Thermal Head With Control Means for Maintaining Head Temperature Within a Range

1. Field of the Invention

The present invention relates to a recording apparatus for recording by causing a plurality of heat generating elements to generate the heat.

Recording apparatuses which can be used in the invention include a printer, a word processor, an electronic typewriter, a facsimile apparatus, a copying apparatus, and the like.

2. Related Background Art

A thermal transfer printer will be described hereinafter as an example of a recording apparatus.

Hitherto, as a heat control system of the thermal transfer printer, a system in which a heat pulse width is controlled by using a thermistor has been known.

That is, a temperature of a print head is measured by the thermistor before printing and a width of heat pulse which is applied to the print head is set on the basis of the temperature value measured, thereby executing the subsequent printing operation by the heat pulse width.

However, in the conventional heat control, the heat pulse width is determined in accordance with the temperature obtained by the thermistor and the printing operation is executed on the basis of the heat pulse width after that. Therefore, when the printing operation is continuously executed, that is, if no white dots exists and black dots are continued, the print head is overheated, so that the heat is accumulated and sometimes printed characters become obscure. Conversely, when the heat is insufficient, the printed characters are partially broken. On the other hand, there is also a problem that erasing characteristics of the character printed when the heat of the print head is accumulated is bad.

To solve the above problems, a system in which the temperature is detected every printing operation by the thermistor and the heat pulse width is controlled is also known. However, this system has a problem in that the high speed printing operation cannot be executed because it takes too much time to detect the temperature.

It is an object of the present invention to provide a recording method and apparatus which can clearly record.

Another object of the invention is to provide a recording method and apparatus which can record at a high speed.

Another object of the invention is to provide a recording method and apparatus which can prevent the accumulation in heat in a recording head and can maintain the temperature of the recording head within a proper range.

Still another object of the invention is to solve the foregoing conventional problems and to provide thermal transfer printer which can keep the temperature of a print head within a proper range and can print at a high speed.

Yet a further object of the invention is to provide a thermal transfer printer for printing which causes a print head having a plurality of heat generating elements to generate the heat by supplying current to the heat generating elements, wherein the printer has detecting means for detecting the temperature of the print head, and when the printer head executes the printing operation, the heat generating elements are heated for a predetermined time in accordance with a predetermined first function on the basis of the result of the temperature detected by the detecting means, and after the elapse of the predetermined time, the heating operation is interrupted for a predetermined time in accordance with a predetermined second function.

FIG. 1 is a block constructional diagram showing an embodiment of the present invention;

FIG. 2 is a timing chart showing the operation in the embodiment of the invention;

FIG. 3 is a flowchart showing an example of a control sequence for the interruption of a stepping motor in the embodiment of the invention;

FIG. 4 is a flowchart showing an example of a control sequence for the interruption of the heating in the embodiment of the invention;

FIG. 5 is a flowchart showing an example of a control sequence when the printing operation is started in the embodiment of the invention;

FIG. 6 is a flowchart showing an example of a control sequence in the case where the invention is applied to a printer having a plurality of ribbons;

FIG. 7 is a timing chart showing the operation in another embodiment of the invention; and

FIG. 8 is a perspective view of a thermal transfer printer to which the heat control of the invention is applied.

According to an embodiment which will be explained hereinbelow, a print head has a plurality of heat generating elements. These heat generating elements are separately and selectively heated for a heat pulse time obtained by using a predetermined first function, that is, a temperature increase function on the basis of the temperature measured by a temperature detecting thermistor, thereby maintaining the temperature of the heat generating element within a temperature range which is suitable for printing. After the passage of the predetermined heat pulse time, the heating operation is interrupted on the basis of a predetermined second function, that is, a temperature decrease function and the temperature information obtained by this function is stored. When the heating operation is again started, the heat generating elements are heated for the heat pulse time obtained by using the temperature increase function on the basis of the temperature information stored, so that the temperature of the heat generating element is held within a temperature range which is suitable for printing. Since the temperature is detected only once before printing, the thermal transfer printer which can print at a high speed is obtained. In the following discussion of the invention, the operation of a single heat generating element will be described. Those having ordinary skill in the art will appreciate that the other heat generating elements of the print heat will operate in like fashion.

An embodiment of a thermal transfer printer to which the invention is applied will be described in detail hereinbelow with reference to the drawings.

