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英文原文The Development and Application of Electronic Technology to Increase Health, Safety, and Productivity in the South African Coal Mining IndustryAndrC de Kock, Member, IEEE. and Jan W.OberholzerAbstract - South Africas coal mining industry is of vital economic importance. This paper describes some of the major electronic achievements within that industry over the past ten to 15 years. Basic design criteria formulated for systems to be used underground is discussed. The experience gained with the implementation of the systems, especially the human factor, is examined. Future directions to be followed by some of the research programs are presented.Key words - Coal mining, health, productivity, remote control, safetyI INTRODUCTIONCoal plays a significant role in the South African economy and is second only to gold in earning foreign exchange. As a result of a lack of alternative energy sources such as oil and large scale hydroelectric power, coal is the countrys major energy source. By supplying 88% of the commercial energy requirements, coal is also, therefore, a major contributor to economic growth and industrialization. Of the 182 million tons produced in 1993, 131.9 million tons (72%) were used domestically l.Escoms coal-fired power stations, together with a few local utilities, consume more than 75 million tons annually. Sasol, the only successful commercial oil-from-coal producer in the world, consumed 39 million tons in 1988, making it the second most important domestic user. Other major users include Iscors metallurgical plants, the cement industry, and the large municipalities.Approximately 90% of the total saleable coal is produced from the Witban/Highvelt coalfields, two of South Africas 19 coalfields. In 1989, South Africas economically recoverable coal reserves, estimated at 58 billion tons, ranked South Africa second among Western nations and fourth in global terms 2. Most of South Africas coal is of a bituminous thermal grade, approximately 2.0% is anthracitic, and some 1.6% is of metallurgical quality.Over the past few years, there has been a growth in demand for steam coal coupled with a supply shortage caused by production, weather, and labor problems that affected most steam coal exporters in the late 1980s. These market conditions were beneficial to the South African coal producers. Of the 182 million tons of coal produced in 1993, about 51 million tons (28% of the annual production) were exported, earning some R4.5 billion of foreign revenue.South African mines tend to be relatively large and, consequently, have numerous underground sections. It is, therefore, not unusual for as many as ten underground sections to be served by a single shaft. An individual mine will usually be dedicated to supply a specific power station or private contract. These requirements, in turn, dictate the overall production rates. Under normal conditions these remain fairly static. The emphasis of a productivity increase is thus not necessarily to increase the output from the mine, but rather to decrease the resources required to extract the coal. This, in turn, leads to a decrease in the number of employees required and is, therefore, met with resistance from the work force.Another aspect of the South African mines is the diversity of mining methods employed. Three mining methods are used in the extraction of the coal. They are bord and pillar, pillar extraction, and longwalls. With bord and pillar and pillar extraction, two mining methods are used. The first is conventional drill and blast, and the second utilizes continuous miners to extract the coal.As can be seen from Fig. 1 3, there was a major increase in the production from the mines between 1970 and 1990. This was a direct result of a significant increase in the mechanization of the coal mines, with the introduction of continuous miners and longwalls.Although the number of longwall faces increased up to 1990, the trend has been reversed in later years, with more attention being given to continuous miner stooping and rib pillar mining. The main motivation for this trend is that continuous miner sections are more flexible with regard to underground geological disturbances in the coal seam. An added advantage is that the capital expenditure per coal production unit is significantly lower. This led the Chamber of Mines Research Organization COMRO, now Counsel for Scientific and Industrial Research (CSIR): Mining Technology) to focus most of its research on continuous miners.During the past few years, the incidence of labor disputes has increased considerably. This has forced management to spend a large amount of time and effort in resolving laborrelated problems. Wage demands have been the major contributor to the escalation in production costs, while increases in productivity have remained low. The work force is, however, beginning to appreciate the need for labor stability and job security.Fig. 1 Production per mining methodII BACKGROUDDuring the 1970s, COMRO spent much time and resources on evaluating continuous miners. It was generally found that the coal in the South African collieries was harder and more abrasive than the coal in the countries from where the continuous miners were originally imported.Research was, therefore, aimed at improving the production rates from the machines by developing a more efficient cutting process. Production figures for the continuous miners were recorded and accumulated in a database. Using this database, Pieterse 4 established that there was only a small change in the overall performance of the continuous miners, although a significant improvement in the cutting process had been brought about by new cutting cycles and locally developed continuous miners.In 1985, Oberholzer 5, using the database, concluded that the availability of a continuous miner for cutting was fairly high. The emphasis of COMROs research then shifted from machine capacity to evaluating the manner in which the continuous miner was utilized.Following this, Oberholzer and Thorpe 6 investigated the cutting rates of continuous miners. They found that the cutting rate of a continuous miner varied significantly, in spite of the fact that the conditions within different sections, including tramming distances, were nearly identical. They further found that the “unproductive” phase within the cutting cycle could be as high as 43% of the total cutting time. This confirmed the assumption that production from a continuous miner was influenced more by external factors included in the overall mining process, as