Pulsed Droplet Ejecting System

Abstract

A facsimile system is described which employs a pulsed ink droplet ejecting system as the record marking means. In such a system, undesired droplets are sometimes deposited on record areas that should receive no ink. The cause of this irregularity is explained and circuit modifications are described which eliminate the undesired marking.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to a system for ejecting droplets of ink on command suitable for use in apparatus such as ink jet printers and facsimile recorders.

2. Description of the Prior Art

Droplet on command ink ejecting systems generally have small nozzles containing ink under zero pressure or small positive or negative pressure. Surface tension in the nozzles prevents flow of ink in the absence of command pulses. When a droplet is desired from a nozzle, a pressure pulse of sufficient amplitude is applied to the ink in the nozzle to overcome the surface tension, thereby causing ejection of a droplet.

In one form of droplet on command system the pressure is exerted by electrostatic attraction when a suitable voltage pulse is applied between the ink in the nozzle and an external electrode. In another form, exemplified by U.S. Pat. No. 3,683,212 granted to Steven I. Zoltan, the pressure is applied by an electroacoustic transducer when a suitable electric pulse is applied to the transducer. The quantity of ink ejected per pulse increases with pulse amplitude and thus such systems are useful in facsimile type recorders in which records are made having controlled variations in shading. However, it has been found that sometimes small ink droplets are deposited in random positions on areas that should be free of spots causing an undesirable background shading.

OBJECT AND SUMMARY OF THE INVENTION

The principal object of this invention is to provide a droplet on command ink ejecting system which may be used in a record marking system without producing the undesirable background above described.

According to this invention an electric pulse actuated ink ejecting system of the type which ejects no ink when the pulse amplitude is below a threshold level, ejects ink droplets with an unacceptable degree of irregularity when the pulse amplitudes fall within a threshold zone extending from the threshold level to a higher second level, and ejects ink with an acceptable degree of regularity in response to each pulse having amplitude above the second level, is driven by a circuit which includes means for preventing the application of pulses having amplitudes which fall in the threshold zone.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates one form of droplet on command ink ejecting system with which the invention may be used;

FIG. 2 illustrates a facsimile system employing the ink ejecting system of FIG. 1 as the record marking means;

FIG. 3 is a graph illustrating the response of a system of the type shown in FIG. 1 to pulses of different amplitudes;

FIGS. 4a, 4b, and 4c illustrate signal wave forms in various parts of the circuit of FIG. 2;

FIG. 4d illustrates the signal wave form of FIG. 4c modified according to this invention; and

FIG. 5 shows a way in which the circuit of FIG. 2 may be modified to embody this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention is useful in connection with droplet on command ink ejecting systems. Examples of one type of droplet on command system are described in U.S. Pat. No. 3,683,212 granted to S. I. Zoltan and in U.S. Pat. application Ser. No. 330,360 filed by J. P. Arndt, both assigned to the same assignee as the present invention. Reference may be made thereto for more detailed descriptions. However, for the purpose of explaining the present invention, the following brief description should suffice.

In FIG. 1, a nozzle 1 is formed at one end of a glass or metal or plastic tube 2 which serves as a section of conduit 4. Plastic tube 5 and metal or glass tube 6 continue conduit 4 to reservoir 7 where it communicated with a supply of ink or other liquid 8. Conduit 4 and nozzle 1 are filled with the liquid. The level of liquid in reservoir 7 is maintained at an elevation which results in zero static pressure or at a slight positive or negative pressure at the nozzle. Surface tension in the nozzle prevents liquid flow in the absence of drive pulses.

A tubular piezoelectric ceramic transducer 10 surrounds conduit section 2 and is secured thereto in stress transmitting engagement by epoxy cement, not shown. Terminal wires 11, 13 connect to electrodes on the outer and inner cylindrical surfaces of the transducer.

When a voltage pulse of suitable polarity is applied between terminals 11 and 13, transducer 10 decreases in diameter for the duration of the pulse. This contraction forces similar contraction of conduit section 2 thereby applying a pressure pulse to the liquid. If the voltage pulse has sufficient amplitude, a droplet 14 is ejected from nozzle 1. As pulses of increasing voltage are applied, the quantity of liquid ejected per pulse also increases.

The simplicity of the droplet ejecting system illustrated in FIG. 1, and its capability to eject quantities of liquid which vary according to pulse amplitude make the system well suited for use as the marking means in facsimile type recorders wherein it is required to record with variations in shading. In order to describe the present invention it is expedient first to describe such a facsimile system.

FIG. 2 illustrates a facsimile system employing the droplet ejecting system of FIG. 1 and which may be modified to incorporate the improvement of the present invention.

The picture or other graphic material 16 to be transmitted is wrapped around and secured to drum 17. A sheet of paper 19 on which a copy of picture 16 is to be made is wrapped around and secured to drum 20. Drums 17 and 20 are caused, by means not shown, to rotate about their respective axes 22, 23 with identical angular velocities, and with the tops of picture 16 and sheet 19 always in the same relative positions. Means for maintaining the synchronization and phasing of the drum rotations are well known and a description is not required for the purpose of explaining the present invention.

