神经网络在电液位置伺服系统中的应用

设计了基于BP神经网络的自适应PID控制器，该控制器充分利用了经典PID控制算法简单的特点，又利用了神经网络良好的自适应和抗干扰能力，通过神经网络的学习和对系统的在线辨识，自适应调整PID参数，使控制系统对参数变化表现出良好的鲁棒性和控制性能，并通过仿真证明了所设计系统的合理性。     

High-temperature performance evaluation of adirectionally solidified nickel-base superalloy

The application of a new approach, design for performance, for high-temperature alloy development, design analysis, and remaining life assessment, based on short-time high-precision testing, is described in this paper. The material tested was a directionally solidified nickel-base alloy, GTD111. It was found that the creep strength at 850 °C was indeed superior to that of a competitive alloy, IN738, but was not necessarily enhanced by the preferred alignment of grain boundaries and crystal orientation. In contrast, the fracture resistance at 800 °C was improved in the longitudinal direction compared with transverse and diagonal orientations in terms of susceptibility to gas phase embrittlement (GPE) by oxygen. Specimens cut transversely and diagonally to the growth direction were more sensitive to GPE than specimens taken from conventionally cast IN738. The new conceptual framework allows account to be taken of GPE and other embrittling phenomena, which may develop in service, leading to rational life management decisions for gas turbine users. Additionally, straightforward design analysis procedures can be developed from the test data, which for the first time allow separate measurements of creep strength and fracture resistance to be used for performance evaluation.     

Some new operational modes and parameters of stress relaxation for the viscoelastic characterization of solid polymers. III. The area ratio mode and the intrinsic “strain–clock” function

The introduced area ratio mode of operation with its corresponding parameters seems to have a fairly high sensitivity to the viscoelastic response of the solid polymer. This appeared from the fact that a good distinction among the linear viscoelastic, the nonlinear viscoelastic, and the viscoplastic ranges of behavior can be made. By using a relevant rheological modeling and its corresponding algorithmical approach, in the case of isotactic polypropylene, this material can be characterized as a morphological three‐phase material consisting of an intraspherulitic crystalline, an amorphous phase, and a interspherulitic para‐crystalline phase. In this sense, the material was simulated using two models: the Poynting–Thomson and the Maxwell–Wierchert, from where a good response of the material to the first model appeared. The so‐called intrinsic “strain–clock” function and its corresponding coefficient of strength of nonlinear viscoelastic behavior, which were relieved by the experimental data, seem to be some powerful and very practical “tools” that can give a proven suplementary characterization of the material. Finally, by this intrinsic function, the existence of permanent internal stresses, was confirmed, in an indirect way, which was mentioned in part II of this study. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 149–158, 2003     

注塑流动与传热分析的自适应隐式控制体积法

在注塑流动与传热分析中采用自适应隐式控制体积法,利用自动控制的参数识别方法来自动调节时间步长,在每个时间步长流动将向前推进大约半个单元大小的距离,厚度方向的温度采用契比雪夫配点法进行计算,压力方程的求解方法是预条件共轭梯度法。计算结果表明,自适应隐式控制体积法的时间复杂度是结点数目的平方,而且1h之内可以求解10000结点规模的问题。     

Analysis and tuning of adaptive generalized predictive control

The adaptive Generalized Predictive Controller (Clarke et al., 1987a, b) is capable of controlling plants with variable dead-time, unknown model orders and unstable poles and zeros. This paper shows how the GPC control law can be written in an equivalent general linear transfer function form which simplifies closed-loop (eg. root locus) analysis. Three recommended strategies for selecting the design parameters during the commissioning stage allow the user to adjust the closed-loop speed of response on-line using only a single active tuning parameter. Experimental runs confirm the ability of adaptive GPC to provide a consistent closed-loop response in spite of large process changes.     

Diluent effects on carbonate mobility in bisphenol-A polycarbonate in the solid state

The effect of miscible low molecular weight additives on the mobility of the carbonate group in bisphenol-A polycarbonate (BPAPC) has been studied using n.m.r. and dielectric relaxation experiments in the solid state. Proton-enhanced dipolar-decoupled carbon-13 n.m.r. spectra of BPAPC, isotopically enriched at the carbonate position, are obtained without magic-angle sample spinning. The resolved chemical shift anisotropy allows study of nuclear spin relaxation for the carbonate groups in the polymer that have different orientations relative to the static magnetic field in the laboratory frame. The spin-lattice relaxation time in the rotating frame (T_1?) is measured at a motional-probe frequency of 50 kHz for the undiluted polymer and for BPAPC-diluent blends containing either dibutylphthalate or dinitrobiphenyl. The T_1? exhibits some dependence on orientation in all systems studied. In the blend containing dibutylphthalate (DBP), T_1? is decreased by a factor of two for all orientations of the carbonate group. This implies that DBP substantially increases the spectral density of 50 kHz motions in the carbonate region of the polymer at ambient temperature. In contrast, dinitrobiphenyl does not significantly alter the Fourier component of thermal fluctuations at 50 kHz. Dielectric relaxation measurements at 10 kHz reveal that the primary (T_g) and secondary (β) motional processes in BPAPC are affected by low molecular weight additives. An intermediate relaxation process appears in the temperature interval between the glass transition temperature (T_g) and the sub-T_g β-relaxation (T_β) in the polymer-diluent blends. The n.m.r. spin-lattice relaxation rate in the rotating frame, T^?1_1?, correlates well with the relative magnitude of the dielectric dissipation factor (tan δ_ε) between T_g and T_β.     