opposed to its coal cutting ability. This was mainly attributed to the machine operators ability, experience, and the techniques he used. These findings then formed the basis of some of the research work that was conducted from 1985.A. Design ConsiderationsIt was decided that the primary aim of all systems that were to be developed was to improve the productivity and working conditions of the operator at the face, without removing the operator from the face. The following were then set as additional criteria that had to be met through the design and implementation of electronic equipment in the program. Needs Driven: Research undertaken has to be needs, and not technology, driven. Therefore, the starting point of projects was to find or develop technology that addressed a specific need of the coal mining industry and not to find applications for existing technology. Intrinsic Safety: All electronic or electric systems have to be designed to conform to South African Bureau of Standards (SABS) standards set out for intrinsically safe or flameproof apparatus, in order to eliminate any possibility of methane ignition. This led to the development of an inexpensive and easy way of manufacturing intrinsically safe battery packs 7.Universal Fitting: The system had to be retrofitable, making it possible to fit the system to all the different types of new and existing equipment used in South African coal mines. The system could then, for example, be transferred between continuous miners in different sections of the mine.Mine-Proof : All the components of a system have to be protected against coal, rock, dust, water, and moisture penetration into electric/electronic enclosures and cables. All parts have to be robust enough to resist mechanical impacts caused by falling materials or movable parts of the equipment.Maintenance and Repair: The designed systems should be simple in that they do not require highly qualified personnel to maintain them in the underground situation. It was therefore decided to use a modular approach where circuit boards could be replaced by unskilled personnel. The faulty modules could then be brought to the surface where they would be checked and repaired. Any installation, maintenance, and repair of systems must not present any restriction IO normal mining operations.Acceptability: It was imperative for the success of systems that the operator and/or other personnel involved in the operation of equipment accept them and not feel threatened by their presence.III TECHNOLOGY DEVELOPMENTSome of the main thrust areas that were investigated during the past ten years were the following.A. Coal Interface Detection (CID)Optimal utilization of available reserves require a mine to remove as much of the in situ coal as possible. In order to achieve this, part of the continuous miner operators goal is to cut as close as possible to the top of the coal seam. To assist the operator in achieving this, two of the most promising techniques that were investigated were natural gamma radiation and vibration analysis.(1) Natural Gamma Radiation: Following the success of using natural gamma radiation for the detection of the coal-stone interface in the United Kingdom, COMRO in 1986 investigated the application of the same method in South Africa. The overlaying strata of the coal seams usually comprise clays, silts, or mud stones. Mixed in with these strata are some naturally occurring radioactive elements, the most common being potassium (K40), thorium (Th232), and uranium (U235 ).The principle of the system was to treat the overlaying strata as a gamma source and the connecting layer of coal as a gamma absorbent. The thickness of the absorbent would then be determined by the amount of attenuation it imposed on the gamma rays emitted from the overlaying strata.The investigation covered 20 collieries using an instrument that had already been proven successful in British collieries. From their findings it was concluded that the use of such a system was not feasible in the South African coal mines. It was found that the presence of uranium within the coal seam tended to cancel the expected attenuation of the gamma rays by the coal in contact with the overlaying strata.(2) Vibration Analysis: Research carried out at COMRO using vibration analysis was not aimed at determining the roof coal interface, but at investigating the condition of the continuous miner. The intention was to investigate the different vibrations produced by the continuous miner. From the vibration spectrum it was hoped to establish a characteristic vibration “fingerprint” for the different activities performed by the continuous miner. It was also envisaged that it might be possible to find a roof/coal interface “fingerprint” that would indicate the thickness of that interface.As a starting point, it was decided to investigate the condition of the picks on the cutter drum. The vibration spectrum of new picks was compared with that of worn picks. Initial results indicated that it might be possible to determine the condition of the picks. A great deal of refinement would, however, have to be made before the system could be used on a continuous miner. However, a significant potential had been indicated. As a result of limited resources and an evaluation of CID, it was decided not to do fundamental research on CID, and research in this field was discontinued.B. ProductivityThe study by Oberholzer and Thorpe 6 showed that the individual continuous miner operator has a major influence on the cutting rates of the continuous miner. Variations in the judgment of different operators appeared to be responsible for the different output rates. These variations, attributed to the human factor, directly affect the control of the sumping and shearing process, as well as the transportation of the coal.The time losses that can be attributed to the operators influence tend to be small, but are significant when viewed in perspective. Management tends to focus more attention on factors causing large time losses and ignore the short, seemingly insignificant, time losses. The small time losses are, however, occurring continuously, and their cumulative effect has a major influence on the total production loss of the section. The most important of these delays are due to the following:poor horizon control leading to premature pick damage, coal contamination, and an excessive amount of time spent on low cutting rates during roof and floor trimming;failure to control the sumping depth and the subsequent inability to cut coal at the optimum rate and to synchronize coal cutting