Abstract:
An electrostatic ink jet recorder of the type controlling charged toner particles contained in ink by using electrophoresis is disclosed. The recorder achieves a miniature and cost-effective configuration by reducing the number of drivers for driving ejection electrodes and auxiliary electrodes. The same amount of toner particles is ejected from all of ejection electrodes despite a scatter in the ejection electrode and a scatter in the position of the ejection electrode relative to auxiliary electrodes and a counter electrode.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an electrostatic ink jet recorder and, more particularly, to an electrostatic ink jet recorder of the type controlling charged toner particles contained in ink by using electrophoresis. 
     An electrostatic ink jet recorder of the type subjecting ink containing charged toner particles to an electric field and ejecting the ink toward a recording medium on the basis of a Coulomb force acting on the particles is conventional. An ink jet recorder of this type includes an electrophoresis electrode for causing the toner particles to gather at ejection ports due to electrophoresis. A plurality of ejection electrodes each ejects the particles gathering at the associated ejection port. A counter electrode is located at the rear of the recording medium while facing the ejection ports. Auxiliary electrodes are so arranged as to intensify electric fields around the ejection electrodes. 
     The conventional recorder of the type described has the following problems (1) and (2) left unsolved. 
     (1) Circuitry for driving the ejection electrodes and auxiliary electrodes is scaled up. Specifically, a single driver must be assigned to each ejection electrode, and a single driver must be assigned to each two auxiliary electrodes. It follows that a multielement head having e.g., electrodes arranged in several ten arrays or a line head having electrodes arranged in several hundred to several thousand arrays needs a prohibitive number of drivers, scaling up drive circuitry. Moreover, an increase in the number of drivers increases the overall size and production cost of the recorder. 
     (2) Even when the same voltage is applied the ejection electrodes, the amount of toner particles ejected differs from one ejection electrode to another ejection electrode, resulting in dots each having a different shape on a recording medium. This is ascribable to scatters ascribable to the head production process, e.g., scatters in the configuration of the ejection electrodes and ejection ports, the position of the auxiliary electrodes relative to the ejection electrodes, and the distance between the ejection electrodes and the counter electrode. The ejection electrodes, for example, promote the concentration of electric fields more positively when provided with sharper tips, increasing the amount of particles to be ejected and the size of a dot on the recording medium. As the distance between a given ejection electrode and the associated auxiliary electrode or the counter electrode decreases, compared to the distance between another ejection electrode and the associated auxiliary electrode or the counter electrode, the size of a dot on the recording medium increases, and vice versa. Such a scatter in dot size is aggravated when the number of ejection electrodes is increased. 
     Technologies relating to the present invention are also taught in, e.g., Japanese Patent Laid-Open Publication Nos. 57-11058, 2-217253, 6-286130, and 8-1942. 
     SUMMARY OF THE INVENTION 
     It is therefore a first object of the present invention to solved the above problem (1), i.e., to provide a miniature cost-effective electrostatic ink jet recorder by reducing the number of drivers for driving ejection electrodes and auxiliary electrodes. 
     It is a second object of the present invention to solve the problem (2), i.e., to provide an electrostatic ink jet recorder capable of ejecting substantially the same amount of toner particles from all of its ejection electrodes despite a scatter in the ejection electrode and a scatter in the position of the ejection electrodes relative to auxiliary electrodes and a counter electrode. 
