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Laser Ultrasound Evaluates Paper

Photonics Spectra
Jun 2002
Michael D. Wheeler

One of the biggest challenges for the paper-making industry is testing the strength and flexibility of paper during production. Engineers must resort to examining samples cut from the ends of completed rolls. Now, engineers from Lawrence Berkeley National Laboratory in Berkeley, Calif., and the Institute of Paper Science and Technology in Atlanta have developed a laser-induced ultrasound system that measures the flexibility of various grades of paper on the fly, in real time.

A laser-induced ultrasound system developed at Lawrence Berkeley National Laboratory and the Institute of Paper Science and Technology samples the strength and flexibility of paper on the production web in real time. Courtesy of Paul Ridgway.

These measurements are made without touching the paper, thereby minimizing the possibility of ripping or tearing lightweight grades such as copy paper or newsprint. The system monitors the propagation of laser-induced sonic waves from the sample, with the velocity of the waves indicating the bending stiffness and rigidity of the paper. "The faster the wave, the stiffer the sheet," explained the laboratory's primary engineer for the project, Paul Ridgway.

The ultrasonic sensor that Ridgway, his colleague Rick Russo and other institute engineers have designed comprises a detection system, data acquisition and signal analysis software, and the laser that generates the ultrasonic wave, which is a standard, Q-switched Nd:YAG that emits at 1.06 µm.

"At 1.06 µm, most of the light that doesn't reflect from the paper passes through without much attenuation," Ridgway said. "So the absorbance that does occur is evenly distributed throughout the sheet-volume absorption, rather than surface absorption, which allows more energy to be absorbed while staying below the damage threshold."

A commercial Mach-Zehnder interferometer, rotating mirror and control electronics are the main components of the system. The mirror moves the interferometer probe beam with the paper, effectively stopping the paper motion with respect to the detection spot. An optical encoder tracks the mirror's rotational position, and an adjustable delay circuit triggers the firing of the laser, ensuring that the ultrasonic wave is detected when the beam is in the proper position on the paper surface. Software was designed to accommodate the paper web's speeds, which can reach 30 meters per second.

In August 2001, the researchers tested the laser ultrasonic sensor at one of MeadWestvaco Paper Corp.'s mills in Ohio. The sensor's signals remained excellent, even at paper speeds of up to 5000 feet per minute, and the laser did not damage the paper. The technique was ineffective on two-ply linerboard, however, and the system remains susceptible to heat and moisture. A commercial system under development is being customized to handle the heavier paper.

"We are preparing an 'industrially hardened' system to withstand the rigors of the paper manufacturing environment," Ridgway said. "Heat and moisture and vibration are all issues we are grappling with."

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