JPH0337512B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a thermal head drive control device for a printer that reduces density unevenness in an image by controlling the drive of each heating resistor according to the resistance value of each heating resistor.

Conventional printer thermal head drive control devices include, for example, a shift register that stores serial recording signals and outputs them as parallel signals;
The parallel signal is input through a gate, and the drive unit is turned on when the signal is "1", and the heating resistor receives power and forms a dot on the recording paper when the drive part is turned on. There are things (for example,
If the recording paper is A4 size, one line is formed by controlling the drive of 1728 heating resistors, and if the recording paper is B4 size, one line is formed by controlling the drive of 2048 heating resistors).

In the above configuration, when the shift register accumulates one row's worth of recording signals, it is outputted to the drive section as one row's worth of parallel signals via the gate. The driving section is turned on and off depending on the "1" and "0" states of the parallel signal, so it is driven according to the recording signal of the heating resistor. The driven heating resistor generates heat by applying a constant voltage over a certain period of time, forming dots on the thermosensitive recording paper or forming dots on the recording paper via the thermosensitive ribbon tape.

However, according to conventional thermal head drive control devices for printers, a constant voltage is applied to the heating resistor over a certain period of time, but the resistance value of the heating resistor varies by about ±20%. Therefore, the power consumption of each heating resistor varies, and there is a limit to the control of image density unevenness.

The present invention has been made in view of the above,
The present invention provides a thermal head drive control device for a printer that controls the drive current or drive time of each heating resistor according to the resistance value of each heating resistor in order to reduce density unevenness in an image.

The thermal head drive control device of the present invention will be explained in detail below.

The accompanying drawings show an embodiment of the present invention, which is applied to a printer in a facsimile machine. The facsimile device includes a network control unit 9 that controls connection to the line, and a demodulator that modulates the transmitted encoded signal (encoded based on the MH or MR method) and demodulates the received modulated signal. It has a modulation section 10, a decoding section 11 that decodes the demodulated encoded signal based on the MH or MR method and outputs a line signal, and a printer section 16 that inputs the line signal and reproduces a recorded image. , performs a receiving function based on this configuration. In addition to this configuration, a reading section 13 that reads the original and outputs a line signal;
It has a storage section 14 that stores line signals over a predetermined number of lines, and an encoding section 15 that encodes and shortens the stored line signals based on the MH or MR method, and includes the network control section 9 described above. The transmitting function is achieved by the configuration with the demodulating section 10 and the demodulating section 10. In order to control these overall configurations, a system control unit 12
is provided.

The printer unit 16 includes a heat generating resistor 1 constituting a thermal head, a switching transistor 2 (drive unit) that controls the drive of the heat generating resistor 1, and a buffer 3 (for example, a amplifier) and charge storage capacitor 5
a, 5b... and charge transfer switch elements (MOS
A charge transfer circuit 4 (based on the outputs of control terminals 12a, 12b, 12c of the system control section 12) that controls the control amount of the buffer 3 according to the resistance value of the heating resistor 1. ) and the control terminal 12d of the system control unit 12
an AND circuit 7 that opens the gate according to the printed signal;
It has a shift register 8 that inputs and accumulates line signals from the decoder 11 and outputs them as parallel image signals. The system control unit 12 controls the entire system, and in order to equalize the power consumption of the heat generating resistor 1, the heat generating resistor 1 is stored in a ROM (not shown).
The resistance values of each are memorized.

