CA2001041C - Nozzle drive control system and method for ink jet printing - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A drive control system is disclosed which automatically maintains nozzle drive voltage within a proper range. The control system monitors the state of the "intermediate satellites"
positioned between ink drops used for printing. When these satellites are neither forward nor backward merging, a first cardinal point designated C(L) is identified. A second cardinal point, C(H), is determined when the drop breakoff point stops decreasing, relative to said nozzle, with increasing nozzle drive voltage. From the two cardinal values, a desired operating range for a particular ink can be computed and the control system automatically set. The computed value is essentially independent of temperature.

Description

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`' NOZZLE DRIVE CONTROL SYSTEM
AND METHOD POR INK JET PRINTING
~ACKGROUND OF THE INVENTION

~ his invention relates to ink ~et printing systems and similar drop marking systems in which a ~upply of electrically conductive ink is provided to a nozzle. The ink is forced through a nozzle orifice while at the same time an exciting voltage is applied to the nozzle to cause the stream of ink to break into droplets which can be charged and deflected onto a substrate to be marked. Such ink jet technology is well known and, for example, see U.S. Patent Nos. 4,727,379 and 4,555,712.

To ensure proper operating conditions for consistent printing guality, the exciting energy or voltage applied to the nozzle must be properly set during operation of th~ system. Presently, most ink ~et printers require manual setting of the en~rgy applied to the ink stream as it ex~ts the nozzle. The appropriate value is either empirically ~etermined ~y comparing what is seen to an existing diagram or by determining the drop separation point and comparing it with machine specifications. In èither case, the re~ulting print quality varies.

Efforts to provide automatic control of the modulation voltage have concentrated on detecting separation point position, relative to a fixed location, such as the charge tunnel. See, for example, published European patent specification EPA 0287373. Another approach is dl~closed in U.S. Patent No. 4,638,325 which utilize~

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a small charging electrode and a downstream electrometer by which the drop separation point can be determined by observing the current at the electrometer as the separation point approaches the small electrode. In the ~325 patent, the maximum current is produced when drop separation is closest to the small charging electrode.

~ he above method does not take into account the basic reason for maintaining consistent drop charging conditions. The drop separation point varies greatly with the surface tension and viscosity of the ink, therefore, simply holding the separation point constant still results in different satellite conditions and variable print quality. In short, maintaining the drop separation point constant is not a satisfactory solution to the problem~

What is desired is a system which can determine a range of proper printing nozzle drive voltages and then compute a satisfactory intermediate value within said range. Such a system should be temperature independent over a wide range of operating temperatures to result in a significantly better control system.

It is accordingly an ob~ect of the present invention to provide such a nozzle drive control system which Lmproves upon known techniques.

It is a further ob~ect of the in~ention to provide a nozzle .~. .

