1.3.1 Flow Metering
The measurements and monitoring of fluid flow or flow metering, is crucial for many industries. Flow metering is used in the petrochemical industry to measure the volumetric flow of natural gas and oil [35]. Water companies use metering to keep track of how much water is being used and recycled, as well as for monitoring water levels. Transducer Path ( L) d v(r) Pipe wall Flow profile θ
Figure 1.13: Schematic diagram of a single path ultrasonic flow measurement using two transducers in pitch-catch mode in laminar flow (parabolic flow pro- file).
Ultrasonic flow metering often uses time-of-flight (TOF) measurements of waves travelling upstream and downstream of the flow. The waves will have a veloc- ity which is the vector addition of the inherent velocity of ultrasound in the specific medium and the velocity of the flow. Hence, for a wave travelling downstream the velocity will be greater and the measured TOF smaller. A typical pitch-catch setup is illustrated schematically in Fig. 1.13. The ultrasound transit times upstream tu
and down streamtd are given by
tu = L c vcos(✓) td= L c+vcos(✓), (1.44)
where L is the length of the ultrasound path, c is the sound speed in the medium with no flow, v is the average flow velocity at and an angle ✓ to the ultrasound path. tu andtd are measured by alternating generator and receiver modes between
the two transducers. Solving for the flow velocity v cancels c and the need of any inherent knowledge of the propagation medium to give
v= d(tu td)
2tutdsin(✓)cos(✓)
, (1.45)
whered is the inner diameter of the pipe, such that L=d/cos(✓). It is important to note that the flow velocity v from equation 1.45 is not enough to work out the volume flow or the mass flow under most circumstances. v is a measure of the average flow velocity along a single line through the pipe and will only correspond to the crossectional average in the specific case of a completely flat flow profile. That is, unless the flow velocityv(x, y) =v the volume flow can only be estimated. The velocityv and the volumetric flow velocity vm can defined as
v= 1 L Z L v(r)dL vm = 1 S Z S v(r)dS, (1.46)
whereSis the crossectional area of the pipe, and assuming that the flow is symmetric about the central axis of the pipe, i.e. axisymmetric flow.
To solve this, a wetted ultrasonic flow meter will in general have several pairs of transceivers, each with a unique path through the pipe [36]. Such a meter, known as a multi-path meter, covers a larger proportion of the crossectional area of the pipe. This allows for more accurate predictions of the volumetric flow vm.
Multi-path meters can also measure swirl. Ultrasonic flow meters with as many as 24 paths are commercially available.
1.3.2 Non-Destructive Evaluation (NDE)
Contactless techniques can be beneficial and sometimes essential in various NDE applications. It has found extensive applications in techniques using Lamb waves (thin plate waves) to evaluate defects in plates [37]. The technique has been shown e↵ective on various materials, including composites such as carbon fibre reinforced polymer (CFRP). Laser ultrasound is another non-contact technique that is used for inspection of composite materials [38]. However, laser ultrasound systems are expensive to set up, and not energy efficient to run.
Time Amplitude (a) (b) Transducer φ θ x y z (c)
Figure 1.14: Schematic diagrams of some typical ACU NDE techniques, includ- ing (a) pulse-echo, (b) spectroscopy/through transmission and (c) back scatter- ing.
There are di↵erent techniques of using ACU for NDE, including pulse-echo, through transmission, ultrasonic spectroscopy and back scattering, which are illus- trated in Fig. 1.14. Pulse-echo (Fig. 1.14a) is one of the most common techniques, where a single transducer emits a short pulse at right angles to the sample surface, and then receives the reflected waves. The depth, and to some extent the size, of
the defect can be determined from the time and amplitude of the associated echo, as well as sample thickness. Through transmission (Fig. 1.14b) can be used for simple defect detection by looking at signal amplitude. By scanning the sample the defect can be e↵ectively sized laterally. The technique is also often used for quickly scanning sample thickness. Ultrasonic spectroscopy, often using a through transmission setup, analyses the frequency content of a signal that has interacted with a sample, and by comparing it to the frequency content of the input pulse, in- formation regarding the sample structure can be deduced. Back scatter techniques (Fig. 1.14c) are most applicable to samples with an inherent non-homogenous sur- face or near-surface structure, such as CFRP. It is similar to the pulse echo setup, but importantly the ultrasonic waves are incident upon the sample at an angle . While is kept constant the sample or transducer is rotated through the angle ✓. The received signal amplitude depends upon the orientation of the sample structure with respect to the incident ultrasound.
1.3.3 Metrology and Proximity Sensing
Ultrasonic techniques have been used for decades for proximity sensing and imaging applications. The field has seen an increased amount activity with the advent of car parking sensors and more recently driverless cars, which need multiple sensor technologies in order to function safely. The mass production of simple yet robust transducers that are needed for these types of applications has been driving the development of low cost, robust and easy to manufacture devices. The flexural transducer is in many ways ideal for this type application, as it only needs a small amount of the more expensive active material (PZT), and the metal housing of the transducer, as was seen in Fig. 1.11a, inherently provides it with the necessary ruggedness and environmental protection. Another advantage is the lack of matching layers, as they can increase the unit cost, the number of production steps, and introduce more points of failure.
Flexural transducers are also used in robotics[39] and for remote location ap- plications, e.g. to monitor water levels due to their low power consumption, allowing them to be driven with simple support electronics and batteries.
Another new use of TOF ACU is touchless gesture control of smart devices[40]. A device, such as a smartphone or tablet, is supplied with ultrasonic sensors. The sensors continuously monitor the distance of objects (usually hands) from the screen. By having a high frame rate movements towards and away from the screen can be detected. Also, by using multiple transducers gestures parallel to the screen and multiple objects can be tracked. This technology is not yet readily available and
is largely still in the development stage, but some patents governing the use of ultrasonic transducers for this purpose have been published[41].