Can underground coal mine operators afford to ignore fibre optics any longer, asks John Ruble*.
Often considered too fragile, too unreliable and too costly, optical-fibre-based data networks generally have not received wide acceptance in the underground coal industry, both in Australia and abroad.
In many cases, mines that have invested in underground optical fibre systems have not realised their promised full potential, the reasons ranging from susceptibility to contamination reducing their effective range, to a range of technical hitches including optical fibre cable jointing in the hazardous atmosphere of an underground coal mine.
Why have robust, high-capacity data networks in the first place? The reasons are numerous, but some of the main ones are a need to “see” what’s going on underground from the surface, increased equipment complexity that in turn increases the data transmission burden, newer and more advanced safety systems such as environmental monitoring, and the need to get information down to where it’s needed most: the working face.
First data networks were auditory, based on bells or whistles. Examples of this were the old bell signals used in mine hoists and skips. Next came copper wires for voice communications (loud-hailers and telephones). The duty of the copper wires was later expanded to include items such as gas monitoring and conveyor control. Next came a need for linking multiple control system sectors using traditional networks such as Modbus. These networks began to take on a dual purpose of information transfer, usually to the surface, and program modification, which could be accomplished from the surface. The networks typically have speed limits of 50Kbps up to more than 1Mbps.
However, even those speeds still aren’t fast enough for today’s “information age”. Larger and larger amounts of information are being requested all over the mine, and these are being mixed with real-time video and information serving. Some of the practical “information superhighway” alternatives include:
— Copper broadband (coaxial cables).
— Microwave radio.
— Optical fibre.
Making a quick comparison of the different media available (copper versus radio vs optical fibre) shows the following:
* Radio would be used to transfer information over, as the name suggests, a wireless medium. Some of the peculiarities include mine roadways acting like waveguides at the radio frequencies that would be typically used (VHF and above). The coal strata generally attenuates VHF+ radio frequency energy quickly, which gives radio a limited range. Low powers are necessary to maintain IS ratings. Radio is susceptible to electromagnetic interference, which means it can interfere with/be interfered by other radio devices. There is a practical bandwith limit which effectively limits the effective information throughput. Some of the advantages of using radio are its ability to provide electrical isolation, while it is also very good for multi-drop applications.
* Copper broadband networks would typically consist of a coaxial-type copper cable dispersed throughout the mine. The main advantages of copper are that it is also good for multi-drop applications, and twisted-pair cables are relatively easy to join. However coax can be more difficult. Higher bandwidths than those possible over twisted pair wires are achievable but only at increased coaxial cable size. The range of copper cables can be good, with practical distances up to 5km for twisted pairs and 25km for coax achievable. Test equipment ranges from cheap to very expensive, and test procedures range from simple to very complicated. Terminal (communications) equipment is normally inexpensive relative to optical fibre. Some of the disadvantages inherent to using copper as a network medium include: Low power necessary to maintain IS rating; copper is susceptible to EM interference; copper cannot provide electrical isolation; and copper can interfere with other devices and be interfered by other devices.
* Optical fibre, as the name implies, relies on the transmission and reception of information over a (typically) glass fibre or fibres. A mine data and communications network set up over optical fibre brings some enormous benefits, among those being very high communication speeds inherent in such a network, enormous bandwidth capacity, and the enormous expandability of the network to accommodate increased information requirements or newer communications technologies. When optical fibre networks are employed, IS requirements become less of an issue (but still must be considered), complete electrical isolation between systems can be achieved, and such networks are unaffected by EM interference and do not cause such interference. The traditional stumbling blocks to using optical fibre in an underground coal mine historically have been the higher up-front cost per metre as compared to twisted-pair cables and some coaxial cables, increased susceptibility to dirt/moisture contamination potentially rendering the link inoperative, and specialised personnel, equipment, and clean conditions to join optical fibre cable. Additionally, no practical means to multi-drop optical fibre cables currently exists, the terminal equipment can be expensive compared to copper based terminal equipment, and the test procedures normally employed with optical fibre can be quite involved and time consuming. Optical fibre cables are starting to be used in underground coal mines both here in Australia and overseas, but their deployment underground has largely been limited to runs between the mine control room and the pit bottom switchroom (or equivalent area), and from the pit bottom area out to conveyor driveheads and other semi-permanent/permanent locations. With a few exceptions, optical fibre cables generally have not been extended to the more mobile sections of the mine such as the development sections and the longwall section. The reasons for this are wide and varied, but generally fall into the three broad categories of expense, reliability, and usefulness. Optical fibre cables have been more expensive to install and maintain relative to copper cables. The reliability of optical fibre cable based networks have historically not been as good as copper based ones, mainly due to the more complicated electronics associated with them and also the relative fragility of optical fibre cable. And finally, there really hasn’t been a need, until recently, for a data network that’s extremely fast and at the same time capable of carrying non-proprietary information such as messaging services (e-mail), print media (electronic drawings and books) and multimedia information.
Another historical impediment to installing and using optical fibre cables in an underground coal mine, especially in the longwall panels, has been the lack of a reliable, affordable, and durable multi-channel connector that would allow the joining of smaller, manageable sections of cable. Those mines that have ventured into installing optical fibre cables in their longwall sections traditionally have resorted to installing very long runs (to keep splice insertion losses low) of industrial grade cable, usually of the “direct-burial type” that’s stiff and difficult to handle.