Ink Jet Printer With Droplet Phase Control Means

Abstract

A charging voltage applied to ink droplets discharged from the nozzle of an ink-jet printer is controlled by varying the phase of the charging voltage placed upon a droplet in dependence upon the time-phase relationship between the droplet and the charging voltage.

Description

This invention relates to ink-jet printing and, more particularly, to phase control of the charging voltage applied to ink droplets.

BACKGROUND

Systems have heretofore been developed for printing documents with a modulated stream of ink. In such systems, a stream of ink passes through a nozzle which is vibrated to cause the stream to break up into droplets. Just before the droplets leave the stream, they pass between a pair of charging plates which place a desired charge on each droplet. The droplets then pass through a constant space field. The reaction between the field and the charge causes the droplets to be deflected. The deflection is proportional to the charge on the drop. Such an ink-jet printer is described in copending U.S. Pat. application Ser. No. 618,923.

The time-phase relationship of the charging voltage and the instant at which the droplets break off from the ink stream is critical. Due to variations in the physical properties of the ink, such as viscosity and conductivity, and because of environmental changes in the equipment such as temperature, the time-phase relationship can change drastically, degrading the resultant printing operation. Therefore, because the breakoff instant of the droplets from the ink stream may vary, there exists a need for compensation therefor.

SUMMARY

In accordance with the present invention, the time-phase relationship between the breakoff instant of the ink droplets and the charging voltage to be placed on the droplets is detected and controlled by varying the charging voltage through a servo loop system responsive to variations in the time-phase relationship. In a more specific aspect, a charging voltage is applied to the charging plates for charging ink droplets which pass therebetween. The droplet path impinges the path of a traveling document. The droplets do not impinge the document during a phase control cycle, but, rather, are collected in an ink catcher. A phase control system is connected to the ink catcher for detecting the voltage on the catcher created upon capture of charged ink droplets. The catcher voltage is compared to a reference voltage in a phase control system to determine the time-phase relationship of the droplet breakoff instant and the charging voltage. The phase control servo will vary the phase of the charging voltage so as to position the charging voltage waveform in the correct time relation with respect to the droplet breakoff instant.

THE DRAWINGS

FIG. 1 illustrates the invention where droplets are electrostatically charged and deflected;

FIG. 2 is a time plot of the droplet charging voltage;

FIG. 3 is a time plot of the droplet charging voltage used during the phase-control cycle;

FIG. 4 illustrates a bar print when the proper time-phase relation exists; and

FIG. 5 illustrates a bar print when an improper time-phase relation exists.

THE PREFERRED EMBODIMENTS

In FIG. 1 a document 100 travels along a line and direction indicated by the arrow 101. Ink-jet printer 10, as indicated by arrow 102, is askew to the direction of document travel. Ink-jet printer 10 comprises a slender conduit or tube 103 through which ink or other suitable writing fluid is fed under pressure. The ink passes through a nozzle or conduit 104 which is under control of a suitable high frequency, low amplitude vibrator, such as a piezoelectric crystal. Piezoelectric crystal 105, energized by alternating current from source 331 by way of channel 332 imparts the desired high speed vibration to the nozzle or conduit 104, a suitable frequency is of the order of 48 kilocycles per second. The nozzle 104 terminates in a tip 107 having a small orifice of, for example, about 0.001 inch diameter. A very fine stream of writing fluid is forced from supply 120 by pump 121 through tip 107.

The high frequency vibration of nozzle 104 causes the flow of fluid from tip 107 to be broken into very small or fine droplets as indicated by the broken stream 108.

A signal voltage from a generator 319 is connected by output channel 328 to two spaced charging plates 109 between which the train of droplets passes. The signal voltage from generators 319 may be varied to deflect the droplets to print bars such as bars 114 on document 100. More particularly, as the droplets fall by gravity or are propelled under suitable pressure or force as applied to nozzle 104, they pass through an electrostatic field created by applying a voltage from source 115 to two spaced deflection plates 110 and 111.

Uncharged ink droplets are not affected by the electrostatic field between plates 110 and 111. They flow or fall without deflection into a catcher 112 which is insulated from plate 110. Catcher 112 may be drained through tube 113. Thus, if no charge is carried by a droplet, it does not land on document 100. However, upon application of a signal voltage to the charging plates 109, the droplets are charged. As the signal voltage increases droplets are each more highly charged. Deflection by the electrostatic field between plates 110 and 111 as they fall on the document 100 increases with increased charge.

