Patent Publication Number: US-6663212-B2

Title: Ink jet device that ejects ink droplets having different volumes

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ink jet device including an actuator that ejects ink droplets by changing an internal pressure of a pressure chamber. 
     2. Related Art 
     There has been known a piezoelectric type ink jet printer head including a cavity plate formed with a pressure chamber and a piezoelectric element positioned adjacent to the pressure chamber. In this type of printer head, an ink droplet is ejected from the pressure chamber when a driving pulse is applied to the piezoelectric element. The rising edge of the driving pulse displaces the piezoelectric element, thereby increasing the volume of the pressure chamber. This volume change decreases an internal pressure of the pressure chamber, and a resultant negative pressure is maintained for a predetermined time, which is equal to the pulse width of the driving pulse, so that ink is introduced into the pressure chamber from a manifold. Then, the lowering edge of the driving pulse releases the displacement of the piezoelectric element, whereby the increased volume of the pressure chamber is restored. This increases the internal pressure of the pressure chamber and ejects an ink droplet through a nozzle onto a recording sheet, which is being transported relative to the printer head. 
     The pulse width of the driving pulse determines the amount of pressure that contributes to ink ejection, which in turn determines the volume of the ink droplet. Because a color-scale of resultant images is determined by the volume of ejected ink droplet, it is possible to obtain a desired color-scale by controlling the pulse width. 
     SUMMARY OF THE INVENTION 
     However, when a driving pulse with a different pulse width is applied in order to change the volume of ink droplets, the ejection speed of these ink droplets also differs, because the pulse width determines a timing at which the pressure change inside the pressure chamber is superimposed on a pressure that restores the deformed condition of the piezoelectric element. When ink droplets are ejected based on predetermined timing-clock signals at different ejection speeds toward a recording sheet that is moving relative to the printer head, impact positions of these ink droplets on the recording sheet will be out of alignment, adversely affecting quality of resultant image. 
     It is an objective of the present invention to overcome the above problems, and also to provide an ink jet device capable of forming dots on desired positions even when the volume of ejected ink droplets varies. 
     In order to achieve the above and other objects, there is provided an ink ejection device including a cavity plate formed with a pressure chamber and a nozzle, an actuator that fluctuates an internal pressure of the pressure chamber to eject an ink droplet from the nozzle onto a recording medium, and a driving unit that selectively outputs a driving signal to the actuator. The actuator fluctuates the internal pressure in response to the driving pulse. The driving unit outputs the driving signal for ejecting a smaller ink droplet at a first timing earlier than a second timing for a larger ink droplet. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a cross-sectional view with a block diagram showing an ink jet head according to an embodiment of the present invention; 
     FIG. 2 is a cross-sectional view of the ink jet head taken along a line II—II of FIG. 1; 
     FIG. 3 is a plan view showing the size and the impact position of three ink droplets ejected at the same timing; 
     FIG.  4 ( a ) is a first pulse signal used in the ink jet head of the present embodiment; 
     FIG.  4 ( b ) is a second pulse signal used in the ink jet head; 
     FIG.  4 ( c ) is a third pulse signal used in the ink jet head; 
     FIG. 5 is a table showing relationships among ejection speeds, volumes, and amounts of shift for each ink droplet; 
     FIG. 6 is a table showing relationships between ejection timings and amounts of shift for each ink droplet; 
     FIG. 7 is a plan view showing the impact position of ink droplets ejected in response to pulse signals output at adjusted timings; 
     FIG.  8 ( a ) is a plan view showing an output timing of the first pulse signal; 
     FIG.  8 ( b ) is a plan view showing an output timing of the second-pulse signal; and 
     FIG.  8 ( c ) is a plan view showing an output timing of the third pulse signal. 
    
