Analysis of postural perturbation responses

People with cerebellar ataxia lack lower limb coordination and dissipate sway motion slowly and inefficiently after a posture perturbation. We report a practical and low-cost “human resonance frequency test” for both laboratory and clinical use to quantify progress in balance and cerebellar rehabilitation. We assumed that the center-of-pressure (COP) oscillation rate of decay following a standing posture perturbation is directly related to resonance frequency; a more rapidly dissipating COP oscillation about the position of equilibrium indicates, by definition, more efficient postural control. We hypothesized that following successful physical rehabilitation, people with cerebellar degeneration will have a faster rate of decay of the COP response to an external perturbation. Because the COP is modulated by a synergy of trunk and lower limb motion strategies, COP decay rate may be a useful measure of lower limb coordination in people with cerebellar ataxia. The method was applied to three subjects with cerebellar ataxia before and after rehabilitation; there was good agreement between the calculated COP decay rate and conventionally used gait stability parameters providing pilot data for this simple approach     

Detecting postural responses to sinusoidal sensory inputs: a statistical approach

A common way for understanding sensory integration in postural control is to provide sinusoidal perturbations to the sensory systems involved in balance. However, not all subjects exhibit a response to the perturbation. Determining whether or not a response has occurred is usually done qualitatively, e.g., by visual inspection of the power spectrum. In this paper, we present the application of a statistical test for quantifying whether or not a postural sway response is present. The test uses an F-statistic for determining if there is significant power in postural sway data at the stimulus frequency. In order to describe the application of this method, 20 subjects viewed sinusoidal anterior-posterior (A-P) optic flow at 0.1 and 0.25 Hz, while their A-P head translation was measured. The test showed that significant postural responses were detected at the stimulus frequency in 12/20 subjects at 0.1 Hz and 13/20 subjects at 0.25 Hz.     

Pulsed electrical breakdown in liquid helium in the μs range

Information about breakdown time-lags under pulsed voltage stress is prerequisite to the insulation design of superconducting devices. Experiments have been carried out using a pulse generator which provides a step voltage with 200 ns rise time and a long decay time constant of more than 1 ms. The present experiments may bridge the information gap between time lag measurements in the ns range using extremely high field strengths, and existing data found with usual lightning impulse voltage. The test device is described in the paper, and some typical results are discussed in detail     

一种改进的PV-AF系统最大功率点跟踪方法

在具有有源滤波功能的光伏并网(Photovoltaic Power Generation and Active Filter,PV-AF)系统中,光伏电池的最大功率点跟踪(Maximum Power Point Tracking,MPPT)方法是其关键技术之一,针对常用的MPPT方法存在的问题,提出了一种改进的MPPT方法。该方法利用扰动观察法在最大功率点附近扰动的正向步数和反向步数基本相同的特点,判断系统工作点是否处于最大功率点(Maximum Power Point,MPP)附近。在此基础上利用变步长的扰动观察法,在远离最大功率点时采用大步长扰动,在最大功率点附近采用小步长扰动。在确定系统工作在最大功率点附近后,采用恒定电压法跟踪。扰动观察法的最大跟踪误差为1.5个电压扰动步长,提出的改进MPPT方法将最大跟踪误差降低为0.5个电压扰动步长。仿真和实验结果表明,该方法兼顾了跟踪的快速性、准确性和稳定性。     

A low-cost, portable system for the assessment of the postural response of wheelchair users to perturbations.

Maintaining seated postural stability presents a serious challenge to wheelchair users in vehicles, even during normal driving conditions. The purpose of this research was to develop a system for the study of seated postural control in response to perturbations similar to those that might be experienced during vehicle turning and braking. A servo-controlled tilt platform was constructed to provide a low-cost, small, and easily transportable device for generating precise and repeatable perturbations. Tilt platform operation was examined for accuracy and reproducibility of a desired perturbation. Repeatability was high with a mean signal-to-noise ratio (SNR) of 45.4 for a given perturbation measured across 11 subjects. An initial comparison of stability results obtained on the tilt platform and in a vehicle showed a correspondence, although differences were apparent. The tilt platform has been used successfully to assess balance in spinal cord-injured subjects and to test wheelchair securement systems.     

