Case Study: Marking and Vision Combo Ensures
Surgical Instruments Are Perfect Match
A US-based company has developed a
software product that allows hospitals
to set up surgical set pick lists and then
ensures that the correct instruments are
loaded into the surgical sets. A 2D Data
Matrix bar code is electrochemically applied to each instrument; this bar code
is scanned during the assembly process
to ensure accurate set assembly and to
track the location of every instrument.
The process of marking the instruments and then accurately reading that
mark proved to be the most difficult part
of the process. The company initially
used a laser to etch the instruments,
then went to an adhesive label with a
2D Data Matrix bar code. However, it
was difficult to maintain adhesion during
wash and sterilisation, so laser bonding
the label was tried. Finally, the company
went to an electrochemical marking
technique that applies a 3-mm Data Ma-
trix bar code. This method provided the
durability and readability needed
to withstand surgery, washing and
sterilisation.
The bar codes were placed on the
shiny surface of a surgical instrument
and were proving difficult to read. In its
search for a compatible ID reader, the
company tested five different generations of readers before it found one that
provided the accuracy, speed and ease
of use required by this critical application. It was also the only reader to meet
the requirements of the Association for
Automatic Identification and Mobility
(AIM) 2D Data Matrix code quality grading requirements.
The marking and vision combination
ensures that all instruments are automatically accounted for (thus eliminating
manual error), can be located quickly,
and are logged for usage (which then
determines when maintenance or replacement is due).
Electrochemical etching (ECE) is a
process whereby the mark is produced as
a result of the oxidation of metal from the
surface being marked through a stencil
impression. ECE is recommended for
round surfaces and for stress-sensitive
parts, and is often used to mark medical
devices.
Ink-jet printers precisely propel ink
drops to the part surface, after which the
fluid that makes up the ink dot evaporates, leaving a coloured dye on the surface of the part creating the pattern of
modules that make up the mark. Ink-jet
marking provides fast marking of moving
parts and offers very good contrast.
Key Dots are small sticky labels with a
Data Matrix code printed on the surface.
They come in a variety of sizes and materials, and are affixed to the product.
accurately read by a device. This is
where machine vision takes up the reins
and ensures that full product traceability
is achieved.
In a typical manufacturing application, the marked part passes in front of
a vision sensor and an image of the Data
Matrix code is captured and processed
using specialised image preprocessing
and identification algorithms. Using this
technology, code reading performance
is unaffected by low contrast or poorly
formed codes, which can result from
marking issues or general wear and tear
of the product.
In addition to reading the data stored
on the code, the sensors can also provide production process feedback on the
quality of the specific marking to ensure
products are marked with the highest
quality 2D codes. Perfecting the quality
of the codes to eliminate waste will lead
to improved overall production efficiency
and reduced operating costs.
Code reader options
Most machine vision systems are integrated
into the production line in the form of
fixed-mount sensors, which identify parts
that are handled and moved automatically by a conveyor, indexer or robot. In
operation, this type of reader is mounted
in a fixed position where the mark can be
repeatedly placed in front of it in continuous or indexed motion. Readers often can
be configured with an integrated or external light source, as required.
To allow maximum flexibility,
advanced ID code readers are also available as hand-held devices. These types of
readers are particularly useful in hospitals
where, for example, surgical instruments
must be scanned before and after surgery.
Although both 1D and 2D readers are
used in medical device manufacturing
environments, 2D area-based imagers are
the most popular. The main reason for
this is that 2D readers are future proofed
because they can read both 1D and 2D
codes, whereas 1D laser scanners can
only read 1D bar codes. In addition, 1D
laser scanners (the preferred method for
bar code scanning) suffer from the need
to have 80% contrast between the foreground and background. By comparison,
2D code readers can read at 20% contrast
levels (and sometimes less) and they are
unaffected by code rotation. All of these
factors should be considered when investing in code reading technology.
Conclusion
Optimised combinations of marking and
vision systems are a significant tool manufacturers can use to overcome everyday
production challenges whilst reaping the
substantial benefits of product traceability.
Code reading
Once the code is marked on a part or
product, it is of little use unless it can be
Leigh Jordan
is Senior Sales Engineer at Cognex UK Ltd,
43 Caldecotte Lake Drive, Sunningdale House,
Caldecotte Lake Business Park
Caldecotte, Milton Keynes, Bucks MK7 8LF, UK
tel: + 44 1908 206 000
e-mail: leigh.jordan@cognex.com
www.cognex.co.uk
emdt.co.uk European Medical Device Technology
June 2010 | 27
