Abstract:
The invention relates to a droplet deposition apparatus including an ink actuator ( 100 ) for ejecting ink droplets; a control unit ( 130 ) for controlling droplet formation; and power supply means for supplying a drive voltage to the control unit ( 130 ); the drive voltage having a voltage amplitude (V 100 ); the control unit ( 130 ) having a plurality of drive signal sources ( 320 ) for defining the wave forms of said electric signals; means ( 470 ) for detecting at least one performance affecting value; and means ( 490 ) for generating an amplitude control signal in response to the performance affecting value; and wherein the power supply means ( 330 ) comprises means ( 350 ) for adjusting the amplitude of the drive voltage in response to the amplitude control signal; said power supply means being separated from said control unit.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a droplet deposition apparatus. 
     DESCRIPTION OF RELATED ART 
     Ink jet printers include an ink actuator for ejecting droplets of ink liquid on demand. Such an ink actuator is disclosed in U.S. Pat. No. 5,016,028. The actuator includes a plurality of channels having side walls which are displaceable in response to electric drive signals. When an electric drive signal is applied to a section of the wall, the wall will move, thereby causing the volume of corresponding channels to increase or decrease. 
     U.S. Pat. No. 4,275,402 describes a circuit arrangement for piezoelectric recording nozzles with control circuits providing control voltages to individual nozzles. A regulating circuit includes a temperature-dependent resistor sensing the environmental temperature to effect temperature regulation of the control voltages in accordance with the environmental temperature. 
     U.S. Pat. No. 5,631,675 describes an apparatus for driving an ink jet recording head having piezoelectric units which expand and contract as controlled by an electric field applied thereto. The apparatus according to U.S. Pat. No. 5,631,675 includes two constant current sources which operate to charge and discharge a capacitor. The voltage of the capacitor is amplified to provide the voltage for driving the piezoelectric units. 
     SUMMARY 
     A problem to which the present invention is directed is to improve print performance and increase the life time of the actuator. 
     This problem is addressed by providing a droplet deposition apparatus including: 
     an ink actuator having a plurality of spaced walls defining ink channels, said walls having opposed sides; said opposed sides being provided with electrodes being adapted to receive electric signals to deform said walls to cause ink in said channels to be ejected therefrom; 
     a control unit including a plurality of current sources for defining the wave forms of said electric signals; 
     a temperature sensor for generating an amplitude control signal in response to a sensed actuator temperature; 
     a power supply for providing a drive voltage to the control unit; said drive voltage having a voltage amplitude; 
     the power supply having means for adjusting the amplitude of the drive voltage in response to the amplitude control signal; wherein 
     said power supply is separated in space from said control unit and from said actuator and in that said electric signals include a controlled current. 
     This solution advantageously leads to reduced operating temperature of the actuator, and to an improved quality of the ejected ink. Since a high operating temperature leads to an accelerated ageing process of the ink in and near the actuator, which in turn leads to deteriorating print quality, this solution results in improved print quality. This solution provides particular improvement in print quality for printing conditions when the droplet deposition apparatus is turned on and operable for long times, but with small amounts of actual printouts. In such cases some ink remains in the actuator for long time periods before being deposited. With a droplet deposition apparatus as defined above, the heat generated by the power supply means is prevented from being transported to the actuator. Whereas an ink volume in the actuator of a prior art apparatus is kept warm for long time durations, such warming is avoided in a droplet deposition apparatus according to the above embodiment of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a print head arrangement including an actuator and a control unit coupled, via a cable, to a power supply and a data interface. 
     FIG. 2 is an exploded partly diagrammatic perspective view of a part of the actuator shown in FIG.  1 . 
     FIG. 3 is a sectional view of an actuator plate. 
     FIG. 4 is a sectional perspective view of a part of the actuator plate shown in FIG.  3 . 
     FIG. 5A is a cross-section of a part of the actuator shown in FIG. 1 and 2 shown in a relaxed state. 
     FIG. 5B is a cross-section of a part of the actuator shown in FIG. 1 and 2 with some channels shown in an expanded state 
     FIG. 5C is a cross-section of a part of the actuator shown in FIG. 1 and 2 with some channels shown in a contracted state. 
     FIG. 6 is a partly schematic view showing electrode connections from an electrical point of view. 
     FIG. 7 illustrates an example of electric signal wave forms at the electrode connections when a maximal number of ink droplets is to be ejected. 
