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Review Article

J. Mech. Des. 2017;139(6):060801-060801-26. doi:10.1115/1.4036351.
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An atlas of 98 microgrippers that recently appeared in Literature is herein presented by using four different forms: (a) a restyled layout of the original mechanical structure, (b) its corresponding pseudorigid body model (PRBM), (c) its kinematic chain, and finally, (d) its related graph. Homogeneity in functional sketching (a) is assumed to be greatly helpful to understand how these grippers work and what are the most significant differences between them. Therefore, a unified and systematic set of aesthetics and proportionality criteria have been adopted. Analogously, unified criteria for obtaining pseudorigid (b), kinematic (c), and graph (d) representations have been also used, which made the atlas easy to be read and inspected. The distinction among lumped and distributed compliance has been also accepted to develop the structure of the atlas. A companion paper has been prepared to present a survey on the variety of operational strategies that are used in these microgrippers.

Commentary by Dr. Valentin Fuster

Research Papers: Design Automation

J. Mech. Des. 2017;139(6):061401-061401-9. doi:10.1115/1.4036396.

Computational approaches have great potential for aiding clinical product development by finding promising candidate designs prior to expensive testing and clinical trials. Here, an approach for designing multilevel bone tissue scaffolds that provide structural support during tissue regeneration is developed by considering mechanical and biological perspectives. Three key scaffold design properties are considered: (1) porosity, which influences potential tissue growth volume and nutrient transport, (2) surface area, which influences biodegradable scaffold dissolution rate and initial cell attachment, and (3) elastic modulus, which influences scaffold deformation under load and, therefore, tissue stimulation. Four scaffold topology types are generated by patterning beam or truss-based unit cells continuously or hierarchically and tuning the element diameter, unit cell length, and number of unit cells. Parametric comparisons suggest that structures with truss-based scaffolds have higher surface areas but lower elastic moduli for a given porosity in comparison to beam-based scaffolds. Hierarchical scaffolds possess a large central pore that increases porosity but lowers elastic moduli and surface area. Scaffold samples of all topology types are 3D printed with dimensions suitable for scientific testing. A hierarchical scaffold is fabricated with dimensions and properties relevant for a spinal interbody fusion cage with a maximized surface-volume ratio, which illustrates a potentially high performing design configured for mechanical and biological factors. These findings demonstrate the merit in using multidisciplinary and computational approaches as a foundation of tissue scaffold development for regenerative medicine.

Commentary by Dr. Valentin Fuster

Research Papers: Design of Mechanisms and Robotic Systems

J. Mech. Des. 2017;139(6):062301-062301-9. doi:10.1115/1.4036216.

Deoxyribose nucleic acid (DNA) origami nanotechnology is a recently developed self-assembly process for design and fabrication of complex three-dimensional (3D) nanostructures using DNA as a functional material. This paper reviews our recent progress in applying DNA origami to design kinematic mechanisms at the nanometer scale. These nanomechanisms, which we call DNA origami mechanisms (DOM), are made of relatively stiff bundles of double-stranded DNA (dsDNA), which function as rigid links, connected by highly compliant single-stranded DNA (ssDNA) strands, which function as kinematic joints. The design of kinematic joints including revolute, prismatic, cylindrical, universal, and spherical is presented. The steps as well as necessary software or experimental tools for designing DOM with DNA origami links and joints are detailed. To demonstrate the designs, we presented the designs of Bennett four-bar and crank–slider linkages. Finally, a list of technical challenges such as design automation and computational modeling are presented. These challenges could also be opportunities for mechanism and robotics community to apply well-developed kinematic theories and computational tools to the design of nanorobots and nanomachines.

Commentary by Dr. Valentin Fuster
J. Mech. Des. 2017;139(6):062302-062302-12. doi:10.1115/1.4036305.

Design synthesis of distributed compliant mechanisms is often a two-stage process involving (a) conceptual topology synthesis and a subsequent (b) refinement stage to meet strength and manufacturing specifications. The usefulness of a solution is ascertained only after the sequential completion of these two steps that are, in general, computationally intensive. This paper presents a strategy to rapidly estimate final operating stresses even before the actual refinement process. This strategy is based on the uniform stress distribution metric, and a functional characterization of the different members that constitute the compliant mechanism topology. Furthermore, this paper uses the underlying mechanics of stress bound estimation to propose two rule of thumb guidelines for insightful selection of topologies and systematically modifying them for an application. The selection of the best conceptual solution in the early stage design avoids refinement of topologies that inherently may not meet the stress constraints. This paper presents two examples that illustrate these guidelines through the selection and refinement of topologies for a planar compliant gripper application.

