Impedance-Controlled Variable Stiffness Actuator for Lower Limb Robot Applications
Impedance-Controlled Variable Stiffness Actuator for Lower Limb Robot Applications
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DOI：
10.1109/tase.2019.2954769
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发表时间：
2020-04-01
影响因子：
5.6
通讯作者：
Misgeld, Berno J. E.
中科院分区：
文献类型：
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作者：
Liu, Lin;Leonhard, Steffen;Misgeld, Berno J. E.
我们提出了一种基于可变刚度致动器（VSA）的辅助/康复机器人特征阻抗控制的新应用，该阻抗控制使用级联位置转矩控制回路。机器人采用自适应阻抗控制模式，根据人体关节力矩达到自适应辅助水平。利用前馈人关节转矩指令协同调节VSA的阻抗控制器和刚度轨迹（此功能架构称为协同控制框架）。通过这种方式，可以提高运动训练过程中的任务性能：1)安全性-例如，当受试者打算贡献相当大的努力时，使用低刚度致动器激活低增益阻抗控制以进一步降低输出阻抗；2)跟踪性能-例如，对于较少努力的受试者，使用高增益阻抗控制，同时追求高刚度以提高扭矩带宽。在安全方面，我们证明了在低刚度下设计的转矩控制器可以在保持跟踪性能的同时对低输出阻抗的干扰敏感。这样做的一个先决条件是分别处理输入干扰。这是由我们之前提出的使用线性二次高斯技术的VSA转矩控制保证的。这里也采用了这种方法，但对观测器设计进行了额外的讨论，以服务于所提出的合作控制方法。在这里，使用VSA原型和人体测试人佩戴的一自由度下肢外骨骼实验验证了所提出的控制系统的有效性。从业人员注意：“物理人机交互”的控制可以通过可变刚度执行器（VSA）的机械部件来实现。然而，刚度变化的机械结构可能会限制实现低输出刚度和快速刚度变化的能力。这些限制在辅助/康复机器人的应用中可能会变得更加明显。为了克服这些限制，阻抗控制方案可以实现可编程的阻抗范围和阻抗变化速度。该控制方案已广泛应用于固定柔度接头，但由于VSA接头现有的机械结构阻抗控制能力，目前尚无办法在其上实施。本文介绍了阻抗控制VSA在下肢机器人上的一种新应用。描述了如何调整作动器的刚度，使之与自适应阻抗控制方案协同工作。基于我们的方法，采用阻抗控制的VSA关节可以扩展机器人的带宽容量和低输出阻抗。这是对阻抗控制固定柔度接头的改进。本文提出的协同控制框架在外骨骼系统上进行了两名健康测试人员的测试，也适用于其他执行器原型。未来的研究旨在将该系统用于实际的患者培训。
We present a novel application of the variable stiffness actuator (VSA)-based assistance/rehabilitation robot-featured impedance control using a cascaded position torque control loop. The robot follows the adaptive impedance control paradigm, thereby achieving an adaptive assistance level according to human joint torque. The feedforward human joint torque command is used to cooperatively adjust the impedance controller and the stiffness trajectory of the VSA (this functional architecture is referred to as the cooperative control framework). In this way, the task performance during movement training can be improved regarding: 1) safety-for example, when the subject intends to contribute considerable effort, low-gain impedance control is activated with a low stiffness actuator to further decrease output impedance and 2) tracking performance-for example, for the subject with less effort, high-gain impedance control is used while pursuing high stiffness to enhance the torque bandwidth. Regarding the safety aspect, we demonstrate that the torque controller designed at low stiffness can be sensitive to the disturbance for low output impedance while maintaining tracking performance. A precondition for this is to treat the input disturbance separately. This is guaranteed by our previously proposed torque control of the VSA using the linear quadratic Gaussian technique. This approach is also employed here, but with additional discussion on the observer design to serve the proposed cooperative control approach. Here, the effectiveness of the proposed control system is experimentally verified using a VSA prototype and a one-degree-of-freedom lower limb exoskeleton worn by a human test person. Note to Practitioners-Control of "physical human-robot interaction" can be achieved by the mechanical parts of the variable stiffness actuator (VSA). However, the mechanical construction for stiffness variation may limit the capacity to achieve low output stiffness and fast stiffness variation in speed. These limitations may become more evident in the assistance/rehabilitation robot applications. To overcome these limitations, the impedance control scheme can be employed to achieve a programmable impedance range and impedance variation speed. This control scheme has been widely applied on the fixed-compliance joint but lacks a way to be implemented on the VSA joint because of its existing capacity to control the impedance with the mechanical construction. This article presents a novel application of the impedance-controlled VSA used on a lower limb robot. We describe how to adjust the actuator stiffness to cooperatively work with the adaptive impedance control scheme. Based on our approach, the robot with the impedance-controlled VSA joint can extend the capacity of bandwidth and low output impedance. This is an improvement on the impedance-controlled fixed-compliance joint. The cooperative control framework presented here was tested on an exoskeleton system with two healthy test persons and is also applicable to other actuator prototypes. Future research aims to employ this system for actual patient training.