1. Field of Inventions
The present inventions relate generally to ink jet printers and, more specifically, to apparatus for use with ink jet printers that reduces systematic print quality defects.
2. Description of the Related Art
Ink jet printers can be used to form text images and graphic images on a variety of printing media including, but not limited to, paper, card stock, mylar and transparency stock. The images are formed on print media by printing individual ink spots (or "pixels") in a two-dimensional array of rows and columns. A row is often referred to as a "dot row" or a "pixel row." Multiple pixel rows are formed to create a pixel array that corresponds to the desired image.
Certain ink jet printers include one or more printer cartridges (or "pens") that are carried on a scanning carriage and are capable of printing multiple pixel rows concurrently to create a larger portion of the pixel array. The printer cartridges typically include a printhead with a plurality of ink ejecting nozzles. A 600 dpi (dots-per-inch) printhead with a 1/2inch swath will, for example, typically have two columns with 150 nozzles in each column. A variety of mechanisms may be used to eject the ink from the nozzles. In one such mechanism, the so-called thermal ink ejection mechanism, ink channels and ink vaporization chambers are disposed between a nozzle orifice plate and a thin film substrate that includes arrays of heater elements such as thin film resistors. The heater elements are selectively energized to heat the ink within selected chambers, thereby causing an ink droplet to be ejected from the nozzles associated with the selected chambers to form ink dots at the desired locations on the print medium.
During a printing operation, the scanning carriage will traverse back and forth over the surface of the print medium. The print medium is advanced in a direction transverse to that of the movement scanning carriage. As the scanning carriage traverses back and forth, a controller causes the nozzles to eject drops of ink at times intended to result in the desired pixel row and, ultimately, the desired pixel array.
One important aspect of printing is image quality which, of course, depends upon the accuracy of the dot placement on the print medium. Variations from perfect dot placement are commonly referred to as dot placement error (DPE). One method of reducing DPE is to simply tighten the tolerances on printer specifications (or DPE specifications) such as drop weight, drop velocity, drop trajectory, medium advancement, printer cartridge/paper spacing, and carriage orientation. This approach is, however, expensive in that meeting relatively tight DPE specification tolerances requires large amounts of design and manufacturing resources to be expended.
At some point, the DPE specification tolerance tightening results in image improvement that is beyond the perception level of a typical viewer. In a relatively high resolution printer (300 dpi or higher), the occasional misdirected ink drop will have essentially no effect on overall image quality. A greater impediment to image quality is visible banding, which occurs when DPEs result in regular repeating patterns. In fact, in many applications, DPE tolerances can be relaxed without a perceptible reduction in image quality if visible banding is eliminated.
One proposed method of reducing banding is disclosed in commonly assigned U.S. application Ser. No. 08/985,641, filed Dec. 5, 1997, and entitled CARRIAGE RANDOM VIBRATION. Here, a vibration inducing element is added to an otherwise conventional ink jet printer to cause minute, random vibrations of the printhead relative to the print medium.