Abstract:
Apparatus including a plurality of droplet ejection devices, an electric source and a controller. Each droplet ejection device includes a fluid chamber having an ejection nozzle, an electrically actuated displacement device associated with the chamber, and a switch having an input connected to the electric source, an output connected to the electrically actuated displacement device, and a control signal input that is controlled by the controller to control whether the input (and thus the electric source) is connected to the output (and thus the electrically actuated device). The electrically actuated displacement device moves between a displaced position and an undisplaced position to change the volume of the chamber as a capacitance associated with the electrically actuated displacement device changes in charge between an actuated condition and an unactuated condition. The controller provides respective charge control signals to respective control signal inputs to control the extent of change in charge on respective capacitances by the time that the respective switch connects the electrical signal to the respective electrically actuated displacement device.

Description:
BACKGROUND  
       [0001]     The invention relates to droplet ejection devices.  
         [0002]     Inkjet printers are one type of apparatus employing droplet ejection devices. In one type of inkjet printer, ink drops are delivered from a plurality of linear inkjet print head devices oriented perpendicular to the direction of travel of the substrate being printed. Each print head device includes a plurality of droplet ejection devices formed in a monolithic body that defines a plurality of pumping chambers (one for each individual droplet ejection device) in an upper surface and has a flat piezoelectric actuator covering each pumping chamber. Each individual droplet ejection device is activated by a voltage pulse to the piezoelectric actuator that distorts the shape of the piezoelectric actuator and discharges a droplet at the desired time in synchronism with the movement of the substrate past the print head device.  
         [0003]     Each individual droplet ejection device is independently addressable and can be activated on demand in proper timing with the other droplet ejection devices to generate an image. Printing occurs in print cycles. In each print cycle, a fire pulse (e.g., 150 volts) is applied to all of the droplet ejection devices at the same time, and enabling signals are sent to only the individual droplet ejection devices that are to jet ink in that print cycle.  
       SUMMARY OF THE INVENTION  
       [0004]     The invention features, in general, apparatus including a plurality of droplet ejection devices, an electric source and a controller. Each droplet ejection device includes a fluid chamber having an ejection nozzle, an electrically actuated displacement device associated with the chamber, and a switch having an input connected to the electric source, an output connected to the electrically actuated displacement device, and a control signal input that is controlled by the controller to control whether the input (and thus the electric source) is connected to the output (and thus the electrically actuated device). The electrically actuated displacement device moves between a displaced position and an undisplaced position to change the volume of the chamber as a capacitance associated with the electrically actuated displacement device changes in charge between an actuated condition and an unactuated condition. The controller provides respective charge control signals to respective control signal inputs to control the extent of change in charge on respective capacitances by the time that the respective switch connects the electrical signal to the respective electrically actuated displacement device.  
         [0005]     Particular embodiments of the invention may include one or more of the following features.  
         [0006]     The actuated condition of the electrically actuated displacement device corresponds to a charged condition, and the unactuated condition corresponds to an uncharged condition. The controller controls the extent of charge placed on respective capacitances by the time that the respective the switch connects the electrical signal to the respective electrically actuated displacement device. Each droplet ejection device can also include a second switch that has a second input connected to a discharging electrical terminal, a second output connected to the electrically actuated displacement device, and a second control signal input to determine whether the second input is connected to or disconnected from the second output, and the controller can provide respective discharge control signals to respective second control signal inputs to control discharge of the charge on respective capacitances.  
         [0007]     Each droplet ejection device can include a first resistance between the electric source and the electrically actuated displacement device. Each droplet ejection device can include a second resistance between the discharging electrical terminal and the electrically actuated displacement device.  
         [0008]     The first resistance can be between the electrical source and the electrically actuated displacement device and can be external of an electrical path from the electrically actuated displacement device to the second switch, and the second resistance can be included in the electrical path from the electrically actuated device to the discharging electrical terminal. Alternatively, a single resistance can be used to charge and discharge a respective capacitance. A plurality of resistors, voltages and switches can be connected to each electrically actuated displacement device and controlled by the controller to change the charge on the capacitance. The discharging electrical terminal can be at ground. The electrical signal can be a controlled voltage signal, a controlled current signal, or a constant current.  
         [0009]     When the first control signal is a constant voltage, the first control signal can terminate the connection of the constant voltage to the electrically actuated displacement device when the charge on the electrically actuated displacement device is at a predetermined value which is less than the constant voltage. The electrically actuated displacement device can be a piezoelectric actuator.  
