Dynamic modulation voltage adjustment

Abstract

A continuous inkjet printer (50) has a drop generator (52) and a controller (58) that varies a modulation voltage applied to the drop generator (52) and measures a corresponding break-up time for ink jets to become drops to obtain a portion of a characteristic of break-up time versus modulation voltage. The controller (58) obtains a range of variation in which the modulation voltage is varied to obtain the portion of the characteristic, calculates a gradient from the portion of the characteristic, compares the gradient to a predetermined gradient, and generates an adjusted range of variation that is shifted in a first direction or an opposite second direction, respectively, relative to the range of variation depending on whether the calculated gradient is less than or greater than the predetermined gradient.

Description

Technical Field

[0001] This invention relates to a continuous inkjet (CIJ) printer, a method of operating a CIJ printer, and a computer program that can be executed by the CIJ printer to implement this method. Background Technology

[0002] CIJ printers typically include: a droplet generator comprising an electromechanical transducer; a driver for applying a periodic (typically sinusoidal) modulated voltage to the transducer to cause the ink jet ejected by the droplet generator to break into droplet streams at a break-off time after ejection from the droplet generator; and a controller for establishing the operating value of the modulated voltage.

[0003] CIJ printers typically also include charging electrodes for applying charge to selected ink droplets, electrostatic deflection plates for deflecting charged droplets onto the printing substrate, and charge detectors for measuring the charge applied to the ink droplets.

[0004] A charge detector is used for "phase fixing," where a phase-fixing signal sequence smaller than the charging signal used to charge the ink droplet (to be used for printing) (e.g., half the duration of the charging signal) is applied to the charging electrode with an increasing phase shift relative to the modulation voltage. By measuring the charge generated on the ink droplet, the optimal phase relationship between the charging signal applied to the charging electrode and the modulation voltage can be identified.

[0005] It should be understood that the optimal phase relationship depends on the break time, and an indication of the break time can be obtained from the optimal phase relationship. However, the determination of the optimal phase relationship, i.e., phase determination, can only be performed using ink droplets not required for printing.

[0006] EP 0 386 049 discloses a CIJ printer that can obtain a portion of a break time parameter indicating the characteristics of the break time relative to the amplitude of the modulation voltage, identify the amplitude of the modulation voltage corresponding to a point having a predetermined gradient on the portion, and establish an operating value that is offset from the identified amplitude of the modulation voltage by a predetermined amount.

[0007] EP 0 386 049 teaches that the operating value of the modulation voltage amplitude can be established every 2 to 10 minutes for the initial warm-up and stabilization phases, and at a lower frequency, typically every 30 minutes to 2 hours, once the operating conditions have stabilized.

[0008] EP 2 209 636 relates to a similar CIJ printer, the difference being that it establishes an operating value corresponding to the modulated voltage amplitude of a point with a predetermined gradient on that part.

[0009] EP 2 209 636 suggests tracking the amplitude of the modulation voltage between printing characters or images onto an object, or after a batch of images or characters has been printed. It does not explain how such tracking can be achieved when there are relatively few ink droplets not required for printing, as is the case when using a CIJ printer on a fast-moving production line to print on closely spaced products. Summary of the Invention

[0010] According to a first aspect of the invention, a continuous inkjet printer is provided, comprising: a droplet generator including an electromechanical transducer; a driver operable to apply a periodic modulation voltage to the transducer to cause an ink jet ejected by the droplet generator to break into droplet streams at a break-time after ejection from the droplet generator; and a controller operable to change a modulation voltage parameter of the modulation voltage and measure a corresponding value of a break-time parameter indicating the break-time to obtain a portion of a characteristic of the break-time parameter relative to the modulation voltage parameter, wherein the controller is operable to: obtain a range of variation; change the modulation voltage parameter within the range of variation to obtain a portion of the characteristic; calculate a gradient from the portion of the characteristic; compare the calculated gradient with a predetermined gradient; and, if the calculated gradient is less than the predetermined gradient, generate an adjusted range of variation shifted relative to the range of variation in a first direction, or, if the calculated gradient is greater than the predetermined gradient, generate an adjusted range of variation shifted relative to the range of variation in a second direction opposite to the first direction.

[0011] The present invention provides a CIJ printer that can adjust modulation voltage parameters in response to changes in printer operating conditions (such as ambient temperature, humidity, or ink viscosity) without waiting for the printing operation (such as printing characters or images onto an object, or printing a batch of images or characters) to complete.

[0012] Preferably, the electromechanical transducer is a piezoelectric element and / or the modulation voltage parameter is the amplitude of the modulation voltage.

[0013] Preferably, the controller is operable to calculate the gradient from the portion of the characteristic by identifying the line that best fits the portion and calculating the gradient of the best-fit line.

[0014] While it is conceivable that the break-off time parameter could be the break-off distance, i.e., the distance from the nozzle of the ink droplet generator when the ink droplet separates from the ink jet, or the phase shift between the modulation voltage and the signal used to apply charge to the ink droplet, preferably, the break-off time parameter is a break-off time that at least indicates the time after the ink ejected from the nozzle of the ink droplet generator separates into ink droplets.

[0015] Preferably, the controller is operable to store the adjusted range of variation in the printer's range of variation memory, and to obtain the range of variation from the range of variation memory.

[0016] The controller therefore operates iteratively, obtaining a range of variation from the range of variation memory, which has been generated by the previous iteration, in each iteration, obtaining a portion of the characteristic indicated by the range of variation, calculating the gradient from the portion of the characteristic, comparing the calculated gradient with a predetermined gradient, generating an adjusted range of variation, and storing the adjusted range of variation in the range of variation memory for use in the next iteration.

