- Email: [email protected]
The use of broadband acoustic transducers and pulse-compression techniques for air-coupled ultrasonic imaging
Ultrasonics 39 (2001) 181±194
www.elsevier.nl/locate/ultras
The use of broadband acoustic transducers and pulse-compression techniques for air-coupled ultrasonic imaging T.H. Gan a, D.A. Hutchins a,*, D.R. Billson a, D.W. Schindel b b
a School of Engineering, University of Warwick, Coventry CV4 7AL, UK MicroAcoustic Instruments Inc., 460 Wilbrod Street, Suite 2, Ottawa, Ont., Canada, K1N 6M8
Received 1 July 2000; received in revised form 1 October 2000
Abstract A pulse-compression technique has been applied to air-coupled testing of solid materials. Capacitance transducers were used to generate wide bandwidth swept-frequency (chirp) signals in air, which were then used to measure and image solid samples in through transmission. The results demonstrate that such signal processing techniques lead to an improvement in the signal to noise ratio and timing accuracy for air-coupled testing. Measurements of thickness and spectroscopic experiments are presented. Images of defects in a wide range of materials, including metals and carbon-®bre composites have also been obtained. This combination of capacitive transducers with pulse-compression techniques is shown to be a powerful tool for non-contact air-coupled ultrasonic measurements. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Capacitance transducer; Air-coupled ultrasound; Signal processing
1. Introduction Ultrasonic immersion testing is a popular technique for the investigation of materials such as metals, ®brereinforced polymers, and many other materials [1±5]. However, the use of water as a coupling medium may not always be suitable for certain inspection situations, e.g. where the material absorbs water, or where contamination or damage would result. There has thus been increased interest in using air as the coupling medium. However, ultrasonic attenuation in air is much greater than in water, especially at high frequencies, and the acoustic impedance of air is much lower, leading to dif®culties in coupling energy into solid samples. Because of such problems, careful attention has to be given to transducer design in terms of sensitivity and bandwidth. The most common transducers used for air-coupled experiments are based on piezoelectric and electrostatic designs. Piezoelectric air transducers are inherently resonant devices, and require special backing and construction to obtain suitable damping coecients. Another
*
Corresponding author. Tel.: +44-1203-523874; fax: +44-1203-418922. E-mail address: [email protected] (D.A. Hutchins).
problem is that the characteristic impedance of the piezoelectric element is very dierent to that of air. A quarter-wavelength thick matching layer at the frequency of interest is thus usually introduced at the front surface [6]. Many types of materials have been tested and found to be useful as the matching layers, including aerogel [7] and silicone rubber [8]. However, the application of a matching layer limits the overall bandwidth of the device. One way of reducing the impedance of the material is to use one to three connectivity piezopolymer composites [9,10], which contain an array of piezoelectric ceramic rods in a polymer ®ller matrix. These have a wider bandwidth than traditional piezoelectric materials such as PZT. However, a matching layer is still required for sensitive work in air, and hence these devices are usually used over a narrow bandwidth. An alternative transducer design is based on the capacitance or electrostatic principle. This has received much interest recently because of the excellent bandwidths that can be obtained. These devices are composed of a thin membrane ®lm and a rigid conducting backplate to form a capacitor. Applied voltages cause the membrane to vibrate, and hence generate ultrasound, whereas a change in charge across the membrane can be used for detection. Metallic backplates can be used [11], many employing a regular grooved backplate,
0041-624X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 1 - 6 2 4 X ( 0 0 ) 0 0 0 5 9 - 7
182
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
which was found to improve the acoustic properties of the transducer [12]. This type of transducer was found to be very sensitive at high bias voltages when a thin polymer membrane was used [13]. Further investigation showed that the sensitivity of the transducer could be increased by modifying the shape of the groove, e.g. to form a V-grooved backplate device [14]. The extension of device sensitivity to higher frequencies requires more careful control of surface features, and hence silicon has been investigated as the backplate material [15±17], with the backplate being etched using micromachining techniques. In the transducer used in the work reported here, the backplate contains small etched holes, which help to produce a wider bandwidth and enhanced sensitivities [18]. These devices can thus be used for non-contact materials inspection and imaging [19±21]. An additional approach is the fully micromachined device, where the complete structure is fabricated using CMOS technology from a silicon wafer. Typically, such designs use a silicon substrate and a silicon nitride membrane [22,23], although other work has used a hexagonal cell structure with polysilicon membranes [24]. The purpose of the present paper is to demonstrate that signal processing techniques can be used to increase the range of application of capacitive devices for ultrasonic materials characterization in air. In particular, it will be demonstrated that pulse-compression techniques can lead to signi®cant improvements in signal to noise ratios (SNRs) when applied to air-coupled throughtransmission testing. Pulse compression has been used for improving the resolution of various measurements in both radar [25] and ultrasonic measurements [26±29]. In the form of pulse compression used here, a broadband frequency-swept ``chirp'' ultrasonic waveform is emitted by the source transducer, which is then detected by a broadband receiver after transmission through the solid sample in air. This paper seems to be the ®rst to use such a technique with broadband capacitance transducers in air, although previous measurements to transmit signals in a gas only (and not through a solid sample) have been reported [30] using piezoelectric transducers. In the following, the pulse-compression technique will be explained, and the advantages over other techniques will be discussed. The resultant waveforms in air are compared to those obtained from conventional transient excitation. The technique will be shown to detect resonances in solid samples, and to measure the presence of defects in ®bre-reinforced composites. 2. Ultrasonic pulse compression One of the major factors limiting the use of ultrasound for non-destructive testing of materials is atten-
uation and scattering, leading to poor signal to noise ratios (SNRs) [31]. In air-coupled testing, the impedance mismatch between air and most engineering materials is also large causing most acoustic energy to be lost by re¯ection. In most situations the SNR problem can be improved by using a high power transmitting pulse, and this usually implies the transmission of a tone-burst signal. Such signals are very convenient, in that gated power ampli®ers can be used to deliver high powers and, when combined with a broad bandwidth transducer, the frequency of operation can be varied. For piezoelectric transducers in particular, tone-burst excitation leads to considerable advantages for air-coupled testing, in that the frequency can be tuned to the through-thickness resonance of the material. This increases the throughtransmission signal levels substantially. There are, however, disadvantages to using a tone burst. First, the voltage excitation level is limited by the type of transducer used, and in the particular case of a capacitance transducer, the voltage must be restricted to avoid dielectric breakdown of the thin polymer membranes. In addition, the exact frequency of excitation must match the through-thickness resonance of the sample to achieve maximum eciency, and this might need to be adjusted if the thickness of the material changed (for instance during an imaging experiment involving positional scanning). It may also be the case that the longitudinal velocity is either not known, or might vary. The main disadvantage in the context of defect detection is that a tone burst leads to relatively poor time resolution [32]. Defects might be dicult to resolve, because multiple re¯ections might overlap in time, although cross-correlation can lead to accurate time-of-¯ight measurements. Because capacitance transducers can operate over a wide frequency range, it is thus better to use a technique which capitalises on this property. The use of a swept-frequency signal, instead of a single transient, allows a high power, broad bandwidth signal to be used which, when combined with suitable processing, also gives excellent time resolution. It is this property that is used in the pulse-compression technique [33±35]. In the context of an air-coupled ultrasound experiment, a tone burst, tuned to the through-thickness resonance, is still likely to give the greatest signal amplitude. However, the advantage of using a wide bandwidth pulse-compression approach, using a broadband swept frequency excitation at the source transducer, is that the full air-coupled spectral response of a material (e.g. with multiple resonances) can be obtained instantaneously. This can be achieved without frequency scanning as has been necessary in the past [27]. Increased accuracy in time-of-¯ight measurements can potentially be obtained, and the technique provides the ability to recover small signals from well below the noise ¯oor, although the pulse-compression method is
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
183
only valid for improving the SNR if the noise is random [34]. The pulse-compression method can be implemented by driving the ultrasonic source with a so-called chirp signal, where the frequency is swept continuously over a pre-determined range. The result is an elongated waveform, with the overall duration of the signal and the rate of frequency sweep de®ning the chirp characteristics. The broadband chirp or linear frequency-modulated signal is also widely used in medical applications [29,36], for measuring gas ¯ow [27] and for spatial characterisation of scattering microstructure [26]. The pulse-compression technique is best described using a simulation, where a chirp signal is buried in noise. This chirp signal can be represented as pB 2 C t sin xs t t 06t6T 1 T
0.2 MHz, and the bandwidth, B, to 1 MHz. The chirp thus extends over the 0.2±1.2 MHz frequency range. Fig. 1(b) shows the corresponding frequency spectrum, obtained via an FFT. It is seen that the signal has a wide bandwidth, but that there are some ripples at the bandedges. These are known as the ÔFresnel ripplesÕ [28], and it is found that the time-bandwidth product of the signal has to be increased to reduce their severity. From the spectrum, it can also be seen that the higher cut-o frequency limit of 1.2 MHz is extended to 1.5 MHz. These disadvantages can be avoided by applying a modulation function such as a bandpass Hanning or Gaussian ®lter. Here we consider a chirp signal that is modulated by a Hanning ®lter, which may be represented as pB 2 t C t H t sin xs t 06t6T 2 T
where xs is the starting angular frequency, B is the bandwidth of the signal, and T is the duration of the pulse. Fig. 1(a) illustrates a typical chirp waveform, generated using Eq. (1). Here, the duration of the chirp, T, was set to 50 ls, the starting frequency fs xs =2p at
where H(t) is the Hanning function given by 1 2pt H t 1 cos 2 T
Fig. 1. (a) Broadband chirp signal with a duration of 50 ls and (b) frequency spectrum of (a).
Fig. 2. (a) Broadband chirp signal with a duration of 50 ls after a Hanning window was applied and (b) frequency spectrum of (a).