     FIG. 8 illustrates an example of an electric signal wave form relating to one wall having two opposing sides with electrodes. 
     FIG. 9 is a block diagram of a printer arrangement including a control unit coupled to an actuator and to a power supply circuit, according to an embodiment of the invention. 
     FIG. 10 is a block diagram of a printer arrangement including a control unit coupled to a power supply circuit and for connection to an actuator, according to another embodiment of the invention. 
     FIG. 11 is a block diagram of a controllable drive signal source, according to an embodiment of the invention. 
     FIG. 12 is a block diagram of a controllable drive signal source, according to another embodiment of the invention. 
     FIG. 13 is a schematic of an embodiment of a controllable drive signal source. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     FIG. 1 is a perspective view of a print head arrangement  90  including an ink actuator  100  mounted on a base plate  110 . The base plate may be arranged on a shuttle in an ink jet printer (not shown). 
     A circuit board  120  is also mounted on the base plate  110 . The circuit board  120  includes a control unit  130  and a connector  140 . 
     A central data processing unit in the printer or in a facsimile machine can be connected to the connector  140  and can supply print orders to the connector  140 . The print orders thus supplied to the print head arrangement  90  are fed to the control unit  130 . The control unit  130  transforms the print orders into electric pulses adapted to cause the actuator assembly  100  to eject ink drops in accordance with the print orders. 
     Ink is supplied from an ink reservoir (not shown) to an ink inlet  150  on the actuator assembly  100 . The ink inlet  150  may include a filter  160 . The ink inlet  150  also includes a sealing unit  170 . The sealing unit  170  may include a rubber strip projecting a few tenths of a millimetre above a surface  160  of the actuator assembly  100 , as shown in FIG. 1, in order to provide a tight seal when pressed towards a corresponding ink duct connector. 
     The actuator  100  comprises an actuator plate  200  and a cover plate  210 . The actuator plate  200  is made from polarised piezoelectric material. The cover plate, which includes the ink inlet  150 , is made from piezoelectric material which is not polarised. 
     FIG. 2 is an exploded partly diagrammatic perspective view of a part of the actuator  100 . 
     The actuator plate  200  includes grooves of a rectangular cross-section forming channels  220 . The channels  220  are separated by side walls  230 . The whole actuator plate is poled in a direction parallel to the Z-axis in FIG.  2 . The direction of polarisation is also illustrated by arrows  240  in FIG.  2 . 
     FIG. 3 is a sectional view of the actuator plate  200 , as seen in the direction of the axis X. 
     According to one embodiment of the actuator assembly there are sixty-six channels  220 . For easy reference the channels are individually referenced C 1 , C 2 , C 3  . . . C 66 . Sixty-four ( 64 ) out of the 66 channels are active while two channels C 1  and C 66  are inactive and not used for expelling ink drops, as described in more detail below. The two inactive channels C 1  and C 66  are the first and the last channels as seen in the direction of the axis y in FIG. 2 or in FIG.  3 . 
     Certain parts of the walls  230  are arranged to move in shear mode in relation to the ink channels  220  when activated by an electric field applied in a direction perpendicular to the direction of polarisation  240  of the wall  230 . The side walls  230  are displaceable transversely relative to the channel axis to cause changes of pressure in the ink in the channels to effect droplet ejection from nozzles F 2 -F 65  in a nozzle plate  265 . The plate  265  is positioned in front of the open ends of the channels  220 , and is provided with nozzle openings for ink droplet ejection. 
     Electrical connections D 1 , D 2 , D 3  . . . D 66  for activating the channel side walls  230  are made to the control unit by bond wires as illustrated in FIGS. 1,  2  and  4 . 
     FIG. 4 is a sectional perspective view of a part of the actuator plate  200 . The bond wire D 1  connects to a thin metal layer  270  (illustrated by dashed lines) arranged on a surface of the actuator plate  200 . The metal layer also covers a part of the surface of the wall  230  facing towards channel C 1  of the wall  230  as illustrated by the shaded area E 1  in FIG.  4 . Another bond wire D 2  connects to metal layers E 2  in channel C 2  in the same manner. The metal layers E 2  form electrodes on the surfaces facing channel C 2  of the walls  230 . The cover plate  210  is cemented onto the actuator plate  220  so as to define, together with the walls  230 , channels  220  with nozzles F 2 , F 3  . . . F 65 . 
