Re: To all: Is there any disadvantage for composite transducers?

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Re: To all: Is there any disadvantage for composite transducers?
Posted by: Bill Grandia , E-mail: wgrandia@aol.com, on September 24, 1996 at 08:51:11:
In Reply to: To all: Is there any disadvantage for composite transducers? posted by : Rolf D. on September 21, 1996 at 13:29:08:
The composite transducer has both advantages and disadvantages. The positive aspects were outlined by G. Fleury and G. Splitt. There are some applications were this extremely nice device is less desirable. To realize what the disadvantages are, requires to analyze the differences in characteristics as compared to conventional transducers.
The name composite transducers means that a solid piezo-electric element has been modified. There a basically two ways to construct a composite element.
a) A solid piezo disk can be broken in small pieces or granules, whereby the spaces between the granules are filled with epoxy. A preferred filler seems to be Spurr epoxy, which in liquid state has a very low viscosity (like water).
A focused composite element can simply be produced by inserting a solid polarized element between concave and convex shaped press-mold halves, hereby submerging the entire mold in liquid epoxy. This method allows the epoxy to flow into the crevices and prevents air from entering into the crushed structure. After the epoxy is cured, the dish-shaped element can be taken out of the mold. Of course, the electrodes need to be restored. There is no need to re-polarize the element, since the granules are staying in their original alignment.
This construction is less popular, since the fill factor for the element is practically fixed. The only feature is that the radial to the thickness mode becomes uncoupled. By this, the effectiveness of the thickness mode vibration is enhanced. There is virtually no reduction in acoustic impedance and dielectric constant. The bandwidth of the element is becomes increased. When a high "Q" element such as PZT5A is used, the signal response of the crushed version more or less behaves like a lead metaniobate element, except that the sensitivity is stronger.
b) A better method is the dicing approach, whereby the element is cut into small pieces and has been described in detail by G. Fleury and G.Splitt.
We need to discuss the disadvantages.
1: The composite element is not desired when a high "Q: transducer is necessary. A "high "Q" means that the transducer has a narrow bandwidth and therefor produces a large amount of ringing. The penetration power of a high Q transducer is strong. When highly attenuative materials are to be inspected, a strong signal is required. Sometimes this can be enhanced by applying a toneburst signal with a frequency equal to the series resonance frequency of the element. At this resonant condition, the electrical impedance of the device becomes minim and therefor the generated sound pressure into the material is strongly enhanced. This is not possible with diced elements. From a receiving standpoint, a strong signal can also be obtained in the narrow band mode. The narrow band application is very popular for the through transmission technique.
2: The composite transducer is less suited as a direct contact transducer for the inspection of steel or ceramics, especially when the transducer needs to be wear resistant.
The lowered acoustical impedance is not impedance matched to the ceramic wearplate or the steel surface.
3: Another characteristic of composite transducers it that the capacitance of the element reduces with the aspect ratio of the total area of the diced elements and the filler epoxy. When this ratio becomes high while trying to make a low acoustic impedance (like good matching to water), the electrical charge produced on the diced element from a reflected signal becomes small. When a long coaxial cable is connected to the transducer, the signal amplitude becomes small. The cable acts now like a parallel capacitance, hereby absorbing the signal to an appreciable extend. Typically, the capacitance of a coaxial cable is 31 pFd / foot. A 10 foot cable therefor is 310 pFd. If the element capacitance is around 300 pFd, the signal amplitude drops to half the value. Of coarse, the can be overcome by using an high input impedance low noise pre-amplifier located very close by the transducer. It is possible to use such pre-amplifiers in the pulse echo mode by inserting an isolation circuit to protect the amplifier from the large damaging transmitting pulse. The use of such an amplifier greatly enhances the performance of the composite transducer by preserving the high resolution wave shape.
This sumps up the few disadvantages of the composite transducers.
Bill Grandia

Follow Ups:

Re: To all: Is there any disadvantage for composite transducers? Gerhard Splitt 16:53:50 10/01/96 (size: 1457) (0)

Re: To all: Is there any disadvantage for composite transducers? Fleury Gerard 20:44:15 9/28/96 (size: 5237) (0)

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: The composite transducer has both advantages and disadvantages. The positive aspects were outlined by G. Fleury and G. Splitt. There are some applications were this extremely nice device is less desirable. To realize what the disadvantages are, requires to analyze the differences in characteristics as compared to conventional transducers. : The name composite transducers means that a solid piezo-electric element has been modified. There a basically two ways to construct a composite element. : a) A solid piezo disk can be broken in small pieces or granules, whereby the spaces between the granules are filled with epoxy. A preferred filler seems to be Spurr epoxy, which in liquid state has a very low viscosity (like water). : A focused composite element can simply be produced by inserting a solid polarized element between concave and convex shaped press-mold halves, hereby submerging the entire mold in liquid epoxy. This method allows the epoxy to flow into the crevices and prevents air from entering into the crushed structure. After the epoxy is cured, the dish-shaped element can be taken out of the mold. Of course, the electrodes need to be restored. There is no need to re-polarize the element, since the granules are staying in their original alignment. : This construction is less popular, since the fill factor for the element is practically fixed. The only feature is that the radial to the thickness mode becomes uncoupled. By this, the effectiveness of the thickness mode vibration is enhanced. There is virtually no reduction in acoustic impedance and dielectric constant. The bandwidth of the element is becomes increased. When a high "Q" element such as PZT5A is used, the signal response of the crushed version more or less behaves like a lead metaniobate element, except that the sensitivity is stronger. : b) A better method is the dicing approach, whereby the element is cut into small pieces and has been described in detail by G. Fleury and G.Splitt. : We need to discuss the disadvantages. : 1: The composite element is not desired when a high "Q: transducer is necessary. A "high "Q" means that the transducer has a narrow bandwidth and therefor produces a large amount of ringing. The penetration power of a high Q transducer is strong. When highly attenuative materials are to be inspected, a strong signal is required. Sometimes this can be enhanced by applying a toneburst signal with a frequency equal to the series resonance frequency of the element. At this resonant condition, the electrical impedance of the device becomes minim and therefor the generated sound pressure into the material is strongly enhanced. This is not possible with diced elements. From a receiving standpoint, a strong signal can also be obtained in the narrow band mode. The narrow band application is very popular for the through transmission technique. : 2: The composite transducer is less suited as a direct contact transducer for the inspection of steel or ceramics, especially when the transducer needs to be wear resistant. : The lowered acoustical impedance is not impedance matched to the ceramic wearplate or the steel surface. : 3: Another characteristic of composite transducers it that the capacitance of the element reduces with the aspect ratio of the total area of the diced elements and the filler epoxy. When this ratio becomes high while trying to make a low acoustic impedance (like good matching to water), the electrical charge produced on the diced element from a reflected signal becomes small. When a long coaxial cable is connected to the transducer, the signal amplitude becomes small. The cable acts now like a parallel capacitance, hereby absorbing the signal to an appreciable extend. Typically, the capacitance of a coaxial cable is 31 pFd / foot. A 10 foot cable therefor is 310 pFd. If the element capacitance is around 300 pFd, the signal amplitude drops to half the value. Of coarse, the can be overcome by using an high input impedance low noise pre-amplifier located very close by the transducer. It is possible to use such pre-amplifiers in the pulse echo mode by inserting an isolation circuit to protect the amplifier from the large damaging transmitting pulse. The use of such an amplifier greatly enhances the performance of the composite transducer by preserving the high resolution wave shape. : This sumps up the few disadvantages of the composite transducers. : Bill Grandia

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