Patent Publication Number: US-8979237-B2

Title: Recording head control method and dot impact printer

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to a method of controlling a recording head that records information by driving recording wires to form dots on recording paper, and to a dot impact printer having the recording head. More particularly, the invention relates to a method of controlling a recording head that can suppress a drop in print throughput while preventing heat damage to the head coil resulting from heat produced while printing, and to a dot impact printer. 
     The invention claims priority based on Japan patent application 2009-294060 filed Dec. 25, 2009, Japan patent application 2010-133686 filed Jun. 11, 2010, and Japan patent application 2010-133687 filed Jun. 11, 2010, the contents of which are incorporated herein by reference. 
     2. Related Art 
     Dot impact printers are used in various fields for the primary purposes of high reliability and overstrike printing on multipart forms. Dot impact printers use a recording head with plural recording wires (wire pins). For each recording wire, the recording head has an electromagnetic coil (head coil) that drives the recording wire, and the recording wires are selectively driven to protrude and form a dot by selectively driving the electromagnetic coils. Dot impact printers have a carriage that carries the recording head, and record information by selectively causing the recording wires of the recording head to strike the recording paper with an ink ribbon therebetween while moving the carriage back and forth widthwise to the recording paper. 
     When the recording wires of a dot impact printer are driven continuously or at a high frequency, the temperature of the head coils (electromagnetic coils) that are driving those recording wires rises quickly. In extreme cases, the coils may burn out. Therefore, to prevent such problems as the head coils burning out, the temperature of the recording head is detected by a thermistor or other temperature detector so that the heat output of the head coils can be reduced when the temperature rises to a level where there is a danger of heat damage. For example, when the temperature of the recording head reaches a preset slowdown temperature setting, the drive frequency of the head coil is reduced and the printing speed is reduced. When the temperature of the recording head reaches a preset stop-temperature setting, the printing operation of the recording head is stopped. Dot impact printers that thus control operation so that the head coils do not reach a burnout threshold temperature are thus known from the literature. See, for example, Japanese Unexamined Patent Appl. Pub. JP-A-2003-127441. 
     However, when the head coil temperature rises suddenly, there are situations in which the head coil may exceed the burnout threshold temperature before the temperature detector detects that the head coil has reached the preset temperature and action to reduce the print speed or stop printing can be taken, and the coil cannot be prevented from burning out. To prevent such problems, the dot impact printer described above presets specific dot patterns (specific recording patterns) that can be expected to produce a sudden temperature rise. When a specific recording pattern is detected during printing, the printing is controlled with the slowdown temperature setting and stop-temperature setting set lower than normal. As a result, when the head coil temperature rises rapidly, the printing speed can be lowered or printing stopped before the head coil reaches the burnout threshold temperature. 
     This dot impact printer changes the slowdown temperature setting and stop-temperature setting to a low temperature even when the specific recording pattern is detected in only one place. As a result, the print speed is reduced or printing is stopped even if the recording head temperature is low enough when the specific recording pattern is printed that the burnout threshold temperature would not be reached by printing the specific recording pattern. More specifically, the dot impact printer described above may reduce the print speed or stop printing even when reducing the print speed or stopping printing is not necessary, and printer throughput therefore drops. 
     SUMMARY 
     The present invention is directed to solving at least part of the foregoing problems, and can be achieved by means of the embodiments and applications described below. 
     A first aspect of the invention is a control method for a recording head of a dot impact printer that prints information on a recording medium by driving the recording wires of a recording head that has a plurality of recording wires while a carriage that carries the recording head traverses the recording medium, each of the recording wires being allocated to printing one dot line in the scanning direction of the carriage, the control method including steps of: during dot line printing, determining before printing if the number of previously defined specific dot patterns contained in the dot line to be printed is greater than or equal to a reference number of 2 or more; and printing the dot line based on the result of the decision. 
     This aspect of the invention enables setting as a specific dot pattern any pattern that could cause the temperature of the recording head to rise suddenly. Whether or not the number of specific dot patterns in the dot line to be printed exceeds a reference number can then be determined, and the dot line can be printed based on the result of the decision. More specifically, whether or not there is a danger of the recording head temperature exceeding the burnout threshold temperature of the head coil during dot line printing is determined, and the dot line can be printed based on the result of this decision. 
     Preferably, the recording head control method according to another aspect of the invention includes a step of determining the drive mode of the recording head for printing the dot line based on the decision during dot line printing. 
     This aspect of the invention determines if the number of specific dot patterns in the dot line to be printed is greater than or equal to a reference number, and selects the drive mode accordingly. More specifically, whether the possibility of a sudden rise in the recording head temperature occurring in the dot line exceeds a reference number is determined, and whether there is a danger of the recording head temperature exceeding the burnout threshold temperature of the head coil when printing the dot line can be determined. As a result, if it is determined that there is a danger of exceeding the burnout threshold temperature of the head coil, printing is accomplished in a drive mode that is set so that burnout does not happen. If it is determined that there is not a danger of exceeding the burnout threshold temperature of the head coil, printing can proceed without changing the drive mode. As a result, a drop in printer throughput can be suppressed while reliably preventing head coil burnout. 
     A recording head control method according to another aspect of the invention also preferably includes steps of: setting a predetermined number of pattern detection areas of a specified width within the maximum line length of the dot line; and during dot line printing determining before printing if the pattern detection area in the dot line to be printed is a specific area containing the specific dot pattern; determining if the number of specific areas in the dot line is greater than or equal to the reference number; and based on the results of these decisions, determining the drive mode of the recording head for printing the dot line. 
     This aspect of the invention enables simply determining if a specific dot pattern is contained in a pattern detection area of a specified width, and counting how many specific areas are contained. The processing load can therefore be reduced compared with when the entire dot line is compared with the specific dot pattern. 
     A recording head control method according to another aspect of the invention also preferably includes steps of: presetting a plurality of drive modes with different heat output as the drive modes of the recording head, and correlating the drive modes with the temperature of the recording head, the temperature of the recording head correlated to each drive mode including at least two temperature settings, a first temperature and a second temperature that is lower than the first temperature; and during dot line printing, detecting the temperature of the recording head before printing, selecting the drive mode corresponding to the detected recording head temperature using the second temperature when the number of specific areas in the dot line to be printed is greater than or equal to the reference number, selecting the drive mode corresponding to the detected recording head temperature using the first temperature when the number of specific areas in the dot line to be printed is less than to the reference number, and printing the dot line using the selected drive mode. 
     This aspect of the invention enables setting plural recording head temperatures correlated to plural drive modes. As a result, two or more different temperatures can be selectively used based on the number of pattern detection areas containing the specific dot pattern to select the drive mode. As a result, a drop in printer throughput can be suppressed by reliably preventing head coil burnout. 
     A recording head control method according to another aspect of the invention also preferably includes a step of: when the recording head temperature detected before printing is less than or equal to a preset threshold temperature, determining the drive mode of the recording head without using the result of determining whether or not the pattern detection areas are the specific area. 
     This aspect of the invention can anticipate that the head coil burnout threshold temperature will not be exceeded even if the specific dot pattern will be printed in plural pattern detection areas, and can prevent a drop in printer throughput. 
     A recording head control method according to another aspect of the invention also preferably includes steps of: relating the print quality of the print data, and the drive mode of the recording head; and during dot line printing, accomplishing dot line printing using the drive mode selected from among the drive modes related to the print quality of the print data containing the dot line to be printed. 
     A dot impact printer changes recording head drive control according to the print quality setting. As a result, the temperature increase differs according to the print quality. The control method according to this aspect of the invention enables setting the drive mode according to the print quality. As a result, head coil burnout can be prevented while also suppressing a drop in printer throughput. 
     A recording head control method according to another aspect of the invention also preferably includes a step of: setting the specific dot pattern based on the number of recording wires driven simultaneously when printing, and a consecutive drive count of a same recording wire. 
     A dot impact printer has many recording wires that are driven simultaneously, and as the number of times the same recording wires are driven continuously rises, that is, as the dot density rises, the temperature can rise quickly. Based on these parameters, this aspect of the invention can set a specific dot pattern that can be expected to produce a sudden rise in temperature. 
     A recording head control method according to another aspect of the invention also preferably includes a step of: during dot line printing, determining a rest time in the dot line printing period based on the decision. 
