Ink jet printer and method of controlling it

Abstract: A control method for an ink jet printer capable of a discharge-free nozzle driving operation for stimulating and vibrating an ink meniscus in each nozzle of the ink jet head to prevent nozzle clogging does not lead to a droplet in print speed. The ink jet head is moved on a carriage. Prior to printing on a printing medium (block B2), this discharge-free driving operation (block B31) is performed while the carriage is moving the ink jet head to the print position. Printing is therefore not delayed by the discharge-free driving operation, and nozzle clogging can be prevented without lowering the print speed.

Description: The present invention relates to an ink jet printer. More specifically, the present invention relates to a method of controlling an ink jet printer so as to efficiently prevent the ink nozzles of the printer to become clogged. In an ink jet printer each of a plurality of ink nozzles of an ink jet head communicates with an ink chamber filled with ink. To eject an ink droplet through a nozzle the ink in the associated ink chamber is exposed to a pressure pulse by means of a pressure generating device or actuator. Among various types of actuators that are currently employed are: a piezoelectric actuator as taught in JP-A-2-51734, a thermal actuator as taught in JP-A-61-59911, and an electrostatic actuator as taught in JP-A-7-81088 and EP-A-0 829 354. A problem common to ink jet printers irrespective of the type of actuator used is that if no ink droplet is ejected from a nozzle for a certain amount of time, the water or other solvent in the ink evaporates causing the viscosity of the ink near the nozzle to increase. Such increase in the viscosity tends to clog the nozzle. This prevents ink droplets from being ejected at all or at the normal speed or volume. After an ink droplet was ejected from a nozzle, the nozzle or the ink chamber needs to be refilled with ink. Another effect of an increased ink viscosity is, that such refilling becomes slower. The ink refilling may become so slow that it does no longer keep up with the ink ejection (supposing ink is still being ejected, i.e., the nozzles are not yet completely clogged); this causes bubbles to become mixed with the ink and further hinders normal ejection of ink droplets. One common technique used to avoid these problems in modem ink jet printers is to cover the nozzles with a cap when the printer is not printing. This prevents the nozzles from drying out and thus prevents an increase in ink viscosity near the nozzles. Another known method to maintain or restore printer performance involves preventively ejecting a small volume of ink from all nozzles at regular intervals between printing operations thereby to prevent ink clogging near the nozzles. Such "preventive" ink ejection will be referred to as "flushing" hereinafter. JP-A-6-39163, for instance, teaches a recovery method wherein the frequency used to drive the ink jet head for flushing is set lower than the highest drive frequency used for printing text and graphics. This makes it possible to reliably expel high viscosity ink from the nozzles without pulling in air through the nozzles and creating bubbles in the ink path. Yet another method for preventing nozzle clogging as a result of dried ink near the nozzles is taught in EP-A-0 829 354. This method prevents clogging by using a signal generator to stimulate a resonance oscillation of the ink in the ink jet head while the printer does not print in order to vibrate the ink meniscus in or near the nozzle orifice without actually discharging ink. Since no ink is discharged, this method will be referred to as a discharge-free recovery. In fact, EP-A-0 829 354 employs a combination of the above mentioned techniques. While the printer is switched off the nozzles are capped. When the printer is switched on a first type of recovery process, referred to "purging" hereinafter is performed. During the purging the ink jet head remains capped and ink is sucked off all nozzles by means of a pump connected to the cap. A second type of recovery process, namely a flushing process, is performed in response to any of two events, lapse of a predetermined time interval since the last purging or flushing process and a command to start printing. During non-printing intervals a third recovery process, namely the above mentioned discharge-free recovery, is applied to all nozzles. During printing this process is applied to all nozzles not used for the printing. The document EP-A-0 574 016 discloses a method of driving an ink jet printer comprising a plurality of nozzles, a corresponding plurality of piezoelectric actuators, and means for transporting said nozzles relative to a printing medium for printing on the printing medium in accordance with print data. The method comprises generating a reference signal of a single frequency, applying to each of said actuators synchronized to the reference signal either a first type electric pulse of an amplitude enabling ink droplet ejection, or a second type electric pulse of an amplitude lower than that of the first type electric pulse for moving ink inside a nozzle without discharging ink. During printing the first type pulse is applied to each actuator associated with a nozzle that is to print a dot, while the second type pulse is applied to each actuator associated with a nozzle that is not to print a dot. Ink near the nozzle associated with actuators energized by the second type pulse is moved but not ejected thereby retarding the nozzle clogging. While the printer is not used at all for an extended period of time, the second type pulse is applied to all pressure generating elements. Though this cannot prevent nozzle clogging it extends the allowable time of non-use after which any nozzle clogging may still be removed by what is called a "flushing operation" in the