Ink-on-Demand Type Ink-Jet Printer

This invention relates to an ink-on-demand type ink-jet printer in which ink droplets are driven toward a paper or other medium each time that a driving pulse is applied to a piezoelectric member attached to one wall of an ink chamber.

Various types of ink-jet printers have been proposed, such as those described in an article entitled "Ink Jet Printing" by Fred J. Kamphoefner published in the IEEE Transactions on Electron Devices. Vol. Ed-19, No. 4, April 1972, pp. 584-593 (Reference 1). An ink-jet printer of the ink-on-demand type is described in detail in U.S. Pat. No. 3,946,398, entitled "Method and Apparatus for Recording with Writing Fluids and Drop Projection Means Therefor" issued to E. L. Kyser, et al., and U.S. Pat. No. 4,106,032 entitled "Apparatus for Applying Liquid Droplets to a Surface by Using a High Speed Laminar Air Flow to Accelerate the Same 38 issued to M. Miura, et al. Some of the many advantages of the ink-jet printer of ink-on-demand type are that it is simple in construction, all the droplets driven out of the ink chamber are projected to a printing surface without requiring a withdrawal of any droplets, and the droplet size may be varied by controlling the energy content of the driving pulse applied to the piezoelectric member, thereby making it possible to record a half tone.

In the conventional ink-on-demand type ink-jet printer, however, an electrostatic deflection method cannot be used for deflecting the ink droplets, because some of the droplets are different in size and weight. When a deflection field is applied to the different droplets, the resulting deflection varies according to the droplet size (weight).

The velocity of the droplets in the ink-jet printer of ink-on-demand type is lower than the velocity in the other types of ink-jet printers. As a result, the different-sized droplets cannot be uniformly accelerated, which directly affects the stability of the printing.

It is, therefore, an object of this invention to provide new and improved ink-jet printers of the ink-on-demand type in which an electrostatic deflection may be applied for deflecting the ink droplets.

It is another object of this invention to provide new and improved ink-jet printers of ink-on-demand type capable of reproducing fine letters and patterns.

According to one aspect to this invention, an ink-jet printer records, or prints, with droplets of conductive ink on a recording medium. The printer has an ink chamber filled with conductive ink. The chamber also has a nozzle and a piezoelectric member attached to one wall thereof. Droplets of ink are driven out the nozzle responsive to driving pulses applied to the piezoelectric member. The size of the droplets is a function of the energy content of the driving pulses. The droplets are charged by a voltage which is applied as they are driven out of the nozzle and that voltage is a function of the energy content of the driving pulses.

Other features and advantages of this invention will be apparent from the following description of preferred embodiments of this invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a cross-sectional view of a first embodiment of this invention;

FIG. 2 is a diagram which illustrates the principle of this invention;

FIGS. 3 and 4 are block diagrams of first and second examples of the driving pulse generator and charging voltage generator used in the first embodiment shown in FIG. 1;

FIGS. 5A through 5D and 6A through 6D are waveform diagrams of signals appearing at various parts of the second example shown in FIG. 4;

FIG. 7 is a block diagram of a third example of the driving pulse and charging voltage generator used in the first embodiment shown in FIG. 1;

FIGS. 8A, 8B, 8C, 9A and 9B are waveform diagrams of signals appearing at various parts of the third example shown in FIG. 7;

FIG. 10 is a cross sectional view of a second embodiment of this invention;

FIG. 11 is a block diagram of a first example of the driving pulse and charging voltage generator used in the second embodiment shown in FIG. 10;

FIG. 12 is a block diagram of a second example of the same generator used in the second embodiment shown in FIG. 10; and

FIGS. 13A through 13E are waveform diagrams of signals appearing at various parts of the second example of the generator shown in FIG. 12.

The first embodiment of the ink-jet printer shown in FIG. 1 includes a print head 10 having an ink chamber 11 which holds conductive ink. Ink is supplied to the chamber 11, from an ink reservoir 12 through a supply tube 13. A nozzle 14 is positioned at one side of the chamber 11, and a piezoelectric member 15 is mounted on the opposite side of the chamber. Piezoelectric member 15 temporarily reduces the volume of the chamber 11 each time that the member 15 is electrically pulsed by a driving pulse P.sub.D supplied from a driving pulse/charging voltage generator 20.

