In the soft robotics field, soft structures are required for safe human interactions, adaptive shape grippers, and haptic interfaces. In contrast, rigid structures are necessary for high force transmission devices and precise position control applications. The tunable stiffness mechanism is a crucial technology for such areas, and various methods have been proposed. Fixing the segments of the string by the tension of a tendon has been studied that can stiffen the structure instantly and can constitute them with rigid materials. However, the conventional wire tensioning mechanism has a limitation in the holding torque, and the optimum segment morphology and stiffening performance is not clear. Here we show a new tendon-driven jamming mechanism called a “radial-layer jamming” that has multiple layered joints and can realize high holding torque. Three basic segment structures are presented, and the design method is shown. We measured the holding torque of the joints and compared the torque between three different designs. Our results show that all three structures can control the holding torque by the pulling force of the wire. Also, we found that the radial layer jamming can keep the joints fixed even if it had an extended structure. The holding force of the radial layer jamming did not largely decrease compared to the other structure even if the number of joints increased. These layered approaches can contribute to the performance improvement of various variable stiffness mechanisms that utilize friction.

Soft robots have attracted much attention in recent years owing to their high adaptability. Long articulated soft robots enable diverse operations, and tip-extending robots that navigate their environment through growth are highly effective in robotic search applications. Robots that extend from the tip can lengthen their body without friction from the environment. However, the flexibility of the thin membrane inhibits the retraction motion of the tip due to buckling. Two methods have been proposed to resolve this issue; increasing the pressure of the internal fluid to reinforce rigidity, and mounting an actuator at the tip. The disadvantage of the former is that the increase is limited by the membrane pressure resistance, while the second method leads to robot complexity. In this paper, we present a tip-retraction mechanism with a hydrostatic skeleton that can prevent buckling and takes advantage of the friction from the external environment. Water is used as the internal fluid to increase ground pressure with the environment, which is different from the conventional methods that use pneumatic. We explore the failure pattern of the retraction motion and propose solutions by using a hydrostatic skeleton robot. Additionally, we develop a prototype robot that successfully retracts by using the proposed methodology. Our solution can contribute to the advancement of mechanical design in the soft robotics field with applications to soft snakes and manipulators.

This letter introduces a balloon pin-array gripper combining shape adaptability to various objects, stable holding by multipoint contact, and isotropic grasping performance. This is particularly useful when the shape or position of the objects cannot be accurately determined because of sensor limitations. This gripper has multiple pins whose tips are covered by flexible balloons. The gripper can adapt to the shapes of objects in two steps: axial sliding of the pins and radial inflation of the balloons. This study focuses on the effect of the layout of pins on grasping and proposes a simulation model to quantify the characteristics of each layout. Simulations showed that the concentric layout enables stable grasping by ensuring many pins contact the object, regardless of misalignment. Experiments using a prototype gripper demonstrated a trend consistent with the simulation results, proving the validity of the simulation model.

Inflatable structures used in soft robotics applications have unique characteristics. In particular, the tip-extension structure, which extends the structure from its tip, can grow without creating friction with the environment. However, these inflatable structures need high pressure to maintain their stiffness under various conditions. Excessive inner pressure limits their application in that it prevents the structure from maintaining its curved shape and from complying with specifications. This study aimed to simultaneously lower the pressure and increase the rigidity of the structure. Our work resulted in the proposal of a mechanism that combines a skeleton structure consisting of multi-joint links with functions to increase the rigidity. Insertion of this mechanism into an inflatable structure obviates the need for high inner pressure, yet enables the structure to bend and maintain the intended shape. We devised a design based on rigid articulated links and combined it with a membrane structure that utilizes the advantages of the tip-extension structure. The experimental results show that the payload of the structure designed to operate at low pressure increases compared to that of the membrane-only structure. The findings of this research can be applied to long robots that can be extended into open space without drooping and to mechanisms that enable structures to wrap around the human body.

Studies on soft robotics have explored mechanisms for switching the stiffness of a robot structure. The hybrid soft-rigid approach, which combines soft materials and high-rigidity structures, is commonly used to achieve variable stiffness mechanisms. In particular, the positive-pressurization method has attracted significant attention in recent years as it can eliminate the constraints on driving pressure. Moreover, it can change the shape holding force according to internal pressure. In this study, a variable stiffness mechanism, comprising 3-axis rotary ball joints and a single chamber, was devised via frictional force using positive pressure. The prototype can change joint angles arbitrarily when no pressure is applied and can hold joint angles when positive pressure is applied. Using a theoretical model of the torque required to hold the joint angle, we simulated the holding torque using finite element modeling analysis and measured the holding torque in the pitch and roll directions when internal pressure was applied. Based on the interaction of the theoretical model, measurement, and FEM analysis, it was confirmed that the value of the holding torque in the roll direction was approximately π/2 times larger than that in the pitch direction for each value of the internal pressure. Further, we evaluated the FEM value, theoretical value, and measured value of the holding torque by performing pairwise numerical comparisons. Our approach will aid the design of effective stiffening mechanisms for soft robotics applications.