Reliable, affordable measuring systems

Metrology system manufacturers are designing cost effective video measurement solutions for high accuracy and productivity.With the metrology industry’s seemingly boundless quest for ultra-high accuracy and sophisticated designs, the question is begged, how much accuracy is absolutely necessary and especially at what cost? Fortunately, recent advancements in video measuring equipment design provide highly accurate, productive, cost-effective solutions that are well within the vast majority of even the most stringent metrology requirements.

The common drivers of today’s requirements include miniaturization, high volume, increased accuracy and 3D measurement – some of the buzzwords heard more frequently in the metrology business – and they dictate the capabilities designed into today’s video and multi-sensor measurement systems.

While the cost factor may seem to be at odds among the other drivers, there are design enhancements that are being incorporated into larger travel systems that come virtually free of charge. In addition, component manufacturers are receiving the same pressure to increase performance while decreasing cost.

Design Enhancements
Nothing demonstrates component optimization at reduced cost better than the changes in the linear encoder industry. Virtually all precision measurement systems utilize linear encoders to monitor the position of their stages. In a manual machine, this simply means that the operator positions the stage to the desired coordinates as displayed on the digital read-out (DRO). A computer numerically controlled (CNC) system would direct a stepper, servo or linear motor drive system to position the transport using various voltages, current and frequency variables, but would adjust and confirm the final position by referencing the linear encoder(s).

While this type of scale is still common, some system designers have switched to adhesive backed tape scales. These are as accurate as the glass scales, and in many cases more so, and can be directly affixed to the structure of the machine, which on larger systems is often granite. Moreover, they are far more immune to damage and misalignment and can be much less expensive than their glass counterparts.

The availability of finite element analysis (FEA) can also deliver more possible improvements. FEA on desktop computers permits the selection of structural elements that decrease cost and weight while sacrificing nothing in the performance category.

The most obvious advantage on a measuring machine relates to the design of the granite base-plate and bearing ways. The bulk of the cost on these items is confined to their precision flatness and squareness. FEA allows the completed structure to be synthetically driven and the resultant distortion to be displayed and analyzed in a range of modes. Small modifications to the support structure can often result in a dramatic decrease in the thickness, weight and cost of the structure.

Computer aided design (CAD) offers another advantage with the advent of parametric modeling. This capability retains the relationship of specific entities so that design changes can be accomplished quickly and with less likelihood for error.

When properly utilized, today’s CAD systems yield many other benefits as well. For example, a rendering of the design model can be created in full 3D and the system can be tested for performance and collision avoidance, as well as analyzed from an ergonomics standpoint. Additionally, these animations can be used for training aids for assembly or for maintenance tasks in the field. Schematics, wiring diagrams, exploded views and bills of material (BOM) can all be generated on the CAD system. This system further offers a foolproof way to store all of the information for a specific machine in archival form on DVD for future reference. All of the above can be achieved for a minimal investment – the utilization of these tools control their payback.

Video, laser, and touch probes can all be integrated into one multi-sensor system with the tools identified above. The interaction of these devices can be plotted, studied and stored. Magnifications can be increased, accuracies can be improved and resolution can be enhanced using these techniques. Multi-sensor systems are ideal for complex, multi-featured parts that may require non-contact, contact and 3D probing measuring routines.

With increasing magnification, vibration isolation will become more important as systems exceed 1,000X – but there is no reason to doubt that we will not see accuracy and repeatability figures in the near future that rival machines of the highest caliber, and at a fraction of the cost. Modern measurement machines map the inherent errors in the operating envelope and store the correction data in a lookup file. These corrections are applied at machine start-up and require no operator intervention. In addition, periodic re-calibrations can be accomplished quickly and efficiently.

Design Examples
Some systems utilize ballscrew drive systems. They can be noisy, expensive, maintenance prone and difficult to install properly. They are also slow compared to competing technologies. The most advanced systems today have either servomotor or linear motor drive systems. When coupled with an air-bearing stage and non-contact linear scales, these systems are virtually friction-free, leading to better repeatability. A further benefit is the absence of lubrication, making their use more compatible for cleanroom environments.

Another no-cost method to increase repeatability on larger systems is to center-drive the moving axes of a system. This eliminates the inherent hysteresis of off-axis drives and contributes significantly to system performance. This is most easily achieved with the use of a split-axis transport system. A compound transport nests one axis within the other. This makes interaction between the axes unavoidable and generally locates the drive system outside the measuring envelope (off-axis). A gantry system fixes the part to the granite base and drives the probe(s) from the side, often mandating the use of dual encoders.

Finally, there is the feature of illumination – an area where enormous benefits can be obtained while decreasing costs. LED lighting offers the advantages of no-heat, incredibly long lamp life, even distribution of light, flexibility for all types of illumination and, of course, low cost.

Thanks to these new components and design techniques, the seemingly unrealistic goal of attaining higher performance while lowering costs is being achieved by today’s metrology systems manufacturers.

The L.S. Starrett Co.
Athol, MA
starrett.com