Research On Wood Moisture Measurement System Based On Single Planar Capacitance Sensor

The People's Republic of China is a country of relatively meager forest resources. How to use wood efficiently has an important economic and social significance. Strict quality detection of forest product materials is the prerequisite to ensure the efficient use of wood. According to statistics, seventy-five percent of wood quality problems are caused by inaccurate estimation of the moisture content of raw materials in processing. In view of this, a wood-moisture-content detection system-considering the capacitance sensor of uniplanar form as the key-was researched and developed, supported by the "948" national project management office (specifically, research into "An Electromagnetic-type, High-precision Measurement System of Wood-moisture-content"). This has an important economic significance related to wood product industry in China, particularly product quality improvement and enhanced competitiveness.The present situations of wood-moisture-measurement systems as well as the moisture measurement techniques, in China and abroad, are summarized respectively in this study; both advantages and disadvantages of each technique were analyzed and compared. The products manufactured by well-known foreign companies and applications are introduced, we find out that the research of non-destructive, non-contact and high-precision measurement system of wood moisture content is the developing orientation of wood-moisture-content measurement technology in the modern wood processing industry. Therefore, this research thesis considers wood-moisture-content measurement system technology of a capacitance sensor, of uniplanar form, as the key.In view of all the complicated wood detection benchmarks which have the capacity of transducer as the detection benchmark of wood-moisture-content in China and abroad, research concerning the constructing of a wood-moisture-content detection benchmark, based on the relationship between the permittivity of wood and wood-moisture-content, was proposed. This allowed the detection system of wood-moisture-content to get rid of the timber size restriction, making the operation simpler and user cost can be reduced. The overall research scheme, therefore, was constructed by studying the characteristics of permittivity of wood and considering the factors of wood-moisture-content, density, the frequency of measurement circuit and texture direction comprehensively.The working principle of a capacitance sensor of unspecific form was also researched. A precise mathematical model of a capacitance sensor to measure permittivity was constructed by means of the theory related to an electrostatic field and the property of conductor and dielectric in the field, when electrodes and measured objects are in the state of arbitrary shapes and random relative positions. The theoretical analysis and solution to the working condition of a spherical capacitance sensor was carried out, and the same result with traditional electromagnetic theory was obtained.The working principles and mathematical model of an uniplanar capacitance sensor were given, based on the research results of the mathematical model of capacitance sensor of unspecific form. For an uniplanar capacitance sensor, its precise analytical solution is difficult to obtain, thus, the finite element method was introduced. A method of obtaining the fitting polynomial equations which is convenient for the detection system to carry through data processing was expounded. Following therein, the functional relationship about the perrnittivity of wood (ε) and the capacity of uniplanar capacitance sensor was established.According to the structure arrangement and working state of the uniplanar capacitance sensor of the wood-moisture-content measurement system, the signal extraction circuit and signal excitation circuit of sensor were designed. The capacity value of the capacitance sensor affected by the measured object was detected. The permittivity of wood (ε), therefore, was obtained. The relationship between wood-moisture-content and permittivity was established through further experiments and theoretical analysis, the detection benchmark database module was constructed based on neural networks. Thus, accurate values of wood-moisture-content can be obtained, according to the permittivity of wood measured.Per industrial field requests, the basic functions of wood-moisture-content detection system were determined. Then, a principle prototype which had the basic functions required was developed. The software of the principle prototype is composed of the detection signal control module and data-processing module. The detection signal control software primarily control and coordinate the relationship between signal processing circuits and mechanical transmission, based on the signals generated by the measured wood position detection circuits of photoelectric sensors. The data processing module is composed of the moduleof detection benchmark database, based on a certain neural net, and the module of wood-moisture-content measurement, these two modules are the core of the entire system.Through testing, the principle prototype has performed well in measuring wood moisture content and the desired functions are achieved.