THERMAL HEAD TEMPERATURE CONTROL DEVICE

Abstract: The temperature of the printing elements in a thermal printing head is controlled by controlling the electrical power supply to the thermal head, and thereby the amount of heat that is generated, said control also taking account of the different printing requirements of ordinary characters, bar codes, and so forth, data on the various such types of printing being stored in electronic memories for reference thereto for the said temperature control. The result is printing that is sharper and more uniform.

Description: This invention relates to a thermal head temperature control device of the type as indicated in the precharacterizing portion of Claim 1. Such a device is known from EP-A2-0 068 702. The temperature of the heating elements of a thermal head at the start of printing differs from that during the course of the printing, even though the amount and duration (the period during which current flows) of the supply of electrical power may be the same in both cases. In particular, because the temperature of the heating element drops following the start of printer operation, the heating temperature becomes very low. Also, what is the optimum temperature for printing by the heating element differs depending upon whether what is being printed is in the form of ordinary characters or whether it is in the form of a bar code or the like. In order to achieve optimum printing under such varied conditions, conventionally the following techniques have been employed. First, in order to eliminate the difference between the temperature at the start of the printing and that during the course of the printing, there in known the preheating of the thermal head upon switching on the power supply to the printer so as to maintain the starting temperature at approximately the same level as the temperature during the course of the printing. While with this technique the temperature at the start of printing will be approximately the same as the temperature during the course of printing, it is impossible to control the temperature in accordance with the nature of the information being printed by the heating element. In order for the heating element to be at a temperature that is suitable for the type of information being printed, there are known techniques for appropriately varying the supply of electricity depending on whether, for example, what is being printed are ordinary characters or bar codes. Such techniques employ power supply circuitry which can generate two or three different voltages, so that the flow of current through the heating element is varied by switching circuits to vary the voltage in accordance with the nature of what is being printed. While this permits some measure of heating temperature control to take account of the type of information being printed, because the switching is limited to two or three different voltages, fine control of temperature during printing is not possible. Taking for example the bar code I shown in Fig. 1, when the printing is proceeding in the direction of the arrow, the temperature of each heating element rises as the printing progresses, giving rise to the problem of a gradual thickening towards the end of the bar. Another technique is to detect the temperature of the thermal head and input to the power supply circuit a signal which corresponds to the detected temperature, so that the power supply supplies the thermal head with an amount of electrical power appropriate to the current temperature as based on the input signal. This allows a more precise level of control, as the right amount of power in view of the current temperature is supplied. However, what is detected is not the change in temperature of each heating element. Instead, detection of the heat of the heating elements in effected via the head board and the like, and as such it is difficult to detect temperature changes precisely, and in addition to this, fine temperature control is impossible because the detection of changes in temperature is delayed by the time it takes the heat to be conducted by the head board and the like. This technique does not therefore present an easy solution to the said problem of the bars of the bar code gradually thickening. Most pertinent EP-A2-0 068 702 discloses a thermal printer in which the amount of energy to be supplied to each heating means in the next printing cycle is determined by the amount of energy which was supplied to said heating means during the previous printing cycle. This means, that then the amount of energy in the previous printing cycle varies, the amount of energy in the subsequent printing cycle will be varied. Therefore, according to the above prior art, the outstanding data are variable ones amended or calculated each time the printing conditions have changed. GB-A-2 109 967 teaches that the length or duration of a coded image signal is measured to determine pulse duration, which also results in that, when the length of the coded image signal varies, the pulse duration is varied. Both of the afore-mentioned prior art devices, although they are provided with data memory means, determine the printing