光伏发电系统最大功率点跟踪控制及拓扑结构研究的新进展

本文针对光伏器件的特点提出两种新的最大功率点跟踪控制方法：短路电流结合扰动观察法及用非对称模糊控制的扰动观察法。前一种方法在短路电流控制方法的基础上引入了优化扰动步长的扰动观察法，它可有效消除光伏器件输出功率在最大功率点的振荡现象。第二种方法把非对称模糊控制引入传统的扰动观察法，它在光伏器件最大功率点两侧的特性采取不同的扰动步长，可有效消除传统方法在最大功率点处的功率振荡。仿真和实验研究证明：上述两种方法可以快速跟踪外部环境变化，并消除系统在最大功率点的振荡现象。同时本文提出一种新型的用于小功率光伏发电的高频逆变电路，它由buck－boost变换器和电流源高频链逆变器构成。由它来实现光伏模块的最大功率点跟踪，得到与电网同步的电压。该电路结构简单、效率高，光伏模块的最大功率点不受负载变化的影响。该方案通过实验验证。     

The control of foot placement during compensatory stepping reactions: does speed of response take precedence over stability?

Rapid, reflex-like stepping movements are a prevalent and functional compensatory reaction to destabilization, however, little is known about the underlying control. In this paper, a model is developed to examine how speed and stability demands affect control of foot placement during forward and backward compensatory stepping reactions. The concept of the velocity stability margin (VSM) is introduced to characterize the degree to which the horizontal velocity of the falling body approaches biomechanical limits on the capacity to decelerate the center of mass; analogous limits on center-of-mass displacement are quantified in terms of the displacement stability margin (DSM). The model is used to predict, for any initial step characteristics, the variation in DSM and VSM that would occur as a function of changes in timing of foot placement. The VSM was found to prevail over the DSM in establishing limits of stability. Model simulations demonstrated that there typically exists a minimum swing duration that maximizes speed of response while meeting minimum requirements for stability (VSM > or = 0), as well as a slower speed of response (longer swing duration) at which stability (VSM) is maximized. Experimental data from platform-perturbation tests in 20 healthy young (22-28) and older (65-81) adults were used, in conjunction with the model, to investigate whether speed or stability takes precedence during natural behavior. Control of single-step reactions appeared to favor stability; although the model predicted that a minimally stable step (VSM = 0) could be attained by swing durations as short as 30 ms, the observed swing durations were, on average, 135 ms longer than this, and the average VSM was nearly as large (80%) as the optimally stable value predicted by the model. Control of the initial step of multiple-step reactions was distinctly different. The average swing duration was only 55 ms greater than the minimally stable value and the average VSM was 81% smaller than in the single-step reactions. This reduction in VSM is consistent with a need to execute additional steps and appears to support the validity of the model. This model may help to provide insight into the biomechanical factors that govern the neural control of compensatory stepping reactions.     

光伏发电系统最大功率追踪算法及其仿真

针对普通的最大功率算法在最大功率点振荡、追踪速度不高等缺点,提出了一种新的最大功率点跟踪控制方法——直线近似法结合变步长扰动观察法的最大功率追踪方法。仿真结果表明,该方法可有效消除传统方法在最大功率点处的功率振荡。     

Directionally Specific Objective Postural Response Assessment Tool for Treatment Evaluation in Stroke Patients

The aim of the research is to develop an objective evaluation tool for use in stroke rehabilitation clinical practice. Stroke patients are prone to particularly high risk of fall, which may differ for various directions of movement. An apparatus enabling perturbations and postural response assessment in eight directions in transversal plane during standing was used to assess data in seven neurologically intact volunteers and 10 stroke patients before and after the rehabilitation. Ground reaction force and center of pressure were acquired during the perturbation, signal processed and compared to Berg Balance Scale (BBS), a clinical outcome measure of balance. The results of the weight load ratio between the affected and unaffected lower extremity demonstrated objective positive outcomes of the rehabilitation and also correlated with the clinical instrument BBS. Additionally, the center of pressure ratio between the anterior/posterior and medial/lateral peak for each perturbation direction have shown identifiable postural response strategies in selected directions of transverse plane. The directional postural information can be helpful when identifying and evaluating the objective rehabilitation progress which can lead to application of targeted rehabilitation techniques. The directional indicator also demonstrated correlation with the BBS in directions indicating rehabilitation progress. When considering the common use with the clinical instrument, the proposed objective rehabilitation progress evaluation tool may also become helpful in directional fall risk indication. The proposed tool may become a powerful instrument, when the balance training and postural response assessment will move to remote or home environment as a telerehabilitation service.      

