JPH022033A - Driving system for thermal head - Google Patents

Abstract

PURPOSE:To enhance recording quality by making recording density constant by detecting drive voltage to control the pulse width of a drive pulse corresponding to the magnitude of said voltage. CONSTITUTION:A CPU 5 reads the pulse width of a drive pulse optimum to set density, that is, the pulse width of each of enable signals ENL1-ENL4 on the basis of the density signal Dn and detection signal Db stored in a RAM 7 from the data table of a ROM 6 to output the same to the drive circuit C of a thermal head 10 through a thermal 1/F 8. These enable signals ENL1-ENL4 are adjusted in their pulse widths corresponding to the voltage of the battery 11 being the power supply of a thermal printer 1 and the pulse widths become long when the voltage becomes low. Therefore, when the voltage Va applied to the thermal head 10 drops, the pulse width of the drive pulse (enable signals ENL1-ENL4) becomes long and recording can be always performed at the density set by an operator.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for driving a thermal head, and more particularly to a method for driving a thermal head that makes printing density uniform.

(Prior Art) Recently, there has been a desire for smaller recording devices, and in response to this demand, thermal recording devices using thermal elements have been developed.

Such a thermal recording device is generally equipped with a thermal head having a plurality of thermal elements for one line in the main scanning direction, and a driving pulse is supplied to each thermal element based on image data. Each line is recorded.

In this case, the recording density is adjusted by setting a density adjustment switch of the recording apparatus. In other words, when the switch for concentration adjustment is set, the pulse width of the drive pulse changes according to the switch setting value, and the time during which the drive current supplied to the thermal element flows changes, causing the heat generation temperature of the thermal element to change. It changes.

Furthermore, since the temperature of the thermal element also changes with changes in the ambient temperature, conventionally the driving pulse width has been controlled in response to the ambient temperature of the thermal head to control the recording density.

(Problem to be Solved by the Invention) However, in such a conventional thermal head drive method, when a density adjustment switch is set, a drive pulse with a drive pulse width set by the set value is is supplied to a thermal head (thermal element) and correction is performed based on the ambient temperature. Therefore, there is a problem in that it is not possible to cope with concentration changes due to changes in the applied voltage due to changes in the power supply voltage.

In particular, in recent years, handy tamper recording devices have become widespread, and batteries are increasingly being used as a power source. In this case, the battery voltage is as shown in Figure 6 for a nickel-cadmium battery that is said to have stable discharge characteristics.
It decreases with the passage of time, and is 1.1v at point A, but becomes 0.9v at point B after L hours. For example, if eight of these batteries are connected in series and used as a power source, L
The pulse width of the driving pulse to the thermal head remains constant despite this change in power supply voltage. the result,
The value of the drive current supplied to the thermal element decreases, the amount of electric power decreases, and the temperature of the thermal element decreases. Therefore, the recording density becomes thinner and the recording quality deteriorates.

(Purpose of the Invention) Therefore, the present invention makes it possible to keep the amount of power of the drive current supplied to the thermal head constant by controlling the pulse width of the drive pulse according to the magnitude of the applied voltage. The purpose is to prevent temperature changes in the thermal head, keep the recording density constant, and improve recording quality.

(Structure of the Invention) In order to achieve the above object, the present invention includes a plurality of thermal elements in the main scanning direction, and supplies a drive current proportional to the voltage to the thermal elements for a time corresponding to the pulse width of the drive pulse. A driving method for a thermal head that records by generating heat from a thermal element is characterized in that the driving voltage is detected and the pulse width of the driving pulse is controlled in accordance with the magnitude of the driving voltage.

Hereinafter, the present invention will be specifically explained based on examples.

1 to 5 are diagrams showing an embodiment of the present invention, which is applied to a thermal printer.

In FIG. 1, 1 is a thermal printer, which includes an input I/F 2, an A/D converter 3.4, and a C
It is equipped with a PU5, ROM6, RAM7, a thermal printer, etc.

An image reading device such as a scanner 9 is connected to the input I/F 2, and an image signal is input from the scanner 9. The image signal input from the scanner 9 is output to the thermal head 10 shown in FIG. 2 via the thermal I/FB. A/D
The converter 3 includes a concentration voltage Vn obtained by dividing a predetermined voltage VO by a concentration setting switch VR provided in the operation section of the thermal printer 1 and resistors R7 and Rz, and a comparison voltage +V.
Raf, VRor Kairi Casatio/'), A/Dit
A unit 3 has the comparison voltage +■8゜1, -■□, and the concentration voltage Vn.
are compared and digitally converted to obtain the density signal Dn.
Output.

The A/D converter 4 is inputted with a detection voltage vb obtained by dividing the voltage vH of the battery 11 of the thermal printer 1 by resistors Ri and R4, and a comparison voltage +■8°t, VRo.
/D converter 4 converts detection voltage vb to comparison voltage +Vilor
, ``R11f, and converts it into a digital signal and outputs it as a detection signal Db.

CPU (Central Processing)
Unit) 5 is ROM (Read Only Mem)
ory) 6 to execute the sequence of the thermal printer 1 by controlling each part according to the program stored in the printer 6, and also execute the pulse width control process of the drive pulse of the present invention. The ROM 6 stores a control program for the thermal printer 1, and also stores data regarding the pulse width of the drive pulse using the concentration signal Dn, which is the setting value of the switch VR, and the detection signal Db, which is the voltage of the battery 11, as parameters, that is, an enable signal. ENL1~EN
The pulse width data of L is stored in the form of a data table. RAM (Random Access M
A work area is formed in memory) 7.

