A Time-of-Flight Extraction Method Based on Time-Sequenced Pulses for Ultrasonic Flow Measurement Using an FPGA-Based Time-to-Digital Converter

The accuracy of transit-time ultrasonic flow measurement depends strongly on stable time-of-flight (TOF) extraction. However, threshold-based TOF methods are susceptible to emission transients, structural ringing, and repeated threshold crossings, which may cause false triggering and timing fluctuations. This paper proposes a time-of-flight extraction method based on time-sequenced pulses for ultrasonic flow measurement using an FPGA-based time-to-digital converter (TDC). The method equalizes signal input paths, combines peripheral path switching with FPGA gating to achieve windowed valid-edge extraction, and exploits the temporal correspondence among consecutive excitation pulses to construct multiple TOF observations from matched pulse pairs, thereby improving extraction stability and timing efficiency. A complete FPGA-TDC ultrasonic flow measurement platform based on a Zynq UltraScale+ device was developed, and TDC linearity, timing precision, and flow calibration experiments were conducted. After code-density calibration, the four channels achieved RMS values of about 20 ps, and within a flow range of 0.7–3.6 m3/h, the relative error with respect to the reference flowmeter remained within ±0.6%, with repeatability errors below 0.3%. The platform operated stably under the present experimental conditions. These results demonstrate improved TOF extraction stability and overall flow measurement performance without complex full-waveform processing.