Patent GB-1423061-A (GB)
Title: Acoustic signal sensing arrangement
IPC: G01N29/24
Filed: 1973-06-05
Abstract: 1423061 Ultrasonic probes WESTING- HOUSE ELECTRIC CORP 5 June 1973 [20 June 1972] 26665/73 Heading H4J A transducer arrangement (Fig. 1) for monitoring metal flaws has a tubular housing 10 with a metal window 22 forming an end wall. A piezoelectric element 12 is acoustically coupled to window 22. An electrode 14 is coupled to the piezoelectric element 12. Electrode 14 and piezoelectric element 12 are electrically insulated from the side wall of tubular housing 10. A layer 20, preferably silicone, seals the piezoelectric element 12 from moisture. Piezoelectric element resonances and spurious housing resonances are damped. A ceramic potting compound 18, or other compound having a porous structure, or a formed ceramic or plastics cup performs the electrically insulating and damping function. Housing 10 and window 22 are preferably of stainless steel. In nuclear applications the housing material has a radioactive half life of about one month. Element 12 is a disc of lead-zirconate-titanate, lead metaniobate or bismuth-strontium-titanate and its thickness and radial resonance frequencies are approximately the same. Electrode 14 is of oxygen-free copper. Housing 10, element 12 and electrode 14 are bonded together with conductive epoxy resin or with a mixture of 3 parts silver to 1 part polyimide cement. Alternatively ultrasonically aided thermo-compression bonding, soldering or brazing using induction can be employed. A complete assembly (Fig. 2) has a stainless steel housing 34 welded to the rear of the tubular housing 10 (Fig. 1). Nickel-plated copper wire 26 with fibreglass braided insulation 28 leads to a transformer 30 having the primary earthed to the housing 34. The secondary is earthed to the outer terminal of a coaxial connector 32, the two earths being isolated by an insulating washer 35. Preferably the window 22 is bonded to a test vessel using an adhesive film (38) Fig. 3 (not shown). In an alternative transducer (Fig. 6) the housing 10, piezoelectric element 12 and electrode 14 are insulated by a cup 16. Window 22 is welded to housing 10. An electrode 22 is brazed to housing 10 for connection to a twisted pair 58. An electrostatic shield 62 and acoustic window 64 protect against electrostatic and electromagnetic interference. An aluminium oxide disc 44 interposed between windows 22 and 64 provides good acoustic coupling. A plastics cup 68 aids in damping housing resonances. Window 64 has a pedestal 70 for contacting the specimen. For high temperature applications a quartz standoff of truncated cone shape has a recess or raised legs 78 of height equal to that of the pedestal. The standoff is one half wavelength long at the frequency used system. The transducer (Fig. 4, not shown) is connected to a 50 ohm transmission line through an isolation and impedance matching transformer. The far end of the line is terminated into a 50 ohm resistor and is coupled to a preamplifier by an impedance matching transformer. The amplified signal is available for spectrum analysis or for counting events per unit time and/or total events. Alternatively (Fig. 5, not shown) there is an electrostatic shield (42) Fig. 5 (not shown) earthed at the test specimen and re-amplifier and embracing the transducer, two transformers and intervening connections. The primary of a first transformer (46) is coupled through twisted leads (58) to the transducer for minimizing electromagnetic interference. The centre tap (48) of the transformer (46) is earthed. A second pair of twisted leads (60) extends to a resistor (50) connected across the primary of a transformer 54. The centre tap (52) of its primary is earthed. The secondary is connected directly to the pre-amplifier.