JPS5945177A - Method and apparatus for driving thermal head - Google Patents

Abstract

PURPOSE:To uniformize surface temperatures between heat generating resistors and ultimately the density of images recorded by controlling heating of the heat generating resistors sequentially from smaller resistance value thereof utilizing a heat storage effect of the heat generating resistors due to heat generation in printing. CONSTITUTION:The heat generating section of a thermal head is provided with heat generating resistors R1...Rn in the number corresponding to the number of image elements in the main scanning direction and current is applied to the heat generating resistors R1...Rn according to image signals to perform a recording on a heat sensitive medium by causing the heat generating resistors alone to generate heat. The heat generating resistors are each provided with proper rectifying diodes D to secure an appropriate heat generation while terminals on the diode D side thereof are kept at a constant potentional. The other terminals thereof are provided with AND gates G1...Gn. Thus, a proper printing pulse is applied through the AND gates to effect heat generation of the heat generating resistors only corresponding to image information.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a thermal head driving force method and apparatus for making the surface temperature between the heat generating resistors of a thermal head constituted by a plurality of heat generating resistors uniform.

Thermal heads used in thermal recording usually have multiple heating resistors corresponding to the number of pixels in the main scanning direction of the thermal recording medium (for example, when recording image information on recording paper No. 4 in column A of the Japanese Industrial Standards). The number of heating resistors required is 1,728 (2,048 in the case of Japanese Industrial Standard B column No. 4 recording paper), and the power is determined according to the image information to be recorded. By supplying the heat to each of the plurality of heat generating resistors separately and causing the required heat generating resistors to generate heat, the thermal recording medium that is in sliding contact with the heat generating portion is colored.

By the way, conventionally, when using such a thermal head, it is not necessary to group the plurality of heat generating resistors according to their heat generation characteristics, but to simply divide the heat generating resistors into groups based on the heat generation characteristics, and conduct electricity at a constant voltage and for a constant time. Prescribed records were being kept.

However, in such a method, since the power consumption between each heating resistor varies depending on the resistance value of the heating resistor, the surface temperature of each heating resistor, which is approximately proportional to the power consumption, varies. will occur. As a result, when recording on a thermal recording medium (including transfer media such as transfer tape in the thermal transfer method), the amount of heat applied to the thermal recording medium varies for each heating resistor, so the recorded image The 'a degree was non-uniform and the print quality was poor.

For example, when recording is performed using the above-mentioned conventional thermal head, even if the variation in resistance value between each heating resistor is suppressed to within 15%, the variation in print density will still be I D
(Image Density) = 0.25 or more may occur.

This invention was made in view of the above-mentioned circumstances, and utilizes the heat storage effect of the heat generating resistor due to heat generated during printing, and performs heating control in order from the heat generating resistor with the lowest resistance value. It is an object of the present invention to provide a method for driving a thermal head and an apparatus therefor, which uniformizes the surface temperature between resistors and thus the density of a recorded image.

In other words, the present invention performs heating control on a plurality of heating resistors that are classified into an appropriate number of groups based on predicted resistance values, starting from the group with the smallest resistance value. For example, the classification information for the heating resistor is stored in an appropriate storage means, and heating control is performed based on this information.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a method for driving a thermal head and an apparatus thereof according to the present invention will be described in detail with reference to embodiments shown in the accompanying drawings.

First, the present invention will be explained in detail with reference to FIGS. 1 to 4.

The heating resistor of the thermal head is as shown in Figure 1.
Due to differences in their inherent heat generation characteristics, that is, differences in their resistance values, the range of resistance values centered around the target resistance value r (in the figure, from resistance value r-△r1 to r+△r
2). In addition, since the power consumption ((voltage value) 2 / resistance value) of the heating resistor and the surface temperature have a relationship as shown in Figure 2 when the energization time is constant, it is necessary to maintain a constant base temperature and constant voltage application. The surface temperature of each heating resistor at this time is approximately inversely proportional to its resistance value. That is, the surface temperature of a heating resistor with a small resistance value is higher than the surface temperature of a heating resistor with a large resistance value. Furthermore, in a thermal head, it is unavoidable that the temperature of the base increases due to the heat storage effect caused by the heat generated by the heat generating resistor during printing, and the base temperature and the surface temperature of the heat generating resistor are almost linear as shown in Figure 3. We have a good relationship. Therefore, the amount of heat stored due to heat generated by the heat generating resistor with a small resistance value (high surface temperature) becomes greater than the amount of heat stored due to heat generated by the heat generating resistor with a high resistance value (low surface temperature).

