Latest Papers

ASME Journal of Mechanisms and Robotics

  • Two Actuation Methods for a Complete Morphing System Composed of a VGTM and a Compliant Parallel Mechanism
    by Xi F, Zhao Y, Wang J, et al. on March 5, 2021 at 12:00 am

    AbstractIn this paper, a complete morphing system consisting of a variable geometry truss manipulator (VGTM) is presented that is fully covered by a flexible panel skin. Two approaches are studied for the morphing control. The first one is to have the VGTM act as a driving mechanism and the flexible panels as a passive system. In this case, the VGTM is composed of active members and passive lockable members. It is shown that the morphing system can reach the desired shapes through intermediate steps. The second method is to have the flexible panels act as drivers and the VGTM as a passive supporting structure. In this case, the VGTM is only composed of passive lockable members. The morphing system can also achieve the desired poses through several steps. The control strategies of the two methods are discussed along with kinematic analysis, a comparison study is conducted to show their pros and cons, two prototypes are fabricated, and experiments are carried out to verify the feasibility of two actuation methods.

  • Design, Kinematics, and Application of Axially and Radially Expandable Modular Soft Pneumatic Actuators
    by Wang N, Chen B, Ge X, et al. on March 5, 2021 at 12:00 am

    AbstractRecently, soft pneumatic actuators (SPAs) have drawn increasing attention due to their ease of fabrication, high customizability, and intrinsic softness. Inspired by modular design, two kinds of SPAs, including an axial elongation soft pneumatic actuator (aeSPA) and a radial expansion soft pneumatic actuator (reSPA), are proposed in this paper, followed by their modeling, fabrication, and application in locomotion robots. The relationships between pressure and displacement of these SPAs are deduced based on the Yeoh model and the principle of virtual work, which has a good agreement with experimental results. Five modular worm-like crawling robots are fabricated by assembling the aeSPAs and reSPAs in different morphologies, and crawling tests are performed under different conditions to show the adaptivity of robots.

  • Clearance-Induced Orientation Uncertainty of Spherical Linkages
    by Chan C, Ting K. on March 5, 2021 at 12:00 am

    AbstractThis paper proposes a kinematic model to evaluate the orientation uncertainty range of spherical linkages caused by the joint clearances. Based on the concepts of imaginary clearance link, spherical N-bar rotatability laws, and the invariant link rotatability, the uncertainty of the output angle can be treated as a mobility problem. The uncertainty region of the end-effector is treated as a workspace problem for the remodeled linkage. This paper highlights the orientation error by isolating the kinematic effects of joint clearance from other error factors. The discussion is carried out through spherical four-bar linkages and five-bar linkages. Numeric examples are presented to demonstrate the uncertainty range of the output angle and the uncertainty region of the end-effector. The result shows that, in the worst case, the error of each joint clearance will be magnified in a closed-loop structure compared with linearly adding all the clearance error. This implies that from a kinematics point of view, closed-loop spherical linkages or parallel manipulators will lead to a greater deviation on the end-effector than their open-loop counterparts. Using more passive joints in the manipulator may result in bigger error possibilities.

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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