energy of pulse duration based on the conditions of the previous printing cycles and use the data memory means to calculate, determine, hold and renew the determined data. Therefore, these prior art devices suffer from a comparatively time-consuming printing speed owing to the real-time processes used to determine the amount of energy to be supplied to the thermal head. It is an object of the present invention to provide a thermal printer of the type as indicated in the precharacterizing portion of Claim 1 which has a comparatively improved printing speed, comparatively more simple circuits and improved reliability with less error. The above object is achieved by the features of the inventive thermal head temperature control device as recited in the characterizing portion of Claim 1. A preferred embodiment of the instant invention is indicated in Claim 2. The primary and fundamental difference between the instant invention and the afore-mentioned prior art devices is that, in the present invention, the data corresponding to the electrical power to be supplied to the thermal head are predetermined, fixed data held beforehand in a power supply data memory, whereas, in the prior art, the outstanding data are variable ones amended or calculated every time the printing conditions have changed. The invention will now be described with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a plane view of an example bar code printed on a continuous strip; Fig. 2 is a block view of the circuit of the overall device; Fig. 3 is an explanatory view showing the specifics of the power supply data memory section; Fig. 4 is a circuit diagram showing details of the thermal head 14; Fig. 5 is a flowchart showing the temperature control process; and Fig. 6 is a flowchart showing the temperature control process of another embodiment of the device according to this invention. DESCRIPTION OF THE PREFERRED EMBODIMENT In Fig. 2, a CPU 1 controls the overall functions, and connected to the CPU 1 is a bus 2 to which are connected a program ROM 3, a printing pattern ROM 4, a power supply data ROM 5, and a printing data RAM 6. Stored in the program memory 3 are the functional sequences for the overall device, the CPU 1 performing the overall control of these function sequences accordingly. The printing pattern memory 4 contains codes corresponding to characters, symbols, bar codes and print patterns. The power supply data memory 5 contains data on printing conditions corresponding to the electrical power to be supplied to a thermal head 14 which is described later. More specifically, as shown in Fig. 3, power supply adjustment data 5a for bar codes, 5b for ordinary characters, power supply data 5c for bar codes and 5d for ordinary characters are stored sequentially at specific addresses. Also, encoded printing data input from an input device (not shown) are stored in the printing data memory 6. The bus 2 further has an I/O port 10 connected thereto, and a D/A conversion circuit 11 is connected to an output terminal of the I/O port 10. The D/A conversion circuit 11 outputs as a current signal digital values input from the CPU 1 via the I/O port 10. Thus, the output terminal of the D/A conversion circuit 11 is connected to the inversion input terminal of an amplifier 12. The non-inversion terminal of the amplifier 12 is grounded, and the output terminal is connected to the input terminals of analogue switches SW1, SW2. A resistor R1 is connected to the feedback loop. The amplifier 12 performs amplification and current - voltage conversion, and the flow of current through the resistor R1 corresponding to the output from the D/A conversion circuit 11 results in an electrical potential at the output terminal that corresponds to the voltage drop produced by the resistor R1. The output terminal of the analogue switch SW1 is connected to the power supply circuit 13. The power supply circuit 13 is for supplying electrical power to the thermal head 14, and is controlled by the potential appearing at the output terminal of the amplifier 12, the electrical power that is supplied corresponding to this potential. That is, the higher the potential, the more the supply of electrical power, and the less the potential, the less the supply of electrical power. The thermal head 14 is also connected to I/O port 10. The numeral 15 indicates a temperature detector for detecting the temperature of the thermal head 14, constituted so that a signal the potential of which increases as the temperature increases and decreases as the temperature decreases is input to the non-inversion input terminal of an amplifier 16. A resistor R2 is connected to the feedback loop of the amplifier 16, and to the inversion terminal is connected a variable resistor R3 one end of which is grounded. The gain can be varied by changing the resistance of the variable resistor R3. The output terminal of the amplifier 16 is connected to the non-inversion terminal of a comparator 17, and the inversion input terminal of the comparator 17 is connected to the output terminal of an analogue switch SW2 and the output terminal is connected to the I/O port 10. The control terminals of