Study of the open and closed loop characteristics of a tractor and a single axle towed implement system

Accurate automatic guidance of towed implements is important for performing agricultural field operations and for gaining the ultimate benefit from such systems. The study of open and closed loop system responses of a vehicle-implement system can be helpful in the design of practical guidance controllers. Open loop analysis of the kinematic and dynamic models revealed that the higher order dynamics captured by the tractor and implement dynamic model had an impact on simulated responses at higher operating velocities and on higher input frequencies. In addition, a dynamic model with tire relaxation length dynamics was also studied. The various model responses were compared with the experimental responses. Closed loop system characteristics were studied by using a linear quadratic regulator (LQR) controller. The tractor position and heading and implement heading states along with respective rate states were fed back to close the loop. Steering dynamics were also added to the dynamic model closed loop analysis, which helped to achieve a realistic closed loop steering angle history. The closed loop system dynamics became faster as the forward velocity was increased. The open and closed loop response analysis performed in this work provided an understanding about the system at various forward velocities, which will help to design and develop efficient and robust tractor and towed implement guidance controller.     

Gemini表面活性剂对疏水缔合聚丙烯酰胺界面吸附膜扩张流变性质的影响

采用小幅低频振荡和界面张力弛豫技术, 考察了疏水缔合水溶性聚丙烯酰胺(HMPAM)在正癸烷-水界面上的扩张黏弹性质, 研究了不对称Gemini表面活性剂C₁₂COONa-p-C₉SO₃Na对其界面扩张性质的影响. 研究发现, 疏水链段的存在, 使HMPAM在界面层中具有较快的弛豫过程, 扩张弹性显示出明显的频率依赖性. 表面活性剂分子可以通过疏水相互作用与聚合物的疏水嵌段在界面上形成类似于混合胶束的特殊聚集体. 表面活性剂分子与界面聚集体之间存在快速交换过程, 可以大大降低聚合物的扩张弹性. 同时, 聚合物分子链能够削弱表面活性剂分子长烷基链之间的强相互作用, 导致混合吸附膜的扩张弹性远低于单独表面活性剂吸附膜.     

Solid‐State NMR Study on Relationships between Miscibility and Chain Mobility in Poly(4‐Methylstyrene)/Poly(Cyclohexyl Methacrylate) Blend

The phase behavior and motional mobility in binary blends of poly(4‐methylstyrene) (P4MS) and poly(cyclohexyl methacrylate) (PCHMA) have been examined by ¹³C solid state NMR techniques. The blend miscibility was studied by measuring the ¹H spin‐relaxation times in the laboratory frame (T₁^H) and in the rotating frame (T_1ρ^H), respectively. Although intermolecular spin diffusion contributes to the proton relaxations in accordance with homogeneity, T_1ρ^H data shows signs of in complete averaging. The T_1ρ^H relaxation behavior indicates the existence of heterogeneous do mains with shortest dimensions in the nanometer range, which is also sup ported by the intermolecular cross polarization experiments with variable contact times. In addition, according to the resuits of carbon T_1ρ relaxation time measurements, it is concluded that mixing is intimate some what enough to cause a reduction in local chain mobility for P4MS and vice versa for PCHMA.     

Nuclear magnetic resonance relaxation studies of polyacrylamide solution

 The cohesive interaction among polymer chains in a polyacrylamide (PAAm)–D₂O solution has been studied by NMR relaxation. The NMR relaxation times of PAAm in the good solvent D₂O were measured at different temperatures. The results show that the solution system has a high local viscosity and that its relaxation characteristic is soft-solid-like. The temperature dependence of the relaxation behavior of the solution is obviously different from that of ordinary polymer solutions. The difference lies in the relaxation behavior of the methylene protons in the main chain of PAAm, as shown by analyzing the relaxation process with single exponential and biexponential decays. As the temperature increases, the solvation is weakened, leading polymer chains to form curling coils, thus hindering the movement of the methylene protons among the main chains. It can be expected from the existence of 80% fast-relaxing protons that there are a zhigh number of entanglements among the polymer chains in PAAm solution. The information about entanglements among the polymer chains can be deduced from the biexponential dependence of the spin–spin relaxation on the concentration of the polymer solutions. Received: 14 April 1999/Accepted in revised form: 12 October 1999     

A large deformation framework for compressible viscoelastic materials: Constitutive equations and finite element implementation

For modeling the constitutive properties of viscoelastic solids in the context of small deformations, the so-called three-parameter solid is often used. The differential equation governing the model response may be derived in a thermodynamically consistent way considering linear spring-dashpot elements. The main problem in generalizing constitutive models from small to finite deformations is to extend the theory in a thermodynamically consistent way, so that the second law of thermodynamics remains satisfied in every admissible process. This paper concerns with the formulation and constitutive equations of finite strain viscoelastic material using multiplicative decomposition in a thermodynamically consistent manner. Based on the proposed constitutive equations, a finite element (FE) procedure is developed and implemented in an FE code. Subsequently, the code is used to predict the response of elastomer bushings. The finite element analysis predicts displacements and rotations at the relaxed state reasonably well. The response to coupled radial and torsional deformations is also simulated.     