MF型音圈电机滑模-自抗扰控制研究

针对系统外部不确定扰动及内部摩擦力等非线性特性对MF型音圈电机控制系统的影响,提出了基于滑模控制与自抗扰控制结合的复合控制。建立了基于改进LuGre动态摩擦力电机模型,设计了内环采用PI控制的双闭环控制,其中外环在系统远离滑模面时采用滑模控制实现快速响应,到达滑模面趋近平衡点时采用自抗扰控制消除抖振,提高系统稳定性和控制精度。通过与PID控制以及滑模控制对比的仿真和试验数据表明:提出的滑模-自抗扰控制显著提升了MF型音圈电机阶跃响应时间、相位滞后时间、控制精度,同时系统对负载变化以及噪声具有较强的抗扰性和鲁棒性。     

窄环隙内层流脉动流动特性分析

为分析脉动流动状态下的热工水力特性,通过建立数学模型,对窄环隙流道内的层流脉动流动特性进行分析。研究结果表明,脉动流对环隙内速度及相位差径向分布有较大影响。当脉动频率较小时,窄环隙内的速度径向分布与稳定流动相同,相位差在整个流道截面为恒定值;随着脉动频率增大,速度径向分布出现"环状效应",相位差沿径向呈现近似抛物线型分布;且脉动频率及流动尺寸越大、或流体粘性越小,脉动对速度及相位差的径向分布影响越明显,而脉动振幅变化对速度及相位差的径向分布没有影响。     

基于改进扰动观察法的直驱波浪发电最大功率跟踪控制

为改善直驱型波浪发电系统在波浪突变情况下的功率捕获效果,分析系统运动频域模型,导出系统输出有功功率与永磁直线发电机等效阻抗的频域空间关系曲线。针对该曲线在三维空间变化特点,提出交替施加变步长扰动的扰动观察法。变步长扰动,易在最大功率点较大范围内陷入局部最优,为此提出依据区域斜率变化,改变扰动步长的控制策略。基于MATLAB/Simulink环境,搭建系统仿真模型,仿真结果表明,在波浪突变情况下,改进控制策略能有效提高系统最大功率跟踪的抗干扰能力,系统能较快地重新追踪到最大功率点附近。     

Implementation of a physiologically identified PD feedback controller for regulating the active ankle torque during quiet stance.

Our studies have recently demonstrated that a proportional and derivative (PD) feedback controller, which takes advantage of the body's position and velocity information to regulate balance during quiet standing, can compensate for long neurological time delays and generate a control command that precedes body sway by 100-200 ms. Furthermore, PD gain pairs were identified that ensure a robust system behavior and at the same time generate dynamic responses as observed in quiet standing experiments with able-bodied subjects. The purpose of the present study was to experimentally verify that the PD controller identified in our previous study can: 1) regulate the active ankle torque to stabilize the body during quiet standing in spite of long neurological time delays and 2) generate system dynamics, i.e., a motor command and body sway fluctuation, that successfully mimic those of the physiologic system of quiet standing. Our real-time closed-loop feedback circuit consisted of a center of mass position sensor and a functional electrical stimulator that elicited contractions of the plantar flexors as determined by the aforementioned PD controller. The control system regulated upright stance of a subject who was partially de-afferented and de-efferented due to a neurological disorder called von Hippel-Lindau Syndrome (McCormick Grade III). While the subject was able to generate a motor command for the ankle joints, he could not regulate the resulting torque sufficiently due to a lack of sensory feedback and motor control. It is important to mention that a time delay was included in the closed-loop circuit of the PD controller to mimic the actual neurological time delay observed in able-bodied individuals. The experimental results of this case study suggest that the proposed PD controller in combination with a functional electrical stimulation system can regulate the active ankle torque during quiet stance and generate the same system dynamics as observed in healthy individuals. While these findings do not imply that the CNS actually applies a PD-like control strategy to regulate balance, they suggest that it is at least theoretically possible.     