    
     PREFERRED EMBODIMENT OF THE PRESENT INVENTION 
     Next, an ink jet device according to an embodiment of the present invention will be described while referring to the attached drawings. The ink jet device of the present embodiment is applied to an ink jet head  1  shown in FIG.  1 . 
     As shown in FIG. 1, the ink jet head  1  includes a cavity plate  10 , a piezoelectric actuator  20 , and a driving device  30 . The cavity plate  10  is formed with an ink supply port  11 , a manifold  12 , a plurality of pressure chambers  14 , and a plurality of nozzles  16 . The ink supply port  11  is in a fluid communication with an ink supply source (not shown) and also with the manifold  12 . The pressure chambers  14  are fluidly connected to the manifold  12  via corresponding restrictors  13 . The nozzles  16  are in one-to-one correspondence with and fluidly connected to the pressure chambers  14  via ports  15 . 
     The cavity plate  10  is formed, for example, of a plurality of 42% nickel alloy plates (42 alloy) with about 50 μm to 150 μm thickness. These plates are laminated one on the other and fixed by an adhesive. Alternatively, the cavity plate  10  could be formed of resin. 
     The piezoelectric actuator  20  is attached to the cavity plate  10  and has a configuration similar to that disclosed in Japanese Patent Application Publication No. HEI-3-274159. Namely, as shown in FIG. 2, the actuator  20  includes a plurality of piezoelectric sheets  21 , a plurality of internal negative electrodes  22 , and a plurality of internal positive electrodes  23 , which are laminated in such a manner that each piezoelectric sheet  21  is sandwiched and fixed by an adhesive between each internal negative electrode  22  and corresponding internal positive electrodes  23 . The internal positive electrodes  23  are aligned with the corresponding pressure chambers  14  in a lamination direction R. When a voltage is applied between the internal negative electrode  22  and the internal positive electrode  23 , bias electric field is in turn developed across the piezoelectric sheet  21  positioned between these electrodes  22 ,  23 . As a result, corresponding portion of piezoelectric sheet  21  deforms in the lamination direction R due to piezoelectric effect, whereby the volume of the corresponding pressure chamber  14  is reduced, so that the internal pressure thereof decreases. In the present embodiment, the voltage is applied to the electrode  22 ,  23  in constant, so that the reduced volume of the pressure chambers  14  is maintained as the normal condition. 
     As shown in FIG. 1, the driving device  30  includes a waveform generation circuit  31 , a clock-signal generation circuit  32 , and an output circuit  33 . The waveform generation circuit  31  stores a plurality of waveform signals each for different ink-droplet volume, and outputs the waveform signals as needed. The clock-signal generation circuit  32  is for generating clock signals that determine ink-ejection timings based on relative movement of the ink jet head  1  and the recording sheet. The output circuit  33  is for generating driving pulse signals based on the waveform signals output from the waveform generation circuit  31  and for outputting the driving pulse signals to the piezoelectric actuator  20  based on the clock signals. 
     In the present embodiment, the driving pulse is selectively applied across the electrodes  22 ,  23  in the condition where the reduced volume of the pressure chamber  14  is maintained. The lowering edge of the applied driving pulse releases the displacement of the piezoelectric sheets  14 , whereby the volume of the pressure chamber  14  is restored, that is, increased to its initial volume, resulting in a negative pressure generated in the pressure chamber  14 . This negative pressure is maintained for a duration of time T corresponding to a duration of time required for a pressure wave to propagate once across the length of the pressure chamber  14 . During the time duration T, ink is supplied into the pressure chamber  14  from the manifold  12 . 
     The duration of time T can be calculated by the following formula: 
     
       
         
           T=L/a  
         
       
     
