复合式光电编码器精密轴系设计及准确度分析

 为了提高复合式光电编码器的轴系准确度,运用一个径向密珠轴承与两个轴向止推密排轴承相结合的结构设计了大空心轴精密轴系,并对轴系误差进行分析.通过该方法设计的精密轴系径向跳动δ＜3 μm,解决了大空心轴轴系准确度差的问题,提高复合式编码器的测角准确度,应用该轴系研制的复合式光电编码器准确度σ＜2″.     

Damping characteristics of elastic-viscoelastic composite curved bars and helical springs

The paper deals with the analysts of damping characteristics of a curved composite shaft. The composite shaft consists of an outer elastic curved tube with a similarly curved rod placed inside. The annular space between the tube and the rod is filled with viscoelastic material. The composite shaft is clamped at one end and a concentrated load (varying harmonically) acts on a radial arm fixed to the outer shell at the free end. It is shown that optimum design values exist for maximizing the total damping capacity of the system. It also is indicated that direct use of this method for increasing the damping capacity is not effective, as the damping factor reduces sharply after reaching a maximum value. It is shown how this difficulty can be overcome and how helical springs, possessing considerable damping capacity, can be designed. Sometimes this may help in simplifying the design of vibration isolators which will take the shape of a simple spring only.     

DYNAMIC RESPONSE AND STABILITY ANALYSIS OF AN AUTOMATIC BALL BALANCER FOR A FLEXIBLE ROTOR

Dynamic stability and time responses are studied for an automatic ball balancer of a rotor with a flexible shaft. The Stodola-Green rotor model, of which the shaft is flexible, is selected for analysis. This rotor model is able to include the influence of rigid-body rotations due to the shaft flexibility on dynamic responses. Applying Lagrange's equation to the rotor with the ball balancer, the non-linear equations of motion are derived. Based on the linearized equations, the stability of the ball balancer around the balanced equilibrium position is analyzed. On the other hand, the time responses computed from the non-linear equations are investigated. This study shows that the automatic ball balancer can achieve the balancing of a rotor with a flexible shaft if the system parameters of the balancer satisfy the stability conditions for the balanced equilibrium position.     

Dynamics of a supercritical composite shaft mounted on viscoelastic supports

The damping in a carbon fiber reinforced plastic (CFRP) laminate is greater than that which occurs in most metallic materials. In the supercritical regime, the damping can trigger unstable whirl oscillations, which can have catastrophic effects. The vibrations occurring in a supercritical composite drive shaft are investigated here in order to predict instabilities of this kind. A simply supported carbon/epoxy composite tube mounted on viscoelastic supports is studied, using an approximation of the Rayleigh–Timoshenko equation. The damping process is assumed to be hysteretic. The composite behavior is described in terms of modulus and loss factor, taking homogenized values. The critical speeds are obtained in several analytical forms in order to determine the effects of factors such as the rotatory inertia, the gyroscopic forces, the transverse shear and the supports stiffness. Assuming that the hysteretic damping can be expressed in terms of the equivalent viscous model, the threshold speed is obtained in the form of an analytical criterion. The influence of the various factors involved is quantified at the first critical speed of a subcritical composite shaft previously described in the literature. The influence of the coupling mechanisms on the unsymmetrical composite laminate and the end fittings is also investigated using a finite element model. None of these parameters were found to have a decisive influence in this case. Those having the greatest effects were the transverse shear and the supports stiffness. The effects of the composite stacking sequence, the shaft length and the supports stiffness on the threshold speed were then investigated. In particular, drive shafts consisting only of ±45° or ±30° plies can be said to be generally unstable in the supercritical regime due to their very high loss factors.     

Transverse vibration of a rotor system driven by a Cardan joint

The transverse vibration of a rotor system driven by a Cardan joint is analyzed and the effect of the transmitted torque on the dynamic stability of the system evaluated. As a result of the analysis, the following facts are proved: when the driving shaft and driven shaft (rotor shaft) are included, both parametric and self-excited vibrations arise due to transmitted torque; asymmetrical stiffness of the rotor supports has the effect of stabilizing this self-excited vibration.     

Bathocuproine/Ag复合电极对于聚合物光伏器件效率和稳定性的影响

采用Bathocuproine/Ag (BCP/Ag)复合电极代替Ca/Al复合电极, 制备PTB7:PC71BM 作为光敏层的聚合物光伏器件, 并通过改变BCP薄膜厚度来研究BCP/Ag复合电极对于器件光电转换器和稳定性的影响. 研究发现: 在光敏层和金属电极之间插入BCP修饰层后, 器件性能得到了显著的改善, 在BCP厚度为5 nm时, 器件的效率达到了6.82%, 且略高于Ca/Al复合电极的器件效率; 相比于采用Ca/Al复合电极的器件, BCP/Ag复合电极增大了器件的短路电流和外量子效率, 使器件效率得到提高; 同时器件的稳定性得到了显著的改善, BCP/Ag 复合电极器件的衰减速率几乎和未插入BCP的器件衰减速率相同, 相对于Ca/Al复合电极器件大幅提高.     

