Ink jet print head drive with normalization

Abstract: A method of normalizing performance of an image forming marking element, the method comprising the steps of operating the marking element with an adjustable operating parameter set to a first test value and quantifying a first value of a quantifiable performance characteristic of the marking element, operating the marking element with the operating parameter set to a second test value and quantifying a second value of the quantifiable performance characteristic, calculating an optimum value of the operating parameter, and adjusting the operating parameter to its calculated optimum value.

Description: This invention relates to ink jet printers and, more specifically, to normalizing the ink jets of a multi-orificed ink jet print head in order to obtain optimum performance from each jet of the print head. U. S. Patent 5,124,716 discloses a multi-orifice ink jet print head for ejecting ink drops onto a print medium, such as paper. The multi-orificed ink jet print head 25 is shown with associated elements in FIG. 1. An acoustic driver, such as a piezoelectric transducer 32, is coupled to a diaphragm 34 for ejecting ink drops from an ink chamber 12, through a nozzle orifice 18, and onto a print medium 19. The piezoelectric transducer 32 comprises first and second conductive electrodes separated by a layer of insulating piezoelectric material. A control signal provided by a signal source 36 is applied to the transducer and the diaphragm 34 is displaced according to the voltage of the control signal. FIG. 2 shows a known unnormalized waveform of a control signal that may be provided by the signal source 56 for driving the piezoelectric transducer 32. The signal has a positive pulse of +Vo volts which lasts for about 5 µs and then returns to 0 volts. The signal remains at 0 volts for a period of time T1. A negative pulse of -Vo volts, follows the period T1 and lasts for a second period T2 before returning to 0 volts. During the positive pulse, the piezoelectric transducer displaces the diaphragm away from the cavity interior, and ink from reservoir 14 is drawn into the cavity 12. In response to the negative pulse, the diaphragm is displaced for compressing the cavity and an ink drop is ejected from the orifice 18 onto the print medium 19. When placing an image on the print medium, the print head 25 shuttles back and forth along the X-axis parallel to the plane of the print medium surface and the print medium advances along the Y-axis perpendicular to the X-axis while the jets of the print head eject drops onto the print medium. The quality of the resulting image depends upon the size and velocity of the drops produced by each jet of the array of jets of the print head. Drop size affects the color density of an image while velocity affects the placement of dots with respect to other dots in the image. Ideally, each jet of the print head performs similarly to the other jets of the print head and each print head is manufactured with optimum parameters for ejecting ink. However, because of limited controls during manufacturing, performance variations exist. Many parameters affect the performance of ink jets. Temperature non-uniformities across a print head will produce variations in ink viscosity for the different jets of the print head. Drop production is affected by driver efficiency, which changes according to parameters such as thickness of the layer of piezoelectric material, stiffness of the diaphragm and the piezoelectric material, density and piezoelectric constant of the piezoelectric material and coupling coefficient between the electrodes and the piezoelectric material. Alignment of the acoustic driver with respect to the ink jet chamber and the coupling interface between the acoustic driver and the diaphragm of the ink chamber also affect drop production. Because of the limited control over these and other ink jet parameters, production lots experience variations in jet performance. By adjusting the waveform of the control signal applied to the acoustic driver, drop size and/or velocity may be altered and variations in jet performance may be partially compensated. It is known from U. S. Patent 5,124,716 to adjust the waveform of the control signal by changing the timing intervals, T1 and T2 of FIG. 3. U.S. Patent 5,212,497 to Stanley et al. (corresponding to European Patent 519,708 and assigned to the assignee of the present invention discloses a normalization technique wherein the drop ejection velocity of a jet is monitored by using a strobe imaging device to strobe ejected drops while adjusting the attenuation of the output signal provided by a signal source to produce the control signal applied to the jet's piezoelectric transducer. Referring to FIG. 3, changing the amplitude of the control signal V cntrl changes the amount by which the acoustic driver 32 displaces the diaphragm 34 of the ink jet and thus affects drop ejection velocity. The control signal received by the piezoelectric transducer is controlled by adjusting a potentiometer R POT , which contributes to the series resistance (R POT + R SA ) of a divider network 36. After adjusting the potentiometer for an optimum ejection velocity, the series resistance is measured and data representative of the optimum series resistance is recorded. This recorded data is sent to a resistor trim production step where the series resistor R SA of the resistor divider network 36 which