Chirp excitation of ultrasonic probes and algorithm for filtering transit times in high-rangeability gas flow metering

Abstract

The signal processing used in an ultrasonic high-rangeability gas flow meter using times of flight is presented. The flow meter under discussion uses a combination of continuous wave and chirp signals to measure at low flow velocities, below 20 m/s, and chirp signals alone to measure high flow velocities, above 20 m/s. Because of the need for a pulse compression technique in the signal waveform design the technique of pulse compression and the choice of signal waveforms are discussed. The advantages and disadvantages of amplitude weighting vis-a-vis frequency domain manipulations of the waveforms are also discussed. To eliminate spurious times of flight, a special filtering technique is used, based on assessing the gradient of ascendingly ordered time series of time-of-flight measurements. A summary of user experience with high-rangeability gas flow meters in use on different offshore platforms and in refineries is given. Long-term tests that examined the accuracy of the high-rangeability flow meter are also described.

Capacitive Micromachined Ultrasonic Transducers: Technology and Application

“…The high efficiency and the relatively large bandwidth of cMUT transducers offer several advantages in flow metering as well. The ability to generate short ultrasound pulses (per large bandwidth) increases the SNR of the received signal and allows the use of chirp signals to improve SNR even further [34]. In low-pressure environments in which acoustical impedance of the medium is lower than air, the low mechanical impedance of cMUTs presents an even greater advantage.…”

“…This also means the acoustic signal can go through solids despite the large impedance mismatch with air, and still be detected (for aluminum, the two-way acoustical mismatch loss is 80dB) with good Signal to noise ratio (SNR). Electrical and acoustical matching, high electromechanical coupling efficiency, and bandwidth are key features for air applications such as ultrasonic ranging [32,33], gas-flow metering [34], and NDT of fracture/crack in solids [6,35]. Ultrasonic ranging is measuring the distance of a target from the ultrasound source, and is done by a pulse-echo measurement, either with a single transducer or with a transmit/ receive pair, and by measuring the time of flight.…”

Ultrasonic flowmeters: Half-century progress report, 1955–2005

“…In some cases, using two frequencies avoids phase ambiguity. Ultimately, chirp might be a better tune to follow, based on the simple idea that if each cycle has a different period, cycle ambiguity ought to be readily eliminated [71].…”

The application of time-frequency analysis to the air-coupled ultrasonic testing of concrete

“…The wide bandwidth allows the use of sweptfrequency "chirp" signals instead of the usual short transient excitation or narrow bandwidth, tone-burst waveform. Cross-correlation combined with these ultrasonic chirp signals has been used in the pulse compression technique, for use in many ultrasonic applications [17]- [20]. The technique provides the ability to recover small signals from well below the noise floor, due to the signal matching algorithm used in cross correlation.…”