QMT Features: September 2007
Straight to the point
Metrology specialists Phil Wilson and Paul Gough break down the technology of video measurement systems and how the latest generation are making life easy for QA/QC specialists.

For more than twenty years, video measurement systems (VMS) have been used in numerous production industries for a wide range of quality assurance and quality control applications (QA/QC). These automated vision systems provide non-contact dimensional measurements of both flat surfaces and three dimensional features with a high degree of speed and accuracy not possible with conventional tactile or manual measurement. 

While precision measurements with manual systems depend greatly on the skill of the operator, automated vision system can provide independent, repeatable and accurate measurements with minimal user interaction and in only a fraction of the time.

A number of potential first-time users, however, have been discouraged by the operational complexities of some models, believing them to be too complicated for the average technician to reliably use, or too costly to fit within the scope of their business. This problem has been compounded by a growing shortage of trained staff with professional knowledge of metrology and video measurement techniques. This has created artificial bottlenecks in production as operators must rely on manual or conventional tactile systems to gather sample measurements. As a result, buyers are now increasingly looking for a VMS ‘magic-box’ solution that can address all their measurement tasks with minimal user training. Recent advances in optics and measurement software are making this a possibility in the form of a number of next-generation VMSs that have overcome previous design limitations, while introducing new functions and applications in lower cost packages.
What’s in a VMS?

Although virtually all video-based measuring systems have the potential to increase the speed of measurement capture and analysis, the precision and accuracy with which they achieve this, as well as the range of parts they are capable of examining, are highly varied parameters between designs. The software included with VMSs has also become a crucial factor to their overall usefulness, with the emphasis being placed primarily on intuitive interfaces and comprehensive application packages, both of which can significantly affect the user experience and the reliability of results.

Every VMS represents a chain of technology consisting of a specialised optical system, dedicated software, a highly accurate stage on which the work piece sits, and often a laser used for focusing, height measurement and, on some higher end systems, for scanning. The old adage ‘a chain is only as strong as its weakest link’ is, however, definitely the case with vision-based measuring systems. Even the smallest deviation in build quality or error in calibration can have a significant impact on the performance of a system, particularly as these instruments are tasked with recording such fine measurements. By separately examining these four components it’s possible for even entry-level operators to gain a better understanding of their technology and the recent advances that are shaping the latest generation of VMS technology.

The optical system

What it does: Comprising the objective lens and an integrated digital camera, the optical system is the central component of all VMSs, gathering the visual information used for sample analysis. High quality objective lenses are vital as aberrations in the optical information may be interpreted as production error in the sample part.

Problems from the past: In previous systems, the objectives used had limited working distances and zoom capabilities, restricting the limits of subsequent measurements and analysis. Lens distortions were also a common problem (and still are in some lower-end systems) that compromised optical resolution.

In the latest generation: In today’s VMS, low distortion, 10x zoom optics (standard on a few systems) can provide crisp resolution at high magnifications, while presenting a wide field of view at lower magnifications. Longer working distances (70+ mm) have also made measurement along the Z-axis a much easier process. Adding to this, the combination of apochromatic lenses and advanced LED illumination can provide increased image contrast, while correcting for chromatic aberrations that would otherwise be encountered throughout the visible spectrum.

More importantly for high throughput laboratories, the adoption of new control algorithms and progressive scan CCD cameras have made vision-based autofocusing (AF) a reality, imparting a generational leap forward in the speed and accuracy of measurement. These systems are capable of performing repeated measurement run sequences quickly and independently, minimising the amount of required user interaction with the system. Vision AF is particularly useful for applications such as surface focus and contrast focus for detecting the fine edges of work pieces.

The laser

In video measurement, lasers are used to focus and obtain Z height information, in most cases this can be achieved in far less time than standard vision-based focus. The manner in which this is achieved, however, is highly variable between systems, with some employing the laser as no more than a pinpoint reference while others offer sophisticated measurement control functions that significantly enhance the user experience.

Problems from the past: Although there are no particular problems with the laser systems used in past VMS designs, their functions were limited. Most simply acted as a ‘gun-sight’ through which users manually targeted reference points for repeat measurements (though some systems did offer automation for this).