document. Even in that case, if the period of non-use of the printer exceeds a certain time, nozzle clogging occurs to such an extent that a "cleaning operation" rather than a flushing operation is required to recover the nozzles. The document describes that the time required for such cleaning operation can be reduced by applying the second type pulse to the pressure generating elements for a given period of time before the cleaning operation is performed. Thus, in this prior art, a series of second type pulses followed by a cleaning operation is performed to recover the nozzles from nozzle clogging rather than to prevent the nozzle clogging. Some of the problems that remain unresolved by the above-noted clogging prevention methods are described below. First, in the prior art employing the discharge-free recovery different voltages need to applied at the same time during normal printing to first and second nozzles, respectively. The first nozzles are those that, in accordance with the print data, are to eject a droplet onto the printing medium. The second nozzles are those that, in accordance with the print data, are not to eject droplets, but are driven so as to vibrate the ink meniscus without ejecting ink droplets. The necessity of applying such different voltages (not including 0 V) at the same time to a head driver requires a complicated circuit structure. Second, even when the time interval between the flushing and the actual start of printing is short it may still be sufficient for a film to form on the nozzles. This is the more likely to happen, the greater the distance is between the position at that flushing is performed and the print start position. Such film prevents ink droplets from being ejected at normal speed and/or volume. It is an object of the invention to provide an ink jet printer and a method of controlling it whereby unnecessary ink consumption can be suppressed, and nozzle clogging can be reliably avoided regardless of how long the printer has been idle. These objects are achieved with an ink jet printer as claimed in claim 1, a method as claimed in claims 5 and a storage medium as claimed in claim 8. Preferred embodiments of the invention are subject-matter of the dependent claims. As has been explained above, if the ink jet head has been inactive in a standby state for a longer time, when a print command instructs printing, it is desirable to perform a flushing operation before the printing is started. However, constantly performing this flushing operation is not desirable because of the increase in unnecessary ink consumption. According to another embodiment of the invention such increase of unnecessary ink consumption is avoided by measuring the time elapsed since the last discharge-free driving operation and the time elapsed since the last flushing operation, and then selecting either the discharge-free driving operation or the flushing operation based on the measured times. Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description of preferred embodiments of the invention taken in conjunction with the accompanying drawings, in which: Fig. 1 is a block diagram of an ink jet printer according to a preferred embodiment of the present invention; Fig. 2 is a perspective view of the printing unit 40 shown in Fig. 1; Fig. 3 is a sectional view of an ink jet head with electrostatic actuators; Fig. 4 is a plan view of the ink jet head shown in Fig. 3; Fig. 5 are sectional views of the ink jet head shown in Fig. 3, (a) showing the standby state, (b) the ink charging state, and (c) ink compression; Fig. 6 is a circuit diagram of one example of the selector 150 shown in Fig. 1; Fig. 7 is a circuit diagram of one example of the head driver 190 shown in Fig. 1; Fig. 8 is a logic chart showing the relationship between driver input and output signals in Fig. 7; Fig. 9 is a timing chart of the printing process, and Fig. 10 is a state transition diagram for an ink jet printer according to the present invention. A preferred embodiment of an ink jet printer according to the present invention is described below with reference to the accompanying figures. Since some elements of this embodiment correspond to elements described in EP-A-0 829 354 reference is made to that document for further details. General structure Fig. 1 is a block diagram of the printer. It comprises a printing unit 40 and a control unit 100. Control unit 100 controls the printing unit 40 based on data and control signals from a host device. A perspective view of the printing unit 40 is shown in Fig. 2. A printing medium 105, simply referred to as paper below, is transported by a platen 300. Ink is stored in an ink tank 301 and supplied to an ink jet head 30 by way of an ink supply tube 306. The ink jet head 30 comprises piezoelectric, thermal, or electrostatic actuators as pressure generating devices and is mounted on a carriage 302. The carriage 302 is driven by a motor 45 (see Fig. 1), and is arranged to travel in a direction perpendicular to the feed direction of paper 105. A cap 304 is provided to cover the nozzles of the ink jet head 30. A pump 303 can be used for an ink jet head recovery process, called "purging" in this text, that draws ink from inside the ink jet head 30 via cap 304 and a tube 308 into a waste ink reservoir 305. This purging process is performed when another recovery process, called "flushing" in this text and described further below, can no longer restore the ink jet head function, such as when nothing has been printed for a long period of time, or there is a bubble in a nozzle. To print on paper 105 ink droplets are ejected from the nozzles while the ink jet head 30 travels within a printing range P, the width of which is approximately equal to that of the platen 300. For cleaning the ink jet head, i.e., for purging and for flushing, the ink jet head is positioned outside of range P at a position R in