FIG. 1 is a block constructional diagram showing an embodiment of the invention. In the diagram, reference numeral 1 denotes a CPU to control the whole printer; 2 indicates a ROM in which a control sequence, control data, and the like of the CPU 1 are stored; 2A is a temperature function table storing a function as explained hereinlater; 3 denotes a RAM to temporarily store data; 3A denotes an area to store the temperature information in the RAM; 4 denotes a keyboard as an input apparatus; 5 denotes an indicator such as CRT, LCD, or the like; 6 denotes an interface section to control the indicator 5; 7 indicates a voltage source; and 8 denotes an interface section to control the thermal head. A stepping motor 9 controls the movement of a carriage 15 and a head motor 10 controls the operation of the head. A character generator 11 stores character patterns and a reference numeral 12 denotes a timer I to control the exciting time of the stepping motor. A reference numeral 13 denotes a timer II to control the output time of a heat pulse to the thermal head and the heat rest time. Further, reference numerals respectively follows: 14 denotes a thermal print head to having a plurality of heat generating elements, 15, a carriage on which the thermal head 14 is mounted; 16, a printer unit including the thermal head 14 and carriage 15; 17, a CPU bus to transmit addresses, data, and control signals; and 18, a thermistor to detect the temperature of the thermal head 14 and heat generating elements.

A timing chart of the embodiment of the invention is shown in FIG. 2.

In the diagram, HC.sub.1 to HC.sub.2 and HC.sub.2 to HC.sub.3 denote heat cycles (HC). An interruption signal to the stepping motor 9 to drive the carriage 15 is generated by the HC.sub.1, HC.sub.2, and HC.sub.3 and the motor 9 is excited.

HP indicates a heat pulse; HP.sub.1, HP.sub.3, HP.sub.5 , . . . denote heating start positions; HP.sub.2, HP.sub.4, HP.sub.6 , . . . represent heating end positions; Tf.sub.1, Tf.sub.2, Tf.sub.3 , . . . heating times; and Tg.sub.1, Tg.sub.2, Tg.sub.3 , . . . denote heat rest times. The interruption of the heating operation is executed at the leading and trailing times of the heat pulse HP. T denotes a temperature of the heat generating element (in the embodiment, about 5.degree. C. to 30.degree.l C.); T.sub.1, T.sub.3, T.sub.5 , . . . indicate temperatures at the start of the heating operation; T.sub.2, T.sub.4, T.sub.6 , . . . represent temperatures at the start of the interruption of the heating operation. y=f(x) denotes a temperature increase function which indicates the relation between the heat pulse time and the heat generating element temperature which increases by outputting the heat pulses to the thermal head 14 for a predetermined time. The function y=f(x) is obtained by the experiments or the like. D.sub.max indicates the highest value of the temperature which is suitable to print and is arrived by the temperature increase function y=f(x) (in the embodiment, about 180.degree. C. to 220.degree. C.). y=g(x) denotes a temperature decrease function which indicates the relation between the heat rest time and the temperature which decreases due to the interruption of the heating operation. The function y=g(x) is obtained by the experiments or the like D.sub.min indicates the lowest temperature value (in the embodiment, about 80.degree. C. to 120.degree. C.) which is suitable for printing and is arrived by the temperature decrease function y=g(x). The temperatures between D.sub.max and D.sub.min are the temperature range which is proper for printing.

When an interruption signal to the stepping motor 9 is generated in the heat cycle HC.sub.1, the stepping motor 9 is excited and the carriage 15 is moved by one step. Next, when the heating operation is interrupted by the HP.sub.1. the difference between the present temperature T.sub.1 which has previously been detected by the thermistor 18 and the objective arrival temperature D.sub.max is calculated. The heat pulse time Tf.sub.1 is determined by the temperature increase function y=f(x) and the heat generating element 14 is heated. At this time, the heat pulse time Tf.sub.1 is set into the timer II and the temperature information in the RAM 3 is also renewed to the objective arrival temperature T.sub.2. After the elapse of the time Tf.sub.1, the heating operation is again interrupted by the HP.sub.2. In this case, the heat pulse is not generated. At this time, the temperature has reached the maximum value D.sub.max suitable to print. The difference between the temperature T.sub.2 obtained by the temperature increase function y=f(x) and the objective down arrival temperature D.sub.min (=T.sub.3) is calculated and the heat rest time Tg.sub.1 is decided by the temperature decrease function y=g(x). The rest time Tg.sub.1 is set into the timer II and the temperature information is also renewed to T.sub.3. After the elapse of the time Tg.sub.1, the heat interruption is again generated by HP.sub.3. The difference between the present temperature T.sub.3 obtained by the temperature decrease function y=g(x) and the objective arrival temperature T.sub.4 is calculated. The heat pulse time Tf.sub.2 is determined by the temperature increase function f(x) and the heat generating element 14 is heated. The temperature increase function y=f(x) which is used in this case is quite the same function as the foregoing function. The time Tf.sub.2 is set into the timer II. After the elapse of the time Tf.sub.2, the heating operation is interrupted by the HP.sub.4. The heat pulse is not generated at the time of HP.sub.4. In this case, the temperature T.sub.4 has reached the highest temperature value D.sub.max which is suitable for printing.

