Wood grain angle estimation in logs with microwave modelling

We study the detection of the wood grain angle using microwaves in order to avoid twist in finished boards. For such a determination of spiral grain, we exploit the anisotropic dielectric properties of wood affecting the polarization of an electromagnetic field. A cylindrical model is used for calculating the electromagnetic scattering from a wooden log. In the model, the material has anisotropic dielectric properties, and is considered homogeneous. Furthermore, the grain angle is small and modelled without radial dependence.Based on the model, we present a measurement strategy using a linearly polarized incident plane wave without z-dependence, together with sensors placed close to the log. The grain angle can be estimated, also when the effect of the cylindrical geometry is substantial, through a comparison between the measured and the calculated field. No information on the log except its radius is required; the moisture content can, e.g., be unknown. We apply the Cramer Rao Lower Bound, to present error estimates for the determined grain angle as well as for the dielectric parameters.