3
The generated signal from Eq. (2) is as shown in Fig. 2(a). The ®gure shows the bell-shaped envelope resulting
184
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
from the Hanning ®lter, and this shape is important to ensure good sensitivity. In addition, the Hanning amplitude weighting also helps to reduce the amount of side lobes in the signal [37]. Fig. 2(b) shows the frequency spectrum of the Hanning-weighted chirp signal. In the ®gure, it can be seen that the lower and higher limits of the frequencies are maintained at 200 KHz and 1.2 MHz. The signal is centred at 700 kHz, and the edge ripples have been removed. It is interesting to demonstrate that a chirp signal is useful for the detection of signals in the presence of high noise levels. The Hanning chirp signal in Fig. 2(a) was thus shifted by 40 ls in time, and mixed into a random noise level of twice the chirp signal amplitude. This simulates the noise levels that would be encountered in a real air-coupled materials inspection experiment. This is as shown in Fig. 3(a). In order to produce the compressed pulse signal, P(t), the received signal CT (t) is bandpass ®ltered within the chirp bandwidth. The ®ltered signal is as shown in Fig. 3(b). The bandpass ®lter removes the noise levels above and below the frequency range of the original chirp driving signal, but the transmitted chirp signal is still not easily visible. The waveform is now cross-correlated with the reference signal C(t). This is represented by Eq. (4) and this process is sometimes known as the matched ®ltering process [38]: P t C t CT t
4
The compressed pulse, P(t), is thus produced by the correlation of the received signal CT (t) with the original reference signal, C(t). The correlated result is as shown in Fig. 3(c), and is in the form of a time signal. The main peak in the pulse-compression output jP tj represents the position in time of the transmitted signal, which is at a time delay of 40 ls. The SNR has been greatly improved compared to Fig. 3(b), as can be seen. The width of the jP tj peak can be reduced to give greater time resolution by increasing the bandwidth, B, of the generated chirp signal, C(t) [34], whereas a greater peak amplitude can be obtained by elongating the time duration, T, for the same bandwidth. It is thus of advantage to use as long a duration T of the chirp as possible, and to also maximise the bandwidth. However, the pulse-compression output can be interpreted much like a conventional ultrasonic waveform, in that the amplitude of the compressed pulse as a function of time is related to the amplitude of the received chirp waveforms as a function of time. Note also that the exact shape of jP tj will be modi®ed if the original chirp pulse shape is distorted by the sample. This will almost always happen in the case of a simple plate at normal incidence, where maximum transmission amplitudes occur at well-de®ned frequencies. In these situations, jP tj also contains information concerning the material through which the signal has travelled. In particular, it can be used to de-
Fig. 3. (a) Transmitted broadband Hanning chirp signal which was (a) embedded in noise, (b) bandpass ®ltered, and (c) shows the result of subsequent pulse compression.
termine the dierent frequencies of resonance that are present. This will be illustrated later in this paper. There are thus three primary reasons why pulse compression is a potentially useful technique when applied to air-coupled measurements. First, because the chirp is a complex coded waveform, it can correlate well only at a single well-de®ned time-of-arrival (for each arriving chirp). Thus, the accuracy of time-of-¯ight measurements could be greatly improved when using
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
pulse-compression techniques. Second, such a coded waveform has the advantage that it can be detected when the received chirp level is well below the noise ¯oor of the detector (i.e., since the noise is random and thus uncorrelated with the chirp shape). Finally, high ultrasonic energy levels can be transferred into a material due to the use of a chirp signal, to give a good SNR. 3. Apparatus and experiment It is informative to compare the performance of the pulse-compression system with that from a conventional set-up using transient excitation. Initially, the characteristics of the capacitive transducer were ®rst determined by aligning a pair of transducers carefully in air, as shown in Fig. 4. The capacitance transducers used for the present measurements were similar to those described in previous work [18,39]. The transducer design is shown in the schematic diagram of Fig. 5. The transducer uses a micromachined silicon backplate, which contains arrays of small cylindrical holes. These holes act as air springs underneath the membrane, which is a metallised polymer ®lm. The backplate is coated with gold to make it conducting, with the outer electrode of the membrane grounded. This type of transducer can be used as either a source or detector. As a
185
source, a transient driving voltage is applied, together with an optional dc bias voltage. As a receiver, the bias voltage is required to give charge variations when the membrane moves. The transducers in the present experiments had an active aperture of 10 mm diameter, and were ®tted into a fully shielded metallic case. The transducer acting as a source had a membrane thickness of 5 lm, so as to withstand the higher excitation voltages without damage to the polymer membrane, and was driven by a Panametrics pulser/receiver (Model 5055PR) together with a 200 V dc bias voltage. This applied a 250 V transient to the transducer, and allowed the transducers to be tested initially under conventional transient excitation. The use of the dc voltage was to ensure phase uniformity across the transducer aperture. The detector, separated by 77 mm from the source, had a ®lm thickness of 2.5 lm, and was connected to a Cooknell CA6/C charge ampli®er with a gain of 250 mV pC 1 . The response was then fed into a Tektronix TDS540 digital oscilloscope for signal averaging. Fig. 6(a) shows a typical waveform of the
Fig. 4. Experimental set-up to determine the characteristics of the aircoupled devices.
Fig. 5. Schematic diagram of the capacitive broadband acoustic transducer.
Fig. 6. Signals transmitted across an air gap of 45 mm with the source driven by the Panametrics pulser/receiver (Model 5055PR): (a) time waveform, and (b) frequency spectrum of the signal obtained via an FFT.
186
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
transmitted signal, and Fig. 6(b) is the corresponding frequency response. These indicate that the transducers had broadband frequency characteristics. The transducers were now placed either side of the sample under test, and the through-transmitted signal recorded. To illustrate this type of experiment, a 2.2 mm thick carbon-®bre-reinforced polymer composite plate with large transverse dimensions was placed between the transducers of Fig. 4. The transmitted signal was found to require signal averaging on the oscilloscope, the small signal level being due to the large dierence in acoustic impedance between air and the composite plate. The recorded waveform in the time domain is as shown in Fig. 7(a), and demonstrates that the received waveform was in the form of an exponential decay, at a centre frequency corresponding to the fundamental throughthickness resonance of the plate. Fig. 7(b) shows the FFT of the signal in Fig. 7(a), and contains the expected resonant peak, centred at a frequency of approximately 0.67 MHz. This behaviour is typical of wide bandwidth air-coupled experiments, and such signals have been used in the characterisation and imaging of a range of materials [19,21]. The low SNRs seen in typical transient
Fig. 7. Signal transmitted through a carbon-®bre composite plate of thickness 2.2 mm: (a) time waveform and (b) frequency spectrum of the transmitted signal.
Fig. 8. Experimental arrangement for through-transmission testing using pulse compression.
air-coupled through-transmission experiments arise from the large acoustic impedance mismatches at air/solid interfaces. The pulse-compression approach was thus designed to improve the signal levels in a through-transmission experiment over an extended bandwidth. The experimental arrangement for through transmission is shown in Fig. 8. The pulse-compression approach was implemented using an NCA1000 pulser/receiver unit from VN Instruments Ltd. This contained an on-board digital signal processor (DSP) to both synthesize output pulses for source excitation, and to run embedded pulse-compression code for on-line processing of received signals. All aspects of the instrument were controlled by a pentium-class computer via a parallel interface and a real-time operating system (QNX). The pulser section contained a 200 W broadband power ampli®er of variable gain, whereas the receiver section contained a variable-gain low-noise receiver ampli®er (maximum gain of 90 dB), followed by an A/D converter, and ®nally the DSP. As before, the output of the instrument was superposed upon a 200 V dc bias voltage. The received response was also ampli®ed by a Cooknell charge ampli®er model CA6/C, with a gain of 250 mV pC 1 , before being fed into the input of the NCA1000. Typical waveforms that resulted when no sample was present between the transducers are shown in Fig. 9. The un®ltered time waveform in Fig. 9(a) shows a typical broadband chirp arriving at the capacitive receiver. It consisted of a frequency sweep from 450 kHz to 1.15 MHz over a time period of 190 ls, and arose by specifying an applied chirp center frequency fc of 800 kHz and a chirp bandwidth B of 700 kHz on the graphics user interface. Fig. 9(b) is a time waveform to which a bandpass ®lter has been applied. This reduces low-frequency oscillations, which can occur if either the samples are of small transverse dimensions (and hence allow a low-frequency air wave to propagate around the outer dimensions), or if plate-like samples become so thin that low-frequency ¯exural modes are additionally excited in the plate. The waveform of Fig. 9(c) is the FFT of the
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
187
Fig. 9. Ultrasonic signals obtained across an air gap using pulse compression: (a) un®ltered time waveform across the air gap, (b) ®ltered time waveform of the transmitted chirp across the air gap, (c) FFT spectrum of the ®ltered time waveform, and (d) compressed pulse of the ®ltered time waveform using the pulse-compression algorithm.
