Latest Papers

ASME Journal of Mechanisms and Robotics

  • Nonlinear Analysis of a Class of Inversion-Based Compliant Cross-Spring Pivots
    by Li S, Hao G, Chen Y, et al. on November 18, 2021 at 12:00 am

    AbstractThis article presents a nonlinear model of an inversion-based generalized cross-spring pivot (IG-CSP) using the beam constraint model (BCM), which can be employed for the geometric error analysis and the characteristic analysis of an inversion-based symmetric cross-spring pivot (IS-CSP). The load-dependent effects are classified into two ways, including the structure load-dependent effects and beam load-dependent effects, where the loading positions, geometric parameters of elastic flexures, and axial forces are the main contributing factors. The closed-form load–rotation relationships of an IS-CSP and a noninversion-based symmetric cross-spring pivot (NIS-CSP) are derived with consideration of the three contributing factors for analyzing the load-dependent effects. The load-dependent effects of IS-CSP and NIS-CSP are compared when the loading position is fixed. The rotational stiffness of the IS-CSP or NIS-CSP can be designed to increase, decrease, or remain constant with axial forces, by regulating the balance between the loading positions and the geometric parameters. The closed-form solution of the center shift of an IS-CSP is derived. The effects of axial forces on the IS-CSP center shift are analyzed and compared with those of a NIS-CSP. Finally, based on the nonlinear analysis results of IS-CSP and NIS-CSP, two new compound symmetric cross-spring pivots are presented and analyzed via analytical and finite element analysis models.

  • Design of the Wearable Spatial Gravity Balance Mechanism
    by Peng Y, Bu W, Chen J. on November 18, 2021 at 12:00 am

    AbstractGravity 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.

  • Locomotion Identification Method of One-Degree-of-Freedom Six-Bar for Jumping Robot
    by Shun G. on November 18, 2021 at 12:00 am

    AbstractExploring the locomotion of creatures is a challenging task in bionic robots, and the existing iterative design methods are mainly based on one or two characteristics to optimize robots. Here, we introduce the thinking of system identification theory to bionic robots, bypassing the exploration of the dynamics and reducing the difficulty of design greatly. A one-degree-of-freedom (DOF) six-bar mechanism (Watt I) was designated as the model to be identified, and it was divided into two parts, i.e., a one-DOF four-bar linkage and a three-DOF series arm. Then, we formed constraints and a loss function. The parameters of the model were identified based on the kinematic data of a jumping marmoset, an animal chosen for its unusually high mass-specific power output. As a result, we obtained the desired model. Then, a prototype derived from the model was fabricated, and the experiments verified the effectiveness of the method. Based on the success of our experiments, we believe our method can be applied to emulate other motions as well.

  • Design and Compliant Control of a Piggyback Transfer Robot
    by Liu Y, Guo S, Yin Y, et al. on November 18, 2021 at 12:00 am

    AbstractPatient transfer, such as lifting and moving a bedridden patient from a bed to a wheelchair or a pedestal pan, is one of the most physically challenging tasks in nursing care. Although many transfer devices have been developed, they are rarely used because of the large time consumption in performing transfer tasks and the lack of safety and comfortableness. We developed a piggyback transfer robot that can conduct patient transfer by imitating the motion when a person holds another person on his/her back. The robot consisted of a chest holder that moves like a human back. In this paper, we present an active stiffness control approach for the motion control of the chest holder, combined with a passive cushion, for lifting a care-receiver comfortably. A human-robot dynamic model was built and a subjective evaluation was conducted to optimize the parameters of both the active stiffness control and the passive cushion of the chest holder. The test results of 10 subjects demonstrated that the robot could transfer a subject safely, and the combination of active stiffness and passive stiffness were essential to a comfortable transfer. The objective evaluation demonstrated that an active stiffness of k = 4 kPa/mm along with a passive stiffness lower than the stiffness of human chest was helpful for a comfort feeling.

  • Design of a Four-Bar Latch Mechanism and a Shear-Based Rotary Viscous Damper for Single-Axis Prosthetic Knees
    by Arelekatti V, Petelina NT, Johnson W, et al. on November 18, 2021 at 12:00 am

    AbstractWith over 30 million people worldwide requiring assistive devices, there is a great need for low-cost and high-performance prosthetic technologies that can enable kinematics close to able-bodied gait. Low-income users of prosthetic knees in the developing world repeatedly report the need for n inconspicuous gait to mitigate the severe socioeconomic discrimination associated with disability. However, passive prosthetic knees designed for these users have primarily focused on stability and affordability, often at the cost of the high biomechanical performance that is required to replicate able-bodied kinematics. In this study, we present the design and preliminary testing of two distinct mechanism modules that are novel for passive prosthetic knee applications: the stability module and the damping module. These mechanisms are designed to enable users of single-axis, passive prosthetic knees to walk with close to able-bodied kinematics on level-ground, specifically during the transition from the stance phase to the swing phase of the gait cycle. The stability module was implemented with a latch mounted on a virtual axis of a four-bar linkage, which can be engaged during early stance for stability and disengaged during late stance to initiate knee flexion. The damping module was implemented with a concentric stack of stationary and rotating pairs of plates that shear thin films of high-viscosity silicone oil. The goal of the resulting first-order damping torque was to achieve smooth flexion of the prosthetic knee within the able-bodied gait range (64 ± 6 deg). For preliminary user-centric validation, a prototype prosthetic knee with the stability module and two different dampers (with varying damping coefficients) was tested on a single subject with above-knee amputation in India. The stability module enabled smooth transition from stance to swing with timely initiation of knee flexion. The dampers also performed satisfactorily, as the increase in the damping coefficient was found to decrease the peak knee flexion angle during swing. The applications of the mechanisms presented in this article could significantly improve the kinematic performance of low-cost, passive prosthetic knees.

Advances in the Theory of Planar Curve Cognates

Abstract

Cognate linkages provide the useful property in mechanism design of having the same motion. This paper describes an approach for determining all coupler curve cognates for planar linkages with rotational joints. Although a prior compilation of six-bar cognates due to Dijksman purported to be a complete list, that analysis assumed, without proof, that cognates only arise by permuting link rotations. Our approach eliminates that assumption using arguments concerning the singular foci of the coupler curve to constrain a cognate search and then completing the analysis by solving a precision point problem. This analysis confirms that Dijksman’s list for six-bars is comprehensive. As we further demonstrate on an eight-bar and a ten-bar example, the method greatly constrains the set of permutations of link rotations that can possibly lead to cognates, thereby facilitating the discovery of all cognates that arise in that manner. However, for these higher order linkages, the further step of using a precision point test to eliminate the possibility of any other cognates is still beyond our computational capabilities.

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