Dynamic stability analysis of torsional vibrations of a shaft system connected by a Hooke׳s joint through a continuous system model

Stability of parametrically excited torsional vibrations of a shaft system composed of two torsionally elastic shafts interconnected through a Hooke?s joint is studied. The shafts are considered to be continuous (distributed-parameter) systems and an approximate discrete model for the torsional vibrations of the shaft system is derived via a finite element scheme. The stability of the solutions of the linearized equations of motion, consisting of a set of Mathieu–Hill type equations, is examined by means of a monodromy matrix method and the results are presented in the form of a Strutt–Ince diagram visualizing the effects of the system parameters on the stability of the shaft system.     

Soil influence on unbalance response and stability of a simple rotor-foundation system

The equations of motion are set up for a simple rotor (Jeffcott or Laval rotor) on a rigid foundation mass resting on an elastic half space (soil). The unbalance response and the stability limit against self-excited vibrations caused by the internal damping of the rotating shaft are calculated. The numerical results presented as response diagrams and stability graphs show that the damping effect of the soil on the system, due to radiation of energy, may have a very positive influence on the smooth running of the rotors.     

Dynamic instability of supercritical driveshafts mounted on dissipative supports—Effects of viscous and hysteretic internal damping

The case of a rotating shaft with internal damping mounted either on elastic dissipative bearings or on infinitely rigid bearings with viscoelastic suspensions is investigated in order to obtain the stability region. A Euler-Bernoulli shaft model is adopted, in which the transverse shear effects are neglected and the effects of translational and rotatory inertia, gyroscopic moments, and internal viscous or hysteretic damping are taken into account. The hysteretic damping is incorporated with an equivalent viscous damping coefficient. Free motion analysis yields critical speeds and threshold speeds for each damping model in analytical form. In the case of elastic dissipative bearings, the present results are compared with the results of previous studies on finite element models. In the case of infinitely rigid bearings with viscoelastic suspensions, it is established that viscoelastic supports increase the stability of long shafts, thus compensating for the loss of efficiency which occurs with classical bearings. The instability criteria also show that the effect of the coupling which occured between rigid modes introducing external damping and shaft modes are almost more important than damping factor. Lastly, comparisons between viscous and hysteretic damping conditions lead to the conclusion that an appropriate material damping model is essential to be able to assess these instabilities.     

Control of hysteretic instability in rotating machinery by elastic suspension systems subject to dry and viscous friction

Most of the undesired whirling motions of rotating machines can be efficiently reduced by supporting journal boxes elastically and controlling their movement by viscous dampers or by dry friction surfaces normal to the shaft axis, which rub against the frame. In the case of dry dampers, resonance ranges of the floating support configuration can be easily cut off by planning a motionless adhesive state of the friction surfaces. On the contrary, the dry friction contact must change automatically into sliding conditions when the fixed support resonances are to be feared. Moreover, the whirl amplitude can be restrained throughout the speed range by a proper choice of the suspension-to-shaft stiffness ratio and of the support-to-rotor mass ratio.This theoretical research deals firstly with the natural precession speeds and looks for Campbell plots in dependence on the shaft angular speed, for several rotor-suspension systems. Then, the steady response to unbalance is investigated, in terms of rotor and support orbits and of conical path of the rotor axis. In this search, the ranges of adhesive or sliding contact are identified in particular for system with dry friction damping. At last, the destabilizing influence of the shaft hysteresis in the supercritical regime is focalized and the counterbalancing effect of the other dissipative sources is verified. In the nonlinear case of dry friction dampers, the control of linear stability is fulfilled by a perturbation procedure, checking the magnitude of Floquet characteristic multipliers on the complex plane. Moreover, the nonlinear stability far from steady motion is tested by the direct numerical solution of the full motion equations. The comparison configuration of suspension systems with viscous dampers and no dry friction is examined through an analytical first approximation approach and closed-form results for stability thresholds are derived in particular for the symmetric case.     

