Controlling a printing system using encoder ratios

1. A method for adjusting an encoder pulse count delay in a printing system for printing on a continuous web of print media, comprising:

providing a first printhead at a first printhead position along a media transport path, the first printhead being adapted to print ink drops onto the print media as the print media travels along the media transport path past the first printhead position;

providing a second printhead at a second printhead position along the media transport path, the second printhead being adapted to print ink drops onto the print media as the print media travels along the media transport path past the second printhead position;

providing a first encoder at a first encoder position along the media transport path, the first encoder generating a first encoder pulse stream having a first encoder pulse rate, each pulse in the first pulse stream indicating the passage of a first defined length of print media past the first encoder position, wherein the first pulse stream controls a timing for printing of ink drops from the first printhead;

providing a second encoder at a second encoder position along the media transport path, the second encoder generating a second encoder pulse stream having a second encoder pulse rate, each pulse in the second pulse stream indicating the passage of a second defined length of print media past the second encoder position, wherein the second pulse stream controls a timing for printing of ink drops from the second printhead;

determining an initial ratio of the first encoder pulse rate to the second encoder pulse rate;

providing an initial encoder pulse count delay for use in printing with the second printhead;

controlling the first printhead responsive to the first encoder pulse stream to print ink drops on the print media in accordance with image data for a first image plane of a first printed image;

controlling the second printhead responsive to the second encoder pulse stream and the initial encoder pulse count delay to print ink drops on the print media in accordance with image data for a second image plane of the first printed image;

determining a subsequent ratio of the first encoder pulse rate to the second encoder pulse rate;

using a processor to determine an adjusted encoder pulse count delay by adjusting the initial encoder pulse count delay responsive to the initial ratio and the subsequent ratio;

controlling the first printhead responsive to the first encoder pulse stream to print ink drops on the print media in accordance with image data for a first image plane of a second printed image;

controlling the second printhead responsive to the second encoder pulse stream and the adjusted encoder pulse count delay to print ink drops on the print media in accordance with image data for a second image plane of the second printed image.

2. The method of claim 1 , wherein the second printhead includes a first jetting module and a second jetting module for printing ink drops onto the print media, the printing from the second jetting module being delayed relative to the printing from the first jetting module as a function of the encoder pulse count delay.

3. The method of claim 1 , wherein the printing of ink drops by the second printhead is delayed relative to the detection of a cue mark on the print media by a cue sensor as a function of the encoder pulse count delay.

4. The method of claim 1 , wherein the printing of image data by the second printhead is delayed relative to the printing of corresponding image data by the first printhead as a function of the encoder pulse count delay.

5. The method of claim 1 , wherein the encoder pulse count delay is used to control an alignment between the printed image data for the second image plane and the printed image data for the first image plane.

6. The method of claim 1 , wherein determining the ratio of the first encoder pulse rate to the second encoder pulse rate includes:

measuring a first encoder pulse rate based on the first pulse stream;

measuring a second encoder pulse rate based on the second pulse stream; and

using a processor to compute a ratio of the measured first encoder pulse rate to the measured second encoder pulse rate.

7. The method of claim 6 , wherein the first and second encoder pulse rates are measured by counting a number of encoder pulses in the respective first and second pulse streams during a defined sampling interval.

8. The method of claim 7 , wherein the sampling interval is a fixed time interval.

9. The method of claim 7 , wherein a cue sensor is used to detect cue marks on the print media as the print media is advanced along the media transport path past the cue sensor, and wherein the sampling interval is the time difference between detecting a first cue mark on the print media and detecting a second cue mark on the print media.

10. The method of claim 7 , further including using the first printhead to print a sequence of cue marks on the print media, wherein the sampling interval is the time difference between printing a first cue mark on the print media and printing a second cue mark on the print media.

11. The method of claim 1 , further including periodically determining an updated ratio of the first encoder pulse rate to the second encoder pulse rates and adjusting the encoder pulse count delay in response to the updated ratio.

12. The method of claim 1 , wherein the second encoder includes an encoder roller that contacts the print media, and wherein the second encoder provides a predefined number of encoder pulses for each revolution of the encoder roller.

13. The method of claim 12 , wherein the second encoder pulse rate varies over time responsive to thermally-induced changes in a diameter of the encoder roller.

14. The method of claim 1 , wherein the adjusted encoder pulse count delay D′ is given by:

D ′=( R s /R i )× D

where D is the initial encoder pulse count delay, R i is the initial ratio of the first encoder pulse rate to the second encoder pulse rate, and R s is the subsequent ratio of the first encoder pulse rate to the second encoder pulse rate.

15. The method of claim 1 , further including shifting the frequency of the pulses in the second encoder pulse stream so that the second encoder pulse rate associated with the second encoder pulse stream matches the first encoder pulse rate associated with the first encoder pulse stream.