Duplex Sheet Registration Using Shared-Edge Skew Correction
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Solution Overview
Problem
Existing duplex printing methods fail to align front and back images accurately, leading to visible misalignments and misregistration of images in stacked sheets, especially when sheets are fed in a skewed position, and this misalignment is exacerbated in duplex printing due to translucency or when documents are bound or scanned.
Innovation Solution
Align the front and back images with respect to the same edge of the sheet, using a detection system to determine the skew angles of leading and trailing edges, and flip the sheet to ensure that this edge aligns with the reference direction, allowing both images to be printed in parallel alignment, and subsequently registered during stacking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the skew angle of the leading edge is corrected before printing the front side image in the first pass, and the skew angle of the leading edge (formerly trailing edge) is corrected by rotating the sheet in the second pass, then the front side image and back side image will be rotated relative to one another by a relatively large angle which is twice the angle between the leading and trailing edges of the sheet
Solution Approach 1:
The patent applies preliminary action by measuring the skew angle of the trailing edge during the first pass and using this measurement to calculate the precise rotation angle needed for the second pass. This preliminary measurement and calculation enable the system to pre-determine the correct alignment, eliminating the need for large corrective rotations that would cause misregistration between front and back images.
2Ease of operation
If the direction of the second edge of the sheet, which is the leading edge in the first pass, would be taken as the reference direction, then the accuracy of the correction would be inferior, because it cannot always be taken for granted that the front side image is really aligned with the second edge (leading edge in the first part) of the sheet which sufficient accuracy
Solution Approach 1:
The patent implements feedback by using the trailing edge as a stable reference and measuring its skew angle, then using this measured information to control the rotation in the second pass. This feedback loop ensures that the correction is based on actual measured data rather than assumptions, maintaining high precision throughout the duplex printing process.
3Productivity
If sheets are fed in a skewed position, then the leading edge of the sheet and the leading edge of the image may be misaligned, but a slight misalignment of the two edges is hardly perceptible with the naked eye
Solution Approach 1:
The patent replaces mechanical alignment methods with an optical/detector-based system that measures the skew angle of the trailing edge. Instead of relying on mechanical guides or manual alignment, the system uses detection means to optically measure the angle and automatically calculates the necessary rotation, achieving high precision without mechanical complexity.
Data Source
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AI summary
An improved method of printing and stacking sheets (41) that improves the relative alignment of images in a stack of printed sheet. The method comprises the steps of: - feeding a sheet (41) at least twice past a printhead assembly (31) for printing an image (I1, I2) on both sides of the sheet (41), wherein in the first and second passes the respective front and back images (I1, I2) are aligned with respect to a first edge (E1) of the sheet (41) by detecting a skew angle (A3) between the first and second edges (E1, E2) and applying the skew angle to align the front and back images; - stacking the sheet (41) wherein the sheet (41) is registered to a predetermined stacking reference direction (Y) by one of its edges, characterized in that during the step of stacking the sheet (41) is registered by its first edge (E1).