Yonghong Liu

Development of an automatic subsea blowout preventer stack control system using PLC based SCADA.

An extremely reliable remote control system for subsea blowout preventer stack is developed based on the off-the-shelf triple modular redundancy system. To meet a high reliability requirement, various redundancy techniques such as controller redundancy, bus redundancy and network redundancy are used to design the system hardware architecture. The control logic, human-machine interface graphical design and redundant databases are developed by using the off-the-shelf software. A series of experiments were performed in laboratory to test the subsea blowout preventer stack control system. The results showed that the tested subsea blowout preventer functions could be executed successfully. For the faults of programmable logic controllers, discrete input groups and analog input groups, the control system could give correct alarms in the human-machine interface.

High-Speed End Electric Discharge Milling of Silicon Carbide Ceramics

Silicon carbide (SiC) ceramics have found a variety of engineering applications due to their superior properties. However, the manufacture of SiC ceramics is not an efficient process. A new process of machining SiC ceramics using end electrical discharge (ED) milling is presented in this research. End ED milling employs a turntable with several small cylindrical rods as the tool electrode, and uses a water-based emulsion as the machining fluid. This process is able to effectively machine a large surface area on SiC ceramic. The machining principle and characteristics of the technique are introduced. The effects of tool polarity, pulse on-time, pulse off-time, peak voltage, and peak current on the process performance such as the material removal rate (MRR), electrode wear ratio (EWR), and surface roughness (SR) have been investigated. In addition, the microstructure of the machined surface is examined with a scanning electron microscope and an energy dispersive spectrometer.

Machining Performance Optimization in End ED Milling and Mechanical Grinding Compound Process

Silicon carbide (SiC) ceramic has been widely used in modern industry for its excellent physical and mechanical properties. However, SiC ceramic is difficult to machine owing to its high hardness and brittleness. This article presents a novel compound process that integrates end electric discharge (ED) milling and mechanical grinding to machine SiC ceramic. The process is able to effectively machine a large surface area on SiC ceramic with a good surface quality. The effect of tool polarity on the machining characteristics such as the material removal rate (MRR), tool wear ratio (TWR), and surface roughness (SR) has been explored. The effects of pulse on-time, pulse off-time, discharge current and open voltage on MRR, TWR, and SR have been investigated with Taguchi experimental design. The experimental data are statistically evaluated by analysis of variance and stepwise regression. The significant machining parameters, the optimal combination levels of machining parameters, and the mathematical models associated with the machining characteristics are obtained. The confirmation experiment results confirm the validity of the used Taguchi method for enhancing the machining performance and optimizing the machining parameters.

Single Discharge Machining Insulating Al2O3 Ceramic with High Instantaneous Pulse Energy in Kerosene

A technique of machining insulating Al2O3 ceramic with a thin copper sheet assisting electrode and high instantaneous pulse energy is developed. The process uses the high peak voltage and large capacitor with high instantaneous pulse energy, and there is no current-limiting resistor in the discharge circuit, so the discharge energy and discharge explosive force are high, and the process is able to effectively machine insulating ceramics. To further explore the process characteristics, the single pulse discharge experiments have been carried out in kerosene. The effects of polarity, capacitance, peak voltage, and current-limiting resistance on the process performance such as the crater volume, crater depth, tool wear ratio (TWR), and assisting electrode wear ratio (AEWR) have been investigated. The microstructure of the discharge crater has been examined with a scanning electron microscope (SEM). The results show that the insulating Al2O3 ceramic materials are mostly removed by spalling, at the center region of the discharge some materials are removed by melting and evaporation.

A review of the current understanding and technology of powder mixed electrical discharge machining (PMEDM)

This paper reviews the current understanding of powder mixed electrical discharge machining (PMEDM), highlighting particularly the mechanisms proposed for PMEDM under the influence of additive of powders in the dielectric fluids. Generally, the mirror surface phenomenon in PMEDM can be explained by the enlarged gap widths which can expand the discharge channel and thereby reduce the electrical power density since lower power density can yield large and shallow craters that evenly distributed on the machined surface. Moreover, the increment of the inter electrodes distance will limit the capacitive effect and enable a small surface roughness even in a large area. Another possible mechanism, multi-discharge model was also presented in this paper. More investigations on the fundamental mechanism in power mixed EDM are highly desirable since it has not been understood clearly. In this paper, the traditional and some recent development technology for PMEDM is also reviewed. Historically, PMEDM was invented to improve the surface quality of the machined components. Nowadays, some new applications of this technology, such as micro machining, rough machining and surface modification were proposed; some novel variations that based on power mixed EDM, such as PMEDM combined with ultrasonic or abrasive, powder mixed near-dry EDM and powder mixed electrical chemical discharge machining (ECDM) were also developed.

Dehydration Efficiency of Water-In-Model Oil Emulsions in High Frequency Pulsed DC Electrical Field: Effect of Physical and Chemical Properties of the Emulsions

The influence of several factors on the electrostatic dehydration process of water-in-model oil emulsions in high-frequency electrical field has been studied using conductivity technique. The ascending and the descending periods of the observed conductivity versus time curves were fit to sigmoid and power function, respectively, which revealed the model parameters that were used to quantify anew dehydration efficiency (DE) index. Experiments results shown that not only water content, surfactant concentration, salt concentration in the aqueous phase and temperature but also homogenization were all found to play a non-negligible role in the electrostatic dehydration process.

Influence of Dielectric Type on Porosity Formation on Electrical Discharge Machined Surfaces

This work presents a comparative study on the performance of three types of dielectrics at different electrical discharge machining (EDM) parameter combinations favorable for generating porosity. The characteristics of the porosity formation in the recast layer formed in water/oil emulsions (W/O) were investigated by comparing them with those present in the recast layer formed in kerosene and deionized water dielectric. The results showed that the porosity characters were influenced significantly by the dielectric type. The formation mechanism of the porosity in different dielectrics is presented in this article.