International Business Machines Corporation

Frank J Horlander

Donald Wayne Stafford

Stanley Dyer

Donald Fred Croley

Kenneth E Edds

Alan E Bohnhoff

John C Bartlett

A thermal printer has a plurality of electrodes arranged in a column for applying heat to a resistive ribbon to transfer material from the resistive ribbon to a recording medium. When the electrode is to be energized in the present cycle but was not energized in the previous cycle or either of the contiguous electrodes is not to be energized in the present cycle, additional power is applied to the electrode during a first portion of the present cycle to increase the total power applied to the electrode. This improves the print quality through enhancing the left leading edge and/or either vertical edge of the character. During a second portion of the cycle, power is applied for the same period of time to all of the electrodes, which are to be activated, during the particular cycle. The additional power applied during the first portion of the cycle is obtained through comparing stored data of the specific electrode as to its activation in the prior cycle and the stored data of the two contiguous electrodes during the particular cycle.

Method and apparatus for controlling print quality of a thermal printer

Print quality controller for a thermal printer

A voltage mode printhead drive for a resistive ribbon 20 employs sensing electrodes 7a contiguous to drive electrodes 1a through 1n. Electrodes 7a sense the return voltage, Vc, from the drive electrodes to ground. Electrodes 7a are mounted in and are a part of the printhead.

Electro resistive printhead drive level sensing and control

Ribbon (22) in thermal printing has an outer layer which adheres to printed characters at intermediate temperatures, lower than printing temperatures. The printhead (7) has a column of electrodes (9) which sweep across the character area. Lift-off is accomplished by the pattern control (40) controlling the current source (38) to provide rapid, square wave pulses displaced in phase 180 degrees at adjoining electrodes (9). The high level of the pulses is about that of the level at printing, and the pulses are sufficiently rapid so that their net effect is to raise the outer layer ribbon (22) to the intermediate temperature. At areas corresponding to underlines of characters, duration of the up period is longer. Good, long term reliability is achieved by the significant erase level being very close to the print level.

Drive for thermal printing lift-off correction

Electrodes 1a through 1n are driven by operational amplifier 4 under control of a reference current source 25. The voltage at each electrode 1a through 1n is monitored by diodes 19a through 19n so that point 21 is set at that of the lowest electrode voltage. A current source provides equal currents through diode 23 and the conducting one of diodes 19a through 19n. The potential of control input 5 is therefore that of the lowest potential of all of electrodes 19a through 19n. Feedback through resistor 27 produces a differential amplifier system in which V2 is set by source 25. The output of amplifier 4 and the magnitude of resistors 15a through 15n are selected so that the nominal voltage on line 13 is reduced by one-half across the resistor 15a through 15n. This approximates constant power to ribbon 2. The voltage regulation and constant power each act to limit power dissipation at the ribbon surface. This reduces debris at the printhead.

Regulated voltage and approximate constant power for thermal printhead

A current-drive circuit (FIG. 1) is provided to drive each of forty electrodes 41. When selected, the circuit forces line 27 to a level of drive voltage Vdr minus a current-level reference voltage Vlev. A constant current is produced across resistor 25. Line 27 is connected through resistor 29a to line 27a, which is the same point in the current-drive circuit of the adjoining electrode 41a (FIG. 3) on one side of electrode 41. Line 27 is similarly connected through resistor 29b to line 27b, which is the same point in the current-drive circuit of the adjoining electrode 41b (FIG. 3) on the opposite side of electrode 41. Selection of the drive circuit also connects line 27 to the associated electrode 41. An unselected drive circuit for an adjoining electrode, such as the one connected to drive electrode 41a, has line 27a floating to the level dictated by Vdr through its resistor 25a, while its electrode 41 is disconnected. Current from the unselected circuit through resistor 29a adds to the current in electrode 41, thereby eliminating lightened-edge printing from current spreading.

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Lexington, KY, United States of America

Frank J Horlander