Modelling of contact deformation for a pinch gripper in automated material handling

Grasping analysis is concerned with relating the characteristics of the grasped object to the requirements of the gripper structure. When a gripper presses a flexible specimen with a size larger than that of the gripping surface, complex deformations are produced (compression, shearing, bending and tension). The boundary conditions are complex too because of the changing compressed region and, for many grippers, a changeable contact area. In this paper a mathematical predictive model is presented for simulating these complex deformations, relating a flat gripper’s performance to the properties of flexible gripped materials using elasticity theory. In particular, the distributions of stress and strain within the materials are derived when the gripper presses them. The main variables in the picking-up action have been identified. Moreover, a contact solution has been derived for calculating the contact length between a sample and a rigid support table. The change in the compressed region within a sample as a function of external load has been calculated. In order to match experimental behaviour the non-linear elastic response of the flexible material and the large deformations have to be incorporated in the model. The model predictions have been tested against experimental data and the results of finite element analysis and reasonably good agreement has been obtained.