A pMUT based flowmeter: A feasibility study

pMUTs are efficient ultrasound sources and receivers in gasses, and are cheap to produce due to their lithographic production process. Making use of their 1
^st
resonance frequency, a good signal to noise ratio (SNR) may be obtained using limited power. This is of interest for low cost-low power gas flow meters. However, using pMUTs in this application also poses challenges; 1) pMUTs are almost omnidirectional and 2) pMUTs are very narrow-banded. A pMUT's resonance frequency mainly depends on the membrane radius, which varies due to production. In the current work the suitability of pMUTs for a flowmeter is investigated. To match the resonance frequencies of the transmitting and receiving pMUTs, a DC bias voltage (from -4V to +4V) was applied to the transmitting pMUT. The flow setup consisted of two pMUTs (radius 0.4 mm, resonance frequency 343 kHz) mounted on the same side of a pipe (crosssection 3.6 cm) with an inter-pMUT distance of 1.7 cm. The transmitting pMUT was excited using 2.5 V Gaussian apodized sine bursts 10 cycles in length. The signals produced by the receiving pMUT were amplified by a custom made pre-amplifier. Pressurized air was used to create a gas flow past the pMUTs. The gas flow speed was calculated using a time delay based algorithm. By varying the DC bias voltage the pMUTs resonance frequency could be shifted by ~5 kHz, which was sufficient to match the resonance frequencies of both pMUTs. The application of a bias Voltage increased the intensity of the emitted sound by 12 dB. The SNR of a single received trace was ~18 dB. The SNR was improved by averaging 1000x for each measurement. The direct sound path between the pMUTs provided a good estimate of the flow velocity. The first arrival in the time signal (at 50 μs) was the direct path between both pMUTs. 55 μs later the echo bouncing off the sidewalls of the rectangular pipe arrived. It was shown that a flow speed of 2 m/s could be measured with an accuracy of ~±0.1 m/s. An initial investigation of the suitability of pMUTs for gas flow measurements was conducted, and encouraging initial results were obtained.