90 resultados para LONG VARIANT


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In the field of motor control, two hypotheses have been controversial: whether the brain acquires internal models that generate accurate motor commands, or whether the brain avoids this by using the viscoelasticity of musculoskeletal system. Recent observations on relatively low stiffness during trained movements support the existence of internal models. However, no study has revealed the decrease in viscoelasticity associated with learning that would imply improvement of internal models as well as synergy between the two hypothetical mechanisms. Previously observed decreases in electromyogram (EMG) might have other explanations, such as trajectory modifications that reduce joint torques. To circumvent such complications, we required strict trajectory control and examined only successful trials having identical trajectory and torque profiles. Subjects were asked to perform a hand movement in unison with a target moving along a specified and unusual trajectory, with shoulder and elbow in the horizontal plane at the shoulder level. To evaluate joint viscoelasticity during the learning of this movement, we proposed an index of muscle co-contraction around the joint (IMCJ). The IMCJ was defined as the summation of the absolute values of antagonistic muscle torques around the joint and computed from the linear relation between surface EMG and joint torque. The IMCJ during isometric contraction, as well as during movements, was confirmed to correlate well with joint stiffness estimated using the conventional method, i.e., applying mechanical perturbations. Accordingly, the IMCJ during the learning of the movement was computed for each joint of each trial using estimated EMG-torque relationship. At the same time, the performance error for each trial was specified as the root mean square of the distance between the target and hand at each time step over the entire trajectory. The time-series data of IMCJ and performance error were decomposed into long-term components that showed decreases in IMCJ in accordance with learning with little change in the trajectory and short-term interactions between the IMCJ and performance error. A cross-correlation analysis and impulse responses both suggested that higher IMCJs follow poor performances, and lower IMCJs follow good performances within a few successive trials. Our results support the hypothesis that viscoelasticity contributes more when internal models are inaccurate, while internal models contribute more after the completion of learning. It is demonstrated that the CNS regulates viscoelasticity on a short- and long-term basis depending on performance error and finally acquires smooth and accurate movements while maintaining stability during the entire learning process.

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Just under half of all energy consumption in the UK today takes place indoors, and over a quarter within our homes. The challenges associated with energy security, climate change and sustainable consumption will be overcome or lost in existing buildings. A background analysis, and the scale of the engineering challenge for the next three to four decades, is described in this paper.

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Just under half of all energy consumption in the UK today takes place indoors, and over a quarter within our homes. The challenges associated with energy security, climate change and sustainable consumption will be overcome or lost in our existing buildings. A background analysis, and the scale of the engineering challenge for the next three to four decades, is described in this paper.

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This paper proposes a single-phase variant of the Brushless Doubly-Fed Machine, the Single-Phase BDFM, SPB. Like the BDFM it is a variable speed generator which requires a converter rated at only a fraction of the machine rating, using structure with no brushes. Unlike the BDFM, most of its power is delivered directly into a single-phase grid connection. As such it is a potential replacement for PM generators in small-ish wind turbines, potentially with a much lower cost. In this paper we give details of its operation, with reference to results from the first prototype SPB. We also suggest how it may be controlled, again with experimental results. © 2011 IEEE.

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Long-term settlement of tunnels has caused concerns about its influence on tunnel safety and serviceability. Aiming to investigate the long-term behaviour of tunnels against the background of Shanghai metro line, two cases of centrifuge modelling were conducted, with efforts to expose the mechanism affecting the consolidation of the ground. Evenly layered ground and transitional ground strata were set for each case separately and the settlement, lining load and pore water pressure were checked against elapsed time up to 20 years. The results verified some previous findings concerning the settlement and lining load development trend, however, it was also shown that the transitional ground made the tunnel response more complicated. The research is expected to provide some basis for further research on other affecting factors, such as lining permeability. © 2010 Taylor & Francis Group, London.