Built-in Sensor System for Monitoring Internal Shear Strain and Stress Distribution in Soft Materials

Soft robotics require flexible sensors that can realize sensitive measurements for which rigid sensors are not viable. Precise and quantitative measurement of the mechanical phenomena is necessary in all the sensors; however, in the field of soft robotics, the complex structure and physical properties of composite materials complicate the construction of a precise sensing model. In this study, a built-in sensor system for internal shear strain and stress distribution measurement is developed by embedding piezoelectric polyvinylidene fluoride polymer films within the objective soft material. Shear-strain sensing models are established by relating the piezoelectricity and cantilever bending mechanics. For validation, experiments are conducted in which the deflection of the embedded film sensors is monitored and digitized using a video camera. The shear-strain sensing model is validated by evaluating the agreement between the modeled and measured physical variations, and a high coefficient of determination of 0.974 is obtained. In addition, based on the linear relation of the arithmetic expression of each sensing model, calibrated results for internal shear strain and stress sensing are successfully obtained. Furthermore, the measurement effect of this type of flexible sensor and its potential applications are discussed.