Evaluation of the characteristics of a viscoelastic material from creep analysis using the universal viscoelastic model. I. Isolation of the elastic component designated as the “Projected Elastic Limit”

In a preceding publication this author introduced a new universal viscoelastic model to describe a definitive relationship between constant strain rate, creep, and stress relaxation analysis for viscoelastic polymeric compounds. One extremely important characteristic of this new model is that it also characterizes secondary creep very well. Because secondary creep is the linear portion of creep after the completion of primary creep, then a straight line with a slope and an intercept can describe secondary creep. To effectively define a straight line in the secondary creep region it was found necessary to obtain averages of the instantaneous slope and the instantaneous intercept strain by averaging over a series of equally spaced data points in the secondary slope region. Most importantly, this average intercept strain was found to be independent of creep stress and creep time. This means that all the secondary creep straight lines must pass through the same intercept creep strain for all creep stresses. The results presented in this study strongly indicate that this secondary creep intercept strain is independent of creep stress and creep time, and appears to increase as the value of the efficiency of yield energy dissipation decreases. Because a decrease in the efficiency of yield energy dissipation, n, appears to correlate with an increase in the elastic solid like character of a material, then it appears that this secondary creep intercept strain should be a direct measure of the strain that the material can survive to retain its full elastic character. Therefore, this secondary creep intercept strain has been designated as the “Projected Elastic Limit” of a given viscoelastic material. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2923–2936, 2003     

Stochastic defect diffusion model for relaxation effects in crystalline polyethylene

It is suggested that some relaxation processes observed in crystalline polyethylene are consequences of the diffusive motion of a particular defect called a point dislocation or twist dispiration loop along the polyethylene stems in lamellar crystals. The motion of the defect, characterized by a diffusion coefficient and a mobility, is described by solutions of the Smoluchowski diffusion equation with boundary conditions that constrain the defect to move along routes that produce experimentally observable results. The fact that passage of the defect causes both a 180° rotation of the chain and moves an extra CH₂ group in the direction of the chain axis is important to the interpretation of the data according to this model. The diffusion coefficient for a defect is estimated to be around 2 × 10^?9 cm² s^?1 at 70°C. This value is shown to be reasonable both from the viewpoint of detailed computer modelling of defect motion and contemporary ideas about scaling.     

Interpretation of quasi-elastic light scattering data for flexible chains: model dependence

The autocorrelation functions and corresponding relaxation times obtained from the forward depolarized quasi-elastic light scattering experiment are exhibited for two quite similar models of flexible polymer chains in solution. A very small change in the chain dynamics is found to be sufficient to change the relaxation time from a relatively short time independent of chain length, with an autocorrelation function suggestive of an unweighted sum of contributions from all the relaxation times in the spectrum of chain motion, to a long time with an autocorrelation function identical with that for the end-to-end vector, strongly dependent upon chain length and dominated by the longest relaxation time in the spectrum. These results raise the question whether widely-used models in which information about short-range chain structure and motion is deliberately omitted can be expected to be appropriate for the interpretation of depolarized scattering experiments.     

Low frequency ac conduction and dielectric relaxation behavior of solution grown and uniaxially stretched poly(vinylidene fluoride) films

The measurements of ac conductivity [σ_m(ω)], dielectric constant [?′(ω)] and loss [?″(ω)] have been performed on solution grown (thickness ∼85 μm) and uniaxially stretched (thickness ∼25, 45 and 80 μm) films of poly(vinylidene fluoride) (PVDF) in the frequency range 0.1 kHz-10 MHz and in the temperature range 77-400 K. The σ_m(ω) can be described by the relation σ(ω) = Aω^s, where s is close to unity and decreases with increase in temperature. Three relaxations, observed in the present investigation, have been designated as the α_c-, the α_a- and the β-relaxations appearing from high temperature side to the low temperature side. The α_c-relaxation could not be observed in the case of uniaxially stretched poly(vinylidene fluoride) films. The α_c- and α_a-relaxations are associated with the molecular motions in the crystalline regions and micro-Brownian motion in the amorphous regions of the main polymer chain, respectively, whereas the β-relaxation is attributed to the rotation of side group dipoles or to the local oscillations of the frozen main polymer chain.