Micro- and macro-tribological properties of SiC ceramics in sliding contact

Surface properties of single-crystalline and polycrystalline silicon carbide (SiC) specimens were measured using atomic force/friction force microscopy, Auger electron spectroscopy and nano-indentation techniques. Running-in behavior during sliding tests in vacuum was studied on self-mated SiC pairs as a function of surface quality produced by machining. Tribological mechanisms were analyzed in short-time tests using a high-resolution micro-tribometer and then were related to results obtained from macro-tribological tests in humid air and in the presence of distilled water. Information on the structure, chemical composition and properties of SiC surfaces resulting from measurements on the nanoscale can be very useful for explaining tribological performance under far different operating conditions such as in vacuum or air, with contact areas ranging from diameters of a few nanometers to one millimeter and initial applied contact pressure from about 1 MPa to 3 GPa. Friction and wear mechanisms are discussed as functions of surface composition and roughness, vacuum and humidity of air.     

Identification of feasible scaled teleoperation region based on scaling factors and sampling rates

The recent spread of scaled telemanipulation into microsurgery and the nano-world increasingly requires the identification of the possible operation region as a main system specification. A teleoperation system is a complex cascaded system since the human operator, master, slave, and communication are involved bilaterally. Hence, a small time delay inside a master and slave system can be critical to the overall system stability even without communication time delay. In this paper we derive an upper bound of the scaling product of position and force by using Llewellyn’s unconditional stability. This bound can be used for checking the validity of the designed bilateral controller. Time delay from the sample and hold of computer control and its effects on stability of scaled teleoperation are modeled and simulated based on the transfer function of the teleoperation system. The feasible operation region in terms of position and force scaling decreases sharply as the sampling rate decreases and time delays inside the master and slave increase.     

ROBUST DECOUPLING CONTROL BASED ON EXTERNAL LOOPS

Decoupling control can substantially simplify the design of multivariable control systems. However, decouplers are usually sensitive to modeling errors, especially for the systems with relative gains being far away from unity. Hence, decoupling control is limited in practical application. In the present study, robust control of a decoupling system with external loops is developed. Design of the control system consists of two steps. The first step is to design the decoupler and the controller based on the nominal plant. The second step is to design the filters in the external loops by using a robust stability criterion. The robust control is implemented without changing the original design of the decoupler and the controller. The control system with external loops for decoupling control significantly improves the control performance. The proposed robust control enhances the applicability of decoupling control.     

DESIGNING PID CONTROLLERS WITH A MINIMUM IAE CRITERION BY A DIFFERENTIAL EVOLUTION ALGORITHM

Although the integral of absolute error (IAE) has long been used as a performance measure for controller evaluation, a systematic approach is still lacking for the design of optimal PID controller with the minimum IAE performance criterion. This may be ascribed to the nonsmoothness of the IAE objective function and the lack of closed-form expression for the IAE performance index. In this paper, we propose the use of a differential evolution algorithm (DEA) to design a minimum-IAE PID controller to overcome the nondifferentiability of the IAE objective function. To achieve also a specified least relative stability margin and least damping ratio, the D-partition technique is used to identify the allowable domain in the tuning parameter space. Three examples are provided to illustrate the effectiveness of the DEA.     

Simultaneous hydrogenation and hydroacylation of vinyl groups in polybutadiene by use of a rhodium catalyst

Vinyl groups in phenyl-terminated polybutadiene (1a) containing 99% unsaturation (27% vinyl group, 73% internal olefin), and for which the average M_n is 3400, were simultaneously hydrogenated and hydroacylated with various aromatic or heteroaromatic primary alcohols in the presence of the catalytic system RhCl₃.H₂O, PPh₃ and 2-amino-4-picoline. Sterically less hindered alcohols, such as benzyl alcohol, showed greater reactivity than sterically more hindered alcohols, such as 2-naphthylmethanol and heteroaromatic primary alcohols. Vinyl groups in phenyl-terminated polybutadiene (1b) containing 99% unsaturation (45% vinyl group, 55% internal olefin) and for which the average M_n is 1300 also showed similar reactivity toward various primary alcohols under identical reaction conditions.     

