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 and Bending Analysis of a Metamorphic Parallel Twisted-Scissor Mechanism


The conventional scissor mechanism is used in modern engineering and robotic applications due to its metamorphic ability. The folding configuration provides space-saving and unfolding provides longer linear expansion capability. However, a conventional scissor suffers unexpected and uncontrolled large bending deformation due to low bending stiffness while unfolding configuration, which may damage its structure. It also has a sudden bending singularity during unfolding, which may also damage the actuator. These limitations impose a significant constraint on real-life applications such as foldable robot arms, space robot arms, and reconfigurable robots. In this paper, we proposed a multi-strands parallel twisted-scissor mechanism (PTSM) to enhance its usability. The PTSM is inspired by a rope structure and designed by introducing a metamorphic segment (MS) using the S-shaped linkage design approach to improve its bending stiffness without affecting conventional scissors’ fundamentals. The PTSM has a unique feature of several automatic-link locking mechanisms to avoid singularity without using additional sensors, mechanism, or control. We experimentally checked the proposed design’s functionality and its feasibility. We formulated a cantilever bending model for foldable PTSM with N metamorphic segments considering revolute joint clearance for bending estimation, experimentally verified, and analyzed the bending deformation in the X–Y and Y–Z planes. Also, it is compared with a conventional scissor. Finally, we found that PTSM is stronger than conventional scissor and can fold/unfold smoothly using a single linear actuator. PTSM can provide large linear displacement with small bending deformation without bending singularity.
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