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control sy~tem whlch can accurately monltor the conditlon of the satelllte drops and the drop breakoff point and compute therefrom a satlsfactory ranqe of nozzle drive voltages for operatlng an lnk ~et prlnter.
A further advantage of this inventlon ls that lt allows automatlon of the nozzle voltage for best quallty prlntlng uslng a contlnuous lnk ~et prlnter regardless of lnk type and temperature.
Thls lnventlon avolds problems wlth recomblnlng satellltes that occur when holdlng the drop separatlon polnt constant while lnk type and temperature vary. These cause unwanted charge variations because a satelllte whlch carrles part of the charge of its parent charged drop wlll transfer that charge to the drop followlng when merglng occurs.
In accordance wlth the present lnventlon there i5 provided a control clrcult for setting the magnltude of the excitlng voltage applled to the nozzle of an ink ~et printer to break a stream of in~ lnto droplets comprlslng.
ta) first means for detectlng the exclting voltage value C~L) at whlch droplet frequency doubles as the magnltude of the excltlng voltage ls slowly lncreased from a mlnlmum value~
(b) second means for detectlng the value C(H) at whlch droplet formatlon flrst occur~ closest to sald nozzle, a~
sald exclting energy ls slowly lncreased from sald Yalue C(L);
(c) thlr~ means recelvlng as lnputs the values C(L) and C(H) for calculatlng the exclting voltage magnltude tQ be A
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2~10~1 73523-4 utlllzed for ~rlntlng therefrom.
In accordance wlth yet another aspect of the inventlon there 15 provlded a control clrcult for ~ettlng the magnltude of the excltlng voltage applied to the noz21e o~ an ln~ ~et prlnter to break a stream of lnk lnto droplets comprlsln~2 (a) flst means for detecting excltlng voltage value C(L) at whlch lntermediate satelllte droplets are produced by sald nozzle as the excltlng voltage is slowly lncreased from a mlnlmum valuel (b) second means detectlng the excitlng voltage value C(~) whlch flrst produces a dlrectlon change ln droplet breakoff polnt relatlve to sald nozzle as the excltlng voltage ls slowly lncreased from C(L);
(c) thlrd means utlllzlng the values C(L) and C(H) for computlng the proper operatlng voltage to be utlllzed for prlntlng.
In accordance wlth a further aspect of the lnventlon there 1~ provlded a method of determlnlng the excltlng voltage to be applled to the nozzle of an lnk ~et prlnter to break a stream of lnk lnto droplets for prlntlng comprlslng the steps o~, (a) slowly lncreaslng the excitlng voltage from a mlnlmum value~
(b) detectlng and recordlng the voltage value C(L) at whlch the droplet frequency doubles due to the formatlon of intermedlate ~non-merglng) satelllte droplets;
tc) detectlng and recordlng and voltage value C(H) at which droplet formatlon first occurs closest to the nozzl~;
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(d) calculatlng the exciting voltage for prlntlng according to the equation.
V~CALC) = alpha [C~L) + C~H)]/2 where alpha ls a value relat~d to the lnk.
In àccordance wlth yet Another aspect of the present lnventlon there ls provlded a circult ~or determlnlng the excitlng voltage to be applled to the nozzle of an lnk ~et printer to break a stream of lnk lnto droplets for printing comprislng.
~ a) means for slowly increaslng the exltlng voltage from a mlnlmum value) ~b) means for detectlng and recordlng the voltage value C(L) at whlch the droplet frequency doubles due to the formatlon of lntermedlate ~non-merglng) satelllte droplets;
~c) means for detectlng and recordlng the voltage value C~H) at whlch droplet formatlon flrst occurs closest to the nozzlet ~d) means for calculatlng the e~clting voltage for prlntlng accordlng to the equationt V~CALC) ~ alpha l~(L) + C~H)]/2 where alpha ls a value related to the lnk.
In accordance wlth a further aspect of the present - lnventlon there ls provlded a control clrcult for determlnlng the excltlng voltage to be applled to the nozzle of an lnk ~et printer to break a stream of lnk lnto droplets for prlnting comprlslng:
(a) means for detecting and recordlng the voltage value C(L) at whlch droplet frequency doubles as the magnitude of 4b .~ - ~ .. , . ~ .
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-`` 20010~1 the excltlng voltage ls slowly lncreased from a mlnlmum value and for detectlng the value C(H) at which droplet formation first occurs clo~est to sald nozzle, as sald excltlng energy ls slowly lncreased from sald value C(L)1 (b) means for recelvln~ as inputs the valùes C~L) and C(H) for calculatlng the exc:Ltlng voltage magnltude to be utlllzed for prlntlng therefrom.
In accordance wlth yet another aspect of the present lnventlon there ls provlded a control clrcult for determlnlng an excltlng voltage to be applled to a nozzle of an lnk ~et printer to break a stream of lnk lnto droplets for prlntlng compri ingS
(a) means for determinlng voltage value C(H) at whlch droplet formatlon flrst occurs closest to the nozzle as sald exclting voltsge ls 810wly lncreased from a maxlmum value, sald determlnlng means lnclu~lng, ~i) means for applying electrlcal test patterns to sald droplets, said patterns varylng in phase relative to droplet timing whereby only some of the teqt patterns wlll 4uccessfully charge sald droplets~
(11) means for detectlng whlch droplets have been successfully charged5 and determlnlng the value C(H) from a change in sequence ~f charge patterns;
(b) means for estimatlng the exitlng voltage for prlntlng according to the equatlon:
- V(est) = C(H) - E

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where E ls a voltage related to the performance of the ink.
These and other ob~ects of the lnventlon wlll be apparent from the remalnlng portlon of thls speclfication.

