EoW September 2012

73 / 110

Technical article
September 2012 71
simultaneously to the decreasing stripe
width. After around 5 seconds, the raw
signal moves only within the main colour
tolerances. The co-extruder was switched
on again at around 50 seconds on x-scale
and stripe signal ramped up in 5 seconds
to normal condition.
The last setup in Table 2 is to test the stripe
to main ratio. As the sensor only detects
the average colour in the scan field, it
isn’t possible to measure the stripe width
directly. In case of a constant longitudinal
product rotation, the time interval of
main tm and stripe ts colour found in
their tolerance interval can be integrated
for certain time T and the resulting time
ratio
should be almost same as the geometrical
ratio. First trials under optimum conditions
gave almost satisfying results with T>10s,
but scan field size, jitter and rotation
irregularities are still a challenge for an
evaluation with high evidence.
Actual device
specifications
and forthcoming
development
The user interface of the device should be
quite easy to control for the line operator
without losing setup flexibility or detail
information, comparable with inline wire
centricity measurement.
Based on an IPC, control of the sensor is
completely transparent for the user. A
non-contact measurement reduces the
risk of sensor damage. Very often the
wire isn’t completely dry while passing
the device. This caused contamination of
the sensor surface but could be solved by
installation of a permanent compressed air
blowing over the sensor.
To protect the optics during production
start/stop and bare wire running, the
sensor is moved into a safe position
until normal production. Temperature
compensation is done automatically.
As already mentioned, colours are
detected as relative measurement, so
the system needs a teach-in for each
geometry/colour combination.
This is done once after the wire runs
in good production and the detected
reference can be stored for further
production of this wire type.
Any number of recipes can be stored. Until
now the recipe database is very simple
and should include search functions in
future software.
Connection to the line PLC should be
possible via simple digital 24V signals
(signal status and device enable) where
fault messages are implemented to
the coil protocol like spark or diameter
faults or with a more complex network
communication via TCP/IP to have
access to the recipe database or to support
host computer link.
Figure 9 shows the main screen that
includes the most important information
for the line operator.
Additional windows can display more
details, like separated L*/a*/b*-channel
measurement, various signal history or
trend information. For later evaluation, a
subsample of raw data is stored on the IPC
hard disk.
To be independent of all production
lighting conditions, the scan field is
covered and illumination is done by an
internal LED light source (see Figure 10,
white arrow).
According the higher luminance variation
compared to a* and b*, a modification of
equation (1) should be tested:
While equation (1) represents a circular
error distribution, the modification is
an elliptical distribution with enhanced
tolerance in the L* channel (for f > 1).
Technical aspects are almost solved; the
main focus is now the improvement of
device interface and handling.
This needs feedback from the wire and
cable production companies. Another
point of discussion should be the
combination of stripe and main colour.
For example main colour red with a
brown stripe is difficult to separate – for
an automatic device as well as for a line
operator.
So this is not a problem in colour
measurement itself but a possible
improvement of quality control by a simple
redefinition of colour pairs with high ∆E. n
References
[1] Edward H Adelson, MIT, 1995
[2]
[3] Joint ISO/CIE Standard: CIE Colorimetry – Part 1-5,
ISO 11664-1··5/CIE S014-1··5/E:2006-2007
[4] Masataka Okabe, Kei Ito (2008-02-15). Color blind
barrier free.
eq (2)
Siebe Engineering GmbH
Industriegebiet Fernthal
53577 Neustadt/Wied
Germany
Email:
Website: