Thermal printer

Abstract: The present invention relates to a thermal printer including a line thermal head (12) having heating elements, and comprises trays (A3, A4, B4, B5) for receiving thermal sensitive cut papers, a plurality of rollers (r1, r2, r3, r4) for feeding the papers from the trays (A3, A4, B4, B5) to the thermal head (12), and an electric power supply for supplying the electric power to the heating elements. The thermal printer also includes a controller (2) for controlling the power supply from the electric power supply in response to a size of the selected papers.

Description: BACKGROUND OF THE INVENTION The present invention relates to a thermal printing apparatus for using a line thermal head. In a conventional thermal printing apparatus using a line thermal head, since the printing width is kept constant and a continuous thermal sensitive paper is used in correspondence with said printing width, the widthwise non-printed area of the thermal sensitive paper depends upon the size of the original. To reduce the resulting waste of paper, on copying small width original various sizes of separated papers may be used. In the line thermal head, in accordance with a content of the original or an output signal of the printing pattern, it is possible to determine which elements of the line thermal head should be heated. However, if a piece of cut paper is used and the size of the paper is selected in error, the width of printing pattern may be larger than expected. In this case, the heating elements which are located outside of the printing width of paper are also heated. Even if the size of paper is selected correctly, it often happens that the width of the printing pattern is as long as the width of paper but that the whole position of the printing pattern signal is slightly offset in the lateral direction. In this case, the heating elements located outside the width of the paper are also heated. Heating elements that are heated without thermal sensitive paper being under them become over-heated and are said to be in a state of "idle heating". Further, if the elements are over-heated repeatedly, the elements are likely to be damaged or break down. Further, if the heating elements which are located close to the edges of paper are repeatedly over-heated, these heating elements are readily damaged with the result that there is a blank line on the printed surface of full width papers in the longitudinal direction. In this case, it is very difficult to replace only the broken and damaged elements of the line thermal head by new ones. Accordingly, the whole head must be exchanged by new head. Therefore, although the waste of paper is reduced by using paper of narrow width for small width originals, the cost of maintenance is increased. Also, in the case of using cut paper, there is the fear that a printing signal will be inputted into a control means for the line thermal head so that the heating elements thereof are accidentally subjected to heat before the paper has arrived at said line thermal head and remain energized after the paper has passed said line thermal head. This is also known as "idle heat". EP-A-181 064 is a prior art that has given thought to some of the above problems. It discloses a fixed head type thermal printer having a plurality of dot printing elements defining a line of print. This printer has means for supplying power sequentially to blocks of said elements, the exact number being determined in accordance with the number of dots to be printed. Of interest is the fact that this known apparatus includes means for detecting the width of the paper moving past said elements and serving to prevent the supply of power to elements positioned adjacent to areas in which said paper is absent. In the particular embodiment of this prior art a handwheel is used to adjust the spacing between two pin tractors to a setting in which they mate with the pin holes in a selected roll width of printing pager. This adjustment rotates a rotary encoder to a setting controlling a de-energizing circuit such that all strobe terminals that are positioned beyond the width of the paper from said roll are not capable of being energized. SUMMARY OF THE INVENTION It is an object of the present invention to provide a thermal printer including a line thermal head and with which a plurality of different sizes of cut paper can be used without any fear that this will result in heating elements in the line thermal head becoming over-heated due to paper widths being used that are less than the width of the line thermal head and due to spaces that may be present between the trailing edge of a leading sheet and the leading edge of the next successive sheet passing under the line thermal head, and thus to prolong the life of the line thermal head. This and other objects are attained by providing a thermal printer including a line thermal head having heating elements, comprising: means for receiving thermal sensitive cut papers having a plurality of sizes; means for selectively feeding said papers from said receiving means to said thermal head; electric power supply means for supplying electric power to said heating elements; and control means for controlling said power supply from said electric power supply means in response to a size of the selected papers, said control means including means for preventing said power supply