Laser Utrasonics for Paper Quality
How can paper's strength and flexibility be measured without touching it? What's more, how can it be done when the paper is moving at 30 meters per second (65 miles per hour)?
EETD's Paul Ridgway and Rick Russo are developing a laser-based ultrasonic technique to answer these questions. This technique measures paper's mechanical properties nondestructively while it's on the papermaking machine, or "web."
Because paper's strength, fiber orientation, and flexibility cannot be measured while the sheet is moving on the web, industry analysts rely on measurements of samples cut from the ends of large paper rolls. The delays that result from having to adjust the process variables cost manufacturers significant time and money. A monitor of paper strength installed on the web would allow real-time feedback control of the process, resulting in valuable savings in feedstock and energy. Such a strength sensor must make nondestructive measurements on paper moving at production speeds. This is the goal of the "Laser Ultrasonics for Paper" project.
Ultrasound is used to probe fiber orientation and to test tensile and compressive strength, bending stiffness, and other important mechanical properties of paper. Ultrasonic analysis using contact transducers is gaining credibility in the papermaking industry; in fact, contact-based techniques for measurement on moving paper have been in development for over 25 years. Commercial systems are just now becoming available for heavy paper grades; however, the pressures required to operate contact transducers damage lighter grades of paper.
Laser Ultrasonics (LUS) is a noncontact, nondestructive method that recently has been used to analyze paper's mechanical properties. A short (less than a microsecond) pulse of laser light generates ultrasonic waves by either a thermal expansion or an ablation shock wave or both. The ultrasonic wave propagates along the paper sheet and is detected at a known distance (several millimeters) using a noncontact interferometric technique. The time-of-flight of the wave over the known distance gives the propagation velocity of the wave. The wave velocity is theoretically related to elastic properties, which in turn are empirically related to strength properties.
Shown schematically in Figure 1, the LUS system for moving paper is composed of three parts: a "web simulator" and generation and detection systems. A photograph of the system is shown in Figure 2. The web simulator, designed by Berkeley Lab engineer Sam Mukherjee and built by machinist Steve Ferreira, rotates a paper belt at up to 30 m/sec. The generation system consists of a pulse Neodymium Yittrium Aluminum Garnet (Nd-YAG) laser and associated optics.
The key components of the detection system are a commercial Mach-Zender interferometer, a rotating mirror, and control electronics. The rotating mirror moves the interferometer probe beam with the paper, effectively stopping the paper motion with respect to the detection spot. An optical encoder is used to track the mirror's rotational position. An adjustable delay circuit, designed and built by Berkeley Lab engineer Paul Barale, triggers the firing of the generation laser pulse so that the ultrasonic wave is detected when the detection beam is in position on the paper surface. Measurements are made on paper moving at up to 30 meters per second, the highest production speeds used in the papermaking industry.
Further development goals include optimizing laser generation wavelength and pulsewidth; extending the method to measure ultrasonic velocities in all directions in the plane of the paper sheet; interfacing the system to a computer for control, data acquisition, and signal analysis; and eliminating the rotating mirror.
For more information, contact:
- Paul Ridgway
- (510) 486-7363; fax (510) 486-7303
- Rick Russo
- (510) 486-4258; fax (510) 486-7303
This research is supported by the U.S. Department of Energy, Office of Industrial Technologies, with direction provided by the Agenda 2020 American Forest Products and Paper Industry Sensors and Controls Task Group.