Piezoelectric member 15 also creates a pressure pulse in the print head chamber 11 which drives out an ink droplet 16, each time a driving pulse P.sub.D is applied from the pulse generator 20 to the piezoelectric member 15. The droplet size is a function of the energy content of the driving pulse P.sub.D, which may be varied by adjusting variables, such as pulse width and pulse amplitude.

Charging electrodes 30 are provided adjacent the nozzle 14. A charging voltage V.sub.c from the generator 20 may be applied to the electrodes 30. First accelerating electrodes 40 which are adjacent electrodes 30 may be charged to a first accelerating voltage from a first accelerating voltage source 41. Deflection electrodes 50, which are adjacent the first accelerating electrodes 40, may be charged with a deflection voltage E generated in a deflection voltage source 51. Second accelerating electrodes 60, which are adjacent deflection electrodes 50, may be charged by a second accelerating voltage from a second voltage source 61.

After an ink droplet 16, having a weight m, is driven out the print head 10, it is charged while passing between the charging electrodes 30. The charge value q is proportional to the charging voltage V.sub.c. The charged ink droplet 16 is then accelerated by the first accelerating electrodes 40. As it passes through a space between the deflection electrodes 50, it is deflected a distance which is determined by the weight m, the charge value q and the deflection voltage E. The deflected ink droplet 16 is again accelerated by the second accelerating electrodes 60 and then it strikes a printing surface of a medium, such as paper 70.

The ink droplet 16 (FIG. 2) is deflected by an angle .theta. while passing through the space between the deflection electrodes 50, inducing a Y-direction velocity component V.sub.y in the droplet 16. The deflected droplet 16 thus strikes the printing surface of the paper 70 at a point which is a deflection distance D from its original path. The deflection distance may be calculated in the following manner:

where l is a distance between the deflection electrodes 50 and the printing surface of the paper 70, and "V.sub.x " is the X-direction velocity of the deflected droplet at the exit of the deflection electrodes 50. Because the change in X-direction velocity component V.sub.x due to the passage of the droplet 16 through the deflection space is negligible, the Y-direction component V.sub.y may be approximate as follows:

where symbol l.sub.d is the length of the deflection space. Therefore, the deflection distance D may be approximated as follows:

According to the invention, to maintain a constant deflection distance D when the weight m of the droplet 16 is varied without changing the voltage E applied to the deflection electrodes 50, the ratio q/m must be maintained at a fixed value. Assuming that the fluctuation in the velocity V.sub.x of the droplets 16 due to the variation in the droplet size is considered to be negligible, the deflection may be accomplished by varying the charge value q in proportion to the weight m of the droplet 16. In this manner, the electrostatic deflection method may be used for deflecting droplets differing from each other in droplet size and weight, by varying the charge value q in proportion to the weight m. Also, the different sized droplets may be uniformly accelerated.

The generator 20 (FIG. 1) produces, upon receipt of an input picture signal PS supplied from an input terminal 21, the driving pulse P.sub.D and the charging voltage V.sub.c. The charging voltage V.sub.c, which determines the charge value q, varies in proportion to the droplet weight m, which is determined by the energy content of the driving pulse P.sub.D.

One embodiment 20A (FIG. 3) of the driving pulse and charging voltage generator 20 comprises a charge voltage generator 101 for generating the charging voltage V.sub.c proportional to the level L (not shown) of the picture signal PS. A driving pulse generator 102 generates the driving pulse P.sub.D. The pulse width and/or pulse amplitude of the driving pulse P.sub.D are varied in response to the picture signal level L so that the energy content C of pulse P.sub.D, and the resulting droplet weight m, are proportional to the picture signal level L. Because of the charging voltage V.sub.c and the energy content C of the driving pulse P.sub.D are proportional to the picture signal level L, the ratio of V.sub.c /C may be maintained at a fixed value. Therefore, it is also possible to maintain the ratio of q/m at a constant level even when the droplet size is varied in response to changes in the picture signal level L. Thus, the different sized droplets 16 may be uniformly accelerated by the accelerating electrodes 40 and 60, and uniformly deflected by the deflection electrodes 50. The deflection distance D is a function of the magnitude of the deflection voltage E, which is controlled by a deflection information signal DS supplied from a deflection signal source 52 (FIG. 1).

