Patent Publication Number: US-2009231373-A1

Title: Charge leakage prevention for inkjet printing

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 10/981,888, filed Nov. 5, 2004. 
    
    
     BACKGROUND 
     The following disclosure relates to droplet ejection devices, such as inkjet printers. 
     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. A flat piezoelectric actuator covers each pumping chamber. Each individual droplet ejection device is activated by applying a voltage pulse to the piezoelectric actuator, which 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. 
     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 a print cycle, a fire pulse is applied to all of the droplet ejection devices at the same time, and enabling signals are sent to only to those droplet ejection devices that are to jet ink in that print cycle. 
     SUMMARY OF THE INVENTION 
     The present disclosure describes methods, apparatus, and systems that implement techniques for preventing voltage drift on a piezoelectric transducer (PZT) element in an inkjet printer. 
     In one general aspect, the techniques feature a method of controlling a droplet ejection device that includes a switch that selectively couples a waveform input signal to a piezoelectric actuator. The method involves controlling the switch to drive the piezoelectric actuator with the waveform input signal during a droplet firing period and controlling the switch to drive the piezoelectric actuator with a constant voltage level during a non-firing period. 
     Advantageous implementations can include one or more of the following features. Controlling the switch can be performed using two different control signals. The method may involve using a channel control signal to control the switch to drive the piezoelectric actuator with the waveform input signal and using a clamp control signal to control the switch to drive the piezoelectric actuator with the constant voltage level. The clamp control signal can prevent charge from accumulating on the piezoelectric actuator when the droplet ejection device is off. The clamp control signal can prevent charge from leaking from the piezoelectric actuator when the droplet ejection device is off. The method may involve selecting either the channel control signal or the clamp control signal to prevent piezoelectric voltage drift. The channel control signal and the clamp control signal may also control multiple switches, including binary-weighted switches. 
     The method may also involve logically combining the channel control signal and the clamp control signal to generate a single drive signal for controlling the switch, which may involve connecting the channel control signal and the clamp control signal to input terminals of an OR gate. An output terminal of the OR gate may have a single drive signal for controlling the switch. 
     The voltage on the piezoelectric actuator may be at a mid-range between a ground potential and a supply potential during the non-firing period. 
     In another general aspect, the techniques feature an apparatus for a droplet ejection device that includes a piezoelectric actuator, a switch to selectively couple a waveform input signal with the piezoelectric actuator, and a controller to control the switch to drive the piezoelectric actuator with the waveform input signal during a droplet firing period and drive the piezoelectric actuator with a constant voltage level during a non-firing droplet period. 
     Advantageous implementations can include one or more of the following features. The switch may have an input terminal to connect with the waveform input signal, an output terminal to couple with the piezoelectric actuator, and a control signal terminal to control an electrical connection of the switch using a first control signal or a second control signal. The waveform input signal may be at the constant voltage level when the second control signal controls the switch. The controller can be coupled with the control signal terminal of the switch and may use the first control signal and the second control signal to control the switch. The controller may involve an OR gate to logically connect the first control signal or the second control signal to the control signal terminal of the switch. A first input of the OR gate can be coupled to the first control signal, a second input of the OR gate can be coupled to the second control signal, and an output of the OR gate can be coupled to the control signal terminal of the switch. The second control signal can control the electrical connection of the switch during non-firing droplet periods of the droplet ejection device, and the first control signal can control the electrical connection of the switch during firing periods of the droplet ejection device. 
     In another general aspect, the techniques feature a system to prevent voltage drift on a piezoelectric actuator of an inkjet printer. The system includes a waveform driving circuit to drive a voltage waveform, a switch to electrically connect the waveform driving circuit with the piezoelectric actuator, and a controller to control the switch during an ink ejection phase and a non-ink ejection phase. The waveform driving circuit drives a constant voltage waveform during the non-ink ejection phase. 
     Advantageous implementations can include one or more of the following features. The controller may electrically connect the waveform driving circuit at an input of the switch with the piezoelectric actuator at an output of the switch during the ink ejection phase and during the non-ink ejection phase. The controller may involve a first control signal to control when the switch is electrically connecting the piezoelectric actuator with the voltage waveform from the waveform driving circuit. The controller may involve a second control signal to control the switch to electrically connect the waveform driving circuit at an input of the switch with the piezoelectric actuator at an output of the switch during the non-ink ejection phase. 
