Quasi-Articulation of a Continuous Robotic Manipulator Enabled by Stiffness-Switching Origami Joints

Soft robots possess a nearly infinite number of kinematic degrees of freedom due to the compliance of their underlying materials which enables them to accomplish incredible feats of movement and adaptation. However, their severely underactuated structures limit their controllability and the degree of precision that can be achieved. As demonstrated by the octopus when fetching prey, it is possible to achieve precise movement in an otherwise “soft” arm by stiffening select sections of the arm while keeping other sections flexible, in effect generating a quasi-articulated structure and reducing the degrees of freedom from practically infinite to a finite number of angles. In this study, we use the bistable generalized Kresling origami to emulate this strategy. Both experimental and computational modeling procedures are conducted to evaluate the bending mechanics of the structure at each of its two stable states (extended and contracted). As the model accurately predicts the major trends observed in experiments, it is used to perform a parametric study on the bending stiffness ratio, defined as the ratio of bending stiffness at the extended state to the bending stiffness at the contracted state. Using the results of the parametric study, we discover that the Kresling design which maximizes the bending stiffness ratio is that possessing the greatest angle ratio λ, the lowest contracted height Lc, and the largest number of sides of the base polygon n, enabling the transformation of the structure from rigid to flexible. To complete the study, we use the optimal Kresling design in the fabrication of a tendon-driven reconfigurable manipulator composed of three Kresling modules. We find that by reconfiguring the Kresling module states (rigid or flexible), the manipulator can effectively transform into 2m different configurations where m corresponds to the number of modules. Through this reconfiguration, the manipulator can generate a quasi-articulated structure which reduces its effective degrees of freedom and enables linkage-like motion. Unlike other methods of stiffness modulation, this solution reduces system complexity by using a bistable structure as both the body of the robot and as a mechanism of stiffness-switching. The structure’s primary reliance on geometry for its properties makes it a scalable solution, which is appealing for minimally invasive surgical applications where both precision and adaptability are vital. The manipulator may also be used as an inspection or exploration robot to access areas that may be inaccessible to humans or rigid robots

Exploiting Multi Stability of Compliant Locking Mechanism for Reconfigurable Articulation in Robotic Arm

This study analyzes a biology inspired approach of utilizing a compliant unit actuator to simplify the control requirements for a soft robotic arm. A robot arm is constructed from a series of compliant unit actuators that precisely actuate between two stable states. The extended state can be characterized as a rigid link with a high bending stiffness. The compressed state can be characterized as a flexible joint with a low bending stiffness. Without the use of an external power source, the bistable mechanism remains in each of the stable states. The unit actuator can demonstrate pseudo-linkage kinematics that require less control parameters than entirely soft manipulators. An advantage of using compliant mechanisms to design a robotic arm is that the bending stiffness ratio between the extended and compressed states is related to the frame and flexural member geometry. Post buckling characteristics of thin flexural members, combined with a cantilever style frame design gives the unit actuator versatile advantages over existing actuator designs like layer jamming and shape memory polymers. To achieve efficient movement with the optimized unit actuator design, experimental validation was performed, and a robotic arm prototype was fabricated. The tendon-driven robotic arm consisted of three modules and proved the capability of transforming and rotating in the eight configurations. The deformations of the robotic arm are accurately predicted by the kinematic model and validate the compliant mechanism arm and simple control system