Piezoelectric particle sizer for measuring bed load using a combination of resonance vibration modes

•Size distributions of sand and stones in the riverbed is important to predict environmental changes.•Passive piezoelectric particle counter for measuring bed load was developed.•The amplitude ratio of two resonance frequency peaks depended on particle size.•The particle sizes could be determined from short-term frequency analysis of the observed voltage waveform.

To predict environmental changes and prevent disasters in rivers it is important to track how the size distributions of sand and stones in the riverbed change over time. In this paper, we used a passive piezoelectric sensor to measure the particle distribution of a simulated bed load. The sensor had a simple, robust structure consisting of a circular aluminum plate and an annular piezoelectric transducer. The detection characteristics were evaluated by measuring the impact of alumina spheres with diameters of 3-8 mm in air. When particles hit the sensor, the impact excited a flexural vibration in the circular plate, generating electric power through the piezoelectric effect. The electric output signal exhibited two main frequency peaks, at 16.3 and 66.7 kHz, whose amplitude ratio depended on particle size. These two frequencies correspond to the fundamental and third resonance vibration modes. When particles hit the sensor surface sequentially, we could determine their particle sizes from short-term frequency analysis of the observed voltage waveform.