This invention relates to moving carriage type thermal ink jet printers, and more particularly, to a thermal ink jet printhead and replenishable reservoir which travels therewith having means to sense the ink content thereof so that it may be refilled at predetermined ink content levels.
Thermal ink jet printing systems use thermal energy selectively produced by resistors located in capillary filled ink channels near channel terminating nozzles or orifices to vaporize momentarily the ink and form bubbles on demand. Each temporary bubble expels an ink droplet and propels it toward a recording medium. The printing system may be incorporated in either a carriage type printer or a pagewidth type printer. A carriage type printer generally has a relatively small printhead containing the ink channels and nozzles. The printhead is usually sealingly attached to an ink supply cartridge and the combined printhead and cartridge assembly is reciprocated to print one swath of information at a time on a stationarily held recording medium, such as paper. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath, so that the next printed swath will be contiguous therewith. The procedure is repeated until the entire page is printed. In contrast, the pagewidth printer has a stationary printhead having a length equal to or greater than the width of the paper. The paper is continually moved past the pagewidth printhead in a direction normal to the printhead length at a constant speed during the printing process. Moving carriage type ink jet printers must either carry the ink reservoir along with the printhead or provide a flexible ink supply line between the moving printhead and a stationary ink reservoir. In addition, thermal ink jet devices require a small negative pressure at the printhead in order prevent ink from weeping from their droplet expelling nozzles.
Generally, thermal ink jet printers of the moving carriage type use a relatively large stationary ink reservoir and a flexible supply line arrangement to a movable small ink reservoir that is attached to the moving printhead. This is because the carriage providing the printhead motion is subjected to periodic accelerations at the ends of scan. Therefore, a large carriage mounted ink reservoir would require a large drive motor along with robust structure in order to achieve rapid carriage accelerations and accurate control of the printhead position. Small carriage mounted disposable ink reservoirs suffer the disadvantage of requiring frequent replacement. Such disposable ink reservoirs generally are bundled with the ink jet printhead so that both are discarded when the cartridge is empty.
U.S. Pat. Nos. 3,967,286; 4,677,448; and 4,757,331 disclose a recording apparatus with a carriage mounted ink tank and a second tank that is fixed. The second tank remains disconnected from the first tank during recording and intermittently is connected to the first carriage mounted tank when the ink is to be supplied from the second tank to the first.
U.S. Pat. No. 4,737,801 discloses an ink supply device comprising a first ink tank and a second ink tank for supplying ink to the first tank. A liquid level sensor is mounted in the first tank. The ink supply system also comprises an overflow sensor.
U.S. Pat. No. 4,788,861 discloses an apparatus for monitoring the ink supply of an ink printing device. The apparatus comprises three electrodes for measuring resistance. A first electrode pair is constantly moistened by the ink to provide an ink comparison resistance. An additional electrode with one of the first pair serves for measuring the resistance change due to fluid level. This arrangement can determine a high and low ink level.
U.S. Pat. No. 4,432,005 discloses an ink control system for an ink jet color printer. The printer comprises three long tubular supply sacks connected to three secondary ink reservoirs mounted on a printhead through flexible umbilical tubes.
U.S. Pat. No. 4,929,969 discloses an ink supply construction for an ink jet printer which uses a foam-filled ink reservoir. The foam structure comprises a network of fine filaments to reduce drooling of the ink and provide a slight negative pressure to the ink at the printhead. While it is known to provide a small foam-filled replenishable ink reservoir attached to a carriage-mounted printhead for movement therewith, such disclosures and prior art printers do not have a reliable, cost effective way to ensure that the printhead replenishable reservoir maintains a sufficient supply of ink by monitoring the ink content in the replenishable reservoir and refilling it at predetermined levels, while concurrently ensuring that the replenishable reservoir is not overfilled.