| Flexible pressure sensor based on polyvinylidene fluoride（PVDF）piezoelectric membrane has high flexibility and piezoelectric effect,moderate wave impedance and elastic modulus,easy to process and install,high integration and simple test circuit,it is widely used in intelligent wear equipment,energy capture device,health monitoring and explosion shock pressure measurement.However,studies have shown that when the flexible pressure sensor with small overall thickness is used for accurate pressure measurement,its measurement performance is affected by many factors such as packaging method,process,material and loading method,but the research on the influence law and mechanism is not sufficient.In addition,the current research on the measurement of explosion shock pressure by PVDF sensor is mainly concentrated in the high pressure range of 101 ～ 103 MPa,and the measurement of ～ 10-1 MPa shock pressure on the structure / personnel surface is very scarce.In this paper,aiming at the problem of flexible shock wave pressure measurement for single soldier wearable equipment,firstly,the influence of different packaging and loading methods on the measurement characteristics of the pressure sensor is studied.Then,based on shock wave calibration experiment and composite piezoelectric effect,a portable,high sensitivity and high precision explosion shock wave sensor is proposed and developed.Finally,the mechanical response of the pressure sensor is studied based on theoretical analysis and numerical simulation.The main research contents and conclusions are as follows :Firstly,CPT sensor with flexible conductive tape as electrode,FCP sensor with polyimide and copper foil as electrode and SLP sensor with strong integrity prepared by local polarization were prepared by traditional sandwich packaging method.The stress and sensitivity coefficients of the three sensors in the range of 10 ～ 400 MPa were calibrated by Hopkinson pressure bar device.The calibration results of the sensitivity coefficients of the three sensors were 35.1 ± 0.6p C / N,32 ± 3.5p C / N and 24.4 ± 0.7p C / N,respectively.The factors affecting the sensitivity coefficient of the sensor were numerically simulated by LSDYNA.The results show that the PVDF piezoelectric membrane element is in onedimensional strain state in the SHPB calibration experiment,and the influence of the internal flatness of the sensor,the packaging thickness and the core material on the measured waveform of the pressure gauge is quantitatively studied.Then,shock waves were loaded on commercial and self-made sensors through shock tube in the pressure range of 0.2 – 0.78 MPa,and the shock wave measurement sensitivity of the sensors was calibrated.The results show that : 1)Large packaging thickness will lead to prominent reflection characteristics of shock wave between layers,resulting in high frequency oscillation of pressure curve;the larger sensitive element area leads to the slow induction of the rising edge of the shock wave and the poor measurement accuracy of the shock wave step characteristics.2)The shock wave measurement sensitivity and dispersion of FCP and SLP sensors are much higher than the SHPB experimental stress wave calibration sensitivity,which are about 75 ～ 300 p C / N and 175 ～ 550 p C / N,respectively,and decrease with the increase of pressure.The charge output shows a nonlinear relationship with the shock wave intensity.The analysis shows that the phenomenon is mainly related to the ultra-high strain rate loading of shock wave and the nonlinear characteristics of mechanical and electrical response of polymer piezoelectric materials in low pressure range.3)By increasing the sensitive element of CPT sensor to 8 mm,the shock wave measurement sensitivity of CPT sensor is more stable than that of FCP and SLP,which is about 10 ～ 40 p C / N,which is consistent with the stress wave calibration results of SHPB experiment.The linearity and dispersion of the mechanical and electrical response of CPT sensor are better than those of FCP and SLP.However,large packaging thickness leads to more high frequency oscillations in the rising edge and exponential decline edge of shock wave measurement.Finally,in view of the strong nonlinearity of PVDF electro-mechanical response under the action of low amplitude shock wave,the theoretical analysis verifies the pressure measurement idea of using the flexible pressure gauge to generate high-order in-plane tensile stress by the way of out-of-plane deflection deformation.The shock wave measurement element with the working mode of composite piezoelectric effect is designed and prepared.The in-plane tensile electromechanical response sensitivity of the flexible sensor was measured by an equal strength cantilever beam and a modified Hopkinson tension bar.The results show that the d31 and d32 of the sensor are 33 p C / N and 3 p C / N,respectively.The measurement results are close to the data in the literature.The shock wave measurement sensitivity of DSP and SLP sensors with in-plane tensile stress working mode measured by shock wave experiment is 882.0 ± 86 p C / N and 1134.3 ± 109.6 p C / N,respectively.The discreteness and individual difference are small,and the linearity of electro-mechanical response is high.Moreover,experiments show that the sensitivity and pulse width of pressure sensor shock wave measurement can be further optimized by adjusting the thickness of the sensor,the packaging material and the diameter of the flexible deformation area.Finally,the mechanism of high sensitivity of pressure sensor shock wave measurement is explained by numerical simulation.The numerical simulation results are consistent with the experimental and theoretical analysis results.The flexible shock wave measurement device with the working principle of composite piezoelectric effect proposed and designed in this paper can provide new technical support for the high integration and intelligence of portable shock wave detection equipment. |
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