the analogue switches SW1 and SW2 are each connected to the I/O port 10, the switches being set to operate in accordance with commands from the CPU 1. The thermal head 14 is comprised of a shift register SR and n heating circuits T₁, T₂, T₃,...,T n . The heating circuits T₁, T₂, T₃,...,T n have AND gates G₁, G₂, G₃,...,G n , transistors Tr₁, Tr₂, Tr₃,...,Tr n , and heating elements r₁, r₂, r₃,...r n . The shift register is for storing one dot-line of printing data; data input terminal DI and clock input terminal CLK are connected to I/O port 10, and each data output terminal DO is connected to one of the input terminals of the AND gates G₁, G₂, G₃,...,G n . The other input terminal of each of the AND gates G₁, G₂, G3,...,G n is connected to the input terminal of I/O port 10, and each of the output terminals is connected to the base of the corresponding transistor Tr₁, Tr₂, Tr₃,...,Tr n . The emitter of each of the transistors Tr₁, Tr₂, Tr₃,...,Tr n is grounded, and the collectors are connected to one side of the corresponding heating elements r₁, r₂, r₃,...r n , and the other side of each of the heating elements r₁, r₂, r₃,...r n is connected to a common terminal y to which electrical power is supplied from the power source circuit 13. The functions of the thermal head temperature control device constituted thus will now be described with reference to Fig. 5. In this embodiment of the present invention, for the explanation the example of the continuous strip shown in Fig. 1 on which a number of labels L are printed will be used. Also, it is to be assumed that the type of information to be printed and the number of printings and the like have been input by a keyboard or some such input means and stored in the data memory 6. First, initialization takes place in step S1. In the initialization process adjustment data E and the current end-of-printing number f are zeroed, the number M of dot-lines to be printed is set to "1", analogue switch S1 is set to ON and analogue switch S2 is set to OFF. In step S2 it is ascertained whether what is to be printed is a bar code, and if it is, reference is made to the power supply data 5c number M for bar codes stored in the power supply memory 5 as power supply data D. That is, as the current dot-line number M is number 1, data corresponding to dot-line 1 is referred to. If what is to be printed is not a bar code, reference is made to the power supply data 5d for ordinary characters, with no relation to the number of dot-lines. In step S5, the digital value V to be supplied to the D/A conversion circuit 11 is computed. This digital value V is the result of deducting adjustment data E from the power supply data D referred to in step S3 or S4, but as at this point the adjustment value E is zero, digital value V is equal to power supply data D. Next, in step S6, the calculated digital value V is supplied to the D/A conversion circuit 11, and us a result of this, D/A conversion circuit 11 acts to cause an amount of current corresponding to the input digital value V to flow in the feedback loop of the amplifier 12, with the resistor R1 producing a voltage drop. Because the inversion input terminal of the amplifier 12 is at zero volts and the resistance of the resistor R1 is constant, a potential appears at the output terminal of the amplifier 12 that is the product of the current flowing in the feedback loop and the resistance of resistor R1. When this potential is input to the power supply circuit 13, the power supply circuit 13 supplies an amount of electrical power corresponding to the said potential to the common terminal y of the thermal head 14. Then, in step S7, an amount of travel and printing that correspond to the first dot-line on the first of the labels L are performed. This printing is carried out in accordance with the one dot-line print pattern data entered in the shift register SR, by reference to the printing pattern memory 4 which is based on printing data that has been input into the printing data memory 6. The print pattern data in the shift register SR is output from the AND gates G₁, G₂, G₃,...,G n for the duration of the time the print command signal is being applied to the common terminals of the AND gates G₁, G₂, G₃,...,G n . In the heating elements r₁, r₂, r₃,...r n corresponding to the AND gates G₁, G₂, G₃,...,G n outputting the signal, a current flows which corresponds to the potential produced across terminals X and Y. As a result, the heating elements r₁, r₂, r₃, ... r n through which the current is passed heat up, and a single dot-line of the pattern is printed on the label L. On completion of the printing of the one dot-line and the accompanying travel, step S8 is carried out, whereby the number of dot-lines M to be printed is incremented by one. In step S9 it is determined whether all of the dot-lines have been printed. If all of the dot-lines have not been printed, the steps S2 through S9 are repeated. Here, because especially high precision is not required if what is being printed are ordinary characters, reference is made continuously to the same power supply data D from the ordinary character supply power data 5d. In the case of a