Magnetically Induced Stiffening for Soft Robotics

Inherent compliance of soft robots renders them safe for many applications such as object manipulation, surgery, haptics, and wearable devices. Stiffness modulation is essential to soft robot design so that these robots can reverse compliance and institute more rigidity when needed. Relevant examples include robot reconfigurability, weight-bearing tasks, and force transmission. This has led to significant research on variable stiffness structures whose designs allow a soft robot to be flexible and compliant until interaction with its environment, when it switches from an inherently low-stiffness state to one of more rigidity. Variable stiffening mechanisms include pneumatically-controlled jamming and thermally-induced phase change materials. In jamming-based stiffening, granules, stacked layers, or fibers are packed into an encasing that as a whole is compliant and of low-density. Upon application of vacuum pressure, the overall structure increases density and solidifies within milliseconds. Despite fast response time, jamming methods lack portability due to reliance on pneumatic pressure lines, which are bulky and cumbersome in wearable or autonomous applications. Thermally-controlled phase change materials, such as shape memory polymers, induce a stiffness change when subject to electronically-controlled heat. They have the advantage of portability via electronic control, but often require high temperatures and have slower response time on the order of tens of seconds.<br/>Combining the advantages of these two stiffening methods, quick response time from pneumatic-based jamming with portability of thermal-controlled phase change materials, we present a magnetically-induced stiffening mechanism that requires scaffolding structures immersed in magnetorheological fluid. Magnetorheological fluids consist of micron-scale iron particles suspended in carrier fluid such as water, and solidify with a characteristic yield stress and viscosity when subjected to an external magnetic field. We exploit the response of magnetorheological fluid to applied magnetic fields to induce rapid stiffness changes, while investigating how the addition of scaffolding structures enhances and increases achievable stiffening ranges. These scaffolding structures are borrowed from pneumatic-based jamming and include stacked 51μm thick polyester layers, 7 μm thick fiber filaments, and 2.4 mm diameter granules. Suspending these materials in 80% iron (by mass) magnetorheological fluid and encasing them in a textile pouch, we create magnetorheological jamming beams (MRJ beams) which are flexible, compliant, and have tunable stiffness with applied magnetic fields.<br/>We investigated the stiffening response of MRJ beams subjected to various magnetic field strengths using two methods: by exploiting the yield stress increase of magnetorheological fluid using a single row of permanent magnets, and by inducing a clutch on the MRJ beam using dual rows of permanent magnets with aligned poles that exploit yield stress effects and compress the beam via magnetic attraction. Scaffolding materials in MRJ beams amplify the stiffening response in the presence of applied magnetic fields in both methods, with stiffness percent changes ranging from ≈10% - 75% and ≈50% - 345% for single and dual row magnet architectures, respectively. We also present an analytical model to provide initial estimates of achievable stiffening ranges as a function of magnetic field to inform soft robot design. We introduce portability by employing electropermanent magnets (EPMs) which require minimal energy to operate, can be electronically-controlled, and can change magnetic field strength in ≈500 μs. Embedding EPMs within an MRJ beam demonstrated electronically-controlled real-time stiffness change in a weight-bearing task. This work paves the way towards customizing soft robotic designs that require portability and fast response time stiffness tuning, such as in many human-robot interactions.