     In accordance with the present invention, an electrostatic ink jet recorder for recording an image on a recording medium by applying an electric field to ink containing charged toner particles and ejecting an ink drop based on a Coulomb force acting on the toner particles includes an electrophoresis electrode for causing the toner particles to concentrate at ejection ports. A plurality of ejection electrodes each ejects the toner particles concentrating at particular one of the ejection ports. A counter electrode faces the ejection ports with the intermediary of the recording medium. A plurality of auxiliary electrodes adjoin the plurality of ejection electrodes for intensifying electric fields. The ejection electrodes are divided into a plurality of groups and applied with a voltage group by group. The auxiliary electrodes are also divided into a plurality of groups and applied with a voltage group by group. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become apparent from the following detailed description taken with the accompanying drawings in which: 
     FIG. 1 is a perspective view showing a head included in a conventional electrostatic ink jet recorder and arrangements surrounding it; 
     FIG. 2 is a plan view of the head of FIG.  1  and the arrangements surrounding it; 
     FIG. 3 shows the waveforms of voltages applied to ejection electrodes and an electrophoresis electrode included in the recorder shown in FIG. 1; 
     FIG. 4 shows the arrangement of auxiliary electrodes also included in the recorder of FIG. 1 for concentrating electric fields; 
     FIG. 5 shows control means included in a first embodiment of the electrostatic ink jet recorder in accordance with the present invention for controlling ejection electrodes and auxiliary electrodes; 
     FIG. 6 shows the waveforms of voltages applied to the ejection electrodes and auxiliary electrodes of the first embodiment; 
     FIG. 7 shows control means representative of a second embodiment of the present invention; 
     FIG. 8 is a perspective view showing a head included in another conventional electrostatic ink jet recorder together with arrangements surrounding it; 
     FIG. 9 is a plan view showing the basic configuration of the head and associated arrangements shown in FIG. 8; 
     FIG. 10 shows the waveforms of voltages applied to the ejection electrodes and auxiliary electrodes of FIG. 8 as well as the waveform of a voltage applied to an electrophoresis electrode; 
     FIG. 11 shows control means representative of a third embodiment of the present invention; 
     FIG. 12 shows the waveforms of voltages applied to ejection electrodes and an auxiliary electrode included in the third embodiment; and 
     FIG. 13 shows control means representative of a fourth embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The problem (1) will be discussed more specifically in order to better understand embodiments of the present invention capable of solving it, i.e., achieving the first object of the present invention. 
     As shown in FIGS. 1 and 2, a conventional electrostatic ink jet recorder includes an ink chamber  402  filled with ink  401  containing toner particles  501 . An electrophoresis electrode  403  causes the toner particles  501  to gather at ejection ports  404 . A plurality of ejection electrodes  406  jet the toner particles  501  gathering at the ejection ports  404  toward a recording medium  405 . A counter electrode  407  is positioned at the rear of a recording medium  405  while facing the ejection electrodes  406 . The ejection ports  404  are partitioned from each other by walls  408  on an ejection electrode basis such that the ink  401  forms a convex meniscus  502  at the tip of each ejection electrode  406 . The ink chamber  402  is communicated to an ink tank, not shown, by tubings, not shown, via an ink inlet  409  and al ink outlet  410 . In this condition, a back pressure acts on the ink existing in the ink chamber  402 , and the ink  401  is forcibly circulated via the ink chamber  402 . 
     Electrophoresis used by the above ink jet recorder is such that when charged toner particles are subjected to an electric field, they migrate in one direction under the electric field. Specifically, as shown in FIG. 3, assume that a preselected voltage V 1  is applied to the electrophoresis electrode  403 . Then, the toner particles  501  of the ink  401  migrate toward the ejection ports  404  at a given electrophoretic velocity. 
     More specifically, assume that one of drivers  503 , FIG.  3 . for ejecting the particles  501  is turned on in order to apply a voltage V 2  shown in FIG. 3, i.e., a pulse-like ejection electrode voltage Vej to the associated ejection electrode  406 . Then, a static electric field is formed between the ejection electrode  406  and the counter electrode  407 . As a result, the particles  501  migrate toward and gather at the tip of the electrode  406 . Such particles  501  overcome the surface tension, viscosity and so forth of the ink  401  due to the electrostatic force. Consequently, the particles  501  fly away from the electrode  406  in the form of a fine mass of particles, or drop,  504  at a timing synchronous with the pulse-like voltage Vej, as shown in FIG.  2 . The drop  504  deposits on the recording medium  405 . Subsequently, ink is fed to the ink chamber  402  via the ink inlet  409  in order to replenish the particles  501 . 
     The above operation is repeated until a desired image has been formed on the recording medium  405 . 
     Generally, the voltage Vej applied to each ejection electrode  406 , as stated above, is as high as about 1,000 V. FIG. 4 shows auxiliary electrodes  701  customarily arranged around the discharge electrodes  406  in order to reduce the voltage to be applied to the electrodes  406 . Specifically, whether or not the drop  504  flies depends on the size of the electric field of the individual ejection electrode  406 . The auxiliary electrodes  701  are therefore used to intensity the electric field around the individual discharge electrode  406  toward the counter electrode  407 . 
     However, the conventional ink jet recorder with the above construction has the problem (1) stated previously. 
