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Transient recovery characteristics of DC voltage are critical to analyze the stability criterion and the fault ride-through capability of voltage source converter (VSC)-based DC power systems. In this paper, the dynamic model of a flexible DC power grid was initially established, and then, the dynamic operation mechanism of the system was presented by analyzing the movement locus of the operating point when disturbances occur in the DC grid. Subsequently, the DC transient voltage limit and the DC fault clearing time were calculated in order to obtain the transient stability criterion of the DC power system. In addition, a novel voltage transient recovery control strategy was proposed by improving the droop control of the DC voltage to extend the transient stability margin of the power system. Finally, a typical multi-terminal flexible DC power grid was developed on a hardware-in-the-loop platform. The results demonstrated that the voltage recovery ability of the DC power system can be quantitatively estimated by the stability criterion, while the system transient stability can be improved by the proposed control strategy.

The inertia support of DC microgrid system is less, the instability of output power of distributed generation and the sudden change of load cause the sharp fluctuation of DC voltage, which endangers the safe and stable operation of DC microgrid. In order to solve the problem of small inertia of DC microgrid, a virtual inertial control strategy with exponential change is proposed based on U-I droop control to slow down the voltage shock caused by disturbance of the system. The DC microgrid system is built in MATLAB/Simulink. The simulation results show that the proposed strategy can increase the inertia of DC system for a short time, suppress DC voltage oscillation, effectively improve the transient response of DC voltage disturbance and improve the operation stability of the system when the power shortage suddenly occurs.

In order to assess the operational reliability of the horizontal machining center (HMC), field failure data for 14 HMCs over one year are collected from an engine plant of a large automobile manufacturing company in China. In contrast with the usual approach, which just pools the data from all copies under the assumption that each copy is modeled by the same power law processes (PLP), a new model based on the generalized linear mixed model (GLMM) is proposed for analyzing the failure data from all copies of HMC. A basic idea of this method is to assume heterogeneity among all copies of HMC; it is also found that the underlying model for each individual copy is a PLP model with different shape parameters and scale parameters in the GLMM model. This method can make inferences about both the population and each individual copy. Meanwhile, the modified Anderson–Darling test is adapted to the goodness-of-fit test of the model. The results of the analysis suggest that this method is effective to analyze reliability of HMC. Copyright © 2010 John Wiley & Sons, Ltd.

Machine line is a type of manufacturing system in which machines are connected in series or in parallel. It is significant to ensure the reliability as well as to reduce the total cost of maintenance and failure losses in the maintenance programs of such systems. Cost-based selective maintenance decision-making, which is the best method for a selected group of machines in machine line is presented under limited maintenance durations. Fault losses and maintenance costs of a single machine under different maintenance actions i.e. minimal repair, preventive maintenance and overhaul on the fault rate of the machine are calculated. An algorithm combining the heuristic rules and tabu search is proposed to solve the presented selective maintenance model. Finally, a case study on the maintenance decision-making problem of a connecting rod machining line in the automobile engine workshop is presented to illustrate the applicability of the proposed method. The end result shows that the fault losses can be further reduced by the optimization of maintenance interval and maintenance duration. Copyright © 2010 John Wiley & Sons, Ltd.

Purpose: This paper introduces a new external beam radiotherapy device named GammaPod that is dedicated for stereotactic radiotherapy of breast cancer. Methods: The design goal of the GammaPod as a dedicated system for treating breast cancer is the ability to deliver ablative doses with sharp gradients under stereotactic image guidance. Stereotactic localization of the breast is achieved by a vacuum-assisted breast immobilization cup with built-in stereotactic frame. Highly focused radiation is achieved at the isocenter due to the cross-firing from 36 radiation arcs generated by rotating 36 individual Cobalt-60 beams. The dedicated treatment planning system optimizes an optimal path of the focal spot using an optimization algorithm borrowed from computational geometry such that the target can be covered by 90%–95% of the prescription dose and the doses to surrounding tissues are minimized. The treatment plan is intended to be delivered with continuous motion of the treatment couch. In this paper the authors described in detail the gamma radiation unit, stereotactic localization of the breast, and the treatment planning system of the GammaPod system. Results: A prototype GammaPod system was installed at University of Maryland Medical Center and has gone through a thorough functional, geometric, and dosimetric testing. The mechanical and functional performances of the system all meet the functional specifications. Conclusions: An image-guided breast stereotactic radiotherapy device, named GammaPod, has been developed to deliver highly focused and localized doses to a target in the breast under stereotactic image guidance. It is envisioned that the GammaPod technology has the potential to significantly shorten radiation treatments and even eliminate surgery by ablating the tumor and sterilizing the tumor bed simultaneously.