Combining X-ray and 3D scanning of logs
In Scandinavia, sawlogs are typically sorted upon arrival at the sawmill. Presorting of sawlogs according to dimension, e.g., using an optical three-dimensional (3D) scanner, is used to make the sawing process more efficient. However, since wood is a biological material with large variability, there will be large differences in the quality of wood sawn from different logs of similar size. For some dimensions of sawn goods, wood quality is very important and low grade products will be difficult to sell, while for other dimensions, grade is less important. This means that if the quality of the sawn goods may be predicted before actual sawing takes place, the production of off-grade products, i.e., products not meeting customer demands, may be reduced.
Some quality information may be obtained using the outer shape information from a 3D scanner and more detailed information on the internal quality of the log can be obtained using an X-ray log scanner. Today, the use of X-ray log scanners is becoming increasingly common and most sawmills installing an X-ray scanner already have a 3D scanner present.
This raises the question of possible benefits from combining the X-ray and 3D scanning techniques. In this thesis, a method has been developed where the log shape measured by the 3D scanner is utilized to estimate the X-ray path lengths through the wood. This allows the calculation of green density profiles of the log, which may in turn be used to calculate quality variables such as heartwood content and heartwood density. The hypothesis of this project has been that the precision in these variables will be improved by the 3D path length compensation of the X-ray data.
The method has been tested on Scots pine (Pinus sylvestris L.) sawlogs, using both simulated and industrial log scanner data. Scanners with one and two X-ray directions have been used. The most important conclusions from the project are that:
- Heartwood diameter predictions are significantly improved by the combination of 3D and X-ray data.
- Dry and green heartwood densities can be calculated with improved precision relative to scanning using X-ray only.
- The method was found suitable for application with both one- and two-directional X-ray scanners.
- For a sawmill already using both 3D and X-ray scanners, there is a large improvement potential available using existing equipment. For sawmills thinking about investing in an X-ray scanner, the possibility of connecting it to the 3D scanner should make the rate of return of the equipment higher and encourage more sawmills to make this investment.
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