     A first and a second embodiment the electrostatic ink jet recorder in accordance with the present invention each is a solution to the problem (1), as follows. 
     1st Embodiment 
     Referring to FIGS. 5 and 6, an electrostatic ink jet recorder embodying the present invention will be described. FIG. 5 shows head drive means including ejection electrode control means and auxiliary electrode control means for promoting the concentration of electric fields. FIG. 6 shows the waveforms of voltages applied to ejection electrodes and auxiliary electrodes. The illustrative embodiment is assumed to include a multielement head having 200 ejection electrodes. 
     As shown in FIG. 5, the auxiliary electrode control means includes auxiliary electrodes  1  for promoting the concentration of electric fields. Let the auxiliary electrodes  1  be serially numbered from  1  through  200 , although not shown specifically. In the illustrative embodiment, each twenty-five consecutive auxiliary electrodes  1  constitute a single group as follows. The auxiliary electrodes # 1 -# 25  constitute a first group while the auxiliary electrodes # 26 -# 50  constitute a second group. In the same manner, the auxiliary electrodes # 176 -# 200  constitute an eighth group. The auxiliary electrodes  1  of each group are connected by the same signal line. The first group is connected to an auxiliary driver  4   a  which is in turn, connected to an auxiliary electrode power source  5 . Likewise, the second to eighth groups are respectively connected to auxiliary drivers  4   b - 4   h  which are also connected to the auxiliary electrode power source  5 . An auxiliary electrode controller  6  selectively turns on or turns off switches included in the drivers  4   a - 4   h  so as to set up or interrupt voltage application to the auxiliary electrodes  1 . 
     The ejection electrode control means includes ejection electrodes  2 . Let the ejection electrodes  2  be also serially numbered from  1  through  200 , as shown in FIG.  5 . In the illustrative embodiment, the ejection electrodes  2  located at every twenty-fifth position constitute a single group. Specifically, the ejection electrodes # 1 , # 26 , . . . , # 176  constitute a first group while the ejection electrodes # 2 , # 27 , . . . , # 127  constitute a second group. In the same manner, the ejection electrodes # 25 , . . . ,  200  constitute a twenty-fifth group. The electrodes  2  belonging to each group are connected by the same signal line. The first group is connected to an ejection driver  7   a  which is, in turn, connected to an ejection electrode power source  8 . Likewise, the second to twenty-fifth groups are respectively connected to ejection drivers  7   b - 7   y  which are also connected to the ejection electrode power source  8 . An ejection electrode controller  9  selectively turns on or turns off switches included in the ejection drivers  7   a - 7   y  so as to set up or interrupt voltage application to the electrodes  2 . 
     How toner particles are ejected from the ejection electrodes  2  will be described with reference to FIG.  6 . As shown, the auxiliary electrode controller  6  sequentially feeds auxiliary control signals T 1 -T 8  to the eight auxiliary drivers  4   a - 4   h,  respectively. The signals T 1 -T 8  are produced by equally dividing a single recording period T into eight with respect to time. Therefore, while the auxiliary control signal T 1  fed to the auxiliary driver  4   a  is in its ON state (labeled  1  in FIG. 1; a period of time of ⅛×T), an auxiliary electrode voltage for concentrating an electric field is applied to the auxiliary electrodes # 1 -# 25  belonging to the first group. Likewise, the auxiliary electrode is sequentially applied to the second group of auxiliary electrodes to the eighth group of auxiliary electrodes for a duration of ⅛×T each. 
     The ejection electrode controller  9  selectively turns on or turns off the ejection drivers  7   a - 7   y  by synchronizing a signal output from an image control section, not shown, to the auxiliary control signals, thereby applying a voltage to the ejection electrodes  2 . For example, when the controller  9  receives an ejection command representative of image data on the ejection electrode # 1 , the controller  9  outputs, via an ejection control signal A, a signal for turning on the ejection driver  7   a  in synchronism with the auxiliary control signal T 1  (labeled [I]). As a result, the auxiliary electrode voltage is applied to the first group of auxiliary electrodes, generating an electrode intense enough to eject toner particles at the ejection electrode # 1 . 