®ltered time waveform. This waveform demonstrates clearly the wideband frequency response of the aircoupled capacitance transducer system (limited by the 700 kHz bandwidth of the applied chirp). Finally, Fig. 9(d) shows the pulse-compression output from the NCA1000 unit, corresponding to jP tj of Eq. (4). This is in the form of a single peak, as expected from the earlier simulation of Fig. 3(c). Note that less than 1 V p±p source drive voltages were applied in Fig. 9 to obtain a result with a good SNR. Fig. 10 shows through-transmission results for a carbon-®bre composite plate of nominal thickness 2.2 mm and large transverse dimensions. Here, the same applied chirp as was used for Fig. 9 was employed. In the un®ltered time waveform of Fig. 10(a), the chirp arriving at 240 ls is associated with the longitudinal wave propagation through the plate, while the small low-frequency oscillations arriving thereafter are due to subsequent excitation of ¯exural modes in the plate. In the ®ltered time waveform, Fig. 10(b), the low-frequency oscillations are eliminated, leaving the longitudinal wave at 240 ls in addition to a second chirp arrival at 480 ls. The second chirp arrival is due to the ®rst echo in the air-gap between the source and composite plate. By applying a fast Fourier transform to only the ®rst longitudinal chirp arrival (i.e., below 475 ls), the FFT
spectrum in Fig. 10(c) results. This ®gure shows a single resonant peak at 0.67 MHz. This value agrees with the frequency indicated in Fig. 7, and was associated with the fundamental longitudinal resonance within the thickness of the plate. Given the measured resonance frequency and thickness of the composite plate, the longitudinal velocity was found to be vl 2935 ms 1 , which was in agreement with the known value for the material. In addition, the response after pulse-compression shown in Fig. 10(d) also exhibited the ringing eect after the arrival of the maximum peak due to multiple echoes within the plate itself. These results show that the ultrasonic pulse-compression technique has the capability to achieve the same outcome as the traditional pulsed system (shown earlier in Fig. 7) but with a much better response. The system was thus used to examine a range of materials and thicknesses, the results of which will be given in Section 4. An experimental arrangement was also set up for imaging, using an X±Y scanning stage, as shown in the block diagram of Fig. 11. This included a digital oscilloscope and an external PC controller. The output chirp voltage from the power ampli®er (typically 300 V p±p) was ®rst superimposed upon a 200 V dc bias using a capacitive decoupling circuit before being applied to a capacitance source of bandwidth 1.5 MHz. The chirp
188
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
Fig. 10. Signals transmitted through a carbon-®bre composite plate of 2.2 mm thickness using pulse compression: (a) un®ltered time waveform, (b) ®ltered time waveform, (c) FFT spectrum of the ®ltered time waveform, and (d) compressed pulse of the ®ltered time waveform.
and the through-transmitted signal received as above by a capacitive receiver. Output from the receiver was again input to the Cooknell CA6/C charge ampli®er, and pulse-compressed data recorded using a Tektronix TDS540 digital oscilloscope, before being transferred to a PC for data storage. The PC was also used to control the X±Y stages. The total area of the scan varied depending on the size of the test sample involved. Usually the scan area was set 50 mm 50 mm, at spatial intervals of 1 mm. All scanning and data acquisition was controlled by L A B V I E W version 5.1 software. This software also extracted data such as the peak amplitude or the time-of-arrival of the transmitted signal from each waveform in the scan, and stored these data in a grid ®le format. The image was then plotted using S U R F E R version 7.0 software. Fig. 11. Experimental arrangement for ultrasonic imaging using pulse compression.
had a centre frequency fc of 400 kHz and a bandwidth B of 700 kHz. The longitudinal waves emitted by the source then propagated through the air to the sample,
4. Results 4.1. Measurements in simple plates Thickness evaluation using through-transmitted signals was performed on dierent types of material placed
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
in the air gap between the transducers, namely brass and aluminium plates of varying thickness, as well as carbon-®bre-reinforced composite plates with a range of layups, including unidirectional (UD) and quasi-isotropic (QI) con®gurations. The measured responses after pulse compression of the ®ltered time waveform for the brass and aluminium plates are as shown in Figs. 12 and 13. It can be seen from the ®gures that there is a shift in the arrival time of the detected signal. This was due to the dierent types of material properties involved. The detected acoustic velocity of each plate was compared with the result measured using the pulse echo contact method. The results are as shown in Table 1. From the measured velocities, about 1% dierence was found by comparing the two methods involved. This was probably due to the greater accuracy of the pulse-compression method, and in particular the fact that no liquid couplant was present (as
189
Table 1 Measured velocities in metal plates
Brass plate Aluminum plate
Air-coupled through transmission method (m s 1 )
Pulse echo contact method (m s 1 )
Percentage dierence (%)
4524 6431
4460 6400
1.4 1
Fig. 14. Variation of time-of-¯ight across aluminum plates of dierent thickness in air.
Fig. 12. Compressed pulse of the ®ltered time waveform across 8.12 mm brass plate.
Fig. 13. Compressed pulse of the ®ltered time waveform across the 6.45 mm aluminum plate.
was required between the sample and the contact piezoelectric transducer). An analysis was also carried out for dierent thicknesses of aluminium plates. The thickness of the plate was ®rst measured with a Kennedy digital caliper model 331±206, and then compared with that of the experimental results. Fig. 14 shows the measured results using both the UPC method and with the caliper. The results show that there is a slight increase in the percentage dierence between the two as the thickness increases, this being 4.7% when measuring the aluminium plate of thickness 9.9 mm. This may be due to the fact that the caliper measures in one location only, whereas the acoustic signal measures the average thickness over the width of the ultrasonic beam in the plate. Fig. 15 shows the transmitted chirp signals across two dierent thickness of the ®bre-reinforced polymer composite. In the ®gure, the dashed line represents the transmitted signal across 3.2 mm thick sample and the solid line represents the signal across a 17.5 mm thickness. Both the signals have a good SNR. The ®gure also shows that the pulse-compression technique not only could be used to detect the shift in the arrival time but also the change in correlated peak amplitude according to the thickness. This reduction in amplitude could be used to measure attenuation. Experiments were also performed in a circular aluminum disk of 30 mm diameter and 3.2 mm thickness.
190
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
Fig. 15. Compressed pulse of the ®ltered time waveform across the 17.5 and 3.17 mm thick composite plates.
Here, resonances and other factors are to be expected. The un®ltered time waveform of Fig. 16(a) shows a combination of high-frequency through-transmission and low-frequency waves wrapping around the disksÕ limited dimensions. Bandpass ®ltering leads to an elimination of the low-frequency waves, as shown in Fig. 16(b). The complexity of the spectral response of the disk is readily apparent from Fig. 16(c), in which the FFT of the ®ltered time waveform is shown. Such complexity occurs because the small transverse dimensions of the sample lead to multiple re¯ections and mode conversions at the boundaries, and the net eect is an excitation of all of the disksÕ natural modes of vibrations within the bandwidth of the experimental system. These natural modes consist of various types of shear, longitudinal and ¯exural resonances, the analysis of which can be quite complicated [40]. The fact that the entire spectral response of a solid sample can be obtained instantaneously in this way is a very powerful advantage of the approach. This is because real-time spectroscopy can be undertaken with this air-coupled system so as to provide a wealth of on-line information about materials in a rapidly scanned non-contact fashion.
4.2. Imaging experiments Further experiments were carried out to image a range of defects within metals, polymers and composites. The ®rst image, shown in Fig. 17, is of a 10.5 mm diameter circular ¯at-bottomed hole, machined into one surface of a 4.1 mm thick Plexiglas plate to a depth of 2 mm. The image, plotted using the peak transmitted signal amplitude, clearly shows the circular shape of the defect of approximately the correct size. The slight differences are to be expected due to the 10 mm aperture size of the transducers. Images were also obtained in a 8.1 mm thick brass plate. Here, the impedance mismatch between air and
Fig. 16. Signals transmitted through a circular aluminum disk of 30 mm diameter and 3 mm thickness: (a) un®ltered time waveform, (b) ®ltered time waveform, and (c) FFT spectrum of the ®ltered time waveform.
brass increased re¯ection coecients at interfaces substantially, so that received amplitudes were greatly reduced (and in fact cannot be measured using a conventional pulse/receiver in air-coupled measurements). To demonstrate that pulse compression can improve this situation, scans were performed of two
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
Fig. 17. Image of a 10.5 mm diameter ¯at-bottomed hole on a Plexiglas plate.
191
with the received amplitude decreasing as the scan passed through the defective areas. This was also con®rmed by plotting images of arrival time from the same signals, as shown in Fig. 18(b), where good agreement in terms of locations and sizes of defects is seen. Further scans were carried out using an aluminium plate containing an internal 18 mm long side-drilled hole of 5 mm diameter. The images obtained using maximum amplitude and time-of-¯ight data are as shown in Fig. 19(a) and (b), respectively. The position of the defect was detected and was clearly shown in these images. Fig. 19(a) shows that most of the transmitted signals were attenuated in the region of the defect and also delay was introduced to the signal in the defect zone. Final scans were performed in anisotropic carbon®bre-reinforced polymer composites. These were 16-ply (2 mm) thick plates containing defects, with the properties shown in Table 2. The defects were introduced during manufacture of the composite, so as to be embedded within it at the mid point of the plate thickness.
Fig. 18. Image of two arti®cial defects within an 8.1 mm thick brass plate, using (a) peak signal amplitude, and (b) time-of-arrival.
arti®cial defects within the plate, and the results are shown in Fig. 18(a). As before, the defects were detected with the correct location and reasonable dimensions,
Fig. 19. Image of an internal defect in an aluminum plate, using (a) peak signal amplitude, and (b) time-of-arrival.
192
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
Table 2 Details of ®bre-reinforced composite samples Lay-up con®guration
Arti®cial defect
QI UD QI
12.5 mm square brass shim Pre-cured area (4 plies) 8 plies of random ®bres
Fig. 21. Image of a pre-cured area (four plies) in a 16-ply thick composite plate, using (a) peak signal amplitude, and (b) time-ofarrival.
Fig. 20. Image of a 12.5 mm square brass shim in a 16-ply thick composite plate, using (a) peak signal amplitude, and (b) time-ofarrival.
Results for the 0.5 in. brass defect are presented in Fig. 20(a) and (b) in terms of maximum amplitude and arrival time respectively. The signals travelling through the defect were attenuated as expected, and also exhibited an increased arrival time due to diraction. The defect was clearly shown in these images, as is disturbance to the plate structure around the defect. Images of precured ply and chopped random ®bres introduced into the material are shown in Figs. 21 and 22. The technique clearly shows the presence of these anomalies, despite the fact that their properties are similar to the sur-
rounding matrix, and the fact that they are only four plies thick. In general, time of ¯ight gives better results. This is to be expected, as the pulse-compression technique gives very accurate time information; small changes in the layup con®guration within the composite lead to small local longitudinal velocity variations (due to a change in structure and hence elastic properties), and these are detected by the technique. A ®nal test was also performed on composite plates containing real damage. Fig. 23 show images of barelyvisible damage caused by a 16 J impact on a pultruded glass-®bre-reinforced composite plate, using time-of¯ight data. Both the damaged area and the internal structure of the composite plate were visible.
