Optimizing Wireless LAN for Longwall Coal Mine Automation

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coal mining automation has been achieved with the successful implementation of wireless LAN (WLAN) technology for com- munication on a longwall shearer.
IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 43, NO. 1, JANUARY/FEBRUARY 2007

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Optimizing Wireless LAN for Longwall Coal Mine Automation Chad O. Hargrave, Jonathon C. Ralston, and David W. Hainsworth

Abstract—A significant development in underground longwall coal mining automation has been achieved with the successful implementation of wireless LAN (WLAN) technology for communication on a longwall shearer. WIreless-FIdelity (Wi-Fi) was selected to meet the bandwidth requirements of the underground data network, and several configurations were installed on operating longwalls to evaluate their performance. Although these efforts demonstrated the feasibility of using WLAN technology in longwall operation, it was clear that new research and development was required in order to establish optimal full-face coverage. By undertaking an accurate characterization of the target environment, it has been possible to achieve great improvements in WLAN performance over a nominal Wi-Fi installation. This paper discusses the impact of Fresnel zone obstructions and multipath effects on radio frequency propagation and reports an optimal antenna and system configuration. Many of the lessons learned in the longwall case are immediately applicable to other underground mining operations, particularly wherever there is a high degree of obstruction from mining equipment. Index Terms—Automation, coal, longwall, mining, WIrelessFIdelity (Wi-Fi), 802.11b.

I. I NTRODUCTION A. Problem

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HE environment on the coal face of an operating longwall is very harsh and does not appear immediately conducive to WIreless-FIdelity (Wi-Fi)-based communication. Fig. 1 shows the primary machinery used on a longwall coal mine face, namely the roof support system, the armored face conveyor (AFC), and the shearer. The shearer moves back and forth across the longwall face along the AFC rail, cutting coal from the face. As the shearer moves along, the individual roof supports move forward to keep the mining area secure, while behind the moving wall of supports the unsupported roof collapses. A typical longwall face is about 250 m across; the face will rise and fall and deviate in and out across this full width. The key design challenges to overcome from a Wi-Fi communications viewpoint are the narrowness of the tunnel formed by the roof support system, the prevalence of metallic structures, the fact that the face is not straight, and the limited options for robust antenna mounting on the shearer body. This Paper PID-06-06, presented at the 2005 Industry Applications Society Annual Meeting, Hong Kong, October 2–6, and approved for publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Mining Industry Committee of the IEEE Industry Applications Society. Manuscript submitted for review October 15, 2005 and released for publication August 7, 2006. The authors are with Exploration and Mining, Commonwealth Scientific and Industrial Research Organisation, Pullenvale, QLD 4069, Australia. Digital Object Identifier 10.1109/TIA.2006.885892

Fig. 1. Longwall shearer, roof supports, and AFC.

nonideal propagation channel produces “dead-band” regions across the face, resulting in communications drop outs with the shearer. In addition, the data rates achieved during the system operation were often lower than required for the growing bandwidth demands (including streaming video) of the automation project. B. Technology The current wireless LAN (WLAN) system used for the longwall automation project is the mature IEEE 802.11b standard. This standard employs the direct sequence spread spectrum (DSSS) transmission method on various channels in the 2.4-GHz range. The use of this modulation technique is one of the factors that make 802.11b so attractive for use in an underground environment with many metallic surfaces, since DSSS has a certain degree of inherent resistance to multipath interference. DSSS spreads each information bit across a range of frequencies using a variety of coding techniques (corresponding to the standard bandwidth capacities of 1, 2, 5.5, and 11 Mb/s) to create a range of symbols for information encoding. When a reflection (due to multipath interference) causes a particular symbol transmission to overlap with the next bit/symbol being transmitted, the despreading applied to the correct symbol to isolate the correct bit value will tend to not despread the undesired (reflected) symbol. However, if there are sufficient reflections in the environment, the multipath delay will be long enough that the more complex encoding techniques will be compromised by misinterpreted bit patterns in adjacent symbols, leading to data corruption and packet retransmission. In this case, the wireless equipment may drop the data rate

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Fig. 2. Network infrastructure from South Bulga colliery.