     To eject toner particles from the ejection electrode # 2 , the ejection electrode controller  9  outputs, via an ejection control signal B, a signal for turning on the ejection driver  7   b  in synchronism with the auxiliary control signal T 1  (labeled [II]). Likewise, to eject toner particles from the ejection electrode # 27 , the ejection electrode controller  9  outputs, via the control signal B, a signal for turning on the ejection driver  7   b  in synchronism with the auxiliary control signal T 2  (labeled [II]). In this manner, then toner particles should be ejected from any one of the ejection electrodes  2  designated by image data, an ejection control signal assigned to the discharge electrode  2  is turned on in synchronism with the auxiliary control signal. 
     2nd Embodiment 
     A second embodiment of the present invention is shown in FIG.  7  and also includes a multielement head having 200 ejection electrodes. As shown, in this embodiment, each twenty-five consecutive ejection electrodes  2  constitute a single group. That is, the electrodes  2  are divided into a first group having the ejection electrodes # 1 , # 2 , # 3 , . . . , # 25 , a second group having the ejection electrode # 26 , . . . , # 50 , and so forth. An eighth group has the ejection electrodes # 176 , . . . , # 200 . On the other hand, the auxiliary electrodes  1  located at every twenty-fifth position constitute a single group. Specifically, a first group has the auxiliary electrodes # 1 , # 26 , # 51 , . . . , # 176 , and a second group has auxiliary electrodes # 2 , # 27 , . . . , # 177 . The last or twenty-fifth group has the auxiliary electrodes # 25 , . . . , # 200 . When voltages are applied to any one of the ejection electrodes  2  and auxiliary electrodes  1  associated therewith at the same time, toner particles are ejected from the ejection electrode  2 , as in the previous embodiment. The illustrative embodiment reduces the required number of ejection drivers from 25 to 8, compared to the first embodiment. This is desirable from the cost standpoint because the ejection drivers turn on and turn off a higher voltage than the auxiliary drivers and are therefore more expensive. 
     With a multielement head having 200 ejection electrodes and auxiliary electrodes associated therewith, it has been customary to assign a single driver to each ejection electrode and a single driver to each two auxiliary electrodes, resulting in 400 drivers in total. By contrast, the first and second embodiments each is capable of driving the ejection electrodes and auxiliary electrodes with thirty-three drivers, i.e., 8+25=33. To further reduce the number of drivers, the auxiliary drivers and ejection drivers may be suitably combined, e.g., 16+13=29. In addition, a combination implementing the lowest production cost may be selected. 
     As stated above, the first and second embodiments each includes head drive means having ejection electrode control means for applying a voltage to a plurality of ejection electrodes while controlling the electrodes group by group and auxiliary electrode control means for applying a voltage to a plurality of ejection electrodes while controlling the electrode group by group. With such head drive means, it is possible to reduce the number of drivers and therefore to scale down the circuitry. The embodiments therefore each implements a miniature cost-effective ink jet recorder. 
     Now, the problem (2) stated earlier will be discussed more specifically in order to better understand other embodiments of the present invention each of which is a solution to the problem (2). 
     FIG. 8 shows another conventional ink jet recorder identical with the conventional recorder of FIG. 1 except that it includes auxiliary electrodes  701  for concentrating electric fields. As shown in FIG. 9, the ejection ports  404  are partitioned by walls  411  on an ejection electrode basis such that the ink  401  forms a convex meniscus at the tip of each ejection electrode  406 . The principle of electrophoresis is also applied to this ink jet recorder. FIG. 10 shows a preselected voltage V 1  applied to the electrophoresis electrode  403 . 
     As shown in FIG. 9, assume that a driver  503  and a driver  503   a  associated therewith are turned on in order to eject the toner particles  501 . The driver  503  feeds a voltage V 2 , FIG. 10, to the associated ejection electrode  406  for a duration of T 2  while the driver  503   a  feeds a pulse voltage V 3  to the associated auxiliary electrodes  701  for the duration of T 2 . As a result, an intense electric field formed between the ejection electrode  406  and the auxiliary electrodes  701  causes the particles  501  to migrate toward and concentrate at the tip of the electrode  406 . The auxiliary electrode  701  is so positioned as to intensify the electric field toward the counter electrode  407 , serving to reduce the voltage to be applied to the ejection electrode  406 . 
     As shown in FIG. 9, the particles  501  having overcome the surface tension, viscosity and so forth of the ink  401  fly away from the tip of the ejection electrode  406  toward the recording medium  405  in the form of a fine mass or drop. Again, the particles  501  are supplemented by ink fed to the ink chamber  402  via the ink inlet port  409 . 
