Flow stress modeling for copper under changing process conditions

Realistic simulation of metal forming processes requires constitutive equations that describe the behavior of a material under varying process conditions. The equations that have hitherto been developed to address this problem are generally too involved and require the determination of many constants. In this paper, a simple approach based on the representative nature of the work hardening rate, which enables the use of an elementary rate equation in such modelling, is introduced. A prediction methodology based on the concept of fading memory is developed and is found to give good predictions in the case of copper. A mechanistic interpretation of the approach is proposed.     

An automated algorithm for small signal analysis of DC-DC powerconverters

Based on a generalized state-space sampled data modeling, an iterative and efficient algorithm for deriving the small signal transfer functions of any DC-DC power converter is proposed. This algorithm is suitable to be implemented in a software program as an analytical tool for automated control analysis of general DC-DC power converters. Such a tool would reduce considerably the time needed by research engineers in modeling existing and new topologies and control methods. The algorithm proposed is applicable to different kinds of linear and nonlinear control methods. It has been verified on different power converters and control methods     

PWM methods to handle time delay in digital control of a UPS inverter

With the popularity of micro-processors, digital controllers are widely used in uninterruptible power supply (UPS) inverters. These digital control systems of UPS inverters require a time interval for sampling and computation, which sometimes affects the performance of inverters. In this paper, the problem of time delay in digital control of a UPS inverter is discussed. Then two novel pulsewidth modulation (PWM) methods, the two-polarity PWM method and the asymmetric PWM method, are proposed to handle the time-delay problem. Both of these PWM methods can achieve a wide range of duty ratio between 0.05-0.95, which is independent of inverter model. Furthermore, they are easy to implement using a digital micro-controller. Experimental results are presented in the paper to verify feasibility of the proposed PWM methods.     

A novel tri-state boost converter with fast dynamics

A challenging problem in the design of boost converters operating in continuous-conduction mode is posed by the dynamically shifting right-half-plane (RHP) zero in the converter's small-signal control-to-output transfer function. The paper proposes a novel tri-state boost converter without such a zero in the transfer function. The additional degree of freedom introduced in the converter in the form of a freewheeling interval has been exploited through an easy control technique to achieve this elimination. The absence of the RHP zero allows the control scheme to achieve larger bandwidth under closed-loop conditions, resulting in fast response. Analytical, simulation and experimental results of the tri-state boost converter have been presented and compared with those of the classical boost converter both under open-loop and under closed-loop operating conditions. The results clearly demonstrate the superior dynamic performance of the proposed converter. The proposed converter can be used in applications wherever fast-response boost action is needed.     

A Simple Single-Input–Single-Output (SISO) Model for a Three-Phase PWM Rectifier

The challenge in controlling a three-phase pulsewidth modulation (PWM) rectifier under balanced conditions arises from the fact that the state-space averaged model reported in literature has a multi-input-multi-output nonlinear structure and furthermore exhibits a nonminimum phase feature. In this paper, a simple single-input-single-output model is constructed by separating the d -axis and the q-axis dynamics through appropriate nonlinear feedforward decoupling while maintaining nearly unity power factor operation. With the proposed model, the nonminimum phase feature inherent in an AC-to-DC rectifier becomes a simple right-half-plane zero appearing in the small-signal control-to-output transfer function. In addition, the model exhibits a close similarity to a DC-DC boost converter under both large-signal and small-signal operating conditions. This makes it possible to extend the system analysis and control design techniques of DC-DC converters to the three-phase PWM rectifier also. The validity of the proposed model has been verified experimentally in the frequency domain under open-loop operation of the PWM rectifier. The usefulness of the model is further demonstrated through closed-loop operation of the rectifier with both voltage mode and inner-current-loop-based schemes.     

Resonant Power Processors, Part II-Methods of Control

The nature of resonant converter control is discussed. Employing the state portrait, different control methods for series resonant converter are identified and their performance evaluated based on their stability, response to control and load changes and range of operation. A new control method, optimal trajectory control, is proposed which, by utilizing the state trajectories as control laws, continuously monitors the energy level of the resonant tank. The method is shown to have superior control properties, especially under transient operation.     