5. Conclusions It is evident from the above that the combination of a pulse-compression technique, and the wide bandwidths
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
Fig. 22. Image of random ®bres in a 16-ply thick composite plate, using (a) peak signal amplitude, and (b) time-of-arrival.
available from micromachined capacitance air-coupled transducers leads to a new range of possible experiments. Using conventional techniques, the testing of metals was not possible over a wide bandwidth, due to the large dierence in acoustic impedance between metals and air. Using broad bandwidth chirp signals increases SNRs signi®cantly. This allows a much wider range of measurements to be made. In addition, the increased accuracy of time measurement leads to many new applications in ¯ow measurement, and where timing accuracy is also important. This was illustrated further by the experiment with the aluminium disk, where spectroscopy of the resonant modes was demonstrated. Such a non-contact technique has many uses in condensed matter physics and materials characterisation for the measurement of elastic properties. This was further illustrated by the imaging of detects in solids. The authors are currently extending this technique to many other application areas, and these results will be reported in due course.
193
Fig. 23. Image of a defect in a composite plate caused by 16 J impact, using (a) peak signal amplitude, and (b) time-of-arrival.
Acknowledgements The authors would like to acknowledge funding for this work from the Defence Evaluation and Research Agency (DERA), UK, and the Canadian Government. Thanks are due to Dr David Bruce and Dr Franklin Wong respectively for their encouragement of this work. References [1] P. Kalyanasundaram, C. Rajagopalan, C.V. Subramaniam, M. Thavasimuthu, B. Raj, Ultrasonic signal analysis for defect characterization in composite-materials, Br. J. Non-destruct. Test. 33 (5) (1991) 221±226. [2] C.S. Karthikeyan, C.R.L Murthy, S. Sankaran, C. Kishore, Characterization of reinforced syntactic foams using ultrasonic imaging technique, Bull. Mater. Sci. 22 (4) (1999) 811±815. [3] M.D. Enderby, A.R. Clarke, M. Patel, P. Ogden, A.A. Johnson, An automated ultrasonic immersion technique for the determination of three-dimensional elastic constants of polymer composites, Ultrasonics 36 (1±5) (1998) 245±249.
194
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
[4] D. Xiang, N.N. Hsu, G.V. Blessing, A simpli®ed ultrasonic immersion technique for materials evaluation, Mater. Evaluat. 56 (7) (1998) 854±859. [5] D.P. Jansen, D.A. Hutchins, J.T. Mottram, Lamb wave tomography of advanced composite laminates containing damage, Ultrasonics 32 (2) (1994) 83±89. [6] W.M.D. Wright, Air-coupled ultrasonic testing of materials, Ph.D. Thesis, University of Warwick, England, 1996. [7] O. Kraur, R. Gerlach, J. Frickle, Experimental and theoretical investigations of SiO2 -aerogel matched piezo-transducers, Ultrasonics 32 (1994) 217±222. [8] J.D. Fox, B.T. Khuri-Yakub, G.S. Kino, High frequency acoustic wave measurements in air, Proc. 1983 IEEE Ultrasonic Symp. 1 (1983) 581±584. [9] G. Hayward, A. Gachagan, An evaluation of 1±3 connectivity composite transducers for air-coupled ultrasonic applications, J. Acoust. Soc. Am. 99 (4) (1996) 2148±2157. [10] A. Gachagan, G. Hayward, S.P. Kelly, W. Galbraith, Characterisation of air-coupled transducers, IEEE Trans. Ultras. Ferr. Freq. Contr. 43 (4) (1996) 678±689. [11] H. Carr, C. Wykes, Diagnostic measurements in capacitance transducers, Ultrasonics 31 (1) (1993) 13±20. [12] M. Ra®q, C. Wykes, The performance of capacitive ultrasonic transducers using v-grooved backplates, Measure. Sci. Technol. 2 (2) (1991) 168±174. [13] J. Hietanen, P. Mattila, J. Storpellinenm, F. Tsuzuki, H. Vaataja, K. Sasaki, M. Luukala, Factors aecting the sensitivity of electrostatic ultrasonic transducers, Measure. Sci. Technol. 4 (10) (1993) 1138±1142. [14] P. Mattil, J. Hietanen, Bandwidth control of an electrostatic ultrasonic transducer, Sens. Actuat. A 45 (3) (1994) 203±208. [15] M.J. Anderson, J.A. Hill, C.M. Fortunko, N.S. Dogan, R.D. Moore, Broadband electrostatic transducers: modelling and experiment, J. Acoust. Soc. Am. 97 (1) (1995) 262±272. [16] L. Pizarro, D. Certon, M. Lethiecq, B. Hosten, Airborne ultrasonic electrostatic transducers with conductive grooved backplates: tailoring their centre frequency, sensitivity and bandwidth, Ultrasonics 37 (7) (1999) 493±504. [17] K. Suzuki, K. Higuchi, H. Tanisaga, A silicon electrostatic ultrasonic transducer, IEEE Trans. Ultras. Ferr. Freq. Contr. 36 (6) (1989) 620±627. [18] D.W. Schindel, D.A. Hutchins, L. Zou, M. Sayer, The design and characterization of micromachined air-coupled capacitive transducers, IEEE Trans. Ultras. Ferr. Freq. Contr. 42 (1) (1995) 42± 50. [19] D.A. Hutchins, W.M.D. Wright, D.W. Schindel, Ultrasonic measurements in polymeric materials using air-coupled capacitance transducers, J. Acoust. Soc. Am. 96 (3) (1994) 1634± 1642. [20] W.M.D. Wright, D.W. Schindel, D.A. Hutchins, P.W. Carpenter, D.P. Jansen, Ultrasonic tomographic imaging of temperature and ¯ow ®elds in gases using air-coupled capacitance transducers, J. Acoust. Soc. Am. 104 (6) (1998) 3446±3455. [21] D.W. Schindel, D.S. Forsyth, D.A. Hutchins, A. Fahr, Aircoupled ultrasonic NDE of bonded aluminum lap joints, Ultrasonics 35 (1) (1997) 1±6.
[22] M.I. Haller, B.T. Khuri-Yakub, A surface micromachined electrostatic ultrasonic air transducer, IEEE Trans. Ultras. Ferr. Freq. Contr. 43 (1) (1996) 1±6. [23] I. Ladabaum, X.C. Jin, H.T. Soh, A. Atalar, B.T. Khuri-Yakub, Surface micromachined capacitive ultrasonic transducers, IEEE Trans. Ultras. Ferr. Freq. Contr. 45 (3) (1998) 678±690. [24] P.C. Eccardt, K. Niederer, B. Fischer, Micromachined transducers for ultrasound applications, Proc. 1997 IEEE Ultrasonics Symp. 2 (1997) 1609±1618. [25] C.E. Cook, Pulse compression-key to more ecient radar transmission, Radar Syst. 3 (1960) 310±315. [26] N.A.H.K. Rao, M. Aubry, Evaluation of a pulse coding technique for spatial structure characterisation, IEEE Trans. Ultras. Ferr. Freq. Contr. 41 (5) (1994) 660±663. [27] T. Folkestad, K.S. Mylvaganam, Chirp excitation of ultrasonic probes and algorithm for ®ltering transit times in high-rangeability gas ¯ow metering, IEEE Trans. Ultras. Ferr. Freq. Contr. 40 (3) (1993) 193±215. [28] M. Pollakowski, H. Ermert, L. Bernus, T. Schmeidl, The optimum bandwidth of chirp signals in ultrasonic applications, Ultrasonics 31 (6) (1993) 417±420. [29] S. Venkatraman, N.A.H.K. Rao, Combining pulse compression and adaptive drive signal design to inverse ®lter the transducer system response and improve resolution in medical ultrasound, Med. Biol. Engng. Comput. 34 (1996) 318±320. [30] M.C. Bhardwaj, I. Neeson, L. Neeson, Ultrasonic analysis of plastics, rubbers and composites by non-contact analyzer ± the NCA 1000, J. Aircraft Engng. Aerospace Technol. 71 (1) (1999) 39±41. [31] B. Koehler, G. Hentges, W. Mueller, A novel technique for advanced ultrasonic testing of concrete by using signal conditioning methods and a scanning laser vibrometer, Proc. 1997 Conf. on NDT in Civil Engineering (1997) 123±134. [32] Y. Izuka, High signal to noise ratio ultrasonic testing system using chirp pulse compression, Insight 40 (4) (1998) 282±285. [33] V. Ermolov, J. Stor-Pellinen, M. Luukkala, Analog pulse compression system for real time ultrasonic nondestructive testing, Ultrasonics 34 (1996) 655±660. [34] N.A.H.K. Rao, Investigation of a pulse compression technique for medical ultrasound: a simulation study, Med. Biol. Engng. Comput. 32 (1994) 181±188. [35] M. Pollakowski, H. Ermert, Chirp signal matching and signal power optimization in pulse echo mode ultrasonic nondestructive testing, IEEE Trans. UFFC 41 (5) (1994) 655±659. [36] N. Rao, S. Mehra, Medical ultrasound imaging using pulse compression, Electron. Lett. 29 (8) (1993) 649±651. [37] N.A.H.K. Rao, S. Mehra, J. Bridges, S. Venkatraman, Experimental point spread function of FM pulse imaging scheme, Ultrason. Imag. 17 (2) (1995) 14±141. [38] R.E. Millet, A matched ®lter pulse-compression system using a nonlinear FM waveform, IEEE Trans. N Aerospace Electron. Syst. AES 6 (1) (1970) 73±78. [39] D.W. Schindel, D.A. Hutchins, W.A. Grandia, Capacitive and piezoelectric air-coupled transducers for resonant ultrasonic inspection, Ultrasonics 34 (6) (1996) 621±627. [40] E.A.G. Shaw, On the resonant vibrations of thick barium titanate disks, JASA 28 (1) (1956) 38±50.