is in the series path between the drive signal source 56 and the acoustic driver 32 is trimmed according to the received data. To produce a normalized print head in which each jet is tuned for uniform performance, the strobe imaging/potentiometer adjustment and the subsequent series resistor trim steps are performed for each jet of the print head. As such, the resistor trim normalization technique requires a significant amount of time for performing the normalization steps for all of the jets of the multiple-jet-array print head. In addition, the divider network dissipates power when attenuating the control signal and therefore consumes extra energy when used to attenuate the control signal and affect jet performance. These problems are solved in the method and apparatus of the present invention. According to a first aspect of the present invention, there is provided a method of normalizing performance of an image forming marking element having an adjustable operating parameter, wherein a quantifiable performance characteristic of the marking element depends on the value of the parameter. The method comprises the steps of operating the marking element with the operating parameter set to at least one test value and quantifying a value of said performance characteristic of the marking element, calculating a value of the operating parameter based on a desired value of said performance characteristic, said at least one test value of the operating parameter, and said value of the performance characteristic, and adjusting the operating parameter to its calculated value. This normalization may be done electronically or manually. According to a second aspect of the present invention there is provided a method of characterizing relative performance characteristics of an array of at least two image forming marking elements, each having an adjustable operating parameter, which method comprises the steps of forming a test image with each marking element of the array with the operating parameter of each marking element set to at least one predetermined value, measuring a quality of each test image representative of each marking element, and quantifying a relative performance characteristic according to the differences in measured qualities between test images representative of the marking elements. For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: FIG. 1 is a schematic fragmentary view of a known piezoelectric, acoustically driven, ink jet print head; FIG. 2 illustrates the waveform drive of the signal that may be used to drive the ink jet print head of FIG. 1; FIG. 3 is a schematic view of a prior art ink jet normalization circuit; FIG. 4 is a schematic illustration of a programmable ink jet in accordance with the present invention; FIG. 5 illustrates the waveform of a drive signal associated with the programmable ink jet of FIG. 4; FIG. 6 is a flow chart representative of an aspect of the present invention; FIG. 7 illustrates an image test pattern corresponding to FIG. 6; and FIGS. 8a-8c show enlargements taken from of FIG. 7 representing different test values. In the drawings, like reference numerals designate similar components. FIG. 4 shows a signal source 56 generating two signals Vpp and Vss. Vpp is a positive going pulse train, with one pulse for each time any of the jets in the print head could need to eject ink. Vss is a negative going pulse train, with a single negative pulse following a fixed delay after the end of each positive Vpp pulse. There may be more than one signal source 56 block for a print head, but typically fewer than there are jets (each one drives multiple jets within the head). For each jet, there is a FET switch 70 connecting Vpp to V cntrl which drives the piezoelectric transducer 32 for that jet. There is also a FET switch 72 connecting Vss to V cntrl for that jet. Diodes 71 and 73 are connected across FET switches 70 and 72 respectively. The FET switches 70 and 72 are controlled from jet logic 76 through level translators 74 and 75 respectively. The level translators convert the standard 0 to 5 volt logic levels from jet logic 76 to the appropriate levels for driving the gates of FET's 70 and 72. Latch 82 within jet logic 76 holds the normalization value in a memory location for that jet. Blocks 70 through 76 are replicated once for each jet. Finally, control logic 77 sends timing, sequencing, and data signals to signal source 56 and to control logic 77. There may be more than one control logic block 77 for a print head, but typically each control logic block 77 will drive multiple jet logic blocks 76 and therefore control multiple jets. V cntrl , the piezoelectric transducer driving voltage, for a given jet is controlled as follows: During the idle times between Vpp and Vss pulses, FET switch 72 is left on to keep V cntrl at zero volts. Since Vpp and Vss are both at zero volts in between pulses, either or both of the FET switches 70 and 72 could be turned on. (Even if neither of the FET switches 70 and 72 were on, V cntrl would remain near zero volts because of diodes 71 and 73.) If the jet is not to fire during a Vpp and Vss pulse pair, then FET switch 70 is kept off during the Vpp pulse and FET switch 72 is kept off during the Vss pulse. The opposite FET switch (72 during the Vpp pulse and 70 during the Vss pulse) may be turned on to help maintain zero volts on V cntrl . If the jet is to fire during a Vpp and Vss pulse pair, then FET switch 72 is kept