A Smooth Solution to a Rough Problem

Laser-based sensor measures the surface roughness of materials in real time.

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To get an idea of how rough a surface is, scientists may run a physical stylus across it to record variations in surface height—what is known as a contact method of measurement. An alternative would be to use optical measurement instruments, a non-contact method that does not damage the surface and allows for higher resolution data.

However, as most optical profilers are large and desk-bound, assessments are typically performed on finished products, when correction of defects may be difficult or wasteful. To make real-time and on-site evaluation of surface roughness possible, scientists from A*STAR’s Advanced Remanufacturing and Technology Centre (ARTC)Nanyang Technological University, Singapore; and the University of Strathclyde in Glasgow, UK; developed an optical sensor that can profile the surface of materials at high resolution, and in real-time.

Their device consists of a laser confocal sensor that concentrates light at a focal point on the material being assessed. Affixed to an industrial robotic arm, the laser-based sensor sweeps across the material surface, while a motion control system minimizes vibration caused by the positioning system or scanning mechanism, allowing for highly precise analysis of material surface properties.

“We found that single-point sensing is very suitable for roughness measurement, but because single-point confocal sensors typically have a small measurement range, we devised a way to increase the horizontal measurement range of the technique using data stitching,” said Fang Cheng, Group Manager at ARTC, a lead author on the study.

With their system, the researchers were able to measure surfaces with a roughness average (Ra) of 0.2-7 micrometres, performing comparably to the Talysurf PGI 800 stylus profilometer, a widely used, high-accuracy standard instrument for surface roughness measurements. Essentially, Ra is the average value of a set of individual measurements of a surface’s microscopic valleys and peaks. A patent application has been filed for the technology.

Moving forward, the team intends to further improve the system by assessing more roughness parameters aside from Ra. “We also need to include surface tracking technology to increase the vertical measurement range, which is critical for free-form surfaces, and provide solutions to isolate vibrations or remove vibration noise,” Cheng added. The group is also looking to further enhance the present optical sensor’s resolution of measurement and lower its cost of production.

The A*STAR-affiliated researchers contributing to this research are from the Advanced Remanufacturing and Technology Centre (ARTC).