Abstract:
An ink-jet printer including an ink jet nozzle for ejecting ink droplets onto the recording paper which is moved relative to the nozzle so that dots of ink are printed in the form of a pattern. The trajectory of flying ink droplets is deflected along the direction of the relative movement under control based on the measurement of the relative movement speed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an ink-jet recording apparatus and, particularly, to an improved ink-jet recording apparatus of a type, in which ink droplets are ejected from a nozzle and impinge on the recording medium to form dots thereon, and the position of the record is affected by the change in the relative speed between the nozzle and the recording medium. 
     It is important for an ink-jet recording apparatus to make a dot record of ink droplets accurately at a specified position on the recording medium. Particularly, in case of color recording, ink droplets ejected from more than one nozzle must produce dots accurately at specified positions on the recording medium. 
     Ink-jet recording apparatus recording information on the recording paper which is rotated on the drum are disclosed in U.S. Pat. No. 3,928,718 by Syoji Sagae et al., and U.S. Pat. No. 3,999,188 by Takahiro Yamada et al. In these apparatus, ink droplets are ejected from the nozzle at a fixed time interval, and therefore if the rotational speed of the drum varies, the dots are not recorded on the correct position of the recording paper, resulting in an uneven pitch of dots. These ink-jet recording apparatus are capable of recording images in color through the arrangement of more than one nozzle for various colors in the circumferential direction of the drum. However, when a certain pattern of image is intended to produce using a plurality of nozzles, recorded patterns by the nozzles would be out of alignment with each other unless each nozzle produces a pattern respectively at a correct position accurately. A possible cause of such a faulty print result is induced by the fluctuation of the drum speed. 
     SUMMARY OF THE INVENTION 
     This invention contemplates to solve the foregoing prior art problem, and its prime object is to provide an ink-jet recording apparatus for recording images accurately at specified positions on the recording paper even under the fluctuating rotation of the drum. 
     The present invention resides in an ink-jet recording apparatus including a device for ejecting ink droplets through a nozzle, a device for moving a recording medium across and relative to the trajectory of the ink droplets, and a device for controlling the trajectory of the ink droplets in accordance with the information signal to be recorded so that each droplet reaches a specified position on the recording medium, wherein the control device comprises a device for producing a signal in terms of the relative speed between the recording medium and the nozzle, a device for charging ink droplets electrostatically in correspondence to the speed signal, and a device for deflecting the ink droplets in the direction along the relative movement to an extent in proportion to the amount of charges on the ink droplets. 
     The inventive apparatus prevents the displacement of a pattern record caused by the variation in the relative speed between the recording medium and the nozzle by controlling the deflection of ink droplets. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration explaining the deviation of a pattern produced by more than one recording head; 
     FIG. 2 is a block diagram of the inventive ink-jet recording apparatus; 
     FIG. 3 is a waveform diagram showing the operation of the above arrangement; 
     FIG. 4 is an illustration used to explain an embodiment of the recording head used in a modified system arrangement; and 
     FIG. 5 is a block diagram showing the inventive color ink-jet recording apparatus using more than one recording head. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 explains the displacement of a pattern record produced by four ink-jet nozzles when the information signal is intended to record at a specified position on the recording paper. The four recording heads A, B, C and D are moved in unison in the axial direction of a drum 7, i.e., perpendicularly to the drawing as shown by symbol ○X . The recording heads A-D have associated nozzles 1a-1d, which are adapted to vibrate at an ultrasonic frequency so that jets of pressurized ink 2a-2d released from the nozzles 1a-1d are formed into ink droplets 4a-4d at the same frequency as of the ultrasonic vibration. The ink droplets 4a-4d are charged in proportion to the information signal components to each head by means of charging electrodes 3a-3d, and the projectile lines of the charged ink droplets are deflected in proportion to the amount of charges by deflection electrodes 5a-5d in the direction shown by the arrow Y which is perpendicular to a plane including the direction shown by the symbol ○X . Gutters 6a-6d are provided at a position partly interfering the flight paths of the ink droplets 4a-4d so that ink droplets unused for recording are caught by them. Each of the recording heads A-D is consistent, but in a 90° rotation, with those disclosed in the above-mentioned U.S. Pat. Nos. 3,928,718 and 3,999,188. 
