JP2001162850A - Method and device for measuring resistance value of thermal head and thermal printer having the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To execute measuring of a resistance of a heating element forming a thermal head by reducing a measuring time period without lowering a resolution of a measuring means. SOLUTION: In a resistance measuring mode, a discharge time period Ts1 is measured from a discharge start voltage E1 to a comparison voltage Vref by discharging from a capacitor by heating of a reference resistor Rs. When the discharge time period Ts1 satisfies a judgment expression of 'Ts1<213t0.rsmin/rmax', it is judged that the number of bits of a counting circuit is not sufficiently used, a discharge time period Ts2 is set based on an arithmetic expression of 'Ts2=(216-1).t0.rsmin/rmax', and then a discharge start voltage E2 is set based on an arithmetic expression of E2=K.Vref/exp (Ts2/Ts1).ln(Vref/E1)} (K<=1.0) by using the discharge time period Ts2. The discharge time period for changing from the discharge start voltage E2 to the comparison voltage Vref is measured by each heating element to calculate the resistant value of each heating element. When the discharge time period Ts1 does not satisfy the judgment expression, the discharge time period is measured by each heating element at the discharge start voltage E1, and the resistance value of each heating element is calculated. In a print mode, image data is corrected based on the resistance value of each calculated heating element and then the printing is executed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for measuring a resistance value of a heating element constituting a thermal head, and a thermal printer having the same.

[0002]

2. Description of the Related Art Thermal printers include a thermal transfer printer using an ink film and a thermal printer that records an image by directly heating a thermal recording material. These thermal printers use a thermal head in which a large number of heating elements (resistance elements) are arranged in a line.

For example, in a color thermal printer, as described in JP-A-61-213169, a magenta thermosensitive coloring layer, a cyan thermosensitive coloring layer, and a yellow thermosensitive coloring layer are sequentially formed on a support. A color thermosensitive recording material is used. In this color thermosensitive recording material, in order to selectively color each thermosensitive coloring layer, its coloring heat energy is different, and a deeper thermosensitive coloring layer requires higher coloring heat energy. Also, when the next heat-sensitive coloring layer is thermally recorded, the heat-fixed heat-sensitive coloring layer is irradiated with a specific electromagnetic wave so as to prevent the heat-sensitive coloring layer therefrom from being recorded again. Is performed.

A thermal printer is provided with a thermal head in which a large number of heating elements are arranged in a line, and applies a coloring heat energy corresponding to a heat-sensitive coloring layer to be recorded to a color thermosensitive recording material. This heat energy for color development is obtained by adding heat energy for color development to a desired density (hereinafter, heat energy for gradation expression) to heat energy immediately before color development (hereinafter, this is called bias heat energy). is there. This bias heat energy is a constant value determined according to the coloring characteristics of the thermosensitive coloring layer. On the other hand, for the gradation expression thermal energy, it is necessary to perform fine heat generation control in order to express high gradation. In general, the heating element is energized for several milliseconds to several tens of ms in bias heating, and the energization of the heating element is controlled in units of several μs to several tens of μs in gradation expression heating.

By the way, in order for such fine heat generation control to be accurately reflected on the printing result, it is necessary that all the heating elements constituting the thermal head have uniform resistance values. However, it is known that the resistance value of the heating element generally has a variation of about 5 to 10%, and changes with time or printing. Therefore, even if each heating element is energized for the same energization time, the thermal energy generated by each heating element changes in accordance with the resistance value of the heating element, which causes inconvenient phenomena such as density unevenness in the recorded image. I do.

To improve this, there is known a color thermal printer which measures the resistance value of each heating element and corrects image data based on the measurement result to prevent the occurrence of density unevenness and the like. (See, for example,
No. 7). In this color thermal printer, a capacitor is connected in parallel with the heating element and the reference resistor, and the discharge time is measured to measure the resistance value of the heating element. This measuring method will be described below using mathematical expressions.

When the discharge start voltage is E, the predetermined voltage is Vref, the capacitance of the capacitor is C, the resistance of the heating element is r, and the discharge time from E to Vref is T, the following equation 1 is established.

[0008]

Vref / E = exp {−T / (C · r)}

By transforming the equation (1), the following equation (2) is obtained.

[0010]

## EQU2 ## r = -T / C / ln (Vref / E)

According to Equation 2, if the capacitance C of the capacitor is known, the resistance value r of the heating element can be calculated by measuring the discharge time T. Further, even if the capacitance C of the capacitor is not known, the reference resistance R whose resistance value rs is known is known.
By separately measuring the discharge time Ts discharged in s, the resistance value r of the heating element can be calculated from the following Expression 3.

[0012]

R = rs · T / Ts

From the above equation (1), the discharge time T is expressed by the following equation (4).