Stability and transient dynamics of a propeller–shaft system as induced by nonlinear friction acting on bearing–shaft contact interface

This paper investigates the friction-induced instability and the resulting self-excited vibration of a propeller–shaft system supported by water-lubricated rubber bearing. The system under consideration is modeled with an analytical approach by involving the nonlinear interaction among torsional vibrations of the continuous shaft, tangential vibrations of the rubber bearing and the nonlinear friction acting on the bearing–shaft contact interface. A degenerative two-degree-of-freedom analytical model is also reasonably developed to characterize system dynamics. The stability and vibrational characteristics are then determined by the complex eigenvalues analysis together with the quantitative analysis based on the method of multiple scales. A parametric study is conducted to clarify the roles of friction parameters and different vibration modes on instabilities; both the graphic and analytical expressions of instability boundaries are obtained. To capture the nature of self-excited vibrations and validate the stability analysis, the nonlinear formulations are numerically solved to calculate the transient dynamics in time and frequency domains. Analytical and numerical results reveal that the nonlinear coupling significantly affects the system responses and the bearing vibration plays a dominant role in the dynamic behavior of the present system.     

VIBRATION CHARACTERISTICS OF SLOTTED SHAFTS

The dynamics of asymmetric rotors has been investigated in the literature. However, considering the importance in the practical fields, the vibration characteristics of slotted shafts are not studied in detail. It needs more attention particularly with the emerging of new materials, such as composites for shafts. The present study aims at the analysis of the slotted shafts including the compensatory inertia slots. Also, the slotted composite shaft has been modelled based on first order shear deformation theory using finite element method with shell elements. Different materials such as, Boron epoxy, Carbon epoxy and Graphite epoxy have been tried for various stacking sequences. The slot parameters, stacking sequences and material properties have found to influence to great extent on the vibrational characteristics of rotors. The results are compared with those of isotropic slotted shaft.     

Influence of load torque on stability of rotor driven by flexible shaft

The influence of the load torque on the stability of a symmetric rotor, driven by a flexible shaft, is studied. Both linear and angular displacements of the rotor are considered. The analysis—which is approximate, and with the deflection, the damping and the load torque assumed to be small—shows that the main destabilizing effect of the load torque is due to the transverse moments acting while the rotor is inclined. The reaction pattern at the ends of the shaft, determined by means of the Kirchhoff equations, indicates that the semitangential mode of loading (conservative) is operative.     

PbSe/TiO2同轴异质结纳米管的制备及光催化性能

通过溶液法合成了PbSe/TiO2复合纳米管,并对其进行了微观形貌、晶体结构等的表征。结果表明,制得的样品是由PbSe和TiO2两种材料构成的复合材料,致密、均匀的TiO2薄膜包覆在PbSe纳米管表面。以氙灯为模拟光源,通过对甲基橙的降解研究了PbSe/TiO2复合纳米管的光催化性能。结果显示,PbSe与TiO2之间形成的异质结使PbSe/TiO2复合纳米管具有较高的光催化性能,比纯PbSe纳米管的催化降解率提高了约4.5倍。另外,对PbSe/TiO2复合纳米管光催化稳定性也进行了研究。     

Effect of drying temperature on a thin PVDF-HFP/PET composite nonwoven separator for lithium-ion batteries

A thin composite separator with polyethylene terephthalate nonwoven membrane as the structural support and polyvinylidene fluoride-hexafluoropropylene as the coating layer for lithium-ion batteries was prepared by a simple dip-coating process. The effect of different drying temperatures on the performance of the composite separator was investigated. The results indicate that 80 °C is the optimal drying temperature, preventing leakage current problems and providing a well-developed porous structure. The drying of the composite separator at 80 °C provides a superior thermal stability, better wettability with electrolyte, higher electrolyte uptake, and ionic conductivity compared to commercially available polypropylene (PP) separator. Furthermore, the electrochemical performance consisting of electrochemical stability, self-discharge, cycle performance, rate performance of the composite separator, and PP were determined. The drying of the composite separator at 80 °C shows almost the same oxidation stability and self-discharge performance, but a better cycling and rate performance than the PP separator.     

Synthesis and characterization of Li4Ti5O12/graphene composite as anode material with enhanced electrochemical performance

The use of graphene as a conductive additive to enhance the rate capability and cycle stability of Li₄Ti₅O₁₂ electrode material has been demonstrated. Li₄Ti₅O₁₂ and its composite with graphene (1.86 wt%) are prepared by ball milling and simple chemical method, respectively. Among the as-synthesized composites, Li₄Ti₅O₁₂ particles uniformly clung to the graphene sheets. When used as an electrode material for lithium ion battery, the composite presents excellent rate performance and high cyclic stability. It is found that the composite displayed high-rate capacity of 118.7 mAh?g^?1 at 20 C. Furthermore, the composite exhibits good cycle stability, retaining over 96 % of its initial capacity after 50 cycles at 10 C. The excellent electrochemical performance is attributed to a decrease in the charge-transfer resistance.     

