1. Field of the Invention
The present invention relates to a drive voltage adjusting method for an office-use or industrial-use ink jet printer with piezoelectric elements for ejecting ink on demand.
2. Description of the Related Art
There are thermal and piezoelectric type on-demand ink jet heads. Thermal type ink jet heads use heaters to boil a portion of ink filling the head, to generate a bubble. Ink is ejected by force of the expanding bubble. Piezoelectric type ink jet heads include a piezoelectric element that deforms a portion of an ink chamber wall, in order to apply pressure to ink in the chamber and eject an ink droplet.
Thermal type heads are advantageous because they can be formed using lithography to a fine nozzle pitch of 100 .mu.m or less. However, thermal type heads can only be driven at an ejection frequency of about 10 to 12 kHz during consecutive ejection. Also, only ink with a boiling point of about 100.degree. C. can be used as the ink to be ejected, which hinders broad use of thermal type heads in industry. In addition, thermal type heads uses a thin-film resistor with a protective layer deposited thereon to generate heat. Generally, because the thin-film resistor and/or the protective layer is corroded before ejecting one hundred millions dots, replacement of the head needs to be performed at a short interval.
With regard to piezoelectric type heads, piezoeleotric elements deform only in small amounts, so the diaphragm in the ink chamber must have a large surface area to produce sufficient deformation for ink ejection. As a result, the nozzle pitch of piezoelectric type heads can not be formed smaller than about 140 .mu.m at the present technology. However, piezoelectric type heads are well suited for high speed printing. That is, the drive frequency depends on the shape of the piezoelectric elements, so piezoelectric elements can be driven at a frequency of 20 kHz or more. Also. piezoelectric type heads are well adapted for industrial use, because in contrast to thermal type heads, they can be used to eject any type of ink and service life is much longer than that of the thermal type.
In a multi-nozzle ink jet printer, a head having a plurality of nozzles is mounted on a carriage, and the carriage is scanned in a horizontal direction across a recording sheet. The head is driven to print while the carriage is scanned horizontally across the recording sheet. Next, the recording sheet is transported in its lengthwise direction, that is, in a direction perpendicular to the horizontal direction. Then, the carriage is again scanned horizontally across the recording sheet. This cycle is repeated until images are printed across the entire recording sheet.
When all nozzles in the nozzle row are driven simultaneously and consecutively while the carriage is scanned across the recording sheet, then the resultant image will appear as a thick strip following the direction of the carriage scan direction. When the all nozzles in the nozzle row are driven simultaneously, but only intermittently while the carriage is scanned across the recording sheet, the resultant images appear as thin lines extending perpendicular to the direction of the carriage scan direction.
Multi-nozzle heads suffer from a problem called cross-talk. Cross-talk occurs when all nozzles of a nozzle row are fired simultaneously. When all nozzles are fired simultaneously, influence from adjacent nozzles changes the ejection speed, usually by reducing the ejection speed compared to when the nozzles are fired individually.
Because cross-talk changes the ejection speed of ink droplets, the position where the ink droplets impinge on the recording sheet can also vary. This can reduce the quality of printed images. The adverse effects of cross-talk are particularly noticeable when all nozzles in a nozzle row are intermittently driven simultaneously. FIG. 6 shows a line printed by driving all nozzles in a row simultaneously once. As can be seen, the resultant line is curved, rather than straight.
It can also be seen in FIG. 6 that the nozzles in the center of the row are more greatly affected by cross-talk. Ink droplets ejected from the nozzles of the center of the row impinge on the recording sheet at positions downward from positions where ink droplets impinge from nozzles in the end of the nozzle row. This is because nozzles near the center of the row are more greatly influenced by cross-talk so that ejection speed of droplets from nozzles in the center of the row is more greatly reduced. In contrast, nozzles near the end of the row are relatively uninfluenced by cross-talk, so that ink droplets ejected from the end nozzles have relatively small reduction in the ejection speed. Because the different nozzles are affected differently, the ink droplets impinge at positions shifted from each other on the recording medium. As a result, a printed line that should be straight is actually curved.
The reason for this is that the drive conditions of the nozzles are individually determined to achieve a desired ejection speed and influence of cross-talk is not taken into account when all the nozzles are driven simultaneously.
Japanese Laid-Open Patent Publication No. HET-10-119260 discloses a configuration with a compensation piezoelectric member for reducing cross-talk. However, this configuration requires provision of the compensation piezoelectric member. Also, in order to eject ink properly, it is necessary to drive the compensation piezoelectric member by applying compensation drive voltages in complete or substantial synchronization with operations for reducing and increasing pressure in the ink chambers. This requires addition of control circuitry for the compensation piezoelectric member. As a result, cost for producing the configuration increases.