front of cap 304. Typically , this position R is also the home position of the carriage 302 and the ink jet head 30. Position R will, therefore, be referred to "home position" hereinafter. It is to be noted, however, that the cleaning position and the home position need not necessarily be the same. The cap 304 can move forward and backward, i.e., advance toward and retract from ink jet head 30. For purging, i.e., to suck ink off the ink jet head 30 , the cap 304 moves forward to cover the nozzles of the ink jet head 30. For flushing after standby the cap 304 may be in its forward position and cover the nozzles. For flushing in between printing, however, the cap 304 may stay in its backward position. In both cases of flushing ink droplets are ejected from all nozzles into the cap 304. When the power is turned on, the ink jet head is then positioned at the home position R with the nozzles covered by the cap 304 and stays there until a print command is received. Turning now to Fig. 1, a receive port 170 is a serial or parallel communications port for receiving print data and control signals from a host device. Image data contained in such signal is stored in a print pattern memory 110, which is typically implemented as a certain area of a RAM. Data at an address in the RAM that is specified by the printer control unit (CPU) 200 can be sequentially output to a selector 150 using an appropriate address signal and read/write signal. A flushing data generator 160 generates flushing data Pd for flushing operations. These flushing data cause ink to be ejected from all nozzles. The flushing data generator 160 outputs the flushing data Pd to the selector 150. The selector 150 selects either the print data from the print pattern memory 110 or the flushing data from the flushing data generator 160 for supply to a drive signal generator 180. The drive signal generator 180 generates drive signals D1 to Dn for corresponding nozzles 11-1 to 11-n based on the data from the selector 150 and timing pulses Tp from the CPU. The pulse widths and the timing of the drive signals that are applied to a head driver 190 are, thus, determined by the timing pulses Tp. The head driver 190 is for stepping up the level of the drive signals from drive signal generator 180 for application to the actuators in the ink jet head 30. Motor driver 195 is for driving the motor 45 based on a control signal Tm output from CPU 200. The control signal is synchronized with the timing pulses Tp in the sense that the period of the timing pulses corresponds to the dot spacing on the paper. A memory block 210 comprises RAM for storing, for example, print commands contained in the signals received from the host device, and ROM for storing a control program for controlling various components of the control unit 100. The CPU 200 appropriately controls the above-noted circuits and functional components based on the control program. A timer or other counter unit 220 according to the present invention comprises a counter 220k for measuring the time Tk since the last discharge-free recovery operation, further described below; a counter 220f for measuring the time Tf since the last flushing operation; a counter 220t for measuring the idle time Tt, i.e., how long the printer is stopped without printing; and a counter 220KJ for measuring the capping time KJ, i.e., the time during which the ink jet head was capped by cap 304. When any of the times measured by these counters passes, the corresponding counter outputs a count-up signal instructing the start of a discharge-free recovery operation or a flushing operation, or sets a flag to announce that a specific time has elapsed. A drive voltage selector 130 applies, based on a control signal from the CPU 200, either a first (V1) or a second voltage (V2) to the head driver 190 as will be explained in more detail later. Ink jet head Fig. 3 is a sectional view of an example of an ink jet head 30 that uses electrostatic actuators, Fig. 4 is a plan view of the same, and Fig. 5 is a partial sectional of the same. While the use of such type of ink jet head is preferred, the invention is not restricted to this ink jet head type. As shown in these figures, ink jet head 30 is a three layer structure comprising a silicon substrate 1 disposed between a silicon nozzle plate 2 and a borosilicate glass plate 3. The thermal expansion coefficient of the borosilicate glass plate 3 is substantially equal to that of silicon. Channels (recesses) etched into the top surface of the silicon substrate 1 form together with the nozzle plate 2, which is bonded to this top surface, a plurality of mutually independent ink chambers 5, a common ink reservoir 6 shared by the ink chambers 5, and ink supply paths 7 connecting each ink chamber 5 to the common ink reservoir 6. A respective nozzle 11 is formed in the nozzle plate 2 at a position near the end of each ink chamber 5. Each nozzle 11 is open to the corresponding ink chamber 5. An ink supply opening 12 communicating with the common ink reservoir 6 is formed in the glass plate 3. Ink can thus be supplied from the ink tank 301 (see Fig. 2) through ink supply opening 12 by way of ink supply tube 306 (see again Fig. 2) to the common ink reservoir 6. Ink supplied to the common ink reservoir 6 is then supplied to the ink chambers 5 through the corresponding ink supply paths 7. Each ink chamber 5 has a thin bottom wall 8. This bottom wall will be referred to a "diaphragm" below because it functions as a diaphragm that is flexibly displaceable upward and downward as viewed in Fig. 3. Shallow recesses 9 are etched in the glass plate's top surface, with which it is bonded to the bottom surface of silicon substrate 1, at positions respectively corresponding to the ink chambers 5. The diaphragm 8 of each ink chamber 5, therefore, opposes the bottom surface 92 of a corresponding recess 9 across a very small gap. At a