A new gripper that acts as an active and passive joint to facilitate prehensile grasping and locomotion

Among primates, the prehensile nature of the hand is vital for greater adaptability and a secure grip over the substrate/branches, particularly for arm-swinging motion or brachiation. Though various brachiation mechanisms that are mechanically equivalent to underactuated pendulum models are reported in the literature, not much attention has been given to the hand design that facilitates both locomotion and within-hand manipulation. In this paper, we propose a new robotic gripper design, equipped with shape conformable active gripping surfaces that can act as an active or passive joint and adapt to substrates with different shapes and sizes. A floating base serial chain, named GraspMaM, equipped with two such grippers, increases the versatility by performing a range of locomotion and manipulation modes without using dedicated systems. The unique gripper design allows the robot to estimate the passive joint state while arm-swinging and exhibits a dual relationship between manipulation and locomotion. We report the design details of the multimodal gripper and how it can be adapted for the brachiation motion assuming it as an articulated suspended pendulum model. Further, the system parameters of the physical prototype are estimated, and experimental results for the brachiation mode are discussed to validate and show the effectiveness of the proposed design.

I. Introduction

The prehensile nature of hands and feet of the primates, with an opposable thumb and toe, respectively, helps them to firmly hold and maneuver on the irregular surfaces of the trees and the branches [1], [2]. The prehensility function is crucial for stable locomotion, brachiation, suspension, foraging and so on, as described in [3]. Various robot designs have been proposed in the literature to emulate the biological systems, which are majorly composed of dedicated mechanisms for achieving locomotion and manipulation capability to suit different tasks and environments. Since our objective is functional mimicking of the prehensility found in primates and increasing versatility, our design approach does not involve replicating the biological form of the primate's hand. Instead, the prehensile functionality has been accomplished by introducing a pair of opposing spring-loaded shape conformable gripping surfaces that can passively conform to the object or substrate for improved stability, as shown in Fig. 1. The shape conformity enables it to grasp a substrate with different geometrical shapes and sizes and performs arm swinging motion without affecting the contact.