Characterization of the tensile behaviour of a co-woven-knitted composite in the continuous and discrete frequency domain

Laplace-transform and Z-transform theories have been applied to analyze the tensile stress–strain curves of a co-woven-knitted (CWK) composite under quasi-static (0.001/s) and high strain rates (up to 2586/s) tension. The transform results were extended to characterize the tension failure and dynamic responses of the CWK composite in the frequency domain. Specifically, the Laplace-transform theory was employed to analyze the stress–strain curves of the CWK composite along 0°, 45° and 90° directions when the composite is assumed to be a continuous system, while the Z-transform theory was used for the discrete system for the composite. From the transformed results, it was found that the stress–strain curves of the CWK composite specimen under different strain rates tension have similar stability behaviours for the Laplace- and Z-transform. For the continuous system, few pole plots are distributed on the left side of the imaginary axis, which means the system is unstable. Nevertheless, the pole-plot distribution is stable before the post-critical deformation of the CWK composite. For the discrete system, most of the poles are located inside the unit circle before post-critical deformation, indicating the system is stable. From the stiffness–time history and fracture morphology, the stability of the pole-plot distribution corresponds to the stiffness stability and fracture uniformity. From continuous and discrete system analyses, it is found that the stress–time and strain-time histories of the CWK composite can be regarded as a digital signal system. Digital signal processing (DSP) methods can be extended to the investigation of the mechanical behaviour of composites.     

Liquid petroleum gas sensing performance of polyaniline-carboxymethyl cellulose composite at room temperature

In the present research, liquid petroleum gas (LPG) sensing properties of polyaniline-carboxymethyl cellulose (PANI-CMC) composite have been investigated. For the purpose of investigation, PANI and PANI-CMC composite are synthesized at room temperature by chemical polymerization of aniline without/with CMC. The prepared materials are characterized by X-ray diffraction (XRD) and Scanning electron microscopy (SEM) techniques. Shift in d-space of PANI in the composite as confirmed by its XRD analysis suggests ordered PANI chain arrangement in the composite. Surface morphology and size of the particles are studied by SEM technique. Nature of the material and mechanism of conduction of both PANI and PANI-CMC composite are investigated by studying complex plane impedance plot in the frequency range of 10²–10⁶ Hz. Based on structural characterizations and impedance studies, sensitivity of PANI and PANI-CMC composite to LPG at room temperature are tested and compared. Further, to examine the efficiency of the composite as a sensor, its stability, recovery and response time have also been studied with a special focus on its ability to work at room temperature.     

Coupled horizontal and vertical bending vibrations of a stationary shaft with two cracks

This paper investigates the coupled bending vibrations of a stationary shaft with two cracks. It is known from the literature that, when a crack exists in a shaft, the bending, torsional, and longitudinal vibrations are coupled. This study focuses on the horizontal and vertical planes of a cracked shaft, whose bending vibrations are caused by a vertical excitation, in the clamped end of the model. When the crack orientations are not symmetrical to the vertical plane, a response in the horizontal plane is observed due to the presence of the cracks. The crack orientation is defined by the rotational angle of the crack, a parameter which affects the horizontal response. When more cracks appear in a shaft, then the coupling becomes stronger or weaker depending on the relative crack orientations. It is shown that a double peak appears in the vibration spectrum of a cracked or multi-cracked shaft.Modeling the crack in the traditional manner, as a spring, yields analytical results for the horizontal response as a function of the rotational angle and the depths of the two cracks. A 2×2 compliance matrix, containing two non-diagonal terms (those responsible for the coupling) serves to model the crack. Using the Euler–Bernoulli beam theory, the equations for the natural frequencies and the coupled response of the shaft are defined. The experimental coupled response and eigenfrequency measurements for the corresponding planes are presented. The double peak was also experimentally observed.     

Size effect in the formation and failure of stable current states in composites based on high-temperature superconductors

The influence of the transverse size of a composite wire based on a high-temperature superconductor on the dynamics of its thermoelectrodynamic properties at constant-rate current input has been studied. The physical mechanism behind the formation of stable regimes, which are characterized by the nonuniform distribution of the electric field and transport current over the cross-sectional area of the composite, has been determined. It has been shown that the critical current density of the superconducting composites determined from their current–voltage characteristic have lower and upper boundaries of electric voltages, which outline the allowable measurement range. It has been found that, when the input current completely penetrates into the composite, conditions for its stability are governed by the size effect. The essence of this effect is that conditions for current state stability in superconducting composites with the same cross-sectional area but various cross size differ. The conditions for the absence of unstable states in the composite the cross section of which is partially filled with the transport current have been formulated.