Hoping to save the paper manufacturing industry millions of dollars in energy costs, Lawrence Berkeley National Laboratory engineers have developed a laser ultrasonic sensor that measures paper’s flexibility as it courses through a production web at up to 65 miles per hour.
‘We’re measuring the elastic properties of paper at manufacturing speeds using a noncontact, nondestructive monitor,’ says Paul Ridgway of Berkeley Lab’s Environmental Energy Technologies Division.
Last summer, Ridgway and colleagues tested the laser ultrasonic sensor at a Mead Paper Company mill in Ohio. They installed the sensor on a pilot paper coating machine and ran six paper grades through the web press, ranging from copy paper to heavy linerboard. The sensor’s signals remained excellent even at paper speeds up to 5,000 feet per minute, and the laser didn’t damage the paper. The effects of the papers’ moisture, tension, basis weight, and speed on the measurements were also examined.
‘The Mead test demonstrated the instrument works in an industrial setting,’ Ridgway says. ‘It’s a successful step toward a mill trial on a paper-making machine in which the environment will be much harsher. It will be hotter and wetter, and there will be more vibrations and fibre debris in the air.’
The sensor is part of the Department of Energy’s Agenda 2020, a collaboration between the wood, paper, and forestry industry and the Department of Energy (DOE) launched in 1994 to improve the industry’s energy and resource efficiency.
To understand how the sensor contributes to this initiative, consider how paper is currently evaluated. After it’s manufactured, a small sample of a three-ton paper roll is manually analysed for its mechanical properties by observing how it bends. If the sample doesn’t meet specifications, the entire roll is scrapped or sold as an inferior grade. To avoid this costly mistake, manufacturers often overengineer paper, erring on the side of caution and using more pulp than necessary to ensure the final product isn’t substandard. Not only does this consume more raw materials, it consumes more energy: The more pulp used per unit of paper, the more heat is required during the drying phase, which even in the most efficient mills requires an enormous amount of energy.
Rather than rely on postproduction evaluation and hope for the best, Ridgway and colleagues have developed a sensor that measures flexibility on the fly, in real time. It also conducts the measurements without touching the paper, an important advantage given that at 30 metres per second the slightest contact can mar lightweight grades such as copy paper and newsprint. This represents an improvement over contact transducers, another real-time evaluation tool that measures paper’s tensile elasticity by placing an ultrasound head directly onto the paper as it’s coursing through the web. Because it touches the paper, this technique can only be used with thicker stock.
In rough terms, the sensor measures the time it takes ultrasonic shock waves to propagate from a laser-induced excitation point to a detection point only millimetres away. The velocity at which the ultrasound waves travel from the ablation point through the paper to the detection point is theoretically related to two elastic properties, bending stiffness and out-of-plane shear rigidity.
More specifically, a detection beam from a commercially available Mach-Zender interferometer is directed toward a quickly rotating mirror. As the mirror spins, the beam is reflected in a circular pattern much like a lighthouse’s beam. During a portion of each revolution, the beam meets the paper as it courses along the production belt and remains with the paper until the beam’s arc leaves the paper’s plane. Because both the beam and the paper are moving at the same speed, the detection beam remains on the same point on the paper throughout their brief contact.
An optical encoder determines when the detection beam is perpendicular to the paper, at which time a specially designed adjustable delay circuit fires the pulsed neodymium-yttrium-aluminum-garnet laser. This microsecond pulse causes a microscopic thermal expansion or ablation on the paper, which is too small to mar the paper and effect how it absorbs ink, but strong enough to send ultrasonic shock waves through the sheet. The waves propagate through the paper until they’re registered by the detection beam. Because the laser is synchronised to only fire when the detection beam is perpendicular to the paper, the distance between the ablation point and detection point is known, and the waves’ speed is calculated.
A full-scale pilot test of the laser ultrasonic sensor is scheduled for the summer of 2003, Ridgway says. And further in the future, the sensor could provide quality-control safeguards and real-time process information for feedback process control in any manufacturing process involving thin, moving sheets such as sheet metals, sheet plastics, polymeric materials, and glass.