Abstract:
A thermal head driving method of driving thermal heads is disclosed. The method includes a step of dividing the thermal heads into plural groups, providing for each of the groups a common potential terminal, a step of using a drive circuit to drive the thermal heads of one or more of the groups, and a step of applying an operating voltage to the common potential terminal of said one or more groups driven by the drive circuit.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to a thermal head which has new driving method and a thermal head drive circuit, and particularly relates to a thermal head driving method and a thermal head drive circuit for driving thermal heads.  
         [0003]     2. Description of the Related Art  
         [0004]     Thermal head drive circuits include IC-mounted type and diode matrix type.  
         [0005]      FIG. 1  is a block diagram showing a configuration of an IC-mounted drive circuit  10 .  
         [0006]     The IC-mounted drive circuit  10  comprises heater elements  11  and a driver IC  12 . Each of the heater elements has an end to which a common potential is applied and the other end connected to the driver IC  12 . The driver IC  12  comprises flip-flops, latch and driver elements equal in number. The driver IC  12  sequentially transfers serially-transferred data items in synchronization with clock, latches the data items into the flip-flops at the time of printing, and drives drivers with strobe signals according to the data items latched in the flip-flops so as to apply current to the heater elements  11 . With this configuration, the driver IC  12  needs to have the same number of drivers as the number of driver elements.  
         [0007]      FIG. 2  is a block diagram showing a configuration of a diode matrix type drive circuit  20 .  
         [0008]     The diode matrix type drive circuit  20  comprises heater elements  21 , a driver IC  22 , and diode drive group control ICs  24 . The heater elements  21  are divided into groups of n heater elements  21  each. The n heater elements  21  in each group are connected through corresponding backflow prevention diodes  23  to the corresponding drive group control IC  24 . The driver IC  22  can drive n heater elements  21  independently at the same time, and can drive the heater elements  21  by group.  
         [0009]     As described above, since IC-mounted drive circuits require the same number of drivers as the number of driver elements, the size of driver ICs is large. On the other hand, diode matrix type thermal head drive circuits require the same number of diodes as the number of driver elements, and therefore the size of the drive circuits is large.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention solves one or more of the above described problems. The present invention is directed to provide a thermal head driving method and a thermal head drive circuit capable of efficiently driving thermal heads with a simple structure.  
         [0011]     According to one aspect of the present invention, there is provided a thermal head driving method of driving thermal heads that comprises a step of dividing the thermal heads into plural groups, providing for each of the groups a common potential terminal, a step of using a drive circuit to drive the thermal heads of one or more of the groups, and a step of applying an operating voltage to the common potential terminal of said one or more groups driven by the drive circuit.  
         [0012]     It is preferable that the above-described thermal head driving method further comprise a step of switching the common potential terminal of groups not driven by the drive circuit into an open-circuit condition.  
         [0013]     It is preferable that the above-described thermal head driving method further comprise a step of applying a predetermined common potential to the common potential terminal of the groups not driven by the drive circuit.  
         [0014]     It is also preferable that the above-described thermal head driving method further comprise a step of driving the thermal heads based on evaluation data, and a step of detecting a failure by detecting a current to be supplied to a power supply terminal.  
         [0015]     In the above-described thermal head driving method, it is preferable that a common potential be supplied to the power supply terminal via plural power supply lines divided into two or more groups so as to perform the failure detection in each of the groups, and a power be supplied via the power supply lines in the group in which the failure is not detected so as to perform printing operation. The above-described thermal head driving method preferably further comprises a step of reducing a printing speed when a failure is detected.  
         [0016]     In one embodiment of the present invention, since a thermal head driving method of driving thermal heads comprises a step of dividing the thermal heads into plural groups, providing a common potential terminal for each of the groups, a step of sharing a drive circuit for driving the thermal heads among the groups, and a step of applying an operating voltage to the common potential terminal of the group to be driven by the drive circuit, the thermal head driving method can be simplified. Moreover, in an embodiment, the thermal heads in the group not being driven can be preheated by application of a small current, thereby allowing high speed printing.  
