Design, Test and Control of a Magnetorheological Universal Gripper for Use On-Orbit

The Magnetorheological (MR) Gripper is a robotic end-effector technology consisting of a bladder filled with magnetorheological (MR) fluid connect to an electromagnetic. MR fluid is a suspension of micron-scale ferro-magnetic particles. The grains in MR fluids form chains and resist deformation when a magnetic field is applied, causing the fluid to exhibit a non-zero yield stress. To operate the MR Gripper, the bladder is pressed over a target (causing the bladder to deform about the target) and the electromagnet is activated. The activation of the electromagnet causes the MR fluid to instantaneously ‘harden’ about the target. Once the electromagnet is deactivated, the MR fluid returns to its liquid state. The MR Gripper could be a useful robotic end-effector in space applications where little is known about the target size and shape pre-launch. Additionally, the symmetry and deformability of the membrane reduce the sensing and control complexity that would be required to actuate this end-effector. The objective of this research was to mature this technology from TRL 2 to TRL 3 through experimental and computational investigations. While robotic end-effectors using MR fluids have previously been investigated in industry, this is the first investigation to publicly release the experimental performance of the MR Gripper to enable reproduction in the wider community. We have experimentally characterized the performance of the MR Gripper for a range of design choices and have found that the Gripper is able to grasp a wide range of targets (ranging from cylinders and spheres to computer mice). Working towards a predictive model of the MR Gripper performance, we have created a simulation of the MR Gripper using LIGGGHTS, an open-source soft-sphere discrete element method (SSDEM) code. For the first time, we have implemented magnetic forces and magnetic induction in LIGGGHTS and reproduced the physical characteristics of MR fluid. These code modifications will be publicly released in the next year. We have also implemented and experimentally validated a hyperelastic bladder in LIGGGHTS. We have also simulated the actuation of the full MR Gripper prototype. Additional analysis is necessary to validate this simulation as a predictive model, and progress the technology to TRL 4. Future work should focus on reducing the Gripper’s mass (through a redesign of the electromagnet housing) and power requirements (through the use of an electropermanent magnet), which will be critical for future space applications. We have provided the first comprehensive investigation of a robotic end-effector actuated solely by electromagnet-activated MR fluid. This technology, or related MR-fluid actuated robotics, may be useful in a variety of spaceflight and terrestrial applications.