[0017] Clearly, the controller operates continuously to change the modulation voltage parameter at a point on a characteristic with a predetermined gradient, which changes in response to variations in the printer's operating conditions.

[0018] The range of variation can be a constant value, such as 10V.

[0019] However, preferably, the range of variation has a portion that varies with the characteristics of the modulation voltage parameters.

[0020] Preferably, the range of variation increases with the magnitude of the modulation voltage amplitude of the component constituting the characteristic.

[0021] For example, for a modulation voltage amplitude of around 50V, the range of variation may be 7V, while for a modulation voltage amplitude of around 150V, the range of variation may be 12V.

[0022] Figure 1 The first characteristic 10 and the second characteristic 12 of the break time (y-axis) relative to the modulation voltage amplitude (x-axis) for the corresponding first and second operating conditions of the printer are schematically shown.

[0023] Reference numeral 14 denotes a portion of the first characteristic 10, wherein the magnitude of the gradient of the characteristic decreases from a first positive value to zero, and then increases from zero to a second negative value. Satisfactory printer operation can be expected along a sub-portion 16 of portion 14 of the first characteristic 10, wherein the magnitude of the gradient decreases from a first positive value to a second non-zero positive value. Reference numerals 18 and 20 denote corresponding portions and sub-portions of the second characteristic 12, respectively.

[0024] It can be seen that part 14 corresponds to a smaller modulation voltage amplitude and a smaller range of modulation voltage amplitudes than part 18. That is, under the operating conditions of the first characteristic 10, a relatively low modulation voltage amplitude is required, and a relatively small change in the modulation voltage amplitude is required to obtain a given change in the break time, while under the operating conditions of the second characteristic 12, a relatively high modulation voltage amplitude is required, and a relatively large change in the modulation voltage amplitude is required to obtain a given change in the break time.

[0025] By increasing the amplitude of the modulated voltage of the constituent parts, the range of variation is increased, enabling the controller to operate over a wide range of printer operating conditions.

[0026] The driver may advantageously include a digital-to-analog converter (DAC). As is common to these devices, the DAC has a non-linear response, such that a given increase in the binary input value of the DAC will not produce the same increase in the DAC output voltage across the DAC's operating range.

[0027] Therefore, preferably, the controller is configured to apply a binary input signal to the DAC, the binary input signal being selected from a subset of a set of binary input signals that can be received by the DAC, the subset being selected such that the output voltages generated by the DAC in response to the binary input signal differ from each other by approximately equal amounts.

[0028] Preferably, the subset is selected such that the output voltages generated by the DAC in response to the binary input signal differ from each other by an amount as close as possible to 1V.

[0029] When the controller is configured to apply a subset of binary input signals to the DAC, the impact of the DAC’s nonlinear response on the controller’s operation is reduced.

[0030] The printer may advantageously include a memory containing a lookup table of the range of sizes of portions of a specified characteristic.

[0031] Preferably, each entry in the lookup table includes a value for the modulation voltage parameter and an indication of those lookup table entries in which the value of the modulation voltage parameter constitutes a part of the characteristic.

[0032] Preferably, each entry in the lookup table includes the modulation voltage amplitude.

[0033] Preferably, each entry in the lookup table includes an index number identifying the entry, a value of the modulation voltage parameter, and a first index number and a second index number for the corresponding first and second terminals of the characteristic portion, wherein the characteristic portion is defined by the value of the modulation voltage parameter of the lookup table entry identified by the first index number and the second index number.

[0034] Preferably, the entries in the lookup table are sorted according to the values ​​of the modulation voltage parameters, and each entry in the lookup table includes indications of the first and second entries of the corresponding first and second ends that define a portion of the lookup table, wherein the values ​​of the modulation voltage parameters of the lookup table entries constitute a portion of the characteristics.

[0035] For each value of the modulation voltage parameter, the controller is advantageously operable to measure the corresponding break time parameter value at least twice and calculate the average break time parameter value from the measured break time parameter values.

[0036] In this way, the impact of any inaccurate measurement of the fracture time parameter value on the gradient calculated from the part of this characteristic is reduced.

[0037] As described above, although the CIJ printer of the present invention is able to change the value of the modulation voltage parameter at any time (even during the jetting of the sequence of ink droplets used for printing), it can only measure the break-time parameter value when it is not printing, because the ink droplets used to measure the break-time parameter value cannot be used for printing.

[0038] CIJ printers typically print messages as a series of strokes on the product being printed on the production line, with ink droplets not used for printing between strokes and messages ejected from a droplet generator. The ink droplets ejected from the droplet generator between strokes and messages can be used for phasing and therefore for measuring break-time parameter values.

[0039] It should be understood that the faster the production line moves, the fewer droplets are ejected between strokes and messages, and the fewer opportunities there are for phasing and measuring breakage time parameters.

[0040] Therefore, advantageously, the printer can be configured to receive an input indicating the production line speed, and to change the number of times the value of the break time parameter, which corresponds to the value of the modulation voltage parameter, is measured according to the input.

[0041] Thus, when the printer is used to print on products on a production line moving at a relatively slow speed, and there are a large number of ink droplets available for measuring the break-time parameter value between strokes and messages, the printer can perform a relatively large number of measurements and average the measured values, and the impact of any inaccurate measurements is correspondingly reduced. However, when the printer is used to print on products on a production line moving at a relatively fast speed, and there are relatively few ink droplets available for measuring the break-time parameter value between strokes (if any) and messages, the printer can perform a smaller number (or even just a single) of measurements of the break-time parameter value, but there is a risk that any inaccurate measurements will affect the gradient calculation of the characteristic portion.

