Thermal Printer

Abstract: The present thermal printer provides a high-speed printing table (7a) and a low-speed printing table (7b) in addition to the thermal printing head (11) containing dot heating elements and a switch for switching the printing mode. The high-speed printing table (7a) contains pulse widths matching to the printing hysteresis of the current to the Nth previous bit and the low-speed printing table (7b) contains pulse widths matching to the printing hysteresis of the current to the Mth previous bit, the M being smaller than the N. The printer further provides a printing control circuit (5) and a central processing unit (1) which operate cooperatively to determine the printing hysteresis of the current to the Nth or Mth previous bit, access to the necessary address locations for the pulse widths according to the printing hysteresis, and allow the current to pass through each necessary dot heating element for the time defined by the accessed pulse width.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal printer, and more particularly to a thermal printer which has at least two printing speed modes, that is, a high-speed mode and a low-speed mode which can be selectively switched and is capable of variably controlling the time when current flows through each required dot heating element contained in a thermal printing head. 2. Description of the Related Art In general, conventional thermal printers include a thermal printing head containing dot heating elements disposed in a vertical line. When printing a character, these dot heating elements are selectively heated to form the character as the thermal printing head travels in the printing direction by a predetermined pitch. Thus, one character is printed by printing the appropriate dots for that character each time the thermal printing head travels by the predetermined pitch. According to this method, some character patterns may allow the same dot heating elements to be repeatedly heated. The surfaces of these heating elements are heated up too much because of the heat retained thereon from being previously heated, and this results in disadvantageously causing the printed characters to have variable density, thereby substantially reducing the character quality and degrading the dot heating elements. To overcome the disadvantage, it is known to continuously monitor the printing histories and variably control the time when current flows through each required dot heating element according to the printing history. Fig.1 is a chart illustrating a table for monitoring the printing histories of the current bit to the fourth previous bit, wherein O denotes printing, X denotes non-printing, and - denotes "Don't Care". Fig. 2 is a chart illustrating pulse widths T1 to T5 defined according to the printing history illustrated in Fig.1 . It is clearly understood from these charts that, as the previous printing becomes further separated in time from the current printing, a larger pulse width is used for the current printing. That is, the larger pulse width increases the time when current flows through each required dot heating element in order to allow for the longer time interval from the current printing, which results in a longer cooling time of the dot heating elements. This achieves substantially uniform density of printed characters. In general, this kind of thermal printer is designed to switch the printing speed between a high-speed printing mode and a low-speed mode. The foregoing table corresponds with the high-speed printing mode in which the dot heating elements do not have enough cooling time because of a shorter printing period. In the low-speed printing mode, however, the dot heating elements energised three bits previously have already cooled down because of the foregoing longer printing period. It is, therefore, unnecessary to consider the printing history before the third previous bit, although, the printing is operated according to the table for the high-speed printing mode. It means that when the printing is done before the third previous bit, the time when current flows through each required dot heating element is made shorter than the appropriate time, resulting in the printed characters being thinner in density. Conversely, if the thermal printer operater according to the table corresponding to the low-speed printing mode, in the high-speed printing mode, excessive heating is applied to the dot heating elements, thereby rapidly degrading the thermal printing head and reducing the life of the head. Published European Patent Application No. EP-A-0 304 916 describes a thermal printing control circuit comprising a first shift register for receiving and storing a series of serial printing image data to be printed by a plurality of thermal print elements, a second register, constituted by a plurality of registers, for storing contents of the first shift register by parallelly and sequentially shifting and receiving the contents thereof, for storing printing history data of a plurality of cycles of the thermal print elements, and a logic circuit for performing a logic operation by using the printing history data of the plurality of cycles of the plurality of thermal print elements, the printing history data being stored in the second shift register, and externally supplied control timing signals and for generating drive signals representing voltage waveforms to be applied in a current cycle to the plurality of thermal print elements. However, there is no suggestion in this document of the provision of two printing modes having different printing speeds. It would therefore be desirable to provide a thermal printer operable in both a high-speed printing mode and a low-speed printing mode which is capable of constantly applying appropriate energy to the dot heating elements for providing printed characters with uniform density when either a high-speed printing mode or a low-speed printing mode is selected. In accordance with the present invention there is provided a thermal printer having a thermal printing head provided with a plurality of dot heating elements and also having a switch for selecting between a high-speed printing mode and a low-speed printing mode, characterised in that said thermal printer comprises: a high-speed printing table containing a respective pulse width for each printing history from a current bit to the Nth previous bit; a low-speed printing table containing a respective pulse width for each printing history from a current bit to the Mth previous bit, where M is smaller than N; means for selecting between said high-speed printing table and said low-speed printing table in dependence on the printing speed mode selected by said switch; means for determining the printing history from the current bit to the Nth or Mth previous bit; means for accessing each necessary address