An ink jet printing apparatus and a method of controlling it

Abstract: A printing apparatus comprises an ink jet head (1) including: a nozzle (11), a pressure chamber (5) having, an ink supply path (6), and an electrostatic actuator (10, 51) comprising a diaphragm (51), as a first electrode, forming one wall of said pressure chamber and an opposing wall (91), as a second electrode, disposed externally to said pressure chamber and opposite to said diaphragm. The diaphragm comprises N continuous segments (51a-51c) and N gaps (G1-G3) are formed in a diminishing size between the N segments of said diaphragm and said opposing wall, respectively. Drive means (21) are provided for applying different drive voltages to said electrostatic actuator at different timings, said different drive voltages including: a first drive voltage capable of forcing all of said N segments of said diaphragm to contact said opposing wall, a second drive voltage capable of maintaining contact between at least one of said N segments of said diaphragm and said opposing wall with other segments of said diaphragm being released, a third drive voltage capable of releasing contact between all of said N segments of said diaphragm and said opposing wall, and a group of drive voltages capable of maintaining contact between only selected ones of said N segments of said diaphragm and said opposing wall.

Description: The present invention relates to a printing apparatus using an ink jet head and to a method of controlling the ink jet head. More particularly the invention relates to a technology for controlling the pressure in the pressure generating chamber that applies an ejecting pressure to the ink contained in the chamber. In general, an ink jet head comprises a pressure generating chamber for applying pressure to ink to eject the ink from a nozzle. One end of the pressure generating chamber is typically connected to an ink tank through an ink supply path, and the other end to a nozzle opening from which the ink droplets are ejected. Part of the pressure generating chamber is made to be easily deformed and functions as a diaphragm. This diaphragm is elastically displaced by an electromechanical conversion means to generate the pressure that ejects ink droplets from the nozzle opening. Recording apparatuses using this type of ink jet head offer outstanding operating characteristics, including low operating noise and low power consumption. They are widely used as hard copy output devices for a variety of information processing devices. As the performance and functionality of information processing devices has improved, demand has also risen for even higher quality and speed printing both text and graphics. This has made urgent the development of technologies enabling even finer and smaller ink droplets to be ejected consistently at even higher frequencies i.e., higher printing speed. (1) Ink ejection frequency Because of the structure of the ink jet head as described above, after ink ejection, vibration remains in the ink inside the pressure generating chamber (also called the ink chamber because it is filled with ink; hereafter "ink chamber"). This residual vibration can easily result in undesirably ejected ink droplets (also called "satellites"). To avoid this, the flow resistance of the ink supply path connecting the ink chamber and ink tank is conventionally set high as a means of accelerating attenuation of residual ink vibration. However, if the flow resistance of the ink supply path is high, the refill supply rate of ink to the ink chamber, after ink ejection, drops, thereby lowering the maximum ink ejection frequency, and thus lowering the printing speed of the printing device. The applicants thus developed and disclosed in JP-A-6-320725/1994 and EP-A-0 573 055 a technology for forming a thin-wall part in the diaphragm to create a flexible wall part that deforms according to the pressure inside the ink chamber. This thin-wall part is used to absorb residual ink vibration in the ink chamber as a means of avoiding undesirable ink ejection or satellite emissions. It is therefore not necessary to set the flow resistance of the ink supply path high because ink ejection does not occur even if there is residual ink vibration, and the ink ejection frequency can therefore be increased. With regard to the technology described in JP-A-6-320725/1994, the compliance (i.e., volume change per unit pressure change) of the ink chamber increases due to the thin-wall part of the diaphragm. While this reduces satellites, the ejection speed required for stable ink ejection cannot be obtained because the pressure generated by the diaphragm for ink ejection is not used effectively for propelling the ink droplets. Furthermore, when the diaphragm drive force is increased to assure sufficient ejection speed, a higher drive voltage is required. This, in turn, increases both the size of the drive device and power consumption. (2) Improving image quality with technologies for varying droplet size Expressing various density gradations by changing the size of the ink droplets formed on the recording medium is a preferred means of improving image quality. The size of the ink droplets output by any recording apparatus, such as a printer, using an ink jet head is determined by various factors, one of which is the size (also called "ink ejection mass") of the ink droplets ejected by the ink jet head. A technology providing plural electrostrictive means of different sizes at the ink chamber, and separately controlling and driving these electrostrictive means to eject ink droplets of various sizes, is described in JP-A-55-79171/1980. When this technology is applied, each of the plural, different size electrostrictive means used to deform