Phased array probes for air-coupled ultrasonic testing based on cellular polymer

Air-coupled ultrasonic testing is an established alternative to conventional ultrasonic methods for testing of light-weight components in aerospace and automotive industry. Compared to direct coupled contact technique or immersion technique it is less invasive, so that it offers numerous financial incentives. While phased arrays are already an industrial standard in fluid-coupled mechanized ultrasonic testing, air-coupled testing is usually performed using two single-element probes and a transmission technique. Therefore, advantages of phased arrays such as active focussing and manipulation of the incidence angle are rarely used with air-coupled testing. This paper presents a newly developed air-coupled phased array device with the emphasis on the characterization of the probes. The device consists of a linear array probe, a transmitter unit and a receiver unit. The main novelty is that the array probe is based on high-voltage excitation of charged cellular polypropylene. Charged cellular polymers are referred to as ferroelectrets or as piezoelectrets. They exhibit an extremely low stiffness and acoustic impedance, so that a matching layer is not necessary. Extremely low mechanical cross-coupling enables independent excitation of the elements of the array. The development of the phased array probes is supported by computations of the sound field using point source synthesis. The main intended applications are transmission with focussing, transmission with an electronic sweep and testing with air-coupled guided waves by using beam steering, applied to carbon-fibre-reinforced plates and similar materials. Air-coupled guided waves enable testing of objects with one-sided access. This work was funded from the “Technologietransfer-Programm Leichtbau” by the German Federal Ministry for Economic Affairs and Climate Action, project 03LB1001A.


Introduction
Since its early beginnings in 1970s, air-coupled ultrasonic testing (ACUT) has found its way into industrial applications [1].Meanwhile it is an established alternative to conventional ultrasonic methods for testing of carbon-fiber-reinforced polymer (CFRP) plates and similar composite materials, as well as adhesive joints, with a total thickness of several millimetres.Testing of light-weight components in aerospace and automotive industry present the largest potential market for air-coupled testing.Compared to directly coupled contact technique or immersion technique it is less invasive, which is advantageous for testing of porous or sensitive materials, and it requires less maintenance, so that it offers numerous financial incentives.
Ultrasonic testing with conventional liquid couplants often involves phased array technology [2].Phased array probes consist of several smaller conventional transducers, which can be controlled independently.This enables a shift in phase and thus steering and focussing of the sound beam.Yet, phased arrays were rarely applied for ACUT [3,4].In this paper we present the development of an air-coupled phased array device based on ferroelectrets, with an emphasis on the development of the probe.The device consists of a linear array probe, a transmitter unit and a receiver unit.

Design of ferroelectret array probes
The main innovation in this work is that the array probe is based on high-voltage excitation of charged cellular polypropylene.Charged cellular polymers are referred to as ferroelectrets or as piezoelectrets [5][6][7].They exhibit an extremely low stiffness (1 MPa at 250 kHz) and acoustic impedance (0.03 Pa s/m 3 ), so that a matching layer is not necessary [8].Our hypothesis is that extremely low mechanical cross-coupling enables independent excitation of the elements of the array, which still needs to be experimentally confirmed.
A coated ferroelectret film was glued onto a printed circuit board (PCB) (Figure 1).Before gluing, commercially available ferroelectret films based on cellular polypropylene (produced by Emfit Ltd.) were coated with a 100 nm or 200 nm thick layer of aluminium using electron beam evaporation, as already reported for single probes [8,9].The coating covers the whole surface of the ferroelectret, so that the layout of the array elements is determined by the structure of the electrodes, which are copper stripes integrated into the PCB.A similar approach was proposed in [10], but using low excitation voltages, while we propose using up to 1.8 kV unipolar square pulses as in [9].This innovation enables generation of much higher sound pressures and therefore inspection of solid plates.

Transmitter and receiver unit
The necessary low noise generation of high voltage causes a complex pulser design.A further requirement was the compatibility with the USPC 4000 AirTech ultrasonic system used for excitation and receiving.To limit the effort, an array layout with eight elements was designed.This enables exploration of acoustic and electronic boundary conditions for functions like panning and focussing of the sound field.
Between single elements, a time delay between 0 and 12 µs with a resolution of 20 ns can be used on transmitter side.At receiver side, a 20 MHz eight channel ADC allows a resolution of 50 ns.The complete setup allows a pulse repetition frequency of ca.200 Hz.