[0014]

## EQU4 ## T = -C.r.ln (Vref / E)

[0015]

The above discharge starting voltage E,
The predetermined voltage Vref, the capacitance C of the capacitor, and the resistance r of the heating element have deviations. If this is expressed in%,
For example, the discharge start voltage E = 20v ± 2%, the predetermined voltage Vre
f = 19v ± 2%, capacitance C of capacitor (± 20
%), And the resistance value r (± 1%) of the heating element. When these values are substituted into Equation 4 and calculated, the deviation of the discharge time T is about + 116% to -83%. If the ratio between the discharge start voltage E and the predetermined voltage Vref approaches 1 in order to shorten the measurement time, the deviation of the discharge time T becomes a very large value due to the effect of the natural logarithm of the equation (4). For this reason, in a relatively inexpensive counter circuit having a small number of bits used as a means for measuring the discharge time T, one unit time must be lengthened so as not to overflow, and the resolution of the counter circuit is reduced. The measurement accuracy of T decreases.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method and an apparatus for measuring the resistance value of a thermal head, which can reduce the measuring time without reducing the resolution of the measuring means, and a thermal printer equipped with the method.

[0017]

The present invention focuses on the fact that the measurement accuracy of the discharge time T is greatly influenced by the setting of the discharge start voltage E and the predetermined voltage Vref from the above equation 4,
The discharge starting voltage E is set appropriately so as not to lower the resolution of the counter circuit. The method for measuring the resistance value of the thermal head according to the present invention is performed in parallel with a plurality of heating elements arranged in a line on the thermal head. Connect a capacitor,
After charging this capacitor, the discharge time until the capacitor is discharged and reaches a predetermined voltage is measured for each heating element,
In the thermal head resistance value measuring method for calculating the resistance value of each heating element based on the measurement result, the capacitor is discharged through a reference resistor having a known resistance value and a predetermined first resistance value is discharged.
The discharge time from the discharge start voltage to the predetermined voltage is measured by the measuring means, and then the second discharge start voltage higher than the first discharge start voltage is reset within a range where the measuring means does not overflow. The discharge time from when the capacitor is discharged from the start voltage to when the predetermined voltage is reached is measured for each reference resistor and each heating element, and the resistance value for each heating element is calculated from the measurement result and the resistance value of the reference resistor. It is.

Further, according to the method of measuring the resistance value of a thermal head of the present invention, a capacitor is connected in parallel to a plurality of heating elements arranged in a line on the thermal head, and the capacitor is charged and then discharged to a predetermined value. In the thermal head resistance value measurement method, which measures the discharge time until the voltage reaches each heating element and calculates the resistance value of each heating element based on the measurement result, the capacitor is passed through a reference resistor whose resistance value is known. A first discharging time from discharging to a predetermined first discharge starting voltage until reaching a predetermined voltage is measured by a measuring means, and when the first discharging time is less than a predetermined number of count bits of the measuring means, A second discharge start voltage higher than the first discharge start voltage is reset within a range where the measuring means does not overflow, and the discharge of the capacitor is performed from the second discharge start voltage. The discharge time until the voltage reaches the predetermined voltage is measured for each reference resistor and each heating element, and the resistance value of each heating element is calculated from the measurement result and the resistance value of the reference resistor. When the number is equal to or more than the predetermined count bit number, the discharge time from the first discharge start voltage until the capacitor is discharged to reach the predetermined voltage is measured for each heating element. The resistance value of each heating element is calculated.

According to the thermal head resistance measuring apparatus of the present invention, a plurality of heating elements arranged in a line on the thermal head, one end of which is connected to the heating element, and a power supply terminal of the thermal head and the thermal head. A first switch means group for turning on / off the connection, a control circuit for controlling the first switch means group so that each heating element is connected to a power supply, and a capacitor connected in parallel to a power terminal of the thermal head. A power supply for driving the heating element, a second switch connected in series with the power supply and the capacitor, and turning on the second switch to charge the capacitor to a predetermined first discharge start voltage. After that, the second switch means is turned off, and the first discharge from when the capacitor starts discharging through the reference resistor having a known resistance value to when the capacitor reaches the predetermined voltage is performed. Measuring means for measuring the interval, a determining means for determining whether the first discharge time satisfies a predetermined count bit number of the measuring means, and a first discharge time corresponding to the predetermined count bit number by the determining means. If it is determined that the second discharge start voltage is not higher than the first discharge start voltage, the capacitor is discharged from the second discharge start voltage to reach the predetermined voltage. The discharge time until reaching is measured for each reference resistor and each heating element, and the resistance value for each heating element is calculated from the measurement result and the resistance value of the reference resistor, and the first discharge time is counted by the measuring means. When it is determined that the number of bits is satisfied, the discharge time from the first discharge start voltage until the capacitor is discharged to reach the predetermined voltage is measured for each heating element, In which the measurement results and the resistance of the reference resistor is constructed from a resistance value calculation means for calculating a resistance value of each heating element.