the diaphragm must be independently driven, resulting in increasing the number of wires needed, and thus making it difficult to achieve a high nozzle density. The number of drivers also increases because of the need to separately drive each actuator, and this makes it difficult to reduce the device size. (3) Improving image quality through a high droplet density Most ink jet heads usually have plural nozzles arrayed in a straight line. Printing devices using such ink jet heads output two-dimensional images by moving the ink jet head across the recording medium in a direction roughly perpendicular to this nozzle line. Therefore, to achieve high image quality by increasing the ink droplet density, it is necessary to reduce the distance between adjacent nozzles (also known as the "nozzle pitch"). An ink jet head using an electrostatic actuator developed and manufactured by the applicants can be manufactured using a production process similar to that used for semiconductor manufacture, and is one of the technologies best suited to achieving a high ink droplet density. The basic structure of this ink jet head is described in JP-A-5-50601/1993, and can be used to reduce the nozzle pitch without changing the size of the ink droplets by narrowing the width and increasing the length of the ink chamber. EP-A-0 629 503 discloses a printing apparatus according to the preamble of claim 1 wherein each electrostatic actuator is associated with a respective pressure chamber. The pressure chamber has an opening in communication with a nozzle (11) and is connected to an ink supply path (6) for supplying ink to the pressure chamber. One wall of the pressure chamber is formed by a flexible diaphragm which has its outer side facing an opposing wall. The actuator comprises to electrodes, one formed by the diaphragm and the other provided on the opposing wall. The diaphragm has a uniform thickness and width throughout its length and the gap between the diaphragm and the electrode on the opposing wall is also uniform. The diaphragm is controlled so as to prevent contact with the electrode on the opposing wall. An ink jet head using electrostatic actuators as described in JP-A-5-50601/1993 (EP-A-0 479 441) can decrease the nozzle pitch without changing the size of the ink droplets. In this case, however, the compliance increases significantly as described below, and a high voltage is therefore required to drive the electrostatic actuator. It is an object of the invention to provide a printing apparatus using an ink jet head, in which the pressure generated by the pressure generating means can be effectively used for ink droplet ejection and satellite emissions can also be suppressed. Another object of the invention is to provide a method of controlling such printing apparatus. These objects are achieved with a printing apparatus as claimed in claim 1 and a method as claimed in claim 4, respectively. Preferred embodiments are subject-matter of the dependent claims. The gap between the diaphragm of the electrostatic actuator and the opposing wall is preferably formed such that the gap size increases from the ink supply path end to the ink nozzle end of ink chamber 5. As a result, by increasing or decreasing the number of segments of diaphragm held in contact with the opposing wall during ink droplet ejection, the compliance of the ink chamber during ink droplet ejection can be changed. Thus, the characteristic vibration frequency of the ink oscillation can be variably controlled. This also means that the volume of the ejected ink droplet can be adjusted. In general, the higher the characteristic vibration frequency of the ink vibration, the finer the ejected ink droplets can be made; and the smaller the displacement volume resulting from diaphragm deflection, the smaller the volume of the ejected ink droplets. In one embodiment of the method according to the invention, a step for selecting one drive voltage from the group of voltages as the second drive voltage according to the print signal may be performed before the second step of the method. It is therefore possible to select the part of the diaphragm contributing to ink droplet ejection. The ejected ink droplet mass can be varied according to the print signal. This technique enables printing various density gradations. When the drive circuit of a printing apparatus embodying the invention comprises a charge/discharge circuit, the control method further preferably comprises a first step for charging the electrostatic actuator to at least the first drive voltage; a second step for discharging the electrostatic actuator to the second drive voltage at a first discharge rate after a first predetermined time has passed after the first step; and a third step for discharging the electrostatic actuator at a second discharge rate after the second process. Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings, in which: Fig. 1 is a simplified longitudinal cross-sectional view, taken along line I - I in Fig. 2, of an ink jet head employing an electrostatic actuator. Fig. 2 is a plan view of the ink jet head shown in Fig. 1. Figs. 3A, 3B and 3C are simplified lateral cross-sectional views, taken along line III - III in Fig. 2; Fig. 3A shows the standby state, Fig. 3B shows the state when ink is supplied, and Fig. 3C shows the state when the ink is compressed or pressurized. Fig. 4 is a graph showing the relationship between the distance from the electrode segment and the force acting on the diaphragm when the diaphragm is displaced. Fig. 5 is a simplified lateral cross-sectional view of an ink jet head according to an embodiment of the present invention. Fig. 6 illustrates the operation of the ink jet head according to the embodiment shown in Fig. 5. Fig. 