     This aspect of the invention enables setting as a specific dot pattern any pattern that could cause the temperature of the recording head to rise suddenly. Whether or not the number of specific dot patterns in the dot line to be printed exceeds a reference number can then be determined, and a rest time can be determined for the dot line printing operation. 
     More specifically, whether the possibility of a sudden rise in the recording head temperature occurring in the printing operation exceeds a reference number is determined, and whether there is a danger of the recording head temperature exceeding the burnout threshold temperature of the head coil when printing the dot line can be determined. As a result, if it is determined that there is a danger of exceeding the burnout threshold temperature of the head coil, printing is accomplished while pausing the printing operation for a rest time that is set so that burnout does not happen. If it is determined that there is not a danger of exceeding the burnout threshold temperature of the head coil, printing can proceed without inserting a rest time. As a result, a drop in printer throughput can be suppressed while reliably preventing head coil burnout. 
     A recording head control method according to another aspect of the invention also preferably includes steps of: setting a plurality of pattern detection areas of a specific width in an area of the maximum line length of the dot line; and during dot line printing, determining before printing if the pattern detection area in the dot line to be printed is a specific area containing the specific dot pattern, determining if the number of specific areas in the dot line is greater than or equal to the reference number, and based on the results of these decisions, determining the rest time between dot line printing operations. 
     This aspect of the invention enables simply determining if a specific dot pattern is contained in a pattern detection area of a specified width, and counting how many specific areas are contained. The processing load can therefore be reduced compared with when the entire dot line is compared with the specific dot pattern. 
     A recording head control method according to another aspect of the invention also preferably includes steps of: presetting a plurality of rest times between dot line printing operations of the recording head, and correlating each rest time to a temperature of the recording head, the recording head temperature correlated to each rest time including at least two temperature settings, a first temperature and a second temperature that is lower than the first temperature; and during dot line printing, detecting the temperature of the recording head before printing, selecting the rest time corresponding to the detected recording head temperature using the second temperature when the number of specific areas in the dot line to be printed is greater than or equal to the reference number; selecting the rest time corresponding to the detected recording head temperature using the first temperature when the number of specific areas in the dot line to be printed is less than to the reference number; and pausing printing during the dot line printing operation based on the selected rest time. 
     This aspect of the invention enables setting plural recording head temperatures correlated to plural rest times. As a result, two or more different temperatures can be selectively used based on the number of pattern detection areas containing the specific dot pattern to select the rest time. As a result, a drop in printer throughput can be suppressed by reliably preventing head coil burnout. 
     A recording head control method according to another aspect of the invention also preferably includes a step of, when the recording head temperature detected before printing is less than or equal to a preset threshold temperature, determining the rest time of the recording head without using the result of determining whether or not the pattern detection areas are the specific area. 
     This aspect of the invention can anticipate that the head coil burnout threshold temperature will not be exceeded even if the specific dot pattern will be printed in plural pattern detection areas, and can prevent a drop in printer throughput. 
     A recording head control method according to another aspect of the invention also preferably includes steps of: relating the print quality of the print data, and the rest time of the recording head; and during dot line printing, pausing printing during the dot line printing operation for the rest time selected from among the rest times related to the print quality of the print data containing the dot line to be printed. 
     A dot impact printer changes recording head drive control according to the print quality setting. As a result, the temperature increase differs according to the print quality. The control method according to this aspect of the invention enables setting the rest time according to the print quality. As a result, head coil burnout can be prevented while also suppressing a drop in printer throughput. 
     Another aspect of the invention is a dot impact printer including a carriage that scans a direction substantially perpendicular to the feed direction of a recording medium on which information is printed; a recording head that is carried on the carriage and has a plurality of recording wires that form print dot rows in a dot line; and a control unit that controls the recording head, determines before printing each dot line if the number of previously defined specific dot patterns contained in the dot line to be printed is greater than or equal to a reference number of 2 or more, and prints the dot line based on the result of the decision. 
     A dot impact printer according to this aspect of the invention can set as a specific dot pattern any pattern that could cause the temperature of the recording head to rise suddenly. Whether or not the number of specific dot patterns in the dot line to be printed exceeds a reference number can then be determined, and the dot line can be printed based on the result of the decision. 
     In a dot impact printer according to another aspect of the invention, the control unit, based on the decision, determines the drive mode of the recording head for printing the dot line. 
     The dot impact printer according to this aspect of the invention determines if the number of specific dot patterns in the dot line to be printed is greater than or equal to a reference number, and selects the drive mode accordingly. More specifically, whether the possibility of a sudden rise in the recording head temperature occurring in the dot line exceeds a reference number is determined, and whether there is a danger of the recording head temperature exceeding the burnout threshold temperature of the head coil when printing the dot line can be determined. As a result, if it is determined that there is a danger of exceeding the burnout threshold temperature of the head coil, printing is accomplished in a drive mode that is set so that burnout does not happen. If it is determined that there is not a danger of exceeding the burnout threshold temperature of the head coil, printing can proceed without changing the drive mode. As a result, a drop in printer throughput can be suppressed while reliably preventing head coil burnout. 
     In a dot impact printer according to another aspect of the invention, the control unit, based on the decision, determines a rest time for the recording head, and pauses printing during the dot line printing operation. 
     The dot impact printer according to this aspect of the invention can set as a specific dot pattern any pattern that could cause the temperature of the recording head to rise suddenly. Whether or not the number of specific dot patterns in the dot line to be printed exceeds a reference number can then be determined, and a rest time can be determined for the dot line printing operation. 
     More specifically, whether the possibility of a sudden rise in the recording head temperature occurring in the printing operation exceeds a reference number is determined, and whether there is a danger of the recording head temperature exceeding the burnout threshold temperature of the head coil when printing the dot line can be determined. As a result, if it is determined that there is a danger of exceeding the burnout threshold temperature of the head coil, printing is accomplished while pausing the printing operation for a rest time that is set so that burnout does not happen. If it is determined that there is not a danger of exceeding the burnout threshold temperature of the head coil, printing can proceed without inserting a rest time. As a result, a drop in printer throughput can be suppressed while reliably preventing head coil burnout. 
     Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an oblique view of the print assembly of a dot impact printer according to a preferred embodiment of the invention. 
         FIG. 2  is a side section view of the print assembly of the dot impact printer. 
         FIG. 3  is a section view of the recording head. 
         FIG. 4  is an exploded view of the recording head. 
         FIG. 5  is a block diagram of the printing control system of the dot impact printer. 
         FIG. 6  shows the distribution of pattern detection areas on each dot line in a first embodiment of the invention. 
         FIG. 7  describes a specific bit image (specific dot pattern) used to determine the drive mode. 
         FIG. 8  is a control table used to determine the drive mode in the first embodiment of the invention. 
         FIG. 9  is a flow chart describing the printing process in the first embodiment of the invention. 
         FIG. 10  shows the carriage drive periods and the distribution of the pattern detection areas on each dot line in a second embodiment of the invention. 
         FIG. 11  is a control table for determining the carriage drive pause time in the second embodiment of the invention. 
         FIG. 12  is a flow chart of the printing process in the second embodiment of the invention. 
         FIG. 13  schematically describes the configuration of a dot impact printer according to a third embodiment of the invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiment 1 
     A first embodiment of the invention is described below with reference to the accompanying figures. Note that for convenience of description or illustration in the figures referenced below, the scale of the horizontal and vertical dimensions of selected members or portions thereof may be shown differently from the actual scale. 
     Configuration of a Dot Impact Printer 
     A dot impact printer that uses a recording head according to this embodiment of the invention is described below with reference to  FIG. 1  and  FIG. 2 .  FIG. 1  is an oblique view of the print assembly used in the dot impact printer, and  FIG. 2  is a side section view of the print assembly. Note that the x-axis shown in  FIG. 1  and  FIG. 2  is the direction of the width of the recording paper that is printed, that is, the direction in which the carriage moves; the y-axis is the direction in which the recording wires of the recording head carried on the carriage move and protrude; and the z-axis is perpendicular to the x-axis and the y-axis. 
     Dot impact printers are used in the sales and distribution industries, for example, and have a recording head that is carried on a carriage that travels across a platen. While the recording head is moved widthwise across the platen, recording wires (pins) that are part of the recording head are driven to strike the recording paper positioned in front of the platen with an ink ribbon between the recording head and the paper, thereby recording information such as text and images onto the recording paper. The recording paper as used herein includes single slips or continuous paper, which may be plain paper or multipart forms paper, passbooks, and envelopes. 
