Patent Publication Number: US-7712854-B2

Title: Piezoelectric fluid injection devices and driving voltage calibration methods thereof

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a micro-fluid injection device, and in particular to a piezoelectric fluid injection device and a driving voltage calibration method thereof. 
     2. Description of the Related Art 
     Recently, fluid injection has been widely utilized in various devices such as inkjet printers and the like. As micro-system engineering increasingly develops, such devices can be further applied in other fields, for example, fuel injection, cell sorting, drug delivery, print lithography, and micro-jet propulsion systems. Inkjet applications generally utilize continuous or drop-on-demand supply. 
     Conventional fluid injection devices also comprise thermal bubble and piezoelectric diaphragm drive types. 
     A conventional control circuit of a piezoelectric inkjet printhead is shown in  FIG. 1 . A piezoelectric inkjet printhead  10  includes a plurality of nozzles  1 ˜X such as  1 ˜ 128 . Each nozzle&#39;s equivalent circuit represents parallel equivalent capacitors C L1 ˜C LX . Each nozzle is driven by a driving cell  20 . Conventionally, the nozzles of printhead are driven by a fixed driving voltage, such as 100V. However, impedance variations among nozzles are produced due to operational variations in piezoelectric diaphragm process or ageing, resulting in formation of various droplet volumes, or even, for some nozzles, no droplets being ejected therefore when the fixed driving voltage is applied, seriously affecting utilization efficiency of the inkjet printhead. 
     Additionally, variations in fluid pressure resulting from alternation of fluid resistance or material property around nozzles may also cause such drawbacks. 
     U.S. Pat. No. 6,286,922 discloses a method of controlling a driving voltage of a piezoelectric inkjet printhead and a feedback procedure. An output driving voltage from a control system is switched via an analog/digital converter and fed back. The feedback voltage is then determined by comparison with an actual required driving voltage by the control system and modified. 
     U.S. Pat. No. 6,286,922 discloses a driving circuit and a control system of a piezoelectric inkjet printhead, capable of controlling ejected droplet volumes and providing preferred printing quality. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides a piezoelectric fluid injection device comprising at least one inkjet printhead comprising a plurality of nozzles, at least one voltage control element connecting to the inkjet printhead, a controller connecting to the voltage control element, a reference capacitor connecting to an auxiliary voltage control element and the controller in parallel with the inkjet printhead. 
     Each nozzle of the inkjet printhead is independently controlled. A driving voltage and its waveform are modified by a feedback circuit, achieving the optimal utilization efficiency of the inkjet printhead. 
     The invention also provides a method of calibrating a driving voltage of a piezoelectric fluid injection device, comprising the following steps. A piezoelectric fluid injection device comprising at least one nozzle and a reference capacitor is provided. The nozzle corresponds to a nozzle driving voltage. The reference capacitor corresponds to a reference driving voltage. The nozzle driving voltage is compared with the reference driving voltage. If the nozzle driving voltage and the reference driving voltage are substantially distinct, the nozzle driving voltage is modified to substantially correspond to the reference driving voltage. The modified nozzle driving voltage is stored in a memory cell and acts as a reference for subsequent calibration. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawing, wherein: 
         FIG. 1  shows a conventional control circuit of a piezoelectric inkjet printhead. 
         FIG. 2A  shows each driving cell of a negative-voltage piezoelectric nozzle control circuit. 
         FIG. 2B  shows each driving cell of a positive-voltage piezoelectric nozzle control circuit. 
         FIG. 3  shows a control system comprising a feedback circuit of a piezoelectric inkjet printhead in accordance with the first embodiment of the invention. 
         FIG. 4  shows a control system comprising a feedback circuit of a piezoelectric inkjet printhead in accordance with the second embodiment of the invention. 
         FIG. 5  shows a control system comprising a feedback circuit of a piezoelectric inkjet printhead in accordance with the third embodiment of the invention. 
         FIG. 6  shows a control system comprising a feedback circuit of a piezoelectric inkjet printhead in accordance with the fourth embodiment of the invention. 
         FIG. 7  shows a control system comprising a feedback circuit of a piezoelectric inkjet printhead in accordance with the fifth embodiment of the invention. 
         FIG. 8  discloses a method of calibrating a driving voltage of a piezoelectric inkjet printhead in accordance with one embodiment of the invention. 
         FIG. 9  discloses a method of calibrating a driving voltage of a piezoelectric inkjet printhead in accordance with the sixth embodiment of the invention. 
         FIGS. 10A˜10B  show the waveforms of the reference voltage and the nozzle voltage (driving voltage) of the sixth embodiment of the invention. 
         FIG. 11  discloses a method of calibrating a driving voltage of a piezoelectric inkjet printhead in accordance with the seventh embodiment of the invention. 