     Particular implementations may provide one or more of the following advantages. For example, using an “all-on clamp” signal to drive a PZT element during non-firing periods can override the effects of parasitic charge leakage on the switch, as well as to prevent potential damage to the PZT element. In another benefit, the all-on clamp signal can be used to control whether the switch is on or off. The all-on clamp signal can prevent damage to the PZT element by holding the PZT element voltage at a constant voltage level during non-firing periods. In another advantage, the all-on clamp signal can prevent degradation in image quality by preventing sudden discharging (or charging) of the PZT element and by preventing a corresponding pressure wave inside an inkjet channel. 
     The details of one or more implementations of the disclosure are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a diagrammatic view of components of an inkjet printer. 
         FIG. 2  illustrates 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. 
         FIG. 3  illustrates a schematic showing electrical components associated with an individual droplet ejection device. 
         FIG. 4  illustrates a timing diagram for the operation of the  FIG. 3  electrical components. 
         FIG. 5  shows an exemplary block diagram of circuitry of a print head of the  FIG. 1  printer. 
         FIG. 6  illustrates a schematic showing an alternative implementation of electrical components associated with the individual droplet ejection device. 
         FIG. 7  illustrates a timing diagram for the operation of the  FIG. 6  electrical components. 
         FIGS. 8A-8B  illustrate schematics showing an alternative implementation of electrical components associated with the individual droplet ejection device. 
         FIG. 9  illustrates a schematic showing an implementation of electrical components associated with the droplet ejection device. 
         FIG. 10A  shows a schematic of electrical components associated with a switch. 
         FIG. 10B  shows a timing diagram for  FIG. 10A . 
         FIG. 11A  shows a schematic of electrical components associated with the switch. 
         FIG. 11B  shows a timing diagram for  FIG. 11A . 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     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 . 
       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 . 
     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 Yvdc the length of pulse  66  should be long enough to cause the output voltage to get as close as desired to Yvdc; 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. 
     In one implementation, the charge voltage applied to droplet ejection device  10  includes a unipolar voltage, in which a DC charge voltage Xvdc is applied at line  14 , and a ground potential is applied at line  15 . In another implementation, the charge voltage applied to the ejection device  10  includes a bipolar voltage, in which a DC charge voltage Xvdc is applied at line  14  and a DC charge voltage that is opposite in potential (e.g., −Xvdc or 180o difference in phase) is applied at line  15 . In another implementation, the charge voltage applied to line  14  could be a waveform. The waveforms may be square pulses, sawtooth (e.g., triangular) waves, and sinusoidal waves. The waveforms can be waveforms of varying cycles, waveforms with one or more DC offset voltages, and waveforms that are the superposition of multiple waveforms. 
     Different firing waveforms (e.g., step pulse, sawtooth, etc.) may be applied to an inkjet to produce different responses, and provide different spot sizes. A field-programmable gate array (FGPA) on a print head can store a waveform table of available firing waveforms. Each image scan line packet transmitted from a computer to the print head can include a pointer to the waveform table to specify which firing waveform should be used for that scan line. Alternatively, the image scan line packet could include multiple points, such as one for each device in the scan line, to specify on a device-specific basis which firing waveform should be used to produce the desired spot size. As a result, print control can be increased over the desired spot size. 
     The waveform table can also include several parameters to increase print control, and produce different responses and spot sizes for each print job. These parameters may be based on different types of substrates (e.g., plain paper, glossy paper, transparent film, newspaper, magazine paper) and the ink absorption rate on those substrates. Other parameters may depend on the type of print head, such as a print head with an electromechanical transducer or piezoelectric transducer (PZT), or a thermal inkjet print head with a heat generating element. The waveform table may have parameters that depend on different types of ink (e.g., photo-print ink, plain paper ink, ink of particular colors, ink of particular ink densities) or the resonant frequency of the ink chamber. The waveform table can have parameters to compensate for inkjet direction variability between ink nozzles, as well as other parameters to calibrate the printing process, such as correcting for variations in humidity. 
     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 . Transistor switch arrays  82  each include the charge and discharge switches  50 ,  54  for 64 droplet ejection devices  10 . 
     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. 
     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 . 
     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 . 
     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 DO 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. 
     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. 
     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. 
     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. 
     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. 
     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 . 
     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. 
     For example, 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. 
     It is also possible to create more complex waveforms. For example, switch  50  could be closed to bring the voltage up to V1, then opened for a period of time to hold this voltage, then closed again to go up to voltage V2. A complex waveform can be created by appropriate closings of switch  50  and switch  54 . 
