Latest Papers

ASME Journal of Mechanisms and Robotics

  • Stable Inverse Dynamics for Feedforward Control of Nonminimum-Phase Underactuated Systems
    on January 25, 2023 at 12:00 am

    AbstractAn enhanced inverse dynamics approach is here presented for feedforward control of underactuated multibody systems, such as mechanisms or robots where the number of independent actuators is smaller than the number of degrees of freedom. The method exploits the concept of partitioning the independent coordinates into actuated and unactuated ones (through a QR-decomposition) and of linearly combined output, to obtain the internal dynamics of the nonminimum-phase system and then to stabilize it through proper output redefinition. Then, the exact algebraic model of the actuated sub-system is inverted, leading to the desired control forces with just minor approximations and no need for pre-actuation. The effectiveness of the proposed approach is assessed by three numerical test cases, by comparing it with some meaningful benchmarks taken from the literature. Finally, experimental verification through an underactuated robotic arm with two degrees of freedom is performed.

Design of the Wearable Spatial Gravity Balance Mechanism

Abstract

Gravity compensation mechanisms are widely used in manipulators and exoskeletons as passive components that generate counter-gravity force and save energy. While there have been making great progresses in the design of gravity compensators, a strict condition that the axes of the gravity compensators are aligned with the axes of the links being balanced (LBBs) exactly is usually assumed implicitly, which is difficult to achieve for exoskeletons in practice. In this paper, the design method of the wearable spatial gravity compensator compatible to the misalignment between the rotation centers of the LBB and the compensator is carefully studied. First, the design of the planar gravity compensation unit (PGCU) is presented for each link when rotating in the yaw plane, and next, the PGCU is adapted into the spatial gravity compensation unit (SGCU) to accommodate the general rotation of the LBB. Then, the type synthesis of the SGCU is conducted followed by the analyses of the acting patterns of synthesized SGCUs on the LBBs and gravity compensation performances when the misalignments occur. Finally, the SGCUs are combined with timing belt mechanisms (TBMs) to construct gravity compensation mechanisms for spatial serial linkages. Simulations of an exoskeleton constructed by SGCUs are conducted to verify the performance of gravity balance and the effectiveness of the proposed design method.

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