www.elsevier.nl/locate/ultras
The use of broadband acoustic transducers and pulse-compression techniques for air-coupled ultrasonic imaging T.H. Gan a, D.A. Hutchins a,*, D.R. Billson a, D.W. Schindel b b
a School of Engineering, University of Warwick, Coventry CV4 7AL, UK MicroAcoustic Instruments Inc., 460 Wilbrod Street, Suite 2, Ottawa, Ont., Canada, K1N 6M8
Received 1 July 2000; received in revised form 1 October 2000
Abstract A pulse-compression technique has been applied to air-coupled testing of solid materials. Capacitance transducers were used to generate wide bandwidth swept-frequency (chirp) signals in air, which were then used to measure and image solid samples in through transmission. The results demonstrate that such signal processing techniques lead to an improvement in the signal to noise ratio and timing accuracy for air-coupled testing. Measurements of thickness and spectroscopic experiments are presented. Images of defects in a wide range of materials, including metals and carbon-®bre composites have also been obtained. This combination of capacitive transducers with pulse-compression techniques is shown to be a powerful tool for non-contact air-coupled ultrasonic measurements. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Capacitance transducer; Air-coupled ultrasound; Signal processing
1. Introduction Ultrasonic immersion testing is a popular technique for the investigation of materials such as metals, ®brereinforced polymers, and many other materials [1±5]. However, the use of water as a coupling medium may not always be suitable for certain inspection situations, e.g. where the material absorbs water, or where contamination or damage would result. There has thus been increased interest in using air as the coupling medium. However, ultrasonic attenuation in air is much greater than in water, especially at high frequencies, and the acoustic impedance of air is much lower, leading to dif®culties in coupling energy into solid samples. Because of such problems, careful attention has to be given to transducer design in terms of sensitivity and bandwidth. The most common transducers used for air-coupled experiments are based on piezoelectric and electrostatic designs. Piezoelectric air transducers are inherently resonant devices, and require special backing and construction to obtain suitable damping coecients. Another
*
Corresponding author. Tel.: +44-1203-523874; fax: +44-1203-418922. E-mail address: [email protected] (D.A. Hutchins).
problem is that the characteristic impedance of the piezoelectric element is very dierent to that of air. A quarter-wavelength thick matching layer at the frequency of interest is thus usually introduced at the front surface [6]. Many types of materials have been tested and found to be useful as the matching layers, including aerogel [7] and silicone rubber [8]. However, the application of a matching layer limits the overall bandwidth of the device. One way of reducing the impedance of the material is to use one to three connectivity piezopolymer composites [9,10], which contain an array of piezoelectric ceramic rods in a polymer ®ller matrix. These have a wider bandwidth than traditional piezoelectric materials such as PZT. However, a matching layer is still required for sensitive work in air, and hence these devices are usually used over a narrow bandwidth. An alternative transducer design is based on the capacitance or electrostatic principle. This has received much interest recently because of the excellent bandwidths that can be obtained. These devices are composed of a thin membrane ®lm and a rigid conducting backplate to form a capacitor. Applied voltages cause the membrane to vibrate, and hence generate ultrasound, whereas a change in charge across the membrane can be used for detection. Metallic backplates can be used [11], many employing a regular grooved backplate,
0041-624X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 1 - 6 2 4 X ( 0 0 ) 0 0 0 5 9 - 7
182
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
which was found to improve the acoustic properties of the transducer [12]. This type of transducer was found to be very sensitive at high bias voltages when a thin polymer membrane was used [13]. Further investigation showed that the sensitivity of the transducer could be increased by modifying the shape of the groove, e.g. to form a V-grooved backplate device [14]. The extension of device sensitivity to higher frequencies requires more careful control of surface features, and hence silicon has been investigated as the backplate material [15±17], with the backplate being etched using micromachining techniques. In the transducer used in the work reported here, the backplate contains small etched holes, which help to produce a wider bandwidth and enhanced sensitivities [18]. These devices can thus be used for non-contact materials inspection and imaging [19±21]. An additional approach is the fully micromachined device, where the complete structure is fabricated using CMOS technology from a silicon wafer. Typically, such designs use a silicon substrate and a silicon nitride membrane [22,23], although other work has used a hexagonal cell structure with polysilicon membranes [24]. The purpose of the present paper is to demonstrate that signal processing techniques can be used to increase the range of application of capacitive devices for ultrasonic materials characterization in air. In particular, it will be demonstrated that pulse-compression techniques can lead to signi®cant improvements in signal to noise ratios (SNRs) when applied to air-coupled throughtransmission testing. Pulse compression has been used for improving the resolution of various measurements in both radar [25] and ultrasonic measurements [26±29]. In the form of pulse compression used here, a broadband frequency-swept ``chirp'' ultrasonic waveform is emitted by the source transducer, which is then detected by a broadband receiver after transmission through the solid sample in air. This paper seems to be the ®rst to use such a technique with broadband capacitance transducers in air, although previous measurements to transmit signals in a gas only (and not through a solid sample) have been reported [30] using piezoelectric transducers. In the following, the pulse-compression technique will be explained, and the advantages over other techniques will be discussed. The resultant waveforms in air are compared to those obtained from conventional transient excitation. The technique will be shown to detect resonances in solid samples, and to measure the presence of defects in ®bre-reinforced composites. 2. Ultrasonic pulse compression One of the major factors limiting the use of ultrasound for non-destructive testing of materials is atten-
uation and scattering, leading to poor signal to noise ratios (SNRs) [31]. In air-coupled testing, the impedance mismatch between air and most engineering materials is also large causing most acoustic energy to be lost by re¯ection. In most situations the SNR problem can be improved by using a high power transmitting pulse, and this usually implies the transmission of a tone-burst signal. Such signals are very convenient, in that gated power ampli®ers can be used to deliver high powers and, when combined with a broad bandwidth transducer, the frequency of operation can be varied. For piezoelectric transducers in particular, tone-burst excitation leads to considerable advantages for air-coupled testing, in that the frequency can be tuned to the through-thickness resonance of the material. This increases the throughtransmission signal levels substantially. There are, however, disadvantages to using a tone burst. First, the voltage excitation level is limited by the type of transducer used, and in the particular case of a capacitance transducer, the voltage must be restricted to avoid dielectric breakdown of the thin polymer membranes. In addition, the exact frequency of excitation must match the through-thickness resonance of the sample to achieve maximum eciency, and this might need to be adjusted if the thickness of the material changed (for instance during an imaging experiment involving positional scanning). It may also be the case that the longitudinal velocity is either not known, or might vary. The main disadvantage in the context of defect detection is that a tone burst leads to relatively poor time resolution [32]. Defects might be dicult to resolve, because multiple re¯ections might overlap in time, although cross-correlation can lead to accurate time-of-¯ight measurements. Because capacitance transducers can operate over a wide frequency range, it is thus better to use a technique which capitalises on this property. The use of a swept-frequency signal, instead of a single transient, allows a high power, broad bandwidth signal to be used which, when combined with suitable processing, also gives excellent time resolution. It is this property that is used in the pulse-compression technique [33±35]. In the context of an air-coupled ultrasound experiment, a tone burst, tuned to the through-thickness resonance, is still likely to give the greatest signal amplitude. However, the advantage of using a wide bandwidth pulse-compression approach, using a broadband swept frequency excitation at the source transducer, is that the full air-coupled spectral response of a material (e.g. with multiple resonances) can be obtained instantaneously. This can be achieved without frequency scanning as has been necessary in the past [27]. Increased accuracy in time-of-¯ight measurements can potentially be obtained, and the technique provides the ability to recover small signals from well below the noise ¯oor, although the pulse-compression method is
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
183
only valid for improving the SNR if the noise is random [34]. The pulse-compression method can be implemented by driving the ultrasonic source with a so-called chirp signal, where the frequency is swept continuously over a pre-determined range. The result is an elongated waveform, with the overall duration of the signal and the rate of frequency sweep de®ning the chirp characteristics. The broadband chirp or linear frequency-modulated signal is also widely used in medical applications [29,36], for measuring gas ¯ow [27] and for spatial characterisation of scattering microstructure [26]. The pulse-compression technique is best described using a simulation, where a chirp signal is buried in noise. This chirp signal can be represented as pB 2 C t sin xs t t 06t6T 1 T
0.2 MHz, and the bandwidth, B, to 1 MHz. The chirp thus extends over the 0.2±1.2 MHz frequency range. Fig. 1(b) shows the corresponding frequency spectrum, obtained via an FFT. It is seen that the signal has a wide bandwidth, but that there are some ripples at the bandedges. These are known as the ÔFresnel ripplesÕ [28], and it is found that the time-bandwidth product of the signal has to be increased to reduce their severity. From the spectrum, it can also be seen that the higher cut-o frequency limit of 1.2 MHz is extended to 1.5 MHz. These disadvantages can be avoided by applying a modulation function such as a bandpass Hanning or Gaussian ®lter. Here we consider a chirp signal that is modulated by a Hanning ®lter, which may be represented as pB 2 t C t H t sin xs t 06t6T 2 T
where xs is the starting angular frequency, B is the bandwidth of the signal, and T is the duration of the pulse. Fig. 1(a) illustrates a typical chirp waveform, generated using Eq. (1). Here, the duration of the chirp, T, was set to 50 ls, the starting frequency fs xs =2p at
where H(t) is the Hanning function given by 1 2pt H t 1 cos 2 T
Fig. 1. (a) Broadband chirp signal with a duration of 50 ls and (b) frequency spectrum of (a).
Fig. 2. (a) Broadband chirp signal with a duration of 50 ls after a Hanning window was applied and (b) frequency spectrum of (a).