bar code, however, the reference is to power supply data D at bar code supply data 5c corresponding to the number of dot-lines M currently to be printed, and a potential that is based on this data is supplied to the D/A conversion circuit 11. If it is determined in step S9 that an amount of printing equivalent to one label L has been completed, in step S10 the number of dot-lines to be printed is set to one, and at step S11 one is added to the number of completed labels f. In step S12, it is determined whether the current number of printing-completed labels f has reached the required number of printed labels F as preset in the printing data memory 6. If the preset number F has not been reached, the program moves on to step S13, while if F has been reached, all operations cease. In step S13, it is determined whether the number of printing-completed labels f is an integer multiple of a constant P derived from a positive natural number stored beforehand in power supply data memory 5. If f is not an integer multiple of P, steps S2 through S13 are repeated until f becomes an integer multiple of P. When f is an integer multiple of P, analogue switch SW1 is set to OFF and analogue switch SW2 is set to ON in step S14, following which the D/A conversion circuit 11 is set to the specified digital value W, in step S15. This digital value W is preferably very small. Because the effect of the D/A conversion circuit 11 is to cause a flow of an amount of current in the feedback loop of the amplifier 12 that corresponds to the digital value W, a voltage drop corresponding to the said current is produced in resistor R1. The result is that at the output terminal of the amplifier 12 there appears a potential which corresponds to the digital value W set on the D/A conversion circuit 11, and this potential is applied, via the analogue switch SW2, to the inversion input terminal of the comparator 17. The comparator 17 compares the potentials of the output terminals of amplifiers 12 and 16 and outputs a signal which corresponds to the polarity of the higher potential. In step S16 it is determined whether the signal output by the comparator 17 is negative, but at the initial stages the comparator 17 outputs a positive signal, because the digital signal set on the D/A conversion circuit 11 is very small and the potential appearing at the output terminal of the amplifier 12 is low. This being the case, by repeating steps S16 and S17 at that point, the digital value W setting of the D/A conversion circuit 11 increases in increments of t until the potential at the output terminal of the amplifier 16 becomes higher than the potential at the output terminal of amplifier 12 and the output of the comparator 17 inverts to negative polarity. At the point when the comparator 17 output goes negative, the digital value W being supplied by the D/A conversion circuit 11 is stored at a specific address in the printing data memory 6, and this value is determined as corresponding to the current temperature of the thermal head 14. In step S19 it is again determined whether the information being printed is in bar code form, in which case in step S20 reference is made to the bar code power supply adjustment data 5a, in the power supply data memory 5, corresponding to the digital value W, and the reference adjustment data E is stored at a specific address in the printing data memory 6. Preferably it is determined what multiple of the constant P the number of printing-finished labels f is, for both bar codes and ordinary characters, and the adjustment data E varied accordingly. More specifically, the value of the adjustment data E gradually becomes larger the higher the multiple. When the adjustment data E has been set, in step S22 the analogue switch SW1 is set ON and the analogue switch SW2 is set OFF, following which the program reverts to step S2. By thus repeating steps S2 through S22, the specified printing is carried out on F number of labels L. Also, in step S13, after it has been determined that the number of printing-completed labels f is a multiple of constant P, that is, after reference has been made to adjustment data E, in order to set the potential to be supplied to the power supply circuit 13, the result of deducting the adjustment data E from the power supply data D is used as the digital value V set into the D/A conversion circuit 11. Thus, with respect to rough temperature changes in the thermal head 14, the power supply to the thermal head 14 is controlled by reference to the bar code power supply adjustment data 5a or the ordinary character power supply adjustment data 5b in accordance with the temperature as detected by the temperature detector 15. With respect to changes in the thermal head temperature for each dot-line, the power supply to the thermal head 14 is controlled by reference to the bar code power supply data 5c for each dot-line. An explanation will now be given with respect to an embodiment in which the detection of the temperature of the thermal head 14 is not performed by means of the temperature detector 15. In such a case, the temperature detector 15, the amplifier circuit 16, comparator 17, the analogue