         [0017]     According to an aspect of the present invention, since a failure can be detected by detecting a current to be supplied to a power supply terminal while driving the thermal heads based on evaluation data, the failure can be detected. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a block diagram showing a configuration of an IC-mounted drive circuit;  
         [0019]      FIG. 2  is a block diagram showing a configuration of a diode matrix type drive circuit;  
         [0020]      FIG. 3  is a block diagram showing a system configuration according to a first embodiment of the present invention;  
         [0021]      FIG. 4  is a block diagram showing a configuration of a control circuit;  
         [0022]      FIG. 5  is a schematic diagram showing a printing unit;  
         [0023]      FIG. 6  is a block diagram showing a configuration of a printing unit;  
         [0024]      FIGS. 7-9  are diagrams for explaining operations according to the first embodiment of the present invention;  
         [0025]      FIGS. 10-12  are diagrams for explaining operations in another condition according to the first embodiment of the present invention;  
         [0026]      FIG. 13  is a circuit diagram showing a common potential supply circuit; and  
         [0027]      FIG. 14  is a flowchart showing operations performed by a control unit. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     First Embodiment  
       [0028]      FIG. 3  is a block diagram showing a configuration of a printing system  100  according to a first embodiment of the present invention.  
         [0029]     The printing system  100  of this embodiment comprises a higher-level device  111 , a printer  112 , and a power supply device  113 .  
         [0030]     The higher-level device  111  comprises a computer system, and is adapted to provide print data to the printer  112 . The printer  112  comprises a control circuit  121 , a printing unit  122 , and a connection cable  123 . The control circuit  121  sends power, data, control signals, etc., to the printing unit  122  via the connection cable  123 . The connection cable  123  comprises a bundle of plural thin connection lines so as to send drive power split into the thin connection lines. The connection cable  123  having this configuration can be easily flexed, and can therefore be easily handled.  
         [0031]     The control circuit  121  controls the printing unit  122  based on the print data provided from the higher-level device  111 . The printing unit  122  is controlled by the control circuit  121  to perform printing based on the print data.  
         [0032]     The power supply device  113  supplies drive power to the printer  112 . The printer  112  is driven by the power supplied from the power supply device  113  to perform printing.  
         [0033]      FIG. 4  is a block diagram showing a configuration of the control circuit  121 .  
         [0034]     The control circuit  121  comprises a control unit  131 , a thermal head drive circuit  132 , a motor drive circuit  133 , and a common potential supply circuit  134 .  
         [0035]     The control unit  131  receives commands and print data from the higher-level device  111  and performs various control operations for printing.  
         [0036]     The thermal head drive circuit  132  is controlled by the control unit  131  to generate drive data for driving the printing unit  122  and provide the generated drive data to the printing unit  122 .  
         [0037]     The motor drive circuit  133  is controlled by the control unit  131  to generate drive signals for driving a paper feeder motor, for example, of the printing unit  122  and provide the generated drive signals to the printing unit  122 .  
         [0038]     The common potential supply circuit  134 , which receives power supply voltage from the power supply device  113 , is controlled by the control unit  131  to generate common potential as drive power and supply the generated common potential to the printing unit  122  via the connection cable  123 . The common potential supply circuit  134  cooperates with the control unit  131  to detect a short circuit in the connection cable  123  and perform a safety operation of stopping power supply to the connection cable  123  upon detection of failure.  
         [0039]      FIG. 5  is a schematic diagram showing the printing unit  122 , and  FIG. 6  is a block diagram showing a configuration of the printing unit  122 .  
         [0040]     Referring to  FIG. 6 , the printing unit  122  comprises a thermal head unit  141 , a platen roller  142 , a motor  143 , and a speed reduction mechanism  144 .  
         [0041]     The thermal head unit  141  comprises a thermal head part  151 , a drive circuit  152 , and a common potential switching circuit  153 , which are mounted on a ceramic substrate. The thermal head part  151  comprises n×m heater elements  161  divided into m groups G 1 -Gm. In other words, each of the groups G 1 -Gm comprises n heater elements  161 .  
