Active piezo sensor

With the aid of a particular class of materials (thin-film piezoelectrics), incorporation of AW devices and conventional integrated circuit components on the same silicon substrate is in fact possible. Under the proper conditions, a number of piezoelectric materials can be deposited in thin-film form, typically by RF sputtering, and retain their piezoelectric nature. For this to occur, the crystallites that grow during deposition must be predominantly oriented in a single, piezo-electrically active crystallographic direction. Two such materials are 2 0 and AIN the former has been used as an overlayer on Si wafers to fabricate all of the FPW devices studied for sensor applications to date, and also for SAW resonators. Because extremely thin piezoelectric films are readily fabricated, both ZnO and AIN have been used to make bulk resonators that operate at much higher... 

Flgnre 16.27 In situ phased array for monitoring multiple damages on an aluminum plate, (a) Circular-fence array of piezo-wafer active sensors placed at plate center (b) simultaneous imaging of multiple crack and hole damage sites on the plate [54]. 

A directional SHM transducer that does not require either phase-array delays or connection switching to achieve steering is described in Xu et al. [63]. This directional sensor achieves tuning into preferential direction at certain discrete frequencies, which are the solution of a firequency-wavenumber equation. The transducer consists of a skew array of piezo-wafer active sensors placed at pitch values d, d.2 and angles a, 5 about the 1 and 2 axes, respectively (Figure 16.29(a)). The transducer achieves directivity at certain fi equencies Figure 16.29(b) illustrates the directionality measured with SLDV on an isotropic aluminum 45° at 105 kHz, 120° at 150 kHz, — 17° at 200 kHz, 88° at 280 kHz. Thus, the transducer directionality is controlled by the excitation firequency in discrete steps. 

The principle of active sensors can be described as an indirect reaction of the sensor system to external influences. Here the environment or the object to be measured is influenced actively by a piezo transducer (e. g. by sending defined ultrasonic waves) while the reaction or the signal response is registered at the sensor position. From the signal transfer function conclusions can be drawn about the structure under examination or about external influences. 

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