position corresponding to the nozzle side end of the ink chamber 5 a protrusion 92b projects from the bottom surface 92 towards diaphragm 8. The part of diaphragm 8 opposite protrusion 92b is referred to as bottom 8b below. The gap between bottom 8b and protrusion 92b is smaller than the gap between the rest of bottom surface 92 and the rest of diaphragm 8, referred to as bottom part 8a below. It should be noted that diaphragm 8 of each ink chamber 5 is used as an electrode. A segment electrode 10 is formed on the bottom surface 92 of each recess 9 so as to oppose a respective diaphragm 8. The surface of each segment electrode 10 is covered with an insulation layer 15 (see Fig. 5) of a certain thickness. The insulation layer 15 is an inorganic glass. Each segment electrode 10 and the corresponding diaphragm 8 form the opposing electrodes of a respective electrostatic actuator. The gap (distance) between these opposing electrodes is G2 near the nozzle, and G1 in the remaining areas. The head driver 190 serves to charge and discharge the respective capacitors formed by these electrostatic actuators based on drive signals from drive signal generator 180 and the timing pulses Tp from CPU 200. Head driver 190 has a plurality of first output terminals and one second output terminal. Each of the first outputs terminals of the head driver 190 is connected directly to a respective segment electrode 10, and the second output terminal is connected to a common electrode terminal 22 formed on silicon substrate 1. The silicon substrate 1 is doped and, therefore, conductive. Hence, it is possible to supply charge carriers from the common electrode terminal 22 to the diaphragms 8. To reduce the contact resistance between the silicon substrate 1 and the common electrode a thin film of gold or other conductive material can be formed by vapor deposition, sputtering, or other technique on the surface of the silicon substrate 1. The silicon substrate 1 and the glass plate 3 are preferably fixed to one another by anodic bonding, and such a conductive film is, therefore, formed on the side of the silicon substrate 1 facing the nozzle plate 2. Fig. 5 is a sectional view through line III-III in Fig. 4. When the head driver 190 applies a drive voltage to the opposing electrodes of an electrostatic actuator, the latter is charged and the electrostatic force produced between its two electrodes causes the flexible one of them, i.e., the diaphragm 8, to deflect towards the other one, i.e., the segment electrode 10. This deflection expands (increases the volume of) the ink chamber 5 (see Fig. 5B). When the charge is then rapidly discharged by the head driver 190, the diaphragm 8 returns as a result of its inherent elastic restoring force, and, thereby, suddenly contracts (reduces the volume of) the ink chamber 5 (see Fig. 5C). The pressure pulse thus generated inside the ink chamber 5 causes part of the ink filling the ink chamber 5 to be ejected as an ink droplet from the corresponding nozzle 11. As explained before, part of the gap between the two electrodes of each actuator is smaller (gap G2) than the other part (gap G1). Compared with bottom part 8a corresponding to the larger gap G1, the bottom 8b corresponding to the smaller gap G2 can be more easily attracted to the protrusion 92b, i.e., deflecting bottom 8b requires a smaller drive voltage than does deflecting bottom part 8a. It is therefore possible to achieve two diaphragm deflection modes depending upon whether a relatively high drive voltage, which causes all of the diaphragm to deflect towards the surface 92, or a relatively low drive voltage, which is only high enough to deflect bottom 8b of diaphragm 8, is applied. More specifically, a deflection mode in which the whole diaphragm 8 is deflected to ejected an ink droplet, and a deflection mode in which only part of diaphragm 8 is deflected, just to move the ink near the nozzle, can be achieved by applying the appropriate drive voltage. The latter deflection mode with only part of diaphragm 8 involved causes minute vibrations of the meniscus formed by the ink at or near the respective nozzle orifice. Drive circuit An example of a drive circuit that is preferably employed is described next with reference to Figs. 6 to 8. Fig. 6 is a circuit diagram showing an exemplary selector 150 (see Fig. 1). Fig. 7 is a circuit diagram of the major components of a head driver 190 comprising a drive voltage selection function. Shown in Fig. 6 are a receive buffer 110, which functions as the print pattern memory 110 in Fig. 1, and the selector 150. It should be noted that receive buffer 110, selector 150, and drive signal generator 180 can be integrated into a single device using a gate array. The receive buffer 110 stores one column of print data, outputting data to the selector 150 and fetching data from the receive port 170 in response to a latch signal La applied from the CPU 200. The column of print data corresponds to one or more lines of nozzles arrayed in or substantially in the direction of paper transport. As shown in Fig. 6, the selector 150 comprises a respective group of two AND gates 152, 153 and one OR gate 154 for each nozzle. Depending upon the state of a selection signal Sel . supplied from CPU 200, the selector 150 selects either the print data from the receive buffer 110, or the flushing data from the flushing data generator 160, and outputs the selected data to the drive signal generator 180. When selection signal Sel is at a low logic level (referred to as "L" hereinafter), NOT gate 151 outputs a high logic level (referred to as "H" hereinafter) causing one input of AND gate 152 to go H. As a result, the print data from receive buffer 110 supplied to the other input of AND gate 152 is passed to the drive signal generator 180. However, if selection signal Sel is H, the flushing data Pd is output to the drive signal generator 