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.

This content is only available via PDF.
You do not currently have access to this content.