[0042] According to a second aspect of the present invention, a method for operating a continuous inkjet printer is provided, the method comprising: applying a periodically modulated voltage to an electromechanical transducer of an ink droplet generator to cause an ink jet ejected from the ink droplet generator to break into ink droplets at a break-time after ejection from the ink droplet generator; changing a modulation voltage parameter of the modulation voltage and measuring a corresponding value of a break-time parameter indicating the break-time to obtain a portion of the characteristic of the break-time parameter relative to the modulation voltage parameter, wherein the method comprises: obtaining a range of variation; changing the modulation voltage parameter within the range of variation to obtain a portion of the characteristic; calculating a gradient from the portion of the characteristic; comparing the calculated gradient with a predetermined gradient; if the calculated gradient is less than the predetermined gradient, generating an adjusted range of variation shifted in a first direction relative to the range of variation, or, if the calculated gradient is greater than the predetermined gradient, generating an adjusted range of variation shifted in a second direction opposite to the first direction relative to the range of variation.

[0043] According to a third aspect of the invention, a computer program is provided that can be executed by a continuous inkjet printer to cause the printer to implement the method of the second aspect of the invention. Attached Figure Description

[0044] The invention will now be described by way of example with reference to the accompanying drawings, in which:

[0045] Figure 1 This is a schematic diagram showing the characteristics of the break time relative to the modulation voltage amplitude under two operating conditions for a CIJ printer.

[0046] Figure 2 This is a schematic diagram of the CIJ printer according to the present invention;

[0047] Figure 3A and Figure 3B yes Figure 2 A flowchart of the operation method for the CIJ printer; and

[0048] Figure 4 It is by Figure 2 The CIJ printer uses a lookup table portion of the computer program it executes. Detailed Implementation

[0049] Figure 2 The CIJ printer 50 includes: a droplet generator 52 comprising an electromechanical transducer in the form of a piezoelectric element 54; and a driver 56 comprising a digital-to-analog converter (DAC) and a power amplifier, operable to apply a sinusoidal modulated voltage to the piezoelectric element 54, causing the ink jet ejected by the droplet generator 52 to break into a stream of droplets at a break-time after ejection from the droplet generator 52. A controller in the form of a programming computer 58 is operable to control the DAC of the driver 56 to vary the amplitude of the modulated voltage applied to the piezoelectric element 54.

[0050] The CIJ printer 50 also includes a charging electrode 60 for applying charge to selected ink droplets generated by the droplet generator 52, a pair of electrostatic deflection plates 62 for deflecting charged ink droplets, a charge detector 64, and a groove 66 for collecting ink droplets not used for printing.

[0051] In the use of the CIJ printer, ink droplets passing through the charging electrode 60 for printing are charged according to a charging signal applied to the charging electrode 60 by a computer 58. The charged ink droplets are deflected away from the lower deflector plate toward the upper deflector plate 62 and impact the substrate 68, which is typically a part of the packaging of the product moving along the production line.

[0052] Uncharged ink droplets continue to pass undeflected through charge detector 64 into slot 66, from where they return to the ink system (not shown) of printer 50.

[0053] The CIJ printer 50 includes an input 70 for connecting to the conveyor belt of the production line to provide the printer with an indication of the speed of the production line movement.

[0054] The phasing of printer 50 occurs when the printer is started and occurs intermittently during printer operation, typically once every 30ms, unless the printing operation prevents it from happening.

[0055] In the phasing operation, a sequence of eight ink droplets ejected from the droplet generator is charged by a phasing signal applied to the charging electrode 60. Sixteen sequences of the eight ink droplets are charged in such a manner that the phasing signal used to charge each sequence of the eight ink droplets is delayed by one-sixteenth of the period of the modulation voltage relative to the previous sequence of the eight ink droplets.

[0056] The phasing signal causes the charge applied to the ink droplet used for phasing to be less than the charge applied to the ink droplet used for printing. The ink droplet used for phasing is not significantly deflected by the deflection plate 62 and passes through the charge detector 64 into the slot 66.

[0057] The phase shift of the phase-determining signal that generates the maximum charge on the sequence of eight ink droplets is used to determine the phase shift of the charging signal relative to the modulation voltage of the ink droplets used for printing. That is, the phase shift of the phase-determining signal that generates the maximum charge detected on the ink droplets used for phase determination is also used to apply the charging signal to the ink droplets used for printing.

[0058] The fracture time parameter, which indicates the fracture time, is measured as follows.

[0059] When the first phase signal in the sequence of eight phase signals is applied to the charging electrode 60, the clock is started. As the eight charged ink droplets approach, pass through, and move away from the charge detector 64, the charge detector 64 generates an output voltage that swings from the initial value in a first direction, reaches a peak value, and swings back through the initial value in the opposite second direction before reaching the peak value and swinging back to the initial value.

[0060] The clock is stopped when the output voltage of charge detector 64 is monitored and exceeds the threshold voltage after the output voltage swings back through the initial value in the opposite second direction.

[0061] Clock count is considered a break time parameter that indicates the break time.

[0062] As described above, by changing the amplitude of the modulation voltage applied to the piezoelectric element 54 and storing a clock count between the first phase signal and the output voltage of the charge detector across the threshold voltage, the printer obtains a break time relative to the amplitude characteristics of the modulation voltage.

[0063] The printer identifies points on the characteristics with a predetermined gradient (e.g., 0.2 μs / V), identifies the modulation voltage amplitude of the characteristics at that point, and sets the modulation voltage amplitude to an initial value that is approximately 7V less than the identified modulation voltage amplitude.

[0064] This is the automatic modulation process described in EP 0 386 049, the disclosure of which is incorporated herein by reference as if disclosed in this specification.

[0065] The operation of the printer 50 described so far is largely routine.

[0066] The operation method of the CIJ printer of the present invention is as follows.

