Method and apparatus for improving print quality of a thermal printer

Details

Inventors: Beach, Harold E.;
Assignee: Hewlett-Packard Company (Palo Alto, CA)
Primary Examiner: Albritton; C. L.
Assistant Examiner:
Attorney, Agent or Firm: Jones; Allston L., Wong; Edward Y.

The uniformity of density of characters printed by thermal printers upon thermally sensitive paper is enhanced by controlling the amount of energy supplied to the print head during subsequent printings before the print head has completely cooled to ambient temperature. To obtain the desired uniformity the energy supplied to the print head for subsequent printings is made proportional to the energy lost by cooling of the print head between printings. This results in the print head being reheated to substantially the same printing temperature for each printing of a character or character segment. By using a dot driver having an R-C circuit that recharges the capacitor between print pulses at a rate that is proportional to the thermal time constant of the print head, the energy stored by the capacitor can then be used to re-heat, or control the re-heating, of the print head to substantially the same selected print temperature. By maintaining the R-C charging time constant substantially between 0.1.tau. and .tau. (.tau. is the thermal time constant of the print head) the resultant printed character segments have substantially uniform density.

DETAILED DESCRIPTION I claim: 1.
Method for printing characters with an electro-thermal transducer type print head, having at least one thermal element, on a thermally sensitive medium, the method comprising, in the order recited, the steps of: charging an energy storage means to a preselected level; electrically energizing a thermal element of said print head from said energy storage means to cause heating of said thermal element to a selected temperature to print a selected character segment of a selected density on the thermally sensitive medium; recharging said energy storage means at a fixed preselected rate that is proportional to the thermal energy loss rate of the print head; subsequently energizing a thermal element of said print head from said energy storage means to reheat the print head to substantially the same selected temperature to print another character segment of substantially the same selected density as that of previously printed character segments on the thermally sensitive medium; and repeating the recharging and subsequently energizing steps a sufficient number of times to complete the desired printing.
2.
Apparatus for printing characters with an electro-thermal transducer type print head, having at least one thermal element, on a thermally sensitive printing medium, the apparatus comprising: gating means responsive to an applied print pulse for initiation of the printing of selected character segments; and energy storage means coupled to the gating means and at least one thermal element on the print head for storing electrical energy, said energy storage means having a fixed preselected time constant that is proportional to the thermal energy loss time constant of the print head when printing is not being performed, and for limiting the amount of energy transferred to said at least one thermal element to an amount that is proportional to the amount of thermal energy lost by the print head between consecutive printings to reheat the print head to substantially the same selected temperature as during each prior printing to cause each character segment printed on the thermally sensitive medium to have substantially the same density as previously printed character segments

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