     wherein L is the length of the pressure chamber  14  (FIG.  1 ); and 
     a is the speed of sound through the ink filling the pressure chamber  14 . 
     Theories on pressure wave propagation teach that at the moment the duration of time T elapses after the lowering edge of the driving pulse, the pressure in the pressure chamber  14  inverts to a positive pressure. The rising edge of the driving pulse applies the voltage to the driving electrodes in synchronization with this inversion so that the volume of the pressure chamber  14  reverts to the reduced volume. 
     The pressure generated when the volume of the pressure chamber  14  is reduced is added to the inverted positive pressure so that a relatively high pressure is generated in the nozzle  16 . This relatively high pressure ejects an ink droplet from the nozzle  16 . 
     The pulse width of the driving pulse is set equal to the time duration T or as an odd integer times the time duration T. Otherwise, the pressure contributing to the ink ejection will decrease, resulting in smaller ink-droplet volume and lower ink ejection speed. 
     Next, ink-ejection operation according to the present embodiment will be described while referring to a specific example. In this example, the ink jet head  1  is attached to a carriage of an ink jet printer (not shown), which moves at a speed of 762 mm/s. The distance between the nozzle  16  and the recording sheet is set to 1.2 mm. Ink droplets with different volumes are ejected through the same nozzle  16 . The time duration T is 6 μsec. 
     As shown in FIG. 3, three types of ink droplets are ejected in this example, that is, ink droplets  40 ,  41 , and  42  having a larger volume in this order. FIGS.  4 ( a ),  4 ( b ),  4 ( c ) show waveforms of driving pulse signals P 1 , P 2 , P 3  for ejecting the ink droplets  40 ,  41 ,  42 , respectively. 
     The waveform of the first pulse signal P 1  includes an ejection pulse p 1  followed by a cancel pulse p 2  with 9 μsec time interval therebetween. The ejection pulse p 1  is for ejecting the ink droplet  40 , and has a pulse width of 6 μsec, which equals to the time duration T. The cancel pulse p 2  has a pulse width of 9 μsec. The cancel pulse p 2  is for reducing residual pressure fluctuation in the pressure chamber  14  after the ink droplet  40  is ejected in response to the ejection pulse p 1 . Specifically, the lowering edge of the cancel pulse p 2  decreases the internal pressure in synchronization with the timing of when the residual pressure inverts to a positive pressure. On the other hand, the rising edge of the cancel pulse p 2  increases the internal pressure when the residual pressure is relatively low. 
     The waveform of the second pulse signal P 2  includes an ejection pulse p 3  followed by a downsizing pulse p 4  with a 3 μsec time interval therebetween. The ejection and downsizing pulses p 3  and p 4  have a pulse width of 6 μsec and 3 μsec, respectively. The downsizing pulse p 4  makes the volume of an ink droplet small by pulling a portion of ejected ink back into the pressure chamber  14 . That is, immediately after ink is ejected through the nozzle  16  in response to the ejection pulse p 3 , the ejected ink is not yet separated from remaining ink inside the pressure chamber  14 . In this condition, the downsizing pulse p 4  generates a negative pressure inside the pressure chamber  14 , whereby a portion of the ink that is ejected but not completely separated is draw back into the pressure chamber  14 . Afterwards, remaining portion of the ejected ink is separated from the ink inside the pressure chamber  14  and forms the ink droplet  41  having a reduced volume. 
     The waveform of the third pulse signal P 3  includes an ejection pulse p 5  followed by a downsizing pulse p 6 . The downsizing pulse p 6  has a pulse width of 2.6 μsec, and there is a 2.6 μsec time interval between the pulses p 5  and p 6 . A pulse width of the ejection pulse p 5  is 6.4 μsec, which is not equal to nor an odd integer times the time duration T. Such a pulse width of the ejection pulse p 5  decreases the volume of resultant ink droplets. The downsizing pulse p 6  further decreases the volume of ejected ink droplets in the same manner as the downsizing pulse p 4 . Moreover, because the time interval between the ejection pulse p 5  and the downsizing pulse p 6  is as small as 2.6 μsec, relatively large portion of ink is draw back into the pressure chamber  14 , thereby reducing the volume even further and generating the ink droplet  42 . 
     Here, the negative pressure generated by the cancel pulses P 2 , P 3  reduces the ejection speed of the ink droplets  41 ,  42  as well as reducing their volume. In addition, the ejection speed of the ink droplet  42  is further reduced due to the pulse width of the ejection pulse p 5 . As a result, the ink droplets  40 ,  41 ,  42  of the present example have the volume of 10 pl, 6 pl, 4 pl, respectively, and the ejection speed of 7.0 m/s, 6.5 m/s, and 6.0 m/s, respectively. 
     Because the ink droplets  40 ,  41 ,  42  have different ejection speeds, if these ink droplets  40 ,  41 ,  42  are all ejected at the same timing, the impact positions of the ink droplets  40 ,  41 ,  42  will be out of alignment and will greatly shift as shown in FIG.  3 . FIG. 5 shows resultant amounts of shift of the ink droplets  40 ,  41 ,  42  from a target position. Here, the target position is where the ink droplet  40  ejected in response to the first pulse signal P 1  that is output at reference timing will impact. The amount of shift is a distance between a center of the target position and a center of the actual impact position with respect to a direction C in which the recording sheet moves relative to the ink jet head  1 . 
     In the present embodiment, the impact positions of the ink droplets  41  and  42  are adjusted by controlling the output timings of the corresponding second and third pulse signals P 2  and P 3  in the following manner. That is, as shown in FIGS.  8 ( a ) to  8 ( c ), the output timing of the first pulse signal P 1  is set as reference timing, and the output timing of the second pulse signal P 2  is set 8 μsec earlier than the reference timing. The output timing of the third pulse signal P 3  is set 15 μsec earlier than the reference timing. The reason for this will be described below. 
     As mentioned above, the ejection speed decreases as the volume of the ink droplet decreases. This means that a smaller ink droplet will take a longer time (flight time) to reach the recording sheet than a larger ink droplet. Therefore, ejecting the smaller ink droplet at a timing earlier than the larger ink droplet can control the impact position of the smaller ink droplet. Here, a time difference t (μsec) between the flight time of the smaller ink droplet and the flight time of the larger ink droplet is calculated in the following formula: 
     
