Laser ultrasonics tests paper quality

17 Jun 2002

Engineers install a laser ultrasound sensor on a pilot paper-coating machine in Ohio, US.

In an attempt to save trees, energy and money, engineers from Lawrence Berkeley National Laboratory, US, have developed a laser ultrasonic sensor that measures paper flexibility while it is manufactured.

Paul Ridgway and colleagues installed and tested the rig at the Mead Paper Company mill in Ohio last year, and plan a full-scale pilot test for 2003.

"We're measuring the elastic properties of paper at manufacturing speeds using a non-contact, non-destructive monitor," said Ridgway.

The sensor includes two optical systems. A microsecond-pulsed Nd:YAG laser to induce the ultrasound in the paper, and a Mach-Zender interferometer to detect it. The YAG pulse causes a tiny thermal expansion in the paper, which is enough to send ultrasonic shockwaves through the paper sheet.

The YAG is synchronized to fire only when the detection beam is perpendicular to the paper. This means that the propagation speed of the ultrasonic waves can be calculated, and this property relates to two elastic properties of the paper - its bending stiffness and out-of-plane shear rigidity.

If it proves successful, the system could end up saving paper manufacturers millions of dollars, as well as helping to conserve trees. This is because the manufacturers currently evaluate paper quality after it is made. To avoid having to scrap inferior rolls, they tend to err on the side of caution and use more pulp than necessary. This consumes more raw materials, more energy and means that more heat is required to dry the paper.

"The Mead test demonstrated that the instrument works in an industrial setting. It's a step towards a mill trial on a paper-making machine in which the environment will be much harsher - it will be hotter, wetter and there will be more vibrations and fiber debris in the air."

Marvin Klein, whose company Lasson Technologies, US, also makes laser ultrasonic sensors, is impressed with the Berkeley team's work but points out: "The big question is not only science or engineering one, but also a business one. Even if the test system provides the required measurement, are the capital and recurring costs low enough to make factory implementation attractive?"

The sensor was developed as part of Agenda 2020, a collaboration between the wood, paper and forestry industries in the US, and the Department of Energy.

Author
Michael Hatcher is technology editor of Opto & Laser Europe magazine.