Resonant transducer

The first second of a stress relaxation step can also show this type of ringing, but it is generally caused by the transducer itself. Thus, the first part of the data may be electronically filtered to remove the transducer ringing by setting a filter cutoff frequency of -40% of the value for the resonant frequency of the transducer and geometry. Some rheometers allow for the measurement of transducer resonant frequency when measuring the geometry inertia. 

One inch immersion transducers (Panametrics) are used with resonance frequencies of respectively 0.48 MHz., 0.70 MHz., 0.90 MHz. and 4.0 MHz. These transducers can be used for measurements between -50°C and 80°C. The two transducers shown in Figure 4.9 are special high temperature transducers (resonance frequency 1.0 MHz.) which can be used up to 200°C. 

Another transducer resonance system is the Fokker Stack Tester, developed to examine ply orientation and stacking sequence in the different layers of CFRP composites. This computer-controlled system displays the signal output in a graphical format. It consists of passing a probe into a rivet or fastener hole and scanning the material surface by a focused laser beam. The intensity of the reflected light is correlated to the orientation of each ply and can provide information on the ply number, the thickness of each ply, and fiber orientation. 

Fig. 4. Electronics block diagram for resonance-method sound-velocity measurements. D, G, and S denote driver transducer, gauge transducer, and specimen. Specimen length is adjusted so that oscillator resonance occurs near the transducer resonance frequency.

Another version of the multiple echo technique (172,173) that has been used to make very accurate sound speed measurements in pol5miers makes use of a single transducer bonded directly to the specimen (110). Pulses are reflected from the opposite face and return as echoes to the front face. By adjusting the pulse repetition rate, an echo from a later pulse can be made to overlap a multiple echo from an earlier pulse. When the repetition rate is adjusted to obtain an in-phase condition, constructive interference will result in a maximum amplitude in the superimposed signals. A value of sound speed accurate to 0.05% can be obtained by observing several repetition rates that 5deld an in-phase condition at the transducer resonant frequency and by repeating the measurements at some other frequency, eg, 10% below resonance, and on other specimens that differ only in thickness. 

Transducers twelve 375 kHz resonant transducers have been used, with a 350 kHz cutoff frequency high pass filter section and a 40 dB preamplifier. 

A resonance in the layered stracture occurs when echoes between two boundaries travel back and forth due to differences in acoustic impedances at the boundaries. For multi-layer structures a number of resonances can be observed depending on their geometry and condition. For each particular defect-free structure and given transducer we obtain a characteristic resonance pattern, an ultrasonic signature, which can be used as a reference. 

Ultrasonic pulses are generated within the sample on the transducer s resonance frequency, pass through sample and are received by the wideband receiver. After received pulse processing in the computer, results analysis is carried out. 

Bulk and surface imprinting strategies are straightforward tools to generate artificial antibodies. Combined with transducers such as QCM (quartz crystal microbalance), SAW (surface acoustic wave resonator), IDC (interdigital capacitor) or SPR (surface plasmon resonator) they yield powerful chemical sensors for a very broad range of analytes. 

For general-purpose use below 1000 Hz, a transducer can be attached to a machine by a magnetic base. Even though the resonant frequency of the transducer/magnet assembly may distort the data, this technique can be used with some success. However, since the magnet can be placed anywhere on the machine, it is difficult to guarantee that the exact location and orientation is maintained with each measurement. Shows common magnetic mounts for transducers. 

The major piezoelectric applications are sensors (pickups, keyboards, microphones, etc.), electromechanical transducers (actuators, vibrators, etc ), signal devices, and surface acoustic wave devices (resonators, traps, filters, etc ). Typical materials are ZnO, AIN, PbTiOg, LiTaOg, and Pb(Zr.Ti)03 (PZT). 

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