| Ultrasonic gas flowmeter (USF) has been developed fast as a new type of electronic flow meter since 1990s because of its great advantages such as high range ratio, high accuracy and double-direction measurement compared with traditional mechanical flowmeter. Meanwhile, with fast development of national economy and projects on natural gas,such as West-East Gas Pipeline project, USF has broad market.However, as a high-accuracy measuring electro-meter, the measurement accuracy and reliability of gas flow meter will be reduced due to the interference from an electromagnetic environment. So the design and research of electromagnetic compatibility (EMC) make great contribution to the accuracy and reliability.The paper focuses on the electromagnetic compatibility of USF and new circuit and circuit board were designed for EMC. The contents are as follow:(1) Anti-interference design of circuitNoise coupling and signal jitter are the most usual problems in USF. The amplified filter circuit was optimized to improve the signal-noise ratio. And also a new method was designed for the measurement to improve the accuracy. Besides, Lighting protection and isolating circuits were designed for the 24V power source and RS485.(2) EMC design of PCBPCB design is a key part in the EMC design due to PCB is also the interference source and interfered equipment. In the paper, the signal processing board was changed from four layers to six layers and wiring such as signal, clock and power source was arranged under EMC rules. Magnetic coupling was analyzed of ultrasonic exciting circuit and transformers were optimized with ANSYS.(3) Structural shielding design of USFUSF can have strong shielding effectiveness with unbroken shield. However gaps and holes make harmful effect. The effect of gap and hole on shield was analyzed. Gaps and holes were sealed by electromagnetic material and wirings of power source and communication were sealed by aviation plugs. Especially,holes and the display window in USF were analyzed and optimized in HFSS.(4) Calibration and EMC experiments of USFCalibration experiment was designed to test the accuracy of USF under the electromagnetic interference. EMC experiments in the national standard were used to measure if USF has good electromagnetic compatibility. Five different experiments were taken and passed in the laboratory based on the design and optimization of gas flow meter pointed above. So new designed gas flow meter can meet the EMC requirement. |
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