Abstract:
An ink-jet printer includes a negative thermal coefficient thermistor for sensing temperature of the print head, and a monostable multivibrator, connected to the thermistor and a capacitor for realizing a pulse duration control circuit, such that the pulse duration control circuit generates a print enable signal with a duration corresponding to resistance of the thermistor. When the printer starts to jet ink, it supplies energy according to the duration of the print enable signal to heat ink, so that if temperature of the print head rises, the duration of the print enable signal decreases and energy supplied to heat ink will become less accordingly, thus degradation of printing due to heat accumulation is avoided.

Description:
BACKGROUND OF INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention provides an ink-jet printer and related apparatus capable of adjusting ink-jet energy according to print-head temperature instantly, and more particularly, a printer and related apparatus for controlling temperature adjustment by a simple circuit structure having a monostable multivibrator with a thermistor.  
         [0003]     2. Description of the Prior Art  
         [0004]     In modern information society, ink-jet printers are one of the most popular types of printers because of low price and outstanding print quality. Information technology companies are eager to develop more progressive ink-jet print techniques to decrease cost and increase quality.  
         [0005]     In general, an ink-jet printer heats ink in nozzles of a print head while printing. The print head connected to an ink cartridge includes a plurality of nozzles, and near each nozzle is a corresponding heating unit (such as a transistor including a heating resistor), which heats nearby ink. When jetting ink, the ink-jet printer transmits heating energy to each heating unit, and then jets an ink drop from a corresponding nozzle to a print document (such as paper or other medium). According to print data, such as words and pictures, the print head controls different nozzles to jet or not to jet ink to the print document repeatedly.  
         [0006]     However, in the above ink-jet process, because the heating unit of each nozzle is heated repeatedly, ink temperature in the print head increases resulting in a heat accumulation phenomenon. In comparison with a heat dissipation situation (for example, as occurs when starting to print), if the printer triggers the heating unit with the same energy and causes heat accumulation (for example, the printer has printed for a long time), owing to both decreased viscosity of hot ink and continuous heating of the heating unit, the nozzles jet too much ink so larger ink drops are printed to the print document. The larger the ink drops, the lower the print resolution (like dots per inch, DPI), print clarity, and quality of ink-jet printing. In order to prevent this negative effect of heat accumulation, several ink-jet printing techniques have been developed.  
         [0007]     Those skilled in the art will recognize that techniques for compensating for heat accumulation can be divided into two types, one type is an open-loop trigger control mode, and the other type is a closed-loop trigger control mode. As disclosed in U.S. Pat. Nos. 5,036,337 and 5,790,144, in the open-loop trigger control mode, an ink-jet printer predicts heat accumulation in a print head according to print data. For example, if the print data have repeatedly triggered a lot of heating units to heat ink in a short time, the ink-jet printer can predict that its print head will encounter more heat accumulation, so that each heating unit is provided with less energy to avoid the ink drops becoming too large. However, what cause print head heat accumulation are not only the print data, but also other factors (such as remaining ink in the print head and the ink cartridge). Therefore, heat accumulation cannot be predicted exactly by the print data; that is, the open-loop trigger control mode cannot prevent heat accumulation completely.  
         [0008]     In addition, the U.S. Pat. No. 6,394,572 discloses a close-loop trigger control mode. In the close-loop trigger control mode, the ink-jet printer controls ink-jet trigger energy by measuring temperature of the print head through a thermistor. Please refer to  FIG. 1  illustrating a block diagram of the prior art close-loop trigger control mode in a printer  10 . The printer  10  is an ink-jet printer that includes an interface circuit  12 , a system control circuit  14 , a non-volatile memory device  15 , a drive circuit  16 , a print head  18 , a measure circuit  20 , and an A/D (analog to digital) converter  22 . The interface circuit  12  receives waiting print data from a data source  24  (or a host such as a PC). The system control circuit  14  controls operations of the printer  10 . The memory  25  registers data for operations of the system control circuit  14  by a non-volatile method. The print head  18  includes K nozzles Np( 1 ), Np( 2 ) . . . Np(K), and heating units Qp( 1 ), Qp( 2 ) . . . Qp(K) each corresponding to the nozzles. The drive circuit  16  triggers drive signals Sp( 1 ), Sp( 2 ) . . . Sp(K) to the heating units Qp( 1 ), Qp( 2 ) . . . Qp(K) under control of the system control circuit  14 . After receiving the corresponding drive signals, each heating unit heats nearby ink corresponding to the nozzles and then jets the ink to a print document  29 .  
