Research On Wood Moisture Detection System And Detection Method

Wood is a renewable material that is widely used in engineering and life.In the processing of wood,moisture content is one of the key parameters affecting the quality and service life of wood.If the moisture content detection is not accurate,it will cause the wood to shrink and swell,and result in the degradation of the wood quality or even loss of value.How to improve the detection accuracy of wood moisture content and reduce the cost performance of wood moisture content detection system is an urgent problem to be solved in the field of wood processing.In this thesis,the microwave transmission method is applied to investigate the influence of excitation frequency,wood temperature,wood texture and other parameters on the detection signal in microwave measurement,and a multi-parameter moisture content detection system based on microwave devices is designed and its performances are verified by experiments.The main work is illustrated as follows:According to the dielectric properties of wood,the previous microwave detection system in our lab based on vector network analyzer is used to investigate the detection performances of wood moisture content at different frequencies,different temperatures and different grain directions.Through experiments,it is found that the parameters such as frequency,temperature and wood texture have a non-negligible influence on the microwave signal.The optimal detection parameters of wood moisture content,and the quantitative relationship between moisture content and the amplitude as well as phase of microwave signals are determined,which lays the foundation for the follow-up work.Then,a water content detection system based on microwave devices is designed.The system combines modulation and frequency reduction functions to simultaneously measure the amplitude and phase of microwave signals.In the preparation of moisture content specimens,the preparation method of standard moisture content specimens is proposed by consideration the changing trend of wood moisture content under different temperature and humidity,which avoids the damage to the specimens caused by improper temperature and humidity.In order to eliminate output fluctuation introduced by the electronic devices,a reference signal is acquired when there is no specimen in detection system,and the difference signal between the detection signal and the reference signal is deemed as the final signal to character the moisture content of woods.After that,a series of experiments are carried out.According to the experiment results,the relative error of the wood moisture content of the designed system is about 1%,and the repeatability of the system is good.Finally,in order to improve the accuracy of the wood moisture content measurement,three detection approach of wood moisture content,including the single-parameter detection method of amplitude difference characterization and phase difference characterization,and the dual-parameter detection method of comprehensive characterization of amplitude and phase are put forward to character the wood moisture content.The experiment result indicated that the relative error of amplitude difference characterization is 0.69% and the relative error of phase difference characterization is1.51%,The detection accuracy of amplitude difference characterization is high,but it is difficult to achieve continuous measurement,and the accuracy of phase difference detection needs to be improved,so the single-parameter detection method has limitations.The dual-parameter detection approach is adopted the amplitude and phase of the detection signal to character the wood water content.The relative error of the dualparameter detection approach is 0.50%,which is 27% higher than that of the singleparameter detection method.