DE69017931T2 - Nozzle drive control and inkjet printing processes. - Google Patents

Description

This invention relates to ink jet printing systems and similar drop marking systems in which a supply of electrically conductive ink is provided to a nozzle. The ink is forced through a nozzle orifice while simultaneously an excitation voltage is applied to the nozzle to cause the ink stream to break up into droplets which can be charged and discharged onto a substrate to be marked. Such ink jet technology is well known and is described, for example, in U.S. Patent Nos. 4,727,379 and 4,555,712.

To ensure appropriate operating conditions for consistent print quality, the excitation energy or voltage applied to the nozzle must be appropriately adjusted during operation of the system. Currently, most inkjet printers require manual adjustment of the energy applied to the ink stream as it leaves the nozzle. The appropriate value is determined either empirically by comparing what is seen with an existing chart, or by determining the drop separation point and comparing this with machine specifications. In both cases, the resulting print quality changes.

Efforts to provide automatic control for the modulation voltage have focused on sensing the separation point location relative to a fixed location such as the charge tunnel. See, for example, published European Patent Specification EP-A-0 287 373. Another approach is disclosed in US Patent No. 4,638,325 which uses a small charging electrode and a downstream electrometer through which the Drop separation point can be determined by observing the current on the electrometer as the separation point approaches the small electrode, with the maximum current produced when the drop separation is closest to the small charging electrode. A system control microprocessor receives the digital inkjet current signal from the electrometer and is programmed to control the gain of an excitation amplifier, the output of which at the piezoelectric transducer is applied to the inkjet printhead, by supplying a reference signal to an automatic gain control circuit. In operation, the excitation amplitude is initially set at a low level to allow the length of the ink filament to approach its natural unexcited length. The excitation amplitude is then monotonically increased as the charge imposed on the ink jet is observed by the electrometer. As the excitation amplitude is increased, the ink filament becomes shorter and the ink drop separation approaches the close charging electrode. In this way, the ink jet current sensed by the electrometer provides a signal proportional to the length of the ink filament. The system control processor is used to increase the excitation reference signal in small increments while monitoring the jet current signal provided by the electrometer. The jet current signal is recorded and a peak value is detected which represents the transition to overdrive, which is an excitation region above the minimum filament length in which satellite drops are produced and drop deflection is found to be difficult to control. The excitation amplitude is then fixed at a predetermined point below the peak value which is found to provide reliable excitation by calculating the reference level as a function of the reference level at the peak value. It is proposed that the reference level is set to the excitation amplitude at the peak minus 25mV, which is a constant offset from the excitation amplitude at the peak.

U.S. Patent No. 4,638,325 accordingly teaches that the excitation amplitude applied to the nozzle of an ink jet printer can be set by a control unit comprising sensing means (in the form of a piezoelectric feedback transducer) for determining the value of the excitation amplitude while slowly increasing its magnitude from a minimum value and for determining the value of the excitation amplitude at which droplet formation occurs closest to the nozzle (that is, the peak value representing the transition to overdrive).

The above teaching does not take into account the root cause of maintaining consistent drop charge states. The drop separation point varies greatly with surface tension and ink viscosity, and therefore simply keeping the separation point constant will still result in different satellite states and variable print quality. In short, keeping the drop separation point constant is not a satisfactory solution to the problem.

What is desired is a system that can specify a range of suitable nozzle drive voltages and then calculate a satisfactory intermediate value within the range. Such a system should be temperature independent over a wide range of operating temperatures to result in a considerably better control system.

According to one aspect of the invention, a detection means is arranged to detect the value C(L) of the excitation voltage at which the drop frequency doubles, and is a Calculation means are provided for receiving as inputs the values C(L) and C(H) and for calculating therefrom the magnitude of the excitation voltage to be used for printing. The detection means may comprise either a capacitive pickup or an optical detector downstream of the nozzle for detecting the electrically charged droplets. The detection means may further comprise circuit means for supplying an output signal to the calculation means when the detected drop frequency doubles.

Means may be provided for applying electrical test patterns to the droplets, the patterns varying in phase relative to the droplet timing so that only some of the test patterns successfully charge the droplets, and the detecting means comprises a sensor for detecting which of the droplets have been charged and the calculating means comprises means for determining the C(H) value from the change in the sequence of the charging patterns. The means for applying the test patterns to the droplets may comprise a charge amplifier and a charge tunnel positioned downstream of the nozzle in the region of droplet formation. Means may also be provided for slowly increasing the excitation voltage from a minimum value.