     In producing a complete record of the information signal using the recording heads A-D at one position on the recording paper 8 placed on the drum 7 rotating in the direction shown by the arrow 9, the voltages carrying information signal components for the heads A-D are applied to the charging electrodes 3b-3d of heads B-D with respective time lags produced by delay circuits, e.g. a shift register, with respect to the time point of voltage application to the charging electrode 3a of recording head A. Namely, the recording head B is activated at a delayed time point when a record ranging 10 to 11 produced by the head A has come to the position ranging 12 to 13. Subsequently, the recording head C is activated at a delayed time point when the record ranging 12 to 13 produced by the head B has come to the position ranging 14 to 15. Finally, the recording head D is activated at a delayed time point when the record ranging 14 to 15 produced by the head C has come to the position ranging 16 to 17, and a composite pattern by the four heads for the information signal is completed. 
     In this case, it is necessary that the range 10-11 of record produced by the head A, the range 12-13 of record produced by the head B, the range 14-15 of record produced by the head C and the range 16-17 of record produced by the head D are coincident with each other on the recording paper 8. However, if the drum speed varies during the recording operations by the four heads, it will arise, for example, that a record is produced by the head B at the position ranging 12-13 when the record ranging 10-11 produced by the head A has come to a position ranging 10&#39;-11&#39;. This results in a displacement of the recording range 12-13 by the head B form the recording range 10&#39;-11&#39; by the head A. The displacement of record also occurs at the ranges 14-15 and 16-17 by the remaining recording heads although it is not shown in the figure. 
     For a recording system with a single recording head, the above-mentioned problem results in an uneven interval of dots aligning in the drum rotational direction, and uneven recording caused by this phenomenon can be prevented as described in the following. 
     The ink-jet recording apparatus shown in FIG. 2 is of the electrostatic modulation type as disclosed in the above-mentioned U.S. Pat. Nos. 3,928,718 and 3,999,188, but with a modification being made such that the recording head is installed in a 90° rotation so that ink droplets are deflected in the direction along the rotational direction of the drum. 
     Referring to FIGS. 2 and 3, an encoder 19 is coupled to the drive shaft (not shown) of the drum 7 so as to produce a rectangular pulse signal 41 having a frequency dependent on the drum speed. The signal 41 is received by a frequency-to-voltage (F/V) converter 20, which produces a voltage signal 24 in proportion to the frequency of the pulse signal 41. Accordingly, when the drum speed varies, the frequency of the pulse signal 41 from the encoder 19 is varied, and thus the voltage signal 24 produced by the F/V converter 20 is varied. As shown in FIG. 3, the voltage signal 24 varies from a voltage V 0  at the normal drum speed to a voltage V 1  in response to a fall in the drum speed, and it varies from V 0  to a voltage V 2  in response to a rise in the drum speed. The voltage signal 24 carrying a voltage level V 0 , V 1  or V 2  is received by a level shift circuit 21, which produces a voltage signal 25 carrying a voltage level v 0 , v 1  or v 2  derived from V 0 , V 1  or V 2 , respectively, but shifted in the negative direction with respect to the reference voltage level Vs. The absolute values of V 0 , V 1  and V 2  are in the order of |V 2  |&gt;|V 0  |&gt;|V 1  |, but as a result of negative shift by Vs the absolute values of v 0 , v 1  and v 2  become in the order of |v 2  |&lt;|v 1  |&lt;|v 0  |. Accordingly, the circuit 21 provides a higher voltage in response to a lower drum speed, and a lower voltage in response to a higher drum speed. 
     The voltage signal 25 is received by a multiplier 22, in which it is multiplied by an information signal 26 supplied from a signal source 45. In FIG. 3, signal levels S1 through S4 in the information signal 26 sampled in the normal drum speed are multiplied by the voltage level v 0  of the voltage signal 25 corresponding to the normal drum speed, and signal levels s1 through s4 are produced in the output 27 of the multiplier 22. For the convenience of explanation, the voltage levels S1-S4 of the information signal 26 are each assumed to be equal to voltage levels s1-s4 of the output 27 from the multiplier 22. 
     Signal levels S5-S8 of the information signal 26 received at a lower drum speed are multiplied by the larger voltage value v 1  of the voltage signal 25, so that they are modified by an increment of +α to larger levels s5-s8 in the output signal 27 than the voltage levels of signals S5-S8 (output signals s1-s4) of the information signal 26. Conversely, signal levels S9-S12 in the information signal 26 received at a higher drum speed are multiplied by the larger voltage value v 2  of the voltage signal 25, so that they are modified by a decrement of -α to smaller levels s9-s12 in the output signal 27. 