180 instead. As a result, flushing data causing an ink droplet to be discharged from all nozzles may be periodically supplied to the drive signal generator 180. Referring to Fig. 7, drive signal generator 180 has a number of stages equal to the number n of segment electrodes 10 (= number of nozzles). Each stage comprises a NAND gate 181 and a NOT gate 182. A timing pulse Tp of a specific pulse width is applied to one input terminal of each NAND gate 181. The drive signals D1 to Dn from the selector 150 are each inverted by the respective NOT gate 182 and then applied to the other input terminal of the respective NAND gate 181. The head driver 190 comprises a common electrode driver 190a for applying a voltage to the common electrode terminal 22 (and thus the diaphragms 8), and a segment electrode driver 190b for applying drive voltages to each segment electrode 10 based on the drive signals D1 to Dn. The common electrode driver 190a can switch the voltage applied to the common electrode terminal 22 between a first voltage V1 and ground (0 V); the segment electrode driver 190b can switch the voltage supplied to the segment electrode 10 between the first or a second voltage V1/V2 and ground (0 V). It should be noted that V1 is greater than V2, and a voltage of either V1 or V1-V2 can be applied across the two opposing electrodes (diaphragm 8 and segment electrode 10) of a respective actuator. Including 0 V, three different voltages can thus be applied to the opposing electrodes. The common electrode driver 190a comprises primarily transistors Q1 and Q2, and resistors R1 and R2. The timing pulse Tp is applied to the input terminal of the common electrode driver 190a. When timing pulse Tp becomes H, transistor Q1 turns on and voltage V1 is applied to the common electrode terminal 22. When the timing pulse Tp goes to L, transistor Q1 goes off and transistor Q2 goes on, thus connecting common electrode terminal 22 to ground. The segment electrode driver 190b has a number of stages equal to the number n of segment electrodes 10; each stage comprises primarily transistors Q3 and Q4 and resistors R3 and R4. Each stage of the segment electrode driver 190b has an input terminal connected to a respective output terminal of the drive signal generator 180. The emitters of transistors Q3 are connected to a terminal Vb to which the voltage selector 130 applies either the first voltage V1 or the second voltage V2. During normal printing, i.e., when, in accordance with the print data, some nozzles are to eject ink droplets and others not, the first voltage V1 is applied to terminal Vb. For the discharge-free recovery, however, the second voltage V2 is applied to terminal Vb. Considering by way of example the x-th nozzle 11-x, when drive signal Dx for nozzle 11-x is H, i.e., nozzle 11-x is to discharge ink, transistor Q4 turns on and the corresponding segment electrode 10-x goes to ground. When drive signal Dx for nozzle 11-x is L, that is, nozzle 11-x is not to discharge ink, transistor Q3 goes on and voltage V2 is applied to the corresponding segment electrode 10-x when the timing pulse Tp is H. When timing pulse Tp is L, transistor Q4 is on keeping segment electrode 10-x at ground irrespective of whether Dx is L or H. The table in Fig. 8 shows the relationship between the logic levels of the timing pulse Tp and the drive signal Dx, the voltages (potentials) applied to the opposing electrodes and the effect on the diaphragm, i.e., its different deflection modes. When timing pulse Tp and drive signal Dx are both H, the voltage between the opposing electrodes is V1, the capacitor of the respective actuator is charged, and all of diaphragm 8 deflects toward the segment electrode (state [1]). When the timing pulse Tp goes L from state [1], the opposing electrodes go to the same potential, the accumulated charge is, thus, discharged, the diaphragm 8 returns to the original position, and an ink droplet is ejected from the corresponding nozzle 11 by the pressure pulse thus generated in the ink chamber 5 (state [2]). When timing pulse Tp is high and drive signal Dx is low, the voltage between the opposing electrodes is V1-V2. Only the part (bottom 8b) of diaphragm 8 opposite to the protruding part 10b of the segment electrode 10 is, therefore, deflected (state [3]). When the timing pulse Tp goes low from state [3], the opposing electrodes go to the same potential, the accumulated charge is discharged, and the diaphragm 8 returns to the original position. The magnitude of the diaphragm's displacement while transiting from state [3] to the original position (state [2]) is sufficiently smaller, however, than that produced by the change from state [1] to state [2]. As a result, the pressure generated in the ink chamber 5 this time is not sufficient to discharge ink from the respective nozzle 11, and the vibration of diaphragm 8 results therefore in moving or oscillating the ink around the nozzle 11 and cause a minute vibration of the ink meniscus in the nozzle. Fig. 8 illustrates only the case where the voltage selector 130 applies the second voltage V2 to terminal Vb. As mentioned before, V2 is applied only for effecting the discharge-free recovery. During printing, however, the first voltage V1 is applied to terminal Vb. As will be appreciated by those skilled in the art, in this case 0 V is applied to the opposing electrodes both in state [2] and in state [3] because the voltage applied in state [3] is V1-V1. The operation of the above circuits is further described below with reference to the timing chart in Fig. 9. The selection signal Sel output from the CPU 200 is set to L in order to print. The column print data read into receive buffer 110 is then passed to the drive signal generator 180 in response to the latch signal La output from the CPU 200. This selection signal Sel remains low while printing continues. As a result, the column