The ultrasonic sensing technology uses the time-of-flight (TOF) of the ultrasonic wave to correlate with the velocity of the medium in the pipe section, and obtains the time difference between the upstream and downstream directions of the ultrasonic wave, and finally measures and calculates the flow rate. This technology is excellent for measuring a wide range of flow rates and is capable of handling liquids such as water and oil as well as gases such as methane.
The TOF-based ultrasonic measurement method measures the flow rate based on the difference in the propagation time of the ultrasonic signals in the upstream and downstream directions. The ultrasonic wave travels faster in the direction of flow of the medium, while the propagation speed is slower in the reverse flow direction. This technology works regardless of whether the transducer is placed inside or outside the pipe. This measurement requires a direct path between the two transducers, which requires careful selection of the mechanical construction of the pipe on which the transducer is mounted. If bubbles are present in the liquid, the technique loses its effect because it causes significant attenuation of the ultrasonic signal.
Because ultrasonic signals travel at different speeds in a single medium and in multiple mixed media, TOF-based ultrasound techniques can also be used to analyze media composition.
Ultrasonic flowmeter configuration
The TOF-based ultrasonic flowmeter has two configurations: plug-in and clip-on. Plug-in flowmeters are intrusive, in which the sensor is installed in the pipeline and in contact with the liquid; the external clip-on flowmeter is non-intrusive, and its sensor is mounted on the outer surface of the tube to penetrate the wall of the tube for acoustic measurement.
Plug-in diagonally mounted transducer layout
Plug-in flowmeters can be installed diagonally, allowing the sensors to be directly opposite, as shown in the image above. Alternatively, the ultrasound can also be reflected from the emission sensor through the surface of the pipe to the receiving sensor, as shown in the following figure. In large-caliber flowmeter applications, two pairs of transducers are usually used to improve performance and solve the problem of large-diameter signal attenuation as shown in the following figure.
Plug-in mutual reflection transducer layout
A major challenge for ultrasonic flowmeters is the need to maintain accuracy over a wide range of flow rates from a few liters per hour to tens of thousands of liters. In some applications, another challenge is to ensure flow rate accuracy over a temperature range of 0°C to 85°C. Since the velocity of the ultrasonic waves in the fluid changes with the temperature of the fluid, the difference in propagation time causes errors in the flow rate measurement when the fluid temperature changes. In general, if the temperature is not considered, a flow rate calculation error of more than 5% will result. In order to improve accuracy, the system will need to install a temperature sensor.
However, we design a test method that does not require temperature measurement. This method requires the absolute time or TOF and time difference of the upstream and downstream propagation to calculate the flow rate of the medium.
Processing advantages based on analog-to-digital converters (ADCs)
We can use a variety of different methods to get the up and down time-of-flight TOF. One method is to detect the zero crossing of the signal using time-to-digital conversion (TDC). Another method is to sample the analog-to-digital conversion (ADC) to the signal received by the transducer for correlation operations.
The TDC technique determines if the signal exceeds the threshold and then calculates the zero crossing of the signal, as shown in the following figure.
In the correlation-based ADC technology, the complete waveform of the signals received by the upstream and downstream transducers is collected and stored. The data is then processed to determine the difference in TOF. The ADC-based approach has three major advantages over the TDC approach:
- Performance. The cross-correlation algorithm also provides low-pass filtering that suppresses noise. This operation is efficiently implemented with a low-power accelerator in the TI MSP430FR6047 MCU. The cross-correlation algorithm can also reduce the standard deviation caused by noise by 3 to 4 times. The correlation filter also suppresses interference such as line noise.
- Robustness of signal amplitude variation. Techniques based on cross-correlation algorithms are insensitive to amplitudes of received signals, differences between transducers, and temperature variations. At high flow rates, changes in signal amplitude are frequently observed. Robustness is a significant advantage as sensor performance degrades over time, as some application flowmeters can last for more than 10 years.
- ADC-based processing can acquire the signal envelope. Obtaining signal amplitude information helps us adjust the sensor frequency. At the same time, you can use the slow variation of the envelope over a long period of time to detect sensor aging. The ADC-based approach is also applicable to Automatic Gain Control (AGC). If the sensor gain degrades over time (reaffirmed, it is due to aging), it can enhance the received signal. Since the correlation-based algorithm can use the amplified received signal while maintaining the output signal level, system performance does not decrease over time even if the sensor ages.
Absolute TOF measurement
It is not necessary to use a temperature sensor and calculate the speed of sound in water when measuring absolute TOF. There are several ways to accurately calculate the absolute TOF. One method is to calculate the envelope of the received signal and then use the maximum value of the envelope change rate as the metering point. The absolute TOF is the time offset after the envelope crosses the threshold. The ADC samples the waveform and envelope for the absolute TOF calculation. The bottom pane gives an enlarged version of the initial waveform.
View TI’s ultrasonic sensing technology for flow metering
The high-performance ultrasonic flowmeter application function module is part of the analog front end (AFE), called the Ultrasonic Sensing Solution (USS) IP module, which operates independently of the central processing unit (CPU) of the MSP430TM MCU. The figure below shows the block diagram. Ultrasonic sensing modules include general purpose USS power supply (UUPS), power sequencer (PSQ), programmable pulse generator (PPG), physical driver and impedance matching network (PHY), programmable gain amplifier (PGA), high speed phase lock Loop (HSPLL), sigma-delta high speed (SDHS) ADC, and acquisition sequencer (ASQ). The ultrasonic sensing module has its own power rail that can be powered up and down independently of other modules on the MSP430FR6047 MCU. You can also reset any other module on the device without affecting it.
Impedance matching in the ultrasonic sensing module is critical for achieving very low drift in deltaTOF measurements over time and water temperature changes. This also allows very low flow rates to be detected.
TI’s latest ADC-based ultrasonic sensing technology enables intelligent water flow meters with high accuracy and accuracy. By integrating an ultrasonic sensing module and a low-power accelerator in the MSP430FR6047 MCU, this performance can be achieved while maintaining low power consumption.