     As shown in  FIG. 1  and  FIG. 2 , the dot impact printer  100  includes at least a print assembly  50  having a frame  20 , a print mechanism  30 , and a paper feed mechanism  40 ; a control device  80 ; and an external case not shown that covers these. 
     The frame  20  includes at least a base frame  21  as the main frame, a paper guide frame  22 , a left side frame  23 , and a right side frame  24 . 
     The print mechanism  30  includes at least a recording head  18 , a carriage  19  on which the recording head  18  is mounted, and a carriage drive mechanism  60  ( FIG. 5 ). 
     The paper feed mechanism  40  includes a platen  41 , a paper guide  42 , a pinch roller  43 , a push tractor unit  44 , a discharge unit  45 , and a paper supply guide  46 . 
     As shown in  FIG. 1 , the left side frame  23  and right side frame  24  are disposed vertically to opposite ends of the frame  20 , and the base frame  21  and paper guide frame  22  (see  FIG. 2 ) are disposed between the left side frame  23  and right side frame  24 . A carriage guide shaft  32  and the platen  41  are disposed freely rotatably between the left side frame  23  and right side frame  24 . The paper guide  42  is fit and fastened to the paper guide frame  22  between the left side frame  23  and right side frame  24 . A tractor unit installation unit and discharge unit installation unit not shown to which the push tractor unit  44  and discharge unit  45  can be installed are disposed to the left side frame  23  and right side frame  24  at the back on the y-axis. 
     The push tractor unit  44  feeds continuous-feed paper used as the recording paper S to the paper feed mechanism  40 . The paper supply guide  46  ( FIG. 2 ) feeds slips used as the recording paper S one sheet (one page) at a time to the paper feed mechanism  40 . The discharge unit  45  discharges the recording paper S, whether continuous-feed paper or slip media, from the paper feed mechanism  40  to the outside of the dot impact printer  100 . As shown in  FIG. 2 , continuous-feed paper is guided to the paper guide  42  of the paper feed mechanism  40  by the action of the pins  47  of a tractor belt  37  when the tractor belt  37  of the push tractor unit  44  turns. The continuous-feed paper then travels through a paper feed path  48  formed between the paper guide  42  and platen  41  to the front of the platen  41  on the y-axis, and is supplied in the direction of arrow (shown in  FIG. 2 . 
     When the push tractor unit  44  is not operating, slips can be supplied one sheet (one page) at a time from the paper supply guide  46  through the paper feed path  48  to the front of the platen  41 . The continuous-feed paper or slip on which text or other content is recorded by the recording head  18  is pulled from the platen  41  part of the paper feed mechanism  40  in the direction of arrow β by rotation of the discharge roller  49  of the discharge unit  45  as described further below. As a result, the continuous-feed paper or slip is conveyed along the y-axis (subscan direction) perpendicular to the x-axis (main scan direction) of the carriage  19 . 
     The carriage  19  shown in  FIG. 1  is fit to slide freely along the carriage guide shaft  32  and carries the recording head  18 . Because the carriage guide shaft  32  and platen  41  are parallel, the carriage  19  can move (scan) on the x-axis aligned with the axes of the platen  41  and the carriage guide shaft  32 . Forward or reverse rotation of a carriage drive motor not shown causes the carriage  19  to move bidirectionally on the x-axis guided on the carriage guide shaft  32  by means of an intervening timing belt  35  ( FIG. 2 ). 
     The recording head  18  has a plurality of recording wires  9  (see  FIG. 3 ), and an ink ribbon  36  ( FIG. 2 ) is positioned in front of the recording wires  9  in the direction in which the recording wires  9  protrude (the y-axis). The recording head  18  causes the recording wires  9  to protrude in a specific printable area that is within the range of carriage  19  movement on the x-axis. By causing the recording wires  9  to strike the ink ribbon  36 , ink contained in the ink ribbon  36  is transferred to the recording paper S (continuous-feed paper or slip) conveyed between the platen  41  and ink ribbon  36 , thereby forming dots on the recording paper S. 
     A single line is recorded by means of plural recording wires  9  on the recording head  18  while the carriage  19  travels left or right on the x-axis. Each time one line is recorded, the platen  41 , push tractor unit  44 , and discharge unit  45  of the paper feed mechanism  40  shown in  FIG. 2  convey the recording paper S a specific distance (normally the line pitch). The recording paper S is normally advanced when the carriage  19  is positioned in a standby zone outside the specific printable area of the range of movement on the x-axis. The recording operation of the recording head  18  is accomplished by repeating these operations. 
     Note that the dot impact printer  100  also has a bottom paper feed opening  39  rendered between the base frame  21  and the paper guide frame  22  for supplying the recording paper S to the paper feed mechanism  40  from the bottom on the z-axis shown in  FIG. 2 . 
     Note also that driving the paper feed mechanism  40 , carriage drive mechanism  60 , and print mechanism  30  that perform the foregoing tasks is controlled by the control device  80 . This control device  80  is rendered on a circuit board, for example, and is disposed, for example, behind the print assembly  50  on the y-axis and below the paper guide frame  22  on the z-axis. The control device  80  is described in detail below. 
     Recording Head 
     The recording head is described next with reference to  FIG. 3  and  FIG. 4 .  FIG. 3  is a section view of the recording head, and  FIG. 4  is an exploded oblique view of the recording head. Note also that the x-axis, y-axis, and z-axis shown in  FIG. 3  and  FIG. 4  are the same as the x-axis, y-axis, and z-axis shown in  FIG. 1  and  FIG. 2 . 
     As shown in  FIG. 3  and  FIG. 4 , the recording head  18  is a serial dot head, and includes a plurality of recording wires  9 , a plurality of head coils (electromagnetic coils)  10 , a head housing  52  housing these, a nose  53 , a heat sink  54 , and a temperature detector (thermistor)  14 . 
     The recording head  18  is configured with the nose  53  contiguous to the head housing  52  on the y-axis in  FIG. 3 , and the heat sink  54  on the outside of the head housing  52 . The recording wires  9  are formed from wire pins that are round in section view, and the face of one recording wire  9  forms one dot. As a result, there may be 9 pins, 16 pins, or 24 pins, for example, according to the dot configuration (dot density) recorded by the recording head  18 . In addition, there is usually one head coil  10  disposed for one recording wire  9 . 
     As shown in  FIG. 3 , the head housing  52  includes a frame  56  that houses the plural recording wires  9  and head coils  10 , and a wire lever  57  and reset spring  59 . The frame  56  is cylindrically shaped, and has plural cores  55  to which the head coils  10  are wound disposed with a specific interval therebetween around the circumference. A recording wire  9  is connected to one end of the wire lever  57 , and when the head coil  10  is energized, the wire lever  57  is pulled to the core  55  and driven. The reset spring  59  urges the wire lever  57  in the direction away from the core  55  pivoting on a pin  58 . With this configuration, the recording wire  9  protrudes to the outside on the y-axis from the nose  53  when the wire lever  57  is pulled to the core  55 . Two head housings  52  are stacked together on the y-axis, which is the direction in which the recording wires  9  advance and retract, and rendered as an integral head housing. Only one head housing  52  is shown in  FIG. 3 , however. 
     The nose  53  guides the in/out operation, including the protruding operation, of the recording wires  9 , and includes a plurality of intermediate guides  62  disposed internally, and a single front guide  63  disposed at the distal end of the nose  53 . The plural recording wires  9  are guided to advance and retract passing through these intermediate guides  62  and the front guide  63 . Note that the plural recording wires  9  are disposed in a line or zigzag pattern along the z-axis in the front guide  63  of the nose  53 . Note, further, that only one recording wire  9  is shown passing through the intermediate guides  62  and front guide  63  in  FIG. 3 , and the other recording wires  9  are omitted. 
     As shown in  FIG. 3  and  FIG. 4 , the heat sink  54  is made from a material with good thermal conductivity, such as aluminum, and is formed in a cylindrical shape with a plurality of fins  64  formed in unison on the outside. The heat sink  54  is rendered around the outside of the head housings  52  of the integral head housing covering each of the head housings  52 . The head coils  10  of the head housings  52  become heat elements and produce heat during the printing operation, that is, when energized. The heat sink  54  dissipates the heat of the head housings  52  produced by heat from the head coils  10  using the fins  64 , and thus functions to cool the head housings  52 . 
