Actuutors A, 25-27 (1991) 549-552
Ultrasonic Transducers with Piezoelectric Polymer Foil F. HARNISCH ELOTECH
GmhH, Lohenstein (F.R.G.)
and W. MANTHEY
Technical University, Chemnitz (F. R.G.)
Abstract The application fields of ultrasonic measurement systems are mainly determined by the properties of ultrasonic transducers. Acoustic features of transducers using piezoelectric polymer foils are considered. The directivity pattern and frequency range can easily be changed by variation of the geometric size and shape of such transducers. Their bandwidth is large in comparison to conventional piezoceramic transducers. Foil transducers offer new applications for ultrasonic sensors in connection with digital signal processing algorithms.
1. Introduction Ultrasonic measurement systems operating in the pulse-echo mode are used for industrial automation and robots. Their main tasks are the contactless distance or position measurement in a range of some meters, the presence detection of objects and object identification by echo-profile evaluation. Ultrasonic sensors supply high resolution in an axial direction (direction of sonic propagation) by measuring the travel time of a pulse-echo signal. In particular, broad-band transducers are of great interest because of their good pulse response behaviour. The use of piezoelectric high polymer foils, e.g., of polarized polyvinylidene fluoride (PVDF), enables low-cost transducers to be constructed with suitable acoustic and electrical properties. Multielement transducers, e.g., with a controllable directivity pattern, can also be built easily. 0924-4247/91/$3.50
2. Properties of Piezoelectric Polymer Foil Transducers Ultrasonic transducers with piezoelectric polymer foils are an alternative to conventional piezoelectric or electrostatic transducers operating in air. The construction principle of a cylindrically shaped foil transducer is shown in Fig. 1 [ 11. The transducer can be used as both transmitter and receiver. By variation of the geometric transducer data (length I, radius of curvature R, opening angle SO, number of parallel curves), the acoustic properties of foil transducers can be adapted to different measurement tasks over a wide range. The following properties are of special interest: frequency response characteristic as both transmitter and receiver, pulse response, directivity pattern and electric impedance. An example of frequency responses of a cylindrical PVDF transducer is shown in Fig. 2. The resonance ratio is small because of high internal mechanical losses within the polymer material. Additional damping is achieved by a foam support. The resonance frequency is nearly inversely proportional to the radius R. In the transmitter mode the
Fig. 1. Construction principle of piezopolymer transducer (1 = piezopolymer foil, 2 = foam support). (a) Cylindrically shaped transducer; (b) multiple curved transducer. 0
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Fig. 2. Frequency response characteristic of a cyhndrical PVDF transducer, R = 2.6 mm, 9, = 1.5 rad. (a) Receiver sensitivity M, (0 dB = 10 V/Pa); (b) transmitter sound pressure level P, (0 dB = 20 PPa) for driving voltage = 5 V, distance = 0.5 m. + + + , Measurement; -, calculation
resonance frequency increases with a decrease of the opening angle &,. The type of PVDF transducers described here is suited for the frequency range 40-200 kHz. Average values of the transducer sensitivity are 0.2-l mV/Pa in the receiver mode (noise voltage level < 1 pV) and 20-50 mPa/ V in the transmitter mode at a distance of 1 m to the transducer. The value of the electric impedance is typically 1- 10 kCL This is very low compared to electrostatic transducers. Therefore the design of the preamplifier can be comparatively simple. Because of the very short decay time of the acoustic signal, PVDF transducers may be used in the alternating transmitter-receiver mode. In this case the minimum measurement distance is about 30 mm. Multilayer transducers consisting of composite piezoceramic and polymer materials show similar good pulse responses, but the transmitter and receiver are separate components . A single-foil transducer also enables distance measurement with small accuracy error to be carried out in the nearfield of the transducer.
Fig. 3. Directivity pattern of a cylindrical PVDF transducer, R=5mm, g,=I.lrad, 1=30mm,f=6OkHz: (a) xz-area, (b) yz-area. + + + , Measurement;p, calculation.