A carriage 25 is transported by lead screw 26 parallel to the axis 22 of drum 17. Carriage 28 is transported by lead screw 29 parallel to the axis 23 of drum 20. The motions of the two carriages also are synchronized, for example by driving them through identical gear trains, not shown, from their respective drums.

A photoelectric system 31 is mounted on carriage 25. It contains a light source, a photosensor, and lenses arranged so that the sensor is illuminated by the light reflected from a very small area of picture 16. Thus as drum 17 rotates and carriage 25 is transported parallel to axis 22 of the drum, the entire picture is scanned and the electric output of system 31 represents instantaneous values of shading of the picture as the scanning proceeds. The output connects to signal conditioner 32 which may amplify the signal, and apply corrections for nonlinearity. The output of signal conditioner 32, at line 33, is zero when the photoelectric system 31 is viewing a white area, and positive when viewing a shaded area with maximum value when it is viewing a black area. Means for performing the functions of photoelectric system 31 and signal conditioner 32 are well known, and further details are not required in the description of the present invention.

The droplet ejecting system of FIG. 1 is mounted on carriage 28 with nozzle 1 close to and aimed directly at sheet 19. Transducer 10 is driven by pulses from pulse generator 34 which are amplified by drive amplifier 35. The pulse rate, and the transport rate of carriage 28 are so related to the surface velocity of drum 20 that sheet 19 may be substantially fully covered by slightly overlapping ink spots by continuously pulsing transducer 10 at high pulse amplitude.

When a picture or other graphic material 16 is being copied, the amplitude of each pulse delivered to transducer 10 is controlled by the signal from signal conditioner 32 via amplifier 41 and modulator 37. Thus ink is ejected repeatedly from nozzle 1, the quantity ejected for each pulse being a function of the darkness of the small area of picture 16 then being viewed by photoelectric system 31. Therefore picture 16 is reconstructed on sheet 19 in the form of closely spaced ink spots with shading determined by the sizes of the spots.

Potentiometer 38 permits adjustment of the overall system sensitivity and in particular the black level. Bias adjustment 40 is provided for a purpose to be described in connection with FIGS. 3 and 4.

FIG. 3 is a graph illustrating how the quantity of ink ejected in response to each pulse varies with pulse amplitude in a droplet ejecting system of the type described in reference to FIG. 1. At some low pulse amplitude, which may be called to threshold level, the curve is discontinuous. At lower pulse amplitudes no ink is ejected. The threshold level depends on a number of design parameters but the exact value is somewhat indeterminate as it appears to vary slightly from time to time for reasons which have not been fully identified.

The numerical values shown in FIG. 3 represent performance of a system having one particular set of parameters, and the threshold level is at about forty volts. In the present example it is assumed that the drum speed and pulse rate have been so selected that the ink ejected in response to repeated pulses of 150 volts results in slightly overlapping spots on record sheet 19, FIG. 2.

FIG. 4a illustrates the picture signal from signal conditioner 32 in FIG. 2 as a horizontal black stripe 36, on a white background in picture 16, sweeps past the scanning area of photoelectric system 31.

FIG. 4b illustrates the modulating signal applied to modulator 37 from amplifier 41. It is the combination of the signal of FIG. 4a and the bias from potentiometer 40 of FIG. 2.

FIG. 4c illustrates the amplitude modulated pulses applied to transducer 10. It can be seen that the bias from potentiometer 40 was adjusted so that pulses 43, and 50, 51, and 52, representing the white background, (zero picture signal) have amplitudes of about 35 volts, the level at which the curve of FIG. 3, extrapolated, indicates zero ink ejection. It can also be seen that potentiometer 38 was adjusted so that pulses 45, 46, 47, and 48 representing the black area of the stripe (maximum picture signal) have amplitudes of 150 volts, which results in ink ejection sufficient to provide overlapping dots on sheet 19. Pulses 44 and 49 have amplitudes intermediate between 35 volts (white) and 150 volts (black) because they occurred as the scanning area embraced both white and black areas.

In actual practice, the bias may be adjusted to a somewhat different value to obtain shading which is most pleasing to the operator of the apparatus.

Pictures and other graphic material have been recorded with good quality using a system such as that illustrated in FIG. 2. However, under some conditions of shading in the original picture an undesirable background of scattered small ink spots was recorded in areas that should have been white, such as in the white background immediately following a black area. This undesirable background could not be alleviated by adjusting bias control 40.

It has been determined that the undesirable background results from the operating characteristics of the ink droplet ejecting system when pulses are applied which have amplitudes falling in a small range extending upwardly from the threshold level. In FIG. 3 this range is identified as the threshold zone and in FIG. 4c pulse 49 falls in that zone. The droplets ejected in response to pulses having amplitudes falling in the threshold zone have very low velocities, and have unstable trajectories. Thus these droplets arrive late at sheet 19, FIG. 2. For certain picture contents, they are deposited on areas that should be free of dots. The scattering of the misplaced spots is due to the unstable trajectories. These difficulties diminish as pulse amplitudes are increased above the threshold level. The level marking the upper limit of the threshold zone depends, among other things, on the tolerance placed on spot position. A representative width of the threshold zone is about 10 volts when the threshold level is about 40 volts.

