Design And Characterization Of Semi-elliptical Flexible Base PVDF Piezoelectric Sensor

Sensors used for wearable devices should have both good flexibility and high sensitivity at the same time,the PVDF piezoelectric film can meet the requirements of the above two points at the same time with its excellent performance.Many studies have suggested that PVDF film sensors with curved flexible substrate have higher sensitivity than those with plannar base.If we change on the plane to round as a gradual process,there are half an ellipse,so this paper presents a half elliptical PVDF film sensor structure of flexible base.In different applications,sensors are classified into equal chord lengths and equal arc lengths depending on the constraints.This paper mainly studies the influence of the size of the semi-elliptical flexible substrate on the sensitivity of the PVDF film sensor under the concentrated force.In this paper,the strain distribution of the base surface is simulated by ANSYS under the effect of 1N concentrated force.The simulation results show that when the length of the short axis b is within a large range,the average strain generated when the semielliptical flexible substrate is subjected to concentrated forces is higher than that of the semi-circular flexible substrate,indicating that the semi-elliptical flexible substrate will increase the sensitivity of the sensor.Then,the output voltages of the two types of sensors,such as equal chord length and equal arc length,were simulated.It was found that the semi-circular and semi-elliptical flexible base PVDF film sensor output voltage is higher than that of the planar sensor,and under the appropriate substrate size,the output voltage of the semi-elliptical flexible base PVDF sensor is higher than that of the semicircle sensor;the output voltage of the semi-oval base PVDF film sensor increases first and then decreases with the increase of the short axis b.When the long sensor obtains the maximum voltage,a/b=2/1,when the arc length sensor obtains the maximum voltage,a/b=9/5.The smaller the size of the flexible substrate,the greater the output of the semi-elliptical and semicircular sensors.In this paper,the size combinations of the long half shaft a and the short half shaft b are(3mm,1mm),(3mm,1.5mm),(3mm,2mm),(4mm,2.5mm),(5mm,3mm).The semielliptical PDMS-based PVDF thin film sensors were respectively numbered 1-5,and the voltage sensitivity and frequency response of each sensor were tested.From the experimental results,it can be seen that the voltage sensitivity of the five sensors is close to the ANSYS simulation result,with good low frequency characteristics,and the No.2 sensor has the highest voltage sensitivity.Then,three pulse sensors No.2,No.4 and No.5 were used to measure the pulse of human body.The measurement results also showed that No.2 sensor had the best measurement effect and had the largest pulse peak.However,during the measurement process,the degree of tightness of the used wristband has a great influence on the measurement result.Only when the wristband has appropriate tightness,the human pulse image has distinct peaks and troughs and clear details.To sum up,in order to make the semi-elliptical PDMS substrate PVDF thin film sensor have higher voltage sensitivity when it is subjected to concentrated force,the semielliptical flexible substrate design rule is: the chord length sensor is in the range of 2~8mm for the long semiaxial a.The length of the long axis a should be twice the length of the short axis b.For the constant arc length sensor with the PVDF film length of 2? to 8?mm,the length of the long axis a should be 1.8 times that of the short axis b.