Laser Ultrasonic Sensors

Description:
LBNL engineers have invented and developed a laser ultrasonic sensor for nondestructive, non-contact, quantitative evaluation of physical properties of web-manufactured material while it moves at production speeds.

Broad Fields of Use:
This technology has applications in all web manufactured materials including paper, nonwovens, plastic film, sheet glass and sheet metal. Measurable properties include all that affect the speed of propagation of mechanical waves, such as elasticity (bending, shear, tensile, and compressive stiffness in the three dimensions), thickness, surface topology/roughness, density, temperature, voids or cracks, and inclusions. The value of this technology is the opportunity it gives for feedback control of the manufacturing process to tighter specifications of the measured property in the product. This can result in savings of energy and natural resources from reduced production of off-specification product (and possible reprocessing costs), and reduced utilization of feedstock to produce in-specification product.

Figure 1. System for LUS analysis of paper.

Comparison with Current Technologies:
There is no other technology available for measurement of paper bending stiffness on-line, during manufacturing. For measuring thickness, contact methods are used on materials that are not too fragile (light weight papers) or too hot (steel and glass). For materials where contact measurements are inappropriate, the non-contact methods do not measure thickness directly. They measure basis weight (mass per unit area) or optical opacity directly, and assume a value for mass or optical density to allow calculation of thickness. If the mass or optical density varies, the thickness calculated is inaccurate. For materials thinner than the ultrasound wavelength (generally less than 0.5 mm), the laser ultrasonic measurement of bending stiffness combined with independent measurement of basis weight is a more reliable measurement of thickness than existing non-contact methods.

Description of Current Application:
We have developed a prototype sensor for measurement of flexural rigidity (or bending stiffness) of paper. Good measurement performance has been demonstrated at paper web speeds up to 25m/sec (5000ft/min). A full-scale paper mill demonstration of the technology is scheduled for summer 2003.

Figure 2. Laser ultrasonic sensor installed on pilot paper coating machine.

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