Numerical verification of a microwave impedance model for a twisted wooden cylinder

摘要

To determine the grain angle under bark for wooden logs, microwaves are suitable in contrast to a laser system that requires that a part of the bark layer is removed. Such measurements can be used to sort out logs in a sawmill in order to avoid problems in the processing, low quality in finished products and unnecessary costs with sawing, drying and transporting. Since the logs are moving with a high speed, the measurements must be done quickly in real time. To get quick algorithms it has been proposed to model the log with a normal surface impedance rather than as a penetrable cylinder. This paper determines the accuracy in such a modelling by comparing results from the two models in two-dimensions. The comparison is based on measured values of dielectric data for wood. The conclusion is that there exists a region of moisture content, frequency and azimuthal angle for which the relative error is less than 1% for the longitudinal and less than 10% for the tangential component of the electric field, and still lower for a technically interesting narrow angular region. The induced model error in the determination of the grain angle is about 5% which is of the same order or less than the error introduced by measurement noise.