Another embodiment 20B (FIGS. 4 through 6) of the driving pulse and charging voltage generator 20 comprises a clock pulse generator which generates a clock pulse CP, as shown in FIGS. 5A and 6A, and a charging voltage generator 104 which generates a charging voltage V.sub.c having saw-tooth waveform, as shown in FIG. 5B, or having a step waveform, as shown in FIG. 6B. Charging voltage V.sub.c is applied to the charging electrodes 30 (FIG. 1). The clock pulse CP (FIG. 4) is applied to the charging voltage generator 104 and to a pulse-amplitude modulator 105. The pulse amplitude modulator 105 produces a PAM signal from the picture signal PS and clock pulse CP, as shown in FIG. 5C or 6C. The PAM signal is supplied to an amplitude-component extractor 106 which produces a control signal responsive to the amplitude component of the PAM signal. The control signal is supplied to a variable delay line 107, which is also supplied with the PAM signal from the modulator 105. Responsive to the receipt of this control signal, the variable delay line 107 delays the PAM signal by a period which is determined by the amplitude component of the PAM signal and produces a delayed PAM signal as shown in FIGS. 5D and 6D.

The delayed PAM signal is the driving pulse P.sub.D, which is applied to the piezoelectric member 15 (FIG. 1). The piezoelectric member 15 produces droplets 16 which are proportional in size to the amplitude component of the delayed PAM signal. These droplets are driven out of the nozzle 14 at each trailing time of the delayed PAM signal. As clearly understood from FIGS. 5B and 5D, or 6B and 6D, the level of the charging voltage V.sub.c at the trailing time of the delayed PAM signal, depends on the delay time, which is related to the amplitude component. Thus, the charge value q of the droplet 16 varies in response to the amplitude component of the delayed PAM signal, which is related to the droplet weight m.

If the droplet weight m (droplet size) is not linearly related to the pulse-amplitude of the driving pulse P.sub.D, the waveform of the charging voltage V.sub.c and the input vs. output characteristic of the amplitude-component extractor 106 must be modified to meet the droplet-weight vs. pulse-amplitude characteristic.

Yet another example 20C (FIGS. 7 and 8) of the driving pulse and charging voltage generator 20 comprises a clock pulse generator 108 and a charging voltage generator 109, which generate a clock pulse CP and a charging voltage C.sub.c respectively, as shown in FIGS. 8A and 8B, and correspond to the generators 103 and 104, respectively (FIG. 4). The clock pulse CP is applied to the charging voltage generator 109, and to a pulse-width modulator 110. The pulse-width modulator 110, which is supplied with the picture signal PS, produces a PWM signal, as shown in FIG. 8C. The pulse width of the PWM signal is a function of the picture signal level L. The charging voltage V.sub.c, shown in FIG. 8B, has different values, depending on the timing of the trailing edge of the PWM signal and thus varies in response to the pulse width.

The PWM signal is the driving pulse P.sub.D supplied to piezoelectric member 15, which drives out a droplet at each of the trailing edges of the PWM signal. At each time that a droplet is driven out, a charging voltage proportional to the pulse width of the PWM signal is applied to the droplet, whereby the charge q is varied in response to the pulse width, i.e., the droplet weight m.

If the droplet weight m vs. pulse width w characteristic is not linear, as shown by a curve 111 in FIG. 9A, a signal having a waveform similar to the curve 111 is used as the charging voltage V.sub.c, as shown in FIG. 9B.

In FIG. 10, the second embodiment of the invention comprises the print head 10, the driving pulse/charging voltage generator 20', the charging electrodes 30, and the deflection electrodes 50. The head 10, and the electrodes 30 and 50 are identical to those in the first embodiment shown in FIG. 1. However, the deflection information signal DS is not supplied to the deflection voltage source 51, but is supplied to the generator 20'. The accelerating electrodes 40 and 60 are omitted in FIG. 10.

The generator 20' (FIG. 10) generates the charging voltage V.sub.c ' and the driving pulse P.sub.D, such tha the charging voltage V.sub.c ' varies in proportion to both the energy content C of the pulse P.sub.D and the deflection information signal DS. The ratio q/m of the droplet 16, which is driven out of the nozzle 14 in response to the driving pulse P.sub.D and then charged in response to the charging voltage V.sub.c ' is thus proportional to the deflection information signal DS.

The charged droplets 16 are passed through the deflection space between the deflection electrodes 50, which are charged with a fixed deflection voltage from the source 51. Thus, the droplet 16 may be deflected by a distance which is in proportion to the deflection information signal DS.

