Automatic Measurement of Roundwood on Vehicles

The delivery of roundwood is a logistical and technical challenge, especially for plants that process large quantities of industrial wood to produce wood products, paper or cellulose. Correct measurement of the delivered wood volume is an important step, both from the point of view of the forest enterprises and the consuming plants. Automatic measuring equipment has the advantage that delivery vehicles do not need to be entered by humans, which significantly increases work safety and can contribute to the rational, cost-saving arrangement of deliveries.

"Structure from motion" is a photogrammetric method for three-dimensional measurement of objects. It has already proven itself in practice for non-contact area measurement on roundwood stacks in forests. Within a research project, Dralle A/S (Hørsholm/Denmark) and PTB have jointly carried out a concept study on the use of structure from motion for the measurement of stacks on vehicles.

In structure from motion, the object to be measured is recorded in motion by electronic cameras. The object can either be moved past the cameras, or the cameras circle the object on oval paths.

In the second step, the images are searched for small patterns that can be reliably recognized from different distances and all spatial directions. If enough common patterns are found in neighbouring images, the images can be matched. Gradually, the viewpoints and the viewing directions used by the cameras can be determined until the entire spatial shape of the object can be reconstructed. Stationary objects in the environment can be automatically detected and separated from the moving object.

Log surfaces usually show large numbers of small irregular structures, which can be well employed in structure from motion. Further requirements for good measurement results are a complete and sufficiently dense coverage of the surface as well as a smart choice of movements during the acquisition. No additional tools are needed, such as structured illumination or optical markers.

Structure from Motion can be performed with colour cameras. The extra colour information can be used to distinguish logs on a loading platform from other moving objects (such as vehicle walls, but also securing devices such as stanchions or chains) and the surroundings, and to detect stack boundaries, which are important for measuring delivered volumes.

In wood processing plants, large quantities of roundwood have to be measured as quickly as possible. The prototype portal built by Dralle A/S as part of the concept study works with seven pairs of cameras mounted on a gantry. The stacks are measured one by one as they pass the portal on the vehicle at walking speed. The seven pairs of cameras allow the stacks to be recorded from all sides along with the loading platform in a single step (Figure 1). The cameras are supplemented by spotlights for measurements at night and in other unfavourable lighting conditions.

For the evaluation of the images, a proprietary software is used, which has been developed by Dralle A/S especially for this task. The software calculates the corresponding surface points on the logs from the images. It assumes the so-called outer and inner orientations of the camera pairs as known, which describe the recording of spatial points with a model.

The orientations of all camera pairs are determined during the installation of the portal by means of a calibration with a special standard, which is recorded in various positions in the measuring volume. The standard is provided with optically detectable markers whose distances are calibrated independently. The orientations of the individual cameras are calculated from the positions of the markers in the images.

The software also detects the direction in which the vehicle passes the portal and can be supplemented by modules for automatic vehicle identification. In the future, this additional information can be used to regulate processes and ensure the safe handling of deliveries.

The metrological evaluation starts with measurements on a barked round pine log. Figure 2 shows the reconstructed point cloud from a measurement where the log was pulled through the portal on a hand cart. With the colour cameras it is possible to capture the spatial shape of the log surface along with its colour. Figure 3 shows the measurement of the length and diameter of the log from Figure 2. The diameter is determined by the cylinder, which is assigned to the shell using the least squares method. The length is measured as the distance between the centroids of the crosscuts in the direction of the cylinder axis.

Structure from motion, as an optical measurement method, offers an economical and reliable solution for the automatic and non-contact measurement of loading volumes on vehicles. The tests carried out so far with logs show that the measurement deviations from the PTB reference values are well below 5 %. Additional information, such as automatic recognition of the vehicle and its movement, can be used for the rational and safe arrangement of roundwood delivery.



Figure 1: Portal for measuring log piles on vehicles. The truck passes through the portal at walking speed. Seven pairs of cameras capture all sides of the pile along with the loading platform.



Figure 2: Reconstruction of a log with a midpoint diameter of 40 cm and a length of 3 m after measurement with seven colour camera pairs providing additional colour information.



Figure 3: Measurement of the length and diameter in of the log from Figure 2. The measured diameter (in Millimetres) deviates by -2% from the reference value, while the deviation in length is about -3%.