Supervisor: Gary S. Schajer
Renewable Resources Laboratory
Wood is a natural material with a wide range of uses spanning from fine furniture to fuel. This versatility derives from the wide range of material properties that exists within the wood supply. However, the highly variable material properties can also create serious difficulties in industrial processes where uniformity is essential for the manufacture of consistent quality products. For example, the grain direction of higher-grade lumber should be closely parallel to the length of each piece. This improves the strength and machinability of the material. Another common requirement is for the moisture content to be low but not bone dry. This minimizes subsequent warping and enables the production of strong glue joints. In some applications, high wood density is desirable to achieve mechanical strength and durability.
To achieve the needed raw material uniformity, it is essential to be able to measure wood physical properties reliably in industrial conditions. Practical measurement devices must be accurate, robust, low-maintenance, fast and preferably non-contacting. Microwave technology provides an interesting wood property measurement opportunity. When a microwave beam propagates through wood, it is depolarized, attenuated and phase shifted. The amounts of these three changes, all of them directly measurable, depend on the wood grain direction, moisture content and dry density. Thus, the three microwave measurements can be used to indicate all three wood properties simultaneously. This simultaneous indication is a key advantage of the microwave technique because it allows separate indication of each wood property, independent of influences of the others. In comparison, most other sensors measure only one property, and are prone to cross-talk from the non-measured properties.
A prototype microwave system for wood property measurements has been developed and built. It has been designed to be robust and to use relatively simple and commonly available microwave components. Thus, the design should be well-suited for industrial use. The system automatically makes a set of eight rapid measurements at each measurement point to identify the attenuation and phase shift in parallel and cross-grain directions. From these measurements, the wood moisture density, dry density and grain direction can be evaluated. The availability of eight measurements to give three results provides a useful data averaging and data self-consistency check, thereby improving the accuracy and reliability of the results.
In a series of experimental measurements, the prototype microwave system successfully identified wood grain angles for hemlock and Douglas fir samples respectively with standard errors of 0.9° and 2.5° (measured range = -90 to +90°), moisture content 1.2% and 1.9% (measured range = 7-28%), moisture density 3.7kg/m3 and 7.9kg/m3 (measured range = 40-150kg/m3), and dry density 16kg/m3 and 30kg/m3 (measured range = 325-625kg/m3).
Improved quality control ability in industrial wood processing facilities allows much more consistent and effective use of the raw material. When sub-standard material can be identified and removed, the full potential of the remaining high-quality material can be achieved. This raises the effective quality of a batch of wood from that of the the poorest material (the “safe” description) to that of the majority material (the “actual” properties) with equal safety. This approach adds value because of the improved quality and the improved consistency of the measured wood.