PLC-based lumber humidity measurement method

Purpose or research. The paper considers the solution of the scientific and practical problem of measuring the humidity content of lumber on the basis of a programmable logic controller (PLC), which makes it possible to implement control systems for drying lumber on inexpensive controllers of wide application with the advantages of modern SCADA systems.Methods. To achieve this goal, modern methods for assessing the humidity content of lumber by electrical resistance have been investigated. We have provided a review and analysis of the known dependences of the electrical resistance of lumber on moisture. It is noted the need to measure resistance in the range of 2.5 KΩ to 25 MΩ. The schematic solution for matching a high-resistance measurement bridge with an additional voltage source with a PLC is described here. We have proposed a method for calculating the matching scheme. Expressions are obtained for calculating humidity according to the data of an analog-to-digital converter. The procedure for setting up the measurement channel according to the readings of two reference measurements is presented.Results. The estimation of the measurement error of humidity and resistance is carried out. It is shown that a resistance measurement error of up to 20% leads to a humidity measurement error of up to 1%, provided that the parameters are correctly configured. To set it up, it is enough to take two reference measurements to complete the procedure for setting the humidity measurement parameters.Conclusion. In the process of designing modern systems for drying lumber, you can use the algorithms and programs of the technological process available and well presented in the literature for implementation on the PLC. The article offers a solution to the main problem of measuring the humidity content of lumber, which is necessary for these algorithms and programs. The material can be useful for researching technological stages of the drying process and developing industrial control systems based on inexpensive controllers.