off during the Vpp pulse and FET switch 70 is kept off during the Vss pulse. FET switch 70 is turned on before the Vpp pulse starts and is turned off during the rising edge of the Vpp pulse. The turn-off time is a function of the value stored in latch 82 within jet logic 76. The larger the value in latch 82, the later FET switch 70 is turned off, and therefore the higher voltage on V cntrl at the time it is turned off. Since the piezoelectric transducer 32 presents a mostly capacitive load on V cntrl , the voltage on V cntrl will substantially maintain the voltage it had at the time FET switch 70 turned off. As Vpp ramps back down to zero volts at the end of its pulse, diode 71 will conduct to pull V cntrl back down near zero. The Vss pulse is handled similarly. Before the start of the Vss pulse, FET switch 72 is turned on. It is turned off during the leading (falling) edge of the Vss pulse at a time determined by the value in latch 82. It should be noted that a different latch could be used if separate control of the positive and negative pulse amplitudes is required. The larger the value in latch 82, the later FET switch 72 is turned off and, therefore, the lower (more negative) is the voltage on V cntrl at the time FET switch 72 is turned off. Again, this voltage is substantially maintained by the capacitive load until Vss ramps back up to zero. As Vss ramps back to zero, diode 73 conducts to ramp V cntrl back almost to zero. The slope of each leading edge of each Vpp and Vss pulse decreases at a knee part way through the leading edge. This allows a given time resolution for turning off FET switches 70 and 72 to result in finer voltage resolution on V cntrl . Since each jet within a print head has its associated jet logic 76 and latch 82, each jet can be driven with a different V cntrl amplitude by storing different values in each of the latches 82. If the values stored in latches 82 are selected such that each jet performs close to an optimum operation point, then the print head can be normalized with this drive method. When generating the normalization data, latches 82 are loaded with a predetermined test value or values, and the desired characteristics of the jet are measured. The best value, or an approximation of that value, for latch 82 for each jet is determined from the measured characteristics, and this data is stored in non-volatile memory within the printer or head. When the printer is operated, the data from this non-volatile memory is loaded into latches 82 to cause each jet to be driven with near its optimum voltage level. Alternatively, latches 82 could be the non-volatile memory avoiding the loading step each time the printer is turned on or used. In one particular normalization mode, normalization is effected by adjusting dot size to produce a desired color density. Color density may be measured by comparing the intensity of light reflected by a test image with the intensity of incident light. The intensity of light reflected by the print medium bearing the test image depends on the proportion of the area of the print medium that remains exposed. This proportion is dependent on the imaged pattern and characteristics of the printer. For a nominal 25% fill shown in FIG. 8, the desired actual test image area coverage is at least π/8 or about 39%. FIG. 6 shows a flow chart for explaining the one particular normalization mode. The normalization mode has a primary objective of normalizing the jet for ejecting ink drops of a given drop size, so that the jet provides a desired color density when used to produce an image on the print medium. In step 102, a desired color density is defined. In step 104, a servo controller simultaneously controls the position of the print head 25 and the printing medium 19 while ejecting drops from the different jets of the multi-jet-array print head onto the print medium in order to create the test images as shown in FIG. 7. The jets are each tested at various test control values during the production of these test images. A first set of test patterns is made with each jet set to a first test control value provided by a normalization controller (not shown), whereupon a new test control value is stored in each of the latches and a new set of test patterns is generated on the print medium. This may be repeated for third and fourth or more test control values, dependent upon the shape of the characteristic curve. When finished generating the test images, the print medium bears an array of test images wherein each test image represents one jet tested with its particular parameter set to a particular test control value. Enlarged views of FIG. 7 are shown in FIGS. 8a, 8b and 8c. When the ink drops are the correct size, FIG. 8b, they occupy the desired percentage of the area of the test image. When the ink drops ejected by the jet are too large, FIG. 8a, the dots occupy a greater proportion of the test image area and the test image produces a low intensity of reflected light. When the drops are too small, FIG. 8c, the dots occupy a lesser proportion of the test image area and the intensity of light reflected is higher. The different test control values used during normalization can cover a sufficient range that at least one test image has a fill ratio greater than the desired percentage and at least one has a fill ratio less than the desired percentage. In step 106, each test image of the print medium is examined by an optical scanning device, for