Thermal printer

Abstract: In a thermal printer for thermo-sensitive recording, a plurality of heating resistors (2) are electrically divided into N units each having M heating resistors, and N driver circuits (3), N latch circuits (4) and N shift registers (5) which are interconnected in tandem are respectively provided in association with the N units of M heating resistors. Dot data signals and associated hysteresis correction signals are collectively applied to the shift registers so that a pulse for print data and a pulse for hysteresis correction data can be applied continuously for printing.

Description: This invention relates to a thermal printer having a thermo-sensitive recording system. Thermo-sensitive recording is suited to efficient maintenance and has therefore been utilised in many terminal printers including facsimiles. In particular, thermo-transfer type thermo-sensitive recording has recently been developed, making it possible to perform polychrome or full colour recording. Conventionally, a thermal printer is controlled for thermo-sensitive printing as will be described below with reference to Fig. 1. The thermal printer as diagrammatically shown in Fig. 1 comprises a plurality of heating elements 31 in the form of heating resistors, driver circuits 32 for powering the heating elements 31 to heat them, a latch circuit 33 for applying dot (heating element) data signals to the driver circuits 32, and a shift register 34 for receiving a print data signal containing the dot data signals and applying the dot data signals to the latch circuit 33. In operation, print data signals for one line are first inputted to the shift register 34. The latch circuit 33 then responds to a strobe signal to latch the print data signal. Subsequently, enable signals are selectively applied to the driver circuits at different phases or timings so that the driver circuits are sequentially actuated to feed currents to the heating elements. As a result, the heating elements are heated in accordance with the dot data signals to perform printing. During the printing operation, correction data signals in association with the respective heating elements 31 are applied to the shirt register 34. A correction data signal is prepared on the basis of a dot data signal for the preceding lane (a hysteresis correction data signal) and a neighboring dot correction data signal, and is used in the same manner as in the case of the above printing operation to correct printing. Problems are encountered in the conventional thermal printer as will be described below with reference to Fig. 2. In high-speed printing, the amount of energy applied for printing is controlled in accordance with contents or the hysteresis correction data signal and neighboring dot correction data signal. As an example, Fig. 2 illustrates a timing chart of one-line printing which is performed in 6.15 msec by using a head of 8 dots/mm density for A4 size paper when the head driving frequency is 1 MHz and 1568 dots (heating elements) of one line are divided into 7 blocks each of which is actuated by an enable signal. The driver circuit is actuated by an enable pulse 1 ○ so as to respond to dot data signals and by an enable pulse 2 ○ to respond to correction data signals, with the result that the two enable pulses can not be applied continuously. This is because the dot data signals for one block must be transferred within a time of 1568 µsec. Since, in the conventional thermal printer, paper feeding is effected in timed relationship with each enable signal, the discontinuity of the two pulses 1 ○ and 2 ○ results in a shear in printing. In addition to the above-mentioned improper application of the hysteresis correction data signal, correct controlling of applied energy could not hitherto be obtained when head temperature and ambient temperature varied. For these reasons, the amount of energy applied for printing can not be controlled properly and accurate printing can not be obtained with the conventional thermal printer. This invention intends to eliminate the above disadvantages and it is a major object of this invention to provide a thermal printer capable of properly controlling the amount of energy applied for printing. Another object of this invention is to provide a thermal printer capable of correctly controlling the amount of energy applied for printing when head temperature and ambient temperature vary with time. According to an embodiment of the invention, in a thermal printer having a thermal head including a plurality of heating elements or dots in the form of heating resistors which are arranged in line on an insulating substrate and which are electrically divided into N units or blocks each having M heating elements, means for selectively powering the heating elements to heat the heating resistors for printing and means for producing enable signals for determining an amount of heating energy to be supplied to said heating elements of said thermal head, the thermal printer includes a head temperature detection sensor for detecting temperatures of said thermal head, first control means responsive to an output signal from said head temperature detection sensor for controlling the pulse width of said enable signals; an ambient temperature detection sensor for detecting ambient temperatures at a point apart form said thermal head, and second control means responsive to an output signal from said ambient temperature detection sensor for controlling, in parallel with the controlling operation of said first control means, the pulse width of said enable signals to change said pulse width by a predetermined amount in