~RIEF DESCRIPTION OF TH~ DRAWINGS
Flgure 1 illustrates the prlnclples of lnk ~et drop formation useful ln understandlng the present lnventlon.
Figure 2 ls a software ~low diagram illustratlng the manner in whlch the processor of the present lnventlon operates.
Flgure 3 ls a clrcult dlagram illustratlng the control clrcult accordlng to the present lnvention.

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Figure 4 is a graph useful in explai~ing the operation of the present in~ention.
Figure 5 illustrates the manner in which intermediate satellite-R may be detected.
Figure 6 is a timing diagram useful in explaining the test pattern used for detecting the upper cardinal point.

DETAI~ED DESCRIPTION

Referring to Figure l, there are a series of nozzles shown.
The nozzle 10 emits therefrom a stream of ink 12. A nozzle drive voltage is applied which voltage causes the stream to break up into a series of discrete drops 14. Smaller drops, known in this art as satellites, form between the drops 14. The satellites 16 behave in a manner which i3 a function of the enQrgy applied to the nozzle ~measured in terms of the nozzle voltage).

Referring to Figure 1, when the applied acoustic power to the ink stream is low, the natural behavior of the satellites is to form independently of the drops and then fall back and merge with the drops which follow. This is referred to a-~ rearward merging satellites or slow satellites and is illustrated in Figure lA. The fall back and merging occurs in approximately ten drop periods depending upon the physical parameters of the ink (vi3cosity, surface tension, specific gravity, etc.).

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. . _ - As the drive to the nozzle is incrQased, a point, dQsignatedherein as C(L), will occur. This term refers to a lower cardinal point. Cardinal is a term borrowecl from optics terminology where it denotes an important point o~ a lens system, i.e., a focal point, a nodal point, or a principal point. For purposès of the present specification, C(L) is an important poin~ because it p represents the point at which ths ~atellites separate from the t,~ IB
leading and the following drops at the same tLme (see Figure ~
Surface tension forces pull these satellites forward and backwa~d with equal force. The result is that the satellites stay at mid or intermediate point between the drops as th~y travel through space. It is this condition, referred to as C(~), that can be detected at a downstream point by detecting the satellites and the drops. At the point C(L~ there will be a doubling of the normal drop freguency which can be detected. In all other cases, the satellites will have merged with sither the leading or the trail~ng drops. Appropriate detectors are illustrated and described in connection with Figure 5 of this disclosure.

Virtually all nozzles used for ink ~et printing systems exhibit such intermediate satellites which are neither forward nor rearward merging. The point C(~) will be detected by frequen~y doubling as the power to the nozzle drive is increased from a l~w level to a level just adequate to form intermediate satellites.

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In one embodLment of the Figure 5 detector, an appropriate - test signal is placed on a charging electrode so that both the drops and the intermediate satellites will be charged. The sensed drop frequency will double when intermediate satellites are present and pass the sensor. Alternatively, an optical detector may be employed which does not require charging of the drops and satellites but will detect a doubling in the number of drop~
passing the detector.

In either ca e, the detector is positioned a sufficient distance downstream from the nozzle orifice to permit the satellites to merge.

In addition to a lower cardinal point, C(L), most ink ~et nozzles also exhibit what can be designated as an upper cardinal point, CIH). This point can be observed by slowly increasing the power to the nozzle and observing the point of drop separation.
As the power to the nozzle is increased from a low level (Figure lA), the drop separation point, designated S, moves closer to the nozzle until it reaches (Figure lG) its minimum distance from the nozzle. This is designated the upper cardinal power point C(H~. Thereafter, the breakoff point moves away from the nozzle (Figure lH). Thls fold back or reversal can be sensed by appropriate circuitry and software. A description of the circuitry and methodology for detecting the upper cardinal point C(H) is provided in connection with a description of Figure 3 . -- , :