The thermal printer according to the present invention is a thermal printer having a thermal head in which a plurality of heating elements are arranged in a line, wherein a first switch means group for turning on / off each of the plurality of heating elements is provided. A control circuit for controlling the first switch means group so that each heating element is connected to a power supply; a capacitor connected in parallel to a power supply terminal of the thermal head; a power supply for driving the heating element; A second switch means connected in series to the capacitor and the capacitor; and turning on the second switch means to charge the capacitor to a predetermined first discharge start voltage, and then turning off the second switch means; Measuring means for measuring a first discharge time from starting discharge through a reference resistor having a known resistance value to reaching a predetermined voltage; A determination unit that determines whether the time period satisfies the predetermined count bit number of the measurement unit; and, if the determination unit determines that the first discharge time does not satisfy the predetermined count bit number, A second discharge start voltage higher than the first discharge start voltage is set within a range where the measuring means does not overflow, and a discharge time from the second discharge start voltage until the capacitor is discharged to reach the predetermined voltage is defined as a reference resistance and each heat generation. Measure each element, calculate the resistance value of each heating element from the measurement result and the resistance value of the reference resistor, and determine that the first discharge time satisfies the number of count bits of the measuring means. Is
A resistor for measuring a discharge time from the first discharge start voltage until the capacitor discharges to reach the predetermined voltage for each heating element, and calculating a resistance value for each heating element from the measurement result and a resistance value of the reference resistor. It comprises a value calculating means and a correcting means for correcting the image data based on the obtained resistance value of each heating element.

[0021]

FIG. 2 shows an embodiment of a color thermal printer. In FIG. 2, a platen drum 10 holds a color thermal recording material 11 on its outer periphery and is rotated by a pulse motor (not shown) during thermal recording. . A clamp member 12 is attached to the platen drum 10, and at least one portion, for example, the tip of the color thermosensitive recording material 11 is fixed to the platen drum 10.

The clamp member 12 has a U-shape, and elongated holes 12a and 12b provided at both ends are fitted into the platen drum shaft 15 and the guide pins 16, respectively. The clamp member 12 is pressed against the platen drum 10 by a spring 17 and is moved away from the platen drum 10 by the solenoid 18 when the color thermosensitive recording material 11 is clamped or released.

On the outer periphery of the platen drum 10, a thermal head 20 and an optical fixing device 21 are provided.
The thermal head 20 is provided with a heating section 22 for sequentially generating constant bias thermal energy and thermal energy for gradation expression corresponding to the color density of the pixel.

The optical fixing device 21 is composed of a rod-shaped ultraviolet lamp 23 having emission peaks at approximately 365 nm and 420 nm as shown by a solid line in FIG. 3 and a cut filter 24 having transmission characteristics as shown by a dotted line. It is configured. The cut filter 24 transmits near-ultraviolet rays of about 420 nm when inserted in front of the ultraviolet lamp 23 by a solenoid or the like.

A conveyance roller pair 28 is disposed in the paper supply / discharge passage 27, and passes through the conveyance roller pair 28.
Is transported. A separation claw 29 for guiding the rear end of the color thermosensitive recording material 11 to the paper supply / discharge passage 27 at the time of paper discharge is provided on the platen drum side of the paper supply / discharge passage 27. In this embodiment, one passage is used for both the paper supply passage and the paper discharge passage, but these may be provided separately.

In FIG. 4 showing an example of a color thermosensitive recording material, a cyan thermosensitive coloring layer 33, a magenta thermosensitive coloring layer 34, a yellow thermosensitive coloring layer 35, and a protective layer 36 are sequentially provided on a support 32. . Each of these thermosensitive coloring layers 33
35 are arranged from the surface in the order of thermal recording. For example, when thermal recording is performed in the order of magenta, yellow, and cyan, the positions of the yellow thermosensitive coloring layer 35 and the magenta thermosensitive coloring layer 34 are determined. Be replaced.

As the support 32, opaque coated paper or plastic film is used.
When manufacturing a sheet, a transparent plastic film is used. The cyan thermosensitive coloring layer 33 contains an electron-donating dye precursor and an electron-accepting compound as main components,
It develops cyan when heated.

As the magenta thermosensitive coloring layer 34, a diazonium salt compound having a maximum absorption wavelength of about 365 nm;
It contains a coupler which reacts with heat and develops magenta. When the magenta thermosensitive coloring layer 34 is irradiated with ultraviolet light near 365 nm after thermal recording, the diazonium salt compound is photolyzed and loses its coloring ability. The yellow thermosensitive coloring layer 35 contains a diazonium salt compound having a maximum absorption wavelength of about 420 nm, and a coupler which reacts with the diazonium salt compound to develop a yellow color. When the yellow thermosensitive coloring layer 35 is irradiated with near-ultraviolet rays of 420 nm, it is light-fixed and loses its coloring ability.

FIG. 5 shows an electric circuit of the color thermal printer.
In FIG. 7, one frame of image data is written in the frame memory 40 in a state of being separated for each color. At the time of the gradation expression heating, the image data of the color to be printed is read line by line from the frame memory 40 and written into the line memory 41. The image data in the line memory 41 is read out for each pixel and
Sent to The comparator 42 compares the image data of each pixel with the gradation data (comparison data), and outputs a signal of “1” when the image data is larger.

The microcomputer 43 includes, for example, 64
In the case of gradation, gradation data of "0" to "3F" is sequentially generated in hexadecimal notation. When the gradation data of “0” is sent from the microcomputer 43, the comparator 42
The image data of each pixel is sequentially compared with the gradation data. As a result, the comparison result for one line is output from the comparator 42 as a serial signal, and sent to the shift register 44 via the switch Sa for switching between the print mode and the resistance measurement mode.