7 illustrates the operation of the ink jet head according to the embodiment shown in Fig. 5. Fig. 8 is a circuit diagram of one example of a drive circuit for an ink jet head according to the embodiment shown in Fig. 5. Figs. 9A-9E are signal timing charts for illustrating the operation of the drive circuit shown in Fig. 8. Fig. 10 is a waveform diagram showing the voltage waves between the opposing electrodes for illustrating the operation of a drive method for an ink jet head according to the embodiment shown in Fig. 5. Fig. 11 illustrates the elastic displacement of the diaphragm in an ink jet head according to the embodiment shown in Fig. 5. Throughout the drawings like reference symbols refer to like parts. To facilitate understanding of the present invention, the general structure and operation of an ink jet head employing electrostatic actuators will first be described with reference to Figs. 1 to 4. Fig. 1 is a cross-sectional view of an ink jet head, Fig. 2 is a partial plan view of Fig. 1, and Figs. 3A-3C are partial cross-sectional views of Fig. 2. In the example shown in these figures, ink jet head 1 is a three-layer lamination which includes a nozzle plate 3 comprising, for example, silicon, a glass substrate 4 comprising, for example, borosilicate having a thermal expansion coefficient close to that of silicon, and a center substrate 2 comprising, for example, silicon. Plural independent ink chambers 5, a common ink chamber 6, and ink supply paths 7 connecting common ink chamber 6 to each of the ink chambers 5, are formed in the center substrate 2 by, for example, etching channels corresponding to each of these components in the surface of center substrate 2 (i.e., the top surface as seen in Fig. 1). After etching, nozzle plate 3 is bonded to the surface of center substrate 2 to complete the formation of the various ink chambers and ink supply paths. Ink nozzles 11 each opening into a corresponding one of ink chambers 5 are formed in nozzle plate 3 at positions corresponding to one end of each ink chamber 5. As shown in Fig. 2, ink supply port 12 opening into common ink chamber 6 is also formed in nozzle plate 3. Ink is supplied from an external ink tank (not shown in the figures) through ink supply port 12 to common ink chamber 6. The ink stored in common ink chamber 6 then passes through ink supply paths 7, and is supplied to each of the ink chambers 5. Ink chambers 5 are provided with a thin bottom wall or bottom wall portion which forms a diaphragm 8 elastically displaceable in the vertical direction as seen in Fig. 1. Shallow recesses 9 are formed by, e.g. etching, in the top side of glass substrate 4 at positions corresponding to each of the ink chambers 5 in center substrate 2. As a result, the diaphragm 8 of each ink chamber 5 faces recess surface 92 with a narrow gap G therebetween. In actual products the gap length may in the range of about 0.2 to 1 µm, the actual value being preferably determined based on the possible precision of manufacturing technology and the other dimensional parameters including the thickness of the diaphragm so as to obtain the desired function with little drive energy being required. Because recesses 9 of glass substrate 4 are disposed opposite diaphragms 8 of ink chambers 5, recesses 9 are referred to as the diaphragm-opposing wall, or simply opposing wall 91. In the described example, the diaphragm 8 of each ink chamber 5 functions as an electrode. An electrode segment 10 is formed on each recess surface 92. The surface of each electrode segment 10 is covered by insulation layer 15 comprising, for example, glass, and having a thickness GO as shown in Figs. 3A-3C. As a result, each electrode segment 10 and the opposing diaphragm 8 of the respective ink chamber form a capacitor having insulation layer 15 in between its electrodes and having an electrode gap of Gn. With one (electrode segment 10) of the electrodes of the capacitor being rigid and the other (diaphragm 8) being flexible, this structure can be used as pressure generating means in the form of an electrostatic actuator. A drive circuit 21 (shown in Fig. 2) is provided for driving the ink jet head by operating the electrostatic actuators (charging and discharging the capacitors) according to a print signal applied from an external source, such as a host computer, not shown in the figures. One output of drive circuit 21 is connected directly to each electrode segment 10, and the other output is connected to common electrode terminal 22 formed on center substrate 2. Drive circuit 21 will be described in detail later. If silicon is used for center substrate 2 it may be doped with impurities to become conductive and capable of supplying charge from common electrode terminal 22 to diaphragms 8. Note that for obtaining a low electrical resistance it is also possible to form a thin-film of gold or other conductive material by vapor deposition, sputtering or other process on one surface of a silicon substrate. Center substrate 2 and glass substrate 4 are bonded by anodic bonding in the described example. A conductive film is therefore formed on the surface of center substrate 2 in which the ink supply paths are formed. Cross-sectional views taken along line III-III in Fig. 2 are shown in Figs. 3A-3C. When a drive voltage is applied from drive circuit 21 to a capacitor formed by the opposing electrodes as mentioned above, a Coulomb force in the form of an attraction force is generated resulting in diaphragm 8 being deflected toward electrode segment 10, thereby increasing the volume of ink chamber 5, as shown in Fig. 3B. When the charge stored in the capacitor is then rapidly discharged by drive circuit 21, diaphragm 8 returns to its original position due to its resiliency