Thermal behavior and properties of naphthalene containing bismaleimide–triazine resins

A series of bismaleimid·triazine (BT) resins were prepared from various dicyanate esters and 2,7-bis(4-maleimidophenoxy)naphthalene (BMPN), which contains a naphthalene group and an aryl ether linkage in the backbone. Their curing behaviors were characterized by differential scanning calorimetry. The exotherm temperature and polymerization reactivity were strongly affected by the chemical structure of the various dicyanate ester monomer. Thermal behaviors were investigated by thermogravimetric analyses and dynamic mechanical analyses. The glass transition temperatures (T_g) of these cured resins with bismaleimide/dicyanate ester at the 1/2 molar ratio were in the range of 250–322°C, and exhibited excellent thermal stability up to 400°C in nitrogen. These results provided a structure–properties relationship for the cured bismaleimid·triazine resins. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1199–1207, 1998     

An experimental study on the pumping performance of molecular drag pumps

The pumping performance of molecular drag pumps (MDP) has been investigated experimentally. The experimented MDPs are a disk-type drag pump (DTDP), helical-type drag pump (HTDP) and compound drag pump (CDP), respectively. In the case of the DTDP, spiral channels of a rotor are cut on both upper surface and lower surface of a rotating disk, and the corresponding stator is a planar disk. In the case of the HTDP, the rotor has six rectangular grooves. The CDP consists with the DTDP, at lower part, and with the HTDP, at upper part. The experiments are performed in the outlet pressure range of 0.2–533 Pa. The inlet pressure and compression ratio are measured under the various conditions of outlet pressure and throughputs, and nitrogen is used for the test gas. At the outlet pressure of 0.2 Pa, the ultimate pressure has been reached to 1.0 × 10⁻² Pa for the HTDP, 1.3 × 10⁻⁴ Pa for the DTDP, and 3.6 × 10⁻⁵ Pa for the CDP. The maximum compression ratio of the CDP is much higher than those of the DTDP or HTDP. Consequently, the ultimate pressure of the CDP is the lowest one.     

Motion Error Compensation Method for Hydrostatic Tables Using Actively Controlled Capillaries

To compensate for the motion errors in hydrostatic tables, a method to actively control the clearance of a bearing corresponding to the amount of error using actively controlled capillaries is introduced in this paper. The design method for an actively controlled capillary that considers the output rate of a piezo actuator and the amount of error that must be corrected is described. The basic characteristics of such a system were tested, such as the maximum controllable range of the error, micro-step response, and available dynamic bandwidth when the capillary was installed in a hydrostatic table. The tests demonstrated that the maximum controllable range was 2.4 /im, the resolution was 27 nm, and the frequency bandwidth was 5.5 Hz. Simultaneous compensation of the linear and angular motion errors using two actively controlled capillaries was also performed for a hydrostatic table driven by a ballscrew and a DC servomotor. An iterative compensation method was applied to improve the compensation characteristics. Experimental results showed that the linear and angular motion errors were improved to 0.12 μm and 0.20 arcsec, which were about 1/15^th and l/6^th of the initial motion errors, respectively. These results confirmed that the proposed compensation method improves the motion accuracy of hydrostatic tables very effectively.     

Fretting cracking behavior of nub of last-stage blade for steam turbine

Journal of Mechanical Science and Technology - Nub-and-sleeve of last-stage blade for steam turbine is a part-span damper. The nub is vulnerable to fretting cracking since it is in contact with the...     

Assessment of two-parameter mixed models for large eddy simulations of transitional and turbulent flows

In the present study, improved two-parameter mixed models for large eddy simulations are proposed based on previous two-parameter mixed models of Salvetti and Banerjee [1] and Horiuti [2]. The subgrid-scale (SGS) stress in our models is decomposed into the modified Leonard stress, modified cross stress and modified SGS Reynolds stress terms. Although the modified Leonard stress term is explicitly calculated based on the scale-similarity, the modified cross stress term is built using an extension of the filtered Bardina model proposed by Horiuti [3] for better predictions of the interaction between resolved and unresolved scales (i.e., energy exchange). The modified SGS Reynolds stress is modeled by the dynamic Smagorinsky model or by a dynamic global model, leading to two unknown model coefficients for the modified cross stress and the modified SGS Reynolds stress terms. In order to demonstrate the reliability of the proposed SGS models, large eddy simulations of two types of flows (i.e., a fully developed turbulent channel flow and a transitional boundary layer flow) are performed. It is shown that the modified cross stress term makes an important contribution to the accurate predictions of such flows because the emergence of negative SGS dissipation (backward scatter) by the modified cross stress term decreases the excessive positive SGS dissipation (forward scatter). A direct comparison of the turbulent statistics with those from previous SGS models shows that the proposed SGS models result in better prediction performance both in transitional and turbulent flows.