Evaluation of wood characteristics: Internal scanning of the material by microwaves

SummaryInternal scanning of wood by microwaves presents many advantages for the automatic detection of defects and the evaluation of the characteristics of wood in the dry wood processing industries. Advantages are: access to more than one variable, efficient coupling in the air, easy automation and reasonable cost. Microwaves are electromagnetic waves the speed and attenuation of which depend of the medium in which they are propagated, especially its electric permittivity which depends on its moisture content and its density. Since wood is anisotropic, this dielectric constant also varies with the direction of fibers. An experimental apparatus was set up that allows to test the feasibility of automatic detection of wood characteristics and defects. It automatically measures the attenuation and dephasing of an ultra high frequency wave sent through a board and moving through two rotating antennas. The graphs and results obtained gave significant information of physical characteristics of wood (density, moisture content, slope of grain) and the detection of defects (knots, metallic objects, sapwood).