JPH02295764A - Thermal head drive - Google Patents

Abstract

PURPOSE:To achieve uniform print density of thermal head by regulating pulse width in response to the temperature of thermal head and controlling the number of pulse in response to a factor other than the temperature. CONSTITUTION:When a single line four division driving of a thermal head is carried out, a counter 8 counts down the number of pulses generated in a pulse generator 9 from n to 0, and a switching circuit 5 is switched the time point of 0 count. An AND gate 10 provides output pulses from the pulse generator 9 to an integrator 7 until 0 outputs are produced four times after starting of count down of the counter 8 on an external count start signal, then the output pulse is converted into a saw-tooth integrated wave at the integrator 7 and fed to a voltage comparator 6. The integrated wave is compared with a reference voltage Vth which is determined by the resistance Rth of a thermistor 4 and driving pulses having period T and unit pulse width tP are produced. When Ro is set considerably higher than Rth, Vth is varied by the resistance of the thermistor 4 and the driving pulse width is varied by temperature variation.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application This invention relates to a thermal head that has a plurality of heating resistors and is capable of printing by energizing the heating resistors.

．． (Ex) Conventional technology It is a well-known fact that the print density of thermal heads tends to vary depending on the temperature, printing cycle, resistance value of the heating resistor, etc. A driving device that controls the current value and energization period for driving the heating resistor has been considered.

Specifically, analog temperature data from a thermistor installed in the thermal head is converted into digital temperature data by an A/D converter and input to the microcomputer, and the microcomputer converts the digital temperature data and print cycle data into digital temperature data. The data in the ROM table is accessed at the address of the ROM table determined from the ROM table address, history data, etc. to obtain drive pulse width data, and the drive pulse width data is input to the pulse width control circuit to control the pulse width. A drive pulse signal (STROBE signal) is generated to maintain a constant printing rate against temperature changes in the thermal head.

(c) Problems to be Solved by the Invention However, in such a conventional thermal head drive device, the memory capacity of the circuit and the amount of control programs of the microcomputer to drive the thermal head from the outside are large. Also, due to the digital processing of the microcomputer, an expensive A/D converter is required. Furthermore, if a thermal head is provided with a control system for driving pulses for the heating resistor, there is a problem in that the shape and dimensions of the thermal head itself become extremely large.

The present invention has been made in consideration of these circumstances, and is intended to provide a thermal head which can provide uniform print density at low cost with a relatively simple circuit and control program, and which is compact in shape.

(2) Means for Solving the Problems This invention is capable of printing by applying a driving voltage corresponding to print data to each heating resistor of a thermal head in which a plurality of heating resistors corresponding to printing dots are arranged. In the thermal head driving device configured as above, the temperature detecting means detects the temperature of the thermal head, the pulse generating means applies a driving voltage consisting of a plurality of pulse voltages to the heating resistor in order to print one dot, and the above-mentioned comprising pulse number control means for controlling the number of pulses per dot of the pulse voltage output by the pulse generation means, and pulse width adjustment means for adjusting the pulse width of the pulse voltage output from the pulse generation means,
This thermal head driving device is characterized in that the number of pulses or pulse width per dot is controlled by the output of a temperature detection means.

In this invention, the drive pulse applied to the heating resistor to print one dot consists of a plurality of unit pulses,
For example, the width of a unit pulse can be controlled inside the thermal head according to the temperature of the thermal head, and at the same time the number of unit pulses can be controlled from Uiro according to other factors, or the number of unit pulses can be controlled inside the thermal head. At the same time as the control, the unit pulse width may be controlled externally. The thermal head usually has 50Q to 5000 heating resistors arranged in a straight line on a ceramic substrate.
It is preferable to form them so that each one can output a pixel. The means for energizing the heating resistor usually includes a shift register into which serial print data is input in synchronization with a clock signal, and a LATCH which inputs the serial print data in synchronization with a clock signal.
A driver IC can be used that includes a latch circuit that latches in response to a signal, and outputs an energization pattern to the heating resistor in accordance with the latched serial print data and the pulse waveform of the STROBE signal. When detecting the temperature of the thermal head, the temperature detecting means includes, for example, Pe.
sOa and MgCr. O. Or MgAltOa solid solution or Nio. A thermistor or the like made by mixing and sintering Mn*Os and COtOs can be used. In that case, it is preferable that the temperature detection means be disposed near the heating element to detect the temperature of the thermal head and convert it into a resistance value.

It is inexpensive and simple in terms of circuitry to use a driving pulse width adjusting means that performs pulse control by converting a sawtooth integral wave into a rectangular wave using a voltage comparator whose reference voltage changes with a change in resistance. A suitable pulse number control means for controlling the number of pulses per dot is, for example, a countdown type counter that can set a preset value and can output an O output.If the pulse number value is preset in the counter,
The number of driving pulses for one printed dot can be controlled.

(E) Effect If the thermal head is equipped with a temperature sensor, the pulse width can be adjusted according to the temperature, and at the same time the number of pulses can be controlled according to factors other than temperature that affect print density, such as the printing cycle. , it is possible to equalize the print density of the thermal head.

(f) Example Embodiments of the invention will be described in detail with reference to the drawings.

．． FIG. 1 is a block diagram showing an embodiment of the present invention, and a thermal head 1 includes a plurality of heating resistors 2 to which a voltage VH is applied, a plurality of driver ICs 3 for driving the heating resistors 2, and a thermal head 1. A thermistor 4 for detecting the temperature of the head 1, a switching circuit 5, and a voltage comparison circuit 6
, an integrating circuit 7, a counter 8, a pulse generating circuit 9, and an AND gate IO.

In the case of fax machines and printers, usually the main CPU
The print data handled by
(bit data), the parallel print data sent from the main CPU is converted into serial print data by a parallel/serial converter (not shown) and then sent to the driver IC3 provided in the thermal head l for driver control. sent along with the signal.

FIG. 2 is a basic circuit diagram of the driver IC 3.

The driver IC 3 consists of a driver (inverter) 3a composed of transistors, an AND gate circuit 3b, a latch circuit 3c, a shift register 3d, an output protection circuit 3e, AND gates 3g and 3h, and an inverter 3g, and input to the driver IC 3. The signals are composed of serial print data, driver control signals (B.E.O signal, LATCH signal and CLOCK signal) and STROBE signal.

The pulse generating circuit 9 in FIG. 1 has a period T. Duty 1/
2 pulses are generated. The counter 8 is a down counter that can be preset, and Uiro sets the number of constituent pulses of the drive pulse as the preset value. FIG. 3 is a waveform diagram of the pulse voltage that drives the heating resistor, and in this case, the number of pulses is n.

Next, using the timing chart in FIG. 4 and the basic circuit diagram of the driver IC 3 in FIG.
The serial print data inputted from the A IN terminal l1 is sent to the shift register 3d in a period synchronized with the CLOCK signal inputted from the CLOIJ terminal l2. Next, Ωfu1
The serial print data of the shift register 3d is latched by the latch circuit 3c by the LATC ■ pulse signal inputted to the terminal l3. Next is B. E. Connect O terminal 14 to B. E. O
After setting the driver 3a to a drivable state by setting the signal to "High", the heating resistor 2 is driven by the STROBE signal inputted from the switching circuit 5 via the STROBE terminal 15, and the counter in FIG. 8 is the pulse generated in the pulse generating circuit 9.
It counts down from 0 to 0, and when it reaches 0, the switching circuit 5 is controlled and STROBE is started as shown in FIG.
Switch I to STROBE4. The AND gate 10 is for sending the output pulses of the pulse generating circuit 9 to the integrating circuit 7 until the fourth 0 output is output after the counter 8 starts counting down in response to an external count start signal. The integrating circuit 7 converts the output pulse of the pulse generating circuit 9 into a sawtooth integrated wave and sends it to the voltage comparing circuit 6.

The voltage comparator circuit 6 compares the reference voltage determined by the resistance value of the thermistor 4 with the integrated wave.
The period T. as shown in the figure. A driving pulse with a unit pulse width tp is generated.

