Controlling residual fine errors of dot placement in an incremental printer

Abstract

A memory holds calibration data that are applied to compensate imperfections in a printhead-carriage guide rod, improving alignment between marks printed with different heads. Commonly heads and a carriage encoder are spaced from the rod at different distances, which interact with rod deviation to form dot-placement errors (DPE) that vary along the rod. The memory holds a single offset value, best a weighted composite of (a) an average of maximum and minimum deviations from straightness, and (b) median deviation, along the rod; or as the carriage moves on the rod the system steps or interpolates between successive offsets, or uses a continuous corrective-offset function. Separate offsets may be stored for adjacent-head pairs. The memory is best a digital unit holding just a few data bits, but may be a mechanical cam or linkage, compensation network or other analog circuit, polynomial coefficients, or codestrip with unequally spaced graduations. A custom strip is used with no further intervention. Calibration data in other memory types are used to modify interhead alignment, carriage-encoder signals, carriage position/speed, printhead-actuation timing or marking rapidity—or image-data position values, color-plane alignment, or pixel structure. Calibration may be prepared by measuring rod-straightness deviations, calculating expectable DPEs between mark pairs made by different heads, and from these finding the needed numbers for storage. Measuring may use conventional instruments but preferably the printer prints patterns (e.g. alternating marks made by two outboard heads) and measures them with an internal sensor. In existing systems—with interhead alignment set in a limited rod segment—the offset is found by comparing DPE ranges over the whole length vs. that segment.