     As shown in  FIG. 4 , a temperature detector  14  (thermistor) that detects the temperature of the frame  56  of the head housing  52  is disposed on the outside of the frame  56  of one head housing  52  of the integral head housing. The temperature detector  14  is housed in a channel  66  formed in the inside surface  65  of the heat sink  54  when the heat sink  54  is installed to the head housings  52  of the integral head housing. The temperature detector  14  detects the temperature of the frame  56  of the head housing  52  and outputs to the control device  80 . 
     Note that because the recording head  18  is thus rendered, the temperature of the head coils  10  is higher than the temperature of the frame  56 . As a result, the temperature of the head coils  10 , or more specifically the heat output, is controlled based on a predetermined correlation between the temperature of the head coils  10  and the temperature of the frame  56 . 
     Controlling the Dot Impact Printer 
     The control system of the dot impact printer is described next with reference to  FIG. 5 .  FIG. 5  is a block diagram showing the main components of the dot impact printer. As shown in  FIG. 5 , the dot impact printer  100  includes a print assembly  50  and a control device  80  that controls the print assembly  50 . The print assembly  50  includes a print mechanism  30  including the recording head  18 , a carriage drive mechanism  60  including a carriage motor not shown, the paper feed mechanism  40 , and a detection unit  68  including the temperature detector  14  (thermistor). 
     The control device  80  includes a control unit  33  that is the main part of the control system, a head driver  82  that controls driving the recording head  18 , a motor driver  84  that drives the paper feed mechanism  40  and carriage drive mechanism  60 , and an interface unit  85 . The control unit  33  includes a CPU  86  (central processing unit), data processing unit  87 , and storage unit  88 . The CPU  86  executes various processes, including processing input signals from other operating channels and detection channels not shown, and the printing process. The data processing unit  87  processes various types of information. 
     The storage unit  88  includes RAM (random access memory) and ROM (read-only memory) not shown. RAM is used for temporarily storing print data and other data input from the host computer  89  through the interface unit  85 , and temporarily storing programs such as a printing process executed by the CPU  86 . The print data specifies the pattern to be printed on the recording paper S by the recording head  18 . 
     Based on a command from the CPU  86 , the head driver  82  controls the recording wires  9  of the recording head  18  individually or in groups. The method of controlling the recording head  18  is described below. 
     The motor driver  84  individually controls the motors of the paper feed mechanism  40  and carriage drive mechanism  60  based on commands from the CPU  86 . 
     The interface unit  85  outputs print data received from the host computer  89  to the control unit  33 , and outputs information received from the control unit  33  to the host computer  89 . 
     Recording Head Control Method of the First Embodiment 
     The recording head control method of the first embodiment of the invention is described next with reference to  FIG. 5  to  FIG. 8 .  FIG. 6  shows the distribution of pattern detection areas in each dot line in the first embodiment of the invention.  FIG. 7  describes a specific bit image as an example of a specific dot pattern that is used to determine the drive mode,  FIG. 7A  showing a first dot pattern that is formed by a single recording wire continuously forming 50 dots, and  FIG. 7B  showing a second dot pattern that is formed by a single recording wire forming 25 dots in the space of 50 continuous dots by skipping every other dot.  FIG. 8  is a control table for determining the drive mode in the first embodiment of the invention. 
     The control unit  33  shown in  FIG. 5  outputs a control signal through the head driver  82  to each of the head coils  10  of the recording head  18  based on the print data and control commands, and controls energizing each of the head coils  10 . Detection signals from the temperature detector  14  disposed to the frame  56  of the recording head  18  shown in  FIG. 3  and  FIG. 4  are input to the control unit  33 . During printing, the control unit  33  determines based on the detection signal from the temperature detector  14  if the recording head  18  has exceeded a preset threshold temperature. Based on the result of this decision, the control unit  33  determines the drive mode of the recording head  18  during printing, that is, sets the energizing control mode of the head coils  10  that drive the recording wires  9 . The storage unit  88  of the control unit  33  stores data used to determine the drive mode and for controlling each drive mode. 
     In this embodiment of the invention as shown in  FIG. 6  for example, if one dot line is a maximum 136 columns wide (one column being the width of one character), five pattern detection areas  15  are set within the range of the maximum column count of one dot line. More specifically, the pattern detection areas  15  are distributed so that a pattern detection area  15  is set at the left and right ends of the 136 columns and the remaining three pattern detection areas  15  are distributed evenly between the end pattern detection areas  15 . If the width of each pattern detection area is 8 columns, the width of the non-detection area  16  between neighboring pattern detection areas  15  is 24 columns. Note the dot line shown in  FIG. 6  is recorded by one recording wire  9  of the recording head  18 . 
     As shown in  FIG. 7A  and  FIG. 7B , the first dot pattern P 1  used as a specific bit image is formed by any same recording wire  9  continuously recording 50 dots, for example, and the second dot pattern P 2  is formed by any same recording wire  9  recording, for example, 25 dots every other dot in the same length as the continuously recorded 50 dots. Before printing each dot line, the control unit  33  determines if either the first dot pattern P 1  or the second dot pattern P 2  is contained in each of the five pattern detection areas  15  of the dot line to be printed. The number of pattern detection areas  15  (specific areas) containing either the first dot pattern P 1  or the second dot pattern P 2  in the dot line to be printed is also counted, and whether this count is greater than or equal to a preset reference number N, or is less than the reference number N, is determined. Based on the result of this decision, the control unit  33  then determines the drive mode of the recording head  18  when printing that dot line. In this embodiment of the invention any of the numbers 2, 3, 4, and 5 can be set as this reference number N. 
     The control table for determining the drive mode shown in  FIG. 8  is described next. In addition to the number of specific areas described above, the parameters for determining the drive mode of the recording head  18  in this embodiment of the invention include the print quality setting specified for the print data containing the dot line to be printed. The print quality is specified by the combination of two parameters, the print mode and the character quality. Either a normal or a copy mode is specified as the print mode. The character quality can be set to either draft or letter quality (LQ). The draft mode is set for print data with a lower resolution than a preset reference resolution. The LQ mode is set for print data with resolution at least equal to the reference resolution. 
     As described above, this embodiment of the invention determines based on the detection signal from the temperature detector  14  if the temperature of the recording head  18  exceeds a preset threshold temperature, and based on the result of this decision adjusts how the head coils  10  are energized when printing each dot line. For example, this embodiment of the invention uses three different threshold temperatures as the threshold temperature. In the control table shown in  FIG. 8 , the values shown in the columns labelled “specific bit image” are used when the number of specific areas is greater than or equal to the reference number N, and the values in columns labelled “not-specific bit image” are used when the number of specific areas is less than the reference number N. As shown in the control table, each of the four print quality settings rendered by two print mode and character quality settings is divided into two cases, that is, whether the number of specific areas is greater than or equal to the reference number N (specific bit image), and whether the number of specific areas is less than the reference number N (not-specific bit image), resulting in 8 patterns for which three different threshold temperature settings are preset. 
     In addition to the set threshold temperatures, the control table also shows the output values of the temperature detector  14  at each threshold temperature, that is, the output values of the thermistor. As shown in  FIG. 8 , the three threshold temperature settings each have two levels, a setting (second temperature) for when the number of specific areas is greater than or equal to the reference number N, and a setting (first temperature) for when the number of specific areas is less than the reference number N. 
     Of the three threshold temperatures, the lowest threshold temperature is the print interval condition for setting the time interval from when energizing to form one dot is completed to when energizing to form the next dot starts (the head coil drive interval or non-printing time). The output of the temperature detector  14  at this threshold temperature is Rin. The control unit  33  compares the temperature detector  14  output with Rin to determine whether to use the initial interval (Tin 1 ) or the high temperature interval (Tin 2 ). In this embodiment of the invention the temperatures used as Tin 1  and Tin 2  differ according to the print quality setting, and the values of Tin 1  and Tin 2  for each print quality setting differ as shown in the control table in  FIG. 8 . 
     The next lowest threshold temperature is the unidirectional (Uni-D) print condition for setting whether to print in the bidirectional print mode in which dots are formed on both the outbound and return passes of the carriage  19  on the x-axis shown in  FIG. 1 , or to print in the unidirectional print mode in which dots are formed only on the outbound pass or the return pass. The output of the temperature detector  14  for this threshold temperature is Rhh. The control unit  33  compares the temperature detector  14  output with Rhh to determine whether to operate in the bidirectional print mode or unidirectional print mode. 