       
           t=g (1/V 1 −1/V 2 )×1000  
       
     
     wherein, g(mm) is the distance between the nozzle  16  and the recording sheet, which is 1.2 mm in this example; 
     V 1  (m/s) is an ejection speed (velocity) of the smaller ink droplet; and 
     V 2  (m/s) is an ejection speed (velocity) of the larger ink droplet. 
     Using the above formula, the time difference t between the flight time of the ink droplet  41  (6.5 m/s ejection speed) and the flight time of ink droplet  40  (7.0 m/s ejection speed) is calculated to be 13 μsec. Accordingly, ejecting the ink droplet  41  at timing 13 μsec earlier than the ink droplet  40  will minimize the amount of shift of the ink droplet  41 . This can be accomplished by outputting the second pulse signal P 2  at timing 13 μsec earlier than the reference timing. 
     Similarly, the time difference t between the flight time of the ink droplet  42  (6.0 m/s ejection speed) and the flight time of the ink droplet  40  (7.0 m/s ejection speed) is calculated to be 29 μsec based on the above formula. Accordingly, ejecting the ink droplet  42  29 μsec earlier than the ink droplet  40  will minimize the amount of shift of the ink droplet  42 . This can be accomplished by outputting the third pulse signal P 3  at timing 29 μsec earlier than the reference timing. 
     Needless to say, the first pulse signal P 1  is output at the reference timing. 
     FIG. 6 shows the above relationships between the output timings and the amounts of shift. Here, shifting the output timings of the pulse signals P 2 , P 3  while maintaining a high frequency of clock signals will cause a problem. That is, as shown in FIG. 7, in order to form a dot on a scan line L+1 with the ink droplet  42 , it is necessary to output the corresponding third pulse signal P 3  at a relatively early timing, for example, when the nozzle  16  reaches a position La shown in FIG.  7 . Accordingly, if a dot has been formed on a scan line L with the ink droplet  40  ejected from the same nozzle  16 , then the time interval between the first pulse signal P 1  and the third pulse signal P 3  will be too small to give a time to reduce enough the residual pressure of the first pulse signal P 1 . As a result, the residual pressure will undesirably affect the subsequent ejection of the ink droplet  42 . 
     Accordingly, the frequency of the clock signals must be set not to cause interference between the first pulse signal P 1  and the following third pulse signal P 3  and to put a time interval sufficient for preventing the residual pressure of the first pulse signal P 1  from interfering the subsequent ejection of the ink droplet  42 . 
     On the other hand, it is unnecessary to achieve the exact and complete target impact positions. Slight displacement of impact positions will hardly affect resultant image quality and so is tolerable. Therefore, it is possible to change the output timings of the second and third pulse signals P 2 , P 3  to be later than the above calculated theoretical timings as long as it is tolerable. This increases the frequency of the clock signals and achieves a higher print speed. 
     As shown in FIG. 6, when the output timing of the second pulse signal P 2  is set 4 μsec earlier than the reference timing, the amount of shift of the ink droplet  41  is 7 μm. Also, when the output timing of the third pulse signal P 3  is set 10 μsec earlier than the reference timing, the amount of shift of the ink droplet  42  is 14 μm. However, these amounts of shift are small enough not to cause any significant effect on image quality. 
     When the output timing of the second pulse signal P 2  is set 8 μsec earlier than the reference timing, the amount of shift of the ink droplet  41  is 4 μm. When the output timing of the third pulse signal P 3  is set 15 μsec earlier than the reference timing, the amount of shift of the ink droplet  42  is 10 μm. These amounts of shift are still small enough not to cause any significant effect on image quality. 
     FIG. 7 shows the impact positions of ink droplets  40 ,  41 ,  42  where the amounts of shift of the ink droplets  41  and  42  are 4 μm and 10 μm, respectively. It is apparent from FIG. 7 that these amounts of shift hardly affect the image quality. Accordingly, as shown in FIGS.  8 ( a ) to  8 ( c ), the output timings of the second and third pulse signals P 2  and P 3  are set 8 μm and 15 μm earlier than the reference timing, respectively. 
     In this manner, the output timings of the pulse signals P 1 , P 2 , and P 3  are controlled in order to provide a high quality image even when ink droplets with different volumes are used. 
     While some exemplary embodiments of this invention have been described in detail, those skilled in the art will recognize that there are many possible modifications and variations which may be made in these exemplary embodiments while yet retaining many of the novel features and advantages of the invention. 
     For example, in the above embodiment the output timing of the first pulse signal P 1  is set as the reference timing, and output timings of the second and third pulse signals P 2  and P 3  are shifted forward with respect to the reference timing. However, the output timing of the third pulse signal P 3 , which is output at a latest timing, could be set as a reference timing, and the output timings of the first and second pulse signals P 1  and P 2  could be set later than the reference timing. Still alternatively, the output timing to form a dot on a previous scan line could be set as a reference timing, and the output timings of all the pulse signals P 1 , P 2 , P 3  could be adjusted to delay from the reference timing by necessity time duration. 
     Further, the waveform generation circuit  31  could store alternative pulse waveforms that include corresponding pulses P 1 , P 2 , P 3  and necessity delay portions also. In this case, such pulse waveforms could be output at the same timing based on the clock signals, regardless of the volume of an ink droplet to eject.