or restoring force, thus rapidly reducing the volume of ink chamber 5, as shown in Fig. 3C and increasing the pressure therein. The increased pressure forces part of the ink contained in ink chamber 5 to be ejected as an ink droplet from the ink nozzle 11 associated with that ink chamber. The relationship between the voltage applied to the opposing electrodes forming a capacitor and the behavior of diaphragm 8 is described next with reference to Fig. 4. Fig. 4 is a graph showing the relationship between the force acting on diaphragm 8 and the distance between the opposing electrodes 10 and 8 when diaphragm 8 is displaced. The restoring force of diaphragm 8 is shown by the straight lines in Fig. 4. Note that the restoring force of diaphragm 8 increases proportionally to the displacement as diaphragm 8 is deflected from the position of gap length G1 toward the electrode segment. The absolute value of the slope of the restoring force line expresses the reciprocal of the compliance of diaphragm 8; thus, as compliance increases, the slope decreases. The curved lines in Fig. 4 indicate the Coulomb force acting on the diaphragm 8; the Coulomb force is inversely proportional to the square of the electrode gap if the applied voltage is assumed constant. Because the Coulomb force is also proportional to the square of the applied voltage, curve (a) shifts in the direction of arrow A as the applied voltage increases, and shifts in the direction of arrow B as it decreases. Fig. 4 illustrates the restoring force of diaphragm 8 for a plurality of (initial) electrode gaps, for example, G1, G2 and G3 between the opposing electrodes as are present in the embodiment of the invention shown in Fig. 5 and described in detail below. G0 in Fig. 4 is the thickness of insulation layer 15 shown in Figs. 3A-3C and represents the minimum distance between the electrodes. The position in which the diaphragm contacts the insulation layer 15 will be referred to below as the "contact position" or the position in which the diaphragm 8 contacts the opposing wall 91 (note that insulation layer 15 is fixed relative to the "opposing wall" 91 which is the member of substrate 4 below recess 9). In case of the gap length G1, values d1 and d2 indicate positions where the restoring force of diaphragm 8 and the Coulomb force acting on it are balanced, d1 being an unstable balance point and d2 being a stable balance point. More specifically, when a certain voltage is applied, diaphragm 8 is deflected from G1 to d2 and then stops. If due to an external force diaphragm 8 is then deflected to a position between d2 and d1, diaphragm 8 will simply return to d2 again when that external force is released. However, if diaphragm 8 is displaced by an external force beyond d1 to a point near the electrode segment, since the Coulomb force is greater than the restoring force, diaphragm 8 will be deflected to the contact position, i.e., to G0, and this contact position will be retained even after the external force is released. A high voltage shown in Fig. 4 as curve (b) is applied to the opposing electrodes to force diaphragm 8 with the gap length of G1 to contact the opposing wall. When this voltage is applied, there are no crossing points of curve (b) and the straight line passing through G1, i.e., balance points d1 and d2, and diaphragm 8 is immediately displaced to the contact position G0. It is to be noted that displacement of diaphragm 8 can be forced to overshoot d1 by suddenly reapplying a voltage after applying a voltage lower than this high voltage if the distance between d1 and d2 is sufficiently small. It is therefore also possible to force diaphragm 8 to the contact position using a lower voltage. In case of gap length G3, the voltage whose curve is denoted (d) in Fig. 4 is required for making diaphragm 8 to contact the opposing wall. This voltage is higher than that required for gap length G1. As described above, it is possible to make the drive voltages required for making individual portions of diaphragm 8 to contact the opposing wall different from each other by using different gap lengths for these portions. To return diaphragm 8 to the original position, the capacitor of the electrostatic actuator is fully or partially discharged as shown in Fig. 4, curve (c). This causes diaphragm 8 to begin moving toward the stable balance point d3 at a rate of acceleration determined by the difference between the diaphragm restoring force and the Coulomb force. As a result, if the applied voltage drops with sufficient speed, the restoring acceleration of diaphragm 8 will be sufficient to propel the ink droplets. Likewise, if the applied voltage is lowered gradually, the restoring acceleration of diaphragm 8 can be kept low enough to prevent ejection of any ink droplets. Diaphragm compliance Because a volume change in the ink chamber is effected by deforming the diaphragm, the term "compliance" is used here also to denote the amount of volume change of the ink chamber resulting from unit pressure change acting on the diaphragm 8. Note that in order to narrow the ink nozzle pitch, diaphragm 8 is designed with the smallest possible dimension in the direction in which the ink nozzles are arrayed, i.e., in the up and down direction as seen in Fig. 2 (the diaphragm "width" hereafter), and a large dimension in the direction perpendicular to the width (hereafter, the diaphragm "length"), e.g., a 3 mm length for a 200 micrometer width in this example. As a result, the rigidity across the width of diaphragm 8, except at the ends in the lengthwise direction of diaphragm 8, determines the amount of deformation in diaphragm 8 when an equally distributed load (pressure or Coulomb force) acts on diaphragm 8 as shown in Fig. 6. The