Research On Measurement Method And Key Technology Of Thermal Conductivity Of Vacuum Insulation Panel

Vacuum insulation panel is one of the most advanced energy saving and environmental protection thermal insulation materials,whose thermal conductivity can reach 3 mW/(m·K).In order to achieve the purpose of producing low thermal conductivity vacuum insulation panel and to detect the aging degree of vacuum insulation panel,it is necessary to accurately and quickly online measure thermal conductivity of vacuum insulation panel.At present,the thermal conductivity of the vacuum insulation panel is measured according to the principle of heat-conditioning plate method,the heat flux is used to calculate the thermal conductivity.The method has high measuring precision,but the stability time is long and the thermal conductivity of the vacuum insulated panel can not be measured online.In view of the present research on the rapid online detection method of thermal conductivity of vacuum insulation panel is not mature enough,this paper is dedicated to research a kind of rapid and high precision available online measurement method of thermal conductivity of vacuum insulation panel based on embedded heat flow meter method.This paper mainly carried out the following aspects of the research work:(1)In view of the traditional measurement methods of thermal conductivity of vacuum insulation panels,there are some problems such as slow detection speed and can not be measured online.Firstly puts forward a novel method for measuring the thermal conductivity of vacuum insulation panel-embedded heat flow meter method(Authorized invention patent number:ZL201410160016.9).Through theoretical analysis,ANSYS simulation analysis and the actual measurement system is constructed,which proves the feasibility of the method.And the hardware implementation method of embedded heat flow meter method is given in detail,including internal measurement module,external measurement module and signal acquisition method.(2)In view of the frequency measurement precision directly affects the measurement accuracy of the thermal conductivity of vacuum insulation panel,in order to eliminate plus or minus 1 count error produced by the direct counting frequency measurement method counting to the measured signal,at the same time in order to overcome the defect that the frequency measurement precision varies with the change of the frequency of the measured signal in the direct counting frequency measurement method,equal precision frequency measurement circuit is developed by using the equal precision frequency measurement method.The theoretical analysis shows that the frequency measurement precision of equal precision frequency measurement method can reach up to 10-7 orders on the whole frequency band.The practical application shows that equal precision frequency measurement circuit meets requirement of high precision and low cost.(3)In order to reduce the power consumption and cost of the internal measurement module,reduce its volume and improve the anti-interference ability of the whole measurement system,the internal measurement module is integrated with Taiwan’s nuvoton0.6um CDMOS process,SOP8 package.The simulation of each function module and the whole circuit is carried out through the Spectre simulator of Cadence.The simulation results show the correctness and rationality of the design.Using Virtuoso platform for the drawing and verification.In view of the fact that the uncertain factors such as process and temperature inevitably affect the magnitude of the reference voltage,a high precision bandgap reference circuit is developed based on the theory of zero temperature coefficient.Using the Miller capacitance compensation method,the phase margin of the op amp is increased and its stability is improved.(4)The calibration method of the measurement model is studied.Aiming at the nonlinear problem between thermal conductivity of vacuum insulation panel and the corresponding frequency variation characteristic value,in this paper,a linear calibration method based on least square method and a nonlinear calibration method based on BP neural network and RBF neural network are proposed.The experimental results show that the nonlinear calibration with neural network is more accurate than the least squares method.In addition,the experimental results show that the use of RBF neural network for nonlinear compensation compared with the use of BP neural network,the former in terms of convergence speed,convergence precision,etc,has more advantages(Mean square error of BP neural network can reach 10-4 orders of magnitude after the 587 step training,Mean square error of RBF neural network can reach 10-4 orders of magnitude after the 18 step training).(5)Aiming at the defects of the BP neural network,which is easy to fall into local optimization and slow convergence speed,we make full use of the global search capability of genetic algorithm to combine the genetic algorithm and BP neural network.The results show that compared with the BP neural network,using genetic neural network for nonlinear calibration has higher prediction accuracy,and the prediction error is less than 0.05 mw/(m·K).In order to improve the generalization ability of RBF neural network,and further improve the performance of traditional RBF neural network,the nonlinear calibration method based on the improved RBF neural network is proposed.The results show that the improved RBF neural network is more accurate than the RBF neural network for nonlinear calibration,and the prediction error is less than 0.04mW/(m·K).Finally,the actual application results show that the nonlinear calibration method based on the improved RBF neural network can greatly improve the measurement precision of the thermal conductivity of vacuum insulation panel.The actual measurement precision is better than 1%,satisfying the measurement requirements of high precision and low cost,has the advantages of small error,high precision and global optimization ability,and has widely application value of promotion.