         [0009]     In order to compensate for heat accumulation in the close-loop trigger control mode, the print head  18  of the printer  10  further includes a thermistor TRp, whose resistance changes as the temperature of the print head  18  changes. In general, heating units and corresponding nozzles are deposited in an ink-jet chip so uniformly that the thermistor TRp layouts surrounding each nozzle (such as the oblique line blocks in  FIG. 1 ) measure temperature of the whole chip. The measure circuit  20  includes two connection ends cp 1  and cp 2  each connected to one end of the circular thermistor, which is equivalent to connecting the connection ends cp 1  and cp 2  through the thermistor TRp. Functions of the measure circuit  20  are measuring resistance of the thermistor TRp, and generating a corresponding outcome  28 A. For example, the measure circuit  20  transmits a stable current to the thermistor TRp to measure the cross voltage of the thermistor TRp; the cross voltage represents the resistance of the thermistor TRp as the outcome  28 A. Because the system control circuit  14  calculates trigger energy according to the resistance of the thermistor TRp in the close-loop trigger control mode, the prior art printer  10  therefore includes an A/D converter  22  to transfer the analog outcome  28 A of the measure circuit  20  to the digital outcome  28 B, and to feedback the outcome  28 B to the system control circuit  14 . Following that, the system control circuit  14  estimates energy of drive signals for each heating unit based on the outcome  28 B. In general, the system control circuit  14  estimates the energy according to a look-up table, and the prior art printer  10  therefore requires space in the memory device  15  for this table.  
         [0010]     As to heat accumulation compensation of the prior art printer  10  in the close-loop trigger control mode, please refer to  FIG. 2  (and  FIG. 1 ), which illustrates a related signal waveform time domain diagram whose X-axis is time scale and Y-axis is waveform amplitude. When printing, the printer  10  receives waiting print data provided by the data source  24  through the interface circuit  12 , and then registers the data into the memory  25 . If the printer  10  starts jetting ink at time point tp 1 , according to the table stored in the memory device  15 , the system control circuit  14  will estimate trigger energy corresponding to the outcome  28 B provided by the measure circuit  20  and the A/D converter  22 . After that, a print enable signal  26 B drops from level H to level L at time point tp 1 , and the system control circuit  14  controls the level L maintenance time. Besides, the print enable signal  26 B will be transmitted to the drive circuit  16 . Meanwhile, the waiting print data registered in the memory  25  is transmitted to the drive circuit  16  as the print data  26 A shown in  FIG. 1 .  
         [0011]     After receiving the print data  26 A, the drive circuit  16  determines which nozzles need to jet ink and which do not. The drive circuit  16  provides an ink-jet drive signal for corresponding ink-jet units. If the print head  18  (in  FIG. 1 ) has a nozzle Np(k) required to jet, the drive circuit  16  will trigger the heating unit Qp(k) to heat ink with a corresponding drive signal Sp(k) as shown in  FIG. 2 . The waveform in  FIG. 2  shows that the drive circuit  16  generates the same pulse wave width of a drive signal Sp(k) as the pulse wave width Tp 1  of the print enable signal  26 B; that is, when the print enable signal  26 B drops from the level H to the level L at time point tp 1 , the drive signal Sp(k) rises from the level Dl to the level Dh. Between time points tp 1  and tp 2 , the print enable signal maintains the level L, so that the drive signal Sp(k) maintains the level Dh, whose corresponding heating unit Qp(k) heats ink continuously to jet ink through corresponding nozzle Np(k). At time point tp 2 , since the system control circuit  14  pulls the print enable signal  26 B to the level H, the drive circuit  16  pushes the drive signal Sp(k) to the level Dl accordingly. Therefore, the heating unit Qp(k) stops heating ink.  