[0067] In the following description, the term "modulation voltage amplitude operating point" will be abbreviated as "voltage operating point".

[0068] The description includes numerical examples to illustrate how the printer operates.

[0069] refer to Figure 3A The initial voltage operating point has been established in step 100 through automatic modulation. In step 102, computer 58 executes the computer program of the present invention to identify lookup table entries including the initial voltage operating point. A portion of the lookup table is displayed. Figure 4 In the figure, it is indicated by reference numeral 200.

[0070] The lookup table is a list of printer voltage operating points, each identified by a lookup table entry index number. Figure 4 Part 200 of the lookup table shown displays lookup table entry index numbers 165 to 185.

[0071] As a numerical example, if the initial voltage operating point is 143.3V, then computer 58 finds a lookup table entry that includes the voltage operating point value of 143.3V. This lookup table entry has... Figure 4 The index number 174 is shown in part 200 of the lookup table. Clearly, if the index number is passed along with the initial voltage operating point value from steps 100 to 102, the computer can find the lookup table entry index number 174 using its index number, rather than the computer looking up the voltage operating point value.

[0072] In addition to the index number, each lookup table entry also has fields for DAC level, voltage operating point value, low index number of test point, high index number of test point, and buffer size.

[0073] As from Figure 4 It can be seen that for lookup table entry index number 174 (represented by reference numeral 210 in the attached figure), the value of the DAC level field is 811, the value of the voltage operating point field is 143.3V, the values ​​of the test point low index number and test point high index number fields are 167 and 181 respectively, and the value of the buffer size field is 15.

[0074] In step 104, the computer 58 stores the break time corresponding to the initial voltage operating point in the computer's buffer memory and obtains the test point low index number 167 and test point high index number 181 from lookup table entry index number 174. (The break time corresponding to the initial voltage operating point is obtained during the automatic modulation process.)

[0075] At step 106, computer 58 searches for the next voltage operating point that is lower than the current voltage operating point.

[0076] Returning to the numerical example, for an initial voltage operating point of 143.3V (as specified in lookup table entry index number 174), the next voltage operating point below the current voltage operating point is 142.5V (as specified in lookup table entry index number 173).

[0077] The computer applies a DAC level value, which includes a lookup table entry for the next voltage operating point below the current voltage operating point, to the DAC of driver 56. This causes a modulation voltage (with the amplitude of the next voltage operating point below the current voltage operating point) to be applied to the voltage operating point of the piezoelectric element 54 of droplet generator 52.

[0078] In the numerical example, the computer applies DAC level 813, specified by lookup table entry index number 173, to the DAC, which results in a modulation voltage of 142.5V being applied to the piezoelectric element 54.

[0079] Phase fixing is performed as described above to identify the phase shift of the phase fixing signal relative to a modulation voltage that maximizes the amplitude of the charge on the ink droplet used for phase fixing. The break-off time corresponding to the amplitude of the modulation voltage is measured as described above.

[0080] Depending on the speed of the production line using the printer, the break time can be adjusted and measured multiple times, and the break time is calculated from the average of the break time measurements to reduce the impact of any inaccurate break time measurements. The break time is stored in the buffer memory of the computer 58.

[0081] At step 108, computer 58 determines whether the current voltage operating point is the voltage operating point specified by the lookup table entry identified by the test point's low index number. If not, the operation returns to step 106.

[0082] In the numerical example, the computer determines whether the current voltage operating point is the voltage operating point specified by lookup table entry index number 167, which is 137V, because lookup table entry index number 167 is specified by lookup table entry index number 174, which contains an initial voltage operating point value of 143.3V.

[0083] Otherwise, at step 110, the computer clears all break times from the buffer memory except for the break time corresponding to the current voltage operating point.

[0084] In the numerical example, the break time stored in the buffer memory corresponds to the break time at the voltage operating point of 137V.

[0085] Surprisingly, the break times between the voltage operating points corresponding to the initial voltage operating point and the voltage operating point specified by the lookup table entry identified by the low index number of the test point were measured and stored in a buffer, and then immediately cleared from the buffer. This was done to avoid large changes in the voltage operating point, which would occur if the modulated voltage amplitude changed in one step from the initial voltage operating point to the voltage operating point specified by the lookup table entry through the low index number of the test point. Such large changes could lead to unstable printer operation.

[0086] The computer sets a direction variable to indicate the value of the increase in the modulation voltage amplitude, which will be used to fill the buffer memory with the break time.

[0087] The computer searches for the next voltage operating point that is higher than the current voltage operating point.

[0088] Returning to the numerical example, the current voltage operating point, specified by lookup table entry index number 167 (identified by the lower index number of the test point), is 137V. The next voltage operating point above the current voltage operating point is 138V (as specified in lookup table entry index number 168).

[0089] The computer applies a modulated voltage with an amplitude higher than the current voltage operating point to the piezoelectric element 54 and performs phasing and measurement of the break time at the new voltage operating point. The computer stores the break time in a buffer memory.

[0090] At step 112, computer 58 searches for the next voltage operating point higher than the current voltage operating point.

[0091] Next, the computer applies a modulated voltage with the amplitude of the next voltage operating point to the piezoelectric element 54 and performs phasing and measurement of the break time at the new voltage operating point. The computer stores the break time in a buffer memory.

[0092] At step 114, the computer determines whether the current voltage operating point is the voltage operating point of the lookup table entry identified by the high index number of the test point. If not, the operation returns to step 112.

[0093] In the numerical example, the computer determines whether the current voltage operating point is the voltage operating point specified by lookup table entry index number 181, which is 149.1V, because lookup table entry index number 181 is specified by lookup table entry index number 174, which contains an initial voltage operating point value of 143.3V.

