JPH02274556A - Ink jet nozzle drive control circuit and control method - Google Patents

Abstract

PURPOSE: To maintain nozzle voltage automatically by providing a first means for detecting the drive voltage at the time of doubling the droplet frequency, a second means for detecting the value at the time of formation of a first droplet closest to a nozzle, and a third means for calculating the magnitude of drive voltage. CONSTITUTION: An analog signal from a capacitive pickup 50 is delivered through a filter 54 to a comparator 56 which produces an output when the input value exceeds a threshold level and points C(L) (B) where the droplet frequency is doubled due to presence of an intermediate associated droplet 16 are detected and transmitted to a controller 42. An output from a pickup 60 communicating electrically with a conductive ink flow is delivered through an integrating preamplifier 62 to a comparator 64 and a digital output is delivered to a controller 42 when the charge of a capacitor associated with the preamplifier 62 exceeds the threshold level of the comparator 64. More specifically, values C(H) (G) are detected at the time of formation of a first droplet closest to a nozzle 10. Nozzle drive voltage is calculated from the values C(L), C(H).

Description

BACKGROUND OF THE INVENTION The present invention relates to ink jet printing systems and similar droplet deposition systems in which electrically conductive ink is supplied to a nozzle. , the ink is forced through the orifice of the nozzle and an energizing voltage is applied to the nozzle to break up the ink stream into droplets, electrically charging the droplets and depositing them onto the substrate to be deposited. It can be deflected. Such ink jet technology is well known;
For example, U.S. Patent No. 4. See '12' no. 1.379 and '12' no. 4.555.712.

- The energizing energy or voltage applied to the nozzle must be set properly during operation of the system to ensure proper operating conditions for consistent print quality. Currently, most ink jet printers require manual setting of the energy applied to the ink stream as it exits the nozzle. An appropriate value is
It can be determined empirically by comparison with what is found in existing diagrams or by determining the point of droplet separation and comparing this to machine specifications.

In either case, the resulting print quality will change.

Research to provide automatic control of the modulation voltage has focused on detecting the location of the separation point relative to a fixed location such as a charge tunnel. See, for example, published European Patent Specification No. EP A 0287373. Another study, disclosed in U.S. Pat. No. 4,638,325, uses a small charged electrode and a downstream electrometer that brings the point of separation close to the small electrode. The separation point of the droplet can be determined by observing the current in the electrometer when the droplet is removed. U.S. Patent No. 4.638.32
In No. 5, the maximum current occurs when the droplet separation is closest to the small charged electrode.

The above method does not take into account the fundamental reason for maintaining the charging conditions of the droplet through-out.

The drop separation point varies significantly with the surface tension and viscosity of the ink, so simply holding this separation point constant still results in different side conditions and variable print quality. In summary, holding the droplet separation point constant is not a sufficient solution to this problem.

What is needed is a system that can determine a range of appropriate print nozzle drive voltages and then calculate a desired intermediate value within this range. Such a system must be temperature independent over a wide range of operating temperatures, resulting in a sufficiently good control system.

It is therefore an object of the present invention to provide such a nozzle drive control system which is an improvement over the known technology.

Another object of the present invention is to provide a nozzle control system capable of accurately monitoring satellite drop conditions and drop breakup points and calculating from these a satisfactory range of nozzle drive voltages for operating an ink jet printer. It is provided by.

Yet another object of the present invention is to enable automation of nozzle voltage for best quality printing using continuous ink jet printers, regardless of ink type and temperature.

The present invention avoids problems associated with satellite drop reassembly that occur in maintaining droplet separation points during variations in ink type and temperature. These findings are due to undesirable charge fluctuations because satellite droplets that carry some of the charge of the parent charged droplet transfer this charge to subsequent droplets when coalescence (reassembly) occurs. Cause. The above and other objects of the invention will become apparent from the rest of the specification.

(Example) In FIG. 1, a series of nozzles is shown.

Nozzle 10 then fires a stream 12 of ink.

A nozzle drive voltage is applied that splits the ink stream into a series of discrete liquids i14. In the abutment procedure, finer droplets 16, known as satellites, are formed between the droplets 14. This appendage 16 behaves in a manner that is a function of the energy applied to the nozzle (measured at 1- with respect to the nozzle voltage).

