# Flexible Shear and Normal Force Sensor Using only One Layer of Polyvinylidene Fluoride Film
## 5 Conclusions  
We have developed a flexible piezoelectric sensor that can sense shear and normal forces and can be fabricated using only one layer of PVDF film. This sensor with a \(2\times 2\) array consists of a PVDF film sealed with a 3D structured PDMS. In particular, this sensor is advantageous in terms of the fabrication process because it can be fabricated by simply patterning the electrode with a laser cutting machine on a commercial PVDF film without any complicated processes. Through the results of the data processing of the sensor output voltages, we obtained a value proportional to the magnitude of the applied force. In particular, the signals generated by the three sensors (P1, P2, and P3) in a single module exhibited different phases, and, therefore, the direction of the applied force could be derived. In addition, since the peak value of the processed value was proportional to the pressure applied to the sensor, the magnitude of the force could also be derived using the peak value after calibrating the sensor. Moreover, we studied the morphology of the PVDF film of one domain by using a finite element method when a force was applied, and the results showed that the difference in sensitivity between the P1 and P3 sensors was due to the connection by one domain of the PVDF film. Furthermore, we presented a system using this sensor array to sense the force applied to the sensor in real time. When unknown forces were applied to the sensor, the system displayed the direction and magnitude of the force in real time on a computer screen. We demonstrated how to make piezoelectric force sensors in a very simple fabrication process that can detect not only the normal but also the shear forces. Previous studies have been able to detect three-axis forces, but electrode patterning is essential, which involves complex processes such as lithography and sputtering [21; 25; 26]. In our method, this work makes it possible to detect biaxial forces simply by embedding a commercial piezoelectric film into the PDMS. Therefore, the main contributions of this work are: (i) developing a design of flexible piezoelectric force sensors capable of detecting shear and normal forces; and (ii) developing a very simple fabrication method for the shear and normal force sensor.  
**Supplementary Materials:** The following are available online at [[http://www.mdpi.com/2076-3417/9/20/4339/s1](http://www.mdpi.com/2076-3417/9/20/4339/s1)]([http://www.mdpi.com/2076-3417/9/20/4339/s1](http://www.mdpi.com/2076-3417/9/20/4339/s1)), Video S1: Test of the real time force sensing system.  
**Author Contributions:** [PERSON] performed the experiment. [PERSON] programmed the real time sensing system. [PERSON] contributed to the setup of the experiment. [PERSON] and [PERSON] analyzed the data and wrote the paper.  
Figure 8: Test of the real time force sensing system. **(a)** Configuration of the window indicating the magnitude and direction of the applied force. Measured direction and magnitude of the force components (\(F_{x}\) and \(F_{z}\)) when (**b**) a shear force and (**c**) a normal force is applied. The red arrows indicate the direction of the applied force.  
**Funding:** This research was supported by the Technology Innovation Program for Development of robotic work control technology capable of grasping and manipulating various objects in everyday life environment based on multimodal recognition and using tools funded by the Ministry of Trade, Industry and Energy under grant number 20001856.  
**Conflicts of Interest:** The authors declare no conflict of interest.