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
