Patent Publication Number: US-2009231371-A1

Title: Apparatus and method for supplying voltage to nozzle in inkjet printer

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     This invention is related to a power supplying apparatus. More specifically, this invention relates to a power supplying apparatus for a print head comprised in a thermal inkjet printer. 
     2. Description of the prior art 
     At the present day, thermal inkjet printers are prevalent printers. Monochrome inkjet printers generally include a print head for providing black ink; color inkjet printers generally further include several print heads for providing inks of basic colors. Each black or color print head has a plurality of nozzles, respectively. Nozzles are the components that actually eject inks. Most inkjet printers are capable of providing a plurality of printing modes; each printing mode corresponds to a respective print resolution and a respective print quality. According to different settings of print resolution, the number of nozzles being used changes. That is to say, a printer controls different nozzles to act based on the printing mode selected by a user. A complete character or figure is formed with lots of tiny ink drops ejected by nozzle. Since each character and each figure are different from other, the number of nozzles being used is uncertain. 
     The working principle of thermal inkjet printers is briefly explained as follows. When a certain nozzle is assigned to act, a voltage impulse is provided to a heater in the nozzle so that the heater can generate heats rapidly. A small bubble is created and gradually grows up in the nozzle due to the heats. An ink drop near the output of the nozzle is then pushed out by the bubble so that the ink drop is ejected to a paper and forms a small ink point. After the voltage impulse is ended, the heater cools down, the bubble disappears, and the other ink in the nozzle shrinks back to the inner side of the nozzle. Please refer to  FIG. 1 .  FIG. 1  shows the typical connecting relation between a power source  10  and the nozzles ( 111 ,  112 , and  113 ) in a print head  11 . The power source  10  is used for providing voltage impulses to the nozzles. Each nozzle respectively includes a heater ( 111 A,  112 A, and  113 A) and a switch ( 111 A,  112 A, and  113 A). 
     The nozzle  111  is taken as an example. When the switch  111 B is opened, the heater  111 A is not conducted to the ground. Therefore, there is no current flowing through the heater  111 A and no voltage drop is formed between the two ends of the heater  111 A. If the nozzle  111  is assigned to act in certain operation, the switch  111 B is closed so that the heater  111  A is conducted to the ground through the switch  111 B. Subsequently, the power supply  10  provides current to the nozzle  111  and there is a voltage drop formed between the two ends of the heater  111 A. An ink drop near the output of the nozzle  111  is then pushed out by the bubble generated by the heater  111 A to form an ink point. 
       FIG. 2  further shows the schematics of the nozzles in actual applications. As shown in  FIG. 2 , each switch of the nozzles can be composed of a sub-switch ( 111 C,  112 C,  113 C) and a MOS device ( 111 D,  112 D,  113 D). When the sub-switch  111 C is closed, the gate of the MOS device  111 D is connected to the power supply  10  and the MOS device  111 D is then conducted. In this way, the heater  111 A can be conducted to the ground through the MOS device  111 D. 
     Each heater, each switch, and even the connecting wires between the power supply  10  and the print head  11  are all loadings for the power supply  10 . As known to people skilled in the art, the output impedance of an ideal voltage supply is zero, thus, regardless of the value of loadings, an ideal voltage supply can provide a constant output voltage. However, in actual situations, the output voltage provide by the power supply  10  varies with different loadings. 
     As shown in  FIG. 1 , if the nozzle  111  and nozzle  112  are assigned to act simultaneously, the heater  111 A and the heater  112 A are parallel. The equivalent loading formed by the two parallel heaters is half of the loading formed by a single heater (assuming the resistances of the heater  111 A and the heater  112 A are the same). Since the number of nozzles being used is uncertain under various conditions, the loading formed by the nozzles also changes for the power supply  10 . Therefore, the voltage drop generated between the two ends of a heater is inevitably affected by the amount of loadings. 
     Presently, most inkjet printers use constant voltages to drive print heads. Once the voltage is affected by different loadings and connecting wires, the voltage supplied to the heaters might be insufficient or too much so that the ink drops in nozzles cannot be ejected or too big. Thus it shows loading problem does decrease printing quality of inkjet printers. 
     Please refer to  FIG. 3 .  FIG. 3  shows the other prior art including two power supplies. The first power supply  20 A is used for providing voltages to the heaters; the second power supply  20 B is used for selectively providing voltages to the gates of the MOS devices. The main difference between  FIG. 3  and  FIG. 2  is that the voltage values for heaters and switches are different; thus, two power supplies are required. The prior art shown in  FIG. 3  also has the aforementioned problem of voltage variations. 
     SUMMARY OF THE INVENTION 
     To solve the aforementioned problems, this invention proposes an apparatus and a method for providing voltages to nozzles included in a thermal inkjet printer. The apparatus and method according to this invention bring the impedances of nozzles into consideration. According to the number of nozzles to be used in different printing modes, this invention previously compensates the voltage variations caused by the impedances of nozzles and connecting wires. Thus, stable and accurate voltages can be provided. One preferred embodiment according to this invention is a power supplying apparatus. The power supplying apparatus includes a look-up table, a selecting module, and an adjustable voltage supply. Plural sets of control signals corresponding to a plurality of printing modes are stored in the look-up table. After receiving a command representing that a user selects one printing mode among the plurality of printing modes as a final printing mode, the selecting module selects one set of control signals among the plural sets of control signals stored in the look-up table as a set of final control signals according to the command. The set of final control signals is corresponding to the final printing mode. The adjustable voltage supply is used for supplying at least one voltage to the plurality of nozzles based on the set of final control signals selected by the selecting module. 
