Inkjet printers advance the paper after each print swath of the inkjet pen. Ideally, the paper is advanced exactly the amount needed to match the nozzle pattern from one print swath to the next. This way the resulting printed image appears to be made by one continuous pen and has high image quality. However, if the paper is advanced too far or too little, even the small discontinuity of the nozzle groupings is easily apparent to the human eye. Paper advances that are too long result in a high contrast white gap in the otherwise continuous print. Paper advances that are too short can appear as a dark band if the nozzle print overlaps. In either case, consistently long or short paper advance errors add up to distort the printed image, e.g. a printed circle could appear to be slightly elliptical. Additionally, the paper may move laterally resulting in text that is no longer co-linear or text with incorrect kerning.
Inkjet printers that use shaft encoders coupled to the drive motor or rollers are prone to swath advance errors as described above. A shaft encoder can give accurate feedback of the rotation of the shaft/roller but that does not necessarily correspond to exact motion of the paper. If a drive roller has a larger than nominal diameter, it will advance the paper more than desired. If it has a smaller than nominal diameter, it will give a smaller than desired advance for the same amount of angular rotation. In addition, if intermediate drive rollers or gears rotate about a center with an eccentric error, they can give the paper advance varying large and small swath errors. Another limitation for the shaft encoders is that they only track motion in one dimension. Side-to-side motion of the print media cannot be detected using the shaft encoder.
U.S. Pat. No. 5,149,980 described an optical encoder developed at Hewlett Packard in the early 1990's using a light emitting diode (LED) source to illuminate the print media at an oblique angle. The oblique illumination of the LED light produces highlights and shadows on surfaces with large roughness features. An image sensor is used to capture the shadow images on the print media and a correlation-based algorithm can be used to determine relative motions between the paper and the encoder. It has been shown that this type of optical encoders may provide highly accurate paper motion feedback; swath advances have been measured to be accurate to one micron per swath advance. Capability of 2D motion encoding of the print media has also been demonstrated using this method.
However, the usability of the oblique LED-based optical encoder is limited by media types with large surface roughness, e.g. white bond paper. Print media with very smooth surface can produce shadow images with contrast too low for the image correlation algorithm to work. In has been found that the oblique LED-based optical encoder does not function on glossy photo paper and transparency films.
The present invention is a motion encoder which accurately measures the actual motion of the paper in both X and Y directions for a Cartesiam coordinate system. An optical encoder uses a coherent or quasi-coherent illumination source. An array sensor may be used to capture images of the surface, speckle patterns, or diffraction images produced from the reflected light off the print media.
In one embodiment, diffraction images are detected. The contrast of the diffraction pattern is related to the degree of coherence of the light source and it is independent of the surface type.
FIGS. 2A-C illustrate a paper transport assembly of the present invention.
The present invention is a printer that includes a two dimensional (2D) optical encoder for tracking motion of the print media. A coherent or quasi-coherent light source, e.g. laser such as a laser diode or a vertical cavity surface emitting laser (VCSEL), a light emitting diode (LED) or a combination of a broadband source and an optical filter, illuminates a surface of the print media. The surface detection technique begins when a preselected angular distribution of reflected light is captured by a detector. The detector, e.g. an array sensor, is preferably positioned to capture the image from the surface. The image may directly correspond to the surface, or be a derivative of surface information, e.g. speckle patterns or diffraction images produced from the reflected light off the print media.
In one embodiment, the images are obtained from the specular direction of the reflected light as shown in
FIGS. 2A-C illustrate a paper transport assembly 10 using an encoder 11 of the present invention. In
As shown in FIGS. 2A-C, the laser-based encoder 11 includes a coherent light source 12, e.g. a laser such as a vertical cavity surface emitting laser (VCSEL), and a detector 16, e.g. a CMOS image array. The coherent light source 12 illuminates the print media with a light beam at an angle of illumination with respect to the surface. The detector 16 is positioned at an angle of reflection with respect to the surface operable to receive a reflected portion of the light beam from the surface, wherein the angle of reflection is substantially equal to the angle of illumination.
When the detected pattern is speckle,
When the detected pattern is diffraction,
Alternatively, the laser-based motion encoder 116 may be used in an ink-jet printer. The ink-jet printer includes a printhead for printing on a medium. The printhead is disposed at a print area for ejecting droplets of ink onto a surface of the medium in a controlled fashion during printing operations. An input supply of print media disposed at an input end of a primary media path. The path guiding the medium from the input supply and past the print area. A laser-based encoder is positioned proximate to the primary media path, generating a signal indicative of the print media position. The inkjet printer further includes first means for advancing a sheet of medium from the input supply into the input end of the primary media path and second means for advancing the medium from a location on the primary path upstream from the print area through the primary feed path to position the medium in relation to the printhead.
All of the aforementioned embodiments generate sequences of images that are analyzed by the control engine to determine the position of the media. Snapshots of the media position may be compared with a desired path. The desired path is known to give the optimal print quality. It is determined by parameters including paper movement and printing mechanism motion. Under closed loop feedback control, the control engine may adjust either the transport assembly or printing mechanism to conform to the desired path. In addition, the control engine can determine the paper velocity to detect a paper jam condition.