example a Hewlett-Packard Scanner Jet IIc trade mark scanner or a JX 450 trade mark scanner from Sharp Electronics, Inc., for obtaining its color density. The color density is determined according to a reflection index, which is a ratio of an average reflected light intensity received from the test image in proportion to an incident light intensity as projected onto the test image. A characteristic curve is then derived in step 107 which defines a "color density versus test control value" relationship. In step 108, an optimum control value is determined for each jet according to the characteristic curve, the test results, test control values of the jet and the desired image color density specified in step 102. After calculating the optimum control value for each jet of the multiple-jet-array print head, the optimum control values are written (in step 110) into previously assigned non-volatile memory locations of, for example, a printer controller (not shown), by an appropriate apparatus, such as a CPU (not shown). In subsequent operation of the print head, the optimum control values are read from the memory array and the control latch 82 of each jet is loaded with its optimum control value. With the print head thus normalized, the ink jet printer will produce images of substantially ideal color density. In a modification of the normalization method described with reference to FIG. 6, a nominal performance curve for a nominal jet may be obtained by collecting a number of test data points, from a number of different jets, from a number of different manufacturing lots, which are tested over a wide range of test control values. The nominal curve is generated from a greater number of control values than would normally be used afterwards for normalizing a single jet as described with reference to FIG. 6. The nominal curve may then be adapted to each particular jet by using a scaling factor. The scaling factor is obtained according to the unique test results of each jet tested at the given test control values. In a normalization procedure for an individual jet, the number of test control values used is only that which is necessary for obtaining the scaling factor and might be only two or three, or could be as few as one. Having obtained the scaling factor, the nominal curve is then scaled to produce a characteristic curve for the particular jet. An optimum control value that produces the desired image color density is then obtained using this characteristic curve for the individual jet. If necessary, an offset could be employed in place of or in conjunction with the scaling factor. As a modification of this characteristic curve technique, a mathematical relationship can be used for characterizing the "color density versus control test value" relationship. The mathematical relationship may be a polynomial equation with the order of the polynomial being less than the number of test control values used during normalization, even as simple as a linear equation, from which the optimum control value would be extrapolated or interpolated. The coefficients taken from either the simple linear equation or the polynomial equation characterize the tested jets. It will be appreciated that the invention is not restricted to the particular embodiments that have been described, and that variations may be made therein without departing from the scope of the invention as defined in the appended claims. For example, the jet might be tested by ejecting an ink drop and making a measurement of the projection path of the ejected ink drop. The jet's projection path would be tested according to different control test values. Based on the test results, an optimum control value would be calculated for providing an optimum ink drop projection path. Measurements need not be limited to quantifying parameters of a printed image on a print medium. The measurement might employ the strobe technique as used in the resistor trim normalization method described above to collect at least one performance characteristic value of the image forming marking element when driven at least one test control value. Further, the strobe technique described above could also be used in its entirety to determine the necessary drive voltage for each jet to obtain the desired performance characteristics. The drive voltage is then used to determine the control values to feed to the multiplicity of latches 82 to normalize the performance of the print head. Desired control or drive voltages can also be obtained by scanning the optical density of the test image. These voltages can then be used to calculate the required resistances to laser trim the resistors integral to print heads, such as those utilized in the Phaser III color printers trade mark sold by Tektronix, Inc. It is to be understood that the adjustable operating parameters discussed herein with regard to controlling the normalization of a print head include, but are not limited to, voltage, pulse width, delay time between pulses, and the rise and fall time of the pulses. The method can also be used to adjust more than one of these parameters by generating test images, for example, with each parameter independently varied while the others remain constant. It is also to be understood that the quantifiable parameters discussed herein for controlling the print head normalization can include, but are not limited to, dot size, drop size, ejection velocity, drop time to target or receiving medium, dot