Generator 20' (FIG. 11) comprises a generator 20 identical to generators 20A (FIG. 3), 20B (FIG. 4) or 20C (FIG. 7), and a variable-gain amplifier 200, which is supplied with the voltage V.sub.c and the deflection information signal DS from the generator 20 and the source 52, respectively. The voltage V.sub.c is controlled by the signal DS to produce the modified charging voltage V.sub.c ', which is proportional to the energy content C of the driving pulse P.sub.D and to the deflection information signal DS.

A second example of the generator 20' (FIGS. 12 and 13) comprises a generator 20B' identical to the generator 20B shown in FIG. 4. The generator 20B' includes a clock pulse generator 103' for generating the clock pulse CP (FIG. 13A), a charging voltage generator 104' for generating charging voltage V.sub.c (FIG. 13C), a PAM modulator 105' for producing the PAM signal (FIG. 13D), an amplitude component extractor 106' and a variable delay line 107'. The structural elements 103' to 107' are identical to corresponding circuits 103 to 107 in FIG. 4, respectively.

The second example of the generator 20' further comprises an arithmetic circuit 201, which is supplied with the amplitude-component signal and the deflection information signal DS (FIG. 13B) from the extractor 106' and the deflection signal source 52, respectively. The arithmetic circuit 201 produces a delay-control signal representing the product of the amplitude-component signal and the deflection information signal DS.

The delay-control signal is applied to the variable delay line 107' to control the delay time. If the deflection information signal DS is zero, as shown by stepwave R1 in FIG. 13B, the delay time is zero, as shown by time T.sub.1 in FIG. 13E, i.e., the PAM signal is passed through the variable delay line 107' without any delay so that the non-delayed PAM signal D1 (FIG. 13E) is produced and supplied as the driving pulse P.sub.D ' to the piezoelectric member 15. As understood from FIGS. 13C and 13E, at the time when the non-delayed PAM signal D1 is applied to the piezoelectric member 15, i.e., at the trailing edge of the signal D1, the charging voltage V.sub.c is zero, so that the squirted droplet 16 is not charged. Thus, it passes through the deflection space without deflection.

If the deflection information signal DS is not zero, as shown by stepwaves R2 and R3 in FIG. 13B, the delay time is varied in response to the amplitude-component signal within time period ranges T.sub.2 and T.sub.3, respectively, as shown in FIG. 13E. The delayed PAM signals D2 and D3 are produced and supplied as the driving pulse P.sub.D ' to the piezoelectric member 15. As understood from FIGS. 13C and 13E, at the trailing edges of the delayed PAM signals D2 and D3, the droplets are driven out and charged by the charging voltage V.sub.c, which is responsive to the energy content, i.e., pulse amplitude of the driving pulse and the deflection information.

In the generator 20' (FIG. 12) of the second embodiment, the deflection voltage applied to the deflection electrodes 50 is fixed. This makes it possible to use the common deflection electrodes even when applied to multi-nozzle print heads.

In both the first and second embodiments of generator 20' described above, the charging voltage V.sub.c is varied in response to the energy content of the driving pulse P.sub.D (FIG. 11) in the first embodiment or both of the energy content and the deflection information signal DS (FIG. 12) in the second embodiment. In other words, the charge value q is varied in response to the droplet size (droplet weight m) or to both the droplet size and the deflection information assuming that there is a negligible fluctuation in the velocity V.sub.x of the droplet 16 due to the variations in droplet size.

If the fluctuations in velocity V.sub.x are not negligible, the charge value q, i.e., the charging voltage V.sub.c must be further responsive to the square of the velocity V.sub.x.sup.2. The velocity V.sub.x is varied as a function of the droplet size, which is determined by the amplitude and pulse width of the driving pulse P.sub.D applied to the piezoelectric member 15. Therefore, the velocity V.sub.x may be determined for various combinations of amplitude and pulse width. The square of the velocity, i.e., V.sub.x.sup.2 may be stored in a digital memory (ROM) having addresses which correspond to the different combinations of amplitude and pulse width of driving pulses P.sub.D. A stored value is read out in response to a particular driving pulse and is supplied to one of the charging voltage generators 101, 104, 109 or 104' or to the variable gain amplifier 200 to control the charging voltage V.sub.c or V.sub.c '.

The charging voltage generators 101, 104, 104', and 109 may include the digital memory (ROM) in which the various voltage levels are stored in addresses designated by the amplitude and pulse width of the driving pulse P.sub.D. The voltage levels may be pre-determined according to the product of the droplet size and the square of the velocity (V.sub.x.sup.2).