A tribo-piezoelectric coupled sensor for force and slip detection in soft grippers

Applying the minimum grasping force to hold fragile objects without slippage is a critical challenge for non-destructive and stable manipulation by soft grippers. Currently, force measurement in soft grippers is mainly based on resistive sensors, which suffer from lower sensitivity or limited linear range. Moreover, limited research has been conducted on slip detection and the minimum grasping force determination on the bending contact surface of soft grippers with contact-driven deformation. In this manuscript, a force and slip detection sensor (FSS) based on tribo-piezoelectric coupled nanogenerators (TPENG) is proposed to determine the minimum grasping force for objects through the slipping threshold in soft grippers. It is shown that in the bending operation state of the soft gripper, the tribo-piezoelectric coupling effect is enhanced by the flexoelectric, triboelectric effects and the piezoelectric effect with d₃₃ and d₃₁ modes, which can provide the FSS with 3.62 times higher sensitivity at a bending radius of 30 mm compared with the flat state. Next, by optimizing the elastic modulus and dimensions of the Polydimethylsiloxane (PDMS) spacers in the FSS, the linear range of the FSS is improved, achieving a high sensitivity of 4.35 V/N over a broad force range of 0–6 N. Moreover, an algorithm is designed for the FSS to simultaneously recognize force and bending radius based on the relationship between the bending radius and the sensitivity of the FSS. Finally, the slipping threshold and the minimum grasping force are determined by monitoring the contact state between the FSS and the object. The results indicate that smaller object mass, reduced surface curvature radius, and grasping method using the finger pad lead to lower slipping threshold and minimum grasping force. The FSS will have good application prospects in the intelligent perception of the environment through soft grippers.