2~01041 Flrst, however, with reference to Figure 4, there ls shown a graph which demonstrates the characterlstlcs of a typlcal lnk used ln an lnk ~et prlntlng system. Thls lnk, manuEactured by the assignee of the present invention, and deslgnated 16-8200, was utillzed wlth a nozzle of the type descrlbed ln U.S. Patent Mo.
4,727,329. The cross hatched area on the graph represent nozzle drive voltages that produce good quallty printlng over a tempera-ture range of approximately 40 degrees F to 110 degrees F. The lower and upper cardinal power points, C(L) and C(H), are also plotted for the same nozzle and lnk composition. From this infor-matlon, it is possible to calculate a voltage value, V(calc), from the followlng equation:
V(calc) = alpha [C(L) + C(H)] ~2 EQ 1 where alpha is a function of the lnk described hereafter.
Values of V(calc) calculated from the foregoing equation are plotted in Figure 4. These values of V(calc) all lie within the cross hatched area of the graph an~ represent nozæle drlve voltages that produce quallty printlng.
Referring to Figures 1 and 3, clrcuitry sultable for practiclng the lnvention will be descrlbed. The nozzle 10 ls connected to an lnk supply 32 via an ink conduit 34. The ink stream is grounded intermedlate the ink supply and nozzle. The nozzle has an acoustlc energy applled to it, as for example, by -..

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means of a piezo-electric device as disclosed in the aforementioned U.S. patent 4,727,379. The drive voltage for the piezo-electric device is provided from a nozzle drive amplifier 38 via line 40.
In turn, the amplifier is controll~d by a processor 42, such as a microcomputer, via a digital to analog converter (D/A) 44. The controller 42 also operates charge amplifier 44 via D/A 46 to control the ~oltage applied to the charge tunnel 48. A~ is well known in this art, the charge tunnel 48 is disposed downstream of the nozzle 10 in the region whère the drops are intended to form as the stream of ink breaks up into drops and satellites. In this manner selected drops can be charged for deflection onto a substrate or, if left uncharged, returned by way of a gutter to the ink supply 32.

According to the present invention, the controller 42 receive~
input signals from a capacitive pickup 50 downstream of the charge tunnel. The signal from the pickup 50 is provided to a preamplifier 52 and to a band pass filter 54 (a notch filter designed to pass a frequency equal to twice the normal drop frequency of the ink ~et system). Thus, the capacitive pickup 50 detects the point C(L) in which the drop frequency has doubled due to the presence of intermediate satellites (Figure lB). That signal, analogue in nature, is passed by the filter 54 to a comparator 56 which pro~ides a digital output when the input exceeds a threshold. This signals the controller that C(L) has been detected. The controller thus stores the corresponding nozzle _g_ ' - ' ' - ' :
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drive voltage valve.
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The second input of interest to controller 42 provides a signal indicating the occurrence of C(H), the fold back point illustrated in Figure lG. This signal is produced on line 58 from a pickup 60 in electrical communication with the elèctrically conductive ink stream. The output of pickup 60 is provided to an integrating preamplifier 62 which, in turn, is provided to a comparator 64. As will be described, if the charge on the capacitor associated with preamplifier 62 exceeds a threshold 8et for comparator 64, a digital output is provided on line 58 to the controller.

To understand the function of the comparator 64, it i8 necessary to refer to Figures 1, 3 and 6. To determine C(H), a test signals are placed on the charge tunnel 48 for a period egual to 30 drop tLmes. For example, the signal denoted Test Video 0 in Figure 6. The wave form illustrated in Figure 6 is referenced to the drop clock wave form which may be, for example, 66 kilohertz.
During the time that the test video 0 signal is high, the charge tunnel 48 attempts to apply a charge to each ink drop formed as tha droplets break off from the ink stre~m. During this period the pickup 60 will detect whether or not the drops are successfully charged. For each drop which is charged an incremental charge is stored on the capacitor associated with the preamplifier 62. If most of the drops are succsssfully charged by the test video -lQ-'.

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signal, the voltage from the preamplifier will exceed the thresholdset on the comparator 64 and signal the controller. This sequence is then repeated for test video signals 1, 2, and 3, all o~ which are illustrated in Figure 6. Each test pattern is a ~uarter lambda out of phase from the preceding test pattern (where lambda is the droplet spacing). As a result, it is possible to accurately determine the location (in quarter lambdas, for example) of the droplet breakoff point relative to the positions of the two cardinal points.

The result of this operation is illustrated in Figure 1 where there is shown for each of Figures lA-H a four bit binary code representing the results of applying the test video signals O
through 3. Thus, for example, with respect to Figure lB, test video 1 and test video 2 are digital ones, while test video O and test video 3 are zero indicating that the latter two test videos did not result in charging of the droplets (This is due to the phase of the test video signals relative to the drop clock).