print data continues to be passed to the drive signal generator 180 and output to the head driver 190. As shown in Fig. 9, timing pulse Tp applied to drivers 190a and 190b is a periodic pulse of period T and pulse width Pw. The time from the start of charging the capacitor of a respective actuator to the start of discharging is determined by pulse width Pw. As mentioned before, motor 45 for moving the carriage 302 is driven synchronized to this timing pulse Tp. A latch signal (which may be the same as latch signal La) applied to the receive port 170 is also synchronized to the timing pulse Tp. Drive signal Dx input to the drive signal generator 180 is controlled to be H synchronized to the timing pulse Tp at positions at which, according to the print data, an ink droplet is to be ejected. As shown in the figure, if the first dot, following an arbitrary position, is to be printed and the second and third dots are not to be printed, drive signal Dx is output in the sequence Dx1=H, Dx2= L, Dx3= L. Note that in Fig. 9, row (2), the trapezoidal pulses denoted Dx1, Dx2 and Dx3 only represent the width of the data in the drive signal sequence, not the logical level. The logical level is indicated by the symbols "•" representing H and "O" representing L. When this sequence of drive signal Dx is output, drive pulses of pulse width Pw and amplitude of V1, V1-V2 and V1-V2 are applied to the opposing electrodes. That is, an ink droplet is ejected at dot 1, but at dots 2 and 3 an ink droplet is not ejected and ink near the nozzle is simply moved. Using a circuit as described above, a low amplitude drive pulse can be applied to move ink near a nozzle, and thereby prevent an increase in ink viscosity near the nozzle, using a simple circuit configuration and without requiring complex control. It is therefore possible to prevent a rise in ink viscosity at infrequently used nozzles. That is, a difference in viscosity at nozzle orifices in the same ink jet head resulting from a difference in the frequency of use can be reduced, the interval between flushing operations can be increased, and unnecessary consumption of ink for these flushing operations can be reduced. This technique is particularly effective with color ink jet printers comprising a plurality of nozzles for each of the used ink colors because it is particularly easy for a difference in the frequency of nozzle use to occur in such ink jet heads. The latch signal La output from the CPU 200 is stopped, and any printing process interrupted when a flushing operation is to be performed. The ink jet head 30 is then moved to the home position R, the selection signal Sel goes H, flushing data causing all nozzles to discharge ink is supplied to the drive signal generator 180, and all nozzles are thus driven to eject a certain number of droplets. The number of droplets to be ejected can be determined, case by case, in accordance with the times KJ and Tf. This helps keeping the ink consumption for the flushing as low as possible. By holding all drive signals D1 to Dn at logic level L while applying the timing pulses Tp as the ink jet head 30 moves to the home position R, a low amplitude drive pulse for moving ink near the nozzles can be applied to all nozzles to suppress an increase in ink viscosity near the nozzles. It should be noted that the drive circuit in this exemplary embodiment is described as driving an ink jet head having an electrostatic actuator as the pressure generating device. It will be obvious, however, that the same effect can be achieved with this type of circuit to drive an ink jet head using a piezoelectric element or a heating element as pressure generating device. Displacement of a piezoelectric element can be varied by controlling the voltage of the drive pulse applied. The heat generated by a heating element can be similarly controlled by the voltage of the drive pulse applied. It is, therefore, possible also in these cases, by applying an appropriate voltage, to only stimulate movement of ink near the nozzle but not cause any ink discharge. Furthermore, the electrostatic actuator of the present embodiment has a step (G1, G2) in the gap between opposing electrodes in the embodiment described above. It will also be obvious, however, that a deflection mode for discharging ink droplets, and a deflection mode for only stimulating movement of ink near the nozzles, can be achieved without such a step in the electrode gap by appropriately controlling the pulse width and voltage of the drive pulse applied to the opposing electrodes. Yet further, it will also be obvious that the drive circuit described in this preferred embodiment can be applied to a serial or line type ink jet head. Control Method Fig. 10 is a state transition diagram used to describe a method of controlling the printer according to a preferred embodiment of the invention. It should be noted that the initialization processes performed when the printer is turned on to initialize the control unit 100 and the printing unit 40 are identical to those of a common ink jet printer. Following such initialization a head recovery (purging) process is performed to remove ink from the nozzles that has increased in viscosity while the printer was not in use. While the printer is operable (power is on) it can be in any one of the following three basic states: a standby state B1, a printing state B2, and a capped state B3 (the capped state differs from the standby state B1 in that the nozzles 11 of the ink jet head are covered with the cap 304 and in that, in the standby state, the ink jet head need not necessarily be in the home position). In addition to these basic states there are the following transitional "states": the purging process (block B11), the flushing process (blocks B21, B22 and B23), and the discharge-free recovery (block B31). At least one of the purging (block B11), the flushing (blocks B21 to