         [0010]     The control signal(s) can be controlled to provide uniform droplet volumes or velocities from the plurality of droplet ejection devices. The control signal(s) can be controlled to provide predetermined different drop volumes or velocities from different droplet ejection devices so as to provide gray scale control. The first and second control signals can be controlled to connect the electrical signal to respective electrically actuated displacement devices for respective predetermined times. A control signal can be controlled to connect the electrical signal to respective electrically actuated displacement devices until respective electrically actuated displacement devices achieve respective predetermined charge voltages. The control signal(s) can be controlled to provide a voltage that is insufficient to eject a droplet, but is sufficient to move a meniscus of a liquid at an ejection nozzle of the droplet ejection device. The control signals can be controlled to inject noise into images being printed so as to break up possible print patterns and banding. The control signals can be controlled to vary the amplitude of charge as well as the length of time of charge on the electrically actuated displacement device for the first droplet out of a droplet ejection device so as to match subsequent droplets.  
         [0011]     In particular embodiments the controller adds a delay to a firing pulse for a displacement device when that device and an adjacent device are called upon to both fire at the same time. The leading edge of firing pulse for the delayed device is delayed by the delay amount after the leading edge of the firing pulse of the undelayed displacement device.  
         [0012]     The apparatus can be an inkjet print head. The controller can include a field programmable gate array on a circuit board mounted to a monolithic body in which the pumping chambers are formed. The controller can control the first switch as a function of the frequency of droplet ejection to reduce variation in drop volume as a function of frequency.  
         [0013]     Particular embodiments of the invention may include one or more of the following advantages. The charging up of an actuator to a desired charge and then disconnecting the electric source results in a saving in electricity over driving a device to a voltage and maintaining the voltage. One can also individually control the charge on devices, the slope of the change in charge, and the timing and slope of discharge to achieve various effects such as uniform droplet volume or velocity and gray scale control.  
         [0014]     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0015]      FIG. 1  is a diagrammatic view of components of an inkjet printer.  
         [0016]      FIG. 2  is a vertical section, taken at  2 - 2  of  FIG. 1 , of a portion of a print head of the  FIG. 1  inkjet printer showing a semiconductor body and an associated piezoelectric actuator defining a pumping chamber of an individual droplet ejection device of the print head.  
         [0017]      FIG. 3  is a schematic showing electrical components associated with an individual droplet ejection device.  
         [0018]      FIG. 4  is a timing diagram for the operation of the  FIG. 3  electrical components.  
         [0019]      FIG. 5  is a block diagram of circuitry of a print head of the  FIG. 1  printer.  
         [0020]      FIG. 6  is a schematic showing an alternative embodiment of electrical components associated with an individual droplet ejection device.  
         [0021]      FIG. 7  is a timing diagram showing the charge voltage on the capacitance for the actuator for the operation of the  FIG. 6  electrical components. 
     
    
     DETAILED DESCRIPTION  
       [0022]     As shown in  FIG. 1 , the 128 individual droplet ejection devices  10  (only one is shown on  FIG. 1 ) of print head  12  are driven by constant voltages provided over supply lines  14  and  15  and distributed by on-board control circuitry  19  to control firing of the individual droplet ejection devices  10 . External controller  20  supplies the voltages over lines  14  and  15  and provides control data and logic power and timing over additional lines  16  to on-board control circuitry  19 . Ink jetted by the individual ejection devices  10  can be delivered to form print lines  17  on a substrate  18  that moves under print head  12 . While the substrate  18  is shown moving past a stationary print head  12  in a single pass mode, alternatively the print head  12  could also move across the substrate  18  in a scanning mode.  
         [0023]     Referring to  FIG. 2 , each droplet ejection device  10  includes an elongated pumping chamber  30  in the upper face of semiconductor block  21  of print head  12 . Pumping chamber  30  extends from an inlet  32  (from the source of ink  34  along the side) to a nozzle flow path in descender passage  36  that descends from the upper surface  22  of block  21  to a nozzle opening  28  in lower layer  29 . A flat piezoelectric actuator  38  covering each pumping chamber  30  is activated by a voltage provided from line  14  and switched on and off by control signals from on-board circuitry  19  to distort the piezoelectric actuator shape and thus the volume in chamber  30  and discharge a droplet at the desired time in synchronism with the relative movement of the substrate  18  past the print head device  12 . A flow restriction  40  is provided at the inlet  32  to each pumping chamber  30 .  