     The highest threshold temperature is the stop printing condition that is used to determine whether or not to stop printing. The output of the temperature detector  14  corresponding to this threshold temperature is Rst. 
     Printing Process 
     The dot line printing process according to the first embodiment of the invention is described next with reference to  FIG. 9 .  FIG. 9  is a flow chart of the process for printing each dot line using the control table. 
     In step S 1  in  FIG. 9  the control unit  33  detects the output value R of the temperature detector  14  and compares it with the output value corresponding to the preset threshold temperature. The threshold temperature that is used in this step is an even lower value than the print interval condition, which is the lowest temperature of the three threshold temperatures described above, and in this embodiment is set to 17° C. (the corresponding output value of the temperature detector  14  is 71.51 kΩ). If the temperature of the recording head  18  is less than or equal to the threshold temperature, that is, if R≧71.51 kΩ (step S 1  returns Yes), the temperature of the recording head  18  is determined to be sufficiently low and control goes to step S 2 . 
     The drive mode is set under these conditions in step S 2 . The control unit  33  sets the head coil drive frequency f to 1.44 kHz, for example, and sets the head coil energize time Pw to Pwn+10 μs in the normal mode, and to Pwc in the copy mode. 
     The head coil drive frequency f is the frequency of the pulse voltage applied to the head coils  10  of the recording head  18 , and means the maximum number of times the recording wire  9  is driven to protrude per unit time (the maximum protrusion operation count). 
     Pwn is the preset reference energize time in the normal mode, and Pwc is the preset reference energize time in the copy mode. Note that the initial interval (Tin 1 ) corresponding to the print quality can be selected from among the values in the not-specific bit image column for the head coil drive interval. Alternatively, a different interval value may be set. Control then goes to step S 3 . 
     In step S 3  the dot line to be printed is printed in the drive mode using the parameters set in step S 2 . The process then ends. 
     If in step S 1  in  FIG. 9  the temperature T of the recording head  18  exceeds the threshold temperature (step S 1  returns No), control goes to step S 4 . 
     The control unit  33  detects and evaluates a specific bit image in step S 4 . More particularly, the control unit  33  detects if the specific bit image (first dot pattern P 1  or second dot pattern P 2 ) is found in the five pattern detection areas  15  shown in  FIG. 6 , and compares the number detected with the reference number N. If the number of pattern detection areas  15  (specific areas) in which a specific bit image is found is greater than or equal to N (step S 4  returns Yes), control goes to step S 5 . 
     In step S 5  the control unit  33  references the control table in  FIG. 8 , and selects the temperatures in the specific bit image column of the print quality specified for the current print data as the three threshold temperatures. The control unit  33  then sets the output values Rin, Rhh, Rst corresponding to the selected threshold temperatures as the criteria to be used to set the drive mode. 
     If in step S 4  the number of pattern detection areas  15  in which the specific bit image was found is less than N (step S 4  returns No), control goes to step S 6 . 
     In step S 6  the control unit  33  references the control table in  FIG. 8 , and selects the temperatures in the not-specific bit image column of the print quality specified for the current print data as the three threshold temperatures. The control unit  33  then sets the output values Rin, Rhh, Rst corresponding to the selected threshold temperatures as the criteria to be used to set the drive mode. 
     If control goes to step S 5  as a result of the decision from step S 4  in  FIG. 9 , the output value R of the temperature detector  14  is compared with Rin, Rhh, Rst in steps S 7  to S 12 , and the drive mode is set based on the results of these steps. The three threshold temperatures (second temperature) set in step S 5  at this time are 65° C., 75° C., 107° C. 
     In step S 7  the output R of the temperature detector  14  is first compared with output Rin corresponding to the print interval condition (65° C.) that is the lowest threshold temperature. If R&gt;Rin (recording head  18  temperature T&lt;65° C.) (step S 7 :No), control goes to step S 8 , and the head coil drive frequency f, and head coil energize time Pw are set to the values corresponding to the current print quality. The head coil drive interval at this time is set to Tin 1 . Control then goes to step S 3 , the dot line to be printed is printed using the parameters set in step S 8 , and the process ends. 
     However, if in step S 7  R≦Rin (recording head  18  temperature T≧65° C.) (step S 7 :Yes), control goes to step S 9 , and the output R of the temperature detector  14  is compared with output value Rhh corresponding to the unidirectional printing condition (75° C.). If R&gt;Rhh (recording head  18  temperature T&lt;75° C.) (step S 9 :No), control goes to step S 10 , and the head coil drive frequency f and head coil energize time Pw are set to the values corresponding to the current print quality. The head coil drive interval at this time is set to Tin 2 . Control then goes to step S 3 , the dot line to be printed is printed using the parameters set in step S 10 , and the process ends. 
     If in step S 9  R≦Rhh (recording head  18  temperature T≧75° C.) (step S 9 :Yes), control goes to step S 11  and the output R of the temperature detector  14  is compared with output value Rst corresponding to the stop printing condition (107° C.). If R&gt;Rst (recording head  18  temperature T&lt;107° C.) (step S 11 :No), control goes to step S 12 , and the head coil drive frequency f and head coil energize time Pw are set to the values corresponding to the current print quality. The head coil drive interval at this time is set to Tin 2 . Control then goes to step S 3 , the dot line to be printed is printed using the parameters set in step S 12 , and the process ends. 
     However, if in step S 11  R≦Rst (107° C.) (step S 11 :Yes), control goes to step S 13  and printing stops for a preset delay time (a time sufficient for the recording head  18  to cool). Control then goes to to step S 1  and processing resumes. 
     If control goes to step S 6  as a result of step S 4  in  FIG. 9 , the temperature R of the recording head  18  is compared with threshold temperatures Rin, Rhh, Rst in steps S 14  to S 19 , and the drive mode is set based on the results of these steps. 
     In step S 6  the threshold temperatures Rin, Rhh, Rst (first temperature) are 102° C., 107° C., 110° C., respectively. Steps S 14  to S 19  are the same as steps S 7  to S 12  except that the three threshold temperatures are higher than the threshold temperatures used in steps S 7  to S 12 . 
     Note that the drive mode settings in steps S 8 , S 10 , and S 12  are the same as the settings in steps S 15 , S 17 , and S 19 , respectively. More specifically, in this embodiment as shown in  FIG. 8  and  FIG. 9 , the threshold temperatures of the recording head  18  for changing the drive mode to the settings shown in step S 8 , S 10 , S 12  are lower in the case of a specific bit image than in the case of a not-specific bit image, and in the case of a specific bit image, the drive mode is configured to produce less heat even if the recording head  18  temperature is the same. 
     The effect of the first embodiment of the invention is described below. 
     As described above, the pattern detection areas  15  in the dot line to be printed are compared with a specific bit image in this embodiment of the invention. If the number of pattern detection areas  15  containing the specific bit image is greater than or equal to a reference number N, the threshold temperature of the recording head  18  used to change to a drive mode that produces less heat and has a slower print speed is set to a low temperature (second temperature). If less than the reference number N, however, the threshold temperature of the recording head  18  used to change to a drive mode that produces less heat and has a slower print speed is set to a higher temperature (first temperature) than the second temperature. As a result, the recording head  18  drive mode does not change to a drive mode that produces less heat and has a slow print speed when the number of pattern detection areas  15  containing the specific bit image is 1 or less and the temperature of the recording head  18  is anticipated to not exceed the burnout threshold temperature of the head coil  10 . Burnout of the head coil  10  can therefore be prevented and a drop in printer throughput can be suppressed. 
     Embodiment 2 
     A second embodiment of the invention is described next with reference to accompanying figures. Note that parts and content of the second embodiment that are the same as in the first embodiment are identified by like reference numerals, and further description thereof is omitted below. 
     Method of Controlling the Recording Head According to the Second Embodiment of the Invention 
     The method of controlling the recording head according to the second embodiment of the invention is described below with reference to  FIG. 5 . 
     The recording head  18  is controlled by the control unit  33  shown in  FIG. 5 . Based on the print data and control commands, the control unit  33  selects a specific drive mode, outputs control signals through the head driver  82  to the head coils  10  of the recording head  18 , and controls energizing each of the head coils  10 . The drive modes are stored in the storage unit  88  of the control unit  33 , and specify, for example, the head coil drive frequency f and head coil energize time Pw. The head coil drive frequency f is the frequency of the pulse voltage applied to the head coils  10  of the recording head  18 , and means the maximum number of times the recording wire  9  is driven to protrude per unit time (the maximum protrusion operation count). 
