine the impact correction delay value t (S 511 ). Since the value B is the power of 2, the least significant bits of the value Y as expressed by binary notation may actually be rounded off by bit shifting. As a result, the impact correction delay value t is determined by Equation (2), shown above.  
         [0055]    As described above, the impact correction delay value t, calculated by the delay value calculating section  114 , is transmitted to the edge trigger delay section  113  (see FIG. 1). Then, print timings are adjusted according to the impact correction delay value t.  
         [0056]    [0056]FIG. 5 is a diagram illustrating the relationship between print timings and an ink droplet impacting position.  
         [0057]    The edge trigger generating section  111  (see FIG. 1) uses an encoder signal  601  to generate an encoder position trigger  602  as a position management signal for the printing head  208 . If high-resolution printing is carried out, triggers are generated which have a period equal to half or quarter the period of the encoder signal. For example, if the period of the encoder signal corresponds to a resolution of 300 dpi, a print resolution of 600 dpi is obtained using triggers having a period equal to half the period of the encoder signal. Further, a print resolution of 1,200 dpi is obtained using triggers having a period equal to quarter the period of the encoder signal. In FIG. 5, for simplification of the description, it is assumed that a printing operation is performed with a resolution corresponding to the period of the encoder signal  601 . That is, the number of encoder position triggers  602  equals the number of print timing triggers  603  generated by the edge trigger delay section  113 .  
         [0058]    In this case, the ink droplet  303  flies in the direction of a vector obtained by synthesizing the vector of the moving speed of the printing head  208  (the moving speed of the carriage  205 ) with the vector of the speed at which the ink droplet  303  is elected. When the printing head  208 , which ejects the ink droplet  303  at a speed Vd, is moved at an ideal speed V, the edge trigger delay section  113  generates a print timing trigger A using a delay value Td. In this case, when the current speed of the printing head  208  is Vf, which is higher than the ideal speed V, the delay value calculating section  114  calculates a smaller impact correction delay value t. The delay value Td correspondingly decreases. Consequently, at this time, a print timing trigger B is generated earlier than the print timing trigger A generated at the ideal speed V. On the other hand, when the current speed of the printing head  208  is Vs, which is lower than the ideal speed V, the delay value calculating section  114  calculates a larger impact correction delay value t. The delay value Td correspondingly increases. Consequently, at this time, a print timing trigger C is generated later than the print timing trigger A generated at the ideal speed V.  
         [0059]    By thus controlling the occurrence timing for the print timing trigger  603 , the deviation of the impacting position of the ink droplet  303 , attributed to a variation in moving speed of the printing head  208 , is corrected to enable the ink droplet  303  to always impact the printing medium at the impacting position  613 , which is obtained when the printing head  208  is moved at the ideal speed. In the calculation, the current moving speed of the printing head  208  (the current moving speed of the carriage  205 ) is the inverse of the period T of the encoder signal  601  corresponding to the position immediately before the current one.  
         [0060]    Other Embodiments  
         [0061]    In the present invention, it is possible to carry out not only unidirectional printing in which a printing operation is performed only when the printing head is moved in one direction but also bi-directional printing in which a printing operation is performed when the printing head is moved in both directions.  
         [0062]    The present invention has been described in detail with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and it is the intention, therefore, in the appended claims to cover all such changes and modifications as fall within the true spirit of the invention.