Next, the difference between the temperature T.sub.4 and the objective down arrival temperature T.sub.5 is calculated and the heat rest time Tg.sub.2 is determined by the temperature decrease function y=g(x). The temperature decrease function y=g(x) which is used here is quite the same function as the foregoing function. After that, the heating operation and the rest operation are repeated by alternately using the two functions y=f(x) and y=g(x). The temperature information is also successively renewed. The interruption cycles HC.sub.2 and HC.sub.3 of the stepping motor are started for the heating and rest periods of time. The motor is excited to move the carriage one step by one. Thus, the printing is executed.

If data to be printed does not exist in the heat cycle HC, that is, in the case where all dots are the white dots, even when the heat pulse is given, the heat generating element 14 is not heated. Therefore, by previously reading the dot information, the heating operation is interrupted.

Therefore, for instance, if no print data exists as shown after the heat cycle HC.sub.3, the heat rest period of time is started. However, in this case, the temperature T.sub.11 after the elapse of the heat rest time is obtained from the preceding renewed temperature information T.sub.10 and the heat rest time by the temperature decrease function y=g(x) and is again renewed as the next temperature information.

That is, when the print data exists at the recording timing HC.sub.4, the difference between the T.sub.11 which was renewed as the temperature information and the objective set temperature D.sub.max is calculated. The heat pulse time Tf.sub.6 is determined by the same temperature increase function y=f(x) as the foregoing function and the thermal head 14 is heated.

FIG. 3 shows a flowchart for the interruption of the stepping motor 9. This sequence shows the operation which is executed for the interruption performed in the heat cycles HC.sub.1, HC.sub.2, and HC.sub.3 in FIG. 2. That is, when the interruption occurs in step S101, the stepping motor is excited to rotate it in step S102. In step S103, the excitation data to be set in the next interruption is obtained. This data is used for the excitation in step S102 at the time of the next interruption. In step S104, the interruption interval time is set into the timer I. This time is equal to the heat cycle shown by HC.sub.1 to HC.sub.2 and HC.sub.2 to HC.sub.3 in FIG. 2. In step S105, data CG is obtained from the character generator 11 in FIG. 1. The data CG is the print data. When the data CG exists, a CG flag in the RAM 3 in FIG. 1 is set ON in step S106. If the data CG does not exist, the CG flag is set OFF. The stepping motor interrupting process is finished.

Further, a control sequence for the interruption of the heating operation will now be described with reference to a flowchart shown in FIG. 4. This sequence shows the operation which is executed for the interruption generated by the HP.sub.1 to HP.sub.6 or the like shown in FIG. 2.

When the heat interruption is generated in step S201, the heating operation is stopped in step S202. In step S203, a check is made to see if the CG flag has been set ON or OFF. If it is OFF, this means that data to be printed does not exist, so that the processing routine advances to step S212. If it is ON, this means that the print data exists, so that step S204 follows. is lower than the lowest temperature value D.sub.min which In step S204, a check is made to see if the temperature is suitable for printing or not. If it is not lower than the D.sub.min, step S210 follows. If it is lower than the D.sub.min the printing operation cannot be executed unless the temperature lies within the temperature range suitable for the printing. Therefore, to raise the temperature, the heating time T.sub.f is decided by using the temperature T.sub.1 of the thermistor which has previously been detected and the temperature increase function y=f(x). In the example, the objective set temperature is set to the highest temperature value D.sub.max suitable to print. In step S206, the temperature information in the area 3A is renewed. In step S207, the heat generating element is heated for the period of time Tf.

Returning to step S204, when the temperature is equal to or higher than the lowest value D.sub.min, that is, when the heat generating element has been heated in step S207 and the temperature has been raised up to the highest value D.sub.max the heating operation must be stopped. In step S210, the heat rest time Tg is determined by using the temperature decrease function y=g(x). In this example, the objective set temperature is set to the lowest temperature value D.sub.min which is suitable to print. In step S211, the temperature information is renewed to the D.sub.min. The heat times T.sub.f and T.sub.g determined in steps S205 and S210 are set into the timer II in step S208. Then, the heat interrupting process is finished.

Returning to step S203, when the CG flag is OFF, step S212 follows and the temperature is obtained by the temperature decrease function y=g(x). At this time, the temperature value stored and the heat rest time are input as mentioned above. In step S213, the temperature information is renewed by the temperature for instance, T.sub.11 obtained. Then, the processing routine is finished.

The renewing process of the temperature information which is executed in step S213 is performed by an output from the function y=g(x) and not by the output data of the thermistor. That is, during the interrupting operation of the print sequence, the renewing processes of the temperature information are all executed in accordance with the temperature function y=g(x).