accordance with the ambient temperatures detected by said ambient temperature detection sensor and without being affected by an output signal from said head temperature detection sensor. Figure 1 is a block diagram illustrating a thermal head of a conventional thermal printer. Figure 2 is a timing chart for explaining the operation of the Fig. 1 head. Figure 3 is a block diagram schematically showing a circuit construction of a thermal printer. Figure 4 is a diagram useful in explaining the operation of an essential part of the Fig. 3 thermal printer. Figure 5 is a block diagram schematically showing a thermal printer according to an embodiment of the invention. Figure 6 is a graph showing commanded controlling curves. Figure 7 is a timing chart useful in explaining the operation of the Fig. 5 thermal printer. The invention will now be described by way of example with reference to the accompanying drawings. The circuit of a thermal printer is diagrammatically illustrated in Fig. 3. The thermal printer comprises a thermal head 1 including a plurality of heating elements 2 in the form of heating resistors which are arranged in line on an insulating substrate and which are electrically divided into N units or blocks each having M heating elements. N driver circuits 3 respectively provide in association with the N units of M heating elements 2, N latch circuits 4 respectively provided in association with the N driver circuits 3 and connected in common to receive a strobe signal, and N shift registers 5 respectively provided in association with the N latch circuits 4 and connected in common to an input line. In the thermal head 1, the heating elements 2 are connected in common, at one end, to a printing power supply and are respectively connected, at the other end, to an output terminals of the driver circuits 3 are connected to output terminals of the latch circuits 4, and input terminals of the latch circuits 4 are connected to output terminals of the shift registers 5. Print data signals are applied to the respective shift registers 5 in parallel with the corresponding latch circuits 4. When enable signals are applied to driver circuits 3, each driver circuit 3 passes the print data signals to provide currents which power the corresponding heating elements 2 so that the corresponding heating resistors are selectively heated to perform thermal printing. Independent enable signals are applied at different phases to the respective driver circuits 3 to control the operation thereof in succession. The thermal printer comprises a print control circuit 6 including a hysteresis correction circuit 7 and a neighbouring dot correction circuit 8. The hysteresis correction circuit 7 comprises a data selector 9 for selectively supplying a print data signal and a correction data signal to the shift registers 5, an AND gate 10 having an output terminal connected to one input terminal of the data selector 9, and an invertor 11 connected to one input terminal of the AND gate 10. The neighbouring dot correction circuit 8 comprises an OR gate 12 having an output terminal connected to the invertor 11 of the hysteresis correction circuit 7, a shift register 13 of two bits having an output terminal connected to one input terminal of the OR gate 12, an AND gate 14 having an output terminal connected to the other input terminal of the OR gate 12, and a shift register 16 for applying signals to input terminals of the AND gate 14 directly and through an invertor 15. In the print control circuit 6, the other input terminal of the data selector 9 included in the hysteresis correction circuit 7 is connected to the other input terminal of the AND gate 10 and to an output terminal, connected to the invertor 15, of the shift register 16 included in the neighbouring dot correction circuit 8. One input terminal of the AND gate 14 is connected directly to the input of the shift register 16. The thermal printer also comprises a print data receiver 17 including three line buffer memories 18, 19 and 20, a read buffer selector 21 and a write buffer selector 22. In the print data receiver 17, any one of the three line buffer memories 18, 19 and 20 is used to receive data for the succeeding line cyclically while the remaining two line buffer memories are being used for printing. More particularly, when reception and printing have been completed for print data in connection with a set of lines, the role of the memories is switched to carry out reception and printing in connection with a set of succeeding lines, as described in Table 1. In the thermal printer constructed as above, a print data signal applied to the read buffer selector 21 of print data receiver 17 is sent to the neighbouring dot correction circuit 8 of print control circuit 6 through the line buffer memories 18, 19 and 20 and write buffer selector 22. In one operational mode of the print control circuit 6, the neighbouring dot correction circuit 8 is adapted to control printing energy applied during printing of a particular dot data signal of a print data signal for the current line in accordance with dot data signals in the neighbourhood of a dot data signal contained in a print data signal for the current line and in accordance with the dot data signal in the preceding line corresponding to the particular dot data signal. In a printer having a printer head of the line type as in the case of the present invention, dot data signals are difficult to transfer each time that individual dots are printed. Therefore, data signals for two lines are transferred and store in advance and a dot data signal for one dot or heating element 2 of the preceding line is