If the document 100 were standing still, the points of impact of droplets having successively increasing charge would lie along a diagonal line corresponding with arrow 102. However, the rate of deposition of droplets and the deflection thereof is coordinated with the rate of travel of the document. As a result, bars may form which are parallel to the leading edge of document 100 and perpendicular to the arrow 101 which indicates the travel direction.

The droplet charging voltage normally applied to plates 109 preferably has the form shown in FIG. 2. It is a stair step voltage synchronized with the sinewave voltage applied to vibrator 105 from generator 331. It is necessary that the droplets employed for forming bars such as bars 114, FIG. 1, leave tip 107 and travel toward document 100 with the charges thereon represented by the levels on the stair step function 150. More particularly, the first droplet 151 in a representative system would have a charge thereon represented by the 200 volt field 152 between plates 110 and 111. The second droplet 153 would have a lower charge represented by the lower voltage 154 on plates 109. The charging voltage is measured relative to the ground potential at which nozzle 104 is maintained.

Because of change in various physical parameters such as the viscosity of the ink, which may change in response to changing temperatures in the system, it has been found that the instant that the ink droplet actually breaks away from the tip may vary relative to the phase of the voltage on vibrator 105.

It has been found that when the time-phase relationship changes, droplet 153 may not pass between plates 109 while plates have charge 154 thereon. Rather, it may break away at a later time, namely the time when the voltage on plates 109 relative to tip 107 is changing from level 154 to level 155. When this is the case, the droplet may have a charge thereon which is any value which may be represented by the difference between the voltages 154 and 155, FIG. 2.

The present invention provides for detection of variations in the instant the droplet is formed and the phase of the sinewave voltage from generator 331 so that is will be assured that the charges on the droplets will be represented by the successive levels on the stair step function 150.

The system includes a line 300 leading from the catcher 112 to the input of a phase detector 301. The phase detector 301 has an input amplifier 302 the second input of which is supplied by way of channel 303 from a reference voltage source 304. The output of amplifier 302 is connected by way of channel 305 to an analog-to-digital converter 306. The analog-to-digital converter has two outputs 307 and 308 which lead to an accumulator 309. An accumulator clock 310 is connected by way of channel 311 to the accumulator 309. Output 312 of accumulator 309 is connected by way of a digital-to-analog converter 313 whose output is connected by way of channel 314 to a phase-shifting unit 315. A system clock 316 is connected by way of channel 317 to the phase-shifter 315 and by way of channel 318 to the sinewave generator 331. The clock pulses from source 316 pass through phase shifter 315 and are applied by way of channel 330 to a drop counter 321 in the function generator 319. The output of drop counter 321 is connected by way of channel 322 to a stair step function generator 323 and by way of channel 334 to a ramp function generator 335.

The generator 323 is connected to a gated output amplifier 326 by way of channel 324. The output channel 328 leads to plates 109. Finally the ramp function generator 335 is connected by way of channel 325 to a gated output amplifier 327 whose output is connected by way of channel 329 to channels 328.

The printing on document 100 is conducted under the control of a control unit such as a computer 336. During the interval that the document is passing along the path in the region of plates 110 and 111, write gates in the form of a change in state of control voltage 156, FIG. 2, are applied to amplifier 326 by way of channel 337 so that the stair step function generated by the unit 323 is applied to the plates 109. In the time interval required for travel of document 100 between locations of bars such as bar 114, and for travel between documents, the write gate on channel 337 is removed and a calibrate gate 157, FIG. 3, is applied to amplifier 327 by way of channel 338 so that a predetermined number of cycles of the ramp function 158 will be applied to the plates 109.

During the interval that the ramp function is applied to plates 109, the charges on successive droplets impinging the catcher 112 are sensed and employed to adjust the phase shift of the system clock pulses applied to the phase-shift unit 315. Thus, for those physical conditions in the system which exist during the calibration interval, the phase will be adjusted so that the droplets will be formed and pass between plates 109 at precise intervals synchronized with the voltage from generator 331. Preferably, such intervals will be maintained within limit 159, FIG. 2. In such case, they pass between plates 109 during time intervals represented by the "treads" on the steps of function 150 rather than on the "riser."

When the droplets pass through the plates 109 in the time-phase relations indicated in FIG. 2, then the droplets will fall on a straight line on document 100, in the manner illustrated in FIG. 4 where bar 114 and two succeeding bars, 114a and 114b, are shown. It is this type of recording that is desired and is controlled in accordance with the present invention. When the synchronization is such that the droplets pass between plates 109 during change of the charging voltage creating the step, as in the case of drop 153a, FIG. 2, then the resultant bars degenerate to be of character such as represented by bars 124a, 124b, and 124c, FIG. 5.