In the latest generation: In a few of today’s machines, such as Nikon’s VMR range, the laser can also be used to scan components to get a surface profile or perform a Z height scan across a line with submicrometer resolution. Using the laser to swiftly focus or scan the work piece surface, optically independent laser autofocusing (AF), or so-called ‘smart-focusing’, represents the very cutting edge of measurement technology, capable of exceptionally precise Z-axis measurements (in most cases, even more precise than Vision AF). In a few systems, this has been combined with specialised software for high-speed data acquisition and surface finish analysis
The chassis

The chassis is the physical body housing the entire system and includes the platform on which samples are placed. The importance of the chassis build is often under-valued by users and even by some manufacturers. This can be a critical flaw as the chassis is the primary means of eliminating the impact of vibrations emanating from within the laboratory and is a crucial element to the overall performance and reliability of a VMS.

Problems from the past: No matter how precise the optical system, errors will always be encountered if the chassis isn’t sufficiently stable. When taking precision measurements at scales of 10-6 m, even the slightest defect in design can have a dramatic effect. This is often the case with systems constructed with components from a mix of manufacturers, making build quality and specification hard to control. For those companies that over rely on off-the-shelf parts, this can have knock-on effects on the overall performance of the finished machine.

In the latest generation: Advances in materials manufacturing involving the use of specialised aluminium alloys have revolutionised the construction of vision systems, even accounting for disturbances caused by fluctuations in the laboratory temperature. Further, a limited number of systems have now been designed with stages capable of automated adjustments in the z-axis. This enables true three dimensional analysis of components and brings a new level of functionality to users.

Machine life and reliability are also crucial aspects of VMS performance and should not be overlooked (particularly for high-throughput laboratories). Due to the associated costs of repairs and maintenance of vision systems, potential users should examine the service contracts of available machines and what parts and options are covered.

The software

The software packages included with VMSs are not only responsible for managing the control of the optical components and the analysis of measurements, they also include the user interfaces and teaching tools that guide users through processes.

Problems from the past: The overriding criticism of previous software versions for VMS systems was simply that that they were too complex for the average operator. Most also lacked pre-defined run routines for common work pieces, meaning that users would have to manually write the code for measurement sequences on software that few were comfortable using. Cheaper software solutions (still in use by certain manufacturers) were also found to be unstable and prone to failure or freezing during run sequences.

In the latest generation: Modern software packages for VMS systems are generally tiered systems designed to match the various technical needs and abilities of different user levels. For example, for entry level users, intuitive software layouts and interactive teaching wizards are available to guide users, step-by-step through what is required to achieve a given task. In a few systems, there will also be the option to set up custom wizards to streamline processes unique to a particular laboratory. For more advanced operators, there are a number of new developments that can support metrologists through complex measurement routines. Examples include computer aided design (CAD) interfaces that enable the importing of the CAD data for a particular work piece to facilitate teaching and shorten working times, and two-dimensional shape analysis programs to judge the profile of pieces that cannot be measured in the normal geometric mode. The programming of custom run sequences has also been simplified in many systems.

More sophisticated software tools include automated shape profilers to evaluate deviations between measurement results and nominal shape data, 3D surface analysis programs, and real-time dynamic data exchange tools that immediately transfer measurement results to spreadsheet programs (e.g. Microsoft Excel). A number of application-specific options have also been developed to assist users through complex measurement routines, for example, gear evaluation software.

A VMS for every need

Given the range of VMS designs, components and software packages available, it’s not uncommon for procurement managers to still be daunted by the prospect of having to search out the VMS market for the ideal investment. In most cases, such a system must be sufficiently easy for shop floor operation, yet still have the power to handle intricate work pieces and demanding measurement applications. Consequently, the modern systems of choice are those that able offer varying user access levels to suit the skill and experience of different operators.

Of near equal (and sometimes greater) importance is that the chosen VMS fit within the confines of the company’s operating budget. Fortunately, although automated VMSs are more costly at the outset than manual systems, their regular operation is generally far less expensive as they are able to run measurements independently and can be safely left to run unsupervised overnight, rather than requiring a dedicated operator. Automation also reduces bottlenecks in production and QA/QC, which in turn increases throughput and enables a more thorough inspection of parts. In the long run this reduces scrap and saves companies money. The pay back of the initial capital expenditure is, therefore, often very rapid. Certain intelligent systems, such as Nixon’s Nexiv VMR series, can also remember previously analysed objects and processes. When a similar object is encountered, they can then apply the same run routine to further cut down the required operator interaction time.

Automated systems have begun to come down in price so that they are now within the grasp of sites that previously would never have considered acquiring an automated vision system. Before acquiring a VMS, however, it is vital that new users educate themselves on the latest available technologies. While older systems may provide few, if any, benefits over traditional tactile machines, truly advanced systems can provide a degree of image quality and data analysis that can genuinely improve QA/QC and significantly reduce operational costs. l

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