Ultrasonic sensing part 2: dynamic fluid flow

This second part looks at measuring fluid flow rates instead of object presence. Find part one here.

Sensing fluid flow

Beyond basic object detection, ultrasonic transducers are used for non-invasive, non-contact measurement of liquid and gas flow rates (velocity). For these applications, the transducers operate at higher frequencies, typically above 200 kHz, to provide the needed measurement resolution.

Figure 1. The principle of the ultrasonic flow meter is based on the difference in speed of acoustic energy propagation when it is going with the fluid flow versus against it. (Image: EFunda)

In a typical flow application, two sensors are placed a known distance apart, as seen in Figure 1. The flow rate can then be calculated given the distance and the transit time that it takes for sound to travel between the two transducers in both directions, as the moving fluid carries the ultrasonic energy at different speeds in each direction.

This time difference is directly proportional to the velocity of the liquid or gas in the pipe. Determining the flowing velocity (V_f) begins with this equation:

V_f = K × Δt/T_L

where K is a calibration factor for the volume and time units used, Δt is the time differential between upstream and downstream transit times, and T_L is the zero-flow transit time.

Of course, there are various compensation and correction factors that are added to this basic equation to adjust for fluid temperature, angle between the traducers and the pipe, and more. In practice, an ultrasonic flow meter requires real-world “hardware” and fittings, shown in Figure 2.

Figure 2. An actual transit-time ultrasonic-based flowmeter requires various fittings and connections that the wiring schematic does not show; note the dual ultrasonic transducers. (Image: Circuit Digest)

Transit-time flow meters work well with viscous liquids, provided that the Reynolds number at minimum flow is either less than 4,000 (laminar flow) or above 10,000 (turbulent flow), but they have significant non-linearities in the transition region between those two regions. They are often used to measure the flow of crude oils and simple fractions in the petroleum industry, and are also widely used for measuring cryogenic liquids down to –300°C, as well as used in molten metal flow metering – clearly two temperature extremes.

[Note that there is another type of ultrasonic-based fluid-flow measurement scheme which uses the Doppler effect and associated frequency shift due to fluid-flow rate. However, this approach requires transducers which can accurately measure frequency rather than time and is very different in principle and implementation compared to the transit-time approach shown.]

PUI offers ultrasonic transducers that are specifically designed for transit-time fluid-flow applications. Their UTR-18225K-TT operates at 225 ± 15 kHz and has the narrow beam angle of just ±15°needed for this application, and which is one consequence of the higher operating frequency. This transmit/receive transducer has a diameter of 18 mm and a depth of 9 mm with 2200 pF capacitance. It can be driven with a 12 V_p-ptrain of square waves and up to 100 V_p-p at a low duty cycle.

It also takes drive and signal-conditioning circuitry

An ultrasonic-based detection system is more than just the piezoelectric transducers themselves. Appropriate and very different circuitry is needed to meet the drive requirements of the transducer in transmit mode and for low-level analog front-end (AFE) signal conditioning in receive mode. While some users build their own circuitry, ICs are available that can conveniently provide the basic drive and AFE functions along with additional features.

For example, the Texas Instruments PGA460 is a 5.00 mm × 4.40 mm, 16-lead IC designed for use with transducers such as the PUI Audio UTR-1440K-TT-R 40 kHz ultrasonic transceiver. This highly integrated system-level IC provides an on-chip ultrasonic transducer driver and signal conditioner and includes an advanced DSP core, as shown in Figure 3.

It features a complementary low-side driver pair that can drive a transducer either in a transformer-based topology for higher drive voltages by using a step-up transformer, or in a direct-drive topology using external high-side FETs for lower drive voltages. The analog front end consists of a low-noise amplifier followed by a programmable time-varying gain stage feeding into an analog/digital converter (ADC). The digitized signal is processed in the DSP core for both near-field and far-field object detection using time-varying thresholds.

Note that the time-varying gain which the PGA460 offers is a feature needed for some ultrasonic-transducer installations where the transmitted signal and its return are both attenuated by the media through which they are traveling. It helps to overcome the unavoidable yet known-in-advance attenuation factor of the acoustic signal energy as it propagates through the medium (air, fluid such as water); this is an especially critical issue for medical ultrasound systems.

For these medium-attenuation situations, the attenuation and the propagation speed are both known, so it is possible to compensate for the unavoidable loss by “ramping up” the AFE gain versus time to effectively cancel the attenuation-versus-distance effect. The result is that the system signal-to-noise ratio (SNR) is maximized regardless of the sensing distance, and the system can handle a wider dynamic range of received signals.

To further explore the use of these transducers, Texas Instruments offers the PGA460PSM-EVM evaluation module, which works with the PUI Audio UTR-1440K-TT-R 40 kHz ultrasonic transceiver in Figure 4.

Figure 4. The PGA460PSM-EVM evaluation module is based on the PGA460 and simplifies exploration and investigation of ultrasonic system operation using the PUI Audio UTR-1440K-TT-R 40 kHz ultrasonic transceiver (Image: Texas Instruments)

This module requires only a few external components plus a power supply for operation (Figure 11). It is controlled by commands received from a PC-based graphical user interface (GUI), and returns data to the GUI for display and further analysis. In addition to basic functionality and the setting of operational parameters, it allows users to display the ultrasonic echo profile and measurement results.

Figure 5. The PGA460PSM-EVM evaluation module connects to a PC with a GUI that allows users to operate and control the transducer and see critical waveforms, among other functions. (Image: Texas Instruments)

Conclusion

Piezoelectric ultrasonic transducers provide a convenient and effective way to sense nearby objects and even measure the distance to them. They are reliable, relatively easy to apply, safe for users, and have no RF spectrum or EMI/RFI regulatory issues. In a different arrangement, they can also be used for non-contact measurement of fluid flow rates. Interface ICs for both their transmit and receive functions simplify integrating them into a system while providing flexibility in setting operating parameters.