     Multiple resistors, voltages, and switches could be used per droplet ejection device to get different slew rates as shown in  FIGS. 6 and 7 . Each droplet ejection device can include one or more resistances connected in parallel between the electric source and the electrically actuated displacement device. A switch can be placed in the path of the electric source and each of the one or more resistances to control the effective resistance of the parallel resistances when charging the device. Alternatively, the resistance can be part of the switch. For example, the resistance may be the source-to-drain resistance of a MOS-type (metal-oxide semiconductor) switch, and the MOS switch may be actuated by switching a voltage on the gate of the switch. Each droplet ejection device can include one or more resistances connected in parallel between the discharging electrical terminal and the electrically actuated displacement device. A switch can be placed in the path of the discharging electric terminal and each of the one or more resistances to control the effective resistance of the parallel resistances when discharging the device. 
       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. 
     The control circuit  100  can serve as a low-pass filter for incoming waveforms. The low-pass filter can filter high-frequency harmonics to result in a more predictable and consistent firing sequence for a given input. In one implementation, the time constant of the low-pass filter can be stated as “Reff×C”, in which Reff is the effective resistance of the resistors that are connected in parallel and C is the capacitance of capacitor  110 . Because Reff can be adjusted depending on which switches are actively connected in parallel, the time constant of the low-pass filter can vary and the resulting waveform across the capacitor  110  can be adjusted (e.g., shaped) accordingly. 
     The slope of the ramp during the charging phase can be determined by the amount of current that can be delivered to charge or discharge the capacitor  110 . The charging (or discharging) of the capacitor  110  is limited by the amount of current that the internal circuitry (not shown) driving the control circuit  100  can deliver to the control circuit  100  to charge (or discharge) the capacitor  110 . The “slew rate” can refer to the rate the capacitor  110  charges (or discharges), and can determine the slope of the charging (or discharging). In one aspect, the slew rate can be stated as the ratio of the current to capacitance (Slew rate=I/C). Alternatively, the slew rate can be stated as the change in voltage across the capacitor  110  divided by the effective resistance multiplied by the capacitance (Slew Rate=ΔV/(Reff*C)). Therefore, the slew rate and the slope of the charging and discharging can be adjusted by varying Reff. For example, if switches  102  and  104  are closed, Reff may represent the effective resistance of the parallel combination of resistors  106  and  108 . However, if switch  102  is open and switch  104  is closed, then Reff can represent the resistance of resistor  108 . 
       FIG. 7  shows a timing diagram of the resulting voltage on the actuator capacitor based on a constant input voltage applied at the input Xvdc. The ramp up at  120  is caused by having switch  102  closed while the other switches are open. The flat portion at  121  represents the voltage across a partially-charged capacitor, in which all the switches are open after having switch  102  partially charge the capacitor during  120 . The ramp up at  122  is caused by having switch  104  closed while the other switches are open. The flat portion at  125  represents a fully-charged capacitor, in which the value of the input voltage Xvdc is across the capacitor  110 . When the voltage across the capacitor  110  has reached the final voltage, Xvdc, all of the switches in the circuit can be opened to save power. At this point, the capacitor  110  effectively “holds” the voltage Xvdc because the charge on the capacitor does not change. 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. The slopes of the ramps up  120 ,  122  and the slopes of the ramps down  124 ,  126  can vary depending on the resistance of the switch that is being activated. Although  FIG. 7  shows one switch being activated at one time, more than one switch can be activated at the same time to vary the effective resistance, and the slope of the ramps. 
     In one implementation, the switches that are activated in the circuit are selected before the waveform is applied to the input of the circuit. In this implementation, effective resistance is fixed during the entire duration of the firing interval. Alternatively, the switches can be activated during the duration of the firing interval. In this alternative implementation, a waveform applied at the input of the circuit can shaped by varying the response of the circuit. The response of the circuit can vary according to the effective resistance, Reff, which can be selected at various instances during the firing interval by selecting which switches are connected in the circuit. 
     In another implementation, a single waveform can be applied across all of the resistances in each resistor&#39;s respective path in which the respective switch of the path is activated. Alternatively, the path of each resistor may use a different waveform in which the respective switch of the respective path is activated. In this case, the resultant waveform at the device can be a superposition of multiple waveforms. In this aspect, waveforms can be provided that are not stored in the waveform table. Hence, waveforms can be supplied from waveform data stored in the waveform table, as well as waveforms that are generated as a result of waveforms that are superimposed across a set of parallel resistor paths. In this aspect, the amount of memory to store a waveform table on the print head can be minimized to generate a limited number of basic waveform patterns, and the control switches can be use to generate additional and/or complex waveform patterns. As a result, a droplet ejection device can have a response that is trimmed or adjusted based on stored waveform data and/or mechanical data for control switches. 