     The above operation is repeated until an image is formed on the recording medium  405 . However, the conventional ink jet recorder having the above configuration has the problem (2). 
     A third and a fourth embodiment of the present invention capable of solving the problem (2) will be described hereinafter. 
     3rd Embodiment 
     Reference will be made to FIGS. 11 and 12 for describing a third embodiment of the present invention. FIG. 11 shows circuitry similar to the circuitry of the first and second embodiments shown in FIGS. 5 and 7, respectively. FIG. 12 shows the waveforms of voltages similar to the waveforms of FIG.  6 . The following description will concentrate on a multielement head having 120 ejection electrodes by way of example. Specifically, this kind of head has fifteen head units each having eight ejection electrodes. 
     Assume that in a given head unit the distance between a given ejection electrode  105  and an auxiliary electrode  101  associated therewith is not constant due to a scatter ascribable to the production process, and that the amounts of toner particles ejected from the ejection electrode  105  # 1  (# 105 - 1  hereinafter; this also applies to the other electrodes) and # 105 - 8  is comparatively small while the amount of particles ejected from the ejection electrode # 105 - 3  is comparatively great. Then, dots formed on a recording medium by the election electrodes # 105 - 1  and # 105 - 8  are small while a dot formed by the ejection electrode # 105 - 3  is large. 
     Auxiliary electrodes  101  for concentrating electric fields are grouped, as follows. A first auxiliary electrode # 101 - 1  and every eight auxiliary electrodes # 101 - 9 , # 101 - 17 , . . . , # 101 - 113  constitute a first group while a second auxiliary electrode  101 - 2  and every eight auxiliary electrodes # 101 - 10 , # 101 - 18 , . . . , # 101 - 114  constitute a second group. Likewise, an eighth auxiliary electrode # 101 - 8  and every eight auxiliary electrodes # 101 - 16 , # 101 - 24 , . . . , # 101 - 120  constitute a fifteenth group. In this manner, the auxiliary electrodes  101  are divided into fifteen groups in total. The auxiliary electrodes of each group are connected by the same signal line. The first group of auxiliary electrodes are connected to one end of an auxiliary driver  102  which is, in turn, connected to an auxiliary electrode power source  103 . The auxiliary driver  102  is turned on and turned off by an auxiliary electrode controller  108 . In this configuration, the output voltage of the power source  103  is fed to each group of auxiliary electrodes via the associated driver  102 . 
     The ejection electrodes  105  are grouped, as follows. A first ejection electrode # 105 - 1  and every fifteen ejection electrodes # 105 - 16 , . . . , # 105 - 106  constitute a fist group connected to an election driver  107  which is, in turn, connected to an ejection electrode power source  106 . Likewise, a second ejection electrode # 105 - 2  and every fifteenth ejection electrodes # 105 - 17 , . . . , # 105 - 107  constitute a second group connected to an ejection driver  109 . In this manner, the ejection electrodes  105  are divided into eight groups each being connected to a respective ejection driver. The ejection drivers are turned on and turned off by an election electrode controller  104 . 
     A voltage duration controller  110  controls the duration of the voltage to be applied from the auxiliary electrode power source  103  to the individual auxiliary electrode group. The auxiliary electrode controller  108  sets up and interrupts the application of the output voltage of the power source  103  to each of the eight groups of auxiliary electrodes  101  via the voltage duration controller  110 . 
     FIG. 12 shows voltages applied to a plurality of ejection electrodes and voltages applied to a plurality of auxiliary electrodes. How the toner particles are ejected from the ejection electrodes in substantially the same amount will be described with reference to FIG.  12 . As shown, the auxiliary electrode controller  108  sequentially feeds to the eight auxiliary drivers auxiliary electrode control signals A-H produced by equally dividing a single recording period T into eight. The durations of the control signals A-H are controlled by the voltage duration controller  110 . 
     T 2 , T 4 , T 5  and T 6  are a default value. T 1  and T 8  are longer than the default value while T 3  is shorter than the default value; the default value is indicated by a dashed line. While the control signal A is in its ON state (T 1 ), it turns on the auxiliary driver connected to the first group of auxiliary electrodes. As a result, the output voltage of the auxiliary electrode power source  103  is applied to the first group of auxiliary electrodes. The control signal B is brought to its ON state after the control signal A. In response, the driver  102  connected to the second group of auxiliary drivers is turned on, feeding the output voltage of the power source  103  to the second group. In this manner, the output voltage of the power source  103  is sequentially fed to the first group to the third group for the durations of T 1 -T 8 , respectively. 