         [0012]     In other words, the level L of the print enable signal  26 B can be seen as an enable level. When the print enable signal  26 B maintains the enable level (during time slot Tp 1 ), the drive signal Sp(k) triggers the heating unit Qp(k) to heat ink with the level Dh signal (which can be seen as a drive level). The longer the print enable signal  26 B in the enable level, the longer the heating unit Qp(k) is active, and the more heating energy is imparted to the ink. The system control circuit  14  controls the enable level L duration (the pulse wave width of the print enable signal) based on the outcome  28 B, so as to control ink-heating energy amount for the heating units. Continuing with  FIG. 2 , if the printer  10  triggers the nozzle Np(k) again at time point tp 3 , the system control circuit  14  transfers the print enable signal  26 B from the level H to the enable level L at time point tp 3 , and the drive circuit  16  transfers the drive signal Sp(k) from the level Dl to the drive level Dh. In this case, if the print head  18  between time points tp 1  and tp 2  has undergoes too much heat accumulation, the resistance of the thermistor TRp will be changed. When the print enable signal  26 B drops to the enable level L at time point tp 3 , the system control circuit  14  re-estimates the enable maintenance time of the print enable signal  26 B according to the outcome  28 B provided by the measure circuit  20  and the A/D converter  22 . Moreover, owing to heat accumulation, the system control circuit  14  maintains the print enable signal  26 B at the enable level for a shorter time slot Tp 2  (compared to the time slot Tp 1 ), so that the drive circuit  16  also maintains the drive signal Sp(k) at the enable level Dh for a shorter time slot. Therefore, the heating unit Qp(k) heats ink with less energy, so as to compensate for the heat accumulation effect.  
         [0013]     One of the drawbacks of the above prior art technique is necessary calculation resources of a printer. As mentioned above, the prior art printer  10  needs the A/D converter  22  to transfer the analog outcome  28 A of the thermistor TRp to the digital outcome  28 B for heat accumulation compensation. Furthermore, the prior art technique occupies both calculation and memory resources (the table stored in the memory device  15 ) of the printer  10 , so as to calculate the enable level maintenance duration of the print enable signal  26 A. Consequently, use of these system resources degrades efficiency of the printer.  
       SUMMARY OF INVENTION  
       [0014]     It is therefore a primary objective of the claimed invention to provide a printer and related apparatus that can adjust ink-jet energy according to print-head temperature in order to compensate for heat accumulation.  
         [0015]     According to the claimed invention, a printer includes: a print head including at least one nozzle, each nozzle for heating ink for jetting ink to a print document; a thermistor disposed in the print head, wherein resistance of the thermistor changes as temperature of the print head changes; a pulse duration control circuit providing a current for a capacitor through the thermistor, generating a print enable signal based on a discharging and charging duration for the current flowing to the capacitor, enabling an enable maintenance duration of the print enable signal corresponding to the discharging and charging duration for the current flowing to the capacitor; and a drive circuit, connected between the pulse duration control circuit and the print head, generating at least one ink-jet drive signal based on the print enable signal, enabling energy of each ink-jet drive signal corresponding to the enable maintenance duration of the print enable signal; each jet ink drive signal corresponding to a nozzle for heating ink with the corresponding nozzle according to the corresponding energy of the jet ink drive signal.  
         [0016]     According to the claimed invention, a method for a printer to adjust energy of each nozzle in the print head for heating ink according to the print head&#39;s temperature, the method includes: providing a thermistor in the print head, the resistance of the thermistor changing as temperature of the print headchanges; processing a wave control step for providing a current for a capacitor through the thermistor, generating a print enable signal based on a discharging and charging duration for the current flowing to the capacitor, enabling an enable maintenance duration of the print enable signal corresponding to the discharging and charging duration for the current flowing to the capacitor; and processing a drive step for generating at least one ink-jet drive signal according to the print enable signal, enabling the energy of each ink-jet drive signal corresponding to the enable maintenance duration of the print enable signal; each ink-jet drive signal corresponding to a nozzle for heating ink by the corresponding nozzle based on the corresponding energy of the jet ink drive signal.  
         [0017]     These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0018]      FIG. 1  illustrates a block diagram of a prior art ink-jet printer when compensating for heat accumulation.  
         [0019]      FIG. 2  illustrates a schematic diagram of each related signal waveform in the time domain when the printer in  FIG. 1  is operating.  
         [0020]      FIG. 3  illustrates a block diagram of a typical monostable multivibrator.  
         [0021]      FIG. 4  illustrates a schematic diagram of each related signal waveform in the time domain when the monostable multivibrator in  FIG. 3  operates.  
