An Assessment of Fused Deposition Modeling for the Manufacturing of Flexural Pivots in an Anthropomorphic Robotic Hand Design
Authors: Hill Scott , Canfield Stephen 
Published: December 2016

There is a significant rise in the design of robots performing ever-more complicated tasks. This has motivated more-anthropomorphic grasping hands for these robots. These hands or grippers are complex machines requiring numerous joints to provide high mobility within a relatively small device. Compliant mechanisms and grippers based on compliant joints provide a viable approach to design improved grippers. The use of compliant joints in the design of a hand yields a number of features that can potentially benefit the design; it allows for more lifelike mobility and can eliminate the need for traditional bearings that yield high contact stresses. This allows for much more variety in material choices. The freedom of choosing from a wider range of materials provides many benefits. For example, plastics can provide softer finger members, improved gripping characteristics and components that are less stiff, making them inherently safer for systems that operate in proximity to people. They can provide the flexibility to more naturally conform to the contour of a particular object when grasping it and reduce the necessary gripping forces to achieve reliable operation. Additionally, a solid-state design compliant mechanism design allows more freedom in designing mechanisms that will be constructed for high mobility and operating in a small space. This approach is further enhanced by the increased availability of additive manufacturing tools that enables ready implementation of compliant mechanism designs with almost any topology. This paper will examine the application additive manufacturing tools to create an anthropomorphic gripper based on compliant mechanism components. The primary contribution of this paper is the empirical evaluation of a set of compliant joints for use as the fingers in an anthropomorphic robotic hand produced using additive manufacturing. Three compliant joints will be considered: the simple straight-axis flexural pivot, cross-axis flexural pivot, and leaf-type isosceles-trapezoidal flexural pivot. Each joint type has demonstrated characteristics that may be suitable for fingers in gripping mechanism and are readily suited to be manufactured using low-cost fused deposition modeling techniques that allow for quick and low-cost production. Further, three materials are evaluated for application as the build material of each compliant joint individually and as a complete solid-state anthropomorphic gripper. These materials are: acrylonitrile butadiene styrene (ABS), Nylon 6, and thermoplastic polyurethane (TPU). Each joint and each material option is compared on the basis of their feasibility for rapid prototyping and suitability for substitution of the interphalangeal joints of the human hand. Deflection tests and finite element analysis are used to gather the empirical data for comparison. An evaluation of the tests is provided to determine which compliant joints are well suited for this application. The paper will also consider the as-built material characteristics relative to their application as gripper elements and will compare and contrast the suitability and any impact on the empirical testing and design. This work will provide information on the combination of joint topology, material and manufacturing processes and can be used to inform the design of soft or highly compliant mechanisms.