As the drive voltage to the nozzle increases, the pattern of the successfully charged drops changes as indicated in Figure 1 in a predictable sequence based upon the phasing of the test video signals. At the cardinal point CtH), however, there is a first phase reversal (additional phase reversals may occur at higher drive voltages). That is, instead of the expected phase pattern 1001 for Figure lH, the pattern 0110 is observed, which pattern is --11-- . .

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exactly the same as Figure lF. Thus, the circuit accurately detects C(H) the first fold back point where drop brea~off within the charge tunnel 48 is at a minLmum distance from the nozzle.

In practice, the comparator 64 is preferably sampled only once, at about 15 drop times after the start of each test video signal. The output from the comparator is a one or zero indicating that the drops were or were not successfully charged.

It will be recognized from the review of Figure 6 that the four test video signals have a pulse width of approxLmately 66% o~
the drop time and that each test video signal is one-quarter drop time out of phase with every other test video signal. The phasing seguence ends after the output of the comparator is recorded for the four video test signals.

As can be seen from Figure 1, the drop separation point occurs earlier (nearer to the nozzle) as nozzle voltage increases. ~his is recognized by the detector as indicated by the pattern of ones marching from right to left in Figures A through G (and wrapping around)~ This continues until the fold back point, C~H) where the sequencing reverses it~elf and the detector signals this voltage value to the controller.

While the Figure 3 embodiment shows separate pickups for C(L) and C(H), it will be recognized by those skilled in the art that .

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the capacitive pickup 50 can be used for both purposes. That is, the pickup 50 can detect the C(L) value and, by connecting preamp 60 and comparator ~4 to the capacitive pickup, it can also detect C(H). Thus, it is not necessary to use a separate pickup 60 ~ehind the nozzle since the capacitive pickup 50 downstream of the charge tunnel can, if desired, perform both functions.

It will be recognized by those skilled in the art that if a separate pickup 60 is utilized for detecting C(H) it i5 then possible to use an optical or an acoustical pickup in place of the capacitive pickup 50 to detect C(L). The advantage of using an optical or acoustical pickup is:that the drops do not have to be charged to be detected.

When the controller has received the information necessary to determine C(L) and C(H), it employs equation one to calculate V(calc). Figure 2 illustrates a software flow diagram suitable for performing the calculations according to the present invention.
It is important to note that knowledge of the ink temperature is not necessary for a determination of a proper nozzle drive voltage.

Referring to Figure 2, determination of the cardinal points will be described. The controller 42, in the case where a capacitive pickup is utilized, sets the charge tunnel voltage to a constant vall~e. It then sets the nozzle drive voltage to a minimum value via line 40. Nozzle drive voltage is slowly .
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increaqed and the capacitive pickup is checked to determine i~
frequency doubling has occurred. If not, voltage increases, in small increments, until frequency doubling is detected. As indicated previously, freguency doubling indicates the condition where intermediate satellites, which are not merging, are being formed. When frequency doubling is detected, the value of th~
nozzle drive voltage is recorded as C(L).

The controller then initiates the phase control portion of its routine to detect C(H). The test video signals shown in Figure 6 are applied to the charge tunnel electrode. The sensor 60, or alternatively the capacitive pickup 50, is monitored to detect whether drops have been successfully charged for each of the four test signals. ~he software then checks to detect whether or not phase reversal has occurred. If not, tha nozzle drive voltage is ~ncreased, in small increments, until phase reversal is detected.
Upon detection, the nozzle drive voltage is recorded as C(H).

Upon obtaining values of C(H) and C(L), the value V(calc) is computed. This value V(calc), which i9 shown in Figure 4 i8 in the middle of the desirable operating range of the system and is thereafter used as the nozzle drive voltage. In summary form, this operation may be stated as follows:
I. A. Assuming an electrical charge detector, begin by applying a constant charge voltage to the charging electrode (charge tunnel).