The present invention eliminates the objectionable background by preventing pulses that would have amplitudes in the threshold zone from driving transducer 10. This is illustrated in FIG. 4d in which pulse 49 of FIG. 4c has been eliminated. It has been found by experiment that graphic material recorded by a system incorporating this invention is generally more pleasing to most observers.

When the circuit is adjusted as described in reference to FIG. 4d, pulses such as 43, 50, 51, and 52 representing white background have amplitudes falling below the threshold level and do not cause ejection of droplets. Thus the "white pulses" need not be eliminated. However, it is expedient to eliminate them and sometimes advantageous to do so because the exact demarcation of the threshold level is uncertain.

FIG. 5 shows one way in which the circuit of FIG. 2 can be modified to incorporate the present invention simply by adding components. Gate 59 is interposed between modulator 37 and transducer drive amplifier 35. When gate control line 61 is plus, the gate passes pulses from the modulator and when line 61 is minus the gate blocks the pulses. Differential comparator 55, which may for example be an operational amplifier without feedback, has its output connected to line 61 to control gate 59. Potentiometer 56 provides an adjustable plus reference voltage to the inverting input of comparator 55. When switch 62 is in position 64, a plus voltage greater than the reference voltage is applied to the non-inverting input of comparator 55. This turns gate 59 on and the system functions as though the circuit of FIG. 2 had not been modified.

When switch 62 is in position 65, picture signal from potentiometer 38 is applied to the non-inverting input of comparator 55. In the absence of picture signal, the reference voltage from potentiometer 56 turns gate 59 off preventing pulses from reaching amplifier 35. When the picture signal rises to equal the reference voltage from potentiometer 56, the output of comparator 55 reverses polarity turning gate 59 on to pass pulses from modulator 37.

Referring to the example of FIG. 4, initial adjustment of the system may be made as follows. With switch 62 in position 64, potentiometer 40 may be adjusted so that when the picture signal is zero, pulses of about 35 volts are applied to transducer 10 as earlier described. With a "black signal" at line 33, potentiometer 38 is set to develop pulses of 150 volts amplitude. Next switch 62 is turned to position 65, and potentiometer 56 is adjusted so that pulses having amplitudes below 50 volts are blocked at gate 59 and pulses above 50 volts are passed. With such adjustment comparator 55 switches state at points 58 on the picture signal envelope in FIG. 4c.

It will be appreciated that many different circuit arrangements can be designed to carry out this invention. As examples, gate 59 could be inserted between amplifier 41 and modulator 37, or the non-inverting input of comparator 55 could receive picture signal plus bias from the output of amplifier 41 rather than receive just picture signal from potentiometer 38, or the bias supply for potentiometer 40 could be obtained from the output of comparator 55 through an inverter thus eliminating the need for gate 59. Another of the many possible variations is to obtain all or part of the bias which is to be added to the picture signal, FIG. 4b, from photoelectric system 31 or signal conditioner 32, rather than from potentiometer 40.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed, therefore, in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

Claims

What is claimed is:

1. A record marking system adapted for use in a facsimile type recorder wherein a record receiving member is successively marked with varying degrees of shading representative of electric signal information supplied to said recorder comprising:

an electric pulse actuated ink ejecting system of the type which ejects no ink when the pulse amplitude is below a first level, ejects ink droplets with an unacceptable degree of irregularity when the pulse amplitudes fall within a threshold zone extending from said first level to a higher second level, and ejects ink with an acceptable degree of regularity in response to each pulse having amplitude above said second level, the quantity of ink ejected per pulse having amplitude above said second level increasing as the pulse amplitude increases; and

circuit means connected to said ink ejecting system and adapted to apply electric pulses thereto, said circuit means including means for preventing the application of pulses having amplitudes which fall in said threshold zone, but permitting the application to said ink ejecting system of pulses having varying amplitudes above said threshold zone.

2. A record marking system according to claim 1 in which said circuit means includes means for preventing the application of all pulses having amplitudes below said second level.

3. In a pulsed droplet ejecting system comprising:

a conduit connected to a nozzle;

liquid filling said conduit and nozzle;

a transducer coupled to said liquid in said conduit and adapted to apply a pressure pulse to said liquid when an electric pulse is applied to said transducer, said pressure pulse causing ejection of liquid from said nozzle when the amplitude of said electric pulse exceeds a limiting level but causing no ejection when the amplitude of said pulse is less than said level; and

circuit means connected to said transducer and adapted to apply thereto electric pulses having a range of amplitudes embracing said level;

the improvement which comprises:

said circuit means including means adapted to prevent the application to said transducer of pulses having amplitudes falling within a selected range of amplitudes which includes said level, while permitting application to said transducer of pulses of varying amplitudes above said selected range of amplitudes.

4. In a pulsed droplet ejecting system as described in claim 3, said selected range of amplitudes extending to zero amplitude.