     FIG. 5A is a cross-section of a part of the actuator assembly  100 , as seen from the nozzle plate  265 . In order to simplify understanding, the three axes x, y and z are shown in FIGS. 2,  3 ,  4  and  5 . Reference numeral  275  indicates the joint where the cover plate  210  is cemented to each wall  230  comprised in the actuator plate  200 . Thus, each wall  230  is firmly attached to the cover plate. 
     The channels C 2 , C 3  . . . C 65  can be activated individually as described above. As described above, the channel C 1  on the far left edge, as seen in FIG. 2, is an inactive channel. The channel C 66  on the far right edge is also an inactive channel, i.e. it is not used for ejecting ink. 
     FIG. 5B illustrates channel C 2  in an expanded state. The expansion is achieved by causing a current to flow from the electrodes E 2  to the electrodes E 1  and E 3 . Due to the impedance between the E 2  and the electrode E 1  there will be a potential U 21  between the electrode E 2  and the electrode E 1 . 
     An electric field is thereby caused in a portion  300  in the wall  230  between the electrode E 2  and the electrode E 1  in a direction substantially perpendicular to the direction of polarisation  240 . This causes the portion  300  of the wall to flex in a shear mode to the position shown in FIG.  5 B. When the wall part  300  flexes, it also forces the complementary part  310  of the wall to bend in the same direction. 
     When channel C 2  expands, it draws in more ink through the ink inlet  150  (best seen in FIG.  2 ). 
     FIG. 6 is a partly schematic view showing the electrode connections from an electrical point of view. The electrodes E 1  in channel C 1  are connected to the control unit  130 , as shown in FIG.  6 . 
     The control unit comprises a current source  320  for each channel. There is thus one current source  320  for each channel C 1 -C 66 . Each current source  320  is coupled to the electrodes E in the corresponding channel, as illustrated in FIG.  6 . 
     Each wall  230  is individually displaceable in dependence on the current between the electrodes on that wall. For example, the wall between channel C 2  and channel C 3  is displaceable in dependence on a current I 23  from electrode E 2  to electrode E 3 . 
     FIG. 7 illustrates examples of electric pulses I 1 -I 6  delivered to the electrodes E 1 -E 6  when a maximal number of ink droplets are to be ejected. 
     FIG. 8 illustrates an example of an electric signal wave form relating to the two opposing electrodes E 2  and E 3  on the wall between channel C 2  and channel C 3 . 
     Certain essential properties of the ink, such as viscosity, change in dependence on ink temperature. In order to compensate for this temperature dependency, the temperature is measured by a temperature sensor and the voltage levels in the pulse wave forms are decreased with rising ink temperature. According to an embodiment of the invention the voltage top value is set to 35 volts when the actuator temperature is 20° C. According to another embodiment of the invention the voltage top value is set to its top value when the actuator temperature is 10° C. The voltage top level is herein referred to as the 100% voltage level. According to an embodiment of the invention the temperature sensor is a thermistor. 
     FIG. 9 is a block diagram of an embodiment of the invention, comprising an actuator control circuit  130 , a power supply circuit  330 , and an actuator  100 . The power supply circuit  330  is coupled to a DC power supply  340 . The power supply  340  may for example provide a substantially constant voltage of 40 volts. The power supply circuit  330  comprises a drive voltage controller  350 , having an input  360  for a power demand signal and a power supply output  370  for delivering a drive voltage with a controlled voltage Vcc. The controlled voltage V cc  may for example be controllable from 10% of V cc(100)  to 100% of V cc(100) , where V cc(100) =35 volts. 
     The actuator control unit  130  comprises a power supply input  380  which is coupled to the output  370  for receiving a controlled drive voltage. The control unit  130  comprises a plurality of controllable current sources  320 , each current source having a drive voltage input  400  which is coupled to the power supply input  380 . There may be provided N current sources, where N is an integer. Each current source  320 : 1 ,  320 : 2  . . .  320 :N has an earth connection  410  and an actuator drive signal output  420 . Each actuator drive signal output is coupled to the electrodes E of a corresponding channel wall in the actuator  100 . 
     Each current source  320  also comprises an input  430  for a current control signal. The current control signal input is coupled to a data conversion unit  440 . The data conversion unit comprises an input  450  for receiving print data indicative of the text or picture to be printed. The input  450  is adapted to be connected to a data interface  460  via a databus  464 . With reference to FIGS. 1 and 9 a plurality of electrical conductors  466  are provided to connect the control unit  130  with the data interface  460  and the power supply circuit  330 . 