         [0042]     The heater elements  161  comprise, for example, resistive elements, and are configured to generate heat when current is applied. First ends of the heater elements  161  in the same group are connected to each other, and the connection point is connected to the common potential switching circuit  153 . The common potential switching circuit  153  applies a common potential to the connection point of the first ends of the heater elements  161  in each group. Second ends of the heater elements  161  are connected to the drive circuit  152 .  
         [0043]     The exemplary drive circuit  152  is connected to the n heater elements  161  as illustrated, and drives the n heater elements  161  independently based on the print data provided from the control circuit  121 . The common potential switching circuit  153  applies a common potential to the connection point of the first ends of the heater elements  161  in one or more groups of heater elements  161  to be driven, while open-circuiting or applying a predetermined potential to the connection point of the first ends of the heater elements  161  in the other groups.  
         [0044]     Operations  
         [0045]      FIGS. 7-9  are diagrams for explaining operations according to a first embodiment of the present invention.  
         [0046]     For ease of explanation, an example is given below in which a group comprising six heater elements A-F and another group comprising six heater elements a-f are provided.  
         [0047]     In a condition where the heater elements A-F are driven and the heater elements a-f are not driven, the heater elements A, B, a, and b operate as described below. With reference to  FIG. 8 , R 00 , R 0 , R 10 , and R 11  indicate the resistances of the heater elements A, B, a, and b, respectively.  
         [0048]     When the heater element A is on and the heater element B is off, an equivalent circuit shown in  FIG. 9  is formed when Vcom 1  is open. The power consumed by each of the heater elements B, a, and b is expressed by the following equation: 
 
{Vcom 0 /3r} 2 ×r=(  1 / 9 )×{Vcom 0   2 /r}, 
 
 wherein the resistances R 00 , R 0 , R 10 , and R 11  of the heater elements A, B, a, and b are R 00 =R 01 =R 10 =R 11 =r and Vcom 0  represents a common potential. 
 
         [0049]     That is, the power consumed by each of the heater elements B, a, and b corresponds to  1 / 9  of the power consumed by the heater elements A. Accordingly, the coloring energy of each of the heater elements B, a, and b is  1 / 9  of the standard coloring energy. Therefore, colors are not developed by the heater elements B, a, and b. Each of the heater elements B, a, and b generates heat with the energy corresponding to  1 / 9  of the standard coloring energy, so that the heater elements B, a, and b are preheated.  
         [0050]      FIGS. 10-12  are diagrams for explaining operations according to the first embodiment of the present invention in another condition.  
         [0051]     The heater elements A, B, C, a, b, and c operate as described below. With reference to  FIG. 10 , R 00 , R 01 , R 02 , R 10 , R 11 , and R 12  indicate the resistances of the heater elements A, B, C, a, b, and c, respectively.  
         [0052]     When the heater element A is on and the heater elements B and C are off, an equivalent circuit shown in  FIG. 11  is formed. The combined resistance of the heater elements B, C, a, b, and c is 2r, wherein the resistances R 00 , R 01 , R 10 , and R 11  of the heater elements A, B, C, a, b, and c are R 00 =R 01 =R 02 =R 10 =R 11 =R 12 =r and Vcom 0  represents a common potential. The power consumed by each of the heater elements B, C, b, and c is  1 / 16  of the power consumed by the heater element A. The power consumed by the heater element a is  1 / 4  of the power consumed by the heater element A.  
         [0053]     When the heater elements A and B are on and the heater element C is off, an equivalent circuit shown in  FIG. 12  is formed. The power consumed by each of the heater elements C and c is expressed by the following equation: 
 
{Vcom 0 /(5r/2)} 2 ×r=(4/25)×{Vcom 0   2 /r}
 
         [0054]     That is, the power consumed by each of the heater elements C and c corresponds to  4 / 25  of the power consumed by each of the heater elements A and B. The power consumed by each of the heater elements a and b is  1 / 25  of the power consumed by each of the heater elements A and B. For this reason, the heater elements C, a, b, and c are not heated enough to develop colors, but are preheated.  
