JPH0611800Y2 - Head drive circuit for thermal transfer printer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to a head drive circuit for a thermal transfer printer, and more particularly to a head drive circuit for a thermal transfer printer that reduces uneven printing.

[Prior Art] Conventionally, in a thermal transfer printer, as a circuit for controlling the print density, a circuit for controlling the print density for each line is known. This circuit detects the temperature of the thermal head for each line by a temperature sensor and controls the amount of energy applied to the thermal head based on the detection result.

By the way, in this circuit, there is a problem that the print density changes line by line due to the change of the ambient temperature, and the difference in density is conspicuous.

Therefore, a circuit has been devised which controls the print density on a page basis instead of controlling the print density on a line basis, and this circuit is currently mainly used.

This circuit detects the temperature of the thermal head by a temperature sensor each time one page is printed, and controls the amount of energy applied to the thermal head based on the detection result. If this circuit is used, the print density is controlled based on the temperature before the printing of one page, so the difference in shade between lines becomes inconspicuous.

[Problems to be Solved by the Invention] However, the above-described circuit for controlling the print density in page units has the following problems. That is, when the number of printed dots per page is large, the print density is high at the beginning of the page, and the heat is accumulated in the thermal head toward the rear of the page, so that the print state becomes dark and bleeding occurs.

The present invention has been made in view of such a background, and an object thereof is to provide a head drive circuit of a thermal transfer printer which has a small difference in density within a page and can make a difference in density between lines inconspicuous. It is in.

[Means for Solving the Problems] The present invention provides a head drive circuit of a thermal transfer printer that detects the temperature of a thermal head and controls the amount of energy supplied to the thermal head based on this temperature. 1) First storage means for storing the temperature of the thermal head when printing the (N is a positive integer) line, and second storage means for storing the temperature of the thermal head when printing the Nth line Storage means, first detection means for detecting a difference between the data in the first storage means and the data in the second storage means, and the temperature difference detected by the first detection means. Second detection means for detecting whether or not a preset temperature difference is larger than a predetermined temperature difference in which the print density does not noticeably change with respect to the temperature change of the thermal head, and the temperature difference is the constant temperature difference. Greater than In the case of, the head driving energy value when the (N-1) th line is printed is increased or decreased by a preset change amount to drive the thermal head to drive the Nth line. Line is printed, and if the temperature difference is smaller than the constant temperature difference, the thermal head is driven by the head drive energy value when the (N-1) th line is printed. A head driving means for printing the Nth line.

[Operation] According to the above configuration, the difference between the temperature of the thermal head when printing the (N-1) th line and the temperature of the thermal head when printing the Nth line is calculated. The value of the temperature difference is compared with a preset constant temperature difference, and when the constant temperature difference is exceeded, the head drive energy value when the (N-1) th line is printed is preset. The Nth line is printed with an increment or decrement of the amount of change made. That is, when the temperature is higher when the Nth line is printed than when the (N-1) th line is printed, the head drive energy amount is decreased by the above change amount, If the temperature has decreased, the head drive energy amount is increased by the change amount.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing the electrical construction of an embodiment of the present invention.

In the figure, 1 is a print data memory in which a dot-converted print data DB is stored. A control unit 2 has an MPU (micro processing unit), a work memory and a program memory. In this case, registers RTa, RTb and R are stored in the work memory.
E is set for each. The control unit 2 has a function of reading the print data DB stored in the print data memory 1 and a function of inputting and outputting control signals and data to various circuits described later.

3 is a shift register circuit 4, a latch circuit 5, a driver 6
And a heating element 7, which is a thermal head. The shift register circuit 4 is a serial-in / parallel-out shift register, which reads the print data DB output from the control unit 2 based on the clock signal CLK supplied from the control unit 2 and outputs the print data DB to the latch circuit 5. The latch circuit 5 reads the output of the shift register 4 based on the latch signal DR output from the control unit 2,
Output to the driver 6. The driver 6 is a NAND gate G
_{1 to} Gn, each of these first input terminals is connected to each output terminal of the latch circuit 5, and each second input terminal is commonly connected. Heating element 7, the heat generating element _TH 1 is composed of a ～THn, one end of each of these heating elements _TH 1 ~THn is connected to the output terminal of the NAND gate _G 1 _~Gn driver 6, the other end They are commonly connected and connected to a positive power source V.

Next, 8 is a timer circuit, which generates a signal RT having a pulse width corresponding to the energy data RE supplied from the control unit 2 when the signal PP is supplied from the control unit 2 and connects the drivers 6 in common. It is output to the end. Reference numeral 9 is a temperature detection element (thermistor) that detects the temperature of the heating element 7, and is attached in close contact with the heating element 7. 10 is A
/ D (analog / digital) converter, which sequentially converts the detection signal (analog value) output from the temperature detection element 9 into a digital signal TM and supplies it to the control unit 2.