The fabrication and creep properties of superalloy-zirconia composites

Many high-temperature structural materials applications require mismatch strain compatibility that can be provided by graded composites. Applications such as ceramic thermal-barrier coatings on superalloy substrates require compatibility to minimize interfacial stresses and degradation effects. The interfaces in such cases could be metal-matrix composites containing a graded distribution of the ceramic phase within the superalloy matrix whose creep properties during elevated temperature service are unknown. This article reviews creep properties of a typical superalloy, René95, containing partially stabilized zirconia. These composites were prepared via powder metallurgy, during which zirconia was found to react with γ′ (Ni₃Al) to form Al₂O₃, resulting in the depletion of γ′ from the superalloy matrix. Due to the combined effects of chemical changes and grain refinement, considerable creep strengthening was achieved at low creep rates, but weakening was observed at higher creep rates. A composite load transfer model is used to isolate the effect of particles on strengthening. For more information, contact A.K. Ghosh, University of Michigan, Department of Materials Science and Engineering, 2300 Hayward Street, Ann Arbor, Michigan 48109-2136; (734) 764-3322; fax (734) 763-4788; e-mail akg@engin.umich.edu.     

Fabrication and creep properties of superalloy-zirconia composites

Graded composite interfaces have been proposed as a means to reduce thermally induced stresses between dissimilar materials. This is expected to be useful in applications such as ceramic thermalbarrier coatings (TBCs) on superalloy substrates. The interfaces, in such cases, are metal-matrix composites containing the ceramic phase within the superalloy matrix, whose creep properties during elevated-temperature service become critically important. This study was carried out to assess the creep properties of a typical superalloy-ceramic combination, namely, a René 95 alloy containing partially stabilized zirconia. Composites of these materials were prepared via powder metallurgy. Microscopy and X-ray work revealed that the zirconia reacted with γ′ (Ni₃Al) to form Al₂O₃, which resulted in the depletion of γ′ from the matrix. The creep behavior of the composites was markedly different from that of the unreinforced matrix. In addition to showing different stress exponents, the composites were stronger than the unreinforced material at low strain rates and weaker at the higher strain rates. A composite load-transfer model is used to isolate the effect of particles on strengthening. It is found that strengthening by the ceramic particles is smaller than strengthening arising from the change in chemistry of the matrix due to the addition of ZrO₂.     

Inductor voltage control of buck-type single-phase AC-DC converter

A novel inductor voltage control (IVC) method cable of achieving any input power factor including unity is being proposed for buck-type AC-DC pulsewidth modulation (PWM) converters. In this method, the input inductor voltage is kept within hysteresis band about a sinusoidal template, thus ensuring sinusoidal input current. This control is much less sensitive to parameter and control signal changes than the existing delta modulation control (DMC). In this paper the IVC method is applied to single-phase buck-type converter. Useful design results based on steady-state analysis have been presented. Simulation and experimental results have been provided to verify the theoretical results. The companion paper extends the applicability of the IVC control method to three-phase converters also. The proposed IVC method has potential in applications requiring AC-DC rectifiers with over-current/short-circuit current limit     

Directed growth of 1D cadmium sulfide by chemically anchored Al2O3 and ZnO nanoparticles

Cadmium sulfide nanowires were synthesized on in-situ generated alumina nanoparticles using a one pot hydrothermal method. As grown hexagonal-phase cadmium sulfide nanowires have diameters ranging from 40 to 50 nm. Mechanistic aspects reveal that the alumina template based growth of cadmium sulfide nanowires is due to the fact that S²⁻ ions will preferably attach to the aluminum due to the Lewis acidic character of Al³⁺ (~ 0.5 on Brown's scale of average Lewis acid strength). To support the proposed mechanism, we have further shown the growth of cadmium sulfide nanowires on zinc ions having similar Lewis acidic strength.