This invention attempts to actively utilize this heat storage effect, and the heating resistor with a small resistance value, that is, the surface temperature is relatively high, is replaced by the heating resistor with a large resistance value, that is, the surface temperature is relatively high. By sequentially heating the heating resistors with lower surface temperatures, the surface temperature of the heating resistors with lower surface temperatures is raised by utilizing the heat storage effect. For example, as shown in Figure 1, the heating resistors of the thermal head are grouped based on their resistance values.
When classified into four groups, Group ■, Group ■, and Group ■, the groups consisting of heating resistors with the lowest resistance value are divided into four groups: Group ■, Group ■, Group ■.
Heating is performed in the order of group ■ and group ■. As a result, as shown in Fig. 4, the surface temperature of the heating resistors in Groups ■, Group ~ 1, and Group The temperature (indicated by diagonal lines in the same figure) rises appropriately compared to the surface temperature of the heating resistor, approaching the surface temperature of Group I, which makes it possible to reduce the variation in the surface temperature of the heating resistor. Possible @ Figure 5 shows the result of this invention constructed based on the above-mentioned principle.
This figure shows an embodiment of a driving device for a thermal head.

As mentioned above, the heat generating part of the thermal head has heat generating resistors lt+X whose number corresponds to the number of pixels in the main scanning direction.
R*...Rn are all false, and by applying a current corresponding to the image signal to each of the plurality of heat generating resistors R+, R1...Rn, the required heat generating resistor is created. Recording is performed on a thermal recording medium by generating heat only. Each heating resistor Rs, Rt...R is provided with an appropriate rectifying diode D for proper heat generation, and each heating resistor Rs, Rt...
...The terminal of the diode D11 of Rn is held at a constant potential. The other gate of each heating resistor that was cut out has an AND gate G], G2, etc.
Gn is here, this ant game h Gt, Gt
By applying an appropriate printing pulse via Gn, only the heating resistor corresponding to the image information generates heat.

Next, memory 111, memory II 12, memory 113
, memory] V 14 stores information for determining to which group each heating resistor belongs, which is classified into four groups, Group I to Group II, based on its resistance value, as described above.

Of course, the information stored in memory ■ corresponds to the heating resistor of group I, and similarly, the information stored in memory ■, memory ■, and memory ■ correspond to group ■, respectively.
This corresponds to the heating resistors of Group ■ and Group ■. The table below shows an example of the storage contents of each of these memories I to Mm.

Namely, memory ■, memory ■, memory ■, memo IJ I
Each V has a bit address area corresponding to at least the total number of heating resistors in the thermal head (in the table above, the dot numbers of the heating resistors correspond to the bit addresses in each memory). , in each memory, only the bit address corresponding to the dot number of the heating resistor of the group corresponding to the memory is stored as a logic value "1", and the other bit addresses are stored as a logic value "0". has been done. For example, according to FIG. 3, the heating resistor with dot number 1 belongs to group I, the heating resistor with dot number 2 belongs to group ■, the heating resistor with dot number 3 belongs to group I, and the heating resistor with dot number 4 belongs to group I. The heating resistor belongs to group (2), and the heating resistor with dot number 5 belongs to group (2).

On the other hand, these memory■, memory■, memory■, memory■
The read operation is performed under the control of the memory control unit.The read operation is performed by first reading out the stored contents of memory I serially according to its bit address, and then serially reading the stored contents of memory II according to its bit address. After that, the contents of memory (2) and memory (2) are read out in the same way.

Therefore, the memory read control unit includes, for example, a ring counter circuit that performs a counting operation based on a transport lock that is a basic clock of each component of this embodiment device, and an output of the ring counter circuit to each memory in a predetermined manner. It can be configured by a selector circuit or the like that switches sequentially based on the order.

