Pneumatic Hyperelastic Actuators for Grasping Curved Organic Objects

Soft robotic grippers often incorporate pneumatically-driven actuators that can elastically deform to grasp delicate, curved organic objects with minimal surface damage. The complexity of the actuator geometry and the nonlinear stress–strain behavior of the stretchable material during inflation make it difficult to predict actuator performance prior to prototype fabrication. In this work, a scalable modular elastic air-driven actuator made from polydimethylsiloxane (PDMS) is developed for a mechanically compliant robotic gripper that grasps individual horticultural plants and fungi during automated harvesting. The key geometric design parameters include the expandable surface area and wall thickness of the deformable structure used to make contact with the target object. The impact of these parameters on actuator displacement is initially explored through simulation using the Mooney–Rivlin model of hyperelastic materials. In addition, several actuator prototypes with varying expandable wall thicknesses are fabricated using a multistep soft-lithography molding process and are inserted in a closed ring assembly for experimental testing. The gripper performance is evaluated in terms of contact force, contact area with the target, and maximum payload before slippage. The viability of the gripper with PDMS actuators for horticultural harvesting applications is illustrated by gently grasping a variety of mushroom caps.