     The drive mode is specified using a combination of two parameters such as the print mode and character quality. The print mode is either normal, which is used for normal printing operations, or copy, which is a mode for printing copies. Note that plural copy modes can be set, such as when printing two copies or four copies, for example. Character quality can be set to draft or letter quality (LQ). The draft mode is set for printing with a lower resolution than a preset reference resolution. The LQ mode is set for printing with resolution at least equal to the reference resolution. 
     Carriage Scanning Control 
     Scanning control of the carriage that carries the recording head is described next with reference to  FIG. 5 ,  FIG. 7 ,  FIG. 10 , and  FIG. 11 . The second embodiment of the invention uses carriage scanning control, or more particularly a rest period in the carriage scanning operation, to reduce excessive rise in the temperature of the recording head.  FIG. 10  shows the carriage scanning period and the distribution of pattern detection areas on each dot line.  FIG. 11  is a control table for determining the rest time. 
     Carriage  19  scanning is controlled by the control unit  33  shown in  FIG. 5 . The control unit  33  outputs a control signal to the carriage motor  69  of the carriage drive mechanism  60  through the motor driver  84  to control carriage  19  scanning. As described above, the carriage  19  is guided by the carriage guide shaft  32  through an intervening timing belt  35  (see  FIG. 2 ) and travels bidirectionally on the x-axis shown in  FIG. 1  as a result of forward or reverse rotation of the carriage motor  69 . The range of carriage  19  travel on the x-axis is referred to below as the scanning period M (see  FIG. 10 ). 
     As shown in  FIG. 10 , the scanning period M of the carriage  19  is segmented into printable period L and standby periods Q. One line composed of a plurality of dot lines L is recorded by a plurality of recording wires  9  of the recording head  18  in the printable period L. Note that because a dot line is formed through the entire printable period L, the same symbol L is used in reference to the dot line. 
     In this embodiment of the invention the printable period L is composed of one dot line that is a maximum 136 columns wide (one column being the width of one character), and five pattern detection areas  15  are set within the range of the maximum column count of one dot line L. More specifically, the pattern detection areas  15  are distributed so that a pattern detection area  15  is set at the left and right ends of the 136 columns and the remaining three pattern detection areas  15  are distributed evenly between the end pattern detection areas  15 . If the width of each pattern detection area is 8 columns, the width of the non-detection area  16  between neighboring pattern detection areas  15  is 24 columns. Note the dot line L shown in  FIG. 6  is recorded by one recording wire  9  of the recording head  18 . 
     As shown in  FIG. 10 , a standby period Q is placed on each end of the printable period L. The recording head  18  does not print in the standby periods Q, which are used to slow the speed of the carriage  19 , reverse direction, pause (stop) scanning as necessary, or accelerate. The recording paper S is generally conveyed while the carriage  19  is in a standby period Q. 
     The control unit  33  shown in  FIG. 5  also outputs control signals to each of the head coils  10  of the recording head  18  as described above, and controls energizing the head coils  10 . The detection signal of the temperature detector  14  disposed to the frame  56  of the recording head  18  shown in  FIG. 3  and  FIG. 4  is also input to the control unit  33 . The control unit  33  determines based on the detection signal from the temperature detector  14  if the recording head  18  exceeds a preset threshold temperature during printing. Based on this decision, the control unit  33  controls the scanning operation of the carriage  19 , that is, determines the scanning rest time setting of the carriage  19 . The storage unit  88  of the control unit  33  stores data used to determine the rest time and control during the rest time. 
     As shown in  FIG. 7A  and  FIG. 7B , the first dot pattern P 1  used as a specific bit image is formed by any same recording wire  9  continuously recording 50 dots, for example, and the second dot pattern P 2  is formed by any same recording wire  9  recording, for example, 25 dots every other dot in the same length as the continuously recorded 50 dots. Before printing each dot line L, the control unit  33  determines if either the first dot pattern P 1  or the second dot pattern P 2  is contained in each of the five pattern detection areas  15  of the dot line L to be printed. The number of pattern detection areas  15  (specific areas) containing either the first dot pattern P 1  or the second dot pattern P 2  in the dot line L to be printed is also counted, and whether this count is greater than or equal to a preset reference number N, or is less than the reference number N, is determined. Based on the result of this decision, the control unit  33  then determines the rest time of the carriage  19  (recording head  18 ) when printing that dot line L. In this embodiment of the invention any of the numbers 2, 3, 4, and 5 can be set as this reference number N. 
     The control table shown in  FIG. 11  for determining the rest time is described next. The number of specific areas and the print quality setting specified for the print data containing the dot line L to be printed are used as parameters for determining the rest time of the carriage  19  in this embodiment of the invention. Print quality is specified by the combination of two parameters, that is, the two print mode settings of normal and copy, and the two character quality settings of draft and letter quality (LQ). 
     As described above, this embodiment of the invention determines based on the detection signal from the temperature detector  14  if the temperature of the recording head  18  exceeds a preset threshold temperature T, and based on the result of this decision determines the rest time of the carriage  19  (recording head  18 ) when printing each dot line L. For example, this embodiment of the invention uses three different threshold temperatures T as the threshold temperature T. In the control table shown in  FIG. 11 , the values shown in the columns labelled “specific bit image” are used when the number of specific areas is greater than or equal to the reference number N, and the values in columns labelled “not-specific bit image” are used when the number of specific areas is less than the reference number N. As shown in the control table, each of the four print quality settings rendered by two print mode and character quality settings is divided into two cases, that is, whether the number of specific areas is greater than or equal to the reference number N (specific bit image), and whether the number of specific areas is less than the reference number N (not-specific bit image), resulting in 8 patterns for which three different threshold temperature settings are preset. 
     In addition to the set threshold temperatures T, the control table also shows the output values R of the temperature detector  14  at each threshold temperature T, that is, the output values of the thermistor. As shown in  FIG. 11 , the three threshold temperature T settings each have two levels, a setting (second temperature) for when the number of specific areas is greater than or equal to the reference number N (specific bit image), and a setting (first temperature) for when the number of specific areas is less than the reference number N (not-specific bit image). 
     The lowest threshold temperature T of the three threshold temperatures T is case  1 , and the output value of the temperature detector  14  at this threshold temperature T is R 1 . The control unit  33  compares the output value R of the temperature detector  14  with R 1  to determine the rest time H. In this embodiment of the invention the setting of each temperature of the rest times H differs according to the print quality setting. The values shown in the control table in  FIG. 11  show the ratio K (%) multiplied by the standard rest time Hst. Note that the standard rest time Hst is preferably a maximum of approximately 1 second. In addition, the ratio K multiplied by the standard rest time Hst is preferably set using two values, ratio K 1  and ratio K 2 , for example. These can be used according to the print data and the drive mode. The temperature may also be divided into smaller increments and the number of ratios increased. 
     The next lowest threshold temperature T is case  2 , and the output value R of the temperature detector  14  at this threshold temperature T is R 2 . The control unit  33  compares the output value R of the temperature detector  14  with R 2  to determine the rest time H. The highest threshold temperature T is case  3 , and the output value R of the temperature detector  14  at this threshold temperature T is R 3 . 
     Printing Process Flow 
     The flow of the dot line printing process is described next with reference to  FIG. 11  and  FIG. 12 .  FIG. 12  is a flow chart showing the dot line printing process using the control table. Note that in the example described below the print mode is normal and the character quality is draft. 
     In step S 1  in  FIG. 12  the control unit  33  detects the output value R of the temperature detector  14  and compares it with the output value corresponding to the preset threshold temperature T. The threshold temperature T that is used in this step is an even lower value than case  1 , which is the lowest temperature of the three threshold temperatures T described above, and in this embodiment is set to 17° C. (the corresponding output value of the temperature detector  14  is 71.51 kΩ). If the temperature of the recording head  18  is less than or equal to the threshold temperature T, that is, if R≧71.51 kΩ (step S 1  returns Yes), the temperature of the recording head  18  is determined to be sufficiently low and control goes to step S 2 . 
