This article presents the design principle and fabrication of a variable stiffness soft robotic gripper for adaptive grasping and robust holding. The proposed robotic gripper is based on a finger design that combines a fiber-reinforced soft actuator and a particle pack. The soft actuator is responsible for the bending motion of the finger, and the particle pack acts as a stiffness-changeable interface between the finger and the object. In the natural state, the particle pack is soft and adaptive to part geometry. It can rapidly stiffen (through vacuum) to resist external load or to freeze the currently bent contour of the finger. Experimental studies have shown that more than a 10-fold stiffness enhancement is achievable. Therefore, the proposed gripper is capable of handling objects with different shapes, weights, and rigidities, which have been a great challenge for robotic grasping. For more effective grasping, a grasping strategy is designed for the proposed soft gripper with simultaneous consideration of grasping adaption and robustness.

The compliance of soft grippers contributes to their great superiority over rigid grippers in grasping irregularly shaped objects and forming soft contact with environments. Due to a relatively small pressure, soft grippers lack the stiffness required for wider applications. Particle jamming has been frequently reported as a means of stiffness control. Unlike previous research using vacuum for particle jamming, this paper proposes a novel passive particle jamming principle that does not need any vacuum power or other control means. The proposed method is by simply patching a silicone rubber soft actuator and a pack (made of strain-limiting membrane) of particles to form an integral gripping finger. The inflation of the soft actuator applies a pressure to the particle pack causing particles inside it to jam. A larger squeezing pressure will result in tighter particle jamming, thus increasing the stiffness of the finger. The stiffness of the finger is controllable as it is proportional to the actuator's air pressure, which has been verified by experiments in this research. The stiffness can increase more than six fold when air pressure changes from 20 to 80 kPa in the experimental studies. The reported discovery may enhance the capabilities of soft robotic grippers so that more robotic picking operations could be performed by soft grippers.

In this article, we have proposed a novel robotic finger design principle aimed to address two challenges in soft pneumatic grippers—the controllability of the stiffness and the controllability of the bending position. The proposed finger design is composed of a 3D printed multimaterial substrate and a soft pneumatic actuator. The substrate has four polylactic acid (PLA) segments interlocked with three shape memory polymer (SMP) joints, inspired by bones and joints in human fingers. By controlling the thermal energy of an SMP joint, the stiffness of the joints is modulated due to the dramatic change in SMP elastic modulus around its glass transition temperature (T_g). When SMP joints are heated above T_g, they exhibit very small stiffness, allowing the finger to easily bend around the SMP joints if the attached soft actuator is actuated. When there is no force from the soft actuator, shape recovery stress in SMP contributes to the finger's shape restoration. Since each joint's rotation can be individually controlled, the position control of the finger is made possible. Experimental analysis has been conducted to show the finger's variable stiffness and the result is compared with the analytical values. It is found that the stiffness ratio can be 24.9 times for a joint at room temperature (20°C) and at an elevated temperature of 60°C when air pressure p of the soft actuator is turned off. Finally, a gripper composed of two fingers is fabricated for demonstration.

This article describes a novel design of bioinspired soft robotic fingers based upon hybrid jamming principle—integrated layer jamming and particle jamming. The finger combines a fiber-reinforced soft pneumatic actuator with a hybrid jamming substrate. Taking advantage of different characteristics of layer jamming and particle jamming, the substrate is designed with three chambers filled with layers (function as bones) and two chambers filled with particles (function as joints). The layer regions and particle regions are interlocked with each other to guarantee load transfer from the fixed finger end to fingertip. With the proposed design, the finger is endowed with bending shape control, as well as variable stiffness capabilities. Theoretical analysis is conducted to predict the stiffness variation of the proposed finger at different vacuum levels, and experimental tests are performed to evaluate the finger's shape control and stiffness tuning effectiveness. Experimental results show that the proposed finger can achieve 5.52 times stiffness enhancement at primary position. Finally, we fabricate a gripper and perform grasping demonstrations on several objects. Results show that the gripper is able to transfer between low stiffness state for adaptive grasping and high stiffness state for robust holding.