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B. Increase the applied nozzle drive voltage 510wly - from a low level, .i.e., le~s than 9 volts, slne wave, peak-to-peak.
C. Nonitor the downstr~am detector for a frequency twice that of the ~rop frequency, that is, search for intermediate ~atellites.
D. Once the doubled frequency is detected, record the voltage level as the lower cardinal power point C tL) .
II. A. Switch t~ the phasing system and apply sequential pha~ing voltages to the charging electrode.
B. Observe the sequential direct~on of "good" phase (in our example "1~8 ) as noz21e drive voltage is incrQased.
C. Record the nozzle voltage as C(H) when the direct~on or sequence of the good phase reverses.
D. Calculate the proper drive voltage from éq(l) for the ink and apply it the nozzle.

Referring again to equation one, it will be noted that the calculation of the value V(calc) requires a ~alue alpha ~e specifled which is ink dependent. This value alpha can ba determined as follows. Since the good printing region lies sand~iched between the lower and upper cardinal power points (see Figure 4) an acceptable solution would be to set alpha = 1. This would locate V(calc) midway between C(L) and C(H), however, some :`
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added tolerance may be gained by choosing slightly smaller or slightly larger values~ A smaller alpha would lower V(calc) and a larger alpha would raise V(calc). It is desirable to ad~ust alpha for each ink to optimize its printing range. This can easily be done by calculating V(calc) for a specific alpha and plotting the results on a graph representing the desirable range of a particular ink. In other words, if desired, alpha may be empirically optimized for each ink composition.

The desirable portion of the range shown in Figure 4 can also be accessed by using only one of the cardinal power points. For example, the following equations can be u~ed for calculating a nozzle drive voltage that will produce good printing from the lower or the higher cardinal points:
V(L) = C(L) + El EQ 2 V(H) = C(H) - E2 EQ 3 where:
El = 15 volts E2 = 2~ volts El and E2 are voltages empirically determined from the good printiny range of a particlar ink. For example, in Figure A, C(L) is about 10 volts. V(calc) is about 25 volts. Therefore, if El is selected a PPrOXI ~c,~e as lS volts, it will reliably a~rixmate v(calc) when used in EQ
2. Both V(L) and V(H) will lie within the cross hatched area on the graph in Figure 4.

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While we have shown and described embodiments of ~he ~ invention, it will be understood that this de~cription and illustrations are offered merely b~ way of example, and that the invention is to be limited in scope only as to the appended claims.

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Claims (19)

1. A control circuit for setting the magnitude of the exciting voltage applied to the nozzle of an ink jet printer to break a stream of ink into droplets comprising:
(a) first means for detecting the exciting voltage value C(L) at which droplet frequency doubles as the magnitude of the exciting voltage is slowly increased from a minimum value;
(b) second means for detecting the value C(H) at which droplet formation first occurs closest to said nozzle, as said exciting energy is slowly increased from said value C(L), (c) third means receiving as inputs the values C(L) and C(H) for calculating the exciting voltage magnitude applied to said nozzle to be utilized for printing therefrom.

2. A control circuit for setting the magnitude of the exciting voltage applied to the nozzle of an ink jet printer to break a stream of ink into droplets comprising:
(a) first means for detecting exciting voltage value C(L) at which intermediate satellite droplets are produced by said nozzle as the exciting voltage is slowly increased from a minimum value;
(b) second means detecting the exciting voltage value C(H) which first produces a direction change in droplet breakoff point relative to said nozzle as the exciting voltage is slowly increased from C(L);
(c) third means utilizing the values C(L) and C(H) for computing the proper operating voltage to be utilized for printing.

3. The control circuit of claim 1 wherein said first detecting means includes a capacitive pickup downstream of said nozzle; said droplets being electrically charged whereby said pickup detects charged droplets.

4. The control circuit of claim 3 wherein said first detecting means further includes circuit means coupled to said pickup for providing an output signal to said calculating means when said droplet frequency doubles.

5. The control circuit of claim 2 wherein first detecting means includes a capacitive pickup downstream of said nozzle; said droplets being electrically charged whereby said pickup detects the charged droplets.

6. The control circuit of claim 5 wherein said first detecting means further includes circuit means coupled to said pickup for providing an output signal to said calculating means when said droplet frequency doubles.

7. The control circuit of claim 1 wherein said first detecting means include an optical detector located downstream of said nozzle, said detector detecting the droplets passing said detector.

8. The control circuit of claim 7 wherein said first detecting means further includes circuit means coupled to said pickup for providing an output signal to said calculating means when said droplet frequency doubles.

