Synthesis and investigation of new cholesteryl carbamate-based liquid crystals

Journal of Materials Science: Materials in Electronics - Cholesteryl chloroformate which is known liquid crystal material was modified to give the new molecules a long-lasting LC phases with the...     

Backstepping approach for design of PID controller with guaranteed performance for micro-air UAV

Flight controllers for micro-air UAVs are generally designed using proportional-integral-derivative (PID) methods, where the tuning of gains is difficult and time-consuming, and performance is not guaranteed. In this paper, we develop a rigorous method based on the sliding mode analysis and nonlinear backstepping to design a PID controller with guaranteed performance. This technique provides the structure and gains for the PID controller, such that a robust and fast response of the UAV (unmanned aerial vehicle) for trajectory tracking is achieved. First, the second-order sliding variable errors are used in a rigorous nonlinear backstepping design to obtain guaranteed performance for the nonlinear UAV dynamics. Then, using a small angle approximation and rigorous geometric manipulations, this nonlinear design is converted into a PID controller whose structure is naturally determined through the backstepping procedure. PID gains that guarantee robust UAV performance are finally computed from the sliding mode gains and from stabilizing gains for tracking error dynamics. We prove that the desired Euler angles of the inner attitude controller loop are related to the dynamics of the outer backstepping tracker loop by inverse kinematics, which provides a seamless connection with existing built-in UAV attitude controllers. We implement the proposed method on actual UAV, and experimental flight tests prove the validity of these algorithms. It is seen that our PID design procedure yields tighter UAV performance than an existing popular PID control technique.     

Spreading of social contagions without key players

Contagion models have been used to study the spread of social behavior among agents of a networked population. Examples include information diffusion, social influence, and participation in collective action (e.g., protests). Key players, which are typically agents characterized by structural properties of the underlying network (e.g., high degree, high core number or high centrality) are considered important for spreading social contagions. In this paper, we ask whether contagions can propagate through a population that is devoid of key players. We justify the use of Erd?s-Rényi random graphs as a representation of unstructured populations that lack key players, and investigate whether complex contagions—those requiring reinforcement—can spread on them. We demonstrate that two game-theoretic contagion models that utilize common knowledge for collective action can readily spread such contagions, thus differing significantly from classic complex contagion models. We compare contagion dynamics results on unstructured networks to those on more typically-studied, structured social networks to understand the role of network structure. We test the classic complex contagion and the two game-theoretic models with a total of 18 networks that range over five orders of magnitude in size and have different structural properties. The two common knowledge models are also contrasted to understand the effects of different modeling assumptions on dynamics. We show that under a wide range of conditions, these two models produce markedly different results. Finally, we demonstrate that the disparity between classic complex contagion and common knowledge models persists as network size increases.     

Synthesis of a side chain liquid crystalline polycarbonate with a chiral backbone

We report the experimental demonstration of a novel and environmentally benign supercritical carbon dioxide (ScCO₂) technique that yields an optically active, side chain liquid crystalline polycarbonate in a single‐step reaction. The obtained polymer is worthwhile, since it is highly stereoregular and can find applications in an enlarged mesomorphic temperature range compared to its acrylic analogues. The synthesized materials were characterized by IR, ¹H‐NMR, and ¹³C‐NMR, while the thermal properties were measured by DSC. Polarized optical micrograph and wide angle X‐ray diffraction were used for the mesogenic property characterization of the copolymer. The transfer chirality from the backbone to the mesophase is demonstrated on the optical microscopy textures. Being consistent with the ¹³C‐NMR, X‐ray implies an ordered polymeric structure. The DSC analysis of the copolymer indicates that the T_i (the clearing point temperature) value does not change dramatically, whereas a pronounced decrease in T_g (the glass transition temperature) value is observed from that of its acrylic analogues. Hence, the obtained polymer exhibits another practical benefit by widening the mesomorphic temperature range. This study is the insightful combination of material processing and chemical design that elucidates the advantages of ScCO₂ application, in terms of liquid crystallinity and the tacticity of the obtained polymer. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1915–1921, 2006     

Machinability of Elongated Coarse Grain Fe-Based Superalloys

□ In conventional metal cutting process, materials are assumed to be homogeneous and isotropic structure. However, some materials with a single crystal or coarse elongated polycrystalline demonstrate strong anisotropic behavior in physical and mechanical properties in machining of some superalloy materials. The anisotropic structure always leads to variation at machinability properties of the material. In this study, machinability properties of ferritic superalloy PM2000, which had elongated a few coarse grains, were investigated. These properties were determined by investigation of chip formation, cutting forces and surface roughness. Machinability was assessed by single-point turning on a CNC lathe and turning forces were measured by using a Kistler Lathe Dynamometer. The chip formation mechanisms in machining of PM2000 at various cutting speeds were determined by using a quick-stop device (QSD). Chip roots and machined surfaces were analyzed by means of scanning electron microscopy (SEM). The results indicated that the machinability properties of the PM2000 were changed by orientated coarse grain structure. Three types chip formation mechanism were determined at the same cutting conditions. Also, surface roughness on the machined each grain changed with changing the grain to be cut. Surface roughness and force fluctuations decreased with increasing the cutting speed; however, tool wearing increased.     

