Influência da geometria e da impedância do sistema de medição na resposta em frequência de filmes finos de PVDF sujeitos a ondas de tensão longitudinal

Abstract

Films of Polyvinylidene fluoride (PVDF), when covered with silver ink, constitute piezoelectric sensors that can be used to detect and measure longitudinal stress waves on structures. Such films can be cut with different geometric dimensions and the final format of the film can influence its own frequency response curve. The objective of this work is to evaluate the influence of the geometrical dimensions of PVDF thin film sensors and electrical impedance of the measurement equipment used into the frequency response of a measurement system used to detect sinusoidal stress waves actuating on a structure. To develop this analysis, a mathematical model was used to simulate the mechanical solicitation, the piezoelectric response of the sensor, the relation between the frequency of the solicitation and the electric output impedance of the sensor and its impact into the signal obtained using an oscilloscope as electrical measurement equipment. Results showed that reduction on length of sensor increased the useful frequency bandwidth for measurement of stress waves and also increased the upper cut-off frequency of operation of the measurement system, allowing its use for measurement of stress waves with higher frequencies. The increase of length and width of the sensor and decrease of its thickness reduced the lower cut-off frequency of the measurement system, improving the detection of stress waves with lower frequencies. Simulations related to the electric output impedance of this sensor showed that the loading effect can significantly affect the amplitude of the measured signal over the frequency bandwidth. For the standard PVDF sensor and typical measurement equipment with 1MΩ of electrical input impedance, the signal attenuation at 1 Hz can be higher than 40dB and the lower cut-off frequency of the measurement system can be higher than 200 Hz . Such factor, if not considered, can generate errors related to the measurement of amplitude of stress waves with lower frequencies propagating on the structure.