When the comparison of the image data for one line is completed, the microcomputer 43 generates the gradation data of “1” and sends it to the comparator 42. Therefore,
By using the gradation data of “0” to “3F”, the image data of each pixel is compared 64 times and converted into 64-bit drive data. The 64-bit drive data is sent to the shift register 44 in 64 times.

The serial drive data is shifted in the shift register 44 by a clock and converted into a parallel signal. The drive data converted into the parallel signal by the shift register 44 is latched by the latch array 45 in synchronization with the latch signal. When the strobe signal is input, the AND gate array 46 outputs a signal of “H” when the input drive signal is “1”. The latch array 45 and the AND gate array 46 are provided with circuit elements for each pixel.

The transistors Tr1 to TrN are connected to the output terminals of the AND gate array 46, respectively. When the output signal is "H", the transistors are turned on. Heating elements R1 to RN are connected in series to these transistors Tr1 to TrN. Each heating element R1
〜RN are resistance elements. The reference resistor Rs and the transistor Trs are connected in parallel with the heating elements R1 to RN and the transistors Tr1 to TrN. As the reference resistor Rs, a resistor having a known resistance value rs and an error of about 1% is used.

A noise absorbing capacitor 50 is connected in parallel with the heating elements R1 to RN and the reference resistor Rs.
The capacitor 50 is connected to the power supply unit 51. The power supply unit 51 includes a switch Sb, a rectifier circuit 52, and a voltage stabilizing circuit 53. The switch Sb is always closed in the print mode, and in the resistance measurement mode,
The microcomputer 43 controls opening and closing each time the resistance values r1 to rN of the heating elements R1 to RN are measured.

A non-inverting input terminal of a comparator 55 is connected to one terminal of the capacitor 50. The comparison voltage Vref of the comparator 55 is the power supply voltage Eref.
{Ra / (ra + rb)} E obtained by dividing the resistance by resistors 62 and 63 having resistance values ra and rb.

The microcomputer 43 is provided with a resistance measuring section 43a. This resistance measuring unit 43a
A counter circuit 59 and a resistance value calculating unit 60 are provided inside,
Each resistance value r1 of the heating elements R1 to RN in the resistance measurement mode
To rN, and write the resistance values r1 to rN to the RAM 43b. The resistance value r1 written in the RAM 43b
ＲrN is backed up by the battery 56.

The measurement of the resistance value of the heating element by the resistance measuring section 43a will be described below using equations. The discharge starting voltage is E, the comparison voltage is Vref, the capacitance of the capacitor 50 is C, the resistance value of the heating element is r, and the discharge time until the voltage of the non-inverting input terminal of the comparator 55 changes from E to Vref is T. Then, these relationships are expressed by the following Expression 5.

[0038]

Vref = E exp {-T / (C r)}

From equation (5), the resistance value r of the heating element is:
It is represented by the following Equation 6.

[0040]

R = -T / C / ln (Vref / E)

By measuring the discharge time Ts discharged by the reference resistor Rs having a known resistance value rs and the discharge time T of each heating element, the resistance value r of each heating element is calculated from the following equation (7). Can be calculated.

[0042]

R = rs · T / Ts

From the above equation (5), the discharge starting voltage E
Is represented by the following Expression 8.

[0044]

E = Vref / exp {−T / (C · r)}

In the equation (8), the capacitance C (≦ Cmax) of the capacitor 50 and the discharge starting voltage E (Emin ≦ E ≦
Emax), the comparison voltage Vref (Vrefmin ≦ Vref ≦ Vrefma)
x) denotes a capacitor 50, a power supply unit 51, a comparator 5
Once the solid (specific components) of No. 5 is determined, there is no significant change except for the temperature characteristics. Further, the resistance value r (r
min ≦ r ≦ rmax) is the upper limit (rmax) depending on the specification.
), The lower limit (rmin) can be determined. The maximum discharge time Tmax is when the discharge start voltage E, the capacitance C and the resistance value r are maximum, and the comparison voltage Vref is minimum.
The following equation 9 holds.

[0046]

E ≦ Emax = Vrefmin / exp ｛−Tmax / (C
max · rmax)｝

A counter circuit 59 is added to Tmax in the equation (9).
By substituting the maximum time measured by
x is determined, and the maximum firing voltage Emax is determined in consideration of the tolerance.
By setting the discharge starting voltage E from the counter circuit 5
It is possible to set the discharge start voltage E such that the number of 9 count bits does not overflow. The discharge start voltage which does not overflow is set as E1, and the discharge time Ts1 until the reference resistor Rs generates heat and the voltage at the non-inverting input terminal of the comparator 55 decreases from the discharge start voltage E1 to Vref is measured. The relationship between E1 and the discharge time Ts1 is represented by the following Expression 10 from Expression 8.