In Figure 1, when the acoustic energy applied to the ink stream is low, the natural behavior of the satellite is to form independently of the droplet and then fall back.
) t, to merge with the following droplet. This is called the posterior fused phloem or the sluggish phloem, and is the 11th
' Shown in NA.

This folding and fusion depends on the physical parameters of the ink (viscosity, surface tension, specific gravity, etc.)
occurs within a droplet period of lAj.

As the energizing energy to the nozzle increases, a point designated C(L) in the text occurs. This term refers to a relatively small cardinal point.

Cardinal is a word borrowed from an optical term that refers to the cardinal point, i.e., focal point, nodal point, or principal point of a lens system. For purposes of this specification, C(L) represents the point where the tail separates from the leading and trailing droplets simultaneously.
is an important point (see first diagram). Surface tension pulls these appendages forward and backward with equal force. As a result, the accompanying portion will be located in the middle between the droplets, that is, at the halfway point of the droplets as they move in space. This condition, referred to as C(L), is determined by downstream detection by detecting satellites and droplets. Point C (
In L), in all cases where there will be twice the normal falling frequency that can be detected, the satellite will merge with the leading or trailing droplet. A suitable detector will be illustrated and explained with reference to FIG. 5 of the main text.

Almost all nozzles used in ink jet printing systems produce such intermediate collaterals that produce neither anterior nor posterior fusion. Point C(L
) will be detected by a doubling in frequency when the power to the nozzle drive is increased from a low level to a level just sufficient to form an intermediate concomitant.

In one embodiment of the detector of FIG. 5, a suitable test signal is applied to the charging electrode such that both the droplet and the intermediate collateral become charged. The detected droplet frequency is
Doubles when intermediate concomitants are present and pass through the sensor. Alternatively, an optical detector is used that does not require a charge on the droplet and its satellites, but detects a doubling of the number of droplets passing through the detector.

In either case, the detector is placed at a sufficient distance downstream from the orifice of the nozzle to allow the satellite parts to fuse.

In addition to the relatively small cardinal point C(L), most ink jet nozzles can also be designated as the upper cardinal point C(H).

This point can be observed by gradually increasing the power to the nozzle and observing the point of droplet separation. As the power to the nozzle is increased from a low level (Figure 1A), the droplet separation point, denoted S, moves closer to the nozzle until it reaches its minimum distance from the nozzle (Figure 1G). . The splitting point then moves away from the nozzle (Figure 1()). This folding or reversal can be detected by appropriate circuitry and software. A description of the circuit and method for is provided with respect to the description of FIG.

However, first the characteristics of typical inks used in ink jet printing systems will be described with reference to the graph of FIG. This ink, designated 1,6-8200, manufactured by the assignee of the present invention, is used with a nozzle of the type described in U.S. Pat. No. 4,727,329, incorporated herein by reference. It is something that The hatched area of the graph is approximately 4 to 43
It represents the nozzle drive voltage that produces good print quality over a temperature range of about 40 to below 110 degrees Celsius. The lower and upper cardinal points C(L) and C(H) are also
Plotted for the same nozzle and ink composition. From this information, the voltage value V (calc
) can be calculated. That is, V (calc)=
alpha[c(L)+C(H)]/2
Equation 1 However, alpha is a function of ink described below.

The value V (calc) calculated from the above formula is plotted in FIG. All of these values of V(calc) lie within the hatched area of the graph and represent the nozzle drive voltages that produce good quality prints.

A circuit suitable for implementing the present invention will be described with reference to FIGS. 1 and 3. The nozzle 10 is connected to an ink supply source 32 via an ink line 34. The ink flow is grounded at line 36 between the ink supply and the nozzle.

The nozzle is described, for example, in U.S. Pat.
Acoustic energy is applied by a piezoelectric device such as that disclosed in US Pat. The driving voltage for the piezoelectric device is line 4
0 from the nozzle drive amplifier 38.

Furthermore, this amplifier is a digital/analog (D/A)
It is controlled by a processor 42, such as a microprocessor, via a converter 44.

Controller 42 also operates a charge amplifier via D/A converter 46 to control the voltage applied to charge tunnel 48 . As is known in the art, a charging tunnel 48 connects the nozzle l in the area where drop formation is intended when the ink stream breaks up into droplets and satellite droplets.
It is located downstream of O.

In this way, selected droplets can be charged to be deflected toward the substrate or, if remaining uncharged, returned to the ink supply 32 by the gutter. I can do it.

