Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2000-12-06
2003-03-11
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S711000, C347S175000
Reexamination Certificate
active
06531883
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thermal printer that forms an image by a thermal head having a plurality of heating elements, and more particularly to a device and a method for measuring resistance of the heating elements, for modifying image data so as to compensate for variations in resistance between the heating elements.
2. Background Arts
As well known, thermal printers may be classified into thermal transfer printers and thermosensitive type printers. The thermal transfer printers use an ink film and transfers ink from the ink film onto a paper by heating the ink film. The thermosensitive type printers heat a thermosensitive recording medium directly to record an image thereon. To heat the ink film or the thermosensitive recording medium, the thermal printer use a thermal head with a linearly arranged array of large number of heating elements. The heating elements are constituted of resistors connected in parallel to one another.
U.S. Pat. No. 4,734,704 (corresponding to JPA No. 61-213169) discloses a color thermosensitive printer that uses a thermosensitive color recording medium. The color thermosensitive recording medium has a cyan thermosensitive coloring layer, a magenta thermosensitive coloring layer, and a yellow thermosensitive coloring layer, which are formed atop another in this order from a base material. These thermosensitive coloring layers, hereinafter called simply as the coloring layers, have different heat sensitivities that become lower as the distance from the outside surface increases. Thus, the deeper the coloring layer, the higher coloring heat energy is required. Furthermore, the coloring layers may be optically fixed, each by electromagnetic rays of a specific wavelength range. Therefore, recording of a full-color image on the thermosensitive color recording medium is performed in the order from the top or outermost coloring layer to the inner coloring layer, while optically fixing the just recording coloring layer prior to recording the next coloring layer, so as to avoid double-recording.
Each heating element applies a different coloring heat energy to the thermosensitive color recording medium in accordance with a characteristic curve of each coloring layer, to form a color dot at a different density. As the coloring heat energy, first a bias heat energy is applied for heating the thermosensitive color recording medium up to a temperature above which a particular color begins to be developed. Next, a gradation heat energy is applied for developing the particular color at a designated density. The bias heat energy is a constant value determined for each color according to the heat sensitivity or characteristic curve of the individual coloring layer. Generally, the heating element is activated for several ms to several tens of ms (milliseconds) to apply the bias heat energy. On the other hand, in order to reproduce fine gradation, the gradation heat energy needs to be controlled with more accuracy, so activation time or power conduction time is controlled by several &mgr;s (micro seconds) to several tens of &mgr;s after applying the bias heating energy.
In spite of such a fine control of heating or conduction time of the heating elements, the consequent image cannot exactly reproduce the desired fine gradation unless all the heating elements of the same thermal head have a completely uniform resistance. This is because the heating elements generate different heat energies if they have different resistances, even while they are driven for the same time. However, the heating elements generally have variations of about 5% to 10% in resistance. Moreover, the resistance of each heating element varies with age and its recording history. For this reason, the printed images tend to have imperfections, such as chromatic unevenness.
In order to eliminate such undesirable phenomena, U.S. Pat. No. 5,469,068 (corresponding to JPA No. 6-79897) proposes a color thermosensitive printer that measures resistances of the respective heating elements, and modify image data based on the measured resistances so as to compensate for the variations in resistance. In this prior art, a capacitor with a known capacitance is fully charged and, thereafter, discharged through each individual heating element, while counting the time required to discharge the capacitor down to a constant voltage level, e.g. a half of a power source voltage. Since the discharge time is proportional to the resistance of the heating element, the resistance of each individual heating element is obtained based on the discharge time and the known capacitance.
Concretely, the resistance R of one heating element is calculated according to the following equations, assuming that it takes a discharge time T for discharging of the capacitor having a capacitance C from a predetermined discharge start voltage E to a predetermined discharge stop voltage Vref through the heating element. In the above prior art, Vref=E/2.
Vref/E=
exp{−
T/
(
C·R
)} (1)
R=−T/C
/ln(
Vref/E
) (2)
If the capacitance C is a known value, it is possible to calculate the resistance R by measuring the discharge time T. Even where the capacitance C is an unknown value, it is possible to calculate the resistance R according to the following equations, by measuring a discharge time Ts required to discharge the capacitor through a reference resistor whose resistance Rs is known.
R=Rs·T/Ts
(3)
T=−C·R·
ln(
Vref/E
) (4)
As a device for measuring the discharge time, a counter circuit or a timer of a microcomputer is used. The counter circuit counts by a predetermined unit time to output a count corresponding to the discharge time. The microcomputer calculates the discharge time by multiplying the unit time by the obtained count. For the sake of accuracy, it is desirable to predetermine the unit time as short as possible. However, with a sufficiently short unit time, the counter circuit is required to count up to a large value where the discharge time to measure is relatively long. That is, an expensive counter circuit with a large bit number is needed. With an inexpensive counter circuit that has a small counter number, the unit time has to be so long that it is hard to achieve sufficiently accurate measurement of the discharge time.
Furthermore, not only the resistance R of the heating elements, but also the above discharge start voltage E, the discharge stop voltage Vref and the capacitance C of the capacitor have variations respectively. For instance, the variation in the discharge start voltage E is ±2% when E=20V, variation in the reference voltage is ±2% when Vref=19V, the variation in capacitance C is ±20%, and the variation in resistance R is ±1%. If these values are applied to the above equation (2), the discharge time T would have a variation of about +116%~−83%. On the other hand, in order to save time for the resistance measurement, it is desirable to set the discharge stop voltage Vref to be closer to the discharge start voltage E. However, as the ratio of the discharge start voltage E to the discharge stop voltage Vref approaches “1”, the variation in the discharge time T comes to be very large, because being affected by a natural logarithm in the equation (4). Also for this reason, it is necessary to set the unit time to be long enough for preventing overflowing of the counter circuit, in order to use an inexpensive counter circuit with a small bit number.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide a device and a method for measuring resistances of heating elements of a thermal head of a thermal printer, that make it possible to measure the discharge time with accuracy and thus calculate the resistance of the heating element with accuracy based on the discharge time, without the need for an expensive counter circuit having a large bit number.
Another object of
Fuji Photo Film Co. , Ltd.
Kerveros James
Le N.
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