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Energy-saving Paper Sensor Passes Major Milestone

The paper industry is one step closer to saving millions of dollars each year. An innovative laser ultrasonic sensor that measures paper quality during production was recently successfully tested at a mill in Jackson, Alabama. The sensor was designed and built by scientists in the Environmental Energy Technologies Division (EETD) at Lawrence Berkeley National Laboratory (Berkeley Lab).

The sensor measures two hallmarks of paper quality-bending stiffness and shear strength-as paper speeds through the production process. These measurements allow manufacturers to ensure that the optimum amount of raw material is used to make the paper, which could reduce the consumption of trees and chemicals and save the U.S. approximately $200 million in energy costs and $330 million in fiber costs each year.

"This is the first full-scale demonstration of the sensor on a commercial paper-making machine while it's in operation," says Paul Ridgway of EETD, who developed the sensor with fellow EETD scientist and principal investigator Rick Russo, working in partnership with the Institute of Paper Science and Technology at Georgia Tech.

The two-week test of the sensor was conducted in February at a mill owned by Boise Cascade. "Boise Cascade's engineers considered the trial to be quite successful and are hopeful that a six-month trial will be conducted at the same mill," says Ridgway.

Eight years in the making, the sensor was funded by the Department of Energy's Office of Industrial Technologies as part of a partnership to improve the energy efficiency of several industries. Under this program, the American Forest and Paper Association created Agenda 2020, which outlines ways for the forest products industry to streamline its production processes.

Papermaking is a prime candidate for improvement. Currently, paper quality is gauged by manufacturing a 15- to 30-ton roll of paper; a few samples from end of the roll are analyzed to determine how well they bend. If the samples don't meet industry specifications, the entire roll is recycled into pulp or sold as an inferior grade. To avoid this costly outcome, manufacturers often over-engineer paper, using more pulp than necessary to ensure that the final product meets the standards.

This method consumes more raw material and energy than necessary, so the EETD team developed a sensor that tracks paper's flexibility in real time. The laser/ultrasonic sensor measures without touching the paper, an important advantage because the paper moves at 20 meters per second (45 miles per hour) during production, and the slightest contact can break the sheet and cause costly machine downtime or mar lightweight grades such as copy paper and newsprint. The recent sensor trial boasted the highest sample speed ever reported for a commercial application of laser ultrasonics.

The sensor measures the time it takes ultrasonic shock waves to propagate from a laser-induced excitation point on the moving paper to a detection point several millimeters away. The velocity at which the ultrasound waves travel from the excitation point through the paper to the detectionpoint is related to two elastic properties: bending stiffness and out-of-plane shear rigidity.

Image of paper making equipment

Figure 1. The laser ultrasonic sensor was successfully tested at a working paper mill.

The sensor works by directing a detection beam from a commercially available interferometer toward a rotating mirror. The spinning mirror reflects the beam onto the paper coursing along the production belt. Because both the beam and the paper are moving at the same speed, the beam remains fixed on the same point on the paper during their brief contact.

Next, an optical encoder determines when the beam is perpendicular to the paper, at which time a circuit fires a pulsed laser. The five-nanosecond pulse causes a microscopic thermal ablation of the paper, which is too small to visibly mar the paper but strong enough to send ultrasonic shock waves through the sheet. The waves propagate until they reach the detection beam. Because the laser is synchronized to fire only when the detection beam is perpendicular to the paper, the distance between the ablation point and detection point is known, which allows the speed of the waves to be calculated. The next step for the researchers on the sensor project is to work with Boise Cascade to link the sensor with sophisticated feedback controls that maintain paper's stiffness during manufacture. ABB Corporation, which participated in the recent trial, is also likely to be part of the upcoming phase.

"Our technology will enable this real-time feedback control," says Ridgway. "And the successful mill trial shows we are one step closer to realizing it."

The mill trial is the latest in a string of successful real-world tests of the sensor. In 2003, Ridgway, Russo, and engineers from the Institute of Paper Science and Technology conducted a pilot-scale test of the laser ultrasonic sensor at Mead Paper Company's research center in Chillicothe, Ohio.

This test demonstrated that the sensor's sophisticated hardware can successfully perform in an industrial environment where conditions are much harsher than in the laboratory.

— Dan Krotz

For more information, contact:

  • Paul Ridgway
  • (510) 486-7363; Fax (510) 486-7303

Dan Krotz is a writer in Berkeley Lab's Public Affairs Department.

This research was funded by the Department of Energy's Office of Industrial Technologies.

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