HP Optical Media Advance System

(OMAS)

Media Advancemotion encoding (1)

encoder

print zone

encoder

final drive roller

final drive roller

mediadrive motor

• Most large format printers measure motion of the media indirectly by encoding an element in the drive system– motor shaft (least accurate)– final drive roller (most accurate)

• The drive system can be calibratedby comparing input (motor) and output (media) motions to minimize systematic errors from...– media physical properties– media slippage

• Indirect measurements are...−adequate for moves less than ~1 inch−subject to errors from

• media slippage• temperature and humidity changes• mechanical tolerances and wear

Media Advancesteady advances in accuracy

• Since 1990, HP has achieved a 20X improvement in the accuracy of encoder-based media advance systems in HP Designjet printers

• Improvements are driven by– larger print swaths, longer moves– need for higher image quality

at higher throughput• Encoder-based systems are reaching

practical limits at ~1um/mm (0.1%)−requires high mechanical precision

• increases manufacturing cost• Wider printheads and HP Double

Swath Technology need longer mediamoves to achieve potential speed – accuracies better than 0.5um/mm

are needed for long moves (~2 inches)– a performance breakthrough is needed

in media advance accuracy

HP Designjets by Year of Introduction

Media Advance Accuracy (um/mm)

0.1

1.0

10.0

100.0

1985 1990 1995 2000 2005

DJ 5500

2010

DJ 4000

target

OMASeffective improvement in image quality

• Results from comparative testing with and without OMAS– internal HP tests with a pool of image quality judges

• controlled viewing environment• best sample rated “100”• images ranked in bands: “better”, “worse”, “acceptable”

– CAD drawing, Normal Mode, HP Bond Paper

Samples printed with OMAS show less banding and better line smoothness.

Consistent OMAS results for printing conditions between 30% and 70% RH.

Line Smoothness(higher number = better)

0

20

40

60

80

100

no OMAS30% RH

no OMAS70% RHOMAS

30% RHOMAS

70% RH

0

20

40

60

80

100 no OMAS30% RH

no OMAS70% RHOMAS

30% RHOMAS

70% RH

Rela

tive im

ag

e q

ualit

ybett

er

mean value

confidence band

Banding(higher number = better)

OMAShow it works (1)

windowA

windowB

XAB

Image A dY

dX

• OMAS stores reference image A from window A• The media moves ~AB in the X-direction

– window B now sees the same region of the media that was stored in reference image A

• OMAS takes new images A’ and B’– window A: image A’ (used for the next AB step)– window B: image B’ (used to match with image A)

• image B’ is now correlated (matched) to image A– algorithms compute the offsets dX and dY

between images A and B’• OMAS reports the media has moved ΔX = AB + dX

−dX and dY will be different on each AB step• dX and dY may be positive or negative

• OMAS takes images A and B in two “windows” • Separation distance AB (~3mm) is measured with

high precision during OMAS manufacture– AB is stored in the OMAS EEPROM

Image B’

media back side