         FIGS. 12A˜12B  show the waveforms of the reference voltage and the nozzle voltage (driving voltage) of the seventh embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
     The invention provides an independent driving circuit for each nozzle to output various driving voltages, overcoming the issue of impedance variations thereamong. In accordance with a preferred embodiment of the invention, a feedback circuit is provided to detect the setted voltage and the output voltage (driving voltage) of each nozzle and modify the output voltage (driving voltage) to correspond to the setted voltage. Additionally, the waveforms of the output voltages (driving voltages) of nozzles, inconsistent due to impedance variations, are synchronized by a waveform control procedure, thus optimizing inkjet time of each nozzle. 
     The invention provides a method of calibrating a driving voltage of a piezoelectric fluid injection device. A piezoelectric inkjet printhead is placed on a printing platform. Each nozzle thereof represents an equivalent capacitive load.  FIG. 1  discloses an equivalent circuit of each nozzle of a piezoelectric fluid injection device. The capacitive loads (impedances) among nozzles are distinct due to operational variations in the print head production process. When a fixed driving voltage is applied thereto, the actuations of nozzles may be inconsistent, deteriorating printing quality. 
     Additionally, when printing is performed, in accordance with input data, only a portion of the nozzles may be simultaneously actuated such that the sum of impedances is altered. Thus, the stability of driving cannot be maintained when the fixed driving voltage is applied. The invention provides an independent driving circuit for each nozzle to avoid the unfixed parallel capacitive loads. 
     A piezoelectric nozzle driving voltage control element includes negative-voltage circuit driving cells and positive-voltage ones which are shown individually in  FIG. 2A  and  FIG. 2B . When each nozzle of print head is independently controlled, proper driving voltage amplitude, waveform width, and circuit are required to control the piezoelectric actuation. A simple negative-voltage driving cell circuit  20 A is disposed near the inkjet printhead to reduce control signal loss during transmission. In  FIG. 2A , Vcc represents a standard logic level voltage and Vss represents a high negative-voltage. When the pulse width control signal is at a low level, a capacitor C L  corresponding to a nozzle is charged by a transistor Q 1 . As driving begins, the pulse width control signal is at a high level and the transistor Q 1  is turned off, the pulse voltage control signal is at a low level and the transistor Q 2  is turned off, too. Next, the pulse voltage control signal is at a high level and the transistor Q 2  is turned on. Meanwhile, the equivalent capacitor C L  is charged to the high negative-voltage until the pulse voltage control signal reaches a low level (Q 2  turn off simultaneously). Finally, the equivalent capacitor C L  achieves a terminal negative-voltage level. Specifically, when the pulse voltage control signal keeps at a high level, the equivalent capacitor C L  is continuously charged to the saturation negative-voltage. Thus, the terminal voltage of the equivalent capacitor C L  is determined by controlling the retention time of the pulse voltage control signal at a high level state. 
     The positive-voltage piezoelectric nozzle control circuit driving cells are shown in  FIG. 2B . A positive-voltage driving cell  20 B is provided that Vcc represents a high positive-voltage and Vss represents a ground. Before driving, the pulse voltage control signal is at a high level and the pulse width control signal is at a low level, thus the transistor Q 1  and Q 2  are turned off. As driving commences, the pulse voltage control signal is at a low level and a transistor Q 1  is turned on. Meanwhile, the equivalent capacitor C L  is charged until the pulse voltage control signal is promoted to a high level (Q 1  is turned off simultaneously). Thus, the equivalent capacitor C L  achieves a terminal positive-voltage level. When the pulse voltage control signal continuously maintains a low level, the equivalent capacitor C L  is continuously charged and finally promoted to the saturation positive-voltage. 
     The invention provides a driving circuit and a control system of a piezoelectric inkjet printhead. Each nozzle ejection behavior will be tune to consistency and uniform, when the independent addressable waveform nozzle control driver which controls includes driving voltage and waveform thereof are modified by a feedback circuit, achieving the optimal utilization efficiency of nozzles. The piezoelectric fluid injection device comprises at least one inkjet printhead comprising a plurality of nozzles, at least one voltage control element connecting to the inkjet printhead, a controller connecting to the voltage control element, a reference capacitor connecting to an auxiliary voltage control element and the controller in parallel with the inkjet printhead. 