A hybrid CDAC-threshold configuring SAR ADC in 28nm FDSOI CMOS

In this paper, a 9-bit 1.3 GS/s single channel SAR ADC is presented. In conventional SAR ADCs, the capacitive DAC size grows exponentially with respect to converter resolution. This results in both signal bandwidth and conversion speed reduction. The proposed architecture implements binary search through a redundant capacitive DAC for the 5 first MSBs and through programmable comparator thresholds for the remaining 4 LSBs. The DAC capacitance at the front-end remains small enough to achieve high sampling rate with increased input bandwidth. Two asynchronously clocked alternate comparators are used additionally to improve conversion speed. The ADC is designed and simulated in 28 nm FD-SOI CMOS. It consumes 4.1 mW from a 1 V supply, while achieving a SNDR of 52.1 dB and a Figure-of-Merit of 11.4 fJ/conversion-step.     

Effect of processing method on microstructure,electrical conductivity and electromagnetic wave interference (EMI) shielding performance of carbon nanofiber filled thermoplastic polyurethane composites

In this study, effect of processing method on microstructure formation and related electrical conductivity and electromagnetic interference shielding effectiveness of carbon nanofiber (CNF) filled thermoplastic polyurethane (TPU) composites, prepared via three different processing techniques; (i) melt compounding (MC) in a twin screw extruder, (ii) simple solution mixing (SM) on a magnetic stirrer, and (iii) solution mixing with sonication (SM-U) were investigated. It was found that the electrical conductivity values of samples decreased in the order of SM > SM-U > MC for a particular amount of CNF. The electromagnetic test results showed that the samples prepared with SM and SM-U methods yielded higher total shielding effectiveness (SE_T) values than those prepared with MC. SE_T values of samples including of 20 phr of CNF prepared with MC, SM-U and SM methods were varied in the range of 10–30 dB, 20–60 dB and 20–80 dB, respectively within a frequency range of 1–12 GHz.     

Free vibration of semi-rigid connected Reddy-Bickford piles embedded in elastic soil

The literature on free vibration analysis of Bernoulli-Euler and Timoshenko piles embedded in elastic soil is plenty, but that of Reddy-Bickford piles partially embedded in elastic soil with/without axial force effect is fewer. The soil that the pile partially embedded in is idealized by Winkler model and is assumed to be two-layered. The pile part above the soil is called the first region and the parts embedded in the soil are called the second and the third region, respectively. It is assumed that the behaviour of the material is linear-elastic, that axial force along the pile length to be constant and the upper end of the pile that is semi-rigid supported against rotation is modelled by an elastic spring. The governing differential equations of motion of the rectangular pile in free vibration are derived using Hamilton’s principle and Winkler hypothesis. The terms are found directly from the solutions of the differential equations that describe the deformations of the cross-section according to the high-order theory. The models have six degrees of freedom at the two ends, one transverse displacement and two rotations, and the end forces are a shear force and two end moments. Natural frequencies of the pile are calculated using transfer matrix and the secant method for non-trivial solution of linear homogeneous system of equations obtained due to values of axial forces acting on the pile, total and embedded lengths of the pile, the linear-elastic rotational restraining stiffness at the upper end of the pile and to the boundary conditions of the pile. Two different boundary conditions are considered in the study. For the first boundary condition, the pile’s end at the first region is semi-rigid connected and not restricted for horizontal displacement and the end at the third region is free and for the second boundary condition, the pile’s end at the first region is semi-rigid connected and restricted for horizontal displacement and the end at the third region is fixed supported. The calculated natural frequencies of semi-rigid connected Reddy-Bickford pile embedded in elastic soil are given in tables and compared with results of Timoshenko pile model.     

The stability of a thin three-layered composite truncated conical shell containing an FGM layer subjected to non-uniform lateral pressure

This paper examines the stability of thin three-layered truncated conical shells containing a functionally graded (FG) layer subjected to non-uniform lateral pressure varying with the longitudinal coordinate. The material properties of the functionally graded layer are assumed to vary continuously through the thickness of the shell, and the variation of properties follows an arbitrary distribution in terms of the volume fractions of the constituents. Further, the fundamental relations for stability and compatibility equations of three-layered truncated conical shells containing an FGM layer have been obtained. These equations, ascertained via Galerkin’s method, have been transformed into a pair of time-dependent differential equations. Then, critical non-uniform lateral pressure has been conclusively obtained. This paper is the result of a detailed parametric study conducted to determine the influences of thickness variations in the FG layer, radius-to-thickness ratio, lengths-to-radius ratio, and the material composition and material profile index on the critical parameters of three-layered, truncated, conical shells. Finally, the results will be validated through the comparison of obtained values with those in the existing literature.