Mechanically Programmable Bend Radius for Fiber-Reinforced Soft Actuators Kevin C. Galloway 1 , Panagiotis Polygerinos 2 , Conor J. Walsh 3 , and Robert J. Wood 4 Abstract— Established design and fabrication guidelines exist for achieving a variety of motions with soft actuators such as bending, contraction, extension, and twisting. These guidelines typically involve multi-step molding of composite materials (elastomers, paper, fiber, etc.) along with specially designed geometry. In this paper we present the design and fabrication of a robust, fiber-reinforced soft bending actuator where its bend radius and bending axis can be mechanically-programed with a flexible, selectively-placed conformal covering that acts to mechanically constrain motion. Several soft actuators were fabricated and their displacement and force capabilities were measured experimentally and compared to demonstrate the utility of this approach. Finally, a prototype two-digit end- effector was designed and programmed with the conformal covering to shape match a rectangular object. We demonstrated improved gripping force compared to a pure bending actuator. We envision this approach enabling rapid customization of soft actuator function for grasping applications where the geometry of the task is known a priori. I. INTRODUCTION The inherent compliance in soft material robotic systems enables capabilities and task versatility not found in tradi- tional rigid-bodied robotic systems. For example, complex motions can be embedded into a monolithic structure, which reduces the mechanical complexity (i.e. no moving parts), manufacturing costs, and can simplify the controls overhead [1], [2], [3]. Soft systems also offer improved safety as these actuators (typically pneumatic or hydraulic) are inherently safe for interfacing with humans, animals or fragile objects due to their natural compliance and back drivability [4]. The soft material actuators found in these soft systems are typically constructed from a combination of elastomeric and inextensible but flexible (i.e. woven and non-woven) materi- als. Upon pressurization, embedded channels or chambers in the soft actuator expand in directions with the lowest stiffness and give rise to linear, bending, and twisting motions. For example, a McKibben actuator swells radially and contracts lengthwise upon pressurization to shorten the overall length [5]. A bending actuator - the focus of this work - combines *Research supported by DARPA award W911NF-11-1-0094. 1 K. C. Galloway is a research engineer with the Wyss Institute at Harvard University, Cambridge, MA 02138 USA (kevin.galloway at wyss.harvard.edu). 2 P. Polygerinos is a post-doctoral research fellow in the Wyss Institute and School of Engineering and Applied Science (SEAS) at Harvard University. 3 C. Walsh is an assistant professor of Mechanical and Biomedical Engineering in the Wyss Institute and SEAS at Harvard Univesity. 4 R.J. Wood is the Charles River Professor of Engineering and Applied Sciences in the Wyss Institute and SEAS at Harvard University. a linear extending actuator with a strain limiting layer along the length. As the actuator is pressurized, part of it grows in length while the strain limited portion cannot, causing the actuator to bend. This concept is well established and a variety of ap- proaches have been demonstrated to achieve this function. For example, in 1967, James Baer patented a bellows- inspired soft actuator that could bend around an object in response to fluid pressure [6]. In another example, [7] presents a multi-degree of freedom bending actuator com- posed of three parallel chambers (120 ◦ apart) contained in a fiber reinforced tubular elastic body. These works and many others [8], [9], [10], [11], [12], [13] present methods for creating bending soft actuators; however, no one has yet - to our knowledge - presented a method for rapidly programming the bend radius and bending axis of a soft bending actuator by modifying the mechanical structure. This has relevance towards improving the agility of soft actuators - the ability to create intricate movements - by enabling the user to adjust the placement and magnitude of a bending actuator’s radius of curvature [2]. As a point of comparison, the rigid mechanical joints in a traditional robotic system enable precise and intricate motions; however, these systems typically have long engineering design cycles with expensive components (e.g. motors, bearings, etc.) and precision machined parts - all requiring time and expense to source, manufacture, assemble and ship. The materials in soft material robotics offers an alternative approach to the design cycle where it is possible to rapidly and inexpensively fabricate custom actuators on-site. II. SOFT ACTUATOR FABRICATION A. Fabrication of a Bending Soft Actuator The fiber reinforced (FR) soft bending actuator (see Fig. 1 for cross-sectional views) used in this study is fabricated using a multi-step molding process. This approach offers complete control over every aspect of the assembled soft actuator including geometry, material properties, and fiber reinforcements. The molds for the actuator were 3D printed with an Objet Connex 500 (Fig. 2). The first rubber layer (labeled ‘rubber’ in Fig. 1 and pictured in Fig. 3a) has a 2mm wall thickness and used a 15.88 mm (0.625 inch) diameter half round steel rod to define the interior, hollow portion of the actuator. After molding the first rubber layer, fiber reinforcements were added to the surface (Figs. 3a-b). Woven fiberglass (S2-6522 plain weave 4 oz. weight) was glued to 978-1-4799-2722-7/13/$31.00 c  2013 IEEE