[0048]

E1 = Vref / exp ｛−Ts1 / (C · r
s)｝

On the other hand, the measured count value of the discharge time Ts1 is equal to the number of count bits of the counter circuit 59, for example, 1
If it is determined that the resolution of the counter circuit 59 is sufficiently utilized when the number of significant digits is 13 bits or more with respect to 6 bits, this determination expression is represented by the following Expression 11. Note that one unit time used by the counter circuit 59 is t
Set to 0. The reason why the right side of the equation is multiplied by "rsmin / rmax" is to prevent the counter circuit 59 from overflowing even when the heater element having the largest resistance value is measured.

[0050]

Ts1 <2 ¹³ t0 · rsmin / rmax

If the measured discharge time Ts1 satisfies Expression 11, it is determined that the resolution of the counter circuit 59 is not sufficiently utilized, and therefore, as shown in FIG. 6, the counter circuit 59 does not overflow. A discharge time Ts2 longer than the discharge time Ts1 is set within the range. The following equation 12 is used to set the discharge time Ts2. Note that Q is the number of count bits of the counter circuit 59.

[0052]

Ts2 = (2 ^Q −1) · t0 · rsmin / rmax

In this embodiment, assuming that Q = 16, the discharge time Ts2 can be obtained from the following equation (13).

[0054]

Ts2 = (2 ¹⁶ −1) · t0 · rsmin / rmax

From the discharge time Ts2, the second discharge starting voltage E2 is obtained from the following equation (14) based on the equation (8).

[0056]

E2 = Vref / exp {-Ts2 / (C.rs)}

From Expressions 10 and 14, the following Expression 15 is obtained.

[0058]

E2 = Vref / exp ｛(Ts2 / Ts1) ·
ln (Vref / E1)｝

In actual operation, when the voltage setting error of the discharge start voltage E2 is on the plus side, the counter circuit 59 may overflow. Therefore, Expression 16 is obtained in which the discharge start voltage E2 is set lower by multiplying the right side of Expression 15 by the voltage setting tolerance K (≦ 1.0).

[0060]

E2 = K · Vref / exp ・ (Ts2 / Ts)
1) · ln (Vref / E1)｝

On the basis of the discharge starting voltage E2 obtained from the equation (16), the counter circuit 59
Is measured for each of the heating elements R1 to RN until the voltage at the non-inverting input terminal decreases from E2 to coincide with Vref. Based on the measured discharge times T1 to TN of the heating elements R1 to RN, the resistance value calculating unit 60 calculates the heating elements R1 to RN from the following Expression 17 where Ts in Expression 7 is Ts2.
The resistance values r1 to rN of the RN are calculated.

[0062]

R = rs · T / Ts2

The measured discharge time Ts1 is given by
Is not satisfied, it is determined that the resolution of the counter circuit 59 is sufficiently utilized, and the discharge times T1 to TN of the respective heating elements R1 to RN are measured while the discharge start voltage E1 is maintained. Then, based on the discharge times T1 to TN, the resistance values r1 to rN of the respective heating elements R1 to RN are calculated from the following Expression 18 where Ts in Expression 7 is Ts1.

[0064]

## EQU18 ## r = rs.T / Ts1

The operation of the thus configured color thermal printer will be described with reference to FIG. During the initial setup of the color thermal printer, the mode is switched to the resistance measurement mode by the switch Sa, and the shift register 44 is connected to the microcomputer 43. The microcomputer 43 turns on only the transistor Trs and turns on the switch Sb while keeping the transistors Tr1 to TrN off.

After the charging voltage of the capacitor 50 reaches the discharge starting voltage E1, the switch Sb is turned off, and at the same time, the counter circuit 59 of the resistance measuring section 43a starts counting. When the voltage at the non-inverting input terminal of the comparator 55 is E
When the voltage drops from 1 to Vref, the counter circuit 59 calculates the discharge time Ts1 by the reference resistance Rs by multiplying the count value Qs1 measured during this time by the unit time t0.

When the discharge time Ts1 satisfies the determination formula 11, it is determined that the number of significant digits of the discharge time Ts1 is less than 13 bits and that the number of count bits of the counter circuit 59 is not sufficiently utilized. , A new discharge time Ts2 is set from Expression 13. Using this discharge time Ts2, E2 is obtained from Expression 16, and this is set as a new discharge start voltage.

After turning off the transistor Trs, the microcomputer 43 turns on the transistor Tr1.
After turning off the transistors Tr2 to TrN, the switch Sb is closed to charge the capacitor 50. After the voltage at the non-inverting input terminal of the comparator 55 reaches the discharge start voltage E2, the switch Sb is opened, and the capacitor 50 is discharged by the heat generated by the heating element R1.

The counter circuit 59 starts counting simultaneously with the opening operation of the switch Sb, and the voltage of the non-inverting input terminal of the comparator 55 decreases from the discharge start voltage E2 to Vre.
The discharge time T1 is obtained from the count number Q1 up to f and the unit time t0. The resistance value calculation unit 60 substitutes the discharge time T1 into Expression 16 to substitute the resistance value r1 of the heating element R1.
Is calculated and written to the RAM 43b.

