Abstract
The scope of this work is to discuss the challenges and demonstrate the potential of microwave testing for applications in the wood processing industry. Microwave technology benefits from the anisotropic dielectric properties of wood to simultaneously identify grain angle, density, and moisture content of wood. Therefore, the theory of free space transmission measurement is thoroughly discussed with emphasis on the characteristics of (and how to deal with) reflections occurring in real measurements. A more sophisticated calculation method for the derivation of the desired physical wood properties is presented. The advantages of a modern laboratory style setup are shown and its possible transition in an industrial-style application is discussed. Moist (moisture content 7.6–14%) and oven-dry spruce samples are tested. The detection of grain angle for moist and oven-dry wood yields an RMSE (root-mean-squared-error) of 0.14° and 0.4°, respectively. Moisture content is evaluated with density- and thickness-independent methods. Adapted regression models are proposed yielding an RMSE for moisture content of 0.45% for a single frequency measurement. The promising advantages of wood moisture estimation with frequency sweeps instead of fixed frequency signals are discussed and demonstrated for all samples (RMSE 0.39%). The dielectric constant of moist and oven dry spruce in the range from 8 to 12 GHz is evaluated in respect to density, moisture content and temperature. The respective constants ε′, ε′′, and tan(δ) are formulated in a general form via a non-linear regression and compared to existing data in literature.
Highlights
Non-destructive evaluation of the physical properties of wood is a key task in modern wood manufacturing processes
The work in this paper revealed that a correction of the measured vector network analyzer (VNA)-ratio in Eq (14) does not yield any noticeable improvement in precision of the derivation of physical properties
Due to the highly precise antenna positioning, grain angle values derived at different antenna positions are compared, which is performed separately for each of the 51 measured frequencies
Summary
Non-destructive evaluation of the physical properties of wood is a key task in modern wood manufacturing processes. Moisture is commonly determined with capacitance type moisture meters using high frequency signals or with resistance type pin moisture meters (Wilson 1999) These moisture meters are able to reasonably cope with industrial needs. Due to their measuring principles, measurements are prone to deviations in the wood grain angle They require density information for a reliable moisture determination. The application of X-rays is successfully established for density measurements enhancing both production speed and quality (Schajer 2001). Still, these systems remain rather costly and require efforts for the protection against hazardous ionizing radiation.
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