Latest Papers

ASME Journal of Mechanisms and Robotics

  • Kinematics and Stiffness Modeling of Soft Robot With a Concentric Backbone
    on November 25, 2022 at 12:00 am

    AbstractSoft 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.

  • Impedance Control on Redundant Manipulators With Zero-Potential-Energy Motions: Theory and Experimental Validation
    on November 25, 2022 at 12:00 am

    AbstractThis article presents an experimental study with theory to identify quantitatively the zero-potential-energy (ZP) motion in Cartesian impedance control of redundant manipulators, based on a new analytical methodology. This ZP mode of motion, analogous to the rigid-body mode in classic mechanical systems, is a result of the redundancy of the robot. When subject to an external perturbation under impedance control, a redundant robot will assume a new equilibrium configuration determined by the ZP motion, governed by the least-energy principle. Consequently, this creates a steady-state deviation from its initial configuration after a perturbation and reaches a new equilibrium. We determine such ZP motion(s) by utilizing a closed-form solution based on vibration theory. Experiments were conducted on a 7-degrees-of-freedom (DoF) redundant Panda robot to determine the new equilibrium after a perturbation. The experimental results are compared with the theoretical prediction of the ZP motions to validate the theoretical results of the zero-potential-energy motions due to stiffness in impedance control. Furthermore, we demonstrated that the ZP motion due to redundancy can be eliminated by removing the redundancy through experimental validation by employing the null-space control, as expected.

  • Thick-Panel Origami Tubes With Hexagonal Cross-Sections
    on November 25, 2022 at 12:00 am

    AbstractRigidly foldable origami tubes can be kinematically regarded as assemblies of spherical linkages. They have exhibited excellent properties for deployable structures. Yet, for the engineering applications, the corresponding thick-panel forms have to be designed. In this paper, the spherical 4R linkages in tubes with hexagonal cross-sections are partially replaced by spatial linkages, leading to a method to construct the thick-panel tubes, which can reproduce kinematic motions equivalent to those realized using zero-thickness origami. Based on the D–H matrix method, the rotational symmetric and symmetric tubes are introduced, together with their four types of vertexes, where the specific spherical 4R linkages are replaced by Bennett and Bricard linkages to obtain the thick-panel foldable tubes. The approach can be applied to multilayered tubes with a straight or curved profile, whose manufacture can be simplified by removing extra links. The results can be readily utilized to the design of deployable tubular structures whose thickness cannot be disregarded.

  • Energy-Based Footstep Planning of Biped on Uneven Deformable Terrain Using Nonlinear Inverted Pendulum
    on November 25, 2022 at 12:00 am

    AbstractIn this work, the motion of a nonlinear inverted pendulum (NIP) on deformable terrain due to foot contact forces is used for energy analysis and footstep placement. In order to include the linear and rotational motions of the NIP, the mass moment of inertia of the body is also considered in the center of mass (CoM) model. The terrain deformation is modeled using a spring-damper contact model, and based on the rigid body dynamics of the NIP, its motion is generated. The energy analysis of the system provides the regions of possible foot placement. Based on the loss of potential energy due to ground deformation, the limits of terrain stiffness are also obtained for walking on uneven deformable terrain.

  • A Reconfigurable 6 R Linkage With Six Motion Modes and Three Topological Structures
    on November 25, 2022 at 12:00 am

    AbstractIn this paper, a new reconfigurable 6R linkage is obtained by combining two identical equilateral Bennett linkages arranged in a plane-symmetric manner, and a detailed kinematic analysis is conducted which shows that there are six distinct motion modes and three topological structures of the derived mechanism without changing the types of kinematic joints. Explicit relationships among the kinematic variables are obtained with D–H method and various modes are discussed in detail. Bifurcation points are derived and the reconfigurations are analyzed. The result shows that the mechanism has six motion modes which contain a special case of a plane-symmetric 6R mode and a special case of a two-fold symmetric 6R mode, an X-shaped motion mode, and two V-shaped motion modes. A physical prototype is fabricated to verify the derivation and it shows that the mechanism can transform among all the motion modes without the need of reassembling.

Flexible-Joint Humanoid Balancing Augmentation via Full-State Feedback Variable Impedance Control


This paper attempts to address the quandary of flexible-joint humanoid balancing performance augmentation, via the introduction of the Full-State Feedback Variable Impedance Control (FSFVIC), and Model-Free Compliant Floating-base VIC (MCFVIC) schemes. In comparison to rigid-joint humanoid robots, efficient balancing control of compliant bipeds, powered by Series Elastic Actuators (or harmonic drives), requires the design of more sophisticated controllers encapsulating both the motor and underactuated link dynamics. It has been demonstrated that Variable Impedance Control (VIC) can improve robotic interaction performance, albeit by introducing energy-injecting elements that may jeopardize closed-loop stability. To this end, the novel FSFVIC and MCFVIC schemes are proposed, which amalgamate both collocated and non-collocated feedback gains, with power-shaping signals that are capable of preserving the system’s stability/passivity during VIC. The FSFVIC and MCFVIC stably modulate the system’s collocated state gains to augment balancing performance, in addition to the non-collocated state gains that dictate the position control accuracy. Utilization of arbitrarily low-impedance gains is permitted by both the FSFVIC and MCFVIC schemes propounded herein. An array of experiments involving the COmpliant huMANoid reveals that significant balancing performance amelioration is achievable through online modulation of the full-state feedback gains (VIC), as compared to utilization of invariant impedance control.
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