Subsequently, the transistor Tr2 is turned on, and the other transistors Tr1 and the transistors Tr3 to TrN are turned off. The heating element R2 is generated by the counter circuit 59.
Is measured, and based on this, the resistance value r2 of the heating element R2 is calculated by the resistance value calculation unit 60 and written into the RAM 43b. Similarly, the resistance values r3 to rN of the heating elements R3 to RN are calculated and the RAM 43
b. These resistance values r1 to rN are maintained until the battery 56 is exhausted.

If the discharge time Ts1 does not satisfy Equation 11, the number of significant digits of the discharge time Ts1 is one.
Since it is determined that the number of count bits of the counter circuit 59 is sufficiently utilized with three or more bits, the discharge time T1 to the discharge time T1 to
The TN was measured, and the resistance values r1 to r1 of the heating elements R1 to RN were measured.
Calculate rN.

In the print mode, the shift register 44 is connected to the comparator 42 by the switch Sa. In the print mode, first, image data of three colors is taken into the frame memory 40. For these image data, correction data is calculated from the difference between the ideal resistance value when the heating elements R1 to RN are completely uniform and the actually measured resistance values r1 to rN, and recorded by the heating elements R1 to RN. The image data is corrected by the correction data so that the image to be printed is printed accurately.

At the time of paper feeding, the platen drum 10 is stopped with the clamp member 12 being vertical in FIG. 2, so that when the solenoid 18 is energized, the clamp member 12 is set to the clamp release position. The conveying roller pair 28 nips the color thermosensitive recording material 11 supplied from a cassette (not shown) and conveys it toward the platen drum 10. The transport roller pair 28 stops once when the leading end of the color thermosensitive recording material 11 enters between the platen drum 10 and the clamp member 12. Thereafter, when the solenoid 18 is turned off, the clamp member 12 is returned by the spring 17, and the color thermosensitive recording material 1
Clamp the tip of 1. After this clamping, the platen drum 10 and the conveying roller pair 28 rotate, so that the color thermosensitive recording material 11 is wound around the platen drum 10.

When the platen drum 10 rotates intermittently at regular steps and the leading end of the recording area of the color thermosensitive recording material 11 reaches the thermal head 20, thermal recording is started. At the time of this thermal recording, the image data of the yellow image for one line is read from the frame memory 40 and once written to the line memory 41.

Next, the corrected image data of each pixel is sequentially read from the line memory 41 and sent to the comparator 42, where it is compared with the gradation data of the gradation level "0". In the pixel for recording the yellow image, the comparator 42
Is "1", and is "0" for a pixel that does not record a yellow image. The comparison result of each pixel is sent to the shift register 44 as serial drive data, and is shifted in the shift register 44 by a clock to be converted into parallel drive data. This parallel drive data is latched by the latch array 45,
The signal is sent to the AND gate array 46.

The microcomputer 43 generates a bias heating pulse having a long width and sends it to the AND gate array 46 as a strobe signal. AND gate array 4
6 outputs the logical product of the strobe signal and the output signal of the latch array 45, so that among the output terminals of the AND gate array 46, the output terminal of the latch array 45 is "1".
Output "1". For example, AND
When the first output terminal of the gate array 46 is “1”, the transistor Tr1 is turned on, so that the heating element R
1 is energized and generates heat. As a result, the heating element R1 is energized for a time corresponding to the bias heating pulse, and gives the bias heat energy to the color thermosensitive recording material 11.

Before the end of the bias heating, the microcomputer 43 generates gradation data having a gradation level of "0", sends it to the comparator 42, and compares it again with the image data of each pixel. By this comparison, serial drive data is formed, and this drive data is stored in the shift register 44.
Is written to. When the bias heating is completed, the microcomputer 43 generates a tone expression pulse having a short pulse width. This gradation expressing pulse is sent to the AND gate array 46 as a strobe signal. The heating element is energized for a short time by this strobe signal, and the yellow thermosensitive coloring layer 35 is colored to the density of the gradation level "1".
Hereinafter, the microcomputer 43 sets the gradation level to “1”.
In order to change in order from to “3F”, drive data corresponding to each gradation level is output from the comparator 42. As a result, the heating elements R1 to RN are energized by the number of times corresponding to the corrected image data, and the color thermosensitive recording material 11 is given a gradation expression heat energy to develop a color at a desired density. For example, in the case of 64 gradations, for a pixel having the maximum density, 64 pulse currents are supplied to the heating element for gradation expression.

When the first line of the yellow image is recorded, the platen roller 10 rotates stepwise by one pixel, and the second rotation of the yellow image from the frame memory 40 is performed.
The image data of the line is read. The second line is thermally recorded on the color thermosensitive recording material 11 based on the image data of the second line of the yellow image. When the portion where the yellow image is thermally recorded reaches the optical fixing device 21, the yellow thermosensitive coloring layer 35 is optically fixed here. In the optical fixing device 21, the cut filter 24 is set in front of the ultraviolet lamp 23, so that near-ultraviolet light near 420 nm is irradiated on the color thermosensitive recording material 11. As a result, the diazonium salt compound contained in the yellow thermosensitive recording material 11 is decomposed, and the coloring ability is lost.