To explain the operation in the above configuration,
When a transmission signal is received from the line via the network control section 9, the demodulation section 10 demodulates it and outputs a coded signal. The encoded signal is decoded by the decoding section 11, outputted as a serial line signal, and stored in the shift register 8. For example, when line signals for one line are accumulated in the shift register 8, a print signal is outputted from the control terminal 12d of the system control unit 12, and the AND circuit 7 is opened and the shift register 8
The accumulated contents are output to the buffer 3. Prior to this, the resistance values of the heating resistors 1 stored as digital values in the ROM of the system control unit 12 are D/A converted in the order of their arrangement, and then the control terminals 12
It is output from a. That is, when the analog control value corresponding to the resistance value of the first heating resistor 1 is output and accumulated in the capacitor 5a (at this time, the control terminal 12b is "0"), the control terminal 12b is changed from "0" to "0". 1" to turn on the switch element 6a and transfer the accumulated value of the capacitor 5a to the capacitor 5b (at this time, the control terminal 12
c is "0"). Next, when the control terminal 12b is set to "0" and the control terminal 12c is set to "1", an analog control value corresponding to the resistance value of the second heating resistor 1 is output from the control terminal 12a, and the value is stored in the capacitor 5a. Also,
The accumulated value of capacitor 5b is transferred to capacitor 5c by turning on switch element 6b. In this way, the analog control value corresponding to the resistance value of each heating resistor 1 is adjusted to the charge storage capacity 5a, 5b corresponding to each heating resistor 1.
It is accumulated in... In this state, when the buffer 3 receives the output of the shift register 8, the corresponding capacitor 5
The amplification factor is controlled by supplying current to the base of the switching transistor 2 according to the accumulated charge amount of the transistors a, 5b, ..., and the amplification factor is controlled based on the constant voltage _VB to each heating resistor 1 according to its resistance value. Pass current. This makes it possible to keep the power consumption of each heating resistor 1 constant, eliminate unevenness in image density, and improve image quality.

In the above embodiments, the current value of each heating resistor 1 was controlled according to the resistance value, but instead of this, the energization time, in other words, the on-time of the switching transistor 2 was controlled according to the resistance value. It's okay. Further, although the printer unit 16 has been described as being for a facsimile machine, the present invention is not limited to this. Furthermore, since the operation of transmitting the image signal is self-evident, the explanation is omitted.

Here, a specific example in which the current value is controlled according to the resistance value will be considered as follows.

If the average resistance value of heating resistor 1 is 500 ohms, the upper limit is 600 ohms due to the variation in resistance value.
The lower limit value is 400 ohms. In normal printing
Since a power of 0.7 W/dot is required, the power supply voltage is set to 18.7 volts based on the average resistance value. The current at this time is 500 ohm...37mA 600 ohm...31mA 400 ohm...47mA, and the power consumption is as follows.

600 ohm...0.6W 400 ohm...0.8W This causes uneven print density, so control the current value as follows to eliminate uneven print density.

600 ohm...34mA 400 ohm...42mA On the other hand, charge according to the resistance value is transferred to the capacitors 5a, 5b,
Considering the case where it is stored in ..., it is as follows.

Current i is i=(1-e ^-t/RC )・V/R Charge Q is Q=∫ ^tc ₀ idt=V/R {t _C −RC (1- e ^-tc/RC )} Heat generating resistance Power consumption of body 1 = I ² Assuming that R = constant, I = βI _b = βv _b /r where β is the current amplification factor of switching transistor 2, I _b is its base current, and r is its base resistance. , v _b is its base voltage.

Therefore, I ² R = (βv _b /r) ² = constant v _b = 1/√R·K (K is constant), and the base voltage v _b of the switching transistor 2 is -1/of the resistance value R. It is sufficient to adopt a value proportional to the square. Therefore, an operational amplifier that converts the voltage given by the Q/C to v _b may be used as the buffer 3.

As explained above, according to the thermal head drive control device for a printer according to the present invention, since the drive current or drive time of each heat generating resistor is controlled according to the resistance value of each heat generating resistor, density unevenness of an image can be prevented. The image quality can be improved by reducing the amount of

[Brief explanation of the drawing]

The accompanying drawings are explanatory diagrams showing one embodiment of the present invention. Explanation of symbols: 1... Heat generating resistor, 2... Switching transistor, 3... Buffer, 4... Charge transfer circuit, 5a, 5b... Charge storage capacitor, 6
a, 6b... switch element, 7... gate circuit,
8...Shift register.

Claims (1)

[Scope of Claims] 1. In a printer that reproduces an image on a recording member by supplying a drive current to a heating resistor array in accordance with an image signal, the resistance value of each heating resistor in the heating resistor array is stored respectively. A thermal head drive control device for a printer, comprising: means for controlling the level of the drive current or the drive time in accordance with the resistance value.