FIG. 5 shows a flowchart for the control procedure to start the printing operation. In step S301, when the print start processing section is activated, the temperature of the heat generating element is measured by the thermistor 18 and the temperature information is set into the area 3A in the RAM 3 in step S302. Next, in step S303, the interruption timer I of the stepping motor is set. In step S304, the heat interruption timer II is set and the processing routine is finished. The temperature is detected by the thermistor 18 only when the printing operation is started.

An example of a control procedure in the case where the invention is applied to a printer having a plurality of ribbons is shown as another embodiment of the invention in a flowchart of FIG. 6. In this case, the function is set every ribbon which is used. In step S401, the process is started. In step S402, the kind of ribbon which is used is checked. If it is CR (Correctable Ribbon), in step S403, the functions y=f(x) and y=g(x) for the correctable ribbon are respectively determined. If it is DR (Dual Color Ribbon), in step S405, the functions y=f(x) and y=g(x) for the dual-color ribbon are respectively decided and step S404 follows. In step S404, the printing operation is started as mentioned above on the basis of the functions which were respectively determined in step S403 or S405 and the processing routine is finished.

Another embodiment of the invention will now be described on the basis of a timing chart shown in FIG. 7.

In the embodiment, the temperature is increased by two steps. That is, the generation of the heat pulse HP is started from the temperature T.sub.1 at the start of the printing in accordance with the temperature increase function y=f(x) and the heat pulses are continuously generated for the time Tf.sub.1 until the temperature becomes the central value T.sub.2 in the temperature range suitable to print. Therefore, the generation of the heat pulses is stopped for the time Tg.sub.1 of tens of msec. and the temperature is reduced until T.sub.3. The heat generating element is again heated for the time Tf.sub.2 until the temperature becomes the highest temperature value (D.sub.max =T.sub.4) suitable to print. In a manner similar to the above embodiment, the heating operation is stopped for the time Tg.sub.2 in accordance with the temperature decrease function y=g(x).

In this manner, by dividing the heating operation into two operations while avoiding that the heat generating element is continuously heated, the life of the heat generating head 14 can be prolonged.

The thermal transfer copying printer to perform the foregoing heat control will now be described with reference to FIG. 8.

In the diagram, reference numeral 20 denotes a platen roller. A recording paper 21 is conveyed by rotating the platen roller 20 in the direction shown by an arrow A (counterclockwise). On the other hand, when the thermal head 14 is put down and presses an ink ribbon 22 onto the recording paper 21, the recording paper 21 is maintained at a predetermined position by the platen roller 20. On the other hand, the carriage 15 is movably attached along a guide shaft 28 attached in parallel with the platen roller 20. The carriage 15 has a cassette loading section 24 for attaching the thermal head 14 so that it can be put up and down and for detachably loading an ink ribbon cassette 23 having therein the ink ribbon 22. A stop member 24a is provided to fixedly hold the cassette 23 onto the loading section 24. The carriage 15 is reciprocated by a driving system comprising: the stepping motor 9; a drive pulley 25; a driven pulley 26; and a belt 27 which is wound around the pulleys 25 and 26 and is fixed to the carriage 15. The thermal head 14 has a plurality of heat generating elements and is swingably attached between a down position to press the platen roller 20 through the ink ribbon 22 and an up position away from the down position.

In the printer of the embodiment, for the recording paper 21 backed up around the platen roller 20, the recording is executed by the thermal head 14 attached to the carriage 15.

In the embodiment, the thermal transfer copying printer has been described as an example of the recording apparatus. However, the invention is not limited to such a printer. For instance, the invention can be also applied to what is called a thermal recording apparatus for recording an image by generating a color from a thermal sheet by applying the heat thereto, what is called an ink jet printer for recording an image by ejecting an ink droplet by applying the heat thereto, or the like. Therefore, the recording heads include the thermal head mentioned in the above embodiment, ink jet head, and the like. In the embodiment, on the other hand, an example in which the thermistor is provided on the back side of the base plate of the head has been described. However, the invention is not limited to such an example. For instance, the thermistor can be also attached to a base body of the apparatus or the like. In such a case, the thermistor detects the ambient air temperature. On the other hand, the recording condition is determined by the kind of ribbon, head voltage, kind of recording paper, and the like.

As will be obvious from the above description, according to the foregoing embodiment, for the heat control of the heat generating elements, the first function to raise the temperature and the second function to decrease the temperature are provided and the temperature information is updated, thereby controlling the heating time and the heat rest time. Therefore, the temperature of the heat generating elements can be always held within a temperature range suitable to print and the clear printed result can be always obtained. On the other hand, since there is no need to measure the temperature of the heat generating element every printing operation, the printing operation can be executed at a high speed.

As mentioned above, according to the invention, a recording apparatus which can obtain a clear recording image can be provided.