EuroWire May 2016

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Technical article
May 2016 53
Optical Wrap Defect
Inspection for Cable
By Craig Girdwood and Andrew McCloskey, Taymer International
Abstract
The optical wrap defect inspection
system significantly improves the quality
control of wrapped cable. The surface
and wrapping structure of the cable is
continuously monitored by a machine
vision system at production line speed.
The wrap surface can be viewed live and/
or recorded for 100 per cent of the cable
length. Surface and wrapping defects
are identified, and images of the defects
are recorded together with position
information.
Wrapping information including wrapping
angle and overlap percentage can be
monitored for changes. This technology
improves the quality of the cable
produced and prevents defective products
from reaching the customer.
1 Introduction
A critical quality issue created during the
production of wrapped cable is surface
defects and wrapping defects. Potential
surface defects include scratches or holes,
and potential wrapping defects include
uneven wrapping, incorrect wrapping
angle, tape peeling and tape tears. These
defects are not only a cosmetic problem,
but they can lead to the core being
exposed to the surroundings, resulting in
short circuits and failure of equipment.
By identifying and detecting these defects
during the production process, immediate
corrective action can be performed
and defective sections of product can
be repaired or discarded. Utilising the
constant and immediate feedback from
the machine, operators and process
engineers are able to pinpoint the root
cause of the defects.
The wrap defect inspection system
utilises one or more high-speed cameras
capturing real-time images of wrapped
cable surfaces. The wrap inspector
software is able to accurately measure
wrapping distance and angle, as well as
determine both the type and size of a wide
range of surface defects and wrapping
defects. Surface and wrapping defects as
small as 0.1mm can be detected and an
alarm triggered. When a defect is detected
a digital image is enhanced, magnified and
displayed on a remote monitor, enabling
the operator to verify the defects. This
allows operators to determine the type
of defect, to identify false positives (eg,
a water droplet), or even detect surface
blemishes (eg, surface discolouring,
scratches).
The wrapping angle and distance
measurements for the entire cable are
saved to a database along with a record
of any surface or wrapping defect
information. The defect information
includes defect type, size and location
on the cable in metres or feet. This allows
defects to be isolated quickly by operators
after production is completed.
The wrap inspector system is able to be
easily integrated into existing production
lines and performs well for various types
of wrapped cables and convoluted
armouring. The wrap inspector system
makes sure that any problems are
discovered prior to any defective products
being sent to customers.
The lifetime of the lights can be over
50,000 hours of usage and they are the
only consumable part.
2 Current Defect
Detection Technique
Limitations
2.1 Laser Diameter Gauges
A laser diameter gauge is used to detect
bulges, and neck-downs. The machine
uses a laser and shadow technology to
measure the diameter of the cable. The
diameter measurement is extremely
accurate and can be used to classify bulges
or neck-downs.
However, there is no way to truly know if
the defect detected is an actual defect. For
example, a dust particle or water droplet
on the cable would increase the diameter
measurement, resulting in a false positive.
2.2 Spark Tester
A spark tester is equipment that can be
used to detect pinhole-type defects. The
machine creates a spark when there is a
pinhole that exposes the core of the cable
– conductive metal. However, if a pinhole
does not expose the core, the spark is
not created. But the pinhole that is not
a through hole is still a defect since over
time the pinhole could expand and expose
the core of the cable. Also, for products
without a core (pipe, hose, tubing) or
products without a conducting core
(fibre optic cable), spark testers are not
applicable.
2.3 Vision Systems
Prior configurations of surface defect
detection systems were unable to identify
and measure convolutions of the product.
These systems were limited to ignoring the
convolutions to identify significant defects
such as pinholes or significant bulges
or shape changes. The wrap inspection
system has been improved with additional
image processing software and alternative
camera configurations that identifies the
wrapped cable profile and can measure
parameters and detect variations in the
wrapping that are considered defects.
3 Machine Vision
Inspection System
3.1 Optical Mechanical Design
The wrap defect inspection system
consists of one or more high resolution
cameras and adequate lighting to capture
images of cable moving at speeds more
than 1,200 feet (400 metres) per minute.
Using multiple cameras, 360-degree
coverage of the cable surface can be
achieved. The system produces high
quality images at these speeds with a very
high frame rate.