Field-On Versus Field-Off Characteristics of Magnetorheological Fluids With an Application in Prosthetic Devices

The study presents an experimental investigation into the trade-offs between field-on versus field-off rheological characteristics of magnetorheological (MR) fluids. This is relevant in a particular application in prosthetic devices where field-off characteristics are of equal importance to the field-on rheological characteristics. The paper introduces a biomechanical prosthetic knee joint that uses an MR fluid to actively control its rotary stiffness while an amputee walks. The knee is a synergy of artificial intelligence, advanced sensors and MR actuator technology. The knee joint is equipped with an MR rotary brake, utilizing the fluid in direct-shear mode. The MR fluid has response time in the order of milliseconds, making it possible to vary the knee’s stiffness in real-time, depending on sensors data. The field-on characteristics of the employed MR fluid define the rigidness of the knee joint while the field-off characteristics define its flexibility in the absence of a magnetic field. Five MR fluid compositions are prepared, each with a different solid loading ranging from 0.25 to 0.35, by volume. All fluids employ a commercially available carbonyl iron powder and a base fluid. The MR fluids are experimentally evaluated in a rheometer, where both field-off and field-on characteristics are measured. An MR fluid figure of merit function is introduced which is used to rate the selected MR fluids for a potential application in the MR prosthetic knee. An MR fluid composition is sought with the highest ratio between the field-on shear yield stress and the off-state viscosity. The research shows the off-state viscosity to decrease faster than the field-on shear yield stress when reducing the solid loading from 0.35 to 0.25. This suggests that an optimum solid loading exists with regards to the defined merit function. The off-state viscosity of suspensions is known to be exponentially dependent on solid loading while the field-on shear-yield stress is known to sub-quadratically dependent on solid loading. Field-on and field-off models are presented from literature. The models compared to the experimental data and used to theoretically predict the optimum solid loading with regards to field-on shear yield stress and off-state viscosity. As a result of the experimental and the theoretical analysis, a prominent MR fluid composition is selected for a potential application in the MR prosthetic knee. This has been shown to help in the development of prosthetic devices and furthering the success of an MR prosthetic knee joint.