following relationship can therefore be defined between the shape and compliance (Cm) of diaphragm 8: Cm = K * L * W 5 /T 3 where K is a constant, and L, W, and T are the length, width, and thickness, respectively, of diaphragm 8. As this equation shows, the compliance (Cm) of diaphragm 8 is proportional to the length (L), proportional to the fifth power of the width, and inversely proportional to the cube of the thickness (T), of diaphragm 8. It will also be obvious that the compliance of diaphragm 8, when diaphragm 8 is in contact with the opposing wall, can be considered equal to zero. This is because even if only a third of the width in the center of diaphragm 8 contacts the opposing wall, the compliance will be less than 1/100 th because compliance is proportional to the fifth power of the width. An embodiment of the present invention is described next with reference to Fig. 5. The gap G between diaphragm 51 and opposing wall 91 in this embodiment is described first. Gap between the diaphragm and opposing wall As shown in Fig. 5, the back of each diaphragm 51 is flat while opposing wall 91 formed on the surface of glass substrate 4 is formed in a stepped pattern descending lengthwise relative to ink chamber 5. This stepped pattern results in plural gaps of different dimensions between glass substrate 4 and diaphragm 51. The smallest gap G1 is formed at the end of ink chamber 5 nearest to ink supply path 7, i.e., between the diaphragm and the first step of opposing wall 91. Adjacent to gap G1 in the middle of diaphragm 51 is formed a second gap G2 greater than gap G1. The third gap G3 formed closest to ink nozzle 11 is the greatest gap between opposing wall 91 and diaphragm 51. These gaps, more accurately, the electrical gaps defined by the distance from the top surface of electrode segment 10 and the bottom of diaphragm 51 correspond to gap Gn in Fig. 3. The corresponding mechanical gaps are defined as these electrical gaps minus the thickness GO of the insulation layer 15. As described above, the gap G between diaphragm 51 and opposing wall 91 is formed sequentially along the length of the ink chamber such that the smallest gap G1, the intermediate gap G2, and the greatest gap G3 are formed in sequence from the ink supply path end to the ink nozzle end of ink chamber 5. As a result, by increasing or decreasing the number of parts of diaphragm 51 held in contact with the opposing wall during ink droplet ejection, the compliance of the ink chamber during ink droplet ejection can be changed. Thus, the characteristic vibration frequency of the ink oscillation can be variably controlled. This also means that the volume of the ejected ink droplet can be adjusted. In general, the higher the characteristic vibration frequency of the ink vibration, the finer the ejected ink droplets can be made; and the smaller the displacement volume resulting from diaphragm deflection, the smaller the volume of the ejected ink droplets. For example, if parts 51b and 51c of diaphragm 51 are driven while holding diaphragm part 51a at the smallest gap G1 in contact with opposing wall 91 as shown in Fig. 6, compliance is reduced by an amount corresponding to the length of part 51a contacting opposing wall 91 because the compliance is proportional to the working length of the diaphragm. The characteristic vibration period of the ink vibration is thus shortened compared with when the entire length of the diaphragm vibrates, and finer ink droplets can be ejected at high speed. In addition, if a part with a small gap G1 is formed, the corresponding part 51a of diaphragm 51 can be easily attracted to opposing wall 91 by applying a noticeably smaller drive voltage than is required with a larger gap. When a partially deflected state is thus formed, this point of partial deflection (i.e., partial contact between the diaphragm and the opposing wall) acts as the starting point for the gradual propagation of elastic displacement along the complete diaphragm as shown in Fig. 7. This is because the other parts of the diaphragm are pulled by part 51a past the unstable balance point, and are displaced until they contact the opposing wall. It is therefore possible to drive an ink jet head thus comprised using a lower voltage than is required when a small gap G1 is not formed. This means that when the same drive voltage is used, the compliance of the diaphragm contributing to ink droplet ejection can be reduced. This is also advantageous for achieving a high ink nozzle density. Specifically, the width of the diaphragm, i.e., the bottom wall of ink chamber 5, must be reduced in order to increase the nozzle density of the ink jet head. Compliance is thus reduced because it is proportional to the fifth power of the width as described above. It is to be noted that these gaps are formed in this embodiment to increase from the ink supply path end to the ink nozzle end of ink chamber 5. Displacement of the diaphragm thus progresses from the ink supply path toward the ink nozzle as shown in Fig. 7. This elastic displacement is propagated toward the nozzle end of the ink chamber. Elastic displacement of diaphragm 51 occurs in order to start an ink flow from ink supply path 7 toward ink nozzle 11, i.e., in the direction supplying ink to ink chamber 5. Ink supply can thus be accomplished quickly. A smooth supply of ink can therefore be achieved, and the ink ejection frequency can be increased. It will also be apparent that while the present embodiment has been described forming gap G in three stages (large, medium, and small gaps), it is also possible to form only a two stage gap, or to form four or more stages. The gap shall also not be limited to a stepped configuration with a finite number of different gaps as described