Media velocity, media present and bubble control in an electrophotographic process

1. An image forming device, comprising:

one or more toner transfer areas operative to transfer toner to a media sheet;

a fuser downstream of a last toner transfer area, the fuser operative to fix the toner to the media sheet;

a media sheet speed sensor downstream of the fuser, the sensor operative to indicate the actual speed of the media sheet through the fuser;

a fuser speed controller operative to adjust the speed of the fuser in response to the media sheet speed sensor;

wherein the media sheet simultaneously traverses at least one toner transfer area and the fuser, and wherein the fuser speed controller is further operative to adjust the speed of the fuser so as to maintain a bubble in the media sheet between the last toner transfer area and the fuser.

2. The device of claim 1 wherein the media sheet speed sensor comprises a rotary encoder and reports at least two rotational positions of the encoder.

3. The device of claim 2 wherein the rotary encoder comprises a plurality of optical windows radially disposed in an arc about a pivoting axis of the encoder.

4. The device of claim 3 wherein the rotary encoder is connected to a flag that is displaced by a leading edge of the media sheet such that the flag displacement imparts rotary motion to the encoder.

5. The device of claim 3 wherein the rotary encoder is a disk, the arc is circular, and the encoder is driven by a wheel contacted and driven by the media sheet.

6. The device of claim 5 wherein the media sheet speed sensor is operative to detect negative velocity of the media sheet.

7. A fuser disposed along a media path in an image forming device, the fuser operative to fix toner to a media sheet, comprising:

a first roller;

a second roller or belt forming a nip with the first roller;

a heat source associated with the second roller or belt and operative to apply heat to a media sheet in the nip;

a media sheet speed sensor disposed downstream of the nip and operative to detect the actual speed of at least a leading edge of the media sheet through the fuser; and

a controller operative to alter the speed of the fuser in response to the media sheet speed sensor, so as to maintain a bubble in the media sheet between the fuser and another nip in the media path.

8. The fuser of claim 7 wherein the media sheet speed sensor comprises an optical encoder operative to generate a signal comprising a plurality of pulses, the spacing of the pulses being indicative of the rotational speed of the encoder.

9. The fuser of claim 8 wherein the optical encoder includes a flag disposed in the media path, and is rotated by the leading edge of the media sheet displacing the flag.

10. The fuser of claim 8 wherein the optical encoder comprises a disk driven by a wheel in contact with the media sheet.

11. The fuser of claim 10 wherein the media sheet speed sensor is operative to detect a negative velocity of the media sheet.

12. A method of forming an image on a media sheet, comprising:

transferring toner to the media sheet in one or more toner transfer nips;

simultaneously with transferring toner to at least part of the media sheet, fixing toner to at least part of the media sheet by applying heat and pressure to part of the media sheet in a fuser nip;

determining, downstream of the toner being fixed to the at least part of the media sheet, an actual media sheet speed at the fuser nip; and

in response to the actual media sheet speed at the fuser nip, controlling the fuser speed to maintain a bubble in the media sheet, between the last toner transfer nip and the fuser nip.

13. The method of claim 12 wherein determining the actual media sheet speed at the fuser nip comprises calculating the actual media sheet speed based on a signal from a media sheet speed sensor associated with the fuser nip.

14. The method of claim 13 wherein the media sheet speed sensor is downstream of the fuser nip.

15. The method of claim 13 further comprising determining the spacing of pulses in the signal from the media sheet speed sensor, and determining the actual media sheet speed based on the pulse spacing.

16. The method of claim 15 further comprising averaging the pulse spacing over a predetermined portion of the length of the media sheet prior to determining the actual media sheet speed.

17. The method of claim 12 further comprising halting the fuser in response to detecting a negative media sheet velocity.