Well, the output terminals of these memories are each OR gate 3.
This OR gate (9) is connected to the input terminal of 0.
After the memory outputs are logically summed by the AND gate 31
It is configured to be input to one of the input terminals. Note that when one memory is executing a read operation, the output signals of the other memories are in a high impedance state, so as not to affect the output of the memory during the read operation.

Next, the register 40 receives image information VD for each main scanning line from an image information reading section (not shown), etc.
Temporarily stores image information @VD for main scanning lines,
The stored image information is sequentially outputted to the AND gate 31 in a predetermined order based on the transport lock. Therefore, its temporary storage capacity requires at least the number of bits equivalent to the total number of heating resistors of the thermal head, or in other words, the number of pixels for a lifetime scanning line. Further, the output operation of the stored image information VD to the AND gate 31 is performed in synchronization with the readout operation of the memory 11, memory 11, memory 2, memory 2, and memory 9, and the register 40 outputs the temporarily stored one main scanning line. The fractional information VD is repeated four times and output to the AND gate 31.

For this reason, the image information VD is generated after the reading operations from the memory +JI to the memory ■ are completed, that is, the image information for one main scanning line temporarily stored in the register 40 is turned over four times and sent to the AND gate 31. After fc, the image information for the main scanning line of the next step is input to the register 40.

Next, the operations of the memories I to 2, the register 40, and the AND gate 31 when recording one main scanning line according to the time chart shown in FIG. 6 will be described. In addition, in this time chart, dot numbers 1 to 8 o of the heating resistor of the thermal head are used as the storage contents of each memory among the examples of the storage contents of each memory shown in the previous table. The operation for the heating resistor of the dot number is omitted.

As shown in the figure, 1. First, in order to perform heating control on the heat generating resistor of the group (■), the memory contents of the memory IJI are sequentially read out according to the bit address, and a register is output in synchronization with the reading operation of the memory. 40 temporary storage contents (image information f1V D corresponding to one main scanning line) are logically ANDed by the AND gate 31 in j order. In this way, among the heating resistors of group (■), only the stored content of the memory I corresponding to the heating resistor that performs heat generating operation based on the image information VD is set to a logic value "1" as a predetermined bit related to a shift register to be described later. It will be shifted to the position. In this time chart, only the heating resistor with dot number 1 as the heating resistor of group (2) is subjected to the heating operation.

When the reading operation of the memory I is completed, the reading operation of the stored contents of the memory 2 is started in order to perform heating control on the heating resistors of the group 2. In this case, the register 40 outputs the temporarily stored image information VD corresponding to one main scanning line to the AND gate 31 again according to the bit address. After the outputs are logically ANDed, an operation is performed to output the outputs to the shift register. In this time chart, the heating resistors of dot number 2 and dot number 6 execute the heating operation as the heating resistors of group (2).

The above-mentioned operation is similarly performed for the heating resistors of groups ① and ②. In this time chart, the heating resistor with dot number 4 is the heating resistor of group ■, and the heating resistor with dot number 7 is the heating resistor of group ■.
After the heating resistor, the next step is the visual emotion? , 13 V D is input to the register 40, and the contents of each memory are transferred from memory ■ to J[[i? It is read out in number K.

Next, the shift register 50 is a serial input parallel output type shift register, and its input terminal is connected to the AND gate 31.
The parallel output terminal is an output terminal provided for each heat generating resistor.
Connect each to one input terminal of n. The shifting operation is performed based on the transfer lock, and the output of the AND gate 31 is appropriately shifted to a predetermined position in order to generate heat in the heating resistors that should be generated for each classified group. ing.

The optimum energization time to be applied to the plurality of heating resistors is stored between the printing pulse generation circuits, and the printing pulse corresponding to the optimum energization time is provided to each heating resistor through fLfl and gates G1, G, ! ...Output to Gn. Of course, the width of the printing pulse output from the printing pulse generation circuit is shorter than the width of the pulse output from the shift register 50, and the output operation of this printing pulse is performed in synchronization with the output operation of the shift register 50. .

