Method and apparatus for automatically correcting the fire timing of a printhead carrier due to linear encoder velocity errors

Abstract

An improved ink jet printer is provided in which its carrier position and velocity information is derived from a linear optical encoder mounted on the carrier, which produces the position information uses a quadrature output signal, and one of the channels of that output signal is used to determine “critical edge” transitions used to provide the actual print fire timing. Two adjacent encoder periods are needed to determine the acceleration of the carrier. By measuring the time intervals of these two encoder periods, and measuring the difference between these time intervals, the present invention provides correction logic that can determine if the second of the two measurements has been corrupted by motion not relative to carrier speed across the print media. If the difference between the previous two periods is greater than a preset maximum, the correction logic will limit the new encoder time (i.e., for purposes of fire pulse width calculation) to the old encoder time, plus or minus the preset limit. In this manner, any significantly large errors in the critical edges of the encoder signals can be temporarily ignored for the fire pulse calculations of the next encoder period, at least within the programmable preset limits. This is particularly useful during printing operations during which the carrier should be traveling at a relatively constant velocity, wherein the false edges that may occur otherwise would significantly degrade the placement of printed pels on the print media if not for the automatic self-correction provided by the present invention.