According to the invention, controller 42 receives an input signal from a capacitive pickup 50 downstream of the charging tunnel. The signal from this pickup 50 is applied to a preamplifier 52 and a bandpass filter 54 (a notch filter designed to pass frequencies equal to twice the normal drop frequency of the ink jet system). .

For this reason, the capacitive pickup 50 is located at point C (
L) is detected. This signal, which is analog, is passed by filter 54 to comparator 56, which produces a digital output when the input exceeds a threshold. This output communicates to the controller that C(L) has been detected.

Thus, the controller stores a corresponding nozzle drive voltage valve.

A second input to controller 42 provides a signal indicative of the occurrence of C(H), the turning point shown in FIG. 1G. This signal is generated on a line 58 from a pickup 60 in electrical communication with the conductive ink stream. The output of pickup 60 is fed to an integrating preamplifier 62, which output is further fed to a comparator 64. As discussed below, if the charge on the capacitor associated with preamplifier 62 exceeds the threshold set for comparator 64, a digital output is provided on line 58 which is connected to the controller.

To understand the function of the comparator 64, see FIGS.
It is necessary to refer to the figures and FIG.

To determine C(H), a test signal is applied to charge tunnel 48 for a period equal to 30 drops.

For example, in FIG. 6, this signal represented test video O. The waveform shown in FIG. 6 is, for example, 66KH.
The droplet clock waveform is referenced to z. This test
During the period when the video O signal is high, the charge tunnel 48 attempts to add a charge to each ink droplet formed as a droplet broken off from the ink stream.

During this period, the pickup 60 will detect whether the droplet is continuously charged. For each charged droplet, an incremental charge is stored in the preamplifier 62 and associated capacitor. If a large portion of these droplets are continuously charged by the test video signal, the voltage from the preamplifier exceeds the threshold set in comparator 64 and transmits a signal to the controller. This sequence is repeated for test video signals 1.2.3, all of which are shown in FIG. Each test pattern is offset from the previous test pattern by λ/4, where λ is the droplet spacing. As a result, Mi! for the positions of the two cardinal points! The location of the splitting point (e.g., λ
/4) can be determined to be jTEJ.

The result of this operation is shown in FIG. 1, where for each of FIGS. 1A-[I, a 4-bit binary code is shown representing the result of applying test video signals O through 3. Thus, for example in FIG. 1B, test video 1 and test video 2 are digital signals, but test video 0 and test video 3 are 0;
The two test videos did not result in droplet charging (this is due to the phase of the test video beam relative to the droplet clock).

As the driving voltage to the nozzle increases, the phase state of the test video signal changes to two predicted levels, as shown in Figure 1.
The pattern of the droplets changes continuously in the sequence of obtaining 2 charges (5). However, at the cardinal point C(1-1), a first phase reversal occurs (another phase reversal occurs at a higher driving voltage). i.e., instead of the expected phase pattern in FIG.
110 is observed, and this pattern is exactly the same as in FIG. 1F. Thus, this circuit consists of a charged tunnel 4
Accurately detect C(H), the first folding point at which droplet breakup within 8 is at the minimum distance from the nozzle.

In practice, comparator 64 is preferably sampled only once at approximately 15 drops after the start of each test video signal. The output from the comparator is 1 or O indicating that the droplet was continuously charged or uncharged.

Referring to Figure 6, it can be seen that the four test video signals have a pulse width of approximately 66% of the droplet time, and that each test video signal has a pulse width of approximately 66% of the droplet time.
The video signal is one-fourth in phase with the other test video signals.
It will be seen that the droplet times are shifted. This phase sequence ends after the comparator outputs have been recorded for the four test video signals.

As can be seen from FIG. 1, the droplet separation point occurs earlier (occurs at a point closer to the nozzle) as the nozzle voltage increases.

This is accomplished by the detector as shown by the right-to-left (and surrounding) pattern in FIGS. 1A-G. This state continues until the turning point C(1-1) where the sequence itself is reversed and the detector transmits this voltage value to the controller.

Although the embodiment of FIG. 3 shows separate pickups for C(L) and CDI, those skilled in the art will appreciate that capacitive pickup 50 can be used for both purposes. That is, the pickup 50 can detect the value C(L), and the preamplifier 60 and comparator 64
By connecting to a capacitive pickup, C(H)
can also be detected. This eliminates the need to use a separate pickup 60 behind the nozzle, allowing the capacitive pickup 50 downstream of the charging tunnel to perform both functions as required.