3
The generated signal from Eq. (2) is as shown in Fig. 2(a). The ®gure shows the bell-shaped envelope resulting
184
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
from the Hanning ®lter, and this shape is important to ensure good sensitivity. In addition, the Hanning amplitude weighting also helps to reduce the amount of side lobes in the signal [37]. Fig. 2(b) shows the frequency spectrum of the Hanning-weighted chirp signal. In the ®gure, it can be seen that the lower and higher limits of the frequencies are maintained at 200 KHz and 1.2 MHz. The signal is centred at 700 kHz, and the edge ripples have been removed. It is interesting to demonstrate that a chirp signal is useful for the detection of signals in the presence of high noise levels. The Hanning chirp signal in Fig. 2(a) was thus shifted by 40 ls in time, and mixed into a random noise level of twice the chirp signal amplitude. This simulates the noise levels that would be encountered in a real air-coupled materials inspection experiment. This is as shown in Fig. 3(a). In order to produce the compressed pulse signal, P(t), the received signal CT (t) is bandpass ®ltered within the chirp bandwidth. The ®ltered signal is as shown in Fig. 3(b). The bandpass ®lter removes the noise levels above and below the frequency range of the original chirp driving signal, but the transmitted chirp signal is still not easily visible. The waveform is now cross-correlated with the reference signal C(t). This is represented by Eq. (4) and this process is sometimes known as the matched ®ltering process [38]: P t C t CT t
4
The compressed pulse, P(t), is thus produced by the correlation of the received signal CT (t) with the original reference signal, C(t). The correlated result is as shown in Fig. 3(c), and is in the form of a time signal. The main peak in the pulse-compression output jP tj represents the position in time of the transmitted signal, which is at a time delay of 40 ls. The SNR has been greatly improved compared to Fig. 3(b), as can be seen. The width of the jP tj peak can be reduced to give greater time resolution by increasing the bandwidth, B, of the generated chirp signal, C(t) [34], whereas a greater peak amplitude can be obtained by elongating the time duration, T, for the same bandwidth. It is thus of advantage to use as long a duration T of the chirp as possible, and to also maximise the bandwidth. However, the pulse-compression output can be interpreted much like a conventional ultrasonic waveform, in that the amplitude of the compressed pulse as a function of time is related to the amplitude of the received chirp waveforms as a function of time. Note also that the exact shape of jP tj will be modi®ed if the original chirp pulse shape is distorted by the sample. This will almost always happen in the case of a simple plate at normal incidence, where maximum transmission amplitudes occur at well-de®ned frequencies. In these situations, jP tj also contains information concerning the material through which the signal has travelled. In particular, it can be used to de-
Fig. 3. (a) Transmitted broadband Hanning chirp signal which was (a) embedded in noise, (b) bandpass ®ltered, and (c) shows the result of subsequent pulse compression.
termine the dierent frequencies of resonance that are present. This will be illustrated later in this paper. There are thus three primary reasons why pulse compression is a potentially useful technique when applied to air-coupled measurements. First, because the chirp is a complex coded waveform, it can correlate well only at a single well-de®ned time-of-arrival (for each arriving chirp). Thus, the accuracy of time-of-¯ight measurements could be greatly improved when using
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
pulse-compression techniques. Second, such a coded waveform has the advantage that it can be detected when the received chirp level is well below the noise ¯oor of the detector (i.e., since the noise is random and thus uncorrelated with the chirp shape). Finally, high ultrasonic energy levels can be transferred into a material due to the use of a chirp signal, to give a good SNR. 3. Apparatus and experiment It is informative to compare the performance of the pulse-compression system with that from a conventional set-up using transient excitation. Initially, the characteristics of the capacitive transducer were ®rst determined by aligning a pair of transducers carefully in air, as shown in Fig. 4. The capacitance transducers used for the present measurements were similar to those described in previous work [18,39]. The transducer design is shown in the schematic diagram of Fig. 5. The transducer uses a micromachined silicon backplate, which contains arrays of small cylindrical holes. These holes act as air springs underneath the membrane, which is a metallised polymer ®lm. The backplate is coated with gold to make it conducting, with the outer electrode of the membrane grounded. This type of transducer can be used as either a source or detector. As a
185
source, a transient driving voltage is applied, together with an optional dc bias voltage. As a receiver, the bias voltage is required to give charge variations when the membrane moves. The transducers in the present experiments had an active aperture of 10 mm diameter, and were ®tted into a fully shielded metallic case. The transducer acting as a source had a membrane thickness of 5 lm, so as to withstand the higher excitation voltages without damage to the polymer membrane, and was driven by a Panametrics pulser/receiver (Model 5055PR) together with a 200 V dc bias voltage. This applied a 250 V transient to the transducer, and allowed the transducers to be tested initially under conventional transient excitation. The use of the dc voltage was to ensure phase uniformity across the transducer aperture. The detector, separated by 77 mm from the source, had a ®lm thickness of 2.5 lm, and was connected to a Cooknell CA6/C charge ampli®er with a gain of 250 mV pC 1 . The response was then fed into a Tektronix TDS540 digital oscilloscope for signal averaging. Fig. 6(a) shows a typical waveform of the
Fig. 4. Experimental set-up to determine the characteristics of the aircoupled devices.
Fig. 5. Schematic diagram of the capacitive broadband acoustic transducer.
Fig. 6. Signals transmitted across an air gap of 45 mm with the source driven by the Panametrics pulser/receiver (Model 5055PR): (a) time waveform, and (b) frequency spectrum of the signal obtained via an FFT.
186
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
transmitted signal, and Fig. 6(b) is the corresponding frequency response. These indicate that the transducers had broadband frequency characteristics. The transducers were now placed either side of the sample under test, and the through-transmitted signal recorded. To illustrate this type of experiment, a 2.2 mm thick carbon-®bre-reinforced polymer composite plate with large transverse dimensions was placed between the transducers of Fig. 4. The transmitted signal was found to require signal averaging on the oscilloscope, the small signal level being due to the large dierence in acoustic impedance between air and the composite plate. The recorded waveform in the time domain is as shown in Fig. 7(a), and demonstrates that the received waveform was in the form of an exponential decay, at a centre frequency corresponding to the fundamental throughthickness resonance of the plate. Fig. 7(b) shows the FFT of the signal in Fig. 7(a), and contains the expected resonant peak, centred at a frequency of approximately 0.67 MHz. This behaviour is typical of wide bandwidth air-coupled experiments, and such signals have been used in the characterisation and imaging of a range of materials [19,21]. The low SNRs seen in typical transient
Fig. 7. Signal transmitted through a carbon-®bre composite plate of thickness 2.2 mm: (a) time waveform and (b) frequency spectrum of the transmitted signal.
Fig. 8. Experimental arrangement for through-transmission testing using pulse compression.
air-coupled through-transmission experiments arise from the large acoustic impedance mismatches at air/solid interfaces. The pulse-compression approach was thus designed to improve the signal levels in a through-transmission experiment over an extended bandwidth. The experimental arrangement for through transmission is shown in Fig. 8. The pulse-compression approach was implemented using an NCA1000 pulser/receiver unit from VN Instruments Ltd. This contained an on-board digital signal processor (DSP) to both synthesize output pulses for source excitation, and to run embedded pulse-compression code for on-line processing of received signals. All aspects of the instrument were controlled by a pentium-class computer via a parallel interface and a real-time operating system (QNX). The pulser section contained a 200 W broadband power ampli®er of variable gain, whereas the receiver section contained a variable-gain low-noise receiver ampli®er (maximum gain of 90 dB), followed by an A/D converter, and ®nally the DSP. As before, the output of the instrument was superposed upon a 200 V dc bias voltage. The received response was also ampli®ed by a Cooknell charge ampli®er model CA6/C, with a gain of 250 mV pC 1 , before being fed into the input of the NCA1000. Typical waveforms that resulted when no sample was present between the transducers are shown in Fig. 9. The un®ltered time waveform in Fig. 9(a) shows a typical broadband chirp arriving at the capacitive receiver. It consisted of a frequency sweep from 450 kHz to 1.15 MHz over a time period of 190 ls, and arose by specifying an applied chirp center frequency fc of 800 kHz and a chirp bandwidth B of 700 kHz on the graphics user interface. Fig. 9(b) is a time waveform to which a bandpass ®lter has been applied. This reduces low-frequency oscillations, which can occur if either the samples are of small transverse dimensions (and hence allow a low-frequency air wave to propagate around the outer dimensions), or if plate-like samples become so thin that low-frequency ¯exural modes are additionally excited in the plate. The waveform of Fig. 9(c) is the FFT of the
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
187
Fig. 9. Ultrasonic signals obtained across an air gap using pulse compression: (a) un®ltered time waveform across the air gap, (b) ®ltered time waveform of the transmitted chirp across the air gap, (c) FFT spectrum of the ®ltered time waveform, and (d) compressed pulse of the ®ltered time waveform using the pulse-compression algorithm.