[0094] Otherwise, at step 116, the computer calculates the best-fit line for the break time stored in the buffer memory.

[0095] In the numerical example, the buffer memory contains the break times corresponding to the voltage operating points specified by lookup table entry indices 167 to 181.

[0096] The computer calculates the gradient of the best-fit line.

[0097] At step 118, the computer compares the calculated gradient with a predetermined gradient of 0.2 μs / V.

[0098] If the gradient of the best-fit line is greater than the predetermined gradient, it indicates that the printer's operating conditions (such as ambient temperature, humidity, or ink viscosity) have changed, thus lengthening the break-time of the characteristic relative to the modulation voltage amplitude along the x-axis, meaning the characteristic has deviated from its intended path. Figure 1 The first characteristic 10 shown changes toward the second characteristic 12. If the gradient of the best-fit line is greater than a predetermined gradient, the operation proceeds to step 120, as follows. Figure 3A As stated above.

[0099] If the gradient of the best-fit line at step 118 is less than the predetermined gradient, it indicates that the break-time characteristic relative to the modulation voltage amplitude has been compressed along the x-axis, meaning the characteristic has deviated significantly from its intended direction. Figure 1The second characteristic 12 shown changes toward the first characteristic 10. If the gradient of the best-fit line is less than a predetermined gradient, the operation proceeds to step 150, as follows (see reference below). Figure 3B As stated above.

[0100] First, the case where the gradient of the best-fit line is greater than the predetermined gradient is handled. In step 120, the computer 58 searches for the next voltage operating point that is higher than the initial or current reference voltage operating point, depending on the situation.

[0101] In a numerical example, the next voltage operating point after the initial operating point of 143.3V, as specified by lookup table entry index number 175, is 144.1V.

[0102] The computer stores the next voltage operating point that is higher than the initial reference voltage operating point or the current reference voltage operating point as the reference voltage operating point.

[0103] It is important to emphasize that the reference voltage operating point does not immediately determine the amplitude of the modulation voltage applied to the piezoelectric element 54. Instead, the reference voltage operating point establishes a lookup table entry where the low and high index numbers of the test point, along with the buffer size value, determine the printer's behavior until the computer selects the next reference voltage operating point.

[0104] Therefore, in the numerical example, the computer stores a reference voltage operating point of 144.1V specified by lookup table entry index number 175, which specifies the test point low index number value 168 and high index number value 182, as well as a buffer size of 15.

[0105] The computer obtains the low index number and high index number of the test point from the lookup table entry for the specified reference voltage operating point.

[0106] At step 122, the computer determines whether the direction variable indicates an increase or decrease in the modulation voltage amplitude. The direction variable at this point, indicating whether the modulation voltage amplitude is increasing or decreasing, is used to fill the break time into the buffer memory.

[0107] In the numerical example, the direction variable indicates the increase in the modulation voltage amplitude because the buffer memory is filled by storing the break times corresponding to the voltage operating points specified by lookup table entries 167 to 181. The current voltage operating point is 149.1V, which is the voltage operating point specified by lookup table entry 181.

[0108] If the direction variable indicates an increase in the modulation voltage amplitude, the operation proceeds to step 124. Otherwise, the operation proceeds to step 126.

[0109] At step 124, the computer determines whether the low index number of the test point specified by the lookup table entry containing the current reference voltage operating point is different from the low index number of the test point specified by the lookup table entry containing the initial or previous reference voltage operating point.

[0110] If the lower index number of the test point is different, the operation proceeds to step 128.

[0111] At step 128, the computer removes from the buffer any breakpoints corresponding to voltage operating points lower than those specified by a lookup table entry identified by a low index number of the test point containing the current reference voltage operating point. The operation then proceeds to step 130.

[0112] In the numerical example, the buffer memory contains break times corresponding to the voltage operating points specified by lookup table entry indices 167 to 181. The lookup table entry containing the current reference voltage operating point is entry index 175, which specifies the test point low index 168. The voltage operating point of 137V specified by lookup table entry index 167 is lower than the voltage operating point of 138V specified by entry index 168. Therefore, at step 128, the computer removes the break time corresponding to the voltage operating point of 137V from the buffer memory.

[0113] If the low index numbers of the test points are the same, the operation proceeds directly from step 124 to step 130.

[0114] At step 130, the computer determines whether the high index number of the test point specified by the lookup table entry containing the current reference voltage operating point is different from the high index number of the test point specified by the lookup table entry containing the initial or previous voltage operating point.

[0115] If the high index number of the test points is different, the operation proceeds to step 132. Otherwise, the operation returns to step 116.

[0116] At step 132, the computer searches for the next voltage operating point higher than the current voltage operating point. The computer applies a modulation voltage with an amplitude higher than the current voltage operating point to the piezoelectric element 54 and performs phasing and measurement of the break time at the new voltage operating point. The computer stores the break time in a buffer memory. The operation proceeds to step 134.

[0117] In the numerical example, the buffer memory contains breakpoints corresponding to the voltage operating points specified by lookup table entry indices 168 to 181. The lookup table entry containing the current reference voltage operating point is entry index 175, which specifies test point high index 182. Test point high index 182 specified by entry index 175 is different from test point high index 181 specified by entry index 174. Therefore, the computer looks up the next voltage operating point higher than 149.1V (i.e., 149.9V as specified by lookup table entry index 182).

[0118] The computer applies the DAC level of 795, specified by lookup table entry index number 182, to the DAC, resulting in a modulation voltage of 149.9V being applied to the piezoelectric element 54. The computer stores the break-time corresponding to the voltage operating point of 149.9V in a buffer memory, thus containing the break-time corresponding to the voltage operating points specified by lookup table entry indices 168 to 182.

