MARKET PLACE
The challenge
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Figure 1: The twin LED episcopic
illumination rings on the video
measuring system.
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Designing any new medical device can involve a battle between the desirable and the feasible. However, there are at least two criteria that can never be compromised: limiting the possibility of failure and ensuring repeatable performance throughout the life of the device. Careful selection of fabrication materials to harness desirable functions such as the durability of metals, the flexibility of plastics and the sealing capacity of rubber, plays a crucial role in meeting both these criteria, as does the ability to continually check that their dimensions meet the required specifications.
Using materials with the correct properties can solve the functional aspect of ensuring that devices do not fail and provide repeatable performance. However, measuring the dimensions of those materials with the speed and accuracy needed to meet the demands of medical device manufacturing is a challenge. This can be particularly important if a manufacturer wants to check as many incoming components as possible on one measuring system. Repeatedly detecting the correct edge for a critical dimension on black rubber, clear plastic and pressed metal is a fundamental problem for manual measuring systems.
As a result, many companies are turning to advanced video measuring systems, because they offer the versatility needed to switch between materials and components of different compositions, colours and surface finishes. This is particularly true if those systems have twin light emitting diode (LED) episcopic illumination rings that can move independently and combine reflections from eight different directions (Figure 1). This feature allows low-contrast edges to be detected, which are usually invisible under episcopic illumination. The inner ring can be used whenever strong illumination from various directions provides the best edge detection. The outer ring facilitates the observation of components that cannot be seen with shallow-angle lighting. The relative position (height) of the rings and the number of LED segments that are illuminated can be varied, and users can change the settings to create and store the conditions that best suit their specific requirements. Video measuring systems also offer the speed needed to check and verify large batches of incoming components without delaying production targets.
Rubber parts
Because rubber parts such as valves and seals are normally black in colour, reliable edge detection can be particularly difficult when manual systems are employed, because operator variability will inevitably be a factor. This makes it hard to achieve the necessary reproducibility and repeatability of measurement required for meaningful quality control (QC) and leads to wider margins of error and a loss of confidence in the results.
However, by selecting appropriate LED illumination settings, video measuring systems can be set up for optimal edge detection on dark components. Furthermore, in automated mode, these settings will always be re-selected for the desired measurement. Working in conjunction with automatic edge detection software, for example, digitally classifying the black and grey portions into 256 graduation level, ensures that the edge is reliably found and re-found, thereby delivering the desired reproducibility and repeatability.
Clear parts
At the other end of the spectrum, clear parts such as polyurethane conduits can also present a challenge because of the inherent difficulty of edge detection. However, appropriate LED illumination allows the edge to be correctly refracted and ensures reliable detection and measurement.
Metal parts
The surfaces of mass-produced, pressed metal components can sometimes be rough. Burrs can make it hard to detect and measure a clean edge. The power of modern video measurement algorithms, which use multi-grey level scaling to compensate for multiple edges within a given calliper, means edges can be readily detected and the appropriate compensation factored in.
Ease of programming
Traditionally, some manufacturers have shunned video measuring systems because they perceive them to be difficult to programme. The advent of interactive teaching and measurement “wizards” that provide step-by-step task-oriented guidance has made it simpler. Nonexperienced staff can set up the systems to automatically recognise components, select the correct measuring routine and deliver the required data sets.
Use in practice
Having to check vital dimensions on components made from a variety of different materials employed in speci-ality medical devices is a challenge faced by Bespak (King’s Lynn, UK) on a daily basis. To perform the exacting QC checks required on bought-in parts, the company relies on video measurement. Although the components are relatively small, ranging from 5 mm in circumference to 12.5 cm in diameter, samples of as many as 2000 components can be subjected to up to 20 separate video-based measurements on its chosen system at a rate of 20 s to 2 min per component.
Another factor in choosing this video measuring system was its ability to directly accept CAD data. This allows it to automatically compare the original specifications against the measurements it acquires from actual products to determine if they are in agreement, without the need to manually programme in the target dimensions. This is particularly useful with unusual shapes such as domes, which are difficult to measure manually, but can be easily rendered topographically using laser scanning techniques.
This choice of video measuring system has also proved beneficial because the manufacturer of many of its pressed metal parts, Clamason Industries (Kingswinford, UK), also checks its own tolerances on the same system, as does the supplier of its valves and seals, Hi-Tech Rubber (Anaheim, California, USA). Having common systems makes it far easier for Bespak to assess the data provided with each shipment and to audit these suppliers’ own QC routines. Furthermore, as part of its Product Introduction Process, the company actively works with its suppliers to ensure the quality of its outsourced products using Advanced Product Quality Planning techniques. Having two of its major suppliers use the same video measuring system makes it much simpler for Bespak to involve them in the product development process and ensure their customer’s brief is met as fully and efficiently as possible.
Chay Keogh is Marketing Manager, Nikon UK Instruments, Kingston-Upon-Thames, UK. Tel. +44 2082 471 718, discover@nikon-instruments,www.nikon-instuments.com