More particularly, clock 316 applies a high frequency clock signal (for example, 48 kilocycles by way of channel 318 to sinewave generator 331. Generator 331 thus provides a synchronized 48 kilocycle sinewave which is applied by way of channel 332 as the crystal excitation voltage to crystal 105. Clock 316 also applies a 48 kilocycle drop clock signal by way of channel 317 to phase-shift unit 315, the operation of which will be described later. Phase-shift unit 315 applies a delayed 48 kilocycle drop clock signal by way of channel 330 to function generator 319.

During each printing cycle, function generator 319 provides a positive charging voltage to the charging plates 109 when bars are to be printed. When a space is desired on the document, the charging voltage will be zero and the charging plates 109 will be at zero potential, thus allowing all drops to pass to the ink catcher 112.

The clock signal on channel 330 is applied to drop counter 321. Drop counter 321 applies a bar rate through channel 322 to stairstep generator 323. In one form counter 321 was a 3- stage, divide-by-two counter having 3 output lines for output pulses which were applied, one pulse per drop, to generator 323. Generator 323 is a digital-to-analog converter that generates a stairstep voltage level, one level for each of eight pulses from counter 321 per bar, thus producing bars of eight drops each.

The stairstep output is applies through channels 324 to write amplifier 326 whose output is a stairstep waveform such as shown in FIG. 2.

When the voltage on the charging plates is a stairstep waveform and droplets are charged on each step, a perfect bar may be formed on the document.

For each bar to be printed computer 336 applies a print command signal through channel 337 to amplifier 326. Amplifier 326 applies the stairstep charging voltage through channel 328 to the charging plates 109. In one embodiment, when the 48 kilocycle signal was used, the voltage is held for a period of approximately 20.8 microseconds at each step in order capacitively to charge each droplet at the instant it breaks off from the ink-jet stream. The charging voltage decreases finite voltage increments properly to charge each of the droplets in the bar. After each droplet has been charged it is individually deflected by the field between plates 110 and 111 in direct proportion to the applied charge on the droplet.

Gating for amplifier 326 may be from suitable control device such as a digital or analog computer. For purpose of this description, such unit has been designated as a control unit 336. Control unit is enabled when a document is in transit past the printing station. Documents may be sensed by unit 339 which is connected to control unit 336 by channel 340. When a bar is to be printed a voltage state as from a source 341 is applied to amplifier 326 by way of an AND gate 342 which energizes the control line 337. For example, movement of a tape 343 through a reader portion of control unit 336 will produce control states on line 344 in accordance with the set 345 which corresponds with bars 114, 114a and 114b.

When the next card is sensed set 346 will be printed thereon and set 347 on the next card, etc.

The present invention prevents deterioration of the bars by providing phase control to correct any change in the time-phase relationship between the droplet breakoff point and the charging voltage.

More particularly, the phase-control system employs a servo loop to change the delay in the synch signal passing from clock 316 to counter 321 by way of the phase-shift unit 315. This phase-control system operates between documents so that normal printing operations are not affected. The phase-control operational period is hereinafter referred to as the calibration period.

It will be noted that the bar rate signal output of drop counter 321 is also applied by way of channel 334 to ramp generator 335. Generator 335, when enabled, converts the bar rate signal to a sawtooth waveform and applies the sawtooth signal by way of channel 325 to a calibration amplifier 327. During the calibration period, control unit 336 applies a calibration command instruction through AND gate 348 and channel 338 to amplifier 327. Amplifier 327 then applies a sawtooth charging voltage by way of channel 329 to charging plates 109. The sawtooth voltage serves as the drop charging voltage during the calibration period.

The charging voltage or calibrate waveform is shown in FIG. 3. This negative calibrate waveform changes from zero to about -200 volts and capacitively charges each drop with a positive charge. The drops apply a positive potential to catcher 112. The charged droplets are analogous to an electrical current since they constitute the movement of charges past a given point in a given time interval. Catcher 112 is isolated from electrical ground and insulated from plate 110.

The voltage created on catcher 112 as a result of collection of the charged droplets is applied through channel 300 to phase detector 301 wherein it is compared by amplifier 302 to a reference voltage. A reference voltage of approximately 200 millivolts has been found to be satisfactory, the same being produced on channel 303 by unit 304. The reference voltage may be selected to represent the desired phase relationship of the droplet breakoff point to the charging voltage. If the voltages on lines 300 and 303 are not equal, the resulting error voltage is amplified and applied by way of channel 305 to analog-to-digital converter 306. If the error voltage indicates a high condition, converter 306 generates an "up" phase error voltage signal on output channel 307. Conversely, a low condition causes converter 306 to generate a "down" phase error voltage signal on output channel 308.