       FIG. 8A  illustrates a schematic showing an alternative implementation of electrical components associated with an individual droplet ejection device.  FIG. 8A  shows an alternative control circuit  850  for an injection device in which multiple (here N) charging control switches Sc_ 1   802 , Sc_ 2   812 , and Sc_N  824  and associated charging resistors Rc_ 1   810 , Rc_ 2   816 , and Rc_N  814  are used to charge the capacitance C  860  of the piezoelectric actuator and multiple (here N) discharging control switches Sd_ 1   832 , Sd_ 2   834 , Sd_N  836  and associated discharging resistors Rd_ 1   840 , Rd_ 2   842 , and Rd_N  844  are used to discharge the capacitance. 
       FIG. 7  can also show the resulting voltage charge on the capacitance for one cycle of a square-pulse waveform Xv_waveform if the waveform is applied prior to  120  and removed after  126 . For example, the ramp up at  120  can be created by having switch  802  closed while the other switches are open. The ramp up at  812  can be created by having switch  104  closed while the other switches are open. The ramp down at  124  can be formed by having switch  832  closed while the other switches are open. The ramp down at  126  can be formed by having switch  834  closed while the other switches are open. Alternatively, any number of switches may be open or closed during ramp up or ramp down. Also, multiple switches may be open or closed during the ramp up or ramp down. 
     In one implementation, all the resistors in the control circuit  850  are of the same resistance. In another implementation, the resistors in the control circuit  850  are of different resistances. For example, the charging resistors Rc_ 1   810 , Rc_ 2   816 , and Rc_N  814  and corresponding discharging resistors Rd_ 1   840 , Rd_ 2   842 , and Rd_N  844  discharging resistors are binary-weighted resistors, in which a resistance in a (parallel) path can vary by a factor of two from a resistor in another (parallel) path. Alternatively, each resistor can have a resistance to allow the effective resistance, Reff, to vary by factors of 2 (e.g., Reff can be R, 2R, 4R, 8R, . . . 32R, etc.). 
       FIG. 8B  illustrates a schematic showing an alternative implementation of electrical components associated with an individual droplet ejection device.  FIG. 8B  shows an alternative control circuit  851  for an injection device in which multiple (here N) charging control switches Sc_ 1   802 , Sc_ 2   812 , and Sc_N  824  and associated charging resistors Rc_ 1   810 , Rc_ 2   816 , and Rc_N  814  are used to charge the capacitance C  860  of the piezoelectric actuator and multiple (here N) discharging control switches Sd_ 1   832 , Sd_ 2   834 , Sd_N  836  and associated discharging resistors Rd_ 1   840 , Rd_ 2   842 , and Rd_N  844  are used to discharge the capacitance. Multiple waveforms (e.g., Xv_waveform_ 1 , Xv_waveform_ 2 , and Xv_waveform_N) can be used as input waveforms into the control circuit  851  to generate a superimposed waveform across the capacitor C  860 . 
     In  FIG. 8A , one waveform is used as a common waveform for each switch-resistance path. For example, the path of Sc_ 1   802  and Rc_ 1   810  has the same waveform at the input of the switch Sc_ 1   802  as switch Sc_ 2   812  for path of Sc_ 2   812  and Rc_ 2   816 . In  FIG. 8B , each charging control switch Sc_ 1   802 , Sc_ 2   812 , Sc_N  824  can have a different waveform (e.g., Xv_waveform_ 1 , Xv_waveform_ 2 , and Xv_waveform_N) at the input of the switch. Hence, each switched-resistance path (e.g., path for Sc_ 1   802  and Rc_ 1   810 , path for Sc_ 2   812  and Rc_ 2   816 , and path for Sc_N  824  and Rc_N  814 ) can have a different waveform across the path. 
     In one implementation, the parallel switches may not increase an overall area of the die of the circuit in  FIG. 6  (or  FIGS. 8A ,  8 B) when compared to using a single switch as shown in  FIG. 3 . In another implementation, the power required by the circuit in  FIG. 6  (or  FIGS. 8A ,  8 B) may not increase power dissipated in the design of the circuit shown in  FIG. 3 . 