B23), and the discharge-free recovery (block B31) is performed in response to each print command, that is, when there is data to be printed; which of the three recovery processes, purging, flushing and/or discharge-free recovery, is performed in response to a print command depends on the values of the counters 220KJ, 220f, 220t, and 220k. As mentioned before, counter 220k counts the time Tk since the last discharge-free recovery process (block B31); counter 220f counts the time Tf since the last flushing operation (blocks B21 to B23); counter 220t counts the idle time Tt during which the printer is not printing (state B1); and counter 220KJ counts the capping time KJ, i.e., the time during which the ink jet head is capped (block B3). The transitions from the respective basic states of the printer and the conditions under which the transitions occur are explained in detail below. (1) No Printing Whenever the printer enters the standby state B1 counter 220t is reset and started to count the idle time Tt. If the printer stays in the standby state B1 for an idle time of Tt ≥ 10 s, there is a state transition to the capped state B3, i.e., the ink jet head 30 is capped. The printer remains in the capped state until it receives a print command. If a print command is received while the printer is in this capped state B3, the next control state is determined according to the duration of the capped state (capping time KJ): If capping time KJ < 15 h, flushing is executed as represented by block B21. This flushing removes ink that has increased in viscosity while the printer was in the standby state B1 or the capped state B3, and nozzle clogging at the resumption of printing is avoided. Counters 220KJ, 220k, and 220f are reset after the flushing. The printer then transits to the printing state B2, i.e., the ink jet head 30 is uncapped and moved to the print start position whereupon the print data is printed. If the capping time KJ ≥ 15 h, purging is performed as represented by block B11, and then the printer transits to the printing state B2. Purging removes ink that has increased in viscosity and any bubbles. Counters 220KJ, 220k, and 220f are reset after the purging. It is to be noted that discharge-free recovery (block B31) can be performed after the purging and before the printer moves to the printing state B2. In this case, it is possible to improve the quality of printing. Experiments have shown that the discharge-free recovery prior to printing ensures ink droplets, especially for the first dot, being ejected at the normal speed or volume which may not be the case otherwise. When the printer is in the standby state B1 it can be forced by a command CL to perform purging as indicated in Fig. 10. (2) Continuous Printing As printing continues, counter 220f measures the time Tf elapsed since the last flushing, and flushing (block B23) is performed every 6 seconds, i.e., each time the counter 220f has counted Tf = 6 s. Counters 220f and 220t are then reset, and printing continues. By interrupting continuous printing operations for flushing in this manner, clogging can be prevented in infrequently used nozzles. It should be noted that while the carriage 302 is moving the ink jet head 30 from the home position R to the printing position after this flushing to resume printing, and while the carriage is moving the ink jet head 30 during such continuous printing from one position to another one with no printing in between, the discharge-free recovery (block B31) is performed. This discharge-free recovery prevents formation of a film on the ink in each nozzle, and can thus prevent nozzle clogging. Since this discharge-free recovery is performed while the ink jet head is not printing and is being moved to the next printing position it does not cause any drop in printing speed. In addition, such discharge-free recovery allows the period of the flushing to be longer (6s in this example) than it would have to be otherwise. (3) Intermittent Printing Intermittent printing means there is a pause (standby state B1) of less than 10 seconds (Tt < 10s) in between successive printing operations. In this case, whether flushing (block B22) or discharge-free recovery (block B31) is performed before the start of the next printing operation (state B2) depends on times Tf and Tk: If time Tk since the last discharge-free recovery is Tk < 2 s, and time Tf since the last flushing is Tf < 6 s, the discharge-free recovery (B31) is performed. After this discharge-free recovery is finished, counter 220k is reset and the printing state B2 entered. It should be noted that the carriage 302 (with the ink jet head 30 mounted on it) is waiting at the home position R or inside the printing range P during the standby state B1, and is moved from there to the actual printing position. According to this preferred embodiment of the invention, the discharge-free recovery is accomplished during this carriage movement before printing starts. Therefore the discharge-free recovery does not lead to a drop in printing speed. All nozzles are driven during this discharge-free recovery while the ink jet head is being moved. If time Tk since the last non-discharge process is Tk ≥ 2 or time Tf since the last flushing operation is Tf ≥ 6 s, flushing (B22) is performed and counters 220f and 220k are reset before the printer enters the printing state B2 again. In this case it is assumed that nozzle clogging cannot be reliably avoided by means of the discharge-free recovery. According to this preferred embodiment, a discharge-free recovery process is employed when a there is only a short printing pause (less than 2 s); otherwise nozzle clogging is prevented by means of flushing. Unnecessary consumption of ink not used for actual printing is therefore less than in the case where flushing is always performed before printing is resumed, and, yet, nozzle clogging can be reliably avoided.