The phase error voltages on channels 307 and 308 are applied to accumulator 309. Accumulator 309 is a seven-bit, up-down counter used as an integrator or loop filter in a servo loop.

After the end of each document, as synchronized by way of channel 349, a clock pulse from the accumulator clock 310 is applied by way of channel 311 to accumulator 309. Upon receiving the clock pulse, accumulator 309 causes the up-down counter to increase up or down one count depending on whether an up or down phase error signal is received from phase detector 301. The accumulator increments only during the calibration period between documents.

The signal from the accumulator 309 is applied over channel 312 to digital-to-analog converter 313 which converts the stored error voltage to a phase-control voltage on channel 314. Channel 314 applies the phase-control voltage to phase-shift unit 315.

Phase-shift unit 315 controls the phase delay between the charging voltage output of generator 319 and the crystal excitation voltage output of generator 331. This phase delay is controlled by the phase error voltage signal from converter 313. If an error is detected by phase detector 301 between the voltage applied to catcher 112 during the calibrate period and the reference voltage of generator 304, the resulting phase error voltage produced in converter 313 applied to phase-shift unit 315 automatically to delay the 48 kc. drop clock signal output of phase-control unit 315 to the function generator 319. Phase-shift unit 315 controls the time-phase relationship between the charging waveform to charging plates 109 and the breakoff point of ink droplets emerging from nozzle 107.

Having described the invention in connection with the foregoing embodiment thereof, it is to be understood that further modifications may now suggest themselves to those skilled in the art, and it is intended to cover such modifications as fall within the scope of the appended claims.

Claims

We claim:

1. In an electrostatically deflected ink jet printer, the method of compensating for variation in the time-phase relation between a periodic driving function and resultant drop formation which comprises:

a. establishing alternate print cycle conditions and calibration cycle conditions;

b. generating repeated stair step functions synchronized with the resultant drop formation for establishing an electrostatic droplet charging field during the print cycle;

c. generating ramp functions synchronized with the resultant drop formation for establishing an electrostatic droplet charging field during the calibration cycle; and

d. adjusting the phase of said stair step functions relative to the resultant drop formation in response to prior variations in charges on the formed droplets during the calibration cycle as a result of the electrostatic charging field established by the ramp functions.

2. A system for compensating for variations in the time-phase relation between a periodically actuated drop forming unit and the instant of drop formation by said unit in an electrostatically deflected ink jet printer which comprises:

a. charging means for establishing an electrostatic droplet charging field;

b. first means for generating repeated stair step functions synchronized with said drop forming unit and connected to said charging means;

c. second means for generating ramp functions synchronized with said drop forming unit and connected to said charging means;

d. means to render said first means and said second means alternately effective to control the charge of said droplets; and

e. a phase shift unit for adjusting the phase of said stair step functions relative to said drop forming unit in response to prior variations in charges on said droplets due to said second means.

3. A system for compensating for variations in the time-phase relation between a periodically actuated drop forming unit and the instant of drop formation by said unit in an electrostatically deflected ink jet printer which comprises:

a. charging means for establishing an electrostatic droplet charging field;

b. means for establishing a uniform deflection field through which said droplets travel;

c. a droplet catcher;

d. first means for generating repeated stair step functions synchronized with said drop forming unit and connected to said charging means;

e. second means for generating ramp functions synchronized with said drop forming unit and connected to said charging means;

f. means to render said first means and said second means alternately effective to control the charge of said droplets; and

g. a phase shift unit connected to said catcher for adjusting the phase of said stair step functions relative to said drop forming unit in response to prior variations in charges on said droplets due to said second means.

4. The combination set forth in claim 3 wherein the means to render said first and second means alternately effective includes means operable between passage of documents through said printer to actuate said second means.

5. The combination set forth in claim 3 wherein means are provided for comparing the voltage on said catcher with a predetermined reference voltage and for setting a delay interval in said phase-shift unit in dependence upon the results of said comparison.

6. The combination set forth in claim 3 wherein an alternating current supply is connected to said drop-forming unit to produce a droplet on each cycle thereof and wherein a system clock controls both said supply and said generators, and wherein said phase-shift unit is connected between said system clock and said generators.