         [0022]      FIG. 5  illustrates a block diagram of the present invention printer.  
         [0023]      FIG. 6  illustrates a schematic diagram of each related signal waveform in the time domain when the printer in  FIG. 5  operates.  
         [0024]      FIG. 7  illustrates a functional diagram of temperature versus pulse wave width when the printer in  FIG. 5  compensates for heat accumulation.  
         [0025]      FIG. 8  illustrates a schematic diagram of the monostable multivibrator in  FIG. 3 .  
         [0026]      FIG. 9  illustrates a schematic diagram of each related signal waveform in the time domain when the circuit in  FIG. 8  is operating. 
     
    
     DETAILED DESCRIPTION  
       [0027]     In the implementation of the present invention, a monostable multivibrator of the present invention directly adjusts pulse wave width of a print enable signal according to resistance of a thermistor. Please refer to  FIG. 3  and  FIG. 4 .  FIG. 3  illustrates a configuration diagram of a typical monostable multivibrator M, while  FIG. 4  illustrates a related signal waveform time domain diagram of the monostable multivibrator M in  FIG. 3 . The X-axis in  FIG. 4  is time scale, and Y-axis is waveform amplitude. The typical monostable multivibrator M includes an input end Mi, an output end Mo, and two connection ends c 1  and c 2 . The input end Mi receives an input signal Vin (such as an input voltage signal); the output end Mo outputs an output signal Vout. The connection ends c 1  and c 2  connect to a capacitor Cx and a resistor Rx respectively as shown in  FIG. 3 , where the voltage V is a stable bias voltage.  
         [0028]     As  FIG. 4  illustrates, the monostable multivibrator M is triggered at the falling edge of the input signal Vin (when the level H changes to the level L). After being triggered, the monostable multivibrator M forms a pulse wave in the output signal, the pulse wave width being directly proportional to the product of the capacitance of the capacitor Cx and the resistance of the resister Rx. For example, as  FIG. 4  illustrates, if the input signal Vin triggers the monostable multivibrator M to operate at time point ta 1 , the monostable multivibrator M will transfer the output signal Vout from the level H to the level L at time point ta 1 , and maintain the output signal Vout at the level L between time points ta 1  and ta 2 . The generated level L pulse wave with a pulse wave width Tw is directly proportional to the product of the capacitance of the capacitor Cx and the resistance of the resistor Rx. At time point ta 2 , the monostable multivibrator M returns the output signal Vout from the level L to the level H automatically.  
         [0029]     According to the same method, after the input signal Vin triggers the monostable multivibrator M at the falling edge at time point ta 3 , the monostable multivibrator M generates the level L pulse wave with the pulse wave width Tw in the output signal Vout; that is, after a duration of the pulse wave width Tw from the time point ta 3 , the monostable multivibrator M returns the output signal Vout to the level H. Similarly, the input signal Vin triggers the monostable multivibrator M at time point ta 5 , and returns to the level H at time point ta 6  after a duration of the pulse wave width Tw. Basically, pulse wave widths of the input signal Vin at time points ta 3 , ta 5 , and ta 7  can be different or very short, such as Ta, Tb, and Tc (in comparison with the pulse wave width Tw), but after being triggered, the monostable multivibrator M can automatically output the level L pulse wave of width Tw according to the product of the capacitance of the capacitor Cx and the resistance of the resistor Rx. In addition, those skilled in the art recognize that the monostable multivibrator M can be implemented in many different ways, however a typical monostable multivibrator changes output signal levels under input signal triggers (such as from the level H to the level L, shown in  FIG. 4 ), and discharges and charges the capacitor Cx through the resistor Rx at the same time. Being discharged and charged, the capacitor Cx triggers the monostable multivibrator level to return (such as from the level L to the level H), so as to output a pulse wave with a pulse wave width proportional to the product of the capacitance of the capacitor Cx and the resistance of the resistor Rx.  