[0119] At step 134, the computer determines whether the current voltage operating point is the voltage operating point specified by the lookup table entry identified by the test point high index number. If not, the operation returns to step 132.

[0120] In the numerical example, the computer determines whether the current voltage operating point is the voltage operating point specified by lookup table entry index number 182, which is 149.9V, because lookup table entry index number 182 is specified by lookup table entry number 175, which contains the current reference voltage operating point.

[0121] Returning to step 126, if the direction variable indicates a decrease in the modulation voltage amplitude, the operation proceeds to this step, where the computer clears all break times from the buffer memory except for the break time corresponding to the current voltage operating point. The computer sets the direction variable to indicate that an increase in the modulation voltage amplitude will be used to fill the buffer memory with break times.

[0122] In the numerical example, the buffer memory will contain break times corresponding to the voltage operating points specified by lookup table entry indices 167 to 181. For example, if the current voltage operating point specified by lookup table entry index 167 is 137V, (the buffer memory will be filled with the value of the reduced modulation voltage amplitude.) Therefore, the computer removes all break times from the buffer memory except for the break time corresponding to the current voltage operating point of 137V.

[0123] The computer determines whether the low index number of the test point specified by the lookup table entry containing the current reference voltage operating point is different from the low index number of the test point specified by the lookup table entry containing the previous reference voltage operating point. (Once the computer has already established a reference voltage operating point, rather than the initial voltage operating point, the operation can only proceed to step 126, because when establishing the initial voltage operating point, the buffer memory is always filled with the value of the increased modulation voltage amplitude.)

[0124] If the lower numbers of the test points are different, proceed to step 136. Otherwise, proceed directly to step 132.

[0125] At step 136, the computer deletes the breakpoint corresponding to the current voltage operating point from the buffer memory. The operation proceeds to step 132.

[0126] In the numerical example, the buffer memory contains the breakpoint corresponding to the voltage operating point specified by lookup table entry index number 167. The lookup table entry containing the current reference voltage operating point is entry index number 175, which specifies the test point low index number 168. The test point low index number 168 specified by entry index number 175 is different from the test point low index number 167 specified by entry index number 174. Therefore, the computer deletes the breakpoint corresponding to the voltage operating point specified by entry index number 167. At this point, the buffer memory is empty.

[0127] Steps 132 and 134 have been described previously.

[0128] Returning to the numerical example, at step 132, the next voltage operating point after the current voltage operating point of 137V, as specified by lookup table entry index number 168, is 138V. The computer applies the DAC level of 823, specified by lookup table entry index number 168, to the DAC, which results in a modulation voltage of 138V being applied to the piezoelectric element 54.

[0129] The computer stores the break time corresponding to the voltage operating point of 138V in a buffer memory.

[0130] At step 134, the computer determines whether the current voltage operating point of 138V is the voltage operating point specified by lookup table entry index number 182, i.e., 149.9V, because lookup table entry index number 182 is specified by lookup table entry index number 175, which contains the current reference voltage operating point.

[0131] The operation iterates through steps 132 and 134 until the computer determines that the current voltage operating point of 149.9V is the voltage operating point specified by lookup table entry index number 182. Here, the buffer memory contains the break times corresponding to the voltage operating points specified by lookup table entry indices 168 to 182.

[0132] Finally, the operation returns to step 116 to calculate the new best-fit line and its gradient.

[0133] Now we turn to the case where the gradient of the best-fit line is less than the predetermined gradient, and refer to... Figure 3B In step 150, the computer searches for the next voltage operating point that is lower than the initial or current reference voltage operating point, depending on the situation.

[0134] In a numerical example, the next voltage operating point after the initial operating point of 143.3V, as specified by lookup table entry index number 173, is 142.5V.

[0135] The computer stores the next voltage operating point below the initial or current reference voltage operating point as the reference voltage operating point, and obtains the low index number and high index number of the test point from the lookup table entry for the specified reference voltage operating point.

[0136] Returning to the numerical example, the computer stores a reference voltage operating point of 142.5V, specified by lookup table entry index number 173, which specifies the test point low index number value 166 and high index number value 180, as well as a buffer size of 15.

[0137] At step 152, the computer determines whether the direction variable indicates an increase or decrease in the modulation voltage amplitude. The direction variable at this point, indicating whether the modulation voltage amplitude is increasing or decreasing, is used to fill the break time into the buffer memory.

[0138] In the numerical example, the direction variable indicates the increase in the modulation voltage amplitude because the buffer memory is filled by storing the break times corresponding to the voltage operating points specified by lookup table entries 167 to 181. The current voltage operating point is 149.1V, which is the voltage operating point specified by lookup table entry 181.

[0139] If the direction variable indicates an increase in the modulation voltage amplitude, the operation proceeds to step 154. Otherwise, the operation proceeds to step 156.

[0140] At step 154, the computer removes all break times from the buffer memory except for the break time corresponding to the current voltage operating point. The computer sets a direction variable to indicate that the reduction in the modulation voltage amplitude will be used to fill the buffer memory with break times.

[0141] In the numerical example, the buffer memory contains break times corresponding to the voltage operating points specified by lookup table entry indices 167 to 181. For example, the current voltage operating point specified by lookup table entry index 181 is 149.1V. Therefore, the computer deletes all break times from the buffer memory except for the break time corresponding to the current voltage operating point of 149.1V.

[0142] The computer determines whether the high index number of the test point specified by the lookup table entry containing the current reference voltage operating point is different from the high index number of the test point specified by the lookup table entry containing the previous reference voltage operating point.

[0143] If the high index numbers of the test points are different, proceed to step 166. Otherwise, proceed directly to step 162.

[0144] At step 166, the computer deletes the breakpoint corresponding to the current voltage operating point from the buffer memory. The operation proceeds to step 162.