Those skilled in the art will recognize that if individual pickups 60 are used for C(H) detection, an optical or acoustic big amplifier can be used in place of capacitive pickup 50 for C(L) detection. It will be. An advantage of using optical or acoustic pickups is that the droplets do not need to be charged for detection.

When the controller receives the information necessary to determine C(L) and C(H), this controller determines V (
Cale) is used to calculate Equation 1.

FIG. 2 shows a flow diagram of software suitable for performing calculations according to the invention. It is important to know that it is not necessary to know the ink temperature to determine the proper nozzle drive voltage.

With reference to FIG. 2, the determination of cardinal points will be described. If a capacitive pickup is used, the controller 42 sets the voltage of the charge tunnel to a constant value.

The controller then sets the nozzle drive voltage to a minimum value via line 40. The nozzle drive voltage is gradually increased and the capacitive pickup is examined to determine if a frequency doubling has occurred. If no doubling occurs, the voltage is increased incrementally until a frequency doubling is detected.

As previously indicated, a doubling of frequency indicates a condition in which non-merging intermediate satellite droplets are forming. When a frequency doubling is detected, the value of the nozzle drive voltage is recorded as C(L).

The controller then begins the phase control portion of the routine that detects C(I().
A video signal is provided to the electrodes of the charged tunnel. Sensor 60, or alternatively capacitive pickup 50, is monitored to detect whether the droplet is sequentially charged during each of the four test video signals. next,
The software checks to detect if a phase reversal has occurred. If not, the nozzle drive voltage is increased in small increments until a phase reversal is detected. Simultaneously with the detection of phase reversal, the nozzle drive voltage changes to C(H)
recorded as.

At the same time as obtaining the values of C(H) and C(L), the value V
(calc) is calculated. This value V shown in FIG.
(calc) is in the middle of the desired operating range of the system and is then used as the nozzle drive voltage. In summary, this operation can be stated as follows.

1. A. Assuming a charge detector, start applying a constant charging voltage to the charging electrode (charge tunnel).

B. Gradually increase the applied nozzle drive voltage from a low level, ie, a sinusoidal peak/peak level below 9 volts.

Downstream detection for a frequency twice the C1 droplet frequency?
; i.e., searching for intermediate collateral sources.

D - Once the doubled frequency is detected, this voltage level is recorded as the lower cardinal power point C(L).

IT, A. Switch to the phase system and apply sequential phase voltages to the charging electrodes; B. Observe the sequential direction of "good" phase ("1" in the text case) as the nozzle drive voltage increases. do.

When the direction or sequence of C1 good phase reverses, record this nozzle drive voltage as C(H).

D. Calculate an appropriate driving voltage from equation (1) for the ink and apply it to the nozzle.

Considering equation (1) again, the value V (calc)
It will be seen that the value α is independent of the ink. This value α can be determined as follows. Since the good printing area is switched between the upper and lower cardinal power points (see FIG. 4), an acceptable solution would be to have α=1. Therefore, the value V (ca
lc) would be found intermediate between C(L) and C(H). However, some tolerance could be obtained by choosing a slightly smaller or slightly larger value. A smaller α will decrease the value V[calc), and a larger α will increase the value V(calc). It is desirable to adjust α to optimize the printing range of each ink.

This can be easily done by calculating V(calc) for a particular α and plotting the result on a graph representing the desired range for a particular ink. In other words, α can be empirically optimized for each ink composition, if desired.

The required portion of the range shown in FIG. 4 can also be accessed by using only one of the cardinal power points. For example, the equation below can be used to calculate the nozzle drive voltage that will result in good printing from the lower or upper cardinal point.

V (L) = C (L) + E+ Formula IV
(H) -C(H)-E2 Formula 2% Formula % E and E2 are voltages determined empirically from the good printing range of a particular ink. For example, in Figure 1, C(L) is approximately 10 volts. V(ca!
e) is approximately 25 volts. Therefore, if El is chosen to be 15 volts, this means that V when used in Equation 2
(c a 1. This would be a good approximation to c.

Both V'(L) and V(H) will be within the hatched area in the graph of FIG.

Although the embodiments of the present invention have been described, the description and illustrations in the main text are merely illustrative. It is to be understood that the present invention is to be limited in scope only by the scope of the following claims.

[Brief explanation of drawings]

Figure 1 is an inkjet if diagram useful for understanding the present invention.
&a is a schematic diagram showing the formation principle; FIG. 2 is a software flow diagram showing the operating state of the processor of the present invention; and FIG.
Figure 4 is a circuit diagram showing a control circuit according to the present invention, Figure 4 is a graph useful for explaining the invention in detail, Figure 5 is a schematic diagram showing a method by which incidental droplets on the way of scattering can be detected, and Figure 6 is is the test used to detect the upper cardinal point.
FIG. 4 is a timing diagram useful in explaining the pattern. lO... Nozzle, 12... Ink flow, I4... Droplet, 16... Associated drop, 32... Ink supply island 1134
... Ink pipe line, 36.40.58 ... Line, 38.
... Nozzle drive amplifier, 42 ... Control o-5, 44
, 46...Digital/analog converter (D/
A) , 48... Charge tunnel, 50... Capacitive pink-up, 52... Preamplifier, 54... Filter, 56.64... Comparator, 60... Big Ala7', 62... - Integrating preamplifier, 64...coverator. Engraving of drawings (no changes in content) F2O, 2 F'/G, / 17G, 5 Answer FIC with middle wI, 6 Test video stoppage

Claims (1)

[Claims] 1. In a control circuit that sets the magnitude of an energizing voltage applied to a nozzle of an ink jet printer to break up an ink flow into droplets, (a) the energizing voltage is As the size gradually increases from the minimum value, the energizing voltage value C(L
); (b) as said energizing voltage is gradually increased from said C(L), a value C( (c) second means for receiving said values C(L) and C(H) as inputs and calculating therefrom the magnitude of said energizing voltage to be used for printing; 3. A control circuit characterized by comprising the means of 3. 2. In a control circuit that sets the magnitude of an energizing voltage applied to a nozzle of an ink jet printer to break up an ink flow into droplets, (a) the magnitude of the energizing voltage varies from a minimum value to a minimum value; a first means for detecting an energizing voltage value at which an intermediate satellite drop is generated by the nozzle as the energizing voltage gradually increases from said C(L); (c) a second means for detecting an energizing voltage value C(H) when a change in direction at the breakup point of the droplet with respect to the nozzle first occurs; (c) the value C(L) and C(H) to calculate an appropriate operating voltage to be used for printing. 3. The first detection means includes a capacitive pickup disposed downstream of the nozzle, and the droplet is electrically charged, so that the pickup detects the charged droplet. The control circuit according to claim 1, characterized in that: 4. The first detection means further comprises circuit means connected to the pickup for providing an output signal to the calculation means when the frequency of the droplet doubles. control circuit. 5. The first detection means includes a capacitive pickup disposed downstream of the nozzle, and the pickup detects the charged droplet when the droplet is electrically charged. 3. The control circuit according to claim 2. 6. The first detection means further comprises circuit means connected to the pickup for providing an output signal to the calculation means when the frequency of the droplet doubles. control circuit. 7. Claim 1, characterized in that said first detection means comprises an optical detector placed downstream of said nozzle, said detector detecting droplets passing through said detector. control circuit. 8. The control circuit of claim 7, wherein the first detection means is further coupled to the pickup to provide an output signal to the calculation means when the frequency of the droplet doubles. 9. Further providing means for applying an electrical test pattern to the droplet, the pattern varying in phase with respect to the timing of the droplet so that only a portion of the test pattern causes the droplet to means for successively charging, said second detection means comprising a pickup for detecting which droplets are charged, and said calculation means for determining a value C(H) from changes in the succession of charge patterns; The control circuit according to claim 1, characterized in that it includes: 10. The control circuit of claim 9, wherein said means for providing said test pattern includes a charge amplifier and a charge tunnel located downstream of said nozzle in a region of droplet formation. 11. Further providing means for applying an electrical test pattern to the droplet, the pattern varying in phase with respect to the timing of the droplet so that only a portion of the pattern continues the droplet; the second detection means includes a pickup for detecting which droplets are charged, and the calculation means determines a value C(H) from changes in the succession of charge patterns. 3. A control circuit according to claim 2, further comprising means. 12. The control circuit of claim 11, wherein said means for providing said test pattern includes a charge amplifier and a charge tunnel located downstream of said nozzle in a droplet formation region. 13. A method for determining an energizing voltage applied to a nozzle of an ink jet printer to break up an ink stream into droplets for printing, comprising: (a) gradually increasing the energizing voltage from a minimum value; Increase (b