9. The control circuit of claim 1 further including means for applying electrical test patterns to said droplets, said patterns varying in phase relative to the droplet timing whereby only some of the test patterns successfully charge said droplets, said second detecting means includes a pickup to detect which droplets have been charged, said calculating means including means for determining the C(H) value from the change in the sequence of charge patterns.

10. The control circuit of claim 9 when said means for applying said test patterns includes a charge amplifier and a charge tunnel positioned downstream of said nozzle in the region of droplet formation.

11. The control circuit of claim 2 further including means for applying electrical test patterns to said droplets, said patterns varying in phase relative to the droplet timing whereby only some of the test patterns successfully charge said droplets, said second detecting means includes a pickup to detect which droplets have been charged said calculating means including means for determining the C(H) value from the change in the sequence of charge patterns.

12. The control circuit of claim 11 when said means for applying said test patterns includes a charge amplifier and a charge tunnel positioned downstream of said nozzle in the region of droplet formation.

13. A method of determining the exciting voltage to be applied to the nozzle of an ink jet printer to break a stream of ink into droplets for printing comprising the steps of:
(a) slowly increasing the exciting voltage from a mini-mum value;
(b) detecting and recording the voltage value C(L) at which the droplet frequency doubles due to the formation of inter-mediate (non-merging) satellite droplets;
(c) detecting and recording the voltage value C(H) at which droplet formation first occurs closest to the nozzle;
(d) calculating the exciting voltage for printing according to the equation:
V(calc) = alpha[C(L) + C(H)]/2 where alpha is a value related to the ink.

14. The method of claim 13 wherein the value C(L) is detec-ted by the sub steps of:
(i) charging the ink droplets;

(ii) detecting the charges on said droplets sufficiently downstream of said nozzle to eliminate the presence of merging satellite droplets.

15. The method of claim 13 wherein the value C(L) is detec-ted by the sub step of:
(i) optically detecting said droplets sufficiently downstream of said nozzle to eliminate the presence of merging satellite droplets.

16. The method of claim 13 wherein the value C(H) is detec-ted by the sub steps of:
(i) applying electrical test patterns to said droplets, said patterns varying in phase relative to the droplet timing whereby only some of the test patterns will successfully charge said droplets;
(ii) detecting which droplets have been successfully charged;
(iii) determining the value C(H) from the change in the sequence of charge patterns.

17. A control circuit for determining the exciting voltage to be applied to the nozzle of an ink jet printer to break a stream of ink into droplets for printing comprising:
(a) means for slowly increasing the exciting voltage from a minimum value;
(b) means for detecting and recording the voltage value C(L) at which the droplet frequency doubles due to the formation of intermediate (non-merging) satellite droplets;
(c) means for detecting and recording the voltage value C(H) at which droplet formation first occurs closest to the nozzle;
(d) means for calculating the exciting voltage for printing according to the equation, V(CALC) = alpha [C(L) + C(H)]/2 where alpha is a value related to the ink.

18. A control circuit for determining the exciting voltage to be applied to the nozzle of an ink jet printer to break a stream of ink into droplets for printing comprising:
(a) means for detecting and recording the voltage value C(L) at which droplet frequency doubles as the magnitude of the exciting voltage is slowly increased from a minimum value and for detecting the value C(H) at which droplet formation first occurs closest to said nozzle, as said exciting energy is slowly increased from said value C(L);
(b) means for receiving as inputs the values C(L) and C(H) for calculating the exciting voltage magnitude to be utilized for printing therefrom.

19. A control circuit for determining an exciting voltage to be applied to a nozzle of an ink jet printer to break a stream of ink into droplets for printing comprising:
(a) means for determining voltage value C(H) at which droplet formation first occurs closest to the nozzle as said exciting voltage is slowly increased from a minimum value, said determining means including (i) means for applying electrical test patterns to said droplets, said patterns varying in phase relative to droplet timing whereby only some of the test patterns will successfully charge said droplets;
(ii) means for detecting which droplets have been successfully charged; and determining the value C(H) from a change in sequence of charge patterns;
(b) means for estimating the exiting voltage for printing according to the equation:
V(est) = C(H) - E
where E is a voltage related to the performance of the ink.