Laser Ultrasonics at 20 m/s in the production environment and on a budget: from dream to reality

A laser-based ultrasonic system for non-contact and non-destructive measurement of the elastic properties of paper was demonstrated on a paper manufacturing machine during commercial operation with paper moving around 20 m/s. We believe this to be the highest sample traveling speed reported to date for a commercial application of laser ultrasonics. Ultrasonic waves were generated in the paper with a pulsed Nd:YAG laser at 1064 nm wavelength and detected with a Mach-Zehnder interferometer coupled with a scanning mirror/timing system to compensate for paper motion. Measurements of the flexural rigidity (FR) and out-of-plane shear rigidity (SR) of the paper web were done automatically by fitting the frequency dependence of the phase velocity of Ao mode Lamb waves to a model wave propagation equation. Variation in FR and SR across the width of the paper sheet (cross-direction profiles), effects of changes in paper manufacturing process variables on measured FR and SR, comparisons with traditional mechanical stiffness tests are presented. The sensor head is fully optical and thus measures the web properties without any contact. This laser-ultrasonics system combines a very reasonable cost with a relatively small footprint and low power consumption due to the low power output of the lasers that are used. Finally, laboratory data indicate that this technology is directly transferable to measurements on sheet metals and possibly other opaque web materials.