®ltered time waveform. This waveform demonstrates clearly the wideband frequency response of the aircoupled capacitance transducer system (limited by the 700 kHz bandwidth of the applied chirp). Finally, Fig. 9(d) shows the pulse-compression output from the NCA1000 unit, corresponding to jP tj of Eq. (4). This is in the form of a single peak, as expected from the earlier simulation of Fig. 3(c). Note that less than 1 V p±p source drive voltages were applied in Fig. 9 to obtain a result with a good SNR. Fig. 10 shows through-transmission results for a carbon-®bre composite plate of nominal thickness 2.2 mm and large transverse dimensions. Here, the same applied chirp as was used for Fig. 9 was employed. In the un®ltered time waveform of Fig. 10(a), the chirp arriving at 240 ls is associated with the longitudinal wave propagation through the plate, while the small low-frequency oscillations arriving thereafter are due to subsequent excitation of ¯exural modes in the plate. In the ®ltered time waveform, Fig. 10(b), the low-frequency oscillations are eliminated, leaving the longitudinal wave at 240 ls in addition to a second chirp arrival at 480 ls. The second chirp arrival is due to the ®rst echo in the air-gap between the source and composite plate. By applying a fast Fourier transform to only the ®rst longitudinal chirp arrival (i.e., below 475 ls), the FFT
spectrum in Fig. 10(c) results. This ®gure shows a single resonant peak at 0.67 MHz. This value agrees with the frequency indicated in Fig. 7, and was associated with the fundamental longitudinal resonance within the thickness of the plate. Given the measured resonance frequency and thickness of the composite plate, the longitudinal velocity was found to be vl 2935 ms 1 , which was in agreement with the known value for the material. In addition, the response after pulse-compression shown in Fig. 10(d) also exhibited the ringing eect after the arrival of the maximum peak due to multiple echoes within the plate itself. These results show that the ultrasonic pulse-compression technique has the capability to achieve the same outcome as the traditional pulsed system (shown earlier in Fig. 7) but with a much better response. The system was thus used to examine a range of materials and thicknesses, the results of which will be given in Section 4. An experimental arrangement was also set up for imaging, using an X±Y scanning stage, as shown in the block diagram of Fig. 11. This included a digital oscilloscope and an external PC controller. The output chirp voltage from the power ampli®er (typically 300 V p±p) was ®rst superimposed upon a 200 V dc bias using a capacitive decoupling circuit before being applied to a capacitance source of bandwidth 1.5 MHz. The chirp
188
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
Fig. 10. Signals transmitted through a carbon-®bre composite plate of 2.2 mm thickness using pulse compression: (a) un®ltered time waveform, (b) ®ltered time waveform, (c) FFT spectrum of the ®ltered time waveform, and (d) compressed pulse of the ®ltered time waveform.
and the through-transmitted signal received as above by a capacitive receiver. Output from the receiver was again input to the Cooknell CA6/C charge ampli®er, and pulse-compressed data recorded using a Tektronix TDS540 digital oscilloscope, before being transferred to a PC for data storage. The PC was also used to control the X±Y stages. The total area of the scan varied depending on the size of the test sample involved. Usually the scan area was set 50 mm 50 mm, at spatial intervals of 1 mm. All scanning and data acquisition was controlled by L A B V I E W version 5.1 software. This software also extracted data such as the peak amplitude or the time-of-arrival of the transmitted signal from each waveform in the scan, and stored these data in a grid ®le format. The image was then plotted using S U R F E R version 7.0 software. Fig. 11. Experimental arrangement for ultrasonic imaging using pulse compression.
had a centre frequency fc of 400 kHz and a bandwidth B of 700 kHz. The longitudinal waves emitted by the source then propagated through the air to the sample,
4. Results 4.1. Measurements in simple plates Thickness evaluation using through-transmitted signals was performed on dierent types of material placed
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
in the air gap between the transducers, namely brass and aluminium plates of varying thickness, as well as carbon-®bre-reinforced composite plates with a range of layups, including unidirectional (UD) and quasi-isotropic (QI) con®gurations. The measured responses after pulse compression of the ®ltered time waveform for the brass and aluminium plates are as shown in Figs. 12 and 13. It can be seen from the ®gures that there is a shift in the arrival time of the detected signal. This was due to the dierent types of material properties involved. The detected acoustic velocity of each plate was compared with the result measured using the pulse echo contact method. The results are as shown in Table 1. From the measured velocities, about 1% dierence was found by comparing the two methods involved. This was probably due to the greater accuracy of the pulse-compression method, and in particular the fact that no liquid couplant was present (as
189
Table 1 Measured velocities in metal plates
Brass plate Aluminum plate
Air-coupled through transmission method (m s 1 )
Pulse echo contact method (m s 1 )
Percentage dierence (%)
4524 6431
4460 6400
1.4 1
Fig. 14. Variation of time-of-¯ight across aluminum plates of dierent thickness in air.
Fig. 12. Compressed pulse of the ®ltered time waveform across 8.12 mm brass plate.
Fig. 13. Compressed pulse of the ®ltered time waveform across the 6.45 mm aluminum plate.
was required between the sample and the contact piezoelectric transducer). An analysis was also carried out for dierent thicknesses of aluminium plates. The thickness of the plate was ®rst measured with a Kennedy digital caliper model 331±206, and then compared with that of the experimental results. Fig. 14 shows the measured results using both the UPC method and with the caliper. The results show that there is a slight increase in the percentage dierence between the two as the thickness increases, this being 4.7% when measuring the aluminium plate of thickness 9.9 mm. This may be due to the fact that the caliper measures in one location only, whereas the acoustic signal measures the average thickness over the width of the ultrasonic beam in the plate. Fig. 15 shows the transmitted chirp signals across two dierent thickness of the ®bre-reinforced polymer composite. In the ®gure, the dashed line represents the transmitted signal across 3.2 mm thick sample and the solid line represents the signal across a 17.5 mm thickness. Both the signals have a good SNR. The ®gure also shows that the pulse-compression technique not only could be used to detect the shift in the arrival time but also the change in correlated peak amplitude according to the thickness. This reduction in amplitude could be used to measure attenuation. Experiments were also performed in a circular aluminum disk of 30 mm diameter and 3.2 mm thickness.
190
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
Fig. 15. Compressed pulse of the ®ltered time waveform across the 17.5 and 3.17 mm thick composite plates.
Here, resonances and other factors are to be expected. The un®ltered time waveform of Fig. 16(a) shows a combination of high-frequency through-transmission and low-frequency waves wrapping around the disksÕ limited dimensions. Bandpass ®ltering leads to an elimination of the low-frequency waves, as shown in Fig. 16(b). The complexity of the spectral response of the disk is readily apparent from Fig. 16(c), in which the FFT of the ®ltered time waveform is shown. Such complexity occurs because the small transverse dimensions of the sample lead to multiple re¯ections and mode conversions at the boundaries, and the net eect is an excitation of all of the disksÕ natural modes of vibrations within the bandwidth of the experimental system. These natural modes consist of various types of shear, longitudinal and ¯exural resonances, the analysis of which can be quite complicated [40]. The fact that the entire spectral response of a solid sample can be obtained instantaneously in this way is a very powerful advantage of the approach. This is because real-time spectroscopy can be undertaken with this air-coupled system so as to provide a wealth of on-line information about materials in a rapidly scanned non-contact fashion.
4.2. Imaging experiments Further experiments were carried out to image a range of defects within metals, polymers and composites. The ®rst image, shown in Fig. 17, is of a 10.5 mm diameter circular ¯at-bottomed hole, machined into one surface of a 4.1 mm thick Plexiglas plate to a depth of 2 mm. The image, plotted using the peak transmitted signal amplitude, clearly shows the circular shape of the defect of approximately the correct size. The slight differences are to be expected due to the 10 mm aperture size of the transducers. Images were also obtained in a 8.1 mm thick brass plate. Here, the impedance mismatch between air and
Fig. 16. Signals transmitted through a circular aluminum disk of 30 mm diameter and 3 mm thickness: (a) un®ltered time waveform, (b) ®ltered time waveform, and (c) FFT spectrum of the ®ltered time waveform.
brass increased re¯ection coecients at interfaces substantially, so that received amplitudes were greatly reduced (and in fact cannot be measured using a conventional pulse/receiver in air-coupled measurements). To demonstrate that pulse compression can improve this situation, scans were performed of two
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
Fig. 17. Image of a 10.5 mm diameter ¯at-bottomed hole on a Plexiglas plate.
191
with the received amplitude decreasing as the scan passed through the defective areas. This was also con®rmed by plotting images of arrival time from the same signals, as shown in Fig. 18(b), where good agreement in terms of locations and sizes of defects is seen. Further scans were carried out using an aluminium plate containing an internal 18 mm long side-drilled hole of 5 mm diameter. The images obtained using maximum amplitude and time-of-¯ight data are as shown in Fig. 19(a) and (b), respectively. The position of the defect was detected and was clearly shown in these images. Fig. 19(a) shows that most of the transmitted signals were attenuated in the region of the defect and also delay was introduced to the signal in the defect zone. Final scans were performed in anisotropic carbon®bre-reinforced polymer composites. These were 16-ply (2 mm) thick plates containing defects, with the properties shown in Table 2. The defects were introduced during manufacture of the composite, so as to be embedded within it at the mid point of the plate thickness.
Fig. 18. Image of two arti®cial defects within an 8.1 mm thick brass plate, using (a) peak signal amplitude, and (b) time-of-arrival.
arti®cial defects within the plate, and the results are shown in Fig. 18(a). As before, the defects were detected with the correct location and reasonable dimensions,
Fig. 19. Image of an internal defect in an aluminum plate, using (a) peak signal amplitude, and (b) time-of-arrival.
192
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
Table 2 Details of ®bre-reinforced composite samples Lay-up con®guration
Arti®cial defect
QI UD QI
12.5 mm square brass shim Pre-cured area (4 plies) 8 plies of random ®bres
Fig. 21. Image of a pre-cured area (four plies) in a 16-ply thick composite plate, using (a) peak signal amplitude, and (b) time-ofarrival.
Fig. 20. Image of a 12.5 mm square brass shim in a 16-ply thick composite plate, using (a) peak signal amplitude, and (b) time-ofarrival.
Results for the 0.5 in. brass defect are presented in Fig. 20(a) and (b) in terms of maximum amplitude and arrival time respectively. The signals travelling through the defect were attenuated as expected, and also exhibited an increased arrival time due to diraction. The defect was clearly shown in these images, as is disturbance to the plate structure around the defect. Images of precured ply and chopped random ®bres introduced into the material are shown in Figs. 21 and 22. The technique clearly shows the presence of these anomalies, despite the fact that their properties are similar to the sur-
rounding matrix, and the fact that they are only four plies thick. In general, time of ¯ight gives better results. This is to be expected, as the pulse-compression technique gives very accurate time information; small changes in the layup con®guration within the composite lead to small local longitudinal velocity variations (due to a change in structure and hence elastic properties), and these are detected by the technique. A ®nal test was also performed on composite plates containing real damage. Fig. 23 show images of barelyvisible damage caused by a 16 J impact on a pultruded glass-®bre-reinforced composite plate, using time-of¯ight data. Both the damaged area and the internal structure of the composite plate were visible.