Claims: An ink jet printer comprising: an ink jet head (30) having a plurality of nozzles (11) each nozzle associated with a respective pressure generating actuator (8, 10), first drive means (110, 150, 180, 190) for selectively driving said actuators (8, 10) so as to apply pressure to the ink of a respective nozzle (11) sufficient to eject an ink droplet from that nozzle, a transport mechanism (195, 45) for causing movement of said ink jet head (30) relative to a printing medium (105), a controller (200), responsive to a print command, for causing said transport mechanism (195, 45) to move said ink jet head (30) into a print start position relative to said printing medium (105) and, subsequently, to activate said first drive means (110, 150, 180, 190) in order to perform a printing operation, second drive means (180, 190) for driving said actuators (8, 10) so as to apply pressure to the ink of a respective nozzle (11) sufficient to move the ink without causing ink to be discharged from that nozzle, third drive means (150, 160, 180, 190) adapted to be activated by said controller (200) for driving all of said actuators (8, 10) to apply pressure to the ink in the respective nozzles (11sufficient to eject ink droplets from the nozzles for flushing the ink jet head (30), and a first counter (220f) for measuring, as a first time (Tf), the time elapsed since the most recent activation of said third drive means (150, 160, 180, 190), characterized by a second counter (220k) for measuring, as a second time (Tk), the time elapsed since the most recent activation of said second drive means (180, 190), wherein said controller (200) is responsive to a print command to activate either said second or said third drive means depending on said first and second times (Tf, Tk). The printer of claim 1, further comprising: a cap (304) for covering said nozzles (11) of the ink jet head (30), and a third counter (220t) for measuring, as a third time (Tt), the continuous time during which said first drive means (110, 150, 180, 190) remains inactive, wherein said controller (200) is adapted to cause said nozzles (11) to be capped with said cap (304) when said third time (Tt) is equal to or longer than a predetermined first value. The printer of claim 2, further comprising: a pump (303) for sucking ink from said nozzles (11) for purging the nozzles; and a fourth counter (220KJ) for measuring, as a fourth time (KJ), the continuous time during which said nozzles (11) remain capped, wherein said controller (200) is responsive to said print command to activate, prior to moving the ink jet head (30) to said print start position, said pump (303) when said fourth time (KJ) is equal to or longer than a predetermined second value, and said third drive means (150, 160, 180, 190) when said fourth time (KJ) is smaller than said second value. The printer according to any one of the preceding claims, wherein said controller (200) is implemented as a program-controlled microprocessor. A method of controlling an ink jet printer as defined in any one of claims 1 to 4, comprising the following steps: (a) detecting a print command, (b) controlling, in response to step (a), said transport mechanism (195, 45) so as to move the ink jet head (30) relative to the printing medium (105) into a print start position, and (c) executing, following step (b), said print command by selectively driving said actuators (8, 10) so as to eject ink droplets from respective nozzles (11) onto the printing medium (105), and (d) driving the actuators (8, 10) in the ink jet head (30) so as to move the ink in each nozzle (11) without discharging ink from the nozzles, (e) flushing the ink jet head (30) by discharging an ink droplet from each nozzle (11), (f) measuring, as a first time (Tf), the time elapsed since the most recent execution of step (e), and (g) measuring, as a second time (Tk), the time elapsed since the most recent execution of step (d), wherein, depending on the times (Tf, Tk) measured in steps (f) and (g) step (e) and/or step (d) is performed is response to a print command. The method of claim 5, further comprising: (h) measuring, as a third time (Tt), the continuous time during which step (c) is not executed, and (i) covering said nozzles (11) with a cap (304) when said third time (Tt) is equal to or longer than a predetermined first value. The method of claim 6, further comprising: (j) measuring, as a fourth time (kJ), the continuous time during which said nozzles (11) remain capped, (k) reading the value of said fourth time (kg) in response to step (a) and performing step (e) when the value read in step (k) is smaller than a predetermined second value, while otherwise (l) purging the nozzles (11) by sucking ink from the nozzles. A machine-readable storage medium carrying a computer program that when executed in a printer as defined in claim 4 implements the method as defined in any one of claims 5 to 7.

Inventors: FUJII MASAHIRO, YAMAZAKI TSUTOMU, FUJII, MASAHIRO, YAMAZAKI, TSUTOMU
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 2000
Main classification: B41J2
IPC codes: B41J2, B41J29