Typewriting machines – Typing by other than type-face or type-die – Thermal
Reexamination Certificate
1999-04-02
2001-03-27
Hilten, John S. (Department: 2854)
Typewriting machines
Typing by other than type-face or type-die
Thermal
C400S120050, C400S120060
Reexamination Certificate
active
06206588
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
This invention relates to a thermal printer comprising a large number of heating elements made of resistors, etc., and an electric motor for transporting recording paper little by little at the recording time.
2. Description of the Related Art
A thermal printer comprising a large number of heating elements divided into groups for activating the groups at different timings (asynchronously) in a time-divisional manner to reduce current capacity of a power supply, etc., is available.
On the other hand, for example, a thermal printer for recording while transporting recording paper one line at a time by a stepping motor may record in both stopping and moving periods of recording paper to raise the recording speed.
In a thermal printer in a related art for activating heating elements in a time-divisional manner and recording also in the moving period of recording paper, all groups are activated only through one order every turn-on period of a driving pulse of a stepping motor. For example, to divide a large number of heating elements of a thermal print head into a left group and a right group relative to the center of the longitudinal direction of the thermal print head and activate the left and right groups separately (each once), a first strobe signal STB
1
is turned on once for activating the left heating elements and then a second strobe signal STB
2
is turned on once for activating the right heating elements every turn-on period of a driving pulse of a stepping motor, as shown in
FIG. 11
, or a first strobe signal STB
1
is turned on more than once for activating the left heating elements and then a second strobe signal STB
2
is turned on more than once for activating the right heating elements every turn-on period of a driving pulse, as shown in FIG.
12
.
However, with the thermal printer in the related art, there occurs unevenness in density on a recorded image because the rotation operation of the stepping motor is unstable; high-quality recording cannot be accomplished.
That is, the rotation speed of the stepping motor is not always constant during the turn-on period of a driving pulse and varies considerably. Particularly, inexpensive stepping motors must often be adopted to reduce the manufacturing costs of the thermal printers. In such a case, the rotation speed of the stepping motor varies hard and even the time between the rising edge of a driving pulse and the rotation start of the actual rotation shaft of the stepping motor is not constant, causing large variations to occur. However, if all groups are activated only through one order every turn-on period of a driving pulse of the stepping motor as in the thermal printer in the related art, the rotation speed of the stepping motor during the activating period varies remarkably from one group to another; for example, the left half portion of an image recorded on recording paper becomes remarkably darker than the right half portion.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a thermal printer capable of recording with high quality regardless of speed variations of an electric motor for transporting recording paper.
In order to achieve the above object, there is provided a thermal printer having the following aspects.
According to a first aspect of the present invention, there is provided a thermal printer comprising: a plurality of heating elements for heating a recording medium to form an image thereon; an electric motor for transporting the recording medium to a direction opposite to a subscanning direction by rotating a predetermined angle each time a driving pulse is input; and an activating control section for dividing the plural heating elements into a plurality of groups and for activating every groups of the heating elements in sequence asynchronously such that the sequential activating of all the groups is repeated more than once while each time the driving pulse is turned on.
According to a second aspect of the present invention, the number of repetition of the sequential activating is determined so the heating elements as to attain activating energy which is able to form the image on the recording medium based on a resistance thereof and voltage applied thereto.
According to a third aspect of the present invention, the sequential activating of all the groups is repeated three to ten times while each time the driving pulse is turned on.
Thus, sequential activating of all the groups is repeated more than once every turn-on period of the driving pulse input to the electric motor, so that high-quality recording can be executed regardless of speed variations of the electric motor for transporting recording paper.
That is, activating of each group of the heating elements is executed at least twice at distant positions in time sequence during the turn-on period of the driving pulse, thus the average speed of the electric motor during the activating period of the heating elements is leveled off for each group. Therefore, the recording density difference between the groups lessens and a high-quality image recording can be conducted.
If sequential activating of all the groups is repeated three to ten times every turn-on period of a driving pulse, damage to the heating elements is avoided as much as possible and a higher-quality image recording can be conducted.
That is, considering only improvement in the quality of a recorded image, the number of repetitions of sequential activating of all the groups should be increased as much as possible. However, if the number of repetitions is increased extremely, a similar effect to that of making a direct current substantially flow into the heating elements is produced and damage to the heating elements occur due to overheating. Therefore, the appropriate number of repetitions is about three to ten times.
According to a fourth aspect of the present invention, the activating control section includes: a plurality of drive circuit sections provided so as to correspond to the respective groups of the heating elements for activating the heating element used for forming the image while a strobe signal is turned on; and a strobe signal generating section for generating the strobe signal to be supplied to the respective drive circuit sections in sequence asynchronously.
According to a fifth aspect of the present invention, turn-on periods of the strobe signals adjacent in time sequence are overlapped with each other in a delay time of the drive circuit section while the strobe signals pass once through all the groups of the heating elements.
According to a sixth aspect of the present invention, the overlapped time is approximately 5-10 &mgr;sec.
In this configuration, a power supply can be driven stably and efficiently.
That is the drive circuits are configured so as to make the actual activating of the heating elements delayed about 5-10 &mgr;sec from the time of the rising edge of the strobe signal to suppress rapid change of the current output of the power supply circuit. As the turn-on periods of the strobe signals adjacent in time sequence are thus made to overlap about 5-10 &mgr;sec, varying of the current output can be suppressed and intermittent output of the same can be prevented. Thereby, the power supply can be efficiently used.
According to a seventh aspect of the present invention, all the turn-on periods of the strobe signals are overlapped with each other in a delay time of the drive circuit section while the driving pulse is turned on.
In this configuration, the varying of the output current of the power supply circuit can be suppressed during the turn-on period of the motor driving pulse.
According to an eighth aspect of the present invention, the strobe signal generating section supplies the first strobe signal to the drive circuit after a lapse of a predetermined time since the driving pulse is tuned on.
According to a ninth aspect of the present invention, the strobe signal generating section supplies the first strobe signal to the drive circuit section
Hilten John S.
Morgan & Lewis & Bockius, LLP
Nolan, Jr. Charles H.
Rohm & Co., Ltd.
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