A model of postural control in quiet standing: robust compensation of delay-induced instability using intermittent activation of feedback control.
A model of postural control in quiet standing: robust compensation of delay-induced instability using intermittent activation of feedback control.
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DOI：
10.1371/journal.pone.0006169
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发表时间：
2009-07-08
期刊：
影响因子：
3.7
通讯作者：
Morasso P
中科院分区：
文献类型：
--
作者：
Asai Y;Tasaka Y;Nomura K;Nomura T;Casadio M;Morasso P
本研究的主要目的是比较两种不同的反馈控制器，以稳定人体安静站立，考虑到固有的踝关节刚度不足，并且反馈回路中存在较大的延迟，导致不稳定：1）标准线性连续时间 PD 控制器和 2）间歇性 PD 控制器，其特征在于相平面中定义的开关函数，在标称平衡状态周围有或没有死区。第一个控制器的稳定性分析是使用线性控制系统的标准工具进行的，而间歇控制器的分析是基于使用相平面中定义的庞加莱图。当 PD 控制关闭时，系统的动力学特征为鞍状平衡，具有稳定和不稳定流形。间歇控制器的切换功能是这样实现的：当状态向量接近鞍座的稳定流形时，PD 控制为“关闭”，否则为“打开”。理论分析和相关仿真研究表明，间歇控制模型比标准模型鲁棒得多，因为在后一种情况下表征稳定行为的反馈控制增益（P vs.D）参数空间中的区域大小比前一种情况大得多。此外，间歇控制器可以使用比标准模型小得多的反馈参数。间歇控制器产生的典型摇摆模式是当 PD 控制关闭时沿着鞍座稳定流形的慢速运动和远离直立平衡的螺旋运动之间交替的结果，该直立平衡由激活具有低反馈增益的 PD 控制确定。值得注意的是，通过巧妙地结合两种不稳定状态（鞍形和不稳定螺旋）可以实现整体动态稳定性。间歇控制器利用鞍座某一部分的稳定作用，让系统在稳定流形上或附近滑动时单独演化；当状态向量进入鞍座的强烈不稳定部分时，它会打开一个温和的反馈，这不应该强加严格的稳定制度，而是为了减轻即将发生的跌落。间歇控制器中死区的存在不会改变稳定性特性，但会提高与生物摇摆模式的相似性。还通过考虑具有加性噪声的模型生成的摇摆序列的功率谱密度（PSD），在频域中对两种类型的控制器进行了比较。与标准连续模型不同，其 PSD 函数类似于无共振的过阻尼二阶系统，间歇控制模型能够表现出人类生理摇摆运动典型的两种幂律缩放机制。
The main purpose of this study is to compare two different feedback controllers for the stabilization of quiet standing in humans, taking into account that the intrinsic ankle stiffness is insufficient and that there is a large delay inducing instability in the feedback loop: 1) a standard linear, continuous-time PD controller and 2) an intermittent PD controller characterized by a switching function defined in the phase plane, with or without a dead zone around the nominal equilibrium state. The stability analysis of the first controller is carried out by using the standard tools of linear control systems, whereas the analysis of the intermittent controllers is based on the use of Poincaré maps defined in the phase plane. When the PD-control is off, the dynamics of the system is characterized by a saddle-like equilibrium, with a stable and an unstable manifold. The switching function of the intermittent controller is implemented in such a way that PD-control is ‘off’ when the state vector is near the stable manifold of the saddle and is ‘on’ otherwise. A theoretical analysis and a related simulation study show that the intermittent control model is much more robust than the standard model because the size of the region in the parameter space of the feedback control gains (P vs. D) that characterizes stable behavior is much larger in the latter case than in the former one. Moreover, the intermittent controller can use feedback parameters that are much smaller than the standard model. Typical sway patterns generated by the intermittent controller are the result of an alternation between slow motion along the stable manifold of the saddle, when the PD-control is off, and spiral motion away from the upright equilibrium determined by the activation of the PD-control with low feedback gains. Remarkably, overall dynamic stability can be achieved by combining in a smart way two unstable regimes: a saddle and an unstable spiral. The intermittent controller exploits the stabilizing effect of one part of the saddle, letting the system evolve by alone when it slides on or near the stable manifold; when the state vector enters the strongly unstable part of the saddle it switches on a mild feedback which is not supposed to impose a strict stable regime but rather to mitigate the impending fall. The presence of a dead zone in the intermittent controller does not alter the stability properties but improves the similarity with biological sway patterns. The two types of controllers are also compared in the frequency domain by considering the power spectral density (PSD) of the sway sequences generated by the models with additive noise. Different from the standard continuous model, whose PSD function is similar to an over-damped second order system without a resonance, the intermittent control model is capable to exhibit the two power law scaling regimes that are typical of physiological sway movements in humans.
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影响因子：
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DOI：
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发表时间：
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