When the platen drum 10 makes one rotation and the recording area comes to the position of the thermal head 20 again, a magenta image is recorded on the magenta thermosensitive coloring layer 34 line by line. The coloring heat energy of the magenta image is larger than the coloring heat energy of the yellow image. However, since the yellow heat-sensitive coloring layer 35 has already been optically fixed, the yellow heat-sensitive coloring layer 35 does not recolor. The color thermosensitive recording material 11 on which the magenta image has been recorded is optically fixed by the fixing device 21 as described above. In this case, since the cut filter 24 is retracted from the front of the ultraviolet lamp 23, all the electromagnetic waves radiated from the ultraviolet lamp 23 are applied to the color thermosensitive recording material 11. Of the electromagnetic waves, the magenta thermosensitive coloring layer 34 is optically fixed by ultraviolet rays near 365 nm.

When the platen drum 10 makes one more rotation and the recording area reaches the position of the thermal head 20 again, a cyan image is recorded on the cyan thermosensitive coloring layer 33 line by line. The cyan heat-sensitive coloring layer 33 does not have light fixability to the cyan heat-sensitive coloring layer 33 because the coloring heat energy has a value that does not cause coloring in a normal storage state. Therefore, in the thermal recording of the cyan thermosensitive coloring layer 33, the optical fixing device 21 is off.

After the thermal recording of the yellow image, the magenta image, and the cyan image is completed, the platen drum 10 and the conveying roller pair 28 rotate in the reverse direction. The reverse rotation of the platen drum 10 causes the rear end of the color thermosensitive recording material 11 to
The sheet is guided to the sheet supply / discharge passage 27 and is nipped by the conveying roller pair 28. Thereafter, when the platen drum 10 reaches the sheet feeding position, the solenoid 18 is energized and the platen drum 10 stops. Solenoid 18
As a result, the clamp member 12 moves against the spring 17 so that the clamp at the front end of the color thermosensitive recording material 11 is released. As a result, the thermally recorded color thermosensitive recording material 11 is discharged to the tray via the paper supply / discharge path 27.

Resistance values r1 to rN of heating elements R1 to RN
Changes with the passage of time and the printing frequency. For example, each time a color thermal printer is set up, a new resistance value r is set.
1 to rN are measured and written to the RAM 43b. Note that a backup power supply may be supplied from, for example, the power supply unit 51 without using the battery 56. Also,
The RAM 43b may be replaced with a flash memory or the like that does not require a backup power supply.

It is also possible to take out only the function of measuring the resistance value of the heating element of the thermal head from the above-described color thermal printer and to make it independent as a thermal head resistance measuring device. In this case, for example, it can be used as a tester when inspecting a thermal head.

In the above embodiment, the first discharge starting voltage E
After measuring the discharge time by causing the reference resistor to generate heat in step 1, it was determined whether or not the resolution of the counter circuit was sufficiently utilized, and the second discharge start voltage E2 was set based on the determination result. Without making a determination, Ts
2, the discharge start voltage E2 may be set, and the resistance value of each heating element may be calculated based on the discharge start voltage E2.

In the description of the above-described embodiment, the order in which the heating element or the reference resistor is turned on in advance and then the capacitor is charged. The discharge may be performed by turning on the reference resistor.

In the above embodiment, a color thermal printer is taken as an example. However, the present invention can be applied to a monochrome thermal printer, a color thermal transfer printer, and the like.
Also, a line printer that moves the color thermosensitive recording material and the thermal head relatively one-dimensionally has been described.
The present invention can also be used for a serial printer whose relative movement is two-dimensional.

[0087]

As described above in detail, according to the present invention, the discharge time from when the capacitor is discharged through the reference resistor having a known resistance value to when the capacitor reaches the predetermined voltage from the predetermined first discharge start voltage is measured. After that, the measuring means resets the second discharge starting voltage higher than the first discharge starting voltage within a range where the overflow does not occur, and sets the discharging time until the capacitor discharges from the second discharging starting voltage to reach the predetermined voltage. Since the resistance value of each heating element is calculated from the measurement result and the resistance value of the reference resistance, the resistance value of each heating element is calculated.
The measurement time can be reduced without lowering the resolution of the measuring means.
Further, in the thermal printer equipped with the resistance measuring device of the present invention, the resistance value of the thermal head is automatically measured and the image data is corrected by this, so that the recorded image can be adjusted without troublesome adjustment before shipping. The occurrence of density unevenness or the like can be prevented.

[Brief description of the drawings]

FIG. 1 is a flowchart of a resistance measurement mode.

FIG. 2 is a schematic view illustrating an example of a color thermal printer.

FIG. 3 is a graph showing characteristics of an ultraviolet lamp and a cut filter of the optical fixing device.

FIG. 4 is an explanatory diagram showing a layer structure of a color thermosensitive recording material.

FIG. 5 is a block diagram showing an electric circuit of the color thermal printer.

FIG. 6 is a graph showing a relationship between a discharge starting voltage of a capacitor and a discharge time.

