Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
1998-10-20
2001-03-20
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C347S191000
Reexamination Certificate
active
06204671
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thermal printer and a resistance data measure device for a thermal head of the same. More particularly, the present invention relates to a thermal printer in which irregularity in performance of heating elements of a thermal head can be overcome in printing operation, and a resistance data measure device for a thermal head of the thermal printer.
2. Description Related to the Prior Art
There are widely used thermal printers including a thermal transfer printer in which ink film is used, and a direct thermal printer in which thermosensitive recording material is heated for directly printing an image.
The color thermal printer of the direct printing is used with a color thermosensitive recording material or recording sheet, in which thermosensitive coloring layers of magenta, yellow and cyan are formed on a support. Selectively to develop colors of the coloring layers, the coloring layers are different in the amount of heat energy (in mJ/mm
2
) to apply to. The lowest heat energy is required for coloring one of the coloring layers located at the obverse of the recording sheet. Higher heat energy is required for coloring the coloring layers according to the closeness to the support. Once a coloring layer is heated, electromagnetic rays are applied to it to fix it before heat energy for another coloring layer next to be colored is applied. This is for the purpose of destroying the further coloring ability of the present coloring layer to prevent it from being colored beyond desired density.
The thermal head includes an array of heating elements as resistors, which are arranged to record pixels arranged in one line. To record an image of each color thermally, heat energy is applied to the recording sheet as a sum of bias heat energy and image heat energy. The bias heat energy has an amount slightly short of causing the coloring layer to develop the one color, and is applied to the recording sheet during the bias heating at the beginning of recording each one pixel. The image heat energy has an amount determined according to the gradation level of one color, namely coloring density of the pixel to be printed, and is applied to the recording sheet during the image heating which succeeds the bias heating.
To reproduce high gradation, the heating operation is controlled finely. The heating elements of the thermal head need to have an equal resistance for the purpose of precise application of the heat as controlled. It is however inevitable that the heating elements have irregularity of 5-10% in the resistance. If the heating elements are driven for an equal duration, generated heat energy differs between the heating elements due to the differences in the resistance. Irregularity in density is likely to occur in an image being recorded.
U.S. Pat. No. 5,469,068 (corresponding to JP-A 6-79897) discloses a thermal printer in which the resistance of the heating elements is measured for the purpose of preventing occurrence of irregularity in printed density by compensating image data. The thermal printer is provided with a capacitor, of which capacitance is known. The capacitor is charged fully, and then discharged via the heating elements connected thereto. Time for decrease in a capacitor voltage is measured. For example, the decrease of the capacitor voltage down to a half of a power source voltage is checked to measure the discharging time. According to the discharging time and the capacitance of the capacitor, the resistance of the heating elements is calculated in view of a proportional relationship between the discharging time and the resistance of the heating elements.
This prior document also suggests use of a reference resistor to which the capacitor is connected, and of which resistance is known. The capacitor is charged fully, and then discharged via the reference resistor. The discharging time for decrease in the capacitor voltage is measured until coming down to a predetermined voltage. Again the capacitor is charged fully, and discharged via the heating elements.
The discharging time for decrease in the capacitor voltage is measured until coming down to the predetermined voltage. According to the resistance of the reference resistor and the discharging times via the reference resistor and the heating elements, the resistance of the heating elements is calculated.
According to U.S. Pat. No. 5,469,068 (corresponding to JP-A 6-79897), the capacitor is fully charged by applying voltage for a predetermined duration, and then discharged down from the capacitor voltage equal to the power source voltage, until the discharging time is measured. To measure the resistance of the heating elements with precision, the duration for the charging operation should be long enough for ensuring the full charging. A problem of the prior art lies in considerable slowness of measuring the resistance of all the heating elements. If the duration for the charging operation is shortened, the capacitor voltage upon the finish of the charging is not kept equal due to the charge having initially remained in the capacitor. Another problem lies in low precision in the measurement of the resistance.
SUMMARY OF THE INVENTION
In view of the foregoing problems, an object of the present invention is to provide a thermal printer in which irregularity in performance of heating elements of a thermal head can be overcome in printing operation, and a resistance data measure device for a thermal head of the thermal printer.
Another object of the present invention is to provide a thermal printer in which resistance of heating elements can be measured precisely and quickly, and a resistance data measure device for a thermal head of the thermal printer.
In order to achieve the above and other objects and advantages of this invention, a thermal head has plural heating elements arranged along a line and connected in parallel with one another. A respective one of the heating elements is connected to one of plural heat control switches. The heat control switches are selectively turned on/off for driving the heating elements individually by applying electrical energy to generate heat. A capacitor is connected in parallel with the plural heating elements. A charging switch is adapted to charging the capacitor A voltage detector detects a capacitor voltage across the capacitor. A reference resistor is connected in parallel with the plural heating elements and the capacitor. An additional switch is connected to the reference resistor. A control circuit controls the heat control switches, the charging switch and the additional switch. The control circuit initially turns on the charging switch to charge the capacitor. The charging switch is turned off to stop charging the capacitor upon an increase of the capacitor voltage to a predetermined high voltage. One selected from a group including the additional switch and the plural heat control switches is thereafter turned on, to discharge the capacitor via the reference resistor or one of the heating elements in association with the selected one being turned on. A timer measures discharging time elapsed in a decrease of the capacitor voltage from the predetermined high voltage to a predetermined low voltage while the capacitor is discharged, in association respectively with the reference resistor and the heating elements. A resistance data determiner determines resistance data of the heating elements in accordance with the discharging time respectively of the heating elements with reference to the discharging time of the reference resistor.
In a preferred embodiment, the resistance data is a ratio of the discharging time of each of the heating elements to the discharging time of the reference resistor, and represents a relative greatness of resistance.
The resistance data measure device is incorporated in a thermal printer in which the heating elements are respectively driven by a drive signal based on bias data and image data, to effect thermal recording to recording material. The thermal printer
Fuji Photo Film Co. , Ltd.
Metjahic Safet
Nguyen Vincent Q.
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