Parameters of the array probe
For experimental evaluation, three testing scenarios are considered.The commonly used scenario of ACUT applications is the through transmission arrangement.The main requirement of this setup to the sending and receiving phased array probes is the flexibility in controlling the focal spot.Another possible optimization approach is electronic scan.This requires controlling active groups in both sending and receiving operation mode, to optimize the overall scanning time by reducing the number of positioning steps of the mechanical system.The third possible application is the control of the incidence angle for the excitation and detection of guided waves.These scenarios are guidelines for the choice of array parameters, most importantly the arrangement of the elements.
The most prominent acoustic effect is the existence of grating lobes, which appear when the element pitch (element width + inter-element gap) is equal or larger than acoustic wavelength (Figure 2), while the steering angle is 0°.More specifically, the relation between pitch size and propagation wavelength to eliminate the appearance of grating lobes is where  is the element pitch,  is the wavelength in the propagation medium and   is the desired steering angle.Sidelobes for pitches up to 3 are shown in Figure 2.For ferroelectret transducers, typical centre frequency is around 250 kHz with some possible variation, so that the wavelength in air is around 1.4 mm.Ideally, the selected pitch would be smaller than 1.4 mm.However, such small elements have a very low electrical impedance, which is challenging for the development of the electrical matching network.
In looking for a compromise between two contradicting requirements, it is tolerable to have some grating lobes if their angle and intensity do not disturb the inspection.For pitch equal to 3λ/2 the grating lobes are at 45° (Figure 2).Therefore, it was decided to set the pitch to 1.6 mm and study the formation of grating lobes in a numerical simulation based on point source synthesis (Huygens principle).The first simulations (Figure 3a) seem to confirm the hypothesis that the grating lobes would not disturb the inspection.However, this still needs to be proven experimentally.

Sound field measurements
Two phased array probes with 32 elements were fabricated and the sound field of each element was measured separately.The elements were 1.4 mm wide, the pitch was 1.6 mm and the element length was 20 mm.The sound field was measured according to newly released guidelines for characterization of air-coupled probes [11] using optical microphone Eta 450 Ultra from Xarion GmbH. Figure 3b shows the C-Scan on one element recorded in a plane containing the acoustical axis and normal to the orientation of the elements (the same plain as in Figures 2 and 3a).The excitation pulse was a unipolar pulse with 1.5 V and 1.73 µs length.
The resulting image shows a broad divergence, as expected for an element with a width equal to the wavelength.These measurements will be evaluated for each element separately to compare their performance.In particular, the variation between the elements in dependence of various production parameters (i.e.gluing) will be studied.An A-Scan was extracted from the same measurement data at the distance 10 mm from the array (Figure 4a).The measured peak-to-peak sound pressure 13 Pa corresponds to 116 dB SPL (sound pressure level), which is promisingly high, having in mind that ultrasound waves of several elements add up at the focal point.This measurement shows one of the advantages of ferroelectret transducers compared to conventional piezocomposites with matching layers, namely low intensity post-ringing of the signal.This can be seen also in the corresponding spectrum (Figure 4b), which has a single maximum corresponding to thickness resonance of the ferroelectret.

Conclusions
This contribution presents air-coupled phased array probes based on ferroelectrets, specifically cellular polypropylene.Results of simulations of the whole array and measurements on single elements indicate that the performance of array elements based on ferroelectret may be suitable for air-coupled ultrasonic testing with phased array technique.Further experiments are necessary to test and to quantify the performance of the phased array system.The combination of the sending and receiving unit with the array probe will be tested in the next phase of the project.The variation between the elements needs to be evaluated and the mechanical cross coupling needs to be studied by measuring the mechanical or electrical response of elements adjacent to the excited element.Furthermore, steering and focussing needs to be evaluated using sound field measurements.The promising perspective of air-coupled ultrasonic arrays is to broaden the application field of air-coupled ultrasonic testing, since they enable electronic steering, focussing, and sweeping without mechanical movements.One of the most compelling possibilities is to excite and detect guided waves without direct contact with the tested object.

Figure 2 .
Figure 2. The main lobe and the side lobes for pitches between λ/2 and 3λ calculated using point source synthesis.

Figure 3 .Figure 4 .
Figure 3. (a) Simulation of the sound field of an array with 8 elements, 1.4 mm wide with a 1.6 mm pitch, 10° steering angle, λ/10 resolution.(b) Measurement of the sound field of a single element 1.4 mm wide, measured with an optical microphone.The signal height (in dB) is relative to 14.2 Pa.