applied once or twice for printing in order to control energy applied to that heating element. Specifically, in the circuit of Fig. 3, a high level pulse is used as a dot data signal for printing a "white" dot and a low level pulse is used as a dot data signal for printing a "black" dot. Thus, when two neighbouring dot data signals on either side of a dot data signal of the print data signal for the current line are "white" or high-level dot signals, the neighbouring dot correction circuit 8 operates to render "white" or high the corresponding dot data signal for the current line to be delivered by the circuit 8, thereby disabling the hysteresis correction circuit 7. For example, when two neighbouring dot data signals on either side of a "black" dot data signal of a print data signal for the current line are "white" and "H, H, L, H, H" are arranged in line in the shift register 16, the neighbouring dot correction circuit 8 renders "white" the corresponding dot data signal for the preceding line to cause the hysteresis correction circuit 7 to produce a "black" hysteresis correction signal, thereby ensuring that one vertical line can be printed clearly or sharply. In the other operational mode of the print control circuit 6, the hysteresis correction circuit 7 operates to control energy applied to a heating element 2 during printing of the current line, in accordance with a dot data signal for the corresponding heating element for the preceding line. More particularly, when a "black" dot data signal occurs in the preceding line, residual heat remains in the corresponding heating element. Accordingly, unless energy applied to that heating element during printing of the current line is reduced by an amount corresponding to the residual heat, excessive energy is applied, resulting in improper density printing. To avoid this disadvantage, the hysteresis correction circuit 7 controls energy applied to a heating element during printing of the current line in accordance with energy applied to the corresponding heating element during printing of the preceding line, as indicated in Table 2. As in the first operational mode of the hysteresis correction circuit 7, a dot data signal for one dot or heating element is applied once or twice for printing in order to control energy applied to that heating element. A hysteresis correction data signal (dot data signal additionally applied to a heating element to perform hysteresis correction) is indicated in Table 3. dot data signal for the current line dot data signal for the preceding line hysteresis correction signal white white white white black white black white black black black white The print data signal thus corrected by the hysteresis correction circuit 7 is applied to the shift registers 5 of thermal head 1. Then, dot data signals are applied from each shift register 5 to the associated driver circuit 3 through the associated latch circuit 4. Each driver circuit 3 is controlled by the corresponding enable signal such that the dot data signals are passed to provide currents which power the corresponding heating elements 2. In this manner, the corresponding heating elements are selectively heated to perform thermal printing. To describe the operation of the above embodiment in greater detail, the dot data signals for the current line are transferred with transfer of necessary hysteresis correction data signals following, in units of one heating element unit or block. The independent enable signals are then applied sequentially at differed phases or timings to the respective driver circuits 3 during an interval of time which is obtained by dividing the time required for printing one line and which is sufficient for the dot data signals and following hysteresis correction signals to pass through each driver circuit. Accordingly, in one heating element unit or block, any one dot data signal is continuous to the associated hysteresis correction signal and printing of each dot can be performed properly without a shear in printing. One enable signal as applied to one heating element unit is illustrated in Fig. 4. For simplicity of illustration, time for passage of dot data signals is totalised within duration A and time for passage of hysteresis correction signals is totalised within duration B. The value of duration B depends on temperatures of the printer head and is controlled such that proper amounts of energy can be applied to the printer head. For example, for printing a sheet of A4 size paper in one minute, a printer head of 8 dots/mm density is used which is driven at a driving frequency of 1 MHz and which has 1568 dots divided into 7 heating element (dot) units or blocks, the total duration C is 700 µsec as a maximum because 6.15 msec of time for printing one line minus 1568 µsec is shared by the 7 heating element blocks as will be seen from Fig. 2 and consequently about 654 µsec can be allotted to each heating element block. In this instance, the duration A is 250 µsec as a minimum because each heating element block has 224 dots and dot data signals therefor are all transferred in 224 µsec. In this manner, the dot data signals can be confined within 250 µsec of the minimum duration A and the hysteresis correction signals can be confined within the remaining duration B to ensure continuous printing of the print data and hysteresis correction data, thereby performing printing without shear. To specifically describe the first operational mode of the print control circuit 6 with reference to Fig. 3, when two neighbouring dot data signals on either side of a "black" data dot signal of the printed data signal for the current line are "white" and "H, H, L, H, H" are arranged in line in the shift register 16, the input signals to the AND gate 14 are all high and the AND gate 14 delivers a high output signal to the OR gate 12. Consequently, the data signal for the preceding line to be applied to the hysteresis correction circuit 7 becomes high or "white" irrespective of the level of the data signal for the preceding line inputted to the neighbouring dot correction circuit 8. This permits the hysteresis correction signal to be "black" when one vertical line is to be printed in order to supply sufficient energy to the corresponding heating element 2, thereby ensuring that the one vertical line can be printed sharply. The amount of printing energy should also be controlled by reflecting temperatures. Conventionally, in this type of thermal printer, the applied energy is controlled, in one way, by consulting only head temperature information produced from a thermistor built in the thermal head or is controlled in another way by consulting a result of calculation of detected values of head temperature and ambient temperature which change with time. However, when applied energy is controlled in the former way, temperature of the printer such as a platen and temperature of the recording medium are not taken into consideration and as a result, print quality differs in accordance with the difference between printer and medium temperatures. When the applied energy is controlled in the latter way, errors in detection of the head temperature and errors in calculation prevent the applied energy from being set correctly. Fig. 5 illustrates an embodiment of the invention which can solve the above problems. Referring to Fig. 5, a thermal head 23 has a built-in thermistor 24 for detection of head temperature. The thermistor 24 produces an output signal which is applied to a pulse generator 25, and a pulse signal of a proper width corresponding to the head temperature is generated from the pulse generator 25. The pulse signal is applied to the output control input terminal of the three-state buffer 26 so as to determine powering duration for a block of heating elements 30 selected by an enable signal delivered by a controller 27. On the other had, a thermistor 28 for detection of ambient temperature is disposed near an atmospheric air in-take port and produces an output signal which is applied through an ambient temperature read circuit 29 to the controller 27 to provide ambient temperature information to the same. Fig. 6 graphically shows an example of a commanded control characteristic in which, for the purpose of providing in accordance with the ambient temperature but independent of the head temperature a predetermined difference in the amount of energy, control curves are plotted by using ambient temperatures as the parameters to be translated with respect to each other in the direction of the ordinate representing applied energy. These control curves can be implemented at timings as illustrated in Fig. 7. Thus, when a trigger signal is applied, the pulse generator 25 generates a pulse signal of a pulse width corresponding to a head temperature. On the other hand, the controller 27 calculates an amount of translation required for a control curve on the basis of information produced from the ambient temperature read circuit. In accordance with the translation amount, the enable signal is retarded with respect to the trigger signal to cause a pulse to fall at a point A, B or C as shown in Fig. 7. The three-state buffer 26 then responds to the output signal from pulse generator 25 as determined by the head temperature alone, and the enable signal retarded in accordance with the ambient temperature supplies to the heating elements 30 a pulse providing energy differing by a predetermined amount in accordance with the ambient temperature but independent of the head temperature. In this manner, the width of the applied pulse can be controlled by the ambient temperature.

Claims: A thermal printer having a thermal head (23) including a plurality of heating elements (30) in the form of heating resistors which are arranged in line on an insulating substrate and which are electrically divided into N units of M heating elements, said heating elements (30) being selectively powered to heat said heating resistors for printing, and means (27) for producing enable signals for determining an amount of heating energy to be supplied to said heating elements (30) of said thermal head (23), wherein the thermal printer includes a head temperature detection sensor (24) for detecting temperatures of said thermal head (23); first control means (25, 26) responsive to an output signal from said head temperature detection sensor (24) for controlling the pulse width of said enable signals; an ambient temperature detection sensor (28) for detecting ambient temperatures at a point apart form said thermal head (23), and second control means (29, 27) responsive to an output signal from said ambient temperature detection sensor (28) for controlling, in parallel with the controlling operation of said first control means (25, 26), the pulse width of said enable signals to change said pulse width by a predetermined amount in accordance with the ambient temperatures detected by said ambient temperature detection sensor (28) and without being affected by an output signal from said head temperature detection sensor (24).

Inventors: ITOH TAICHI, NISHIYAMA RYUJI, NOMURA YOSHIKAZU, TSURU YOSHIKAZU, ITOH, TAICHI, NISHIYAMA, RYUJI, NOMURA, YOSHIKAZU, TSURU, YOSHIKAZU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1988
Main classification: B41J2
IPC codes: B41J2