[Explanation of symbols]

 Reference Signs List 11 color thermosensitive recording material 20 thermal head 43 microcomputer 43a resistance measuring unit 43b RAM 50 capacitor 51 power supply unit 55 comparator 59 counter circuit 60 resistance value calculating unit R1 to RN heating element Rs reference resistance Sa, Sb switch

Claims (4)

[Claims] A capacitor is connected in parallel with a plurality of heating elements arranged in a line on a thermal head, and after charging the capacitor, a discharging time until the capacitor reaches a predetermined voltage is set for each heating element. In the thermal head resistance value measuring method of calculating the resistance value of each heating element based on the measurement result, a capacitor is discharged through a reference resistor having a known resistance value and a predetermined first discharge starting voltage is applied to a predetermined value. The discharge time until the voltage is reached is measured by the measuring means, and then the second discharging start voltage higher than the first discharging starting voltage is reset within a range where the measuring means does not overflow. The discharge time until the voltage reaches the predetermined voltage is measured for each reference resistor and each heating element. Resistance measuring method for a thermal head and calculating the resistance value. 2. A method of connecting a plurality of heating elements arranged in a line to a thermal head in parallel with a capacitor, and charging the capacitor and discharging the capacitor until the capacitor reaches a predetermined voltage. In the thermal head resistance value measuring method of calculating the resistance value of each heating element based on the measurement result, a capacitor is discharged through a reference resistor having a known resistance value and a predetermined first discharge starting voltage is applied to a predetermined value. The first discharging time until the voltage reaches the voltage is measured by the measuring means. If the first discharging time is less than the predetermined number of count bits of the measuring means, the first discharging time is set within the range in which the measuring means does not overflow.
The second discharge start voltage higher than the discharge start voltage is reset, and the discharge time until the capacitor is discharged from the second discharge start voltage to reach the predetermined voltage is measured for each reference resistor and each heating element. The resistance value of each heating element is calculated from the measurement result and the resistance value of the reference resistor. If the first discharge time is equal to or greater than a predetermined number of count bits, the capacitor is discharged from the first discharge start voltage to discharge the predetermined voltage. A method for measuring a resistance value of a thermal head, comprising: measuring a discharge time for each heating element until the heating element reaches the threshold value; and calculating a resistance value for each heating element from the measurement result and a resistance value of a reference resistor. 3. A plurality of heating elements arranged in a line on the thermal head, and a first switch having one end connected to the heating element and for turning on / off a connection between the heating element and a power supply terminal of the thermal head. A group of means, a control circuit for controlling the first group of switch means so that each heating element is connected to a power supply, a capacitor connected in parallel to a power supply terminal of the thermal head, and a power supply section for driving the heating element. A second switch connected in series to the power supply unit and the capacitor; and turning on the second switch to charge the capacitor to a predetermined first discharge start voltage.
Measuring means for turning off the second switch means and measuring a first discharge time from when the capacitor starts discharging through a reference resistor having a known resistance value to when a predetermined voltage is reached; and Determining means for determining whether a predetermined count bit number of the means is satisfied; and if the determination means determines that the first discharge time does not satisfy the predetermined count bit number, the measuring means overflows. A second discharge start voltage higher than the first discharge start voltage is set within a range not to be measured, and a discharge time from the second discharge start voltage until the capacitor is discharged and reaches the predetermined voltage is measured for each of the reference resistance and each heating element. Then, the resistance value of each heating element is calculated from the measurement result and the resistance value of the reference resistance, and the first discharge time satisfies the number of count bits of the measuring means. If it is determined that the first
Resistance value calculation for measuring the discharge time from the discharge start voltage until the capacitor reaches the predetermined voltage for each heating element, and calculating the resistance value for each heating element from the measurement result and the resistance value of the reference resistor. Means for measuring the resistance of a thermal head. 4. A thermal printer having a thermal head in which a plurality of heating elements are arranged in a line, wherein a first switch means group for turning on / off each of the plurality of heating elements, and each of the heating elements. A control circuit for controlling the first group of switch means so that is connected to a power supply;
A capacitor connected in parallel to a power terminal of the thermal head, a power unit for driving the heating element, a second switch unit connected in series with the power unit and the capacitor, and a second switch unit turned on. After charging the capacitor to a predetermined first discharge start voltage, the second switch means is turned off, and a first discharge time period from when the capacitor starts discharging through a reference resistor having a known resistance value to when the capacitor reaches a predetermined voltage. Measuring means for measuring the first discharge time, and determining means for determining whether or not the first discharge time satisfies a predetermined count bit number of the measuring means, and the determination means determines that the first discharge time is equal to the predetermined count bit number. If it is determined that the second discharge start voltage is higher than the first discharge start voltage within a range where the measuring means does not overflow, the second discharge start voltage is set. The discharge time from the start voltage to the discharge of the capacitor to the predetermined voltage is measured for each reference resistor and each heating element, and the resistance value of each heating element is calculated from the measurement result and the resistance value of the reference resistor. If it is determined that the first discharge time satisfies the number of count bits of the measuring means, the discharge time from the first discharge starting voltage until the capacitor discharges to the predetermined voltage is determined for each heating element. Resistance value calculating means for calculating the resistance value of each heating element from the measurement result and the resistance value of the reference resistance, and correction means for correcting image data based on the obtained resistance value of each heating element. A thermal printer comprising: