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首页> 外文会议>19th International conference on adaptive structures and technologies 2008 >Multi-objective Optimization of a Magneto-rheological Prosthetic Knee Actuator
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Multi-objective Optimization of a Magneto-rheological Prosthetic Knee Actuator

机译：磁流变假肢膝关节致动器的多目标优化

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Regaining biomechanical function, comfort and quality of life is a prime consideration when designing prosthetic limbs. Recently, microprocessor-controlled prosthetic knees, which rely on magneto-rheological (MR) technology, have become available and have the potential to meet these needs. One of these promising products is a prosthetic knee manufactured by the company Ossur Inc. The knee is a synergy of artificial intelligence, advanced sensors and MR actuator technology. The prosthetic knee uses the variable rheological properties of MR fluids to control the knee's variable stiffness while an amputee walks. The 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 MR actuator in the prosthetic knee. The actuator is a variable stiffness MR rotary brake, utilizing MR fluids in shear-mode. MR fluids are fluids that change their rheological properties upon the application of a magnetic field. MR fluids consist of a carrier liquid, immersed with solid particles. The paper addresses the design of the MR rotary brake actuator, with respect to three important qualities. The three design qualities are the maximum obtainable field-induced braking torque of the actuator, the minimum obtainable stiffness in the absence of a magnetic field, and the weight of the MR actuator. It is important to investigate the trade-offs between these qualities. Maximizing the field-induced braking torque of the knee is important for the knee to be capable of supporting heavy amputees. Minimizing the off-state stiffness of the knee is important for fast movements of the knee, in load-free movements. Furthermore, minimizing the weight of the actuator is important for patients comfort. It is realized that these design goals can not be addressed separately and to some extend, the design goals are contradictory. Mathematical models are presented that describe the design goals 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. Calculating the weight of the actuator requires the geometry of the actuator and the density of its materials. The actuator employs a silicone-based MR fluid. The presented models rely on off-state viscosity measurements of an MR fluid from the actuator. The field-induced behavior of the fluid is determined with a shear-yield stress model of a perfluorinated polyether-based (PFPE) MR fluid. The maximum torque, the minimum torque and the weight of the actuator are specified mathematically in the form of three objective functions. A direct search optimization algorithm is presented that investigates, simultaneously, the trade-offs between the three objective functions. The optimization is restricted by practical manufacturing design constraints. Mapping the dependency between the maximum torque, minimum stiffness and the weight of the MR actuator gives valuable insight into further development of the prosthetic knee.
机译：在设计假肢时，重新考虑生物力学功能，舒适性和生活质量是首要考虑因素。最近，依靠磁流变（MR）技术的微处理器控制的假肢已经问世，并且有潜力满足这些需求。这些有前途的产品之一是Ossur Inc.公司制造的假肢。膝盖是人工智能，先进的传感器和MR执行器技术的结合。假肢使用MR流体的可变流变特性来控制截肢者行走时膝盖的可变刚度。流体的响应时间约为毫秒，因此可以根据传感器数据实时更改膝盖的刚度。本文的重点是假肢膝关节的MR执行器。该执行器是可变刚度MR旋转制动器，利用剪切模式的MR流体。 MR流体是在施加磁场后会改变其流变性的流体。 MR流体由浸有固体颗粒的载液组成。本文从三个重要方面论述了MR旋转制动执行器的设计。这三个设计质量是执行器的最大可获得的场致制动扭矩，在没有磁场的情况下最小的可获得的刚度以及MR执行器的重量。重要的是要研究这些质量之间的权衡。对于膝盖来说，能够支撑重截肢者，最大程度地增加膝盖的场致制动扭矩非常重要。在无负载运动中，最小化膝盖的断态刚度对于膝盖的快速运动很重要。此外，使致动器的重量最小对于患者的舒适度很重要。已经认识到，这些设计目标不能分开解决，并且在某种程度上，这些设计目标是矛盾的。介绍了数学模型，这些数学模型将设计目标描述为所选设计参数的函数。确定励磁制动力矩需要进行磁性有限元分析，以评估MR流体中的磁通密度和MR流体的剪切屈服应力曲线。评估断态刚度需要MR流体的断态粘度，以及轴承和油封中的摩擦力。计算致动器的重量需要致动器的几何形状及其材料的密度。执行器采用硅树脂基MR流体。提出的模型依赖于来自执行器的MR流体的断态粘度测量值。流体的场致行为是通过全氟化聚醚基（PFPE）MR流体的剪切屈服应力模型确定的。以三个目标函数的形式在数学上规定了执行器的最大转矩，最小转矩和重量。提出了一种直接搜索优化算法，该算法同时研究了三个目标函数之间的取舍。优化受到实际制造设计约束的限制。绘制最大扭矩，最小刚度和MR执行器重量之间的依存关系，可以为假肢膝关节的进一步发展提供有价值的见解。

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