Multi-Sensor Technology – Crossing Thin Blue Line

Advanced coordinate metrology equipment has traditionally been classified into three types namely coordinate measuring machines (CMMs), optical digitizers and scanners (ODS), and video measuring machines (VMMs).

Originally the three coordinate measuring products were principally focused in different market sectors:

  • CMM – machined, fabricated and medium and large plastic parts
  • ODS – reverse engineering
  • VMM – small parts, electronic components and assemblies

Today, with the increased sensor and software capabilities of ODS and VMM, increasingly, these original markets sectors are becoming blurred. Multi-sensor capability CMMs have emerged whereby the different sensors technologies can be utilized in a single measurement set-up allowing all part topology and geometrical features to be inspected.

Multi-sensor, multi-axis, integrated motorized probe head – Optiv Dual Technology from Hexagon manufacturing Intelligence

Multi-sensor machines have been available in the market for some time whereby tactile, optical and laser sensors are integrated onto a single measurement platform. Sensors being either manipulated around the part using a combination of machine and motorized head motion (CMM) or the part is presented to the sensor using a combination of linear and rotary machine motions (VMM). Some larger and more sophisticated VMMs also have an integrated motorized probe head providing an increased combination of machine and sensor motions for the most complex part inspections.

As part inspection moves out of the laboratory and onto the production floor a single measuring station, with multiple sensing technologies, is demanded by many applications to ensure the capture of 100% of part geometry in a single measuring cycle. With the multi-sensor equipment configurations came the need to expand metrology software’s to incorporate the numerous sensing technologies into a seamless user experience and incorporating a common inspection part program.

All of major CMM manufacturers have been adding multi-sensor capabilities to traditional tactile CMMs over past years as well as VMM manufacturers adding additional sensors to their machine structures to extend measurement capabilities. Today, all of the top global CMM suppliers now also supply multi-sensor VMM’s ensuring they can offer a multi-sensor solution regardless of the customer application requirement.

The penetration of structured light scanning sensors, mounted onto a standard 6 axis industrial robot, has penetrated the automotive sheet-metal and car body inspection markets over past years ousting the horizontal coordinate measuring machine – traditionally used for these applications. The robotic system offers a shop-floor production solution, with rapid capture and generation of complete 3D surface geometry, immediate CAD comparison and analysis. The overwhelming trend of industry to capture everything and extract only the needed part geometry for process control is a far different approach to the traditional CMM and multi-sensor CMM approach whereby the CMM software is instructed what to measure feature by feature.

The CMM and VCMM frames remain the most accurate sensor carriers available today. Despite attempts to enhance the 6 axis robot measuring accuracy performance by error mapping, externally tracking robot position or use of photogrammetry techniques, the robot measuring solution does not match the accuracy needs of many part manufacturing processes.

Over past decades CMMs have become faster, more accurate, and cheaper. Developments by CMM manufacturers have included making structures stiffer, lighter and thermally compensated for use outside of the traditional temperature controlled quality laboratory. There has been much speculation over the past decade associated with the demise of the CMM but in reality what is the alternative where high accuracy is a prerequisite? The CMM is affordable, reliable, repeatable and accurate. Any replacement technology needs to be all of these …  and more so its no surprise that the CMM ‘soldiers on’ and is now finding a new role in integrating with the next generation of sensor technologies.

The recent introduction by Renishaw of its RFP structured light optical scanning probe adds another dimension to the CMM platform as a multi-sensor carrier and acknowledges the advantages found by manufacturers who implemented robotic structured light measuring solutions. The RFP probe increases the multi-sensor capability of the Renishaw REVO system which now offers five interchangeable probe families, including optical and surface finish. The RFP probe is designed for the inspection of freeform surfaces and complex geometry, rapidly delivering patches of surface data with a high point density. Unlike other non‑contact structured light systems, the RFP fringe probe does not require reference markers to stitch together data from different part areas.

The original demarcation between CMM, VMM and ODS have all but been eroded. The emerging sensor technologies have now thrust the multi-sensor platform into main stream shop-floor metrology applications. It will be interesting to witness the penetration of these advanced multi-sensor dimensional metrology platforms as blue-light optical scanning sensors jumps the line from robot to CMM and starts to ‘capture’ the higher accuracy part inspection requirements.

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