The calculation means is preferably arranged to calculate the magnitude of the excitation voltage to be used for printing according to the equation:

V(CALC) = alpha [C(L) + C(H)]/2,

where alpha is an ink related value.

According to another aspect of the invention, a method for determining the excitation voltage applied to a nozzle of an inkjet printer to break up an ink stream into droplets for printing, in which the value C(L) of the existing voltage at which the droplet frequency doubles due to the formation of non-mixing intersatellite droplets is detected and that the value V(calc) of the existing voltage to be used for printing is calculated using the equation:

V(CALC) = alpha [C(L) + C(H)]/2,

where alpha is an ink related value. This method may also include sensing the value C(L) by charging the ink droplets and sensing the charges on the droplets sufficiently downstream of the nozzle to eliminate the presence of mixing satellite droplets. The method may include sensing the value C(L) by optically sensing the ink droplets sufficiently downstream of the nozzle to eliminate the presence of mixing satellite droplets. The method may include sensing the value C(H) by

(i) electrical test patterns are applied to the ink droplets so that the patterns vary in phase relative to the ink droplet timing, whereby only some of the test patterns will successfully charge the droplets,

(ii) detecting which droplets have been successfully charged, and

(iii) the value C(H) is determined from the change in the sequence of the charging patterns.

The present invention enables a nozzle control system to accurately monitor the condition of the satellite droplets and the droplet break-off point and to to calculate a satisfactory range of nozzle drive voltages to operate an inkjet printer.

Another advantage of the present invention is that it allows for the automation of nozzle voltage for best quality printing using a continuous inkjet printer regardless of ink type and temperature. It can also avoid satellite re-merging problems that occur when the drop separation point is held constant while ink type and temperature change. These lead to undesirable charge changes because a satellite carrying some of the charge of the charged drop from which it has split will transfer that charge to the subsequent drop when mixing occurs.

Figure 1 illustrates the principles of inkjet droplet formation useful for understanding the present invention.

Figure 2 is a software flow diagram illustrating the manner in which the processor of the present invention operates.

Figure 3 is a circuit diagram illustrating the control circuit according to the present invention.

Figure 4 is a graph useful for explaining the operation of the present invention.

Figure 5 illustrates the manner in which intermediate satellites can be detected.

Figure 6 is a timing diagram useful for explaining the test pattern used to detect the upper cardinal point.

Referring to Figure 1, there is shown a series of nozzles. The nozzle 10 emits a jet of ink 12. A nozzle drive voltage is applied, which voltage causes the jet to break up into a series of separate drops 14. Smaller drops, referred to in the art as satellites, form between the drops 14. The satellites 16 behave in a manner that is a function of the energy supplied to the nozzle (measured as the nozzle voltage).

Referring to Figure 1, when the acoustic energy supplied to the ink jet is low, the natural behavior of the satellites is to form independently of the drops and then fall back and mix with the subsequent drops. This is called backward mixing satellites or slow satellites and is shown in Figure 1A. The fall back and mixing occurs in about ten drop periods depending on the physical parameters of the ink (viscosity, surface tension, density, etc.).

As the nozzle drive is increased, a point referred to herein as C(L) will occur. This term denotes a lower cardinal point. Cardinal is a term borrowed from optics terminology where it denotes an important point of a lens system, i.e. a focal point, nodal point or principal point. For the purposes of the present description, C(L) is an important point because it represents the point at which the satellites separate from the preceding and following drops simultaneously (compare Figure 1D). Surface tension forces pull these satellites forward and backward with equal force. The result is that the satellites remain at a midpoint or intermediate point between the drops as they move through space. It is this condition, referred to as C(L), that can be detected at a downstream point by detecting the satellites and the drops. At point C(L) there will be a doubling of the normal drop frequency that can be detected. In all other cases the satellites will have mixed with either the preceding or following drops. Suitable detectors are shown and described in connection with Figure 5 of this disclosure.

All nozzles used in inkjet printing systems are designed to produce such intermediate satellites that do not mix either front or back. The point C(L) is detected by the frequency doubling when the jet propulsion energy is increased from a low level to a level just adequate to produce intermediate satellites.

In one embodiment of a detector of Figure 5, a suitable test signal is placed on a charging electrode so that both the drops and the intermediate satellites are charged. The drop frequency detected will double when intermediate satellites are present and pass the sensor. Alternatively, an optical detector can be used which does not require charging of the drops and the satellites but will detect a doubling of the number of drops passing the detector.

In any case, the detector is positioned at a sufficient distance downstream from the nozzle opening to allow the satellites to mix.

In addition to a lower cardinal point C(L), most inkjet nozzles also form what may be referred to as an upper cardinal point C(H). This point can be observed by slowly increasing the energy to the nozzle and observing the point of drop separation. As the energy to the nozzle is increased from a low level (Figure 1A), the drop separation point, designated S, moves further toward the nozzle until it reaches its minimum distance from the nozzle (Figure 1G). This is referred to as the upper cardinal energy point C(H). Subsequently, the break point moves further away from the nozzle (Figure 1H). This folding back or reversal can be detected by appropriate circuitry and software. A description of the circuitry and method for detecting the upper cardinal point C(H) is provided in conjunction with a description of Figure 3.

First, however, referring to Figure 4, there is shown a graph showing the properties of a typical ink used in an ink jet printing system. This ink, manufactured by the assignee of the present invention and designated 16/8200, was used with a nozzle of the type described in U.S. Patent No. 4,727,329, which patent is expressly incorporated by reference. The cross-hatched area on the graph represents nozzle drive voltages that produce good quality printing over a temperature range of about 4.44°C to 43.33°C (40°F to 110°F). The upper and lower cardinal energy points C(L) and C(H) are also plotted for the same nozzle and ink composition. From this information it is possible to calculate a voltage value V(calc) from the following equation:

V(calc) = alpha [C(L) + C(H)] /2 GL 1,

where alpha is a function of the ink, which is described below.

Values for V(calc) calculated from the above equation are plotted in Figure 4. These values of V(calc) all lie within the cross-hatched area of the graph and represent nozzle drive voltages that produce quality printing.

Referring to Figures 1 and 3, a circuit suitable for carrying out the invention will be described. The nozzle 10 is connected to an ink source 32 via an ink line 34. The ink stream is grounded between the ink source and the nozzle 36. Acoustic energy is applied to the nozzle by, for example, a piezoelectric device as disclosed in the aforementioned U.S. Patent 4,727,379. The drive voltage for the piezoelectric device is provided by a nozzle drive amplifier 38 via a line 40. The amplifier, in turn, is controlled by a processor 42 such as a microcomputer via a digital-to-analog (D/A) converter 44. The controller 42 also operates a charge amplifier 44 via D/A 46 to control the voltage applied to the charge tunnel 48. As is well known in the art, the charge tunnel 48 is located downstream of the nozzle 10 in the region where the drops are to form as the stream of ink breaks up into droplets and satellites. In this way, selected drops can be charged for discharge to a substrate or, if left uncharged, returned to the ink source 32 via a gutter.

In accordance with the present invention, the controller 42 receives input signals from a capacitive pickup 50 downstream of the charge tunnel. The signal from the pickup 50 is fed to a preamplifier 52 and a bandpass filter 54 (a notch filter designed to pass a frequency equal to or twice the normal drop frequency of the ink jet system). Thus, the capacitive pickup 50 detects the point C(L) where the drop frequency has doubled due to the presence of intermediate satellites (Figure 1B). The signal, thus analog, is passed through the filter 54 to a comparator 56 which provides a digital output when the input exceeds a threshold. This signals to the controller that C(L) has been detected. The controller thus stores the corresponding nozzle drive voltage value.

The second input to the controller 42 of interest provides a signal indicative of the occurrence of C(H), i.e. the turning point shown in Figure 1G. This signal is produced on a line 58 by a pickup 60 in electrical communication with the electrically conductive ink stream. The output of the pickup 60 is fed to an integrated preamplifier 62 which in turn is fed to a comparator 64. As will be described, a digital output on line 58 is fed to the controller when the charge on the capacitor associated with the preamplifier 62 exceeds a threshold set for the comparator 64.

To understand the operation of comparator 64, it is necessary to refer to Figures 1, 3 and 6. To determine C(H), test signals are placed on charge tunnel 48 for a period equal to 30 drop times. For example, the signal labeled test video 0 in Figure 6. The waveform shown in Figure 6 is referred to as the drop clock waveform, 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 that is formed as the droplets break off from the ink stream. During this period, the pickup 60 will sense whether or not the drops are sufficiently charged. For each drop that is charged, an incremental charge is stored on the capacitor associated with the preamplifier 62. When most of the drops have been sufficiently charged by the test video signal, the voltage from the preamplifier will exceed the threshold set on the comparator 64, signaling this to the controller. The sequence is then repeated for test video signals 1, 2, and 3, all of which are shown in Figure 6. Each test pattern is one quarter lambda out of phase from the previous test pattern (where lambda is the droplet spacing). As a result, it is possible to accurately determine the position (in quarter lambdas, for example) of the droplet break point relative to the positions of the two cardinal points.

The result of this operation is shown in Figure 1, where for each of Figures 1A through H a four bit binary code is shown representing the results of applying test video signals 0 through 3. Thus, for example, referring to Figure 1B, test video 1 and test video 2 are digital, while test video 0 and test video 3 are zero, indicating that the latter two test videos did not result from charging the droplets (this is due to the phase of the test video signals relative to the drop clock).

As the nozzle drive voltage increases, the pattern of successfully charged drops changes as indicated in Figure 1 in a predictable sequence based on the phase relationship of the test video signals. However, at the cardinal point C(H), a first phase reversal occurs (additional phase reversals may occur at higher drive voltages). That is, instead of the expected phase pattern 1001 for Figure 1H, the pattern 0110 is observed, which pattern is exactly the same as Figure 1F. Thus, the circuit detects exactly C(H) as the first fold-back point where droplet break-off occurs within the charge tunnel 48 at the minimum distance from the nozzle.

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

From inspection of Figure 6, it will be seen that the four test video signals have a pulse width of approximately 66% of the drop time and that each test video signal is one-quarter drop time out of phase with every other test video signal. The phase sequence ends after the output of the comparator for the four video test signals has been recorded.

As can be seen from Figure 1, the drop separation point occurs earlier (closer to the nozzle) as the voltage increases. This is detected by the detector as indicated by the pattern of those marching (and wrapping) from right to left in Figures A to G. This continues until the fold-back point C(H) where the sequence reverses itself and the detector signals this voltage value to the controller.

While the embodiment of Figure 3 shows separate sensors for C(L) and C(H), it will be recognized by those skilled in the art that the capacitive sensor 50 can be used for both purposes. That is, the sensor 50 can sense the C(L) value and by connecting the preamplifier 62 and the comparator 64 to the capacitive sensor, it can also sense C(H). Thus, it is not necessary to use a separate sensor 60 downstream of the nozzle since the capacitive sensor 50 downstream of the charge tunnel can perform both functions if desired.

It will be appreciated by those skilled in the art that if a separate sensor 60 is used to detect C(H), it is then possible to use an optical or an acoustic sensor instead of the capacitive sensor 50 to detect C(L). The advantage of using an optical or an acoustic sensor is that the drops do not need to be charged in order to be detected.

Once the controller has received the information necessary to determine C(L) and C(H), it uses Equation 1 to calculate V(calc). Figure 2 illustrates a software flow chart suitable for performing the calculations according to the present invention. It is important to note that knowledge of the ink temperature is not required for determining an appropriate nozzle drive voltage.

The determination of the cardinal points will be described with reference to Figure 2. The controller 42 sets the charge tunneling voltage to a constant value in the case where a capacitive pickup is used. It sets then the jet drive voltage is increased slowly over line 40 and the capacitive pickup is checked to see if frequency doubling has occurred. If not, the frequency is increased in small increments until frequency doubling is detected. As previously indicated, frequency doubling indicates the condition in which non-merging intermediate satellites are being formed. When frequency doubling is detected, the jet drive voltage value is recorded as C(L).

The controller then begins the phase control portion of its routine to detect C(H). The test video signals shown in Figure 6 are applied to the charge tunneling 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. The software then checks to detect whether or not a phase reversal has occurred. If not, the nozzle drive voltage is increased in small increments until a phase reversal is detected. Upon such detection, the nozzle drive voltage C(H) is recorded.

Upon obtaining the values for C(H) and C(L), the value V(calc) is calculated. This value V(calc), shown in Figure 4, is in the middle of the desired operating range of the system and is thereafter used as the nozzle drive voltage. In summary, this operation can be stated as follows:

I. A. Assuming a detector for an electric charge, we begin by applying a constant charge voltage to the charging electrode (charge tunnel).

B. Slowly increase the applied jet driver voltage from a low level, i.e. below 9 volts, in a sine wave fashion, peak to peak.

C. Observation of the downstream detector for a frequency twice the drop frequency, i.e. search for intermediate satellites.

