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Product category: Form/co-ordinate, optical and vision instrumentation
News Release from: OGP UK | Subject: OGP Smartscope
Edited by the Engineeringtalk Editorial Team on 25 October 2002

Noncontact measurement helps pregnancy
testers

An OGP Smartscope is provides high-speed noncontact co-ordinate measurement crucial to quality control of ClearBlue pregnancy testing devices at medical diagnostics company Unipath.

So successful is the UK operation of medical diagnostics company Unipath that it manufactures more than 20 million home pregnancy testing kits a year Holding the position as one of the world leading women's health consumer diagnostics companies, its 'one minute' ClearBlue pregnancy testing devices, are produced around the clock at the prestigious, purpose developed headquarter site just south of Bedford

Here, some 450 people perform research and development in new products and are involved with production of a range of professional medical diagnostic devices.

Indeed, with the high volumes associated with ClearBlue, three purpose developed automatic assembly and packaging machines, which have a work rate up to 100 devices a minute, means there is little room for error in the various components.

As consistency of production is critical, Unipath decided noncontact, video-based measurement was the most appropriate solution to its high level quality requirements via an OGP Smartscope installed by Disc GB of Hatton some two years ago.

Originally, the quality team under Anne Colgrove, Unipath's Technical Quality Manager, investigated tactile probing through 3D co-ordinate measuring machine (CMM) technology to verify plastic injection mouldings and a laminated band of nitro-cellulose bonded onto glass fibre tape.

However, conventional CMM technology would not account for any variation in visual appearance, was slow to provide support for the high throughput capability of the three high speed assembly machines and being tactile, meant it was detrimental to the fragility of some of the components.

As Michael Hoskins, Quality Control Analyst maintains: "The more we discussed the needs of quality control, our research and development and design requirements, the case for noncontact vision-based measurement came to the fore".

This led to the contract with Disc GB to install the OGP SmartScope ZIP 600 to which Keith Faulkner, Senior Quality Control Scientist relates: "It has become a key process in not only providing functional checks on component quality, but providing a means to develop specifications from our R and D for design and production".

The ClearBlue device is not much larger than a thick biro pen with a push-on plastic cap.

The main casing is made from two halves each injection moulded in the shape of a fan made up of 10 components.

Each moulding has various location and fixing pegs and slots which retain the internal laminate band and an absorbent wick and are simply pressed together.

While tolerances are not tight, typically +0.1mm, the smooth exterior profile of the casing is based on the ergonomics of a rounded elliptical shape which is easy to hold.

Therefore, any mismatch between the two halves would create a serious visual defect.

In addition, any error in geometric relationships of features would be detrimental to the efficiency of the automatic assembly process.

As Hoskins explains: "For volume based automatic assembly, it is imperative that the mouldings are consistent but you can get variations because they are made from several dies.

This also applies to the laminated band of nitro-cellulose.

Each band is flexible and sliced into 39 separate strips from which one strip is fitted into each body during the assembly process.

To help position the strip a series of feeder holes are prepunched to +0.05mm tolerance for location in the plastic body and also used as a guide for the assembly process.

However, because the strip is very fragile and any contact with the nitro-cellulose surface would destroy the coating, this means the noncontact SmartScope is ideal as a measuring process".

Unipath has devised a series of simple fixtures with datum holes for inspecting the different parts.

These also include the relative positioning of the fan shape moulded body casings which is important as the assembly process progressively picks each half in turn from the centre hub of the moulding.

So any positional error could cause a misload.

"You cannot check these positions by contact methods and the only conventional way is via a shadowgraph but the SmartScope is far quicker and will also record measurement data.

In fact, we can now easily check all the features of some 200 moulded components in under eight hours, if required.

The checks by SmartScope also account for elements such as moulding flash, which can be critical due to the up to 20 different segments that can contribute to tool wear".

As Keith Faulkner maintains, Unipath can easily determine the capability of the process with non-contact video measurement.

"We can have positive discussions with suppliers based on factual evidence presented from a screen dump and printed out through a video graphics printer.

We can also determine from our captured data records if it is a natural tolerance drift or, say, a new problem such as a change in material specification causing a size shift".

Michael Hoskins adds that it used to take him a whole day to compile trend graphs.

"They can now be created from the SmartScope in less than two hours".

He implies that an important feature of the machine is the programmable z-axis which is excellent for checking the relative heights of different moulded features.

"We could not do this in a practical fashion before the ZIP machine was installed", he says.

The SmartScope is able to measure parts up to 500 x 660 x 300mm and with the patented Accucentric 10.1 auto-calibrating zoom optics, between 32 to 320 times on-screen magnification can be obtained to a scale resolution of 0.1 micron.

To this Michael Hoskins comments: "This feature combines the best advantages of an optical microscope with a digital camera".

The capability of the SmartScope is enhanced through its programmable illumination using a substage profile light with electronically controlled iris.

It is able to automatically synchronise the illumination to the operating magnification while a fibre-optic ring light and OGP's patented SmartRing gives added flexibility of oblique illumination through an eight sector/six ring light LED configuration.

This capability gives added flexibility to the process especially when different features such as tags, holes and tapers are measured in the same cycle and the results could be influenced by shadows.

While Anne Colgrove has overall responsibility for quality and sees the SmartScope as a validation tool and an important element in the process of trouble shooting, she maintains it has also become an important tool to design and research and development engineers.

"On new products we can verify the parts are to drawing, but the SmartScope has also proved to be highly efficient in measuring features and the development of specifications for a moulding".

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