The thermal I/F 8 outputs various signals for controlling the driving of the thermal head 10.

As shown in FIG. 2, the thermal head 10 includes a thermal element T, a drive circuit C, and the like.

A plurality of thermal elements T, for example, 1728 thermal elements T are arranged along one line in the main scanning direction, and are divided into four blocks GP to GP4 due to power supply capacity and the like. One professional 7ri GP of thermal element T, ~GP,
One of the drive circuits C can be shown in detail as shown in FIG.

In FIG. 3, the drive circuit C has gates 00 to G&! ,
Latches LT, ~L T b x and shift register S
Equipped with Ro~SR&3, etc., each gate G0~c6
Thermal elements T0 to T6:l are connected to i. Thermal element T. -T6. A drive voltage Va supplied from the battery 11 is applied to the drive voltage Va, and a drive current proportional to the drive voltage Va is supplied to the drive voltage Va. Enable signal ENL and enable signal ENH are input to gates G0 to G63, and U.7chi LT, ~LT,, ff
When the enable signal ENL is at low level "L'" and the enable signal ENH is at high level "°H," the value of the image signal SV latched in the latches LT, ~LT6. Accordingly, a drive current flows through the thermal elements T0-T, 3. An image signal is input as parallel data from the shift register S Ro-5Rbs to the latches LT, ~LTo, and the latches LT, ~LT63 input the image signal according to the latch signal LAT. latches the gates G0 to Gi,
Output to 3. Image signals (serial data) are input to the shift registers SR, ~SR4, and the shift register SRo~5R=z sequentially reads the image signals according to the clock signal CK, and when the image signals for the line are read, the parallel data is input. It is converted into and output to latch LTo=LTbs.

Next, the action will be explained.

When recording, the operator sets the density by adjusting the density setting switch VR in the printing section of the thermal printer 1.

When the concentration is set, the setting value of this switch VR is
The converter 3 detects it and outputs it as a concentration signal Dn.

On the other hand, the voltage of the battery 11 is constantly detected by the A/D converter 4,
It is output as a detection signal Db.

These density signal Dn and detection signal Db are read by the CPU 5 and stored in the RAM 7. CPU5 is RAM7
The pulse width of the drive pulse is optimal for the set density based on the density signal Dn and the detection signal Db stored in , that is, the enable signal ENL.

~ENL, is read from the data table in the ROM 6 and output to the drive circuit C of the thermal head 10 via the thermal I/F 8.

On the other hand, in parallel with setting the pulse width of the drive pulse, the CPtJ5 reads the image signal SV input from the scanner 9 etc. in synchronization with the clock signal CK as shown in FIG. The signal is output to the shift registers SRo to 5R63 of the thermal head 10 through the thermal head 10. The image signal S for one line is transferred to the shift register SR, ~SR6:1
etc., the latch signal LAT is shifted to latch LT,
-LT, 3 etc. and latches the enable signals ENL, -EN for each block GP, -GP4.
L.

is output and recording is performed for each block CP, -GP.

As mentioned above, the pulse width of the enable signals ENL, ~ENL, is adjusted according to the voltage of the battery 11, which is the power source of the thermal printer 1, and as shown in FIG. , the pulse width becomes longer. Therefore, when the voltage Va applied to the thermal head 10 decreases, the drive pulse (enable signal E N L + ~E
The pulse width of NL4) becomes longer, and it is possible to always record at the density set by the operator. As a result, the quality of recorded images can be improved.

In addition, in the above embodiment, the battery voltage is detected as a representative of the driving voltage of the thermal head.
The method is not limited to this, and the driving voltage may be directly detected.

(Effects) According to the present invention, it is possible to prevent the temperature of the thermal head from changing due to a change in the driving voltage of the thermal head, and it is possible to keep the recording density constant and improve the recording quality.

[Brief explanation of the drawing]

1 to 5 are diagrams showing an embodiment of the thermal head driving method of the present invention, FIG. 1 is a circuit block diagram of a thermal printer to which the thermal head driving method is applied, and FIG. 2 is a circuit block diagram of a thermal printer to which the thermal head driving method is applied. The circuit diagram of the thermal head, Figure 3 is an enlarged circuit diagram of the main parts of the thermal and throat, and Figure 4 is the timing chart of various drive signals for the thermal and throat.
FIG. 5 is a diagram showing the relationship between the battery voltage and pulse width. FIG. 6 is a diagram showing the discharge characteristics of the battery. Tom: Thermal printer, 2: Input 1/F, 3.4: A/D converter, 5: CPU. 6...ROM. 7...RAM. 8... Thermal I/F, 9... Scanner 10... Thermal head, 11... Battery, T... Thermal element.

Claims (1)

[Claims] In a method of driving a thermal head that has a plurality of thermal elements in the main scanning direction and supplies a drive current proportional to the voltage to the thermal elements for a time corresponding to the pulse width of a drive pulse to cause the thermal elements to generate heat for recording, the drive A thermal head drive method that detects a voltage and controls the pulse width of a drive pulse according to the magnitude of the drive voltage.