Ink jet printing involves ejecting ink droplets from orifices in a print head onto a receiving substrate to form an image. The image is made up of a grid-like pattern of potential drop locations, commonly referred to as pixels. The resolution of the image is expressed by the number of ink drops or dots per inch (dpi), with common resolutions being 300 dpi and 600 dpi.
Ink-jet printing systems commonly utilize either direct printing or offset printing architecture. In a typical direct printing system, ink is ejected from jets in the print head directly onto the final receiving substrate. In an offset printing system, the print head jets the ink onto an intermediate transfer surface, such as a liquid layer on a drum. The final receiving substrate is then brought into contact with the intermediate transfer surface and the ink image is transferred and fused or fixed to the substrate.
In many direct and offset printing systems, the print head and the final receiving substrate or the intermediate transfer surface move relative to one another in two dimensions as the print head jets are fired. Typically, the print head is translated along an X-axis in a direction perpendicular to media travel (Y-axis). The final receiving substrate/intermediate transfer surface is moved past the print head along the Y-axis. In this manner, the print head "scans" over the medium/substrate and forms a dot-matrix image by selectively depositing ink drops at specific pixel locations. To increase image density and allow for greater speeds, multiple print heads may be utilized.
Image resolution, print quality and speed are among the most important considerations in designing a printing system. Where greater speeds are paramount, it is known to utilize one or more stationary print heads to eliminate the necessity of scanning across the transfer surface or media. Multiple stationary print heads increase speeds while also allowing for greater image density and increased image width.
One challenge with a multiple print head architecture, whether scanning or stationary, is to maintain proper alignment among the print heads. If one print head is misaligned relative to the other print heads in the array, printing artifacts such as banding and misregistration can occur. Additionally, whenever a print head is installed in the print head array, it must be precisely aligned with the other print heads.
Alignment among multiple print heads may be expressed as the position of one print head relative to another print head within a coordinate system of multiple axes. For purposes of discussion, the X-axis will refer to a direction perpendicular to the media/intermediate transfer surface travel direction past a print head, the Y-axis will refer to a direction parallel to the media travel direction and the Z-axis will refer to a direction perpendicular to the X-Y axis plane. It will be appreciated that in this three dimensional coordinate system, a print head has six degrees of freedom of movement--three degrees of freedom of translation along the X, Y and Z axes, and three degrees of freedom of rotation about the three axes.
For optimal placement of ink drops on the receiving substrate, each print head in a multiple print head system should be aligned with the other print heads with respect to all six degrees of freedom of movement. It will be noted, however, that the printed image is a two-dimensional pattern of pixels arranged in the X-Y plane on the receiving substrate. Thus, the alignment of the print heads with respect to their position along the X and Y-axes and their angular rotation or roll about the Z-axis, also referred to as .theta., will have the most impact on print quality and printing artifacts.
Prior art multiple print head systems have disclosed alignment mechanisms that utilize operator input to perform print head alignment along two axes. For example, in U.S. Pat. No. 5,428,375 to Simon et al. (the '375 patent), each print head is supported by a platform that carries X and Y translation actuators. The X translation actuator moves the platform along a fixed lead screw in an X-axis direction. The Y translation actuator drives a plunger back and forth to move the platform in a Y-axis direction. An operator examines output from the printer for visual artifacts and manually adjusts the X and Y actuators to reposition the print heads. This mechanism does not allow for adjustment of individual print head "roll" or .theta. correction.
U.S. Pat. No. 5,241,325 to Nguyen (the '325 patent) discloses a scanning or "swath type" printer that includes a mechanism for aligning two print cartridges with respect to a single axis of movement. One print cartridge is mounted in a fixed-position retaining shoe and the other print cartridge is mounted in a pivoting retaining shoe. Both retaining shoes are mounted on a carriage that scans across the media in an X-axis direction.
The print cartridges print test lines and an optical scanner measures the distance between test line segments. Horizontal or X-axis misalignment between the two print cartridges is addressed by adjusting the timing of the ink jet nozzle firing as the cartridges scan across the media. Vertical or Y-axis misalignment is addressed by nozzle selection and by mechanically adjusting the angular position about the X-axis of the adjustable retaining shoe relative to the fixed-position retaining shoe.
The mechanical adjustment is performed by advancing the print cartridges along the X-axis until a cam lever on the carriage engages an actuator arm. Movement of the cam lever rotates a position adjustment cam that bears against a cam follower flange on the adjustable retaining shoe. This rotates the adjustable retaining shoe and associated print cartridge about the X-axis while the fixed-position shoe and cartridge remain stationary.
One drawback to the adjustment mechanism in the '325 patent is that it is limited to scanning or "swath type" printing systems, as movement of the print cartridges in the X-axis direction is required to actuate the mechanism. This mechanism is also limited to rotational adjustments about the X-axis. Additionally, like the mechanism in the '375 patent, the mechanism in the '325 patent does not allow for adjustment of print head "roll" or .theta. correction.
The present invention addresses the drawbacks of the prior art by providing an apparatus and method for automatically adjusting the relative position of multiple print heads with respect to three axes of movement, including rotational or .theta. adjustment about the Z-axis. The present invention also provides a method for automatically adjusting the position of a single print head with respect to its angular rotation about the Z-axis.