         [0055]     In this embodiment, as described above, the drive circuit of the heater elements is simplified. Moreover, since the heater elements can be preheated while being off, they can be turned on by application of reduced energy. This enables increasing the printing speed.  
         [0056]     In this embodiment, a common potential Vcom 1  is not applied. However, the common potential Vcom 1  corresponding to (  1 / 3 ) Vcom 0  may be applied so that  1 / 9  of the energy applied to the heater elements being on is applied to each of the heater elements being off.  
         [0057]     Protection Operations  
         [0058]     The following describes protection operations performed by the control unit  131  and the common potential supply circuit  134 .  
         [0059]      FIG. 13  is a circuit diagram showing the common potential supply circuit  134 .  
         [0060]     The common potential supply circuit  134  comprises a common potential generation circuit  211 , a first switching circuit  212 , a second switching circuit  213 , a first current detection circuit  214 , a second current detection circuit  215 , and rectifier diodes D 1  and D 2 .  
         [0061]     The common potential generation circuit  211  receives a power supply voltage from the power supply device  113 , and generates, for example, two different levels of common potentials based on the power supply voltage received from the power supply device  113 . The common potential generated by the common potential generation circuit  211  is sent to a first power supply line  231 , which is a part of the connection cable  123 , via the first switching circuit  212 , the first current detection circuit  214 , and the rectifier diode D 1 , and sent to a second power supply line  232 , which is a part of the connection cable  123 , via the second switching circuit  213 , the second current detection circuit  215 , and the rectifier diode D 2 .  
         [0062]     In the illustrated exemplary embodiment, each of the first power supply line  231  and the second power supply line  232  comprises, for example, four leads L 1 -L 4 . With this configuration, since the power is supplied through the plural leads, thinner leads can be used compared to the case where the power is supplied through one lead. As mentioned above, the connection cable  123  with thinner leads can be handled more easily.  
         [0063]     The first switching circuit  212  and the second switching circuit  213  are turned on or off in response to a switching signal from the control unit  131 .  
         [0064]     The first current detection circuit  214  comprises a detection resistor Rs, an error amplifier  221 , a comparator  222 , and a reference supply  223 .  
         [0065]     The detection resistor Rs is connected serially between the first switching circuit  212  and the first power supply line  231 , and configured to generate at both ends thereof a voltage corresponding to a current sent to the first power supply line  231 . The error amplifier  221  is connected at its non-inverting terminal to a connection point between the detection resistor Rs and the first switching circuit  212 , and is connected at its inverting terminal to a connection point between the detection resistor Rs and the first power supply line  231 . The error amplifier  221  outputs a detection signal corresponding to the voltage generated by the detection resistor Rs. The detection signal output from the error amplifier  221  is provided to a non-inverting terminal of the comparator  222 . The reference supply  223  applies a reference voltage to an inverting input terminal of the comparator  222 . The reference supply  223  can be controlled by the control unit  131 .  
         [0066]     The comparator  222  switches its output to high if the detection signal output from the error amplifier  221  is greater than the reference voltage, and switches its output to low if the detection signal output from the error amplifier  221  is smaller than the reference voltage. The output of the comparator  222  is provided to the control unit  131 .  
         [0067]     The second current detection circuit  215  is connected between the second switching circuit  213  and the second power supply line  232 , and has the same configuration as the first current detection circuit  214 .  
         [0068]     The control unit  131  detects the output of the comparators  222  while the printing unit  122  is driven based on evaluation data provided from the control unit  131 , and thus detects a leak current from the first power supply line  231  and the second power supply line  232 , and controls the first switching circuit  212  and the second switching circuit  213 .  
         [0069]      FIG. 14  is a flowchart showing an exemplary embodiment of operations performed by the control unit  131 .  
         [0070]     The control unit  131  provides the evaluation data to the printing unit  122  in Step S 1 - 1 , and issues a command of data latch in Step S 1 - 2 . In Step S 1 - 3 , the control unit  131  outputs strobe signals so as to drive the printing unit  122  based on the evaluation data. As the evaluation data used herein do not require printing or coloring, current is applied to heater elements  161  for a short period of time.  
