Abstract
The double-threshold method has been widely used in ultrasonic flow measurement to determine time-of-flight (TOF) due to its low cost and ease of implementation. Performance of this method is negatively affected by the cycle-skip phenomenon which occurs frequently under inconstant working conditions, especially varied fluid temperature. This paper proposes a method to suppress the phenomenon to facilitate reliable determination of TOF in ultrasonic flow measurement. First, the double-threshold method is used to generate a feature point to segment the signal. Second, based on the correlation coefficient and signal power, judgement factors of individual signal periods are calculated to determine signal onset. Finally, a valid zero crossing which has a constant lag from the onset is selected to determine the TOF. Thus, the cycle-skip phenomenon is suppressed. Two additional modifications are proposed to eliminate the influence of varied signal frequency and low sampling rate. The proposed method was validated by an experiment based on an ultrasonic water flow sensor. Results showed that the frequently appearing cycle-skip phenomenon can be successfully suppressed by the proposed method.
摘要
目 的
超声波渡跃时间 (TOF) 的准确检测是超声波流量测量中最重要的一步. 测量环境 (如流体介质、 温度等因素) 的变化, 会导致超声波波形发生变化, 进而引起 TOF 检测的跳波问题, 带来流量测量误差. 本文旨在提出一种 TOF 检测算法, 避免跳波问题的出现.
创新点
1. 根据超声波信号的波形特征, 提出了基于单一超声波信号的起振点判定方法, 进而抑制跳波问题的发生; 2. 针对实际应用中常见的频变和采样率低的问题, 提出了优化方法, 使所提方法更具实用性.
方 法
1. 依据超声波信号的周期性和幅值特征, 提出单周期信号间相关系数和平均功率相结合的判定因子, 据此对超声波信号进行判定并寻找起振点, 再根据起振点来确定 TOF, 从而抑制跳波问题; 2. 应用基于过零点的信号分割方法和基于 FFT 的信号插值方法, 解决信号频率变化和采样率低带来的实用性问题; 3. 根据超声波信号波形易受温度影响的特性, 利用流量标定台设计并进行相应实验, 使用自制的超声波流量传感器采集大量波形剧烈变化的信号用于计算 TOF, 并与传统方法进行对比, 验证所提方法在抑制跳波问题方面的有效性; 4. 使用标定台在不同温度下对超声波流量计进行标定, 使用不同的 TOF 确定方法, 展示跳波问题对流量计精度的影响, 并对此进行理论分析.
结 论
1. 介质温度等因素会影响超声波信号的波形, 进而引起 TOF 检测的跳波问题. 2. 超声波信号在周期性和单周期的平均功率上都与噪声信号有所差异; 所提方法从这两方面出发, 可准确找到信号起振点, 进而抑制跳波问题.
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Ze-hua FANG raised the idea. Cheng-wei LIU completed and implemented the method. Yong-qiang LIU helped to build the experimental setup. Cheng-wei LIU, Liang HU, and Wei-ting LIU wrote the original draft. Rui SU and Liang HU revised and edited the final version.
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Cheng-wei LIU, Ze-hua FANG, Liang HU, Yong-qiang LIU, Rui SU, and Wei-ting LIU declare that they have no conflict of interest.
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Project supported by the Science Fund for Creative Research Groups of National Natural Science Foundation of China (No. 51821093)
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Liu, Cw., Fang, Zh., Hu, L. et al. A method to avoid the cycle-skip phenomenon in time-of-flight determination for ultrasonic flow measurement. J. Zhejiang Univ. Sci. A 22, 695–706 (2021). https://doi.org/10.1631/jzus.A2000284
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DOI: https://doi.org/10.1631/jzus.A2000284
Key words
- Ultrasonic flow measurement
- Time-of-flight (TOF)
- Correlation coefficient
- Signal power
- Double-threshold method