Determination of time-of-flight (TOF) is crucially important in liquid ultrasonic flow meters. There are some deficiencies of previous TOF detection methods in practical applications. Inspired by our last work and the knowledge of clustering algorithms, we proposed a TOF determination method based on ultrasonic signal onset (USO) detection and multiple-zero-crossing technique to solve cycle-skip problem caused by waveform change. This method categorizes the ultrasonic and the noise signal, and identifies the rough position of USO. A time value from the obtained zero-crossing time values is selected as the TOF. The main advantage of the proposed method is the high insensitivity towards waveform change, i.e., the effect of amplitude characteristics change on TOF detection is negligible. Numerical experiments were done to evaluate the performance of the proposed method and its anti-noise ability. Experimental validation on a liquid ultrasonic flow meter showed the proposed method is capable of obtaining the accurate TOF.

飞行时间（TOF）的确定在液体超声波流量计中至关重要。在实际应用中，先前的TOF检测方法存在一些缺陷。在最后的工作和聚类算法的知识的启发下，我们提出了一种基于超声波信号发生（USO）检测和多次过零技术的TOF确定方法，以解决由波形变化引起的周期跳跃问题。该方法将超声波和噪声信号进行分类，并确定USO的大致位置。从获得的过零时间值中选择一个时间值作为TOF。该方法的主要优点是对波形变化不敏感，即幅度特性变化对TOF检测的影响可以忽略不计。通过数值实验评估了该方法的性能及其抗噪能力。在液体超声波流量计上的实验验证表明，该方法能够获得准确的TOF。