Many computer printers, including some low resolution ink jet printers, scan a print head back and forth across a print medium (e.g., paper) to print graphics and text images thereon. Printing typically occurs while the print head is scanned in each direction, thereby employing relatively fast bidirectional printing.
An ink jet printer projects microscopic ink droplets from the print head onto the paper to form a printed image. Since the print head is typically not in contact with the paper, the droplets are projected to the paper through air. Accordingly, there is a propagation time during which the droplets propagate from the print head to the paper. This propagation time is dependent upon the velocity at which the droplets are ejected from the print head and the distance between the print head and the paper.
The print head is scanned across the paper at a scanning velocity. A droplet projected from the print head will have the scanning velocity in the direction the print head is being scanned. A droplet projected toward an image location on the paper must, therefore, be ejected from the print head at an ejection time that occurs before the print head is aligned with the image location. Nominally, the ejection time precedes the alignment of print head with the image location by about the propagation time of the droplet.
When printing takes place in only one scan direction, all droplets are subjected to the same scanning velocity. As a result, the alignment of droplets ejected during successive scans is substantially independent of the propagation time of the droplets.
In bidirectional printing, however, droplets are subjected to different scanning velocities during the successive scans in opposite directions. As a result, the alignment of droplets ejected during successive scans is dependent upon the propagation time of the droplets (i.e., the velocity at which the droplets are ejected from the print head and the distance between the print head and the paper).
The velocity at which the droplets are ejected can be precisely controlled by the print head. Accordingly, the distance between the ink jet print head and the paper must be accurately maintained to provide adequate alignment of the droplets ejected during successive scans in opposite directions. In low speed, low resolution ink jet printers of the type presently available, the distance can be maintained automatically, for example, by a semi-rigid follower wheel that rolls across the paper with the print head as it is scanned. In some other printers, a manual adjustment cam of the type used on conventional typewriters allows a user to select the desired distance.
These conventional spacing mechanisms do not provide spacing to within a tolerance adequate for high resolution ink jet printers. Such printers can form images with about 120 dots/cm and require that the distance between the print head and paper be maintained at within a tolerance of about .+-.0.025 mm. Moreover, such printers are sometimes adapted to print onto media having a wide range of thicknesses.