     The modified signal levels s1-s4, s5-s8 and s9-s12 in the output 27 are amplified by an amplifier 23 and supplied to the charging electrodes (not shown) in the recording head 18. Ink droplets 28 charged electrostatically by the charging electrodes in proportion to the voltage levels s1-s12 are deflected for their flight path by the deflecting electrodes (not shown) by amounts in proportion to the respective charges along the drum rotational direction shown by the arrow 9, and they reach the specified points (not shown) on the recording paper 8 set on the drum 7. 
     Namely, when the drum 7 rotates at the normal speed, sampled signal levels S1-S4 in the information signal 26 are recorded at the specified position on the recording paper 8 by the ink droplets 28 which are charged to the voltage levels s1-s4 equal to S1-S4, respectively. When the drum speed falls, sampled signal levels S5-S8 are recorded at the specified position on the paper 8 by the ink droplets 28 which are charged to the voltage levels s5-s8 larger than S5-S8 to cause an increased deflection angle, i.e., a longer trajectory distance, in the direction of drum rotation. When the drum speed rises, sampled signal levels S9-S12 are recorded at the specified position on the paper 8 by the ink droplets 28 which are charged to the voltage levels s9-s12 smaller than S9-S12 to cause a decreased deflection angle, i.e., a shorter trajectory distance, in the direction of drum rotation. 
     The foregoing embodiment illustrated in FIGS. 2 and 3 can be applied identically to the arrangement with more than one recording head. 
     FIG. 4 shows an embodiment of the recording head according to the present invention, and FIG. 5 shows the arrangement for color recording using four recording heads each shown in FIG. 4. 
     In FIG. 4, the amplitude of ultrasonic vibration applied to a nozzle 32 of a recording head 18 is controlled so that ink droplets 28a having a larger diameter and ink droplets 28b having a smaller diameter are produced alternately at the frequency of the ultrasonic vibration. Charging-deflecting electrodes 29a and 29b are applied with voltage pulses supplied from information signal sources 30a and 30b that are superimposed by bias voltages supplied from voltage sources 31a and 31b respectively. 
     The larger ink droplet 28a flies faster than the smaller ink droplet 28b. Both ink droplets 28a and 28b are charged in proportion to the pulse voltage representing the information signal, and in this case the amount of charges given to the larger ink droplet 28a is more than that given to the smaller ink droplet 28b. Accordingly, by application of the bias voltages provided by the voltage sources 31a and 31b to the charging-deflecting electrodes 29a and 29b, the larger ink droplet 28a is deflected in a greater angle than the smaller ink droplet 28b. On this account, when the recording paper 8 is moved at a constant speed in the direction shown by the arrow 9 along the deflecting direction, the larger ink droplet 28a flies faster on a longer projectile line and the smaller ink droplet 28b flies slower on a shorter projectile line resulting in the arrival of both droplets 28a and 28b, without merging, at specified positions 34, 35, and so on on the recording paper 8. When both ink droplets 28a and 28b are not charged and, thus, not deflected, the smaller ink droplet 28b&#39; is merged into the larger ink droplet 28a&#39; because of their different flight speed, and such unused ink droplets are collected by a gutter 33. 
     Although the foregoing recording head is designed to produce larger and smaller ink droplets 28a and 28b for making pattern records of information signal at the specified positions 34, 35, 36 and so on on the recording paper 8, it can also be applied to ink-jet recording apparatus of the on-demand type producing ink droplets of separate flight speeds only when necessary, as disclosed in U.S. Pat. No. 3,946,398 by Edmond L. Kyser et al. 
     FIG. 5 shows the arrangement for color recording employing recording heads 18A, 18B, 18C and 18D of the type shown in FIG. 4 for making pattern records at specified positions 37, 38, 39 and 40, respectively, on the recording paper 8 through the control of the deflection angle for compensating the displacement of recording position due to different flight speeds of larger and smaller ink droplets 28a and 28b and the displacement of recording position due to the fluctuation of the drum speed as described previously. In operation, the recording head 18A is first activated to produce larger and smaller ink droplets 28a and 28b so that a pattern record is made at the specified position 37. Thereafter, when the drum 7 has rotated in the direction shown by the arrow 9 so that the position 37 becomes coincident with the position 38, the recording head 18B is activated to produce larger and smaller ink droplets so that the same position 37 is recorded again this time by the head 18B. In this manner, recording takes place when the initial recording position 37 has arrived at the head positions 39 and 30 successively, and a color pattern record is completed. 
     Although in the foregoing embodiments ink droplets 28, 28a and 28b are deflected in the direction along the drum rotational direction shown by the arrow 9, the same effect is achieved by deflecting ink droplets in the direction opposite to the drum rotational direction.