Towards pneumatic spiral grippers: Modeling and design considerations

There are a number of instances in nature where long and slender objects are grasped by a continuum arm spirally twirling around the object, thereby increasing the area of contact and stability between the gripper and the object. This paper investigates the design and modeling of spiral grippers using pneumatic fiber-reinforced actuators. The paper proposes two reduced order models, a pure helical model, and a spatial Cosserat rod model to capture the deformed behavior of the gripper using the mechanics of fiber-reinforced actuators in the presence of self-weight. While the former model can yield closed form expressions that aid in design, the deformation parameters deviate by greater than 40% of its length for actuators longer than 200 mm. However, the Cosserat rod model deviates by less than 8% of its length for two different prototypes validated in this work. The deformation of the gripper is then correlated to the number of spiral turns achievable about the object, which determines the quality of the grip. Together, they enable a systematic framework where the gripper parameters can be designed for a given range of object sizes to be handled. This framework is experimentally validated by successful gripping of a range of slender objects that lie between a 20 mm diameter tubelight and a 60 mm diameter PVC pipe.