     In accordance with the first embodiment of the invention, a control system comprising a feedback circuit of a piezoelectric inkjet printhead is shown in  FIG. 3 . A control system  100   a  of a piezoelectric inkjet printhead comprises a control system  130 , a plurality of voltage control cells  120 , a piezoelectric inkjet printhead  110 , a voltage down cell  150 , an analog switch  160 , a comparator  170 , and a reference capacitor C L    140 . After placing the inkjet printhead into a printing system, starting the printing system, or using the inkjet printhead for a period of time, each nozzle voltage (driving voltage) is calibrated to correspond to the set value as follows. After setting the voltage, a reference voltage is produced from a reference capacitor C L    140  and dropped to a proper voltage level. A nozzle voltage (driving voltage) is dropped by a voltage down cell  150  and switched via an analog switch  160  to output a voltage signal into a comparator  170  to compare with the reference voltage. The nozzle voltage (driving voltage) is then continuously modified by a controller until corresponding to the reference voltage. The foregoing steps are repeated until calibration of all nozzles is completed. The calibrated voltage parameters are stored in a data storing cell of the controller and used when printing is performed. 
     In accordance with the second embodiment of the invention, a control system comprising a feedback circuit of a piezoelectric inkjet printhead is shown in  FIG. 4 . An analog/digital converter  180  is added. Compared to the control system of  FIG. 3  in which a relative reference voltage is obtained, a real reference voltage is obtained by the analog/digital converter  180 . A control system  100   b  of a piezoelectric inkjet printhead comprises a control system  130 , a plurality of voltage control cells  120 , a piezoelectric inkjet printhead  110 , a voltage down cell  150 , an analog switch  160 , a comparator  170 , a reference capacitor C L    140 , and an analog/digital converter  180 . After placing the inkjet printhead into a printing system, starting the printing system, or using the inkjet printhead for a period of time, each nozzle voltage (driving voltage) is calibrated to correspond to the set value as follows. After setting the voltage, a reference voltage is produced from a reference capacitor C L    140  and generated to a proper voltage level. The reference voltage is then switched via the analog/digital converter  180  and fed back and modified to correspond to the setting value. A nozzle feedback voltage is generated by a voltage down cell  150  and switched via an analog switch  160  to output a feedback voltage signal into a comparator  170  to compare with the reference voltage. The nozzle voltage (driving voltage) is then continuously modified by a controller until corresponding to the reference voltage. The foregoing steps are repeated until calibration of all nozzles is completed. The calibrated voltage parameters are stored in a data storing cell of the controller and used when printing is performed. 
     In accordance with the third embodiment of the invention, a control system comprising a feedback circuit of a piezoelectric inkjet printhead is shown in  FIG. 5 . Compared to the control system of  FIG. 3 , the reference capacitor C L  is replaced by one of the nozzles (reference nozzle) of the inkjet printhead. The control system of  FIG. 5  saves a reference capacitor C L  circuit. More reserved nozzles, however, are required for preventing the malfunction of the reference nozzle. 
     In accordance with the fourth embodiment of the invention, a control system comprising a feedback circuit of a piezoelectric inkjet printhead is shown in  FIG. 6 . Compared to the control system of  FIG. 5 , an analog/digital converter  180  is added, thereby obtaining an accurate output voltage. Similarly, more reserved nozzles are required for preventing the malfunction of the reference nozzle. 
     In accordance with the fifth embodiment of the invention, a control system comprising a feedback circuit of a piezoelectric inkjet printhead is shown in  FIG. 7 . Compared to the control systems of  FIGS. 3˜6 , the comparator  170  is removed, thereby accurately controlling each nozzle voltage (driving voltage), however, prolonging the calibration time. 
     The invention provides a method of calibrating a driving voltage amplitude and a driving waveform of a piezoelectric inkjet printhead, overcoming the issue of impedance variations among nozzles. A procedure of voltage modification is provided which is calibrated after placing the inkjet printhead into a printing system, using the inkjet printhead for a period of time, or setting an action voltage. After the voltage calibration, the driving waveforms are calibrated to achieve uniformity. The invention provides two waveform calibration methods comprising aligning the rising curve central point of each driving waveform and aligning the terminal voltage of each driving waveform to improve the uniformity of droplets. 
     In accordance with one embodiment of the invention, a method of calibrating a driving voltage of a piezoelectric inkjet printhead is shown in  FIG. 8 . After placing a new inkjet printhead into a printing system, using the inkjet printhead for a period of time, or setting a driving voltage, nozzle voltage (driving voltage) calibration is performed S 210 . Next, a nozzle voltage (driving voltage) is compared with a reference voltage S 220 . The nozzle voltage (driving voltage) is then calibrated until corresponding to the reference voltage S 250 . Next, other nozzle voltages (driving voltages) are continuously compared with reference voltages S 230  until the nozzle voltage (driving voltage) calibration is completed S 240 . Additionally, the calibrated voltage parameters and nozzle capacitive loads are recorded in a memory cell S 260 . A voltage parameters-nozzle capacitive loads table is obtained and fed back the control system S 270 . 
