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Laser Ultrasonic Stiffness Sensor Wins R&D 100

Image of Paul Ridgway on left and Rick Russo on right

Figure 1. The Berkeley Lab's Laser Ultrasonic Sensor team: Paul Ridgway (left) and Rick Russo (right)

R&D Magazine has bestowed four of its prestigious R&D 100 Awards for 2006 on researchers at Lawrence Berkeley National Laboratory and their colleagues. One award went to the laser ultrasonic stiffness sensor (LUS), developed by Rick Russo and Paul Ridgway (Figure 1) of the Environmental Energy Technologies Division (EETD) along with colleagues Emmanuel Lafond, Chuck Habeger, and Ted Jackson of Georgia Tech's Institute of Paper Science and Technology. The R&D 100 awards honor the magazine's choices for the 100 most significant proven technological advances of the year.

The LUS will substantially improve the cost effectiveness and efficiency of paper manufacturing (Figure 2). Paper is often engineered to exceed minimum stiffness specifications. This results in the use of more raw materials, chemicals and energy than necessary to produce the end product.

In some cases, process engineers manually test each three-ton paper roll after it has been made and reject and discard the whole roll if doesn't meet specifications. The LUS reduces waste on the manufacturing line by measuring the stiffness of 30 meters of paper per second without touching the product. The LUS works by measuring the speed of laser-induced ultrasonic waves in the sheet.

A detection beam is reflected from a rotating mirror in a circular pattern, briefly traveling with the paper as it courses along the production belt. When the beam is perpendicular to the paper, a laser fires a nanosecond pulse that causes a microscopic thermal expansion, too small to mar the paper or affect how it absorbs ink but strong enough to send ultrasonic waves through the sheet. The waves propagate until they're registered by the detection beam. The velocity at which the ultrasound waves travel from the excitation point through the paper to the detection point is related to two elastic properties of importance in paper and other sheet materials: flexural rigidity (or bending stiffness) and out-of-plane shear rigidity.

Image of the Laser Ultrasonic Sensor

Figure 2. The Laser Ultrasonic Sensor

Flexural rigidity is a property that is of great importance to a wide variety of paper grades. For example, flimsy paper will jam in copiers and printers. Shear rigidity is important in packaging grades because it strongly influences downstream operations such as fluting, scoring, creasing and bending.

Software developed as a part of this project analyzes the LUS signals in real time to measure the flexural rigidity in lightweight papers, and the flexural rigidity and shear rigidity in heavier paperboard grades.

A significant advantage of the LUS is that it does not require any contact with the paper, so it does not break or create defects in the paper. This is important because papermaking machines move the sheet at speeds up to 30 meters per second (60 miles per hour), and the slightest contact could break a sheet, cause costly machine downtime, and mar lightweight grades such as copy paper and newsprint.

An additional groundbreaking feature of the LUS is that it enables the integration of feedback process controls based on real-time stiffness data: LUS measurements can be fed back into the paper machine's process control system allowing it to continuously adjust process variables to keep bending stiffness adequate while minimizing use of fiber feedstock. This saves energy, chemicals, and natural resources.

If this technology were to be widely adopted by U.S. paper mills, the researchers estimate it could save more than 30 million trees, eight trillion watt-hours of electricity, and about five hundred millions dollars annually.

For more information, contact:

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

Laser Ultrasonic Sensors

Energy-Saving Paper Sensor Passes Major Milestone

Laser Ultrasonic Sensor Streamlines Papermaking Process

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