location for the pulse width in the selected table according to the printing history determined by said determining means; means for adjusting an elapse time of a current flowing through each of the necessary dot heating elements in accordance with the time defined by the pulse width contained in the selected table; means for selecting any one of said dot heating elements; and means for supplying a current pulse to each of said selected dot heating elements so that each of said selected dot elements is individually heated by said current pulse supplied, said current pulse supplied to each of said dot heating elements being determined in accordance with said selected table. The supplying means is preferably so arranged that the current pulse supplied to each of the dot heating elements is also determined according to an elapse time since the previous current pulse supplied to that dot heating element. In this case, the supplying means is preferably arranged to supply the current pulse to each of the selected dot heating elements in such a manner that the width of the current pulse is less than the width of a previous pulse, both of the high-speed printing table and the low-speed printing table containing data wherein the pulse widths become longer as the elapse time increases. For the high-speed printing mode, the present thermal printer operates to select the mode, determine the printing history from the current bit to the N-th previous bit, have access to the time-width address location on the high-speed printing table according to the printing hysteresis, and pass current through each necessary dot heating element contained in the thermal printing head for a time defined by the accessed time width. Likewise, for the low-speed printing mode, the present thermal printer operates to select the mode, determine the printing history from the current bit to the M-th previous bit, have access to the time-width address location on the low-speed printing table according to the printing history, and pass current through each necessary dot heating element contained in the thermal printing head for a time defined by the accessed pulse width. As mentioned above, the thermal printer of this invention provides the high-speed printing table and the low-speed printing table which respectively contain the printing histories and the pulse width matched to the printing histories of the high-speed or the low-speed printing mode. The printer operates to select the proper table for the printing speed mode. Hence, it can constantly apply proper energy to the dot heating elements contained in the printing head in a manner to adapt to the various conditions so that it can achieve uniform density for printed characters. Further, the printer can keep the life of the thermal printing head as long as possible, because it serves to positively refrain application of excessive energy which would otherwise degrade the thermal printing head overly. Further advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a chart illustrating a table used in a related art; Fig. 2 is a chart illustrating pulse widths contained in the table used in the related art; Fig. 3 is a block diagram showing a thermal printer according to an embodiment of the present invention; Fig. 4 is a flowchart showing the operation of the thermal printer according to the embodiment; Fig. 5 is a chart illustrating a high-speed printing table; Fig. 6 is a chart illustrating pulse widths contained in the high-speed printing table; Fig. 7 is a chart illustrating a low-speed printing table; and Fig. 8 is a chart illustrating pulse widths contained in the low-speed printing table. DESCRIPTION OF THE PREFERRED EMBODIMENT Hereinafter, a description will be directed to one preferred embodiment of the invention with reference to the drawings. At first, the description is directed to the arrangement of a thermal printer related to the embodiment with reference to Fig. 3. 1 denotes a central processing unit (abbreviated as a CPU), which serves to control the printer in accordance with a program stored in a read-only memory 4. The CPU 1 serves to process text data entered on a keyboard 2 and send the result to a random access memory (abbreviated as a RAM) 3. The RAM 3 temporarily stores the processed text data. Then, in response to a printing command entered on the keyboard 2, the CPU 1 reads the text data from the RAM 3 to a printing control circuit 5. The printing control circuit 5 reads each character contained in the text data, that is, a character pattern at a predetermined row and column from a character generator 6 and then continuously outputs each column data of the character pattern to a head driver 10. The head driver 10 controls a thermal head providing dot heating elements so that it may selectively pass current through the dot heating elements corresponding to the column data. Each time each dot heating element finishes its selective heating, the CPU 1 sends out a motor-driving pulse to a carriage driver 8 through the printing control circuit 5 so that the carriage driver 8 can operate a carriage motor 9 one pitch by one pitch, thereby travelling the thermal head 11 in the printing direction one pitch by one pitch. 7 denotes a table memory for saving a high-speed printing table 7a and a low-speed printing table 7b. One of these tables is selected in response to the specification done on the keyboard 2. The printing control circuit 5 serves to have access to the corresponding address locations contained in the table 7a or 7b. The address locations to be accessed correspond to the printing history of the current bit to the N-th or M-th previous bit and concerns with the pulse widths. Then, the printing control circuit 5 serves to pass current through each dot heating element for the time defined by the pulse width of the accessed address location. Fig. 5 is a chart illustrating the high-speed printing table 7a for managing the printing histories of the current bit to the N-th previous bit. Fig. 7 is a chart illustrating the low-speed printing table 7b for managing the printing histories of the current bit to the M-th previous bit. In both tables, O denotes printing, X denotes non-printing, and - denotes "Don't Care". The pulse widths defined for the printing histories is illustrated in Figs. 6 and 8. For the high-speed printing table 7a, the previous bit from the current bit matches to a pulse width T₁, the second previous bit from the current bit matches to a pulse width T₁ + T₂, the third previous bit from the current bit matches to a pulse width T₁ + T₂ + T₃,... ... the (N-2)th previous bit from the current bit matches to a pulse width T₁ + ... ... + T N-2 , the (N-1)th previous bit from the current bit matches to a pulse width T₁ + ... ... + T N-1 , and the N previous bit from the current bit matches to a pulse width T₁ + ... ... + T