# Piezoelectric Sensor with a Helical Structure on the Thread Core
## 4 Results
### Output Voltage of the Smart Textile According to Bending Input Variation  
First, the electrical response of the smart textile was investigated with a change in \(\Phi\). Flexural loadings were applied to the smart textile under identical conditions of the piezoelectric sensor. Figure 7 shows the relationship between the sensor output voltage and the bending angle. The \(\Phi_{0}\)-\(V_{0}\) relation of the smart textile was also modeled with the power function using Equation (1) and had \(\psi\) of 5.354 and \(\eta\) of 1.263. The modeled result showed high resemblance to the experimental result, with an R-squared value of 0.9981. Thus, the flexural loading applied to the smart textile can also be predicted by the  
Figure 5: Peak-to-peak bending angle according to the peak-to-peak voltage of the sensor. The black squares, red circles, green upward-pointing triangles, and blue downward-pointing triangles represent the voltage outputs of the sensor at the operating frequencies of 0.8, 1.2, 2, and 3 Hz, respectively. The red and purple solid lines represent the power-fitted bending angle and linear fitted bending angle within the bending angle range from 5\({}^{\circ}\) to 29\({}^{\circ}\) according to the output voltage at \(f=1.2\) Hz, respectively.  
Figure 6: Peak-to-peak voltage of the sensor according to the operating frequency. The maximum, minimum, and averaged peak-to-peak voltages of the repetitive experiments are represented as error bars. The black, red, and blue squares represent the peak-to-peak voltages according to the operating frequency at \(\Phi\) of 10\({}^{\circ}\), 20\({}^{\circ}\), and 29\({}^{\circ}\), respectively.  
power function. The results indicated that the sensing output of the piezoelectric sensor had a higher voltage level compared with the sensing output of the smart textile under the same flexural loading. This can be attributed to the fact that the surrounding threads of the smart textiles might undermine the strain applied to the sensor. The sensitivity and resolution of the smart textile were also studied under an identical dynamic range with the textile-based piezoelectric sensor, that is when flexural loadings having a range of 5\({}^{\circ}\)\(\leq\)\(\Phi_{0}\)\(\leq\) 29\({}^{\circ}\) were applied to the smart textile. The experimental result of the smart textile under the dynamic range showed a good match with the linearly fitted result with an R-squared value of 0.9971. Additionally, the sensitivity and resolution of smart textile were 8.501\({}^{\circ}\)/V and 2.6 \(\times\) 10\({}^{-3}\)\({}^{\circ}\), respectively.  
In addition, we observed the electrical response of the smart textile according to the variation in the operating frequency. Figure 8 shows the peak-to-peak voltage of the smart textile according to the operating frequency. Therein, the variation in frequency also had no effect. The maximum difference with the variation in the frequency was 3%. We believe that the woven structure surrounding the sensor acted as a damping system [41], resulting in stabilizing the sensing output of the smart textile.  
Figure 8: Peak-to-peak voltage of the smart textile according to the operating frequency. The maximum, minimum, and averaged peak-to-peak voltages of the repetitive experiments are represented as error bars. The black, red, and blue squares represent the peak-to-peak voltages according to the operating frequency at \(\Phi\) of 10\({}^{\circ}\), 20\({}^{\circ}\), and 29\({}^{\circ}\), respectively.  
Figure 7: Peak-to-peak bending angle according to the peak-to-peak voltage of the smart textile. Black squares, red circles, green upward-pointing triangles, and blue downward-pointing triangles represent the voltage output of the sensor at the operating frequency of 0.8, 1.2, 2, and 3 Hz, respectively. The red and purple solid lines represent the power-fitted bending angle and linear fitted bending angle within the bending angle range from 5\({}^{\circ}\) to 29\({}^{\circ}\) according to the output voltage at \(f=1.2\) Hz, respectively.