         [0030]     Please refer to  FIG. 5 , which illustrates a block diagram of an implementation of a present invention printer  30 . The printer  30  includes an interface circuit  32 , a system control circuit  34 , a drive circuit  36 , a print head  38 , a pulse duration control circuit  40 , and a memory  46 . The interface circuit  32  can receive print data from an electronic print document provided by a data source  42  (such as a PC, or a card reading machine for reading data from a memory card). The system control circuit  34  controls operations of the printer  30 , and the memory  46  registers data for operations of the system control circuit  34 . Furthermore, the print head  38  includes a plurality of heating units Q( 1 ) to Q(K), and corresponding nozzles N( 1 ) to N(K). The heating units Q( 1 ) to Q(K) can receive corresponding drive signals S( 1 ) to S(K) through the drive circuit  36 . When the printer  30  is operating, the interface circuit  32  transmits the waiting print data to the system control circuit  34 , and then registers the data in the memory  46 . When the printer  30  starts to jet ink, the system control circuit  34  triggers a print trigger signal  48 B, and transmits a waiting print data  48 A stored in the memory  46  to the drive circuit  36 . The drive circuit  36  determines which nozzles need to jet ink according to the print data  48 A, and maintains the drive signals at the drive level, which is equal to the pulse wave width of the print enable signal  48 C. During the maintenance duration of the drive signals, the corresponding heating units heat ink continuously for jetting ink to a print document  49 , thereby completing ink-jet printing.  
         [0031]     As mentioned above, the pulse wave width of the print enable signal  48 C controls the heating energy amount of each heating unit. In order to compensate for heat accumulation, the print head  38  includes a negative thermal coefficient thermistor TR for temperature detection, so that the pulse duration control circuit  40  can adjust the pulse wave width of the print enable signal  48 C according to resistance of the thermistor TR. In  FIG. 5 , the present invention achieves functions of the pulse duration control circuit  40  with the monostable multivibrator M in  FIG. 3 . As  FIG. 5  illustrates, the input end Mi of the monostable multivibrator M receives the print trigger signal  48 B provided by the system control circuit  34  as an input signal, and two connection ends c 1  and c 2  connected to a capacitor Cx with constant capacitance and the thermistor TR. Please notice that the configuration composed of the connection ends c 1 , c 2 , the capacitor Cx, and the thermistor TR in  FIG. 5  makes the thermistor TR equivalent to the resistor Rx in  FIG. 3 . In other words, when the print trigger signal  48 B triggers, the monostable multivibrator M in  FIG. 5  adjusts the pulse wave width in the output end Mo according to the product of the capacitance of the capacitor Cx and the resistance of the thermistor TR. The output signal of the monostable multivibrator M can be taken as the print enable signal  48 C, and can make the drive circuit  36  capable of controlling heating energy accumulation of heating units Q( 1 ) to Q(K) in accordance with the pulse wave width of the output signal. While the temperature of the print head  38  rises, the resistance of the thermistor TR decreases (because of its negative thermal coefficient). Therefore, both the monostable multivibrator M outputs a shorter print enable signal  48 C, and the drive circuit  36  curtails heating duration, which prevent negatives effect of heat accumulation.  
         [0032]     In the present invention, seeing that the pulse duration control circuit  40  can adjust the pulse wave width of the print enable signal  48 C, the system control circuit  34  does not occupy system resources for calculating and adjusting the pulse wave width, but triggers the pulse duration control circuit  40  with a stable pulse wave width provided by the print trigger signal  48 B. As to this condition, please refer to  FIG. 6  (and  FIG. 5 ), which illustrates a related signal waveform diagram in the time domain when the printer  30  in  FIG. 5  operates. The X-axis is time scale, and the Y-axis is waveform amplitude. If the printer  30  starts to jet ink at time point t 1 , the system control circuit  34  can transfer the print trigger signal  48 B from the level H to the level L at time point t 1 , so as to trigger the monostable multivibrator M at the falling edge to transfer the print enable signal  48 C from the level H to the level L (or enable level). Therefore, the pulse wave width Tw 1  of the print enable signal  48 C in the enable level L is proportional to the product of the capacitance of the capacitor Cx and the resistance of the thermistor TR. According to the print data  48 A, if some nozzle N(k) is required to jet ink, the drive circuit  36  transfers the corresponding drive signal S(k) from the level Dl to the drive level Dh with the print enable signal  48 C at time point t 1 , and then maintains the drive signal S(k) in the drive level for a duration of the pulse wave width of the print enable signal  48 C. Therefore, the heating unit Q(k) heats ink during the duration, so as to jet ink from the nozzle N(k).  