N . These pulse widths individually matched to the previous bits are made smaller as the bit are more previous from the current bit. These previous bit number N and their corresponding pulse widths are computed from each printing period defined in the high-speed printing mode. For the low-speed printing table 7b, the arrangement is analogous to that of the table 7a except that the pulse width is represented by t and the most previous bit to be managed is M. N for the high-speed printing mode is larger than M for the low-speed printing mode, because the numbers N and M are computed from the printing period and in the high-speed printing mode, the printing period, that is, a head-cooling period is shorter than that in the low-speed printing mode. In operation, the thermal printer operates on the flow shown in Fig.4. In response to a printing command entered on the keyboard 2, the printing control circuit 5 determines if the high-speed printing mode is specified (step S1). If it is specified, the circuit 5 selects the high-speed printing table 7a from the table memory 7 (step S2). Then, the circuit 5 reads the first column data of the character pattern for the printing character from the character generator 6 (step S3). This first column data corresponds to the heating-commanded dot heating elements contained in the thermal printing head 11. Next, the circuit 5 checks the printing history of the current bit to the Nth previous bit about the heating-commanded dot heating elements in the first column data (step S4). The circuit 5 has access to the address locations for pulse widths on the high-speed printing table 7a according to the checked printing history (step S5). It controls the head driver 10 to allow the current to pass through each heating-commanded dot heating element for the time defined by the pulse width of the accessed address location (step S6). Finally, the thermal head 11 serves to thermally record the dots. After the thermal recording, it is determined if all the characters to be printed are printed (step S7). If not, the printing control circuit 5 reads the next column data of the character pattern (step S8). Then, the process jumps to the step S4, where the operation is executed from the steps S4 to S8 until the printing is finished. If, on the other hand, the low-speed printing mode is specified, like the foregoing high-speed printing mode, the printing control circuit 5 selects the low-speed printing table 7b from the table memory 7 (step S12) and then reads the first column data of the character pattern for the printing character from the character generator 6 (step S13). This first column data corresponds to the heating-commanded dot heating elements contained in the thermal printing head 11. Next, the circuit 5 checks the printing current bit to the M previous bit about the heating-commanded dot heating elements of the first column data (step S14). The circuit 5 has access to the address locations for pulse widths on the low-speed printing table 7b according to the checked printing history (step S15). It controls the head driver 10 to allow the current to pass through each heating-commanded dot heating element for the time defined by the pulse width of the accessed address location (step S16). Finally, the thermal head 11 serves to thermally record the dots. After the thermal recording, it is determined if all the characters to be printed are printed (step S17). If not, the printing control circuit 5 reads the next column data of the character pattern (step S18). Then, the process jumps to the step S4, where the operation is executed from the steps S4 to S8 until the printing is finished. As described above, the thermal printer of the present invention provides the high-speed printing table and the low-speed printing table respectively having the printing history and the pulse width matching to the high-speed and low-speed mode printing periods, so that it can select one of these tables according to the printing speed mode. Hence, the printer is capable of constantly applying proper energy to each dot heating element of the printing head in a manner to adapt to the various conditions, resulting in being able to print high-quality, that is, uniform-density characters as well as positively refrain application of excessive energy which would otherwise degrade the thermal printing head overly.

Claims: A thermal printer having a thermal printing head (11) provided with a plurality of dot heating elements and also having a switch (2) for selecting between high-speed printing mode and a low-speed printing mode, characterised in that said thermal printer comprises: a high-speed printing table (7a) containing a respective pulse width for each printing history from a current bit to the Nth previous bit; a low-speed printing table (7b) containing a respective pulse width for each printing history from a current bit to the Mth previous bit, where M is smaller than N; means (5) for selecting between said high-speed printing table (7a) and said low-speed printing table (7b) in dependence on the printing speed mode selected by said switch (2); means (5) for determining the printing history from the current bit to the Nth or Mth previous bit; means (5) for accessing each necessary address location for the pulse width in the selected table (7a or 7b) according to the printing history determined by said determining means (5); means (5) for adjusting an elapse time of a current flowing through each of the necessary dot heating elements in accordance with the time defined by the pulse width contained in the selected table (7a or 7b); means (5) for selecting any one of said dot heating elements; and means (10) for supplying a current pulse to each of said selected dot heating elements so that each of said selected dot elements is individually heated by said current pulse supplied, said current pulse supplied to each of said dot heating elements being determined in accordance with said selected table (7a or 7b). The thermal printer according to claim 1, wherein said supplying means (10) is so arranged that said current pulse supplied to each of said dot heating elements is also determined according to an elapse time since the previous current pulse supplied to that dot heating element. The thermal printer according to claim 2, wherein said supplying means (10) is arranged to supply said current pulse to each of said selected dot heating elements in such a manner that the width of said current pulse is less than the width of a previous pulse, both of said high-speed printing table (7a) and said low-speed printing table (7b) containing data wherein the pulse widths become longer as said elapse time increases.

Inventors: SUGIMOTO YUKIHIKO, SUGIMOTO, YUKIHIKO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1990
Main classification: B41J2
IPC codes: H04N1, B41J2