placement, optical density, drop break off time, variation of drop size or velocity as a function of drop ejection frequency, peak negative pressure within the jet, PZT diaphragm deflection and ink meniscus resonance amplitude. In the case of the embodiment described with reference to FIG. 4, time at which the FET switches 70 and 72 turn off need not be a linear function of the data value in latch 82. The latch value to turn-off time function could be modified to compensate for non-linearities in the leading edge ramp of Vss and Vpp, and/or for non linearities in the ink jet performance curve. While the invention has been described above with references to the specific embodiments thereof, it is apparent that many changes, modifications and variations in the materials, arrangements of parts and steps can be made without departing from the inventive concept disclosed herein. For example, the invention is not limited to the marking element being an ink jet, but is applicable also to the marking element for a bubble-jet printer, thermal transfer wax printer, or a dot matrix printer. Normalization also might involve determining different optimum control values for the positive and negative pulses, in which case the latch 82 could be used for positive pulses and a different latch (not shown) could be used for negative pulses.

Claims: A method of marking a print medium (19) comprising the steps of operating a marking element (25) to apply a marking medium to the print medium, the performance of the marking element relative to an optimum standard being quantifiable in terms of a value of a measurable performance parameter; operating a control signal means connected to the marking element to input a control signal to the marking element whereby the marking element is actuated in response to the control signal such that the performance parameter is dependent on the amplitude of the control signal; storing a predetermined control value in a latch means (82) of the control signal means; and determining by operation of the control signal means the amplitude of the control signal in dependence upon the control value; characterised by the operation of the control signal means comprising the steps of inputting to a switching means (70,72) an input signal having at least one transition with a finite slew rate, and actuating the switching means to produce the control signal as a function of the control value by selectively connecting the input signal to the marking element and disconnecting the input signal therefrom at a time that depends on the control value, whereby the amplitude of the control signal depends on the control value and the slew rate. A method as claimed in Claim 1 wherein the input signal comprises a pulse having a first transition from a first voltage level to a peak voltage, a flat peak voltage from the end of the first transition, and a second transition from the peak voltage to the first voltage level and wherein said control signal is produced equal to or less than the input signal and at a voltage level corresponding to said control value. A method as claimed in Claim 2 wherein the input signal further comprises a second pulse of the opposite polarity to the first-mentioned pulse and having a first transition from said first voltage level to an opposite polarity peak voltage, a flat peak voltage from the end of the first transition and a second transition from the opposite polarity peak voltage to said first voltage level. A method as claimed in any preceding claim wherein said marking element is a jet element of an ink jet print head and wherein the control signal is applied to a driving means (32) of the jet element, whereby the jet element ejects fluid according to said control signal. A method as claimed in any preceding claim including a normalizing step in which the control value to be stored in the latch means is determined; comprising the steps of (a) operating the marking element with a test control value in the latch means and quantifying a corresponding test value of said performance parameter of the marking element; (b) calculating the control value based on a desired value of said performance parameter, said test control value, and said corresponding test value of the performance parameter; and (c) storing the control value so calculated in the latch means. A method as claimed in Claim 5 and including the following step carried out between steps (a) and (b), namely the step of operating the marking element with at least one other test control value in the latch means and quantifying at least one other test value of said performance parameter, and wherein step (b) comprises calculating said control value based on said desired value of said performance parameter, the test control values, and the corresponding quantified test values of said performance parameter. A method as claimed in any of Claims 5 and 6 wherein the marking element is a marking element of a print head having an array of M marking elements and said method comprises performing steps (a) through (c) for each of the M marking elements. A method as claimed in any of Claims 5 to 7 wherein step (a) comprises employing the marking element to form a test image within a test area on a print medium and measuring a characteristic of the test image to obtain said test value of performance parameter. A method as claimed in Claim 8 wherein the marking element is employed to apply a marking medium of a