     The other preferred embodiment according to this invention is a method of controlling an adjustable voltage supply for a print head comprised in a thermal inkjet printer. In this embodiment, a look-up table is previously provided for storing plural sets of control signals corresponding to a plurality of printing modes. The first step in the method is receiving a command representing that a user selects one printing mode among the plurality of printing modes as a final printing mode. The second step is selecting one set of control signals among the plural sets of control signals stored in the look-up table as a set of final control signals according to the command received in the first step. The set of final control signals is corresponding to the final printing mode. The third step of this method is adjusting the adjustable voltage supply such that the adjustable voltage supply provides at least one voltage corresponding to the set of final control signals to the plurality of nozzles. 
     The advantage and spirit of the invention may be understood by the following recitations together with the appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE APPENDED DRAWINGS 
         FIG. 1  shows the typical connecting relation between a power source and the nozzles in a print head. 
         FIG. 2  further shows the schematics of the nozzles in  FIG. 1 . 
         FIG. 3  shows the other prior art including two power supplies. 
         FIG. 4  shows the block diagram of the power supplying apparatus of the first preferred embodiment according to this invention. 
         FIG. 5  shows the block diagram of the power supplying apparatus of the second preferred embodiment according to this invention. 
         FIG. 6  shows the flowchart of the method of the third preferred embodiment according to this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     The main purpose of this invention is proposing an apparatus and a method for providing voltages to a print head included in a thermal inkjet printer. The print head includes a plurality of nozzles. The thermal inkjet printer is capable of providing a plurality of printing modes; each printing mode is corresponding to a respective number of nozzles included in the print head. 
     The first preferred embodiment according to this invention is a power supplying apparatus. Please refer to  FIG. 4 .  FIG. 4  shows the block diagram of this power supplying apparatus. The power supplying apparatus  30  includes an adjustable voltage supply  30 A, a selecting module  30 B, and a look-up table  30 C. Each nozzle shown in  FIG. 4  includes a heater, a MOS device, and a switch. 
     In this invention, the loadings caused by the impedances of nozzles and connecting wires under different conditions are previously calculated. Then, the adjustment amounts corresponding to different loadings are also evaluated for the power supplying apparatus  30 . Thus, the power supplying apparatus  30  can be adjusted in advance to supply accurate voltages. The look-up table  30 C is used for storing plural sets of control signals for the power supplying apparatus  30  corresponding to different loadings. 
     In actual applications, the adjustable voltage supply  30 A is connected to the plurality of nozzles via a bus line. A bus line generally has serial impedances that also diminish voltages. Furthermore, due to the effects of parasitic resistances and parasitic capacitances, the time when the nozzles receive voltages (or currents) is later than the time when the power supplying apparatus  30  starts to send out voltages (or currents). To timely provide enough power required at the instant that nozzles are opened, information about the delay time can also be stored in the look-up table  30 C according to this invention so that the voltage supplied by the power supplying apparatus  30  can be adjusted in advance. That is to say, the plural sets of control signals can also be relative to the serial impedances of bus lines. 
     After receiving a command  31  representing that a user selects one printing mode among the plurality of printing modes as a final printing mode, the selecting module  30 B selects one set of control signals among the plural sets of control signals stored in the look-up table  30 C as a set of final control signals according to the command  31 . In actual applications, the look-up table  30 C and the selecting module  30 B can be integrated as a control unit. The adjustable voltage supply  30 A is used for supplying at least one voltage to the plurality of nozzles ( 111 ,  112 , and  113 ) based on the set of final control signals selected by the selecting module. 
     Please refer to  FIG. 5 .  FIG. 5  shows the block diagram of the power supplying apparatus of the second preferred embodiment according to this invention. The working principle of the power supplying apparatus  40  is the same as the power supplying apparatus  30  in  FIG. 4 . In this embodiment, the voltage values for heaters and switches are different. Accordingly, the power supplying apparatus  40  provides two voltages. The first voltage is provided to the heater of each nozzle through the bus line  42 ; the second voltage is provided to the switch of each nozzle through the bus line  43 . 
     The third preferred embodiment according to this invention is a method of controlling an adjustable voltage supply for a print head comprised in a thermal inkjet printer. In this embodiment, a look-up table is previously provided for storing plural sets of control signals corresponding to a plurality of printing modes. Please refer to  FIG. 6 .  FIG. 6  is the flowchart of this method. The first step S 61  is receiving a command representing that a user selects one printing mode among the plurality of printing modes as a final printing mode. The second step S 62  is selecting one set of control signals among the plural sets of control signals stored in the look-up table as a set of final control signals according to the command received in the first step. The set of final control signals is corresponding to the final printing mode. The third step S 63  is adjusting the adjustable voltage supply such that the adjustable voltage supply provides at least one voltage corresponding to the set of final control signals to the plurality of nozzles. 
     Because the impedances of nozzles are brought into consideration, this invention can previously compensate the voltage variations caused by the impedances of nozzles and connecting wires according to the number of nozzles to be used in different printing modes. Thus, stable and accurate voltages are provided. 
     With the example and explanations above, the features and spirits of the invention will be hopefully well described. Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teaching of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.