US 4,391,535 AGranted

Method and apparatus for controlling the area of a thermal print medium that is exposed by a thermal printer

AssigneeIntermec Corporation

InventorRoger C. Palmer

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Abstract

After discussion of a typical prior art thermal printer that is capable of providing precise printing of machine-readable characters on a thermal print medium through the use of an electrically-resistive thermal print element, a thermal model of the thermal print element and the heat transfer relationships between the thermal print element and its surrounding environment is constructed. From this thermal model, an equivalent electrical model is constructed which provides a signal representing the estimated temperature of the thermal print element. This signal is then used to control the exposure time of the thermal print (the time that is sufficient to raise the temperature of only an incremental area of the thermal print medium in uniform contact with the thermal print element to or above the threshold temperature) and to control the rest time of the thermal printer (the time following the exposure time that is sufficient to allow the temperature of the thermal print element to decrease to a value that will not result in additional exposure of the thermal print medium upon movement thereof).

An apparatus for controlling the area of a thermal print medium that is exposed by a thermal printer, the thermal print medium being such that any portion thereof is exposed when its temperature equals or exceeds a predetermined threshold temperature, the thermal printer including an electrically-resistive thermal print element having a surface that is in good thermal contact with the thermal print medium and that has an area equal to the desired exposure area of the thermal print medium, the thermal printer further including driver means for applying an electrical signal having a substantially constant amplitude to the thermal print element, said apparatus comprising:

electrical energy storage means representing the thermal mass of the thermal print element, said electrical energy storage means being adapted to provide a first signal whose amplitude is proportional to the instantaneous amount of electrical energy stored in said electrical energy storage means;
first means for transferring electrical energy into said electrical energy storage means at a rate proportional to the power being supplied to the thermal print element by the application of said substantially constant amplitude electrical signal thereto;
second means transferring electrical energy to and from said electrical energy storage means in relation to the heat transferred between the thermal print element and the environment in heat transfer relationship with the thermal print element, whereby the amplitude of said first signal is proportional to the instantaneous temperature of the thermal print element;
third means providing a second signal whose amplitude is related to the threshold temperature of the thermal print medium;
and,
fourth means concurrently enabling the driver means of the thermal printer and said first means, and concurrently disabling the driver means of the thermal printer and said first means whenever the amplitude of said first signal exceeds that of said second signal.

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Abstract

Claims10