[0145] In the numerical example, the buffer memory contains the breakpoint corresponding to the voltage operating point specified by lookup table entry index number 181. The lookup table entry containing the current reference voltage operating point is entry index number 173, which specifies test point high index number 180. Test point high index number 180 specified by entry index number 173 is different from test point high index number 181 specified by entry index number 174. The computer therefore deletes the breakpoint corresponding to the voltage operating point specified by entry index number 181, leaving the buffer memory empty.

[0146] At step 162, the computer searches for the next voltage operating point lower than the current voltage operating point. The computer applies a modulation voltage with the amplitude of the next voltage operating point lower than the current voltage operating point to the piezoelectric element 54 and performs phasing and measurement of the break time at the new voltage operating point. The computer stores the break time in a buffer memory. The operation proceeds to step 164.

[0147] Returning to the numerical example, at step 162, the next voltage operating point below the current voltage operating point of 149.1V, as specified by lookup table entry index number 180, is 148.2V. The computer applies the DAC level of 799, specified by lookup table entry index number 180, to the DAC, which results in a modulation voltage of 148.2V being applied to the piezoelectric element 54.

[0148] The computer stores the break time corresponding to the voltage operating point of 148.2V in a buffer memory.

[0149] At step 64, the computer determines whether the current voltage operating point is the voltage operating point specified by the lookup table entry identified by the test point's low index number. If not, the operation returns to step 162.

[0150] In the numerical example, the computer determines whether the current voltage operating point is the voltage operating point specified by lookup table entry index number 166, which is 136V, because lookup table entry index number 166 is specified by lookup table entry index number 173, which contains the current reference voltage operating point.

[0151] Returning to step 156, if the direction variable indicates a decrease in the modulation voltage, the operation proceeds to this step, where the computer determines whether the high index number of the test point specified by the lookup table entry containing the current reference voltage operating point is different from the high index number of the test point specified by the lookup table entry containing the initial or previous reference voltage operating point.

[0152] If the high index number of the test points is different, the operation proceeds to step 158.

[0153] At step 158, the computer removes from the buffer any breakpoints corresponding to voltage operating points higher than the voltage operating point specified by a lookup table entry identified by a test point high index number containing a lookup table entry of the current reference voltage operating point. The operation then proceeds to step 160.

[0154] In the numerical example, the buffer memory contains break times corresponding to the voltage operating points specified by lookup table entry indices 167 to 181. The lookup table entry containing the current reference voltage operating point is entry index 173, which specifies the test point high index 180. The voltage operating point of 149.1V specified by lookup table entry index 181 is higher than the voltage operating point of 148.2V specified by entry index 180. Therefore, at step 158, the computer removes the break time corresponding to the voltage operating point of 149.1V from the buffer memory.

[0155] If the high index numbers of the test points are the same, the operation proceeds directly from step 156 to step 160.

[0156] At step 160, the computer determines whether the low index number of the test point specified by the lookup table entry containing the current reference voltage operating point is different from the low index number of the test point specified by the lookup table entry containing the initial or previous voltage operating point.

[0157] If the lower index number of the test point is different, the operation proceeds to step 162. Otherwise, the operation returns to step 116.

[0158] At step 162, the computer searches for the next voltage operating point lower than the current voltage operating point. The computer applies a modulation voltage with the amplitude of the next voltage operating point lower than the current voltage operating point to the piezoelectric element 54 and performs phasing and measurement of the break time at the new voltage operating point. The computer stores the break time in a buffer memory. The operation proceeds to step 164.

[0159] In the numerical example, the buffer memory contains breakpoints corresponding to the voltage operating points specified by lookup table entry indices 167 to 180. The lookup table entry containing the current reference voltage operating point is entry index 173, which specifies test point low index 166. Test point low index 166 specified by entry index 173 is different from test point low index 167 specified by entry index 174. Therefore, the computer looks up the next voltage operating point below the current voltage operating point of 137V, i.e., 136V as specified by lookup table entry index 166.

[0160] The computer applies the DAC level of 827, specified by lookup table entry index number 166, to the DAC, which results in a modulation voltage of 136V being applied to the piezoelectric element 54. The computer stores the break-time corresponding to the voltage operating point of 136V in a buffer memory, thus containing the break-time corresponding to the voltage operating points specified by lookup table entries index numbers 166 to 180.

[0161] At step 164, the computer determines whether the current voltage operating point is the voltage operating point specified by the lookup table entry identified by the low index number of the test point. If not, the operation returns to step 162.

[0162] In the numerical example, the computer determines whether the current voltage operating point is the voltage operating point specified by lookup table entry index number 166, which is 136V, because lookup table entry index number 166 is specified by lookup table entry number 173, which contains the current reference voltage operating point.

[0163] Otherwise, the operation returns to step 116 to calculate the new best-fit line and its gradient.

[0164] It should be understood that although the voltage operating point changes continuously, the reference voltage operating point selected at step 120 or 150 effectively tracks the change in break time relative to the modulation voltage amplitude characteristics, thereby causing the voltage operating point to change around a point with a predetermined gradient.

[0165] Figure 4The display shows that the DAC level decreases by two from one lookup table entry to the next. That is, the DAC level values ​​available to the computer from portion 200 of the lookup table are a subset of those values ​​that can be received by the DAC, namely the odd-numbered DAC level values ​​from 829 to 789. This makes the difference between the voltage operating points of consecutive lookup table entries as close to 1V as possible.