基于改进扰动观察法的光伏阵列最大功率点跟踪

常规的扰动观察法具有算法简洁、跟踪效率高的特点,在光伏阵列的最大功率点跟踪系统中得到广泛的应用。然而常规扰动观察法由于采用固定的扰动步长,难于同时获得较高的响应速度和稳态跟踪精度。本文提出了一种固定电压启动的变步长扰动观察算法,并与常规的扰动观察法进行了对比实验,实验结果证明了该方法的有效性。     

海上风电场三电平VSC-HVDC系统仿真研究

随着土地资源紧缺和海上风电成本的不断降低,海上风电开发与建设受到各国重视,且随着海上风电场规模的扩大,急需采用柔性直流输电变流器装置.这里研究了海上风电场柔性直流输电变流器拓扑结构及控制方式,其中,主电路采用中点箝位式三电平结构,通过建立该结构的等效数学模型,根据空间矢量调制算法,检测交流侧电流方向以及直流侧电容电压,...     

Determining Parameters of Turbine's Model Using Heat Balance Data of Steam Power Unit for Educational Purposes

In this paper, the procedure for deriving the parameters of an IEEE governor-turbine model (particularly turbine model) based on a practical study case consisting of a 200-MW tandem compound, single reheat steam unit and its available heat balance data is presented. The main focus of the work is on presenting a regular procedure and using only available heat balance data of the steam unit, to be suitable for training the principles and details of such an approach for educational purposes. Unavailable parameters are approximated with simple thermodynamic assumptions, resulting in good correspondence to typical values. The model response to step changes for special scenarios is simulated and presented as well.     

动态电压振荡型失稳边界分析与算法研究

提出了全参数空间下的最近动态电压振荡型失稳边界的计算方法。在每次迭代中,采用逐步增长负荷的迭代方式找到振荡型失稳边界,求取失稳边界的超平面法向量。将该法向量作为新的负荷增长方式寻找下一步失稳边界,最终找到系统发生电压振荡型失稳的最近边界及其对应的最危险负荷增长方向。相对于摄动方法而言,该方法将各种因素对于电压稳定域的影响考虑在内,且不需要求取控制参数的海森矩阵,具有计算速度快、精度高的特点。选取系统有功负荷作为控制参数,研究了负荷的不同增长方式对系统振荡失稳边界的影响,并根据超平面的法向量求解出系统发生振荡型失稳的最近边界以及所对应的最危险的负荷增长方向。提出2类电压稳定指标,所提指标有助于综合评估系统动态电压稳定裕度。采用3节点和IEEE 14节点仿真系统对算法的有效性进行了验证。     

Compliant grasp in a myoelectric hand prosthesis

A novel myoelectric hand prosthesis consisting of electromyogram (EMG) signal processing units, a microprocessor-based dc motor servo system, and a 1 degree-of-freedom (DOF) end effector has been developed. The flexion angle and compliance of the finger of this prosthesis can be voluntarily controlled with EMG signals. The biomimetic controller for the myoelectric hand incorporated a model of the neuromuscular control system constructed from an analysis of the force response to length perturbation of the flexor pollicis longus muscle, processing of EMG signals, and the configuration of the hand. Basic functions of the human neuromuscular control system are realized by using position control, force feedback, and variable gain, modulated by EMG signal amplitude. A limb-absent person and four healthy subjects were able to voluntarily control the finger angle and compliance of the prosthesis and were able to easily grasp a soft object after a short training period.     

正反馈有源频率扰动孤岛检测的一种改进算法

反孤岛保护是光伏并网发电系统的主要特性指标。这里结合周期性扰动的正反馈有源频率扰动(AFDPF)法和2N周期电流扰动法的优点,提出一种新型AFDPF孤岛检测法。该方法通过检测电流频率扰动后输出端电压频率的变化来判断扰动效果,实现了孤岛保护。基于PSIM和Matlab软件平台对孤岛保护效果进行仿真,而后在6 kW的并网逆变器实验平台进行验证。仿真和样机实验结果表明,该方法可快速检测出孤岛状态。     

配直吹式制粉系统的超临界机组调节系统的改进及应用

配中速磨直吹式制粉系统的超临界机组,制粉系统滞后时间长、惯性大,而超临界锅炉本身蓄热较小且具有强耦合性和非线性。针对以上特点,对机组协调控制系统及其子系统进行了改进和优化,克服纯迟延、大惯性环节对控制系统的影响,加快锅炉侧动态响应,稳定燃烧与给水之间的平衡关系,改善调节品质。