Research And Design On The High Precision Weak Capacitance Detection System

Nowadays,domestic electric power almost comes from coal-fired power which depends on Pulverized coal fired boiler. If the pulverized coal flow can’t be controlled, it would cause pulverized coal blockage which is a potential safety hazard. So the measurement of pulverized coal flow is extremely important. High pulverized coal injection technology is widely used in boilers.High pulverized coal injection technology can reduce environmental pollution as well as the cost of production, it can also eliminate the Potential danger. As the gas-solid two-phase flows systems are complex and uncontrollable, the measuring solid phase’s concentration becomes a technical difficulty in the pneumatic transmission systems. It is necessary to find a reliable and accurate method of measuring pulverized coal flow.A High Precision Stray-immune Weak Capacitance Measurement System is proposed to measure the gas-solid two-phase flows in this thesis. This thesis introduces the characteristics of gas-solid two phase flow and the common used detection method of gas-solid two-phase flow parameters. The system mainly includes the corresponding weak capacitance detection circuit and two capacitance sensors. The main content of this thesis are as follows:1.According to a great deal of literature and expertise, this thesis presents a research above many common methods, their merits and demerits are analyzed too. By contrasting about the suitable condition and the result of all measuring methods, we select the best measuring method which is the capacitance measurement.2.This part has introduced the structure and principle of the capacitance sensor and designs a new capacitance sensor, which has two earthed screen. Since the stray capacitance is much larger than the variation of measured capacitance, the stray capacitance should be eliminated by the measurement system. So this thesis puts forward a weak capacitance detection circuit.3.The system software flow chart and the principle of cross-correlation algorithm are introduced, and then this part gives the flow chart about the cross-correlation algorithm.4.In order to validate the validity of this system, it was tested by many times under different conditions, including the test of static performance and dynamic performance, then this part gives the MATLAB simulation results about the cross-correlation algorithm.The experimental results show that the system has good linearity, high measurement accuracy and stray-immune. The result of the percentage error is about 0.672%, the system has a measureresolution of 8.5fF and the non-linear error is less than 2.375%. The sampling rate is very fast. The measuring system has good accuracy and can meet the needs of industry.