         [0033]     At time point t 3 , if the printer  30  continues to print un-printed data (and make the nozzle N(k) jet ink), the system control circuit  34  will trigger the pulse duration control circuit  40  again at time point t 3  at the falling edge, hence the monostable multivibrator M will generate the enable pulse wave at time point t 3  according to the temperature of the thermistor TR. Moreover, if the temperature of the print head  38  has risen because of the heat accumulation, resistance of the thermistor at time point t 3  is decreased, so that the monostable multivibrator M reduces the pulse wave width Tw 2  at time point t 3 . Therefore, the drive circuit makes the pulse wave width of the drive signal S(k) in the drive level Dh decreased, so as to prevent the heating unit Q(k) from outputting too much heating energy lest print quality is degraded.  
         [0034]     Similarly, if the printer  30  starts to print again (and make the nozzle N(k) jet ink) at time point t 5 , the monostable multivibrator M will determine the pulse wave width of the print enable signal  48 C according to the resistance of the thermistor (and the capacitance of the capacitor Cx). Besides, if the temperature of the print head  38  is still high (higher than that between time points t 1  and t 4 ), the resistance of the thermistor TR will be decreased much more (smaller than that between time points t 1  and t 4 ), with the result that the monostable multivibrator M will make the pulse wave width Tw 3  of the print enable signal  48 C smaller than the pulse wave widths Tw 1  and Tw 2 . Therefore, the drive circuit  36  will trigger the heating unit Q(k) with a much shorter drive level pulse wave in the drive signal S(k) to compensate the heat accumulation effect.  
         [0035]     As mentioned above, the monostable multivibrator M of the present invention realizes functions of the pulse duration control circuit  40 , and changes the pulse wave width of the print enable signal  48 C according to different resistance of the thermistor, so as to compensate for heat accumulation. That is, the printer of the present invention does not need the same measure circuits and A/D converters as the prior art printer  10  does, so that calculation and memory resources needed for the present invention are reduced. To further illustrate pulse wave width adjustment of the pulse duration control circuit  40 , please refer to  FIG. 7  (also  FIG. 5  and  FIG. 6 ), which illustrates a functional relationship diagram of the pulse wave width of the print enable signal  48 C in the pulse duration control circuit  40 . The X-axis in  FIG. 7  is temperature of the print head  38  (the unit is centigrade), and the Y-axis is pulse wave widths of the print enable signal  48 C when in the drive level (the unit is μs, microsecond). As  FIG. 7  illustrates, as temperature of the print head  38  jumps from 20 degrees to 80 degrees, the pulse wave width of the pulse duration control circuit  40  drops from 2.7 μs to about 1.6 μs. An ideal functional relationship between temperature and pulse wave widths can be provided by adjusting the capacitance of the capacitor Cx and material characters of the thermistor for compensating for heat accumulation.  
         [0036]     There are many ways to implement the monostable multivibrator M, and the following illustrates one implementation for example. Please refer to  FIG. 8  and  FIG. 9  (also  FIG. 3  and  FIG. 4 ).  FIG. 8  is an implementation circuit diagram of the monostable multivibrator M in  FIG. 3 , while  FIG. 9  illustrates a related signal waveform diagram in the time domain when the monostable multivibrator M in  FIG. 8  is operating. The X-axis in  FIG. 9  is time scale, and the Y-axis is waveform amplitude. In  FIG. 8 , the monostable multivibrator M can achieve its functions with two inverters I 1  and I 2 , two inverse OR gates Nor 1  and Nor 2 , a resistor Rx, and a capacitor Cx though two connection ends c 1  and c 2 . The inverters I 1 , I 2  and the inverse OR gates Nor 1 , Nor 2  are biased between direct voltage V and G (such as ground voltage). The inverter I 1  receives an input signal Vin in the input end Mi and generates a signal voltage V 1 . After performing an inverse OR on the voltage V 1  and V 4 , the inverse OR gate Nor 1  generates the voltage V 2  in the connection end c 1 . Through the capacitor Cx and the resistor Rx connected to the connection end c 1  and c 2 , the voltage V 3  is input to two input ends of the inverse OR gate Nor 2 , which generates the signal voltage V 4 . Finally, the output signal Vout is generated in the output end Mo through the inverter I 2 .  