) detecting and recording the voltage value C(L) at which the frequency of said droplet doubles due to the formation of an intermediate (non-merging) satellite drop; A method characterized in that the method comprises the steps of: detecting and recording a voltage value C(H) occurring at the time when the voltage C(H) occurs; and (d) calculating the energizing voltage for printing according to the following formula. V(CALC)=α[C(L)+C(H)]/2However,
α is a value related to ink. 14. The value C(L) is determined by (i) charging a droplet of ink and (ii) continuously detecting the charge on the droplet well downstream of the nozzle to prevent coalescence of satellite droplets. 14. A method according to claim 13, characterized in that the detection is performed by the substep of removing the presence. 15. Claim characterized in that said value C(L) is detected by the substep of: (i) continuously detecting said droplets downstream of said nozzle to eliminate the presence of coalescence of satellite drops; The method according to item 13. 16, the value C(H) is such that (i) by applying to said droplet an electrical test pattern that is out of phase with respect to the timing of the droplet, only a portion of said test pattern continues said droplet; (ii) detect which droplets are successfully charged; (iii) determine the value C(
14. The control circuit according to claim 13, wherein the control circuit is determined by the substep of determining H). 17. In a circuit for determining the energizing voltage to be applied to a nozzle of an ink jet printer to break up a stream of ink into droplets for printing, the method comprises: (a) gradually increasing said energizing voltage from a minimum value; (b) means for detecting and recording a voltage value C(L) when the frequency of said droplet is doubled due to the formation of an intermediate (non-merging) satellite drop; (c) means for detecting and recording a voltage value C(L) when the droplet formation is (d) means for calculating the energizing voltage for printing using the following formula; Features energizing voltage determining circuit. V(CALC)=α[C(L)+C(H)]/2However,
α is a value related to ink. 18. A method for determining an energizing voltage to be applied to a nozzle of an ink jet printer to break up a stream of ink into droplets for printing, comprising: (a) gradually increasing said energizing voltage from a minimum value; , (b
) Detect and record the voltage value C(L) when the frequency of the droplet is doubled due to the formation of an intermediate (unfused) satellite drop; (c) calculate the energizing voltage for printing by the following formula: A method comprising the steps of making a prediction. V(est)=C(L)+(E) However, E is the voltage related to the performance of the ink. 19. A method for determining an energizing voltage to be applied to a nozzle of an ink jet printer to break up a stream of ink into droplets for printing, comprising: (a) gradually increasing said energizing voltage from a minimum value; , (b
) detecting and recording the voltage value C(H) when droplet formation first occurs closest to the nozzle; (c) predicting the energizing voltage for printing by the following formula: A method characterized by: V(est)=C(L)-(E) where E is a voltage related to the performance of the ink. 20. In a control circuit for determining the energizing voltage to be applied to a nozzle of an ink jet printer to break up a stream of ink into droplets for printing, wherein: (a) said energizing voltage is gradually increased from a minimum value; means for detecting and recording the voltage value C(L) when the frequency of the droplet is doubled due to the formation of an intermediate (non-merging) satellite drop; (b) for printing according to the following formula; and means for predicting the energizing voltage. V(est)=C(L)+(E) However, E is the voltage related to the performance of the ink. 21. In a control circuit for determining the energizing voltage to be applied to a nozzle of an ink jet printer to break up a stream of ink into droplets for printing, (a) said energizing voltage is gradually increased from a minimum value; means for detecting and recording the voltage value C(H) when droplet formation first occurs closest to the nozzle; (b) determining the energizing voltage for printing by the following formula; A control circuit comprising a means for predicting. V(est)=C(H)+(E) However, E is the voltage related to the performance of the ink. 22. In a control circuit for determining the energizing voltage to be applied to a nozzle of an ink jet printer to break up a stream of ink into droplets for printing, (a) the magnitude of said energizing voltage is from a minimum value to As the droplet frequency increases gradually, the energizing voltage value C(L
) and record the voltage value C(H) at which droplet formation first occurs closest to the nozzle as the energizing voltage is gradually increased from the value C(L). (b) means for receiving said values C(L) and C(H) as input and calculating therefrom the magnitude of said energizing voltage to be used for printing. A control circuit featuring: