Robotic end effectors are needed for a variety of terrestrial and space-based tasks. End effectors are commonly inspired by the human hand, with a range of complexity, including varying number of fingers, joints and degrees of freedom. Hand and pincher-type grippers can require complicated actuation and/or a significant sensing and control system to ensure proper orientation of the gripper about the target prior to gripping as well as to avoid crushing the target. Universal grippers, most notably exemplified by the universal Jamming Gripper (Brown et al 2010 Proceedings of the National Academy of Science 107, 18809-18814), provide an alternative design that aims to simplify the actuation, sensing and control of robotic fixturing end-effectors. The universal Jamming Gripper is actuated pneumatically, by applying vacuum to granular media, thereby causing the grains to jam, when confined by an external bladder. We present the design and performance of a universal gripper that is actuated by applying a magnetic field, thereby eliminating the need to supply vacuum (and a reinflation source) to actuate the gripper. The magnetic field actuates a magnetorheological fluid composed of a bi-disperse mixture of carbonyl iron grains suspended in a silicone oil. The performance of this novel gripper design is characterized for a range of target sizes and shapes, and gripper design characteristics.
Magnetorheological (MR) valves are an attractive way to make reliable valves with no moving parts. MR fluid valves operate by powering an electromagnet positioned near a constriction through which MR fluid is flowing. However, these valves are high-power devices, consuming on the order of watts of power while closed, and the electromagnets and flow paths are relatively bulky. Due to their power draw and size, they are unsuitable for many miniaturized and portable applications which would otherwise benefit from a solid state valve. In this paper, we introduce a low power, jamming MR valve that makes use of an electropermanent magnet, which can provide either a strong magnetic field or no field, with no continuous power draw and no moving parts. The resulting valve has overall dimensions of 4× 4×6mm, a mass of 0.476g, material costs of $7.32 per valve USD at quantity 100, holds over 415 kPa of pressure, and leaks only 0.02g of fluid over a 24h period when held at 105 kPa. These valves are well suited for use in soft robots, e.g. robots composed of stretchable elastomers and may allow for increased degrees of freedom in soft robotic designs. We discuss the design considerations for making MR valves, study the effect of different fluids and valve sizes, develop a numerical framework for simulation and further valve design, and demonstrate the use of a MR valve to control the actuation of a soft robotic appendage.
The design and test of a magnetorheological fluid (MRF)-based universal gripper (MR gripper) are presented in this study. The MR gripper was developed to have a simple design, but with the ability to produce reliable gripping and handling of a wide range of simple objects. The MR gripper design consists of a bladder mounted atop an electromagnet, where the bladder is filled with an MRF, which was formulated to have long-term stable sedimentation stability, that was synthesized using a high viscosity linear polysiloxane (HVLP) carrier fluid with a carbonyl iron particle (CIP) volume fraction of 35%. Two bladders were fabricated: a magnetizable bladder using a magnetorheological elastomer (MRE), and a passive (non-magnetizable) silicone rubber bladder. The holding force and applied (initial compression) force of the MR gripper for a bladder fill volume of 75% were experimentally measured, for both magnetizable and passive bladders, using a servohydraulic material testing machine for a range of objects. The gripping performance of the MR gripper using an MRE bladder was compared to that of the MR gripper using a passive bladder.
We simulate magnetorheological fluids (MRF) using open source LIGGGHTS soft sphere discrete element method code, extended by us to include a mutual dipole magnetic model. Our simulations take advantage of the many pair forces available in the LIGGGHTS framework, including SJKR cohesion, friction, and rolling resistance. In addition, we have included an uncoupled, Couette flow background carrier fluid. The simulated particles in this work are polydisperse, with distributions made to match the distributions used to produce magnetorheological fluids in literature, increasing the fidelity of the simulations. Using the accurate particle size distributions, high heritage contact models, and an uncoupled fluid model, we are able to match experimental MRF yield stress results more closely than with monodisperse simulations.
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