     Assume that while the control signal A is in ON state, the ejection electrode controller  104  receives a signal indicative of image data on the ejection electrode # 105 - 1  from an image control section, not shown. Then, the controller  104  feeds an ejection electrode control signal D 1  to the ejection driver  107  connected to the ejection electrode # 105 - 1  and thereby turns it on (labeled [I]). Consequently, the output voltage of the ejection electrode power source  106  is fed to the first group of ejection electrodes # 105 - 1 , # 105 - 16 , . . . , # 105 - 106  via the driver  107 . Because a particular voltage is applied to each of the auxiliary electrodes  101  and ejection electrode # 105 - 1 , an electric field intense enough to eject the toner particles is generated. As a result, a fine mass of particles is ejected from the tip of the ejection electrode  105 - 1  toward the counter electrode  112  for the duration of T 1 . The particles deposit on the recording medium and form a dot thereon. 
     In the above condition, although the other ejection electrodes # 105 - 16 , # 105 - 31 , . . . , # 105 - 106  are applied with the same voltage as the ejection electrode # 105 - 1 , they do not eject the particles because they are not applied with the auxiliary electrode voltage. Specifically. FIG. 12 shows a condition wherein the electrode # 105 - 17  does not eject the particles (labeled ( 1 )), but the electrode # 105 - 113  ejects them for the duration of T 1  (labeled [ 1 ]). 
     While the control signal B remains in its ON state for the the duration of T 2 , the electrode # 105 - 2  (labeled [II]) and electrode # 105 - 17  (labeled ( 2 )) each ejects the particles for the duration of T 2 , but the electrode # 105 - 113  (labeled [ 2 ]) does not eject them. Further, while the control signal H is in its ON state for the duration of T 8 , the electrode # 105 - 8  (labeled [III]) and electrode # 105 - 120  (labeled [ 8 ]) each ejects the particles for T 8 , but the electrode # 105 - 33  (labeled ( 8 )) does not eject them. 
     As stated above, each ejection electrode ejects the toner particles only when it and its associated auxiliary electrodes each is applied with a particular voltage. In the illustrative embodiment, in each eight-element head unit, the ejection heads # 105 - 1  and # 105 - 8  eject the particles for a period of time longer than the default value while the ejection head # 105 - 3  ejects them for a period of time shorter than the default value. This successfully makes up for the scatter among the ejection electrodes and thereby guarantees substantially the same amount of ejection from all of the ejection electrodes  105 . 
     4th Embodiment 
     A fourth embodiment of the present invention will be described with reference to FIG.  13 . This embodiment is identical with the third embodiment as to the configuration of the electrodes. In this embodiment, a particular voltage is applied to each of the auxiliary electrode group, and a voltage controller  301  is substituted for the voltage duration contrdller  110 . The voltage controller  301  uses the fact that the intensity of electric field and therefore the amount of toner particles to be ejected increases with an increase in voltage. 
     Specifically, as shown in FIG. 13, a voltage lower than a default value is assigned to the first and eighth groups of auxiliary electrodes  101  while a voltage higher than the default value is assigned to the third group of auxiliary electrodes  101 . In this condition, the potential difference is greatest between the ejection electrodes and the auxiliary electrodes corresponding to each of the first and eighth auxiliary electrode groups, intensifying the electric fields around the ejection ports. By contrast, the above potential difference is smallest between the ejection electrodes and the auxiliary electrodes corresponding to the third auxiliary electrode group, slightly weakening the electric fields around the ejection ports. This successfully sets up a substantially uniform electric field distribution throughout the groups and thereby substantially uniforms the amount of particles to be ejected. 
     As stated above, in the third and fourth embodiments, the duration of a voltage to be applied to the auxiliary electrodes or the voltage itself is varied in order to vary the amount of toner particles to be ejected on an ejection electrode basis. With this scheme, it is possible to absorb a scatter among heads ascribable to the production process and therefore to allow all the ejection electrodes to eject substantially the same amount of toner particles. 
     Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.