to a part of said heating elements located in a predetermined width on each side of said line thermal head in response to a width of the selected papers whereby any waste idle heat of said heating elements may be avoided; said control means including detecting means provided on an inlet side of said line thermal head and generating a detection signal for a first period of time commencing from the detection of the leading end of a sheet of paper; and generating a detection signal for a second period of time commencing from the detection of the trailing end of said sheet of paper; said control means being such that it causes energization of said heating elements at the end of said first period of time and causes de-energization of said heating elements at the end of said second period of time. When cut papers are used, paper is not continuously below the thermal head. After a sheet of paper has passed through the thermal head, there is no paper below the thermal head until the next sheet passes through. Therefore, the heating elements of known thermal printers are idle-heated by flowing current unnecessarily to the thermal head until the next sheet of paper is fed below the thermal head. In the present invention, the paper is detected at the inlet side of the line thermal head and the line thermal printer prevents the current from flowing through the line thermal head before a lapse of a predetermined time lag from a time when a detection signal has been generated. Therefore, there is no problem that the heating elements are idle-heated. In the same way, with respect to the trailing edge of the paper, since the thermal printer prevents a current from flowing through the thermal head after another predetermined time lag from a time when the detection signal has been terminated. With respect to the width of the paper, the thermal printer of the present invention prevents current from flowing to those elements of the line thermal head under which there is no paper passing. Therefore, there is no problem that heating elements of the line thermal head will be idle-heated. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Fig. 1 is a diagram showing a thermal printer according to an embodiment of the present invention; and Fig. 2 is a circuit diagram showing primary portion of the embodiment. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows a whole system of a thermal printer according to one embodiment of the present invention. In Fig. 1, reference characters A3, A4, B4, and B5 representative of various sizes denote trays for receiving thermal sensitive cut papers. The trays are inserted into shelves provided in a printer body. Papers A3, A4, B4, and B5 are received in the trays A3, A4, B4 and B5, respectively. Reference characters P1, P2, P3 and P4 indicate limit switches for detecting sizes of the trays inserted in the printer body. If the tray is inserted into the printer body, the associated limit switch is turned on. Feeding rolls r1, r2, r3 and r4 are each provided in contact with a printing surface of the thermal sensitive paper received in each tray. The feeding rolls are operated, respectively, by a signal fed from a control unit 2 (later described) for feeding papers. A pair of nip rolls 10 are provided in the midway of a paper feed path from each tray to the printing position. The nip rolls 10 are provided to operate at once. A pair of the nip rolls 11 are commonly used for each size of paper. The nip rolls 11 are provided on an inlet side of the paper to the printing position. A printing unit includes a line thermal head 12, and a platen roll 13. The line thermal head 12 is normally lifted away from the platen roll 13. The line thermal head 12 is driven so that the head 12 is lowered only when the paper passes below the head 12. A pair of nip rolls 14 are used for discharging the paper. The paper is fed along a paper path 20. A light emitting element 15 and a light receiving element 16 are provided between the nip rolls 11 and platen roll 13. The light emitting element 15 and light receiving element 16 are provided to face each other across the passage 20 so as to detect the existence of the paper. A cable 1 is used for interconnecting a computer and a printer to each other. A computer (not shown) applies signals of the printing pattern. The control unit 2 is used to select one signal for designating the size of the paper and the other signal for a printing pattern image in accordance with the signals fed from the computer. Both kinds of the signals are restricted by the computer so that the size designation signal is first outputted and subsequently the image signal is outputted therefrom. The control unit 2 serves to select the size designation signal, to judge the designated size, to store it, to feed a suitable signal to a paper feed control unit 5, to temporarily drive the feed roll (any one of rolls r1 to r4) corresponding to the signal sensitive paper where the paper reaches the nip roll 10. All of the nip rolls 10, 11, and 14 are simultaneously rotated and the thermal sensitive paper is fed until the paper is discharged by the nip rolls 14. During this operation, when the paper has passed below the element 16, the incident light is blocked off by the paper so that the light receiving element 16 outputs the signal for detecting the paper. The control unit 2 detects an initial rise of the