Figure 3 shows directivity patterns of a cylindrically curved PVDF transducer as illustrated in Fig. l(a). In the xz-area the acoustic aperture angle is about 40 degree. In the yz-area the aperture is very small if 1% A(1 = wavelength of sound in the propagation medium). A multiple curved transducer as shown in Fig. l(b) supplies a sharp directivity pattern in both the xz- and yz-areas. 3. Multielement Transducer Figure 4(a) shows an example of a linear curved transducer using multielement piezoelectric polymer foil [ 31. For every single element I < 2, so in the yz-area the aperture angle of a single element is large. The electric parallel switching of all single elements effects a sharp directivity pattern in the yz-area, as
4. Echo Profile Evaluation The application of signal-processing techniques requires the analog-digital conversion and storage of the received acoustic signal which is called the ‘echo profile’. Because of the limited digital memory, short pulses from broad-band transducers are advantageous. Figure 5 shows the experimental arrangement for the investigation of objects with a characteristic surface profile. An example of an echo signal s(t) is illustrated in Fig. 6(b). The signal s(t) represents the superposition of a number of time-shifted single echoes with different amplitudes corresponding to surface details of the reflector.
Fig. 4. Multielement transducer: (a) construction principle; (b) measured directivity pattern (At = k/( 16 .f)). Number of elements, 8; I = 5 mm; d =6 mm; S= 42 kHz.
illustrated in Fig. 3(b). By means of timeshifted driving of every single element, the direction of the sound radiation maximum can be controlled electronically. In the yzarea the deflection angle increases with the delay time At, see Fig. 4(b). The value of the deflection angle depends on the ratio i/d. In the receiver mode it is possible to use only one single element with a large aperture angle. Otherwise time-shifted superposition of the output signals from every single element effects a higher signal-to-noise ratio and better local resolution, but more complex electronic circuitry is necessary. The directivity pattern which will be obtained using all transducer elements corresponds to the transmitter characteristic given in Fig. 4(b). A linear multielement transducer enables objects to be scanned in a defined area with a sharp ultrasonic beam and without mechanical motion of the transducer. The parallel arrangement of linear multielement transducers leads to an array structure which is the base for ultrasonic scanning within a limited solid angle.
Fig. 5. Experimental arrangement. hl = 10 mm, h2 = 4 mm, h, = 300 mm.
Fig. 6. Echo profile evaluation (ultrasonic frequency f= 65 kHz). (a) Echo of a plane reflector; (b) echo of an object corresponding to Fig. 5; (c) deconvoluted signal. Results: h, = 10.4 mm, h, = 4.4 mm.
The acoustic transmission system is assumed to be linear, so s(t) can be written as s(t) = 1 ai . ~~(t - ti) where s,(t) = echo of a plane reflector, see Fig. 6(a), ai = amplitude coefficient of partial reflector i, ti = travel time corresponding to partial reflector i. The signal s(t) can also be written as a convolution of a sequence of time-shifted Dirac-like pulses with weighting coefficients ai and the echo s,(t): s(t) =
c a, . d(t
In Fig. 6(c) the deconvoluted sequence ~~(8) corresponding to the echo profile s(t) is illustrated. The axial resolution of neighbouring partial reflectors increases with the ultrasonic frequency and the bandwidth of the spectral weighting function F(f). That is why broad-band transducers are very suitable for this application. The maximum resolution of surface details in the axial direction is about A/2 if a PVDF transducer is used. The resolution of lateral details requires scanning of the object. For this purpose the multielement transducers described above can be used. References
*so(t) = SR(t)*S&)
The function sR(t) represents an image of the reflector’s surface within the aperture angle of the transducer. The method of evaluating the sequence +(t) is described elsewhere 121.After Fourier transformation, division of the complex spectral functions S(f) /S,( f) and multiplication with a weighting function F(S), the signal sR(t) is obtained by inverse Fourier transformation.
1 W. Manthey and N. Kroemer, Ultraschallsensoren auf der Basis piezoelektrischer Polymere, Tech. Mess., 56 (1989) 377-384. 2 P. Kleinschmidt and V. MBgori, Ultrasonic robotic-
sensors for exact short range distance measurement and object identification, Proc. IEEE Ulrrusonics, Symp., San Francisco, CA, U.S.A., Oct. 1985, pp. 457-462. 3 N. Kroemer, W. Manthey, W. Kiinstler, R. Danz
and D. Geiss, Ultrasonic transducers using piezoelectric PVDF films, Proc. 6th Int. Symp. Elecu+efs, Oxford, 1988, pp. 379-384.