Originally Published MPMN March 2001
PROFILE: Better Vision System Boosts Inhaler Box Output
With a new pair of eyes watching a once-troublesome production process, an inhaler maker is getting more accurate inspections and fewer false rejects.
At the U.S. facility of Bespak plc, a UK-based manufacturer
of drug-delivery devices, a machine vision system measures the dimensions
of orifices in injection-molded plastic inserts that are assembled
into inhaler boxes. To meet customer specifications, the vision
system is supposed to reject orifices that don't fall within a ±0.002-in.-diam
tolerance. The system also checks to make sure that the orifices
are no more than 20% oval (i.e., that there is no more than a 20%
difference between any two diametric measurements of the same orifice).
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Automated assembly of the inhalers is performed at the Bespak facility in Apex, NC. |
But Bespak's eight-year-old vision system wasn't up to the job. The system couldn't pass the most basic validation tests or meet the company's repeatability and reliability testing requirements. "Our internal requirement was no more than 20% variability in performance, and our ultimate goal was to achieve less than 10%," explains Jim Gallion, manager of manufacturing engineering at Bespak's facility, located in Apex, NC. "With that system we were struggling to get below 3040% variability, which was leading to an excessive amount of falsely rejected products."
A Better Alternative
Gallion and his colleagues investigated other alternatives. After
evaluating several products, they settled on a Checkpoint 900 vision
system from Cognex Corp. (Natick, MA). The vision system includes
a Pentium-equipped computer with a Checkpoint 900 PC plug-in card,
two Sony CCD cameras, and a remote-controlled Fostec fiber-optic
light source.
In Bespak's plant, inhalers are assembled by an automated
machine. The machine takes inserts from feeder bowls and places
them two at a time on a rotary indexing table. The table indexes
the parts into the view of the two vision cameras, which are mounted
side by side. Motion stops while each camera captures an image of
the part in its line of view. Extreme magnification is necessary
to get high-quality images of the tiny orifices, which measure approximately
0.02 in. diam.Extreme magnification is required to capture
images of the 0.02-in.-diam orifices in the inhaler box inserts.
Image data goes to the Checkpoint card, which uses PatMax geometric pattern matching software to determine the exact position of the orifices. This is no easy task, due in part to the size of the orifices and in part to machine vibration. "Even slight vibration can shake the field of view and cause the pattern's position to vary," explains Joe Grove, a Bespak manufacturing engineer. But with its rapid-reset CCD cameras and high-speed processor, the vision system can capture high-resolution images of a vibrating target, Grove reports.
When the system receives an image, it checks the orifice diameter using Checkpoint's Edge and Arc Gauge tools. These tools acquire 72 radial points, which are used to calculate an average diameter. The system also uses the radial points to determine whether the shape of the orifice meets the oval-shape specifications. The entire inspection process takes less than a second.
Light Concerns
During inspection, the Fostec light illuminates the orifice from
the inside. If light intensity diminishes, "the vision system is
not able to detect the crisp, well-defined edge of the orifice,
which affects the accuracy and reliability of the results," Grove
says.
To maintain adequate light intensity, Grove came up with a data-carrying
loop. A key component of the loop is Checkpoint's Light Meter tool,
which assigns a "light value" to each image the system captures.
This value is transferred from Checkpoint software to Visual Basic
through bridging software called Checkpoint Communication, or CPComm.
The transfer is necessary because, unlike the Checkpoint software,
Visual Basic can be used to communicate with the Fostec light. If
the light value is too low, the system sends a Visual Basic message
to the light source to boost intensity. Then the light meter checks
the intensity again. If it's still too low, another Visual Basic
message is dispatched to the light. The process is repeated until
the proper intensity level is reached. "This is a pretty high-level
feedback loop that solves the major problem with vision, which is
lighting variation," Gallion says.
When the vision system spots a defect, it's logged in a database.
Defective parts are dropped in a reject bin. Parts that pass inspection
are placed on an offload conveyor, which carries them to a packaging
station. During the inspection process, a special graphical user interface
appears on a 17-in. monitor built into the assembly machine. Developed
by Bespak engineers, the interface displays the parts being inspected
and keeps a running tally of the number of rejects. By hiding Checkpoint's
more complex features, the interface makes it easier for plant personnel
to operate, test, and calibrate the system. Passing the Test
Since it was installed and validated last year, the new vision
system has been inspecting more than 20,000 inserts a day. "The
Checkpoint system [has] proved capable of measuring the inserts
within strict tolerances," Gallion reports. "And it passed the original
validation tests and many others with flying colors." According to Bespak, the system has been very reliable, producing
fewer false rejects and less scrap than its predecessor. The system
has also reduced variability to well below Bespak's target of 10%.
Even with tighter-than-normal tolerances, variability now ranges
between 3 and 4%. According to Gallion, variability that low "is
completely unheard of for automatic vision inspection at these speeds."
Encouraged by this experience, Bespak has much bigger plans
for the Checkpoint vision system. "We will use this everywhere,"
says Gallion, adding that his company plans to use vision data to
make process adjustments that will prevent the manufacture of faulty
parts. "This is where we see the world going."
William Leventon
Copyright ©2001 Medical Product Manufacturing
NewsThe machine vision system inspects more than
20,000 inserts daily.