       FIG. 9  illustrates another schematic showing an alternative implementation of electrical components associated with the individual droplet ejection device.  FIG. 9  shows a control circuit  900  for an injection device in which multiple (here 4) control switches Sc_ 1   902 , Sc_ 2   912 , Sc_ 3   922 , and Sc_ 4   932  and associated resistors Rc_ 1   906 , Rc_ 2   916 , Rc_ 3   926 , and Rc_ 4   936  are used to charge and discharge the capacitance C  960  of the piezoelectric actuator. Instead of using separate discharging control switches and associated discharging resistors as shown in  FIGS. 3 ,  6 ,  8 A, and  8 B, an amplifier  950  can be used to drive an input signal, Xinput, to charge and discharge capacitance C  960  using control switches Sc_ 1   902 , Sc_ 2   912 , Sc_ 3   922 , and Sc_ 4   932  and associated resistors Rc_ 1   906 , Rc_ 2   916 , Rc_ 3   926 , and Rc_ 4   836 . The amplifier  950  can supply both the charging current and the discharging current for the capacitor C  960 . The input signal, Xinput, may be a constant voltage input (i.e., DC input) or may be another type of waveform, such as a sawtooth waveform, or a sinusoidal-type waveform, and the like. In one implementation, each of the control switches can be preset to an opened or closed position before the input signal is applied and driven by the amplifier  950 . After the input signal has been applied and the capacitance C  960  has been charged or discharged to a final value by the amplifier  950 , each of the control switches can be reset to a different opened or closed position for a successive input signal to be applied to the circuit  900 . The successive input signal may be a same type of input signal as applied for the previous signal, or may be a different type of input signal, such as a sawtooth waveform followed by a sinusoidal-type waveform. 
       FIG. 10A  shows a schematic of electrical components associated with a switch.  FIG. 10B  shows a timing diagram corresponding to the switch in  FIG. 10A . The input of the switch is driven by a drive waveform signal  1010 , and the output of the switch is connected to the PZT element  1014 . The channel control signal  1020  turns the switch  1022  “on” (or “off”), and electrically connects (or disconnects) the drive waveform signal  1010  with the PZT element  1014 . Analog switch  1022  has parasitic leakage currents I 1   1026  and I 2   1028  that can change an amount of charge stored on the PZT capacitor element  1014 , and can result in a change in PZT voltage  1012  when the PZT element  1014  is not being driven by the drive waveform signal  1010 . 
     For an ideal PZT voltage  1064  (i.e., when there is no leakage current (I 1 =I 2 =0) from the switch), the PZT voltage is held at a constant voltage during the non-firing periods  1042 ,  1046 ,  1050 —that is, when the droplet ejection device does not eject ink—because the PZT element  1014  does not lose charge. For this implementation, the droplet ejection device ejects ink according to the drive waveform  1060  when the charge control signal  1062  is held high. As a result, when the ideal PZT voltage  1064  is in the drop firing cycle  1040 ,  1044 ,  1048 , the droplet ejection device fires the drive waveform  1060  when the channel control  1062  is held high or turned “on”. Ideally, the amount of charge on the PZT element remains the same during the non-firing periods  1042 ,  1046 ,  1050  and when the channel control is held low or turned “off” because there is no leakage current. 
     For a case of when an actual PZT voltage  1066  has leakage currents I 1 &gt;I 2 , the current leakage I 1   1026  from the voltage supply  1024  is greater than the current leakage I 2   1028  to the ground potential  1016 . As a result, the amount of charge on the PZT element  1014  increases when the channel control is “off” (at  1042 ,  1044 ,  1046 ,  1050 ), and the PZT voltage increases until the PZT voltage  1066  reaches a level of the voltage supply (shown at the end of  1050 ). 
     For a case of when an actual PZT voltage  1068  has leakage currents I 1 &lt;I 2 , the current leakage I 1   1026  from the voltage supply  1024  is less than the current leakage I 2   1028  to the ground potential  1016 . As a result, the amount of charge on the PZT element  1014  decreases when the channel control is “off” (at  1042 ,  1044 ,  1046 ,  1050 ), and the PZT voltage decreases until the PZT voltage  1068  reaches a level of the ground potential (shown at the end of  1050 ). 