switches SW1 and SW2 and the resistors R2 and R3 are not required, but as the other portions are the same as the embodiment shown in Fig. 2, the circuit diagrams thereof are omitted. The functioning in this embodiment is as shown in Fig. 6; as steps S2 through S13 have already been explained with reference to Fig. 5 and are the same for this embodiment, explanation of these steps is here omitted, so here only the different functions will be described. First, in the initialization process of step S1, the number of conditions-matched repetitions A for the number of printing-finished labels f to become an integer multiple of constant P, value B which is the product of said repetitions A multiplied by constant D, and the number of printing-completed labels f are all set to zero, while the number of dot-lines to be printed M is set to 1. Also, if in step S13 it is determined that the number of printing-completed labels f is a multiple of constant P, in step S23 one is added to the number of conditions-matched repetitions, and the process moves to step S24, in which the said repetitions A is multiplied by the constant Q, and the result, value B, is stored at a specific address in the printing data memory 6. Also, the digital value V supplied to the D/A converter in the subsequent functions becomes the result of deducting the said value B from the power supply data D obtained by reference to the bar code power supply data 5c or the ordinary character power supply data 5d. That is, in the embodiment shown in Fig. 5, the temperature detector 15 detects the temperature of the thermal head 14 each time P number of labels L have been printed, and on the basis of the detected temperature, reference is made to adjustment data 5a or 5b and the supply of electrical power to the thermal head 14 is varied accordingly. In the embodiment shown in Fig. 6, the digital value supplied to the D/A conversion circuit 11 after each printing of P number of labels L is decremented by Q, reducing the amount of power supplied to the thermal head 14 from the power supply circuit 13. Also, in the embodiment being described with respect to Fig. 5, the temperature of the thermal head 14 as detected by the temperature detector 15, 1 is detected in accordance with the program stored in the program memory 3, reference is made to power supply adjustment data 5a and 5b and a digital value V is provided that is adjusted with respect to D/A conversion circuit 11, the electrical power supplied from the power supply circuit 13 to the thermal head 14 being thereby adjusted. However, it is also possible for the power to be adjusted by supplying the signal output by the amplifier 16 directly to the power supply circuit 13 and for the power supply circuit 13 itself to correct temperature fluctuations in the thermal head 14, with no detection using changes in the output of comparator 17 in accordance with a program stored in program memory 3. Also in the embodiment described with reference to Fig. 6, adjustment value B is computed each time P number of labels L are printed. However, it is also possible for reference to be made to the adjustment data each time P number of labels L are printed by storing the adjustment data to be referred to in the power supply data memory 5 beforehand. As described in the foregoing, the present invention comprises the provision of a power supply data memory in which is stored beforehand data for each printing condition which correspond to the power to be sent to the thermal head, so that power is supplied to the thermal head in accordance with printing conditions with reference to the data stored in this power supply data memory. Therefore, it is possible to effect fine control of the heat of the heating elements of the thermal head in response to the nature of what is being printed, as the printing proceeds, providing consistently clear, sharp printing results.

Claims: A thermal head temperature control device comprising: a power supply circuit (13) for supplying electrical power to a thermal head (14); a power supply data memory (5) in which is stored data which correspond to the electrical power to be supplied to the thermal head (14), and control means (1) for controlling said power supply circuit (13) with reference to the data stored in said power supply data memory (5), characterized in that the data corresponding to the electrical power to be supplied to the thermal head (14) are experimentally predetermined and fixedly stored beforehand in the power r supply data memory (5) and that after a predetermined number of printing cycles the electrical power to be supplied to the thermal head is reduced. A thermal head temperature control device according to Claim 1, further comprising temperature detecting means (15) for detecting the temperature of said thermal head (14) after said number of printing cycles, said control means (1) control the reducing of power to be supplied to the thermal head (14) by reference to the temperature as detected by said temperature detecting means (15).

Inventors: OKUDA, AKIRA
Applicants: KABUSHIKI KAISHA SATO
Application year: 1986
Main classification: B41J2
IPC codes: B41J2