Latest Papers

ASME Journal of Mechanisms and Robotics

  • Mechanical Characterization of Compliant Cellular Robots. Part I: Passive Stiffness
    by Singh G, Nawroj A, Dollar AM. on June 23, 2022 at 12:00 am

    AbstractModular active cell robots (MACROs) are a design paradigm for modular robotic hardware that uses only two components, namely actuators and passive compliant joints. Under the MACRO approach, a large number of actuators and joints are connected to create mesh-like cellular robotic structures that can be actuated to achieve large deformation and shape change. In this two-part paper, we study the importance of different possible mesh topologies within the MACRO framework. Regular and semi-regular tilings of the plane are used as the candidate mesh topologies and simulated using finite element analysis (FEA). In Part 1, we use FEA to evaluate their passive stiffness characteristics. Using a strain-energy method, the homogenized material properties (Young's modulus, shear modulus, and Poisson's ratio) of the different mesh topologies are computed and compared. The results show that the stiffnesses increase with increasing nodal connectivity and that stretching-dominated topologies have higher stiffness compared to bending-dominated ones. We also investigate the role of relative actuator-node stiffness on the overall mesh characteristics. This analysis shows that the stiffness of stretching-dominated topologies scales directly with their cross-section area whereas bending-dominated ones do not have such a direct relationship.

  • Efficient Computation of Large Deformation of Spatial Flexure-Based Mechanisms in Design Optimizations
    by Dwarshuis K, Aarts R, Ellenbroek M, et al. on June 23, 2022 at 12:00 am

    AbstractDesign optimizations of flexure-based mechanisms take a lot of computation time, in particular when large deformations are involved. In an optimization procedure, statically deformed configurations of many designs have to be obtained, while finding the statically deformed configuration itself requires tens to hundreds of load step iterations. The kinematically started deformation method (KSD-method) (Dwarshuis, K. S., Aarts, R. G. K. M., Ellenbroek, M. H. M., and Brouwer, D. M., 2020, “Kinematically Started Efficient Position Analysis of Deformed Compliant Mechanisms Utilizing Data of Standard Joints,” Mech. Mach. Theory, 152, p. 103911) computes deformed configurations fast by starting the computation from an approximation. This approximation is obtained by allowing the mechanism only to move in the compliant motion-direction, based on kinematic equations, using data of the flexure joints in the mechanism. This is possible as flexure-based mechanisms are typically designed to be kinematically determined in the motion directions. In this paper, the KSD-method is extended such that it can also be applied without joint-data, such that it is not necessary to maintain a database with joint-data. This paper also shows that the method can be used for mechanisms containing joints that allow full spatial motion. Several variants of the KSD-method are presented and evaluated for accuracy and required computation time. One variant, which uses joint-data, is 21 times faster and shows errors in stress and stiffness below 1% compared to a conventional multibody analysis on the same model. Another variant, which does not use joint-data, reduces the computation time by a factor of 14, keeping errors below 1%. The KSD-method is shown to be helpful in design optimizations of complex flexure mechanisms for large range of motion.

  • Mechanical Characterization of Compliant Cellular Robots. Part II: Active Strain
    by Singh G, Nawroj A, Dollar AM. on June 23, 2022 at 12:00 am

    AbstractModular active cell robots (MACROs) is a design approach in which a large number of linear actuators and passive compliant joints are assembled to create an active structure with a repeating unit cell. Such a mesh-like robotic structure can be actuated to achieve large deformation and shape-change. In this two-part paper, we use finite element analysis (FEA) to model the deformation behavior of different MACRO mesh topologies and evaluate their passive and active mechanical characteristics. In Part I, we presented the passive stiffness characteristics of different MACRO meshes. In this Part II of the paper, we investigate the active strain characteristics of planar MACRO meshes. Using FEA, we quantify and compare the strains generated for the specific choice of MACRO mesh topology and further for the specific choice of actuators actuated in that particular mesh. We simulate a series of actuation modes that are based on the angular orientation of the actuators within the mesh and show that such actuation modes result in deformation that is independent of the size of the mesh. We also show that there exists a subset of such actuation modes that spans the range of deformation behavior. Finally, we compare the actuation effort required to actuate different MACRO meshes and show that the actuation effort is related to the nodal connectivity of the mesh.

  • Kinematic Performance and Static Analysis of a Two-Degree-of-Freedom 3-RPS/US Parallel Manipulator With Two Passive Limbs
    by Li X, Qu H, Guo S. on June 23, 2022 at 12:00 am

    AbstractIn this paper, a new 3-RPS (the limb consisting of one revolute, one prismatic, and one spherical joint)/US (universal joint and spherical joint) parallel mechanism with two degrees-of-freedom (DOFs) is obtained by adding a US passive limb into the 3-RPS parallel mechanism with the aim of obtaining a high load-bearing capacity. The moving platform possesses two rotational motions, analyzed by the Grassmann line geometry and screw theory. Then, the kinematic performance of the mechanism is analyzed, including inverse kinematics, overall Jacobian matrix, workspace, and singularity. On this basis, the mapping between the driving force and the load on the moving platform is deduced and verified by simulation. Next, the static of the proposed parallel mechanism is compared with that of the 3-RPS mechanism. The results show that the load-bearing capacity of the mechanism is improved by introducing the US passive limb. Finally, a case study verifies the potential application of the mechanism as a dual-axis tracking photovoltaic bracket.

Development of a Hydraulic Driven Bionic Soft Gecko Toe

Abstract

Geckos can climb freely on various types of surfaces using their flexible and adhesive toes. Gecko-inspired robots are capable of climbing on different surface conditions and have shown many important applications. Nonetheless, due to poor flexibility of toes, the movements of gecko-inspired robots are restricted to flat surfaces. To improve the flexibility, by utilizing design technique of soft actuator and incorporating the characteristics of a real gecko’s toe, the design of new bionic soft toes is proposed. The abilities of this bionic toe are verified using modeling and two soft toes are manufactured. One is Type A toe having varied semi-circle cross sections as the feature of real gecko toe and the other is Type B toe with a constant semi-circle cross section. The bending behaviors of the bionic toes subjected to a range of hydraulic pressure are also experimentally studied. It demonstrated that both toes can perform similarly large bending angles for the adduction (attachment) and abduction (detachment) motions. In comparisons, Type B toe exhibits larger output force, which is ascribed to the fact that at proximal section of Type B corresponds to larger volume for bearing fluid. Both toes can not only provide sufficient adhesion but can be quickly detached with low peeling forces. Finally, different curved surfaces are used to further justify the applicability of these bionic toes. In particular, the flexible toes developed also have the advantages of low cost, lightweight, and simple control, which is desirable for wall-climbing robots.
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