Magneto-rheological (MR) fluids have been successfully introduced to prosthetic devices. One such a device is a biomechanical prosthetic knee joint that uses a 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 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 focus of this paper is on the design of the magnetic circuit of the actuator and on the geometry of the fluid chamber. The paper describes the function of the MR rotary actuator and shows how design optimization techniques can aid in the development of the actuator. The design is optimized, with respect to three important design objectives. These objectives are: the maximum obtainable field-induced braking torque, the minimum obtainable rotary damping in the absence of a magnetic field, and the weight of the actuator. Multi-objective design optimization techniques are presented and applied to the prosthetic knee actuator design problem. Trade-offs between design objectives are investigated giving valuable information on the development of the actuator. Maximizing the field-induced braking torque is important for the knee to be capable of supporting heavy amputees. Minimizing the off-state stiffness is important for fast movements of the knee, in load-free movements. Furthermore, minimizing the weight of the actuator is important for allowing heavy components like batteries to be installed. It is realized that these design objectives can not be addressed separately and to some extend, the design goals are contradictory. Mathematical models are presented that describe the design objectives as a function of selected design parameters. Determining the field-induced braking torque requires a magnetic finite element analysis, to evaluate the magnetic flux density in the MR fluid, and the shear-yield stress curve of the MR fluid. Evaluating the off-state stiffness requires the off-state viscosity of the MR fluid, along with friction in bearings and oil seals. The models are based on rheological measurements of the MR fluid employed in the knee. Evaluating the weight of the actuator requires the geometry of the actuator and the density of its materials. The optimization is restricted by practical manufacturing design constraints. Mapping the dependency between the maximum torque, the minimum damping, and the weight of the MR actuator gives valuable insight into the design of the prosthetic knee actuator.
- Aerospace Division
A Multi-Objective Design Optimization of a Smart Magneto-Rheological Prosthetic Knee
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Gudmundsson, KH, Jonsdottir, F, & Thorsteinsson, F. "A Multi-Objective Design Optimization of a Smart Magneto-Rheological Prosthetic Knee." Proceedings of the ASME 2009 Conference on Smart Materials, Adaptive Structures and Intelligent Systems. Volume 2: Multifunctional Materials; Enabling Technologies and Integrated System Design; Structural Health Monitoring/NDE; Bio-Inspired Smart Materials and Structures. Oxnard, California, USA. September 21–23, 2009. pp. 187-194. ASME. https://doi.org/10.1115/SMASIS2009-1237
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