Photoacoustic and Fluorescence Spectroscopy of Metallomesogens Containing Lanthanide Ions

Metallomesogens Ln(bta)3L2 (Ln3+: La3+, Eu3+, and Ho3+; bta: benzoyltrifluoroacetonate; L: Schiff base) were prepared. Photoacoustic (PA) spectroscopy was used to study physicochemical properties of the liquid crystalline metal complexes. In the region of ligand absorption, PA intensity increases for Eu(bta)3L2, La(bta)3L2, and Ho(bta)3L2, in that order. It is found that the PA intensity of the ligand bears a relationto the intramolecular energy transfer process. For the first time, phase transitions of Eu(bta)3L2 from glass state to smectic A (SmA) phase and SmA phase to isotropic liquid are monitored by PA and fluorescence (FL) spectroscopy from two aspects: nonradiative and radiative transitions. The results show that PA technique may serve as a new tool for investigating the spectral properties and phase transitions of liquid crystals containing metal ions.     

Synthesis of Nonionic Oligomeric Manganese（ Ⅱ ） Complexes and Investigation of Their Toxicity and T1 -Relaxation Enhancement

Six nonionic oligomeric manganese （ Ⅱ ） complexes with oligomeric phosphate-polyglycol-EDTA ester ligands were synthesized and characterized. The longitudinal relaxivities of these complexes were measured. One of these complexes, which showed the highest relaxivity, was chosen to be used in the acute toxicity test and the T1-weighted imaging experiment. It has been found that compared to Gd-DTPA, this nonionic oligomeric Mn（ Ⅱ ） complex exhibits no acute toxicity, generates highly enhanced MRI signals and increases the intention time in the rat liver tissue.     

From dynamic modulus via different relaxation spectra to relaxation and creep functions

The main goal of the paper is to compare predictive power of relaxation spectra found by different methods of calculations. The experimental data were obtained for a new family of propylene random copolymers with 1-pentene as a comonomer. The results of measurements include flow curves, viscoelastic properties, creep curves and rubbery elasticity of copolymer melts. Different relaxation spectra were calculated using independent methods based on different ideas. It lead to various distributions of relaxation times and their “weights”. However, all of them correctly describe the frequency dependencies of dynamic modulus. Besides, calculated spectra were used for finding integral characteristics of viscoelastic behaviour of a material (Newtonian viscosity, the normal stress coefficient, steady-state compliance). In this sense all approaches are equivalent, though it appears impossible to estimate instantaneous modulus. The most crucial arguments in estimating the results of different approaches is calculating the other viscoelastic function and predicting behaviour of a material in various deformation modes. It is the relaxation and creep functions. The results of relaxation curve calculations show that all methods used give rather similar results in the central part of the curves, but the relaxation curves begin to diverge when approaching the high-time (low-frequency) boundary of the relaxation curves. The distributions of retardation times calculated through different approaches also appear very different. Meanwhile, predictions of the creep curves based on these different retardation spectra are rather close to each other and coincide with the experimental points in the wide time range. Relatively slight divergences are observed close to the upper boundary of the experimental window. All these results support the conclusion about a rather free choice of the relaxation time spectrum in fitting experimental data and predicting viscoelastic behaviour of a material in different deformation modes. Received: 15 March 2000 Accepted: 18 September 2000     

Approximations to Isentropic Planar Magneto-Hydrodynamics Equations by Relaxed Euler-Type Systems

In this paper, the authors consider an approximation to the isentropic planar Magneto-hydrodynamics (MHD for short) equations by a kind of relaxed Euler-type system. The approximation is based on the generalization of the Maxwell law for nonNewtonian fluids together with the Maxwell correction for the Amp`ere law, hence the approximate system becomes a first-order quasilinear symmetrizable hyperbolic systems with partial dissipation. They establish the global-in-time smooth solutions to the approximate Euler-type equations in a small neighbourhood of constant equilibrium states and obtain the global-in-time convergence towards the isentropic planar MHD equations. In addition, they also establish the global-in-time error estimates of the limit based on stream function techniques and energy estimates for error variables.