         [0024]      FIG. 3  shows the electrical components associated with each individual droplet ejection device  10 . The circuitry for each device  10  includes a charging control switch  50  and charging resistor  52  connected between the DC charge voltage Xvdc from line  14  and the electrode of piezoelectric actuator  38  (acting as one capacitor plate), which also interacts with a nearby portion of an electrode (acting as the other capacitor plate) which is connected to ground or a different potential. The two electrodes forming the capacitor could be on opposite sides of piezoelectric material or could be parallel traces on the same surface of the piezoelectric material. The circuitry for each device  10  also includes a discharging control switch  54  and discharging resistor  56  connected between the DC discharge voltage Ydc (which could be ground) from line  15  and the same side of piezoelectric actuator  38 . Switch  50  is switched on and off in response to a Switch Control Charge signal on control line  60 , and switch  54  is switched on and off in response to a Switch Control Discharge signal on control line  62 .  
         [0025]     Referring to  FIGS. 3 and 4 , piezoelectric actuator  38  functions as a capacitor; thus, the voltage across piezoelectric actuator ramps up from Vpzt_start after switch  50  is closed in response to switch charge pulse  64  on line  60 . At the end of pulse  64 , switch  50  opens, and the ramping of voltage ends at Vpzt_finish (a voltage less than Xvdc). Piezoelectric actuator  38  (acting as a capacitor) then generally maintains its voltage Vpzt_finish (it may decay slightly as shown in  FIG. 4 ), until it is discharged by connection to a lower voltage Ydc by discharge control switch  54 , which is closed in response to switch discharge pulse  66  on line  62 . The speeds of ramping up and down are determined by the voltages on lines  14  and  15  and the time constants resulting from the capacitance of piezoelectric actuator  38  and the resistances of resistors  52  and  56 . The beginning and end of print cycle  68  are shown on  FIG. 4 . Pulses  64  and  66  are thus timed with respect to each other to maintain the voltage on piezoelectric actuator  38  for the desired length of time and are timed with respect to the print cycle  68  to eject the droplet at the desired time with respect to movement of substrate  18  and the ejection of droplets from other ejection devices  10 . The length of pulse  64  is set to control the magnitude of Vpzt, which, along with the width of the PZT voltage between pulses  64 ,  66 , controls drop volume and velocity. If one is discharging to Y vdc  the length of pulse  66  should be long enough to cause the output voltage to get as close as desired to Y vdc ; if one is discharging to an intermediate voltage, the length of pulse  66  should be set to end at a time set to achieve the intermediate voltage.  
         [0026]     Referring to  FIG. 5 , on-board control circuitry  19  includes inputs for constant voltages Xvdc and Ydc over lines  14 ,  15  respectively, D 0 -D 7  data inputs  70 , logic level fire pulse trigger  72  (to synchronize droplet ejection to relative movement of substrate  18  and print head  12 ), logic power  74  and optional programming port  76 . Circuitry  19  also includes receiver  78 , field programmable gate arrays (FPGAs)  80 , transistor switch arrays  82 , resistor arrays  84 , crystals  86 , and memory  88 .  
         [0027]     Transistor switch arrays  82  each include the charge and discharge switches  50 ,  54  for  64  droplet ejection devices  10 .  
         [0028]     FPGAs  80  each include logic to provide pulses  64 ,  66  for respective piezoelectric actuators  38  at the desired times. D 0 -D 7  data inputs  70  are used to set up the timing for individual switches  50 ,  54  in FPGAs  80  so that the pulses start and end at the desired times in a print cycle  68 . Where the same size droplet will be ejected from an ejection device throughout a run, this timing information only needs to be entered once, over inputs D 0 -D 7 , prior to starting a run. If droplet size will be varied on a drop-by-drop basis, e.g., to provide gray scale control, the timing information will need to be passed through D 0 -D 7  and updated in the FPGAs at the beginning of each print cycle. Input D 0  alone is used during printing to provide the firing information, in a serial bit stream, to identify which droplet ejection devices  10  are operated during a print cycle. Instead of FPGAs other logic devices, e.g., discrete logic or microprocessors, can be used.  