     During long periods of non-firing  1050  for actual PZT voltages  1066 ,  1068 , the resulting voltage on the PZT element can damage the PZT element. During shorter periods of non-firing  1042 ,  1046  when the PZT voltage does not reach the level of ground or the voltage supply, the charge on the PZT element can be suddenly discharged (or charged) to the voltage level of the drive waveform voltage  1060  when the channel control signal  1062  is turned on. The sudden discharge (or charge) of the PZT element to the voltage level of the drive waveform voltage can create a pressure wave inside the inkjet channel, which can interfere constructively or destructively with energy intentionally introduced in a subsequent firing cycle. As a result of the sudden discharge (or charge) on the PZT element, an overall image quality may degrade. 
       FIG. 11A  shows a schematic of electrical components associated with the switch.  FIG. 11B  shows a timing diagram corresponding to the switch in  FIG. 11A . The schematic shows that the channel control signal  1020  and an all-on clamp signal  1030  can be connected by an OR gate  1018  to control the “on” and “off” functionality of the analog switch  1022 . The switch  1022  can electrically connect the drive waveform signal  1010  to the PZT element  1014  whenever either the channel control signal  1020  or the all-on clamp signal  1030  is turned “on” or high. In one aspect, the all-on clamp signal  1030  can prevent damage to the PZT element  1014  as described in  FIGS. 10A-10B  by holding the PZT element voltage  1012  at a constant voltage level during non-firing periods  1042 ,  1046 ,  1050 . In another aspect, the all-on clamp signal can prevent degradation in image quality by preventing sudden discharging (and charging) of the PZT element and the corresponding pressure wave inside the inkjet channel. 
     For an ideal PZT voltage  1074  for which there is no leakage current (I 1 =I 2 =0) from the switch, the PZT voltage is held at a constant voltage during the non-firing periods  1042 ,  1046 ,  1050  when the droplet ejection device does not eject ink because the PZT element  1014  does not lose charge and/or because the all-on clamp signal can maintain the voltage constant. The all-on clamp signal  1080  can be turned on during the non-firing periods  1042 ,  1046 ,  1050  to keep the PZT voltage at the level of the drive waveform signal. For this implementation, the droplet ejection device ejects ink according to the drive waveform  1070  when the charge control signal  1072  is held high. As a result, when the ideal PZT voltage  1074  is in the drop firing cycle  1040 ,  1044 ,  1048 , the droplet ejection device fires the drive waveform  1070  when the channel control  1072  is held high or turned “on”. The PZT voltage can remain constant during the non-firing periods  1042 ,  1046 ,  1050  and when the channel control is held low or turned “off”. The PZT voltage also can be driven to a constant voltage during the non-firing periods  1042 ,  1046 ,  1050  when the all-on signal is turned on. 
     For cases of when the actual PZT voltage  1076  has leakage currents I 1 &gt;I 2   1076  or I 1 &lt;I 2   1078 , the all-on clamp signal  1080  can be turned on during the non-firing periods  1042 ,  1046 ,  1050  to keep the PZT voltage constant. For these non-firing periods  1042 ,  1046 ,  1050 , the drive waveform is held at a constant voltage level, and the all-on clamp signal  1080  turns on the switch  1022  to electrically connect the drive waveform  1070  to the PZT element. When the channel control  1072  and the all-on clamp  1080  are off and the droplet ejection device is in a drop firing cycle  1044 , the PZT element is not electrically connected to the drive waveform and current leakage may begin to change the PZT voltage as charge begins to accumulate (or leave) the PZT element. The actual PZT voltage  1076  or  1078  may be restored (at  1046 ) to the drive waveform voltage if the channel control signal  1072  or the all-on clamp  1080  signal is turned on to connect the PZT element to the drive waveform signal. 
     In one aspect, using the all-on clamp signal to drive the PZT element during non-firing periods can override the effect of parasitic charge leakage on the switch. In another aspect, the all-on clamp signal can be used to override the switch control of the channel control signal. 
     Other implementations of the disclosure are within the scope of the appended claims. For example, the switch and resistor can be discrete elements or may be part of a single element, such as the resistance of a field-effect transistor (FET) switch. The resistances shown in  FIGS. 3 ,  6 ,  8 A-B, and  9  can be designed based on the power dissipation of the droplet ejection device. In another example, the resistances shown in  FIGS. 3 ,  6 ,  8 A-B, and  9  can be designed based on the effective charging and/or discharging time constant of the droplet ejection device. In  FIGS. 10A and 11A , the switch  1022  may be a complementary metal oxide semiconductor (CMOS) device. In another implementation, other types of logic functions may be used instead of an OR gate  1018  in  FIG. 11A . Also, one all-on clamp signal  1030  can control the functionality of multiple switches in an array.