         [0071]     In Step S 1 - 4 , the control unit  131  determines whether the output of the first current detection circuit  214  is at an abnormal level. If, in Step S 1 - 4 , the output of the first current detection circuit  214  is determined to be at an abnormal level, there might be a failure such as a short circuit in the first power supply line  231 . The control unit  131  therefore turns off the first switching circuit  212  in Step S 1 - 5 .  
         [0072]     Then, in Step S 1 - 6 , the control unit  131  determines whether the output of the second current detection circuit  215  is at an abnormal level. If, in Step S 1 - 6 , the output of the second current detection circuit  215  is determined to be at an abnormal level, there might be a failure in the second power supply line  232 . The control unit  131  therefore turns off the second switching circuit  213  to stop sending a current to the second power supply line  232  in Step S 1 - 7 , and reports a failure in the connection cable  123  to the higher-level device  111  in Step S 1 - 8 . In this case, as there might be failures in the first power supply line  231  and the second power supply line  232 , printing is not performed.  
         [0073]     If, in Step S 1 - 6 , the output of the second current detection circuit  215  is not determined to be at an abnormal level, the control unit  131  reports to the higher-level device  111  that there is a failure in the first power supply line  231 . In this case, since a small drive current can be sent via the second power supply line  232 , the control unit  131  reports the failure to the higher-level device  111  in Step S 1 - 9  and performs printing in a low-speed printing mode in Step S 1 - 10 . In the low-speed printing mode, printing is performed at lower speed and with smaller drive current than usual. Accordingly, printing can be performed with the small current sent via the second power supply line  232 .  
         [0074]     If, in Step S 1 - 4 , the output of the first current detection circuit  214  is not determined to be at an abnormal level, then in Step S 1 - 11  the control unit  131  determines whether the output of the second current detection circuit  215  is at an abnormal level. If, in Step S 1 - 11 , the output of the second current detection circuit  215  is determined to be at an abnormal level, there might be a failure in the second power supply line  232 . The control unit  131  therefore turns off the second switching circuit  213  to stop sending a current to the second power supply line  232  in Step S 1 - 12 , and reports the failure in the second power supply line  232  to the higher-level device  111  in Step S 1 - 13 .  
         [0075]     In this case, a small drive current can be sent via the first power supply line  231  to the printing unit  122 . Accordingly, in this embodiment, in Step S 1 - 14 , the control unit  131  performs printing in the normal-speed printing mode.  
         [0076]     If, in Step S 1 - 11 , the output of the second current detection circuit  215  is determined not to be at an abnormal level, the control unit  131  performs printing in a normal printing mode in Step S 1 - 14 .  
         [0077]     In this embodiment, the control unit  131  detects a current flowing in each of the first power supply line  231  and the second power supply line  232  for supplying a common potential to the printing unit  122  while the printing unit  122  is driven based on the evaluation data provided from the control unit  131 , and thus can detect a failure such as a short circuit in each of the first power supply line  231  and the second power supply line  232 . Accordingly, failures such as short circuits in the first and second power supply lines  231  and  232  can be detected.  
         [0078]     As described above, the first power supply line  231  and the second power supply line  232  for supplying a common potential to the printing unit  122  are provided. This configuration allows supplying power from the second power supply line  232  to the printing unit  122  if there is a failure in the first power supply line  231 , and supplying power from the first power supply line  231  to the printing unit  122  if there is a failure in the second power supply line  232 . Moreover, by switching the printing mode to the low-speed printing mode, printing can be performed without imposing a large workload on the first or second power supply line  231  or  232 .  
         [0079]     The first and second power supply lines  231  and  232  may be provided for each block so as to detect failure in each of the first power supply line  231  and the second power supply line  232 . With this configuration, more detailed control can be implemented. For example, a driver element not being driven can be driven at  1 / 3  potential or can be off depending on the conditions of the first power supply line  231  and the second power supply line  232 .  
         [0080]     The present application is based on Japanese Priority Application No. 2005-253863 filed on Sept. 1, 2005, with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.