     In response to print data received on the input  450  the data conversion unit  440  converts the print data into individual current control signals for each current source  320 . For this purpose the data conversion unit  440  comprises a control signal output  471  corresponding to each current source  320 , and hence a current control signal for each channel in the actuator. 
     The data conversion unit in co-operation with the controllable current sources  320  operates to generate drive currents on the outputs  420  such that the wave forms of the drive signals delivered to each actuator wall causes a controlled movement of each wall. 
     The voltage amplitude of the drive current on each output  420  depends on the voltage on the power supply input  380 . 
     For the purpose of controlling the voltage level so as to compensate for the temperature dependency of the viscosity of the ink, the actuator includes a temperature sensor  470 . The temperature sensor  470  provides a temperature signal which indicates the power demand for driving the actuator with optimum performance. The power demand signal input  360  of the power supply circuit is adapted to receive the signal from the sensor  470 , or a demand signal derived from the sensor  470 . 
     According to one embodiment of the invention the power demand signal delivered to the input  360  is derived from the signal from sensor  470  in combination with other performance affecting variables. FIG. 10 is a block diagram of another embodiment of the ink jet printing arrangement. The embodiment according to FIG. 10 differs from the embodiment according to FIG. 9 in that the sensor  470  is coupled to an evaluation circuit  490 , which operates to generate a voltage demand signal in dependence on sensed temperature. The output of the evaluation circuit  490  is coupled to the input  360  of the power supply circuit  350 . 
     According to one embodiment of the invention the evaluation circuit  490  comprises an input  520  for receiving additional data relating to the performance affecting variables such as for example actuator efficiency and/or type of liquid. Such data includes for example data defining the temperature dependency of the liquid to be ejected by the actuator. The evaluation circuit  490  is, according to a preferred embodiment, integrated with the control unit  130 . 
     The additional data relating to performance of the actuator  100  are generated by an actuator status circuit  530 . The actuator status circuit  530 , also integrated in the control unit  130 , includes a memory for storing data derived from measurements of the performance of the individual actuator control unit combination. 
     According to a preferred embodiment of the invention, the actuator control circuit  130  and the actuator  100  are arranged on a movable shuttle in a printer, while the data interface  460  and the drive voltage controller  350  are stationary parts in the printer. The set of conductors  466  is bendable so as to enable having one end attached to the firmly mounted power supply  330 , and the other end connected to the movable shuttle which carries the control unit  130  and the actuator. Hence, the power supply  330  is separated in space from the control unit and from the actuator  100 . As the shuttle with the control unit and the actuator  100  moves during printing operations the separating distance between them changes. The separation in space leads to reduction or elimination of warming of the actuator  100  by thermal radiation from the variable voltage supply  330 . 
     Moreover the heat dissipation from the control unit to the actuator is reduced since the voltage drop in the control unit  130  is minimized. The signal sources  320  are designed for minimized voltage drop between the power input  380  and the outputs  420 . The reduced power losses in the control unit thereby decreases the amount of heat generated in the immediate vicinity of the ink actuator, so as to reduce heat conduction from the control unit to the actuator. 
     FIG. 11 is a block diagram illustrating two of the controllable current signal sources  320 : 1  and  320 : 2  shown in FIG. 9, according to an embodiment of the invention. FIG. 11 also shows how two current sources  320 : 1  and  320 : 2  co-operate to provide a push-pull drive signal, as illustrated in FIG. 8, to an actuator wall  230  between channels CHk and CHk+1. 
     Hence, generally each actuator wall is connected to two individually controlled current sources  320 :k and  320 :k+1. As indicated by FIGS. 8,  9  and  11  an actuator wall is connected so as to receive a push-pull signal from one pair of current sources  320 :k and  320 :k+1 whereas other walls receive push-pull signals from other pairs of current sources  320 :j and  320 :j+1; where k and j are positive integers, and j never equals k. In other words a first actuator wall is coupled to receive a drive signal from a first pair of push-pull connected signal sources, and a second actuator wall is coupled to receive a drive signal from a second pair of push-pull connected signal sources, where the second pair is different from the first pair. 
     According to the invention there is provided a plurality of current signal sources  320 , each such current source  320  being connected to at least one actuator wall. In this manner an improved print quality is enabled. This advantageous effect is obtained since control of the deflection of each wall is enabled by controlling the current delivered to it. In the embodiment shown in FIG. 9 there is provided one current source  320  for each actuator channel, and the current through one wall is determined by the current sources connected to the channels bordering that wall. 