Commentary by Dr. Valentin Fuster
J. Mech. Des. 2017;139(6):062303-062303-11. doi:10.1115/1.4036395.

Continuum robots have excited increasing attention and efforts from the robotic community due to their high dexterity and safety. This paper proposes a design for a type of multimodule continuum robot equipped with an elastic backbone structure and tendon-driven actuation system. The kinematic model of the robot is formulated where the maximum bending angle of a module is obtained by identifying the interference between the backbone structure and the tendons. A superposition method is then used to determine the configuration space of the robotic module. Finally, an approximation method is presented to estimate the workspace of the tendon-driven continuum robot that reduces the computational complexity in comparison with the previously used scanning method. Experiments are provided to validate the proposed methods.

Commentary by Dr. Valentin Fuster
J. Mech. Des. 2017;139(6):062304-062304-12. doi:10.1115/1.4036306.

Epicyclic gear trains (EGTs) are used in the mechanical energy transmission systems where high velocity ratios are needed in a compact space. It is necessary to eliminate duplicate structures in the initial stages of enumeration. In this paper, a novel and simple method is proposed using a parameter, Vertex Incidence Polynomial (VIP), to synthesize epicyclic gear trains up to six links eliminating all isomorphic gear trains. Each epicyclic gear train is represented as a graph by denoting gear pair with thick line and transfer pair with thin line. All the permissible graphs of epicyclic gear trains from the fundamental principles are generated by the recursive method. Isomorphic graphs are identified by calculating VIP. Another parameter “Rotation Index” (RI) is proposed to detect rotational isomorphism. It is found that there are six nonisomorphic rotation graphs for five-link one degree-of-freedom (1-DOF) and 26 graphs for six-link 1-DOF EGTs from which all the nonisomorphic displacement graphs can be derived by adding the transfer vertices for each combination. The proposed method proved to be successful in clustering all the isomorphic structures into a group, which in turn checked for rotational isomorphism. This method is very easy to understand and allows performing isomorphism test in epicyclic gear trains.

Commentary by Dr. Valentin Fuster

Research Papers: Design of Direct Contact Systems

J. Mech. Des. 2017;139(6):063301-063301-9. doi:10.1115/1.4036353.

A recent addition to the many milling processes used in manufacturing to cut straight bevel gears (SBGs) is a new face-hobbing (FH) method that uses a virtual hypocycloid straight-lined mechanism to produce straight-lined teeth. Despite earning much attention because of its high productivity, however, this method is unable to handle lengthwise crowning on tooth surfaces, which results in poor contact performance. This paper therefore proposes a novel lengthwise crowning method, applicable on a modern six-axis computer numerical control (CNC) bevel gear cutting machine, in which the gear blank performs a swinging motion during machining. This swinging motion is enabled by machine setting modifications, which here are derived from a mathematical model of a double (profile and lengthwise) crowned gear. After the model's correctness is confirmed using ease-off and tooth contact analyses, a final investigation examines the effect of two key parameters related to contact performance indexes whose interrelations are graphed to provide a designer reference.

Commentary by Dr. Valentin Fuster

Design Innovation Paper

J. Mech. Des. 2017;139(6):065001-065001-10. doi:10.1115/1.4036304.

This paper presents a case study in engineering for global development. It introduces the Village Drill, which is an engineered product that has—5 years after its introduction to the market—enabled hundreds of thousands of people across 15 countries and three continents to have access to clean water. The Village Drill creates a 15 cm (6 in) borehole as deep as 76 m (250 ft) to reach groundwater suitable for drinking. The case study presents facts for the actual development and sustaining and are unaltered for the purpose of publication. This approach provides the reader with a realistic view of the development time, testing conditions, fundraising, and the work needed to sustain the drill through 5 years of sales and distribution. The purpose of the case study is to provide sufficient and frank data about a real project so as to promote discussion, critique, and other evaluations that will lead to new developments that inspire and inform successful engineering for global development. As part of the case, the paper describes six fundamental items: the product, the customer, the impact, the manufacturing, the delivery, and the revenue model of the drill.

Topics: Drills (Tools) , Water
Commentary by Dr. Valentin Fuster

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