     The rest time H under this condition is set in step S 2 . The control unit  33  selects case C 0  from the control table shown in  FIG. 11 . In case C 0  the ratio K multiplied by the standard rest time Hst is 0% and the rest time H is therefore 0, that is, the carriage  19  is set to decelerate, change direction, and accelerate in standby period Q with setting a rest time H. Control then goes to step S 3 . 
     In step S 3  the dot line L to be printed is printed in the preset drive mode. The process then ends. 
     If in step S 1  in  FIG. 12  the temperature t of the recording head  18  exceeds the threshold temperature T (step S 1  returns No), control goes to step S 4 . 
     The control unit  33  detects and evaluates a specific bit image in step S 4 . More particularly, the control unit  33  detects if the specific bit image (first dot pattern P 1  or second dot pattern P 2 ) is found in the five pattern detection areas  15  shown in  FIG. 10 , and compares the number detected with the reference number N. If the number of pattern detection areas  15  (specific areas) in which a specific bit image is found is greater than or equal to N (step S 4  returns Yes), control goes to step S 5 . 
     In step S 5  the control unit  33  references the control table in  FIG. 11 , and selects the temperatures in the specific bit image column of the print quality specified for the current print data as the three threshold temperatures T. The control unit  33  then sets the output values R 1 , R 2 , R 3  corresponding to the selected threshold temperatures as the criteria to be used to set the rest time H. 
     If in step S 4  the number of pattern detection areas  15  in which the specific bit image was found is less than N (step S 4  returns No), control goes to step S 6 . 
     In step S 6  the control unit  33  references the control table in  FIG. 11 , and selects the temperatures in the not-specific bit image column of the print quality specified for the current print data as the three threshold temperatures T. The control unit  33  then sets the output values R 1 , R 2 , R 3  corresponding to the selected threshold temperatures T as the criteria to be used to set the drive mode. 
     If control goes to step S 5  as a result of the decision from step S 4  in  FIG. 12 , the output value R of the temperature detector  14  is compared with R 1 , R 2 , R 3  in steps S 7  to S 12 , and the rest time H is set based on the results of these steps. The three threshold temperatures (second temperature) set in step S 5  at this time are 65° C., 75° C., 107° C. 
     In step S 7  the output R of the temperature detector  14  is first compared with output R 1  corresponding to case  1  (65° C.), that is, the lowest threshold temperature. If R&gt;R 1  (recording head  18  temperature t&lt;65° C.) (step S 7 :No), control goes to step S 8 , and the rest time H is set according to these conditions. Because the ratio K multiplied by the standard rest time Hst is 40% as shown in the control table in  FIG. 11 , the control unit  33  determines the rest time H to be 0.4×Hst, that is, sets a rest time H of 0.4×Hst in the standby period Q. Control then goes to step S 3 . 
     In step S 3  the dot line L to be printed is printed in the preset drive mode. The process then ends. 
     However, if in step S 7  R≦Rin (recording head  18  temperature t≧65° C.) (step S 7 :Yes), control goes to step S 9 , and the output R of the temperature detector  14  is compared with output value R 2  corresponding to case  2  (75° C.). If R&gt;Rhh (recording head  18  temperature t&lt;75° C.) (step S 9 :No), control goes to step S 10 , and the rest time H is set according to these conditions. Because the ratio K multiplied by the standard rest time Hst is 50% as shown in the control table in  FIG. 11 , the control unit  33  determines the rest time H to be 0.5×Hst, that is, sets a rest time H of 0.5×Hst in the standby period Q. Control then goes to step S 3 . 
     In step S 3  the dot line L to be printed is printed in the preset drive mode. The process then ends. 
     If in step S 9  R≦Rhh (recording head  18  temperature t≧75° C.) (step S 9 :Yes), control goes to step S 11  and the output R of the temperature detector  14  is compared with output value R 3  corresponding to case  3  as the stop printing condition (107° C.). If R&gt;R 3  (recording head  18  temperature t&lt;107° C.) (step S 11 :No), control goes to step S 12 , and the rest time H is set according to these conditions. Because the ratio K multiplied by the standard rest time Hst is 50% as shown in the control table in  FIG. 11 , the control unit  33  determines the rest time H to be 0.5×Hst, that is, sets a rest time H of 0.5×Hst in the standby period Q, and determines to print on only one pass of the bidirectional carriage scan. Control then goes to step S 3 . 
     In step S 3  the dot line L to be printed is printed in the preset drive mode. The process then ends. 
     However, if in step S 11  R≦R 3  (recording head  18  temperature t≧107° C.) (step S 11 :Yes), control goes to step S 13  and printing stops for a preset delay time (a time sufficient for the recording head  18  to cool). Control then goes to step S 1  and processing resumes. 
     If control goes to step S 6  as a result of step S 4  in  FIG. 9 , the output value R of the temperature detector  14  corresponding to temperature t of the recording head  18  is compared with the output values R 1 , R 3 , R 3  of the temperature detector  14  corresponding to the threshold temperatures T in steps S 14  to S 19 , and the rest time H is set based on the results of these steps. 
     In step S 6  the threshold temperatures T (first temperature) are 102° C., 107° C., 110° C., respectively. Steps S 14  to S 19  are the same as steps S 7  to S 12  except that the three threshold temperatures T are higher than the threshold temperatures T used in steps S 7  to S 12 . 
     Note that the rest time H settings in steps S 8 , S 10 , and S 12  are the same as the settings in steps S 15 , S 17 , and S 19 , respectively. More specifically, in this embodiment as shown in  FIG. 8  and  FIG. 9 , the threshold temperatures T of the recording head  18  for changing the rest time H to the settings shown in step S 8 , S 10 , S 12  are lower in the case of a specific bit image than in the case of a not-specific bit image, and in the case of a specific bit image, a rest time H that can be expected to have a better heat dissipation (cooling) effect is set even if the recording head  18  temperature is the same. 
     While an example in which the print mode is set to normal and the character quality is set to draft is described above, the same process is executed based on the control table shown in  FIG. 11  when a different mode is set, such as when the print mode is set to copy or the character quality is set to letter quality. 
     The effect of this embodiment is described below.
     (1) The recording head  18  control method described above can prevent an excessive rise in the temperature of the head coils  10  of the recording head  18  by resting the recording head  18  in a standby period Q that is outside the printable period L of the recording head  18 . In addition, the control unit  33  determines if the number of first dot patterns P 1  and second dot patterns P 2 , which are specific bit images that could cause a sudden temperature rise in the head coils  10 , contained in the dot line L to be printed exceeds a reference number N, and determines the rest time H in the printing operation of the recording head  18  accordingly. As a result, a sudden rise in the temperature of the head coil  10  can therefore be determined by a simpler method than when controlling energizing the head coils  10 , for example.   (2) The recording head  18  control method described above can prevent a sudden temperature rise in the head coils  10  while the drive mode of the recording head  18  remains the same. Controlling the dot impact printer  100  can therefore be prevented from becoming more complicated.   (3) The recording head  18  control method described above compares the pattern detection areas  15  in the dot line L to be printed with a specific bit image, and if the number of pattern detection areas  15  containing the specific bit image is greater than or equal to a reference number N, changes the threshold temperature of the recording head  18  used to set a rest time H to a low temperature (second temperature). If less than the reference number N, however, the threshold temperature of the recording head  18  for setting the rest time H is set to a higher temperature (first temperature) than the second temperature. As a result, the rest time H can be set to a short time when the number of pattern detection areas  15  containing the specific bit image is 1 or less and the temperature of the recording head  18  is anticipated to not exceed the burnout threshold temperature of the head coil  10 . Burnout of the head coil  10  can therefore be prevented and a drop in printer throughput can be suppressed.   

     A third embodiment of the invention is described next with reference to accompanying figures. Note that parts and content of the third embodiment that are the same as in the first or second embodiment are identified by like reference numerals, and further description thereof is omitted below. Note that this third embodiment of the invention differs from the first and second embodiments in the configuration of the dot impact printer. 
     Dot Impact Printer Configuration 
     The configuration of a dot impact printer according to the third embodiment of the invention is described first below.  FIG. 13  schematically describes the configuration of a dot impact printer according to the third embodiment of the invention. 