Next, how this driving pulse width t p b'< is determined will be explained with reference to FIGS. 5 to 8. FIG. 5 is an example of the integrating circuit 7 and voltage comparator circuit 6 in FIG. 1, and the waveforms Va, Vb, and Vc of each are shown in FIG. 6. In FIG. 5, the pulse generation circuit 9 of FIG. When a pulse Va with a duty of 1/2 is input, the waveform of the output vb of the integrating circuit 7 consisting of a resistor R1, a capacitor C, and a diode D is converted into a sawtooth integrated wave as shown in FIG. On the other hand, the reference voltage vth of the voltage comparison circuit 6 is determined by the resistance value of the thermistor 4. Therefore, the integrated wave is compared with the reference voltage vth by the comparator OP of the voltage comparison circuit 6, and the pulse width tp is compared with the reference voltage vth.
A driving pulse VC is output. The output voltage vb of the integrator circuit 7 is defined as vb=v(1-.''(t/IICI) at t<T/2, assuming that the high-level voltage of the input pulse V& is the rise of the v1 pulse at time 1=0. ,,,
・,・Q). If the time for Vc=Vth is 11 days, then Vth=V (1 − e −《t””c
>>) −−(2), and t P= t/2+
V th (3) On the other hand, if the resistance value of the thermistor 4 is Rth, the thermistor 4 has a voltage V as shown in FIG.

resistor R from the power source. Since the current is supplied through Above (2). The drive pulse width tp is calculated from equations (3) and (4). Next, the principle of how the drive pulse width changes due to temperature changes will be explained with reference to FIGS. 7 and 8. As shown in FIG. 7, temperatures t1 and t2 (tl
<t2) The resistance value of the thermistor 4 corresponding to R l
, R t (R r > R *), the reference voltage V of the voltage comparison circuit 52 at temperatures tl and t2 is
tb+. V tht is determined by equation (4). Therefore, R0 is R. ,R. By making vth sufficiently larger than , it is possible to make the change in vth with respect to the resistance value of the thermistor 4 noticeable.

FIG. 7 shows the reference voltage vth. and V tL (V th
t > V tht ), the drive pulse width LpI.
Shows jlh. j+)+ > tpt, which means that the drive pulse width is controlled to be wide when the temperature is low and narrow when the temperature is high.

Note that if the resistor R0 is replaced with a variable resistor, more accurate temperature control can be performed.

As described above, drive pulse width control can be performed independently based on temperature inside the thermal head with a relatively simple circuit. Furthermore, by changing the number of pulses set by Uiro as necessary, it is possible to control the energy applied to the heating resistor from the outside.

Although the above embodiment describes the four-division driving of the thermal head, the present invention is not limited thereto.

(G) Effects of the Invention According to the present invention, it is possible to maintain a constant print density against temperature changes using a simple and inexpensive circuit provided uniquely to the thermal head. At the same time, the energy applied to the heating resistor can also be controlled from the outside in response to factors that cause variations in print density, so a predetermined print density can be obtained with high accuracy.

[Brief explanation of the drawing]

Fig. 1 is a configuration explanatory diagram showing one embodiment of the present invention, Figs. 2 and 5 are detailed views of the main parts of Fig. 1, and Fig. 3 is a drive for the heating resistor of the embodiment shown in Fig. 1. FIG. 4 is a timing chart explaining the operation of the embodiment shown in FIG. 1. FIGS. 6 and 8 are timing charts showing waveforms of each part of the electric circuit shown in FIG. Figure 7 is the first
It is a graph which shows the temperature-resistance characteristic of the thermistor of the Example shown in a figure. 2. Figure 1: Thermal head, 2: Group heating resistor, 3: Driver IC. 4...Thermistor, 5...Switching circuit, 6...Voltage comparison circuit, 7...
Integrating circuit, 9...counter, 9...pulse generation circuit, lO...and gate. 3rd l2! B. E. ○ Figure No. Bay View Map

Claims (1)

[Claims] 1. In a thermal head drive device configured to perform printing by applying a driving voltage corresponding to print data to each heating resistor of a thermal head in which a plurality of heating resistors corresponding to printing dots are arranged, temperature detection means for detecting temperature; pulse generation means for applying a driving voltage consisting of a plurality of pulse voltages to the heating resistor in order to print one dot; and pulse voltage per dot output by the pulse generation means. pulse number control means for controlling the number of pulses, and pulse width adjustment means for adjusting the pulse width of the pulse voltage output by the pulse generation means, and the number of pulses per dot or the pulse width can be adjusted according to the output of the temperature detection means. A thermal head driving device characterized by controlling.