Next, the concept of this invention will be described with reference to FIGS. 2 (a) and 2 (b). In addition, FIG.
5 is a characteristic graph showing changes in print density with respect to temperature, in which the horizontal axis represents temperature and the vertical axis represents print density. Further, FIG. 6B is a characteristic graph showing a change in the head drive energy value with respect to the temperature of the heat generating element 7, in which the horizontal axis represents temperature and the vertical axis represents head drive energy.

In FIG. 6A, a temperature difference in which the print density does not change remarkably with respect to the temperature change of the heating element 7 is obtained by an experiment. For example, this temperature difference is (th2-th1), and the value of this difference is D1. The D1 may vary depending on the temperature range of the heating element 7 (for example, the high temperature region and the low temperature region), and there may be a plurality of D1 depending on the temperature range. next,
When the temperature of the heating element 7 becomes larger or smaller when the Nth line is printed than when the (N-1) th line is printed, and when the temperature difference at this time exceeds the value D1. , Nth
The head drive energy value when printing the th line is a value obtained by increasing or decreasing this value by a certain amount based on the head drive energy value when printing the (N-1) th line. For example, in the same figure (b), it is assumed that the temperature of the heating element 7 when printing the (N-1) th line is th1, and the temperature of the heating element 7 when printing the Nth line. Be th3. In this case, since th3> th2, the temperature difference (th3-th1) exceeds the value D1. Therefore, as shown in FIG. 6B, for example, a temperature difference (th4-th) between the temperature th1 and the temperature th4.
The reference energy value corresponding to 1) is determined in advance, and this reference energy value is subtracted from the head drive energy value required when the (N-1) th line is printed to print the Nth line. Head drive energy value when That is, a value smaller than the temperature th2 is set as the temperature th4, and a value smaller than the head driving energy value that compensates for the temperature difference between the temperatures th1 and th2 is set as the reference energy value. In the above case, the temperature of the heating element 7 when printing the Nth line is higher than the temperature of the heating element 7 when printing the (N-1) th line. On the contrary, it is assumed that the temperature of the heating element 7 when printing the (N-1) th line is th3 and the temperature of the heating element 7 when printing the Nth line is th1. For example, the reference energy value is added to the head drive energy value required when printing the (N-1) th line to obtain the head drive energy when printing the Nth line.

As described above, the temperature difference in which the print density does not noticeably change with respect to the temperature change of the heating element 7 is set in advance as a constant value, and the (N-1) th line is set every time one line is printed. The difference between the temperature of the heating element 7 when printing and the temperature of the heating element 7 when printing the Nth line is compared with the above constant value. Then, when the temperature difference exceeds this constant value, the head drive energy when the (N-1) th line is printed is increased or decreased by the preset reference energy value to obtain the Nth line. The head drive energy value for printing is determined. This allows
Since the head drive energy difference is not extremely given,
The shade difference between the -1) th line and the Nth line becomes inconspicuous, and the print quality of the entire page improves.

Therefore, the circuit devised based on the above method is the first
It is the circuit shown in the figure. The operation of this circuit will be described below with reference to the flowchart of FIG.

Here, the operation of this circuit is performed by taking the case where the printing of the Nth line is started as an example. In addition, when the printing of the Nth line is started, the register RTb of the control unit 2 is
Stores the temperature data TDD (N-1) when the (N-1) th line is printed, and the register RE stores the (N-1) th line when printed. It is assumed that the head drive energy data RED (N-1) is stored. In addition, the shift register circuit 4 includes
It is assumed that the print data DBD (N) for the Nth line is held.

First, in step S1, the control unit 2 reads the temperature data TDD (N) indicating the temperature of the heating element 7 at the present time and stores the temperature data TDD (N) in the register RTa. Next, in step S2, the difference ΔT between the temperature data TDD (N) in the register RTa and the temperature data TDD (N−1) in the register RTb is calculated. This value ΔT is compared with a preset value D1 in step S3. In this case, if ΔT> D1, the process proceeds to step S4, and at step S4, the temperature data T
The magnitude of D (N) and the temperature data TD (N-1) are compared. In this case, if TD (N)> TD (N-1), the process proceeds to step S5. Then, in step S5,
A predetermined value D2 is subtracted from the energization time data RED (N-1) in the register RE, the subtracted value is stored in the register RE again, and then the process proceeds to step S7. If TDD (N) <TDD (N-1) in step S4,
A predetermined value D2 is added to the energization time data RED (N-1), and the added value is stored again in the register RE as the energization time data RED (N). Then, the process proceeds to step S7. Further, in the above step S3, ΔT <D1
If so, the process directly proceeds to step S7. Next, in step S7, the energization time data RE in the register RE
D (N) is supplied to the timer circuit 8. Next, in step S8, the control unit 2 sends the gate signal PP to the timer circuit 8.
To supply. When the gate signal PP is supplied to the timer circuit 8, the energization time data RE
A pulse RT having a pulse width corresponding to D (N) is generated and supplied to each input terminal of the drivers G _{1 to} Gn. As a result,
Printing of the Nth line is performed. In addition, the shift register circuit 4 is activated while the Nth line is being printed.
, The print data DBD (N + 1) of N + 1 lines is supplied from the print data memory 1 via the control unit 2. When the printing of N lines is completed, the printing paper is driven one pitch.

As described above, according to this invention, the temperature difference in which the print density does not noticeably change with respect to the temperature change of the heating element 7 is set to the constant value D.
The temperature difference ΔT between the temperature of the heating element 7 when printing the (N−1) th line and the temperature of the heating element 7 when printing the Nth line is set in advance as 1. , And the above constant value D1. Then, the temperature difference ΔT is a constant value D1.
When the (N-1) th line is printed, the head drive energy value supplied to the heating element 7 when printing the (N-1) th line is increased or decreased by a predetermined value D2 to print the Nth line. It is the head drive energy value supplied to the heating element 7 at that time. With such a configuration, the (N-1) th
Even if the temperature difference between the temperature of the heating element 7 when printing the Nth line and the temperature of the heating element 7 when printing the Nth line becomes large, The head drive energy value supplied to the heating element 7 does not follow and increase. That is, the head driving energy having a value directly corresponding to the temperature difference of the heating element 7 is not supplied to the heating element 7, but the head driving energy required for printing the (N-1) th line is used. , The (N-
1) The supply is performed by gradually increasing or decreasing by a constant change amount such that the difference in shade between the 1st line and the Nth line does not appear extremely. As a result, even if the print density is controlled on a line-by-line basis, the difference in shade between lines is not noticeable. On the other hand, since the print density is controlled in line units instead of page units, the phenomenon that occurs when controlling print density in page units, that is, the beginning of a page is lighter and the end of a page is darker There is no.

Optimal values for the above-mentioned values D1 and D2 are determined by experiments. In the embodiment of the present invention, the heating element 7
The control of the head drive energy value supplied to is performed by changing the energization time, but in addition, the voltage value may be changed.

As described above, according to the present invention, in the head drive circuit of the thermal transfer printer that detects the temperature of the thermal head and controls the amount of energy supplied to the thermal head based on this temperature, First storage means for storing the temperature of the thermal head when printing the (N-1) th line (N is a positive integer), and the temperature of the thermal head when printing the Nth line. Second storage means, first detection means for detecting a difference between data in the first storage means and data in the second storage means, and temperature detected by the first detection means Second detection means for detecting whether or not the difference is larger than a predetermined temperature difference in which the print density does not noticeably change with respect to the temperature change of the thermal head, and the temperature difference is constant. temperature If the difference is larger than the difference, the thermal head is driven by an energy value obtained by increasing or decreasing the head drive energy value when the (N-1) th line is printed by a preset change amount. When the Nth line is printed and the temperature difference is smaller than the predetermined temperature difference, the thermal head is driven by the head driving energy value when the (N-1) th line is printed. Further, since the head driving means for printing the Nth line is provided, it is possible to make the density difference between the lines inconspicuous and thereby reduce the density difference within the page.

[Brief description of drawings]

FIG. 1 is a block diagram showing an electrical configuration of an embodiment of the present invention, FIGS. 2 (a) and 2 (b) are characteristic graphs for explaining the gist of the embodiment, and FIG. 3 is the same embodiment. 5 is a flowchart for explaining the operation of FIG. 2 ... Control unit (having RTa, RTb, first detecting means and second detecting means), 3 ... Thermal head, R
Tb ... Register (first storage means), RTa ... Register (second storage means), 8 ... Timer circuit (2 and 8 are head drive means).

Claims (1)

[Scope of utility model registration request] 1. A head drive circuit for a thermal transfer printer, which detects (a) the temperature of a thermal head and controls the amount of energy supplied to the thermal head based on this temperature. (B) The (N-1) th First storage means for storing the temperature of the thermal head when printing the line (N is a positive integer); and (c) Second storage means for storing the temperature of the thermal head when printing the Nth line. (D) first detecting means for detecting a difference between the data in the first storing means and the data in the second storing means, and (e) by the first detecting means. Second detecting means for detecting whether or not the detected temperature difference is larger than a preset constant temperature difference in which the print density does not noticeably change with respect to the temperature change of the thermal head; The temperature difference is the constant temperature In the case of larger than the difference,
Printing the Nth line by driving the thermal head with an energy value obtained by increasing or decreasing the head driving energy value when the (N-1) th line is printed by a preset change amount. When the temperature difference is smaller than the constant temperature difference, the (N-1) th
A head drive circuit of a thermal transfer printer, comprising: a head driving unit that drives the thermal head according to a head driving energy value when printing the Nth line to print the Nth line.