     As shown in  FIG. 13 , the dot impact printer  100   a  has a carriage guide shaft  32  extending transversely between the left and right side panels  23 ,  24  of the printer frame. A carriage  19  that carries a recording head  18  is configured to move bidirectionally along this carriage guide shaft  32  widthwise to the printer based on output from a carriage motor  69  by means of a drive mechanism  19   b  such as a timing belt  35  and pulley not shown. A platen  41  opposes the recording head  18  with a constant gap therebetween. Recording paper is conveyed passed the printing position determined by the platen  41  through a recording paper transportation path that passes the printing position. The recording paper is conveyed in conjunction with the printing operation of the recording head  18  through the recording paper transportation path by a paper feed mechanism  40 , and is discharged outside the dot impact printer  100  when printing ends. 
     An ink ribbon  36  pulled from an ink ribbon cassette not shown is loaded into the gap between the recording head  18  and platen  41  across the head surface  18   a  of the recording head  18 . The ink ribbon  36  is configured so that the used portion can be sequentially rewound on a takeup spindle not shown. The recording head  18  has a plurality of recording wires  9  and a head coil  10  such as a solenoid coil for driving each recording wire  9 . The distal ends of the recording wires  9  can be driven to protrude from the head surface  18   a  to the platen  41  by means of drive power from the corresponding head coil  10 . The plural recording wires  9  are arrayed in the head surface  18   a  so that each recording wire  9  forms one dot of the print data. For example, 24 recording wires  9  can be arrayed in two rows of 12 in the head surface  18   a.    
     When the head coil  10  is energized, the recording wire  9  corresponding to the energized head coil  10  strikes the recording paper on the platen  41  through the ink ribbon  36 . As a result, a dot is formed on the recording paper by the ink from the ink ribbon  36 . When pressure sensitive paper is conveyed with the recording paper, a dot is also formed at the same position on the pressure sensitive paper. The dot impact printer  100   a  prints one dot line at a time by energizing the head coils  10  based on the print data to cause the corresponding recording wires  9  to protrude at the necessary time while the carriage  19  traverses over the paper. When printing one dot line is completed, the recording paper is advanced one dot line and the next dot line is printed. 
     The recording head control method and printing process flow are the same as the method, control, and processes of the first embodiment and second embodiment described above. 
     As in the first embodiment and the second embodiment, this embodiment of the invention compares the pattern detection areas  15  in the dot line to be printed with a specific bit image. When the number of pattern detection areas  15  containing the specific bit image is greater than or equal to reference number N, the threshold temperature of the recording head  18  for changing to a drive mode with less heat output and a slower printing speed is set to a low temperature (second temperature). If less than the reference number N, however, the threshold temperature of the recording head  18  used to change to a drive mode that produces less heat and has a slower print speed is set to a higher temperature (first temperature) than the second temperature. As a result, the recording head  18  drive mode does not change to a drive mode that produces less heat and has a slow print speed when the number of pattern detection areas  15  containing the specific bit image is 1 or less and the temperature of the recording head  18  is anticipated to not exceed the burnout threshold temperature of the head coil  10 . Burnout of the head coil  10  can therefore be prevented and a drop in printer throughput can be suppressed. 
     Preferred embodiments of the invention are described above, and it will be obvious to one with ordinary skill in the related art that the foregoing embodiments can be changed in various ways without departing from the scope of the accompanying claims. Examples of such variations are described below. 
     Variation 1 
     The foregoing embodiments describe two types of specific bit images, a first dot pattern P 1  that is formed by a single recording wire continuously forming 50 dots, and a second dot pattern P 2  that is formed by a single recording wire forming 25 dots in the space of 50 continuous dots by skipping every other dot. Other dot patterns can be used as a specific bit image, however. 
     For example, the number of dots that are formed continuously (that is, the number of times the corresponding head coil  10  is driven continuously) when the specific bit image is a series of consecutive dots can be set according to the width of each pattern detection area  15 , the gap between neighboring pattern detection areas  15 , or the maximum length (maximum column count) of the dot line, for example. Alternatively, a dot pattern formed by more than a specified percentage of the dot count equal to the width of the pattern detection area  15  could be used as a specific bit image. Further alternatively, a configuration in which a specified number or more of the plural recording wires  9  in the recording head  18  simultaneously form the first dot pattern P 1  or second dot pattern P 2  is treated as a specific bit image is also conceivable. 
     Variation 2 
     In the foregoing embodiments plural pattern detection areas  15  are set in the range of the dot line, and the specific bit images are detected only in the pattern detection areas  15 . However, a configuration that detects how many specific bit images are within the maximum length of the dot line without setting pattern detection areas  15 , and determines the drive mode based on how many are detected, is also conceivable. Alternatively, the number of pattern detection areas  15  may be increased and the width of the non-detection areas between neighboring pattern detection areas  15  could be shortened. 
     Variation 3 
     Processing does not change according to the content of the detected specific bit images (that is, whether first dot pattern P 1  or second dot pattern P 2  is detected) in the foregoing embodiments, but the drive mode settings could be changed according to the content of the detected specific bit image. Yet further, the interval between plural pattern detection areas  15  in which a specific bit image is detected, the dot density of areas near the specific bit image, or the printing content (print history) before printing the dot line to be printed, for example, could be used as parameters for setting the drive mode. 
     Although the present invention has been described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims, unless they depart therefrom. 
     Text in the Figures 
     
         
         
           FIG. 5 
         
         HOST COMPUTER  89   
         INTERFACE UNIT  85   
         CONTROL DEVICE  80   
         CONTROL UNIT  33   
         CPU  86   
         DATA PROCESSING UNIT  87   
         STORAGE UNIT  88   
         HEAD DRIVER  82   
         MOTOR DRIVER  84   
         PRINT ASSEMBLY  50   
         PRINT MECHANISM  30   
         RECORDING HEAD  18   
         HEAD COIL  10   
         CARRIAGE DRIVE MECHANISM  60   
         CARRIAGE MOTOR  69   
         paper feed MECHANISM  40   
         DETECTION UNIT  68   
         TEMPERATURE DETECTOR  14   
           FIG. 6 ,  FIG. 10   
           1  DOT LINE 
         
           FIG. 8 
         
         PRINT MODE 
       
    
     NORMAL 
     COPY
     PRINT INTERVAL CONDITION   UNIDIRECTIONAL PRINT CONDITION   STOP PRINTING CONDITION   HEAD COIL DRIVE INTERVAL   SYMBOL   CHARACTER QUALITY   DRAFT MODE   LETTER QUALITY MODE   NOT-SPECIFIC BIT IMAGE   SPECIFIC BIT IMAGE     FIG. 9     START   S 3  PRINT   S 4  SPECIFIC BIT IMAGE DETECTED?   S 5  SET THRESHOLD TEMPERATURE BASED ON SPECIFIC BIT IMAGE COLUMN IN CONTROL TABLE   S 6  SET THRESHOLD TEMPERATURE BASED ON NOT-SPECIFIC BIT IMAGE COLUMN IN CONTROL TABLE   S 8  HEAD COIL DRIVE INTERVAL: Tin 1     S 13  STOP PRINTING   S 19  HEAD COIL DRIVE INTERVAL: Tin 2 ; UNIDIRECTIONAL PRINTING   END   Key   f: HEAD DRIVE FREQUENCY (kHz)   Pw: HEAD COIL ENERGIZE TIME   N: NORMAL MODE   C: COPY MODE   Pwn: REFERENCE ENERGIZE TIME IN NORMAL MODE   Pwc: REFERENCE ENERGIZE TIME IN COPY MODE   Tin 1 : HEAD COIL DRIVE INTERVAL (INITIAL)   Tin 2 : HEAD COIL DRIVE INTERVAL (HIGH TEMPERATURE)   Rin: PRINT INTERVAL CONDITION   Rhh: UNIDIRECTIONAL PRINTING CONDITION   Rst: STOP PRINTING CONDITION     FIG. 11     PRINT MODE   

     NORMAL 
     COPY
     CASE C 1     PAUSE TIME   CHARACTER QUALITY   DRAFT MODE   LETTER QUALITY MODE   NOT-SPECIFIC BIT IMAGE   SPECIFIC BIT IMAGE     FIG. 12     START   S 3  PRINT   S 4  SPECIFIC BIT IMAGE DETECTED?   S 5  SET THRESHOLD TEMPERATURE BASED ON SPECIFIC BIT IMAGE COLUMN IN CONTROL TABLE   S 6  SET THRESHOLD TEMPERATURE BASED ON NOT-SPECIFIC BIT IMAGE COLUMN IN CONTROL TABLE   S 12  SIMPLEX PRINTING   S 13  STOP PRINTING   S 19  SIMPLEX PRINTING   END