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Laser sensor could save millions - on paper
Published:  01 May, 2002

Laser sensor could save millions — on paper

American researchers are working on a laser-based sensing system that they say could save millions of dollars every year in manufacturing processes involving thin moving sheets of materials such as paper, metals, plastics and glass.

Initial work is focusing on applications in the paper industry. The non-contact sensor, being developed by researchers at the Lawrence Berkeley National Laboratory (LBNL) and the Institute of Paper Science & Technology (IPST), measures the paper`s flexibility as it whizzes through a production web at speeds of up to 100km/h.

Traditionally, paper manufacturers have assessed the flexibility of paper by analysing small samples of paper taken from a three-tonne roll after it has been manufactured. If the sample doesn`t meet specifications, the roll is scrapped or sold as an inferior grade. To avoid this, the manufacturers often over-engineer the paper, using more pulp than necessary. This is costly, not only in terms of the extra materials, but also in terms of the extra energy needed to dry the heavier paper.

The new sensor should avoid these problems by measuring paper flexibility on the fly, in real time, and allowing adjustments to be made. Unlike ultrasonic sensing systems, which can damage a roll if they touch the paper coursing past at 30m/s, the new system makes its measurements without making contact with the paper.

The sensor measures the time it takes ultrasonic shock waves to propagate from a laser-induced excitation point to a detection point a few millimetres away. The laser beam follows the paper at the speed of the web, and when an optical encoder determines that the beam is perpendicular to the paper, a nanosecond pulse of energy is fired at the paper. This sends ultrasonic shock waves through the paper - the speed with which these propagate depends on the paper`s stiffness.

Last year, the US researchers tested their system on a paper mill in Ohio and found that it produced good results even with paper moving faster than 25m/s. They are now planning a full-scale pilot test in a much harsher environment.

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