above, and a continuously variable range of gaps can also be formed using a smooth curved or sloping surface. Ink jet head drive circuit A drive circuit suitable as voltage application means 21 (shown in Fig. 2) used to apply a voltage and thus drive an ink jet head constructed as described above is described below with reference to Fig. 8, which shows a circuit diagram of the drive circuit, and Fig. 9, which shows a timing chart of drive circuit operation. While the circuit shown in Fig. 8 is a preferred circuit, as would be appreciated by one of ordinary skill in the art, other circuit designs may be utilized. Charge signal IN1 in Fig. 8 is used to accumulate charges on the opposing electrodes (diaphragm 51 and electrode segment 10) to displace diaphragm 51, and is input through level-shift transistor Q1 to first current source circuit 400. First current source circuit 400 comprises primarily transistors Q2 and Q3, and resistor R1, and charges capacitor C with a constant current value. Discharge signal IN2 is used to discharge the charge on the opposing electrodes, and thus restore diaphragm 51 to the standby (non-displaced) state. Ejection volume control circuit 410 comprises first and second one-shot multivibrators MV1 and MV2. First one-shot multivibrator MV1 outputs a signal of pulse width Tx when discharge signal IN2 is input. Pulse width Tx output by first one-shot multivibrator MV1 may be one of three different pulse widths selectable by an ink ejection control signal in this embodiment. More specifically, the time constant of the time constant circuit which determines the output pulse width of the one-shot multivibrator MV1 is changed by switching with a resistance switcher SW the connected resistances R sw . Note that resistance switcher SW can be easily achieved using transistors or various other known switching circuit technologies. Second one-shot multivibrator MV2 outputs a signal of pulse width Td synchronized to the trailing edge of the pulse output from first one-shot multivibrator MV1. The output of first one-shot multivibrator MV1 is input to a second current source circuit 420, and the output of second one-shot multivibrator MV2 is input to a third current source circuit 430. Second current source circuit 420 comprises primarily transistors Q4 and Q5, and resistor R2; its purpose is to discharge the charge stored to capacitor C at a constant rate during period Tx based on the signal input from first one-shot multivibrator MV1. Third current source circuit 430 comprises primarily transistors Q10 and Q11, and resistor R3, the resistance of which is greater than that of resistor R2. Third current source circuit 430 is comprised to discharge the charge stored to capacitor C at a constant rate that is slower than the discharge rate of second current source circuit 420 during period Td based on the signal input from second one-shot multivibrator MV2. The terminals of capacitor C are connected to the output terminal OUT via a buffer comprising transistors Q6, Q7, Q8, and Q9. The common electrode terminal 22 of the ink jet head is also connected to the output terminal OUT, and the output of transistor T is connected to the respective electrode segment 10. While charge signal IN1 is active, capacitor C is charged with a constant current level. If the transistor T corresponding to the electrode segment of the nozzle from which a droplet is to be ejected is also on at this time, the corresponding pair of opposing electrodes will be charged to the same voltage level as the capacitor C. Because the capacitor C is discharged when the discharge signal is input, the charge stored on the opposing electrodes is also discharged through the corresponding diode D. The operation of a drive circuit thus comprised is described further below with reference to the timing chart in Fig. 9. When charge signal IN1, as shown in Fig. 9A, becomes active, the leading edge of the charge signal turns level-shift transistor Q1 and transistor Q2 of first current source circuit 400 sequentially on. Capacitor C is thus charged using a constant current value determined by resistor R1. The terminal voltage of capacitor C thus rises linearly from 0 volt with a constant slope 1 as shown in Fig. 9C, during the period TO (0 to time t1) (Fig. 9E). This slope 1 is determined by the resistance of resistor R1 and the capacity of capacitor C. Thus, by increasing the resistance of resistor R1, the charge rate of capacitor C and that of the opposing electrodes connected thereto through the buffer can be set low. This charge rate is determined with consideration given to, for example, the ink supply rate to the ink chamber. Ink thus flows from common ink chamber 6 into ink chamber 5 through the ink supply path because diaphragm 51 is displaced toward electrode segment 10, and ink chamber 5 expands. When charge signal IN1 becomes inactive after time TO has passed (at time t1), transistors Q1 and Q2 become off and charging of capacitor C thus stops. The voltage corresponding to the charges stored on the opposing electrodes is thus held at voltage VO at time t1, and diaphragm 51 stops while being in contact with electrode segment 10 via insulation layer 15. After a predetermined period Th has passed, discharge signal IN2 becomes active (Fig. 9B). Transistor Q4 of second current source circuit 420 is thus turned on by the signal (Fig. 9C) output from first one-shot multivibrator MV1 in ejection volume control circuit 410, and the charge stored to capacitor C is discharged during period Tx at a rate determined by resistor R2. The voltage between the terminals of capacitor C thus drops linearly with slope ð 2 based on the resistance of resistor R2. When a period determined by the output pulse width Tx of first