Resonance transducer

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

The output signal approaches that of a non-resonant transducer while retaining the versatility and capability of measuring long duration events... 

ZnO is a wide band gap semiconductor, which is used for various applications. Based on textured ZnO films one can build highly effective piezo field emitters. On the other hand ZnO is a very effective electron-excited phosphor. ZnO films easily withstand electron fluence more than 1 W/cm. ZnO films doped with Al, Ga, or In have a low resistivity of about 10 " Qcm and a high transparency of about 90%. This is sufficient for applications as a front contact in solar cells, liquid crystal displays etc. Dielectric ZnO films have a high electromechanical coupling factor that allow using ZnO in various surface acoustic wave (SAW) devices such as delay lines, delay-line filters, resonators, transducers and SAW convolvers. 

In pulse-echo-based techniques, the time of flight in a sample cannot be determined simply from the observation of the time span between adjacent echoes in the echo pattern if plane parallel transducers operated at resonant frequencies are employed. Transducers introduce substantial errors if the velocity is derived from such measurements, especially if relatively short samples are used. Various correction approaches have so far been developed in order to consider the influence of resonant transducers and the effects of diffraction [31-33]. The need for corrections can be avoided and a broad operational bandwidth obtained by using short pulses of duration equal to or shorter than the transduction [34] this requires a time resolution better than the transit time in the transducer. This short-pulse excitation (e.g. the maximum for a 10-MHz transducer is 50 ns) requires a high-power wide-band ultra-linear amplifier to ensure the detection of US signals with sufficient resolution under non-resonant conditions. 

Guckel, H. Sniegowski, J.J. Christenson, T.R. Raissi, F. The application of fine-grained polysilicon to mechanically resonant transducers. Sens. Actuators A 1990, 346-351. 

Mainly electrochemical (amperometric, potentiometric, impedimetric, or conductometric) and optical (IR, Raman, fluorescence, absorption, reflection, evanescence field, or surface plasmon resonance) transducers are used as the basis for biosensors. However, beside these there are other, less often employed transducers that make use of the piezoelectric effect, surface acoustic waves, or detection of heat generated in enzyme reactions [40, 41]. In the context of this work, the focus is on the specific features of electrochemical transducers. An overview showing the different fields of apphcation can be found in Sect. 2.11.1.5 (Table 2). 

Graphene has been demonstrated to act as a resonator, transducer, signal mixer, and high sensitivity and selectivity sensors, thanks to the exceptional intrinsic electrical, mechanical and thermal properties for applications in sensing science as an optimum material for each different type of sensor the convenient fabrication of the devices, and atomic cross section allow graphene to be the perfect material for measurement of chemical and physical parameters. 

There are two main types of transducer. The most sensitive is the resonant transducer, which has its maximum sensitivity at the resonance frequency. Typical sensors have resonances in the range 70-600 kHz. 

The signal from a resonant transducer resembles an AM radio signal. It has a carrier wave, at the resonance frequency of the transducer, which is amplitude modulated by the process. The information about the process is in the modulation envelope. An RMS-to-DC (root mean square-to-direct current) converter is used to demodulate the signal. The output of this device is the amplitude of the envelope. 

However, most sensors used currently in AE applications for concrete are manufactured in a more traditional way, showing either a resonant behavior or several particular resonances. These sensors, which are called multi-resonance transducers, have a higher sensitivity than sensors with a backward mass used outside of their resonance fiequency. Such sensors should not, however, be considered as (true) broadband and it is essential to know their frequency response function. Otherwise, signal characteristics from the source are not distinguishable from artifacts introduced by incorrect knowledge of the frequency response. A calibration of the sensors frequency response, as well as understanding of the direction sensitivity, is important for many applications of AET. 

The zone location method is frequently used to monitor large structures such as buildings, pipes, vessels, etc. Sensors are distributed over a wide area on the surface of the structure and/or concentrated at the most critical locations. To enhance the detection radius, resonant transducers, which are more sensitive than broadband sensors, are the first choice. Fig. 5.7 shows an example of a sensor configuration used to monitor leaks in pipes. The leak or failure must be within the detection range of the sensor, to allow the AE to be detected. If AE is recorded by a particular sensor, the technician should inspect the vicinity next to this sensor for leaks or cracks. 

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