Research And Development Of A Capacitively-coupled Electromagnetic Flowmeter Signal Converter

Electromagnetic flowmeter has been widely used in the field of industry, due to its simple structure, no pressure losses, high accuracy and wide measurement range. Capacitively-coupled electromagnetic flowmeter(CEMF) is a special electromagnetic flowmeter, which detects the flowrate signal in the capacity coupled mode. Besides of the conventional features, the CEMF can also be used to measure the flowrate of liquids such as high-concentration slurry fluids, adhesive fluids, corrosive fluids and low-conductivity fluids, because of its non-contract measurement character. Those liquids can not be measured by ordinary electromagnetic flowmeter. Thus the CEMF has more application field. However, it's difficult to pick up the flowrate signal in this kind of flowmeter, because the coupled signal is too weak compared with the noise. There are more difficulties and higher requirement in the development of the CEMF sensor and signal converter.Based on previous works and the related literatures, this thesis focuses on the research and development of the CEMF signal converter with high signal-to-noise ratio. The main works are listed as follows:1. With the signal characteristics of the CEMF being analyzed, a new cross-correlation signal processing method for the electromagnetic flowmeter with rectangular wave excitation is proposed, and an improved rotary capacitor filter is designed to realize the flowrate signal cross-correlation detection. This method can effectively detect the flowrate signal under strong noise background while keeping low zero drift.2. A CEMF signal converter prototype based on ARM processor has been developed. The signal processing circuit is designed with the rotary capacitor filter as the core part, and adopts a high-resolution A/D converter to improve precision. A complex programmable logic device(CPLD) is used to implement time logical control. The signal converter also includes excitation circuit, man-machine interface and communication module.3. Using the CEMF prototype, an experimental study on the characteristics of electromagnetic flowmeter in the oil-water two-phase flow is implemented. The experimental results show that the performance of the CEMF is better than the conventional electromagnetic flowmeter in the oil-water two-phase flow measurement.