         [0029]     Resistor arrays  84  include resistors  52 ,  56  for the respective droplet ejection devices  10 . There are two inputs and one output for each of  64  ejection devices controlled by an array  84 .  
         [0030]     Programming port  76  can be used instead of D 0 -D 7  data input  70  to input data to set up FPGAs  80 . Memory  88  can be used to buffer or prestore timing information for FPGAs  80 .  
         [0031]     In operation under a normal printing mode, the individual droplet ejection devices  10  can be calibrated to determine appropriate timing for pulses  64 ,  66  for each device  10  so that each device will eject droplets with the desired volume and desired velocity, and this information is used to program FPGAs  80 . This operation can also be employed without calibration so long as appropriate timing has been determined. The data specifying a print job are then serially transmitted over the D 0  terminal of data input  72  and used to control logic in FPGAs to trigger pulses  64 ,  66  in each print cycle in which that particular device is specified to print in the print job.  
         [0032]     In a gray scale print mode, or in operations employing drop-by-drop variation, information setting the timing for each device  10  is passed over all eight terminals D 0 -D 7  of data input  70  at the beginning of each print cycle so that each device will have the desired drop volume during that print cycle.  
         [0033]     FPGAs  80  can also receive timing information and be controlled to provide so-called tickler pulses of a voltage that is insufficient to eject a droplet, but is sufficient to move the meniscus and prevent it from drying on an individual ejection device that is not being fired frequently.  
         [0034]     FPGAs  80  can also receive timing information and be controlled to eject noise into the droplet ejection information so as to break up possible print patterns and banding.  
         [0035]     FPGAs  80  can also receive timing information and be controlled to vary the amplitude (i.e, Vpzt_finish) as well as the width (time between charge and discharge pulses  64 ,  66 ) to achieve, e.g., a velocity and volume for the first droplet out of an ejection device  10  as for the subsequent droplets during a job.  
         [0036]     The use of two resistors  52 ,  56 , one for charge and one for discharge, permits one to independently control the slope of ramping up and down of the voltage on piezoelectric actuator  38 . Alternatively, the outputs of switches  50 ,  54  could be joined together and connected to a common resistor that is connected to piezoelectric actuator  38  or the joined together output could be directly connected to the actuator  38  itself, with resistance provided elsewhere in series with the actuator  38 .  
         [0037]     By charging up to the desired voltage (Vpzt_finish) and maintaining the voltage on the piezoelectric actuators  38  by disconnecting the source voltage Xvdc and relying on the actuator&#39;s capacitance, less power is used by the print head than would be used if the actuators were held at the voltage (which would be Xvdc) during the length of the firing pulse.  
         [0038]     Other embodiments of the invention are within the scope of the appended claims. E.g., a switch and resistor could be replaced by a current source that is switched on and off. Also, common circuitry (e.g., a switch and resistor) could be used to drive a plurality of droplet ejection devices. Also, the drive pulse parameters could be varied as a function of the frequency of droplet ejection to reduce variation in drop volume as a function of frequency. Also, a third switch could be associated with each pumping chamber and controlled to connect the electrode of the piezoelectric actuator  38  to ground, e.g., when not being fired, while the second switch is used to connect the electrode of the piezoelectric actuator  38  to a voltage lower than ground to speed up the discharge.  
         [0039]     It is also possible to create more complex waveforms. For example, switch  50  could be closed to bring the voltage up to V 1 , then opened for a period of time to hold this voltage, then closed again to go up to voltage V 2 . A complex waveform can be created by appropriate closings of switch  50  and switch  54 .  
         [0040]     Multiple resistors, voltages, and switches could be used per droplet ejection device to get different slew rates as shown in  FIGS. 6 and 7 .  FIG. 6  shows an alternative control circuit  100  for an injection device in which multiple (here two) charging control switches  102 ,  104  and associated charging resistors  106 ,  108  are used to charge the capacitance  110  of the piezoelectric actuator and multiple (here two) discharging control switches  112 ,  114  and associated discharging resistors  116 ,  118  are used to discharge the capacitance.  FIG. 7  shows the resulting voltage charge on the capacitance. The ramp up at  120  is caused by having switch  102  closed while the other switches are open. The ramp up at  122  is caused by having switch  104  closed while the other switches are open. The ramp down at  124  is caused by having switch  112  closed while the other switches are open. The ramp down at  126  is caused by having switch  114  closed while the other switches are open.