one-shot multivibrator MV1 has passed, transistor Q4 becomes off, and discharging by second current source circuit 420 stops. At the same time, transistor Q10 in third current source circuit 430 is turned on by the signal (Fig. 9D) from second one-shot multivibrator MV2 in ejection volume control circuit 410, and discharging of the charge held in capacitor C begins again, this time through resistor R3. The resistance of resistor R3 is greater than the resistance of resistor R2, and the voltage between the terminals of capacitor C thus drops linearly but at a more gradual slope ð 3 (i.e., at a slower rate). Note that the pulse width Td of the signal output from second one-shot multivibrator MV2 is set with consideration given to both the ink ejection frequency and the time needed to completely discharge the charges on the opposing electrodes. Ink jet head drive method The drive method for the ink jet head described above is described next below with reference to Figs. 10 and 11. Fig. 10 shows one example of the voltage waveform between the opposing electrodes. They are charged so that the terminal voltage V10 rises to a peak voltage VO at time t1, and the peak voltage VO (V11) is then held until time t2. The terminal voltage is then decreased as described below to eject ink. The discharge process of the charges on the opposing electrodes (the "gap charge" below) is divided into two periods: a first period V12 in which the slope of the voltage drop relative to time is steep, and a second period contiguous to the first period but with a more gradual slope of the voltage drop curve. Specifically, discharging begins at time t2 following a known period from time t1 during which the gap charge is held at the peak voltage V0. The gap charge thus drops to voltage Va at time t3 following the rapid voltage drop curve of the first discharge period V12, and then drops to zero from time t3 following the more gradual voltage drop curve of the second period V13. It should be noted that the voltage drop target value of the first period V12 can be varied by drive circuit 21 of this embodiment between voltages Va, Vb and Vc, for example, as shown in Fig. 10. This can be specifically achieved by selecting the output pulse width of first one-shot multivibrator MV1 described above. For example, if the voltage drop target value is selected as voltage Vb or Vc, the voltage drops first to the selected target voltage and then to zero during period V14 or V15 at the same discharge rate used in period V13. Diaphragm 51 operates as described below when the gap charge is discharged in the first period V12 to Va at time t3, and then from time t3 to 0 V following the more gradual discharge slope of period V13. While the gap charge drops to voltage Va, part 51c of diaphragm 51 where the electrode gap G3 is greatest separates from surface 91a of opposing wall 91 first, and is elastically displaced toward the inside of ink chamber 5. This elastic displacement of diaphragm 51 is shown by the solid line in Fig. 11. As the voltage continues to drop gradually from this point, part 51b (at intermediate gap G2) and part 51a (at the narrowest gap G1) are separated sequentially from opposing wall 91, and are displaced into ink chamber 5 by their inherent elastic restoring force. When these parts 51b and 51a separate from opposing wall 91, however, ink droplet ejection is already completed. As a result, ink droplet ejection is effectively accomplished by the ink pressure generated inside ink chamber 5 by the elastic restoring energy of diaphragm part 51c disposed to the largest gap G3. During ink droplet ejection part 51b at intermediate gap G2, and part 51a at the smallest gap G1, respectively contact surfaces 91b and 91a of opposing wall 91, and the compliance of the ink vibration system is thus low. The characteristic vibration period can therefore be shortened, and fine ink droplets can be ejected at high speed. After ink droplet ejection, parts 51b and 51a of the diaphragm separate from opposing wall 91, and the compliance of the ink oscillation system is increased. Satellite emissions resulting from vibration of the ink are thus prevented. When the gap charge drops to voltage Vb at the slope of first period V12, and then drops gradually to zero on slope V14, parts 51c and 51b of diaphragm 51 corresponding to the large and intermediate gaps G3 and G2, respectively, separate nearly simultaneously from parts 91c and 91b of the opposing wall, and are displaced into ink chamber 5 by the elastic restoring force to eject ink from the nozzle. In this case, part 51a of diaphragm 51 corresponding to the smallest gap G1 remains in contact with surface 91a of opposing wall 91, and does not contribute to ink ejection. The compliance of the ink oscillation system during ink ejection is thus greater than during the ink ejection operation achieved by only part 51c of the diaphragm (shown by the solid line in Fig. 11). The amount of ink ejected is also greater because a greater proportion of the diaphragm displacement contributes to ink ejection causing the vibration frequency to be lowered. If the gap charge is discharged rapidly to voltage Vc, all of diaphragm 51 is elastically displaced into the ink chamber by the elastic restoring force as shown by the dot-dot-dash line in Fig. 11, and contributes to ink droplet ejection. No part of the diaphragm remains in contact with opposing wall 91 in this case, compliance is greatest, and a large ink droplet can therefore be ejected. It is therefore possible to change the ink droplet ejection characteristics, particularly the ink droplet speed and size, of ink nozzle 11 by changing the voltage drop characteristics when discharging the gap charge, i.e., by changing the discharge rate.