D. Once twice the frequency has been acquired, record the voltage level as the lower cardinal energy point C(L).

II. A. Switch to the phasing system and apply successive phasing voltages to the charge electrode.

B. Observe the successive direction of "good" phase (in our example, "1"s) while increasing the nozzle drive voltage.

C. Record the nozzle voltage as C(H) when the direction or sequence of the good phase reverses.

D. Calculate the appropriate drive voltage from Equation (1) for the ink and apply it to the Nozzle.

Referring again to equation 1, it will be noted that the calculation of the value V(calc) requires that a value alpha be specified which is ink dependent. This value alpha can be determined as follows. Since the good printing area lies between the upper and lower cardinal energy points (see Figure 4), an acceptable solution would be to set alpha = 1. This would place V(calc) midway between C(L) and C(H), however some additional Tolerances can be gained by choosing slightly smaller or slightly larger values. A smaller alpha would decrease V(calc) and a larger alpha would increase V(calc). It is desirable to adjust alpha for each ink to optimize the print range. This can be done simply by calculating V(calc) for a specific alpha and plotting the results on a graph representing the desirable range for a particular ink. In other words, alpha can be optimized empirically for any ink composition.

Claims (12)

1. A control circuit for determining the magnitude of the excitation voltage applied to the nozzle of an ink jet printer to break up an ink stream into droplets, comprising sensing means for determining the value of the excitation voltage while slowly increasing its magnitude from a minimum value and sensing the value C(H) of the excitation voltage at which droplet formation occurs closest to the nozzle, characterized in that the sensing means (50, 52, 54, 56) is arranged to sense the value C(L) of the excitation voltage at which the droplet frequency doubles, and in that computing means (42) is provided for receiving as inputs the values C(L) and C(H) and computing therefrom the magnitude of the excitation voltage to be used for printing. 2. A control circuit according to claim 1, characterized in that the detection means comprises a capacitive pickup (50) downstream of the nozzle (10) for detecting the electrically charged droplets. 3. A control circuit according to claim 1, characterized in that the detection means comprises an optical detector arranged downstream of the nozzle (10) for detecting the droplets passing the detector. 4. A control circuit according to claim 2 or 3, characterized in that the detecting means further comprises circuit means (52, 54, 56) for supplying an output signal to the calculating means (42) when the detected droplet frequency doubles. 5. A control circuit according to any preceding claim, characterized in that means (42, 44, 46, 48) are provided for applying an electrical test pattern to the droplet, the patterns varying in phase relative to the droplet timing so that only some of the test patterns successfully charge the droplet, and wherein the detecting means comprises a sensor (50) for detecting which of the droplets have been charged, and the calculating means (42) comprises means for determining the C(H) value from the change in the sequence of charging patterns. 6. A control circuit according to claim 5, characterized in that the means (42, 44, 46, 48) for applying the test patterns to the droplets comprises a charge amplifier (44) and a charge tunnel (48) positioned below the nozzle (10) in the region of droplet formation. 7. A control circuit according to any preceding claim, characterized in that means (38, 40, 42, 44) are provided for slowly increasing the excitation voltage from a minimum value. 8. A control circuit according to any preceding claim, characterized in that the calculating means (42) is arranged to calculate the magnitude of the excitation voltage to be used for printing according to the equation: V(CALC) = alpha [C(L) + C(H)]/2, where alpha is an ink related value. 9. A method for determining the excitation voltage to be applied to the nozzle of an inkjet printer to break up an ink stream into droplets for printing, comprising (a) that the excitation voltage is slowly increased from a minimum value, (b) that the existing voltage is detected, (c) that the value C(H) of the excitation voltage at which droplet formation first occurs closest to the nozzle is detected and recorded, and (d) that the value of C(H) is used to calculate the value V(calc) of the excitation voltage to be used for printing, characterized in that in addition, the value C(L) of the existing voltage at which the droplet frequency doubles due to the formation of non-mixing intersatellite droplets is detected and that the value V(calc) of the excitation voltage to be used for printing is calculated using the equation: V(CALC) = alpha [C(L) + C(H)]/2, where alpha is an ink related value. 10. A method according to claim 9, characterized in that the value C(L) is sensed by charging the ink droplets and by sensing the charges on the droplets sufficiently downstream from the nozzle to eliminate the presence of intermixing satellite droplets. 11. A method according to claim 9, characterized in that the value C(L) is detected by optically detecting the ink droplets sufficiently downstream of the nozzle to eliminate the presence of intermixing satellite droplets. 12. A method according to any one of claims 9 to 11, characterized in that the value C(H) is detected by (i) electrical test patterns are applied to the ink droplets so that the patterns vary in phase relative to the ink droplet timing, whereby only some of the test patterns will successfully charge the droplets, (ii) detecting which droplets have been successfully charged, and (iii) the value C(H) is determined from the change in the sequence of the charging patterns.