blocking signal. After the predetermined time lag longer than required for the leading edge of the thermal sensitive paper to enter between the thermal head 12 and the platen roller 13, the thermal head 12 is lowered so that the signal is fed to the computer to allow the issuance of the print pattern image signal from the computer. Thereafter, the image signal is sent from the computer so that the thermal head is energized to start the printing operation. When the trailing edge of the paper has passed below the light receiving element 16, no signal for detecting the paper is issued. When this state is detected by the control unit 2, after a predetermined time lag the image signal issuance is stopped to interrupt the energization of the thermal head 12. If the printing is continued, the signal for continuing the printing is sent from the computer. The control unit 2 operates to repeat the above-mentioned operation in accordance with the signal which is sent from the computer. Fig. 2 shows a circuit for setting a heating preventing zone for preventing the thermal head from being heated at both end portions. Reference character T shows respective heating elements each of which is connected to one thermal of a power source through a gate element g. A number of heating elements which are arranged in the unit portion of the width of the to-be-used paper having the smallest size are connected directly to the other terminal of the power source. All the rest elements of both the sides of the thermal head are divided into several groups and are connected in the last-mentioned terminal of the power source through the switch elements S1 to S3 of every group. Reference character B shows a buffer register picking up image signal for one line. A latch circuit R reads the output of the buffer register B by reading pulses. The gate elements g are opened or closed in accordance with the signal of the image. As a result, the current flows to the predetermined heating elements. If the control unit 2 selects and judges the signal for designating the size, switch elements are selectively turned on according to the designated size. For example, if the size is A3, the current flows through all the switch S1 to S3, if the size is B4, the current is shut off only by switch S1, if the size is A4, the current is shut off by switches S1 and S2, and if the size is B5, the current is shut off by all the switches S1 to S3. Such a selected ON/OFF condition is held until a series of the printing operations have been finished. According to the embodiment of the present invention, the tray A3, A4, B4, and B5 can be inserted into the same portion of the printer body as desired. Therefore, the apparatus can be made compact. In this case, a limit switch is available for feeding a suitable signal of the paper size. In the prior art, a continuous thermal sensitive paper has been used by the printer having a line thermal head. Therefore, the size of the printing pattern is often less than the width of paper and thus results in waste paper. However, according to the present invention, cut paper can be used, since the size of the paper can be selected in accordance with the width and length of the printing pattern. Therefore, waste paper can be decreased. It is also possible to reduce maintenance costs for the printer without any fear that the serving life of the thermal head will become short.

Claims: A thermal printer including a line thermal head (12) having heating elements (T), comprising: means (A3, A4, B4, B5) for receiving thermal sensitive cut papers (A3, A4, B4, B5) having a plurality of sizes; means (5) for selectively feeding said papers from said receiving means to said thermal head (12); electric power supply means for supplying electric power to said heating elements (T); and control means (2) for controlling said power supply from said electric power supply means in response to a size of the selected papers, said control means (2) including means for preventing said power supply to a part of said heating elements (T) located in a predetermined width on each side of said line thermal head in response to a width of the selected papers whereby any waste idle heat of said heating elements may be avoided; said control means (2) including detecting means (16) provided on an inlet side of said line thermal head (12) and generating a detection signal for a first period of time commencing from the detection of the leading end of a sheet of paper; and generating a detection signal for a second period of time commencing from the detection of the trailing end of said sheet of paper; said control means (2) being such that it causes energization of said heating elements (T) at the end of said first period of time and causes de-energization of said heating elements (T) at the end of said second period of time. The thermal printer according to the Claim 1, wherein said detector means includes a light source (15) and an optical sensor (16), said light source and said optical sensor being provided facing each other across a path along which said paper sheets are conveyed.

Inventors: OGUSHI AKIRA C O KANZAKI WORKS, UEMURA HISASHI C O KANZAKI WOR, OGUSHI, AKIRA C/O KANZAKI WORKS, UEMURA, HISASHI C/O KANZAKI WORKS
Applicants: KANZAKI PAPER MFG CO LTD, KANZAKI PAPER MFG. CO., LTD.
Application year: 1989
Main classification: B41J2
IPC codes: B41J2, B41J13, B41J29