     A current signal source  320  comprises a current source  500  receiving a drive voltage from the drive voltage input  400 , and a control signal from the control signal input  430 . The output of the current source  500  is coupled to a switch  515  for connecting the driver output  420  either to the output of the current source  500  or to ground  410 . The switch  515  is also controlled by the signal from the control signal input  430 . FIG. 11 illustrates a switch setting when current source  320 : 1  can drive a current via switch  515 : 1  through the wall between channels CH 1  and CH 2  and via switch  515 : 2  to ground. 
     FIG. 12 is a block diagram of a controllable drive signal source  320 , according to another embodiment of the invention. Tests made by the inventors have shown that drop velocity depends on the slew rate of the drive signal shown in FIG.  8 . In order to control the slew rate of the voltage pulse the drive signal source is constructed with four output current sources  500 :A,  500 :B,  500 :C,  500 :D. Current source  500 :B provides twice the current of current source  500 :A., current source  500 :C provides twice the current of current source  500 :B, and current source  500 :D provides twice the current of current source  500 :C. Hence, a current ratio  1 : 2 : 4 : 8  is obtained. A switch unit  514 , having switches  514 :A,  514 :B,  514 :C and  514 :D controls the activation of the individual current sources  500 :A,  500 :B,  500 :C and  500 :D, respectively. The current sources  500  include output devices, wherein the geometric area of an output device is directly proportional to the current it can provide, thereby making slew rate control possible. According to an embodiment the output devices  500 :A,  500 :B,  500 :C,  500 :D are integrated circuit MOS transistors. 
     The driver stage  320  is push-pull connected. The outputs of the current sources  500 :A,  500 :B,  500 :C,  500 :D are coupled to a switch  515  for connecting the driver output  420  to the outputs of the current sources  500 :A,  500 :B,  500 :C,  500 :D or to ground  410 . According to an embodiment there is provided a number of current sources (not shown) between the switch  515  and ground  410  so as to enable control of the negative slope of the pulse signal delivered on output  420 . These current sources are pulling current sources of values corresponding to the pushing current sources  500 :A,  500 :B,  500 :C,  500 :D, and these current sources are also controlled by the switch means  514 . According to another version there is provided a separate switch for controlling the pulling current sources. 
     FIG. 13 is a schematic of an embodiment of a controllable drive signal source  320 . Each of the N actuator channels is coupled to a non-inverting drive signal source  320 . The actuator load appears as a large capacitor and parallel resistor strung between each neighbouring driver output. The dielectric of these capacitors is formed by the piezoelectric material in the wall  230  (FIG.  2 ). In order to draw in liquid in the k:th channel the driver  320 :k drives the output  420 :k to the positive rail, whilst the outputs  420 :k−1 and  420 :k+1 of the neighbouring channels (C k−1  and C k+1 ) are held at the negative rail. This charges the two capacitors of the walls of channel C k . During the droplet ejection stage, a reverse polarity pulse is applied, see FIGS. 5,  7  and  8 , reversing the charge polarity of the wall capacitor. Again, this deflects the channel walls so as to contract the channel (FIG.  5 C). Finally, during a recovery stage the potential across the wall  230  is restored to zero as the wall capacitance is discharged to their initial state. 
     With reference to FIG. 13 there is provided a two output bipolar NPN-transistors  540  and  550  forming the switch  515 . A number of MOS transistors form the current sources  500 :A,  500 :B,  500 :C and  500 :D, as described above, for driving the output  420  to the positive. In a similar manner a number of NPN-transistors  560 A,  560 B,  560 C, and  560 D act as current sources  560  for driving the output  420  to the negative rail. The output drive capacity of the output bipolar NPN-transistors  540  and  550  is determined by the MOS transistors  500 :A,  500 :B,  500 :C,  500 :D and by the NPN-transistors  560 A,  560 B,  560 C, and  560 D. The MOS transistors  500  and the NPN-transistors  560  limit the available base current for the NPN-transistors  540  and  550 , thereby determining the slew rate when switching these devices in a controlled manner. The output state is determined by control signals GA, GB, GC, GD, BA, BB, BC and BD which are related to the signal on control signal input  430 , as described above. With reference to FIG.  13  and FIG. 12 in conjunction with the associated description, a switch  514 , e.g. in the form of a fusible link memory, may be provided between the input  430  and the terminals GA, GB, GC, GD, BA, BB, BC and BD.