As described above, in this embodiment device, the heating resistors of the thermal head are classified into four groups based on their resistance values, and the classification information is stored separately in four memories, and the memories are stored in accordance with the resistance values. The data is read sequentially from memory corresponding to small groups. Therefore, the heating resistor group with a small resistance value, that is, the heating resistor group with a relatively high surface temperature, will be heated later, and the group with a large resistance value, that is, the heating resistor group with a relatively low surface temperature, will be heated later. The surface temperature of the body group will be raised due to the effect of the heat storage effect described above. As a result, variations in the surface temperature of the heating resistor can be reduced.

In addition, memory ■, memory ■, memory ■ of this embodiment device
, Memo IJ IV may be any memory as long as it can appropriately store classification information of heat generating resistors, but it is recommended to use a programmable ROM that is writable and non-volatile, for example. For example, it is now possible to achieve the above-mentioned good printing quality by simply changing the memory contents according to the resistance value variation of the heating resistor of the thermal head, which differs for each device. It is possible to provide a thermal head with a wide range of properties, and also to save labor and reduce costs during manufacturing.

In order to speed up the apparatus of the embodiment shown in FIG. 5, for example, when the number of dots to be printed is small, the memory 11 and the memory (2) are first made active at the same time by adding a memory read control section to read the stored contents. If the heating resistors of groups ■ and group ■ are read out in parallel and the heating resistors of groups ■ and group ■ are made to generate heat, then the heating resistors of groups ■ and group ■ are made to generate heat at the same time in the same way, the classification is essentially divided into two groups. The printing time can be halved.

Furthermore, in the above-mentioned embodiment, the heating resistors were classified into four groups according to their resistance values, but the number of classifications is of course not limited to four, and the resistance value of each thermal head Any number of groupings may be performed depending on the variation. When the number of classifications is increased, variations in the surface temperature of the thermal head can be further reduced.

1. The configuration of the thermal head driving device according to the present invention is not limited to the embodiment shown in FIG. Any other configuration may be used as long as the heating is controlled in order from the group with the lowest resistance value to equalize the surface temperature between the heating resistors.

By the way, there is a control method that shortens the heating time in order to compensate for the temperature rise of the heating resistor when continuous pulses are applied or due to the adjacent effect. It is also possible to realize a thermal head with even better printing quality by attaching it to the printer.

As explained above, according to the method and device for driving a thermal head according to the present invention, the resistance values of the heating resistors of the thermal head classified into an appropriate number of groups based on the predicted resistance values are determined. The heating resistors are heated in order from the small group of resistance values, that is, the heating resistor group with a relatively high surface temperature, and the surface temperature of the heating resistor group, which is heated later, with a large resistance value, that is, the heating resistor group whose surface temperature is relatively low, is increased by the heat storage effect. By doing so, it is possible to reduce variations in the surface temperature of each heating resistor, which in turn makes it possible to reduce variations in recorded image density and improve print quality.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the distribution of the heating resistors of the thermal head according to their resistance values, and the classification of the heating resistors into four groups based on their resistance values.・' FIG. 5 is a block diagram showing an embodiment of the thermal head driving device according to the present invention, and FIG. 5 is a time chart showing an operation example of the embodiment device shown in FIG. 5. IL 1.2.13.14...Memory, Nod...Memory read control unit, 40...Register, 50...・Shift register, (a)...Print pulse generation circuit. Applicant's agent Takashi Kimura 11i11 state envy 4 rotation≦ヰ4 □□□#-he

Claims (2)

[Claims] (1) Driving a thermal head characterized in that heating control is performed on a plurality of heating resistors classified into an appropriate number of groups based on each resistance value trap, in order from the group with the smallest resistance value. Method. (2) A memory that stores information for each classified group to identify which group the plurality of heating resistors belong to, which are classified into an appropriate number of groups based on the predicted resistance value, and a predetermined memory. temporary storage means for temporarily storing image information for each number of bits until reading of the storage contents of the memory is completed; and temporary storage means for temporarily storing image information for each number of bits until the reading of the storage contents of the memory is completed, and the storage contents of the memory are stored in the classified groups in order from the group with the smallest resistance value. readout control means for controlling readout; and for selecting a heat generating resistor to be heated from among the plurality of heat generating resistors for each of the classified groups based on the sequential outputs of the temporary storage means and the memory. A thermal head driving device that forms information and performs heating control in order from the small group of heating resistors.