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.

A New Cam-Follower Safety Joint Mechanism Design Based on Variable-Length Four-Bar Linkage for Robot Safety


Humans come into physical contact with various machines such as robots in daily life. This leads to the underlying issue of guaranteeing safety during such human-robot interactions. Thus, many devices and methods have been studied for impact damage reduction. A safety joint mechanism (SJM) using four-bar linkages has been highlighted as an impact cutoff device owing to its capabilities of nonlinear load transfer. This paper focuses on a new design and testing for a kinematic element of an SJM based on four-bar linkages to improve the impact cutoff performances. In the present work, a set of variable-length floating link designs is proposed, and the mechanism is implemented by mechanical contact surface profile shaping between the cams and followers. The performance of the cam-follower mechanism is evaluated depending on the variable length of the floating link, by using the equivalent stiffness method, which successfully quantifies the performance of the proposed mechanism. Based on this design and analysis, two SJMs having symmetrical arrangements for four numbers of cam-follower mechanisms are fabricated: one SJM has fixed-length floating links and the other has variable-length floating links. The effect of the new kinematic elements on the performance improvement is verified by comparing the absorbed impact rates of the two SJMs by impact hammer-like drop tests. Consequently, it is confirmed that the rapid length change of the floating link is the core element for improving the performance of the safety mechanism.
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