Closed-loop control of lumber drying based on acoustic emission peak amplitude

After preliminary drying runs to establish the experimental procedure and acoustic emission (AE) parameters, several runs were made to determine a method of using feedback to control the drying process. The basis for using AE in drying control is that it directly measures stress development that leads to checking. In past studies, the approach has been to rely on changes in AE event rates to accelerate or decelerate the drying process. One of the major difficulties in using AE has been the very limited area from which AE could be sensed. The approach in this study, which has been previously reported, is the use of a metal sticker inserted into the load of lumber that acts as an accumulator of AE during the drying process without affecting airflow. Because of the multiple board contact, the AE is much greater in numbers than direct contact with a single transducer and more reflective of the response of the load. This paper reports on a new method of using automatic feedback control from a ratio of peak amplitudes to more accurately reflect the development of checking. Three runs were made with a difficult to dry hardwood (tanoak), of which the first two were made with conventional sample board controlled schedules to establish the AE control strategy. The final run was made solely under AE control, with the end result of a reduction in drying time to the fiber saturation point of about 40 percent with degrade no greater than the conventional runs.