predetermined color to the print medium and the characteristic of the test image is color density. A method as claimed in Claim 9 wherein the step of measuring the characteristic of the test image comprises illuminating the test area with incident light, measuring the intensity with which light is reflected by the test area and calculating said color density according to the intensity ratio of the reflected light and said incident light. A method as claimed in any of Claims 5 to 10 and comprising, between steps (b) and (c) the steps of assigning a memory location to the marking element, writing correction data representative of said calculated value of the control value into the memory location, and employing the correction data in said memory location to adjust the control signal. A method as claimed in Claim 11 wherein the step of employing the correction data includes reading the correction data from said memory location. A method as claimed in any of Claims 5 to 12 wherein step (a) comprises loading a test control value into the latch means, receiving an input signal, generating a control signal by processing the input signal according to the test control value loaded into the latch means and controlling said marking element according to said control signal. A method as claimed in Claim 5 wherein the normalising step comprises the steps of:- (a) operating the marking element with a first test control value in the latch means and quantifying a first value of said performance parameter of the marking element; (b) repeating step (a) at least once with at least one other test control value in the latch means; (c) determining a mathematical polynomial relationship between the quantified values of said performance parameter and said test control values wherein the order of the polynomial is less than the number of test control values; and (d) characterizing said marking element according to the coefficients of said polynomial. Apparatus for marking a print medium, the apparatus comprising a marking element (25) operable to apply a marking medium to the print medium (19), the performance of the marking element relative to an optimum performance standard being quantifiable in terms of a value of a measurable performance parameter, control signal means connected to the marking element and operable to input a control signal to the marking element whereby the marking element is operable in response to the control signal such that the value of the performance parameter is dependent on the amplitude of the control signal; the control signal means comprising latch means (82) operable to store a control value, and the control signal means being operable to determine the amplitude of the control signal in dependence upon the control value, characterised in that the control signal means comprises switching means (70,72) operable to receive an input signal having at least one transition with a finite slew rate, the switching means being operable to produce the control signal as a function of the control value by selectively connecting the input signal to the marking element and disconnecting the input signal therefrom at a time that depends on the control value whereby the amplitude of the control signal depends on the control value and the slew rate. An apparatus as claimed in Claim 15 wherein the switching means includes a time function controller which determines the control signal amplitude by the time of disconnection of the input signal. An apparatus as claimed in Claim 16 wherein the time function controller is operative to enable and disable the switching means. An apparatus as claimed in any of Claims 15 to 17 wherein the switching means includes at least a first FET (70) and a first diode (71) attached across the drain and source of the first FET. Apparatus as claimed in any of Claims 15 to 18 comprising:- (a) a source (14) of ink coloring agent to apply to the print medium and constituting said marking medium; (b) an ink jet print head having a plurality of ink jet elements constituting said marking elements through which the ink coloring agent is propelled to be applied to the print medium; and (c) control signal means connected to the print head to drive the ink coloring agent from the plurality of ink jet elements, the control signal means being operable to input a respective control signal for each of the plurality of ink jet elements, different ones of the plurality of ink jet elements being driven at different control signal amplitudes, the control signal means further being able to increase or decrease the control signal amplitudes multiple times. An apparatus as claimed in Claim 19 wherein the control signal means is able to change the control signal amplitudes during the individual marking of a print medium. An apparatus as claimed in Claim 19 wherein said control signal means has a memory location constituting said latch means for each of the plurality of ink jet elements, the memory location containing a control value representing the control signal amplitude value corresponding to each individual ink jet element. An apparatus as claimed in any of Claims 15 to 21 wherein the marking elements have a sufficiently capacitive load to substantially maintain voltages present at the times of disconnection of the control signals.

Inventors: KNIERIM DAVID L, KNIERIM, DAVID L.
Applicants: TEKTRONIX INC, TEKTRONIX, INC.
Application year: 1993
Main classification: B41J2
IPC codes: B41J2, B41J29