FBG-Based Methods for Monitoring Critical Slip Point During Flexible Fingers Clamping

Clamping is a necessary ability to perform operational tasks for flexible fingers. However, when flexible fingers clamp an object, an excessive contact force can damage it, and a small contact force can cause it to slide. Therefore, real-time monitoring of the contact state is the key to clamp the object stably and nondestructively. This article develops a monitoring system for the contact state of flexible fingers based on fiber Bragg grating (FBG). The mapping relationship between the FBG wavelength shift and the normal contact force derived from the fingertip contact model can determine the critical slip point (CSP) during the clamping process. Algorithms based on wavelet transform and maximum root mean square difference (MRD) are proposed to monitor and predict the CSP (MPCSP) in FBG wavelength signal. Experiments in flexible fingers clamping objects with different shapes, stiffnesses, and masses show that the critical and predicted slip points are about 0.4 and 1.0 s earlier than the sliding. Moreover, feedback and adjustment of the contact force at the critical and predicted slip points can realize the stable clamping. The research results have important guiding significance on the feedback control of the nondestructive grasping for soft fingers.