     In accordance with sixth embodiment of the invention, a method of calibrating a driving voltage of a piezoelectric inkjet printhead is shown in  FIG. 9 . After placing a new piezoelectric inkjet printhead into a printing system, using the piezoelectric inkjet printhead for a period of time, or setting a driving voltage, nozzle voltage waveform (driving voltage waveform) calibration is performed S 310 . Next, a nozzle voltage waveform (Δtx=(tx end −tx start )) (driving voltage waveform) is compared with a reference voltage waveform (Δtp=(tp end −tp start )) S 320 . The nozzle voltage waveform (Δtx=(tx end −tx start )) (driving voltage waveform) is then calibrated until corresponding to the reference voltage waveform (Δtp=(tp end −tp start )) S 350  and S 360 , satisfying the two conditions including X nozzle pulse offset=tp start −(Δtx−Δtp)/2 or X nozzle pulse offset=tp start +(Δtx−Δtp)/2. Next, other nozzle voltage waveforms (driving voltage waveforms) are continuously compared with reference voltage waveforms S 330  until the nozzle voltage waveform (driving voltage waveform) calibration is completed S 340 . Additionally, the offset values of the rising curve central points of the nozzle voltage waveforms (driving voltage waveforms) are recorded in a memory cell S 370 . 
     The widths of different nozzle voltage waveforms (driving voltage waveforms) are distinct due to variations in impedance among nozzles. The invention provides a calibration method to align the rising curve central points of different nozzle voltage waveforms (driving voltage waveforms), unifying injection behavior at nozzles. The time (Δtp) of the reference voltage waveform (Δtp=(tp end −tp start )) is compared with the time (Δtx) of the nozzle voltage waveform (driving voltage waveform) (Δtx=(tx end −tx start )). If Δtx is larger than Δtp or the inverse, the rising curve central points thereof are then aligned. After alignment, such relevant waveform parameters are stored. 
     In the invention, the start point of the reference voltage waveform is a basis of calibration, but is not limited thereto. 
     The waveforms of the reference voltage and the nozzle voltage (driving voltage) of the sixth embodiment are shown in  FIGS. 10A and 10B . The voltage waveforms of  FIG. 10A  are not yet calibrated. In  FIG. 10A , although the start points are the same, the end points are distinct due to variations in impedance. However, after the rising curve central point calibration, no deviation occurs between the two voltage waveforms as shown in  FIG. 10B , unifying injection behavior at nozzles. 
     In accordance with seventh embodiment of the invention, a method of calibrating a driving voltage of a piezoelectric inkjet printhead is shown in  FIG. 11 . After placing a new piezoelectric inkjet printhead into a printing system, using the piezoelectric inkjet printhead for a period of time, or setting a driving voltage, nozzle voltage waveform (driving voltage waveform) calibration is performed S 410 . Next, a nozzle voltage waveform (Δtx=(tx end −tx start )) (driving voltage waveform) is compared with a reference voltage waveform (Δtp=(tp end −tp start )) S 420 . The nozzle voltage waveform (Δtx=(tx end −tx start )) (driving voltage waveform) is then calibrated until corresponding to the reference voltage waveform (Δtp=(tp end −tp start )) S 450  and S 460 , satisfying the two conditions including X nozzle pulse offset=tp start −(Δtx−Δtp) or X nozzle pulse offset=tp start +(Δtx−Δtp). Next, other nozzle voltage waveforms (driving voltage waveforms) are continuously compared with reference voltage waveforms S 430  until the nozzle voltage waveform (driving voltage waveform) calibration is completed S 440 . Additionally, the offset values of the end points of the nozzle voltage waveforms (driving voltage waveforms) are recorded in a memory cell S 470 . 
     The widths of different nozzle voltage waveforms (driving voltage waveforms) are distinct due to variations in impedance among nozzles. The invention provides a calibration method to align the end points of different nozzle voltage waveforms (driving voltage waveforms). The time (Δtp) of the reference voltage waveform (Δtp=(tp end −tp start )) is compared with the time (Δtx) of the nozzle voltage waveform (driving voltage waveform) (Δtx=(tx end −tx start )). If Δtx is larger than Δtp or the contrary, the end points thereof are then aligned. After alignment, such relevant waveform parameters are stored. 
     The waveforms of the reference voltage and the nozzle voltage (driving voltage) of the seventh embodiment are shown in  FIGS. 12A and 12B . The voltage waveforms of  FIG. 12A  are not yet calibrated. In  FIG. 12A , although the start points are the same, the end points are distinct due to variations in impedance. However, after the end point calibration, no deviation occurs between the two voltage waveforms as shown in  FIG. 12B , unifying injection behavior at nozzles. 
     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.