[0166] Figure 4 Section 200 of the lookup table shown indicates the buffer size, which is the number of voltage operating points used to calculate the gradient of the break time relative to a portion of the modulation voltage amplitude characteristic (the number of voltage operating points increases with the modulation voltage amplitude). For example, lookup table entry index 167 uses 13 voltage operating points, while lookup table entry index 181 uses 16 such operating points. Given that the differences between the voltage operating points of consecutive lookup table entries are arranged to be as close as possible to 1V, increasing the buffer size causes the range of modulation voltage amplitude variation to increase with increasing modulation voltage amplitude.

[0167] It should be understood that the above description relates to only one embodiment of the present invention, and the present invention covers other embodiments as defined by the claims.

Claims

1. A continuous inkjet printer, comprising: An ink droplet generator, the ink droplet generator including an electromechanical transducer; a driver operable to apply a periodically modulated voltage to the transducer so that an ink jet ejected by the ink droplet generator breaks into an ink droplet stream at a break-off time after being ejected from the ink droplet generator. and a controller operable to change the modulation voltage parameter of the modulation voltage and measure a corresponding value of the break-time parameter indicating the break-time to obtain a portion of the characteristic of the break-time parameter relative to the modulation voltage parameter, wherein the characteristic is a curve of the break-time parameter changing with the modulation voltage parameter, and the portion is a sub-part in which the magnitude of the gradient in the characteristic gradually decreases from a first positive value to a second non-zero positive value. The controller is operable to: obtain a range of variation of the modulation voltage parameter; change the modulation voltage parameter within the range of variation to obtain a portion of the characteristic; calculate a gradient of the portion of the characteristic from the portion of the characteristic; compare the calculated gradient with a predetermined gradient; and, if the calculated gradient is less than the predetermined gradient, generate an adjusted range of variation shifted in a first direction relative to the range of variation, or, if the calculated gradient is greater than the predetermined gradient, generate an adjusted range of variation shifted in a second direction opposite to the first direction relative to the range of variation.

2. The printer according to claim 1, wherein the modulation voltage parameter is the amplitude of the modulation voltage.

3. The printer of claim 1 or 2, wherein the controller is operable to calculate the gradient from the portion of the characteristic by identifying a line that best fits the portion and calculating the gradient of the best-fit line.

4. The printer according to claim 1 or 2, wherein the break-off time parameter is at least an indication of the time required for ink ejected from the nozzle of the droplet generator to separate into droplets.

5. The printer according to claim 1 or 2, wherein the controller is operable to store the range of variation of the adjustment in the range of variation memory of the printer, and to obtain the range of variation from the range of variation memory.

6. The printer of claim 2, wherein the range of variation increases with the magnitude of the modulation voltage amplitude constituting the portion of the characteristic.

7. The printer of claim 1 or 2, wherein the driver includes a digital-to-analog converter (DAC), and the controller is configured to apply a binary input signal to the DAC, the binary input signal being selected from a subset of a set of binary input signals that can be received by the DAC, the subset being selected such that the output voltages generated by the DAC in response to the binary input signal differ from each other by approximately equal amounts.

8. The printer of claim 7, wherein the subset is selected such that the output voltages generated by the digital-to-analog converter in response to the binary input signal differ from each other by an amount as close as possible to 1V.

9. The printer of claim 1 or 2, wherein the printer includes a memory containing a lookup table specifying the size of the range of variation of the portion of the characteristic, and each entry of the lookup table includes a value of the modulation voltage parameter and an indication of those lookup table entries in which the value of the modulation voltage parameter constitutes the portion of the characteristic.

10. The printer of claim 9, wherein each entry in the lookup table includes an index number identifying the entry, a value of the modulation voltage parameter, and a first index number and a second index number for a corresponding first and second terminal defining the portion of the characteristic, the portion of the characteristic being defined by the value of the modulation voltage parameter of the lookup table entry identified by the first index number and the second index number.

11. The printer of claim 9, wherein the entries of the lookup table are ordered according to the values ​​of the modulation voltage parameter, and each entry of the lookup table includes indications of a first entry and a second entry for a corresponding first end and a second end defining a portion of the lookup table, wherein the values ​​of the modulation voltage parameter of the lookup table entries of the portion of the lookup table constitute a portion of the characteristic.

12. The printer according to claim 1 or 2, wherein for each value of the modulation voltage parameter, the controller can be operated to measure the corresponding break time parameter value at least twice and calculate an average break time parameter value from the measured break time parameter values.

13. The printer of claim 12, wherein the printer is configured to receive an input indicating a production line speed and to change the number of times a break time parameter value corresponding to a value of the modulation voltage parameter is measured based on the input.

14. A method of operating a continuous inkjet printer, the method comprising: A periodically modulated voltage is applied to the electromechanical transducer of an ink droplet generator to cause the ink jet ejected by the ink droplet generator to break into ink droplets at a break-time after ejection from the ink droplet generator. The modulation voltage parameter of the modulation voltage is varied, and the corresponding value of the break-time parameter indicating the break-time is measured to obtain a portion of the characteristic of the break-time parameter relative to the modulation voltage parameter, wherein the characteristic is a curve of the break-time parameter changing with the modulation voltage parameter, and the portion is a sub-part in which the magnitude of the gradient in the characteristic gradually decreases from a first positive value to a second non-zero positive value. The method includes: obtaining a range of variation of the modulation voltage parameter; changing the modulation voltage parameter within the range of variation to obtain a portion of the characteristic; calculating a gradient of the portion of the characteristic from the portion of the characteristic; comparing the calculated gradient with a predetermined gradient; if the calculated gradient is less than the predetermined gradient, generating an adjusted range of variation shifted in a first direction relative to the range of variation, or if the calculated gradient is greater than the predetermined gradient, generating an adjusted range of variation shifted in a second direction opposite to the first direction relative to the range of variation.

15. A computer program executable by a continuous inkjet printer to cause the printer to perform the method of claim 14.

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