and coal transportation; low boom lift rates due to operator caution and the inability to identify the correct sumping height due to poor visibility.To address the problems identified in the production of coal using a continuous miner, as discussed above, a control system was designed 7. The “total” control system consists of a horizon control system, an advance control system, the continuous miner operator, the machine controls, and the required horizon. By using only the visual indicators of the horizon and advance control system, the operator will compare the actual position of the cutter head with that of the required position. If there is a difference between the two positions, the operator will close the loop by activating the machine controls and move the boom until the actual and required position are the same. With this approach, the operator remains the crucial link in the system, as he retains overall control.The two problems addressed by the horizon control are the boom lift times and control of the roof and floor horizon to which the coal seam is cut. The boom lift time is dependent on the ability of the operator to lift the cutting boom at maximum speed until the cutter head is as close to the required position as possible. Similarly, the location of the horizon is dependent on the ability of the operator to stop the cutting boom at the correct roof and floor positions every time. Without the horizon control system, the operator “feels” for the required position. This is done by cautiously lifting the boom in small increments when it is close to the desired position. These practices result in increased boom lift times and the uneven cutting of the horizon.The other section activity on which the continuous miner operator has a major influence is the synchronization of coal cutting and transportation of the coal in the section. During normal operating conditions there is always a shuttle car waiting to be loaded at the continuous miner. If the continuous miner fills the shuttle car before the cut is complete, it has to wait while the shuttle cars change, and the process of filling one car with one cut loses synchronism. This problem was addressed by the advance control, where the depth of sump is controlled to allow a shuttle car to be filled with one sump and shear cycle.Various methods of determining the sumping depth were investigated 3, 7. Each of the methods is based on using a different approach for establishing a reference from which to measure the forward movement of the continuous miner. The three reference planes considered were:the coal face in front of the continuous miner;a target fixed to the roof behind the continuous miner;the roof above the continuous miner.During the quantification of the advantages of the systems, it was found to be possible to cut smooth floor and roof horizons. The cycle time of the cutting process was improved by 36% by using the horizon control system.C. Communication ChannelsIn addition to the normal two-wire systems found underground, channels for the transmission of data were investigated 8. The channels were infrared, optical fiber, radio, and powerline carrier.(1) Infrared: Without special optical devices, in the presence of coal dust (not exceeding a level of 100 mg/m3) and with a total LED radiation power of 200 mW, reliable transmission distances of 3040 m were achieved. The addition of a single lens at either the transmission or receiving end increased the distance to between 60-80 m. Due to the defusing of infrared radiation on road and pillar surfaces, and scattering from coal, rock, or dust particles, communication is possible even without a direct propagation path between the infrared transmitter and receiver.A mathematical model was developed to determine the full power of infrared radiation incident on the photosensitive surface of the photoreceiver.(2) Powerline Carrier: The principle of this channel is the use of the continuous miner, shuttle car, or roof bolter power trailing cable as a communication medium. The method incorporates clip-on inductive antennae which can be clamped around both sides of a trailing power cable at the machine and the switchgear. This is the most attractive method as it provides quick, in-mine installation of a communication channel. The main difference between this and a conventional radio system is the antenna system and propagation medium. Tests were carried out where a signal was coupled into the trailing cable of a continuous miner. The results have shown that communication is possible over a distance of 200 m. The system is based on a nonintrusive method of coupling that was developed by COMRO.(3) Optical Fibers: Optical fibers are fabricated from materials which are electrical insulators. This makes them ideal for communication use in an electrically hazardous environment, as they cannot cause any arcing or spark hazard.Modulation of several gigahertz over a distance of a few kilometers without the intervention of electronics (repeaters) is possible. The information-carrying capacity and bandwidth of optical fiber systems is far superior to the best copper cable systems or wide-band radio systems.With the advantages offered by optical fiber systems, it is foreseen that most copper wire systems will eventually be replaced with optical fiber systems. For the successful implementation of optical fiber as a channel, the optical fiber has to be manufactured as an integral part of the power cable of equipment. As this type of cable is becoming available, it is opening a whole new field of available channels that can be used for transmitting vast amounts of information.(4) Radio: Radio communication channels have some underground mining applications for speech and data communication. Without some casual waveguide, such as cables, rails, steel ropes, and pipes, a radio channel cannot provide long distance out-of-sight communication. In most cases, a leaky feeder has to be installed throughout the mine to provide reliable links between portable and mobile radio stations or terminals. Radio waves are highly susceptible to electromagnetic fields generated by the power cables of the mining equipment in the section. Some of the previously mentioned systems are more suitable for transmission of data throughout a mine.IV RESULTSEven though a large amount of time and effort was spent on devising an implementation strategy, it was not possible to implement the systems in a mine beyond the prototype stage.With the implementation of the horizon control system, problems were
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