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1. Kinematics and Stiffness Modeling of Soft Robot With a Concentric Backbone

   https://doi.org/10.1115/1.4055860  

   Xiao, Qingyu; Musa, Mishek; Godage, Isuru S.; Su, Hao; Chen, Yue (October 2023, Journal of Mechanisms and Robotics)

   Abstract Soft robots can undergo large elastic deformations and adapt to complex shapes. However, they lack the structural strength to withstand external loads due to the intrinsic compliance of fabrication materials (silicone or rubber). In this paper, we present a novel stiffness modulation approach that controls the robot’s stiffness on-demand without permanently affecting the intrinsic compliance of the elastomeric body. Inspired by concentric tube robots, this approach uses a Nitinol tube as the backbone, which can be slid in and out of the soft robot body to achieve robot pose or stiffness modulation. To validate the proposed idea, we fabricated a tendon-driven concentric tube (TDCT) soft robot and developed the model based on Cosserat rod theory. The model is validated in different scenarios by varying the joint-space tendon input and task-space external contact force. Experimental results indicate that the model is capable of estimating the shape of the TDCT soft robot with an average root-mean-square error (RMSE) of 0.90 (0.56% of total length) mm and average tip error of 1.49 (0.93% of total length) mm. Simulation studies demonstrate that the Nitinol backbone insertion can enhance the kinematic workspace and reduce the compliance of the TDCT soft robot by 57.7%. Two case studies (object manipulation and soft laparoscopic photodynamic therapy) are presented to demonstrate the potential application of the proposed design. 

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2. Model-based contact detection and position control of a fabric soft robot in unknown environments

   https://doi.org/10.3389/frobt.2022.997366  

   Qiao, Zhi; Nguyen, Pham H.; Zhang, Wenlong (October 2022, Frontiers in Robotics and AI)

   Soft robots have shown great potential to enable safe interactions with unknown environments due to their inherent compliance and variable stiffness. However, without knowledge of potential contacts, a soft robot could exhibit rigid behaviors in a goal-reaching task and collide into obstacles. In this paper, we introduce a Sliding Mode Augmented by Reactive Transitioning (SMART) controller to detect the contact events, adjust the robot’s desired trajectory, and reject estimated disturbances in a goal reaching task. We employ a sliding mode controller to track the desired trajectory with a nonlinear disturbance observer (NDOB) to estimate the lumped disturbance, and a switching algorithm to adjust the desired robot trajectories. The proposed controller is validated on a pneumatic-driven fabric soft robot whose dynamics is described by a new extended rigid-arm model to fit the actuator design. A stability analysis of the proposed controller is also presented. Experimental results show that, despite modeling uncertainties, the robot can detect obstacles, adjust the reference trajectories to maintain compliance, and recover to track the original desired path once the obstacle is removed. Without force sensors, the proposed model-based controller can adjust the robot’s stiffness based on the estimated disturbance to achieve goal reaching and compliant interaction with unknown obstacles. 

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3. Dynamically Reconfigurable Discrete Distributed Stiffness for Inflated Beam Robots

   https://doi.org/10.1109/ICRA40945.2020.9197237  

   Do, Brian H.; Banashek, Valory; Okamura, Allison M. (May 2020, IEEE International Conference on Robotics and Automation (ICRA))

   null (Ed.)

   Inflated continuum robots are promising for a variety of navigation tasks, but controlling their motion with a small number of actuators is challenging. These inflated beam robots tend to buckle under compressive loads, producing extremely tight local curvature at difficult-to-control buckle point locations. In this paper, we present an inflated beam robot that uses distributed stiffness changing sections enabled by positive pressure layer jamming to control or prevent buckling. Passive valves are actuated by an electromagnet carried by an electromechanical device that travels inside the main inflated beam robot body. The valves themselves require no external connections or wiring, allowing the distributed stiffness control to be scaled to long beam lengths. Multiple layer jamming elements are stiffened simultaneously to achieve global stiffening, allowing the robot to support greater cantilevered loads and longer unsupported lengths. Local stiffening, achieved by leaving certain layer jamming elements unstiffened, allows the robot to produce "virtual joints" that dynamically change the robot kinematics. Implementing these stiffening strategies is compatible with growth through tip eversion and tendon steering, and enables a number of new capabilities for inflated beam robots and tip-everting robots. 

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4. A Comparative Study on the Effect of Mechanical Compliance for a Safe Physical Human–Robot Interaction

   https://doi.org/10.1115/1.4046068  

   She, Yu; Song, Siyang; Su, Hai-Jun; Wang, Junmin (June 2020, Journal of Mechanical Design)

   Abstract In this paper, we study the effects of mechanical compliance on safety in physical human–robot interaction (pHRI). More specifically, we compare the effect of joint compliance and link compliance on the impact force assuming a contact occurred between a robot and a human head. We first establish pHRI system models that are composed of robot dynamics, an impact contact model, and head dynamics. These models are validated by Simscape simulation. By comparing impact results with a robotic arm made of a compliant link (CL) and compliant joint (CJ), we conclude that the CL design produces a smaller maximum impact force given the same lateral stiffness as well as other physical and geometric parameters. Furthermore, we compare the variable stiffness joint (VSJ) with the variable stiffness link (VSL) for various actuation parameters and design parameters. While decreasing stiffness of CJs cannot effectively reduce the maximum impact force, CL design is more effective in reducing impact force by varying the link stiffness. We conclude that the CL design potentially outperforms the CJ design in addressing safety in pHRI and can be used as a promising alternative solution to address the safety constraints in pHRI. 

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5. Tunable Anisotropic Stiffness with Square Fiber Jamming

   https://doi.org/10.1109/RoboSoft48309.2020.9116030  

   Aktas, Buse; Howe, Robert D. (May 2020, 2020 3rd IEEE International Conference on Soft Robotics (RoboSoft))

   null (Ed.)

   Jamming is a phenomenon in which a collectionof compliant elements is encased in an airtight envelope, anda vacuum-induced pressure enhances frictional and kinematiccoupling, resulting in dramatic changes in stiffness. This paperintroduces the jamming of square cross-sectioned fibers, whichallow for tunable and programmable anisotropic stiffness. Atheoretical model captures the effect of major geometric designparameters on the direction-variant bending stiffness of theselong and slender jamming elements. The model is experimen-tally validated, and a 13-fold stiffening in one direction anda 22-fold stiffening in the orthogonal direction is achievedwith a single jamming element. The performance of a square-fiber-jamming continuum robot structure is demonstrated in asteering task, with an average reduction of 74% in the measuredinsertion force when unjammed, and a direction-variant 53%or 100% increase in the measured tip stiffness when jammed. 

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