Air-Coupled Ultrasonic Transducers

Summary:

We have been working on air-coupled ultrasonic transducers for more than 20 years. The main result is a robust, efficient and flexible technology able to produce different transducers in terms of centre frequency, sensitivity, bandwidth and field geometry that can be adapted to different applciations that include NDT, materials characterization, surface metrology, etc.

We started by making transducers for our own research. Now we make transducers for academic collaborations or for industrial partners under contract.

We have tested our technology for different applications, different clients and in different industrial enviroments, including food, agriculture, aeronautical, farma, building, and energy industries, with industrial partners that include: ACCIONA, AIRBUS, Oi2GO, Qi2, SIRO, TECNATOM, TETRAPAK, etc.

Our air-coupled transducers are the result of research in five main areas:

Figure 1. Fields involved in the desing, optimization and fabrication of air-coupled piezoelectric transducers.

1. Passive materials for the transducers. Including backing and low-impedance and low attenuation materials to fabricate impedance matchingt layers.
2. Piezoelectric materials, adapted for air-coupled operation and optimized to get either optimum sensitivity or bandwidth or both.
3. Tools to precisely model the transducer response under different operation modes and to calculate the transducer figure of merit that is used for transducer optimization. Different definitions of the figure of merit are used depending on the application.
4. Metaheuristic optimization techniques, able to cope with the high dimensionality of the problem, the multicriteria optimization search, avoid local maxima, and efficiently consider the multiple restrictions and non-ideal design fetaures present in this problem
5. Transducer fabrication routes and assembly procedures that are compatible with all previous areas, that is, that permit to reproduce the simulated and optimized transducer response and allows scaling up and industrialization.

This permit us to produce efficient air-coupled transducers. Three main standard designs are available:

1. Flat general purpose air-coupled transducers, where both bandwidth and sensitivity are optimized simultaneously. This transducers can be used as both transmitters and receivers. They can be operated in either through transmission or pulse-echo mode.  Frequency range is 0.2-1.0 MHz. Two different configurations:
   • Monolithic.
   • Linear array.
2. Flat narrow-band paired air-coupled transducers. A pair of transmitter and receiver finely tuned to one single frequency to offer an enhanced sensitivity at the cost of a reduced bandwidth.  Normally intended for inspection of highly attenuating materials they are mainly orineted towards low fequency applications (200-500 kHz). Two different configurations:
   • Monolithic
   • Linear array.
3. Focused air-coupled transducers to provide enhanced spatial resolution. Specially designed for demanding NDT application where both sensitivity and resolution are required. Two field geometries available:
   • Spot focus
   • Linear focus
4. Pulse-echo transducers. Transducers specially desingned for pulse-echo operation where aixal resolution is key parameter. Hence more emphasis is given to obtain a flat and wide band resonse over sensitivity. Normally oriented towards high frequency air-coupled applications: 0.8-1.5 MHz
5. High frequency tansducers (> 1.5 MHz).

One of the main characteristics of these transducers is that they can be used with any comercial ultrasonic electronic equipment currently available (probably for NDT purposes of for water inmerison or for medical applications) for both trasmission and reception stages. We currently use Krautkramer, Olympus and DASEL pulser/receivers. But any other more general equipment can also be used. For example, excellent results are also obtained in the transmission stage by using a conventional function generator (e.g. Agilent) that can be supported by a power amplifier (e. Falco). The same for the reception stages where standalone amplifiers or preamplifiers can also be used (Olympus, DASEL, Phoenix, etc.)

I. Flat general-purpose air-coupled transducers.

These flat transducers are designed to optimize both sensitivity and bandwith. The standard desing (with a BNC back connector) is illustrated in Fig. 2. Hosing is cylindrical and made of aluminum, with a front ring.

 Figure 2. Schematic representation of the flat general-purpose transducers.

Available frequency range and main transducer characteristics are shown in table I

These transducers are characterized as illustrated in figure 3.

Figure 3. Transducer characterization set-up

As an example, figure 4 and 5 shows impulse response (excitation 100 V,  one semicycle of square wave with a Olympus 5077 PR and reception directly into a Tektronix digital oscilloscope, 1 Mohm).

Figure 4. 2 MHz air-coupled trasnducer. Impulse response and sensitivity frequency band.

Figure 5. 0.25 MHz air-coupled trasnducer. Impulse response and sensitivity frequency band

II.  Flat narrow-band paired air-coupled transducers.

Under construction

III. Focused air-coupled transducers

Under construction

IV. Pulse-echo transducers. 

Under construction

V. High frequency tansducers (> 1.5 MHz).

Under construction