Microwave Methods for Strength Grading of Timber and for Automatic Edging of Boards

Microwave methods have been succesfully applied in the saw-mill industry to the strength grading of timber and automatic edging of boards. Microwave detection of defects in sawn goods is based on inhomogenity or anisotropy associated with imperfections of wood material. A microwave bridge with freespace transmission through the timber in both branches of the bridge is used for the knot measurement of the strength grading equipment. Slope of grain is determined as a change of polarization of the transmitted microwave field. In the automatic edging system the microwave detection of knots is used together with optical methods. The knot detection is achieved using TM01-mode detectors to catch the dielectric wavequide mode HE11 associated with the knot.