5. Conclusions It is evident from the above that the combination of a pulse-compression technique, and the wide bandwidths
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
Fig. 22. Image of random ®bres in a 16-ply thick composite plate, using (a) peak signal amplitude, and (b) time-of-arrival.
available from micromachined capacitance air-coupled transducers leads to a new range of possible experiments. Using conventional techniques, the testing of metals was not possible over a wide bandwidth, due to the large dierence in acoustic impedance between metals and air. Using broad bandwidth chirp signals increases SNRs signi®cantly. This allows a much wider range of measurements to be made. In addition, the increased accuracy of time measurement leads to many new applications in ¯ow measurement, and where timing accuracy is also important. This was illustrated further by the experiment with the aluminium disk, where spectroscopy of the resonant modes was demonstrated. Such a non-contact technique has many uses in condensed matter physics and materials characterisation for the measurement of elastic properties. This was further illustrated by the imaging of detects in solids. The authors are currently extending this technique to many other application areas, and these results will be reported in due course.
193
Fig. 23. Image of a defect in a composite plate caused by 16 J impact, using (a) peak signal amplitude, and (b) time-of-arrival.
Acknowledgements The authors would like to acknowledge funding for this work from the Defence Evaluation and Research Agency (DERA), UK, and the Canadian Government. Thanks are due to Dr David Bruce and Dr Franklin Wong respectively for their encouragement of this work. References [1] P. Kalyanasundaram, C. Rajagopalan, C.V. Subramaniam, M. Thavasimuthu, B. Raj, Ultrasonic signal analysis for defect characterization in composite-materials, Br. J. Non-destruct. Test. 33 (5) (1991) 221±226. [2] C.S. Karthikeyan, C.R.L Murthy, S. Sankaran, C. Kishore, Characterization of reinforced syntactic foams using ultrasonic imaging technique, Bull. Mater. Sci. 22 (4) (1999) 811±815. [3] M.D. Enderby, A.R. Clarke, M. Patel, P. Ogden, A.A. Johnson, An automated ultrasonic immersion technique for the determination of three-dimensional elastic constants of polymer composites, Ultrasonics 36 (1±5) (1998) 245±249.
194
T.H. Gan et al. / Ultrasonics 39 (2001) 181±194
[4] D. Xiang, N.N. Hsu, G.V. Blessing, A simpli®ed ultrasonic immersion technique for materials evaluation, Mater. Evaluat. 56 (7) (1998) 854±859. [5] D.P. Jansen, D.A. Hutchins, J.T. Mottram, Lamb wave tomography of advanced composite laminates containing damage, Ultrasonics 32 (2) (1994) 83±89. [6] W.M.D. Wright, Air-coupled ultrasonic testing of materials, Ph.D. Thesis, University of Warwick, England, 1996. [7] O. Kraur, R. Gerlach, J. Frickle, Experimental and theoretical investigations of SiO2 -aerogel matched piezo-transducers, Ultrasonics 32 (1994) 217±222. [8] J.D. Fox, B.T. Khuri-Yakub, G.S. Kino, High frequency acoustic wave measurements in air, Proc. 1983 IEEE Ultrasonic Symp. 1 (1983) 581±584. [9] G. Hayward, A. Gachagan, An evaluation of 1±3 connectivity composite transducers for air-coupled ultrasonic applications, J. Acoust. Soc. Am. 99 (4) (1996) 2148±2157. [10] A. Gachagan, G. Hayward, S.P. Kelly, W. Galbraith, Characterisation of air-coupled transducers, IEEE Trans. Ultras. Ferr. Freq. Contr. 43 (4) (1996) 678±689. [11] H. Carr, C. Wykes, Diagnostic measurements in capacitance transducers, Ultrasonics 31 (1) (1993) 13±20. [12] M. Ra®q, C. Wykes, The performance of capacitive ultrasonic transducers using v-grooved backplates, Measure. Sci. Technol. 2 (2) (1991) 168±174. [13] J. Hietanen, P. Mattila, J. Storpellinenm, F. Tsuzuki, H. Vaataja, K. Sasaki, M. Luukala, Factors aecting the sensitivity of electrostatic ultrasonic transducers, Measure. Sci. Technol. 4 (10) (1993) 1138±1142. [14] P. Mattil, J. Hietanen, Bandwidth control of an electrostatic ultrasonic transducer, Sens. Actuat. A 45 (3) (1994) 203±208. [15] M.J. Anderson, J.A. Hill, C.M. Fortunko, N.S. Dogan, R.D. Moore, Broadband electrostatic transducers: modelling and experiment, J. Acoust. Soc. Am. 97 (1) (1995) 262±272. [16] L. Pizarro, D. Certon, M. Lethiecq, B. Hosten, Airborne ultrasonic electrostatic transducers with conductive grooved backplates: tailoring their centre frequency, sensitivity and bandwidth, Ultrasonics 37 (7) (1999) 493±504. [17] K. Suzuki, K. Higuchi, H. Tanisaga, A silicon electrostatic ultrasonic transducer, IEEE Trans. Ultras. Ferr. Freq. Contr. 36 (6) (1989) 620±627. [18] D.W. Schindel, D.A. Hutchins, L. Zou, M. Sayer, The design and characterization of micromachined air-coupled capacitive transducers, IEEE Trans. Ultras. Ferr. Freq. Contr. 42 (1) (1995) 42± 50. [19] D.A. Hutchins, W.M.D. Wright, D.W. Schindel, Ultrasonic measurements in polymeric materials using air-coupled capacitance transducers, J. Acoust. Soc. Am. 96 (3) (1994) 1634± 1642. [20] W.M.D. Wright, D.W. Schindel, D.A. Hutchins, P.W. Carpenter, D.P. Jansen, Ultrasonic tomographic imaging of temperature and ¯ow ®elds in gases using air-coupled capacitance transducers, J. Acoust. Soc. Am. 104 (6) (1998) 3446±3455. [21] D.W. Schindel, D.S. Forsyth, D.A. Hutchins, A. Fahr, Aircoupled ultrasonic NDE of bonded aluminum lap joints, Ultrasonics 35 (1) (1997) 1±6.
[22] M.I. Haller, B.T. Khuri-Yakub, A surface micromachined electrostatic ultrasonic air transducer, IEEE Trans. Ultras. Ferr. Freq. Contr. 43 (1) (1996) 1±6. [23] I. Ladabaum, X.C. Jin, H.T. Soh, A. Atalar, B.T. Khuri-Yakub, Surface micromachined capacitive ultrasonic transducers, IEEE Trans. Ultras. Ferr. Freq. Contr. 45 (3) (1998) 678±690. [24] P.C. Eccardt, K. Niederer, B. Fischer, Micromachined transducers for ultrasound applications, Proc. 1997 IEEE Ultrasonics Symp. 2 (1997) 1609±1618. [25] C.E. Cook, Pulse compression-key to more ecient radar transmission, Radar Syst. 3 (1960) 310±315. [26] N.A.H.K. Rao, M. Aubry, Evaluation of a pulse coding technique for spatial structure characterisation, IEEE Trans. Ultras. Ferr. Freq. Contr. 41 (5) (1994) 660±663. [27] T. Folkestad, K.S. Mylvaganam, Chirp excitation of ultrasonic probes and algorithm for ®ltering transit times in high-rangeability gas ¯ow metering, IEEE Trans. Ultras. Ferr. Freq. Contr. 40 (3) (1993) 193±215. [28] M. Pollakowski, H. Ermert, L. Bernus, T. Schmeidl, The optimum bandwidth of chirp signals in ultrasonic applications, Ultrasonics 31 (6) (1993) 417±420. [29] S. Venkatraman, N.A.H.K. Rao, Combining pulse compression and adaptive drive signal design to inverse ®lter the transducer system response and improve resolution in medical ultrasound, Med. Biol. Engng. Comput. 34 (1996) 318±320. [30] M.C. Bhardwaj, I. Neeson, L. Neeson, Ultrasonic analysis of plastics, rubbers and composites by non-contact analyzer ± the NCA 1000, J. Aircraft Engng. Aerospace Technol. 71 (1) (1999) 39±41. [31] B. Koehler, G. Hentges, W. Mueller, A novel technique for advanced ultrasonic testing of concrete by using signal conditioning methods and a scanning laser vibrometer, Proc. 1997 Conf. on NDT in Civil Engineering (1997) 123±134. [32] Y. Izuka, High signal to noise ratio ultrasonic testing system using chirp pulse compression, Insight 40 (4) (1998) 282±285. [33] V. Ermolov, J. Stor-Pellinen, M. Luukkala, Analog pulse compression system for real time ultrasonic nondestructive testing, Ultrasonics 34 (1996) 655±660. [34] N.A.H.K. Rao, Investigation of a pulse compression technique for medical ultrasound: a simulation study, Med. Biol. Engng. Comput. 32 (1994) 181±188. [35] M. Pollakowski, H. Ermert, Chirp signal matching and signal power optimization in pulse echo mode ultrasonic nondestructive testing, IEEE Trans. UFFC 41 (5) (1994) 655±659. [36] N. Rao, S. Mehra, Medical ultrasound imaging using pulse compression, Electron. Lett. 29 (8) (1993) 649±651. [37] N.A.H.K. Rao, S. Mehra, J. Bridges, S. Venkatraman, Experimental point spread function of FM pulse imaging scheme, Ultrason. Imag. 17 (2) (1995) 14±141. [38] R.E. Millet, A matched ®lter pulse-compression system using a nonlinear FM waveform, IEEE Trans. N Aerospace Electron. Syst. AES 6 (1) (1970) 73±78. [39] D.W. Schindel, D.A. Hutchins, W.A. Grandia, Capacitive and piezoelectric air-coupled transducers for resonant ultrasonic inspection, Ultrasonics 34 (6) (1996) 621±627. [40] E.A.G. Shaw, On the resonant vibrations of thick barium titanate disks, JASA 28 (1) (1956) 38±50.