Claims: A printing apparatus, comprising: an ink jet head (1); and drive means (21) for driving said ink jet head; wherein said ink jet head comprises at least one ink jet head unit which includes: a nozzle (11), a pressure chamber (5) having an opening in communication with said nozzle (11), an ink supply path (6) for supplying ink to said pressure chamber (5), and an electrostatic actuator (10, 51) for generating pressure to cause ink vibration in said pressure chamber (5) for ejecting ink droplets through the nozzle, said electrostatic actuator comprising a diaphragm (51), as a first electrode, forming one wall of said pressure chamber and an opposing wall (91), as a second electrode, disposed externally to said pressure chamber and opposite to said diaphragm, and said electrostatic actuator elastically displacing said diaphragm according to a drive voltage applied between said first and second electrodes, characterised in that said diaphragm comprises N continuous segments (51a-51c), N being greater than two, and N gaps (G1-G3) are formed in a diminishing size between the N segments of said diaphragm and said opposing wall, respectively; and wherein said drive means (21) comprises means for applying different drive voltages to said electrostatic actuator at different timings, and said different drive voltages include: a first drive voltage capable of forcing all of said N segments of said diaphragm to contact said opposing wall, a second drive voltage capable of maintaining contact between at least one of said N segments of said diaphragm and said opposing wall with other segments of said diaphragm being released, a third drive voltage capable of releasing contact between all of said N segments of said diaphragm and said opposing wall, and a group of drive voltages capable of maintaining contact between only selected ones of said N segments of said diaphragm and said opposing wall. The printing apparatus according to Claim 1, wherein said drive means (21) further includes charge/discharge means for charging and discharging said electrostatic actuator, and wherein said charge/discharge means comprises: charging means (400) for charging said electrostatic actuator to at least the first drive voltage (V0), first discharge means (420) for discharging, at a first discharge rate (τ 2 ), said electrostatic actuator to a first selected voltage in said group of drive voltages, and second discharge means (430) for discharging, at a second discharge rate (τ 3 ), said electrostatic actuator from said first selected voltage in said group of drive voltages, wherein the second discharge rate is slower than the first discharge rate. The printing apparatus according to claim 2 wherein said drive means further comprises: switching means for controlling said charge/discharge means to individually charge and discharge said first and second electrodes of a plurality of electrostatic actuators corresponding to a plurality of ink jet head units, according to externally supplied print signals. A method of controlling the printing apparatus as defined in claim 1, comprising the steps of: (a) applying a first drive voltage (VO) to the electrostatic actuator so that all of the N segments (51a-51c) of the diaphragm (51) contact the opposing wall (91); (b) after a first predetermined time (Th)has passed after step (a), applying a second drive voltage (Va; Vb; Vc) to the electrostatic actuator for maintaining contact between at least one, but less than all of the N segments of the diaphragm and the opposing wall with remaining segments of the diaphragm being out of contact; and (c) after a second predetermined time (Tx) has passed after step (b), applying a third drive voltage to the electrostatic actuator for releasing contact between all of the N segments of the diaphragm and the opposing wall. The control method according to Claim 4, further comprising, after step (a), applying a drive voltage to the electrostatic actuator for maintaining contact between selected ones of the N segments (G1-G3) of the diaphragm (51) and the opposing wall (91), The control method according to Claim 4, wherein step (a) comprises charging the electrostatic actuator to the first drive voltage (V0); wherein step (b) comprises a first discharging step for discharging the electrostatic actuator to the second drive voltage (Va; Vb; Vc) at a first discharge rate (τ 2 ); and a second discharging step, following the first discharging step, for discharging the electrostatic actuator at a second discharge rate (τ 3 ) from the second drive voltage for maintaining contact between selected ones of the N segments of the diaphragm and the opposing wall; wherein the second discharge rate is slower than the first discharge rate. The control method according to Claim 6, wherein step (a) and said first and second steps of discharging are performed according to externally supplied print signals.

Inventors: FUJII MASAHIRO, SAKAI SHINRI, FUJII, MASAHIRO, SAKAI, SHINRI
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1996
Main classification: B41J2
IPC codes: B41J2