Extremum Seeking Control for Stiffness Auto-Tuning of a Quasi-Passive Ankle Exoskeleton
Extremum Seeking Control for Stiffness Auto-Tuning of a Quasi-Passive Ankle Exoskeleton
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DOI：
10.1109/lra.2020.3001541
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发表时间：
2020-06
影响因子：
5.2
通讯作者：
Saurav Kumar;Matthew Richard Zwall;Edgar A. Bolívar-Nieto;R. Gregg;N. Gans
中科院分区：
文献类型：
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作者：
Saurav Kumar;Matthew Richard Zwall;Edgar A. Bolívar-Nieto;R. Gregg;N. Gans
最近，有研究表明，重量轻、被动的脚踝外骨骼具有基于弹簧的能量储存和释放机制，可以减少人类行走的肌肉力量。这种装置中弹簧的刚度必须适当地调整，以便将肌肉的作用力降到最低。然而，这种肌肉作用力会随着运动条件的不同而变化(例如，行走速度)，从而导致最佳弹簧刚度也不同。现有的被动外骨骼在操作过程中具有固定的刚性，使其无法对行走条件的变化做出反应。因此，需要一种设备和自动调整算法，在保持被动外骨骼优势的同时，将不同行走条件下的肌肉力量降至最低。在这封信中，我们开发了一种准被动踝关节外骨骼，具有可自我调节的变刚度机构。由于不同行走速度下的肌肉作用力与最优弹簧刚度之间的关系是未知的，因此实现了一种无模型的离散时间极值寻优控制算法(ESC)来实时优化弹簧刚度。对一名健全的受试者进行的实验表明，随着使用者行走速度的改变，ESC会自动调整脚踝关节的扭转刚度。慢走时，胫前肌和比目鱼肌的均方根肌电读数分别下降了26.48%和7.42%。
Recently, it has been shown that light-weight, passive, ankle exoskeletons with spring-based energy store-and-release mechanisms can reduce the muscular effort of human walking. The stiffness of the spring in such a device must be properly tuned in order to minimize the muscular effort. However, this muscular effort changes for different locomotion conditions (e.g., walking speed), causing the optimal spring stiffness to vary as well. Existing passive exoskeletons have a fixed stiffness during operation, preventing it from responding to changes in walking conditions. Thus, there is a need of a device and auto-tuning algorithm that minimizes the muscular effort across different walking conditions, while preserving the advantages of passive exoskeletons. In this letter, we developed a quasi-passive ankle exoskeleton with a variable stiffness mechanism capable of self-tuning. As the relationship between the muscular effort and the optimal spring stiffness across different walking speeds is not known a priori, a model-free, discrete-time extremum seeking control (ESC) algorithm was implemented for real-time optimization of spring stiffness. Experiments with an able-bodied subject demonstrate that as the walking speed of the user changes, ESC automatically tunes the torsional stiffness about the ankle joint. The average RMS EMG readings of tibialis anterior and soleus muscles at slow walking speed decreased by 26.48% and 7.42%, respectively.