         [0037]     As  FIG. 9  illustrates, before time point tb 1 , the input signal stays at the level H (this can be the level of the bias voltage V), while the voltage V 1  stays at the level L through the inverter I 1  (this can be the level of the bias voltage G). In a stable situation, the capacitor Cx should have no current flow, hence the voltage V 3  nears the bias voltage V or the level H, consequently the voltage V 4  provided by the inverse OR gate Nor 2  is at the level L. Furthermore, the voltage V 4  feedbacks to the inverse OR gate Nor 1 , which is combined with the voltage V 1  in the inverse OR gate Nor 1  for the output voltage V 2  at the level H. Besides, the voltage V 4  makes the output signal Vout in the level H after the inverter I 2 .  
         [0038]     Suppose that, at time point tb 1 , the input signal Vin, which changes from the level H to the level L, triggers the monostable multivibrator M, while the voltage V 1  changes from the level L to the level H. After the inverse OR gate Nor 1  finishes the inverse OR operation of the voltage V 1 , the voltage V 2  drops a difference voltage DV from the level H to near the level L, so that the voltage across the capacitor Cx decreases the difference voltage DV at the same time because the capacitor Cx cannot change its charge amount rapidly. As a result, the voltage V 3  drops to near the level L, and the voltage V 4  jumps to the level H. Finally, the output signal Vout changes from the level H to the level L.  
         [0039]     Although the capacitor Cx cannot discharge and charge rapidly for the voltage V 3  to descend along with the voltage V 2 , the bias voltage V charges the capacitor Cx through the resistor Rx after time point tb 1 , so that the voltage V 3  increases continuously. At time point tb 3 , the voltage V 3  is charged to a threshold voltage Vth, which is near the level H and can be seen as a digital “ 1 ” (the level L is a digital “ 0 ”). In other words, at time point tb 3 , the inverse OR gate Nor 2  transfers its output voltage V 4  to the level L because the voltage V 3  becomes a digital “ 1 ”. Therefore, the monostable multivibrator M returns the output signal Vout to the level H, and generates the level L pulse wave with the pulse wave width Tw 0  between time point tb 1  and tb 3 . Please notice that the voltage V 4  stays at the level H after time point tb 1  (until time point tb 3 ), so that even if the input signal Vin returns to the level H at time point tb 2 , the voltages V 2  and V 3  are disturbed (as are the voltages V 4  and Vout).  
         [0040]     As discussed above, the pulse wave width Tw 0  of the output signal Vout is determined by the duration of the voltage V 3  charging to the threshold voltage Vth. The shorter the duration, the shorter the pulse wave width Tw 0 . Because the voltage V 3  is accumulated by charging the capacitor Cx from the resistor Rx, the charging duration of the voltage V 3  is determined by the product of the capacitance of the capacitor Cx and the resistance of the resistor Rx (which is a time constant of a capacitor-resistor circuit). In normal situations, the charging duration of the voltage V 3  is directly proportional to the time constant, the product of the capacitance of the capacitor Cx and the resistance of the resistor Rx. Therefore, the present invention establishes the thermistor of the print head as the resistor Rx.  
         [0041]     In  FIG. 8  and  FIG. 9 , the monostable multivibrator (and the pulse duration control circuit) of the present invention is a simple, efficient, low-cost circuit. Therefore, cost of the present invention can be decreased efficiently, and so can system resources for compensating for heat accumulation. Certainly, alternative solutions regarding the monostable multivibrator M exist. For example, in some circuit configurations, a pulse wave width of the output signal can be determined by a discharge duration of the capacitor Cx through the resistor Rx. Accordingly, the monostable multivibrator of the present invention can achieve the heat accumulation compensation via discharging and charging the capacitor Cx through the resistor Rx under the input signal triggering, and then triggering changes of the output signal based on discharging and charging duration of the capacitor.  
         [0042]     In summary, although the prior art printer can measure a print head&#39;s temperature through a thermistor, it needs both a high-cost A/D converter to convert resistance of the thermistor to digital and high system resources for calculation and adjustment. This makes the prior art printer high cost, but low in efficiency. In contrast the present invention can achieve functions of a pulse duration control circuit with a simple/low-cost monostable multivibrator, which can not only reduce cost and system resources effectively, but also compensate for heat accumulation and promote printer efficiency.  
         [0043]     Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.