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.

A Mobile Mathieu Oscillator Model for Vibrational Locomotion of a Bristlebot

Abstract

Terrestrial locomotion that is produced by creating and exploiting frictional anisotropy is common amongst animals such as snakes, gastropods, and limbless lizards. In this paper we present a model of a bristlebot that locomotes by generating frictional anisotropy due to the oscillatory motion of an internal mass and show that this is equivalent to a stick–slip Mathieu oscillator. Such vibrational robots have been available as toys and theoretical curiosities and have seen some applications such as the well-known kilobot and in pipe line inspection, but much remains unknown about this type of terrestrial locomotion. In this paper, motivated by a toy model of a bristlebot made from a toothbrush, we derive a theoretical model for its dynamics and show that its dynamics can be classified into four modes of motion: purely stick (no locomotion), slip, stick–slip, and hopping. In the stick mode, the dynamics of the system are those of a nonlinear Mathieu oscillator and large amplitude resonance oscillations lead to the slip mode of motion. The mode of motion depends on the amplitude and frequency of the periodic forcing. We compute a phase diagram that captures this behavior, which is reminiscent of the tongues of instability seen in a Mathieu oscillator. The broader result that emerges in this paper is that mobile limbless continuum or soft robots can exploit high-frequency parametric oscillations to generate fast and efficient terrestrial motion.
Read More
Journal of Mechanisms and Robotics Open Issues