in order to transmit longer symbols with greater information redundancy and better noise immunity. The main mechanism for testing the Wi-Fi link is the devicespecific received signal strength indicator (RSSI), which is defined in the IEEE 802.11 standard [2] as an indicator that “is intended to be used in a relative manner.” This particular metric is manufacturer specific, and “absolute accuracy is not specified.” For the current 802.11b equipment being used for the longwall automation project, the manufacturer is Cisco Systems, and the devices used are from the Aironet 350 series (both access points and workgroup bridges), with the system being used in the infrastructure (as opposed to ad hoc) mode. The Cisco equipment provides a signal strength measure that is based on 100 separate divisions proportional to the signal power at the receiver. The signal strength is thus shown as a range from 0% to 100%, which corresponds approximately to a relative power measure at the receiver between −113 and −10 dBm [3]. The second metric for characterizing a Wi-Fi link is the signal quality, which is defined in the standard as “a measure of the PN code correlation quality received by the DSSS PHY” [4]. The “PN code correlation” refers to the degree of correlation between a received (measured) DSSS symbol and the actual DSSS code for that symbol. Signal quality is thus a measure of the link integrity based on actual useful information transfer, regardless of signal strength. This measure can be particularly important in a cluttered environment where multipath interference is common. The 802.11b Wi-Fi equipment was first used with the longwall automation project at the South Bulga colliery, as described in [1]. The current paper describes the next phase of the Wi-Fi implementation at the Beltana coal mine and the preparation and testing for the third site at the new Broadmeadow longwall mine. There may be good grounds for investigating the newer 802.11g and 802.11a standards, as will be described below. The Wi-Fi link to the shearer is the last leg of an exten-

sive mine network infrastructure (Fig. 2) that uses both optical fiber and Category-5 copper cable to provide a connection from the surface all the way to the longwall face (typically a distance of between 3 and 5 km). C. Prior Results and New Sites The initial positive results for Wi-Fi testing at the South Bulga colliery provided the impetus to continue research at the newer sites involved in the longwall automation project. Ranges of over 150 m were achieved at this site, which was far more than expected given that a typical indoor performance range for 802.11b equipment is closer to 50 m. It was speculated that this surprising range was due at least partially to the highly reflective metallic environment at the mine, which caused the received signal strength to register higher than it would in areas with more absorbent material. Whether this measured range could sustain a reasonable rate of data transfer was as yet uncertain. Whereas at South Bulga only a single access point was installed at one end of the longwall, at the Beltana site, a second access point was placed at the opposite end in an attempt to provide full-face coverage. This second access point was joined to the mine network via a fiber optic link that passed along the back of roof supports across the length of the longwall face. It was felt that the two access points could be sufficient to provide full coverage for a 250-m face given the ranges measured at South Bulga for a single access point. This system was installed at Beltana in late 2002, and the results were monitored. It became clear almost at once that while the system was providing a high level of coverage for the face, there was consistently a period of time when the shearer was between the ends of the panel (the “gate ends”) in which the network connection to the shearer was lost. This situation was acceptable since the nature of the data acquisition required by the system for the particular stage of the automation project

HARGRAVE et al.: OPTIMIZING WIRELESS LAN FOR LONGWALL COAL MINE AUTOMATION

at the time could tolerate a reasonable degree of latency in the delivery of the data. When the shearer came into range at either gate end of the face, the data acquired in the out-ofrange segment would be uploaded to the automation system. It was observed with interest that this mid-panel dead spot was not fixed, instead varying over time with the changing shape of the longwall face, sometimes growing quite large (over 100 m) in bad conditions, at other times shrinking to around 30 m. Full-face coverage, however, was not achieved, and it was recognized that the WLAN infrastructure would have to be improved in order to support the bandwidth requirements of the automation project. Nevertheless the two-access-point solution installed at Beltana provided sufficient coverage to allow the first instance of streaming video being transmitted live from a longwall shearer, which was used to provide valuable thermal imaging data for part of the project goals. This achievement again bolstered confidence that the Wi-Fi approach could provide a solution for the communications needs of the project, provided that a more robust site infrastructure could be established. The opportunity to design a more complete Wi-Fi infrastructure was presented in the form of the third mine site to become involved in the longwall automation project: the Broadmeadow longwall coal mine in northern Queensland, Australia. Broadmeadow is a new mine, and it was possible to exploit this fact to insist upon the inclusion of additional access point base stations in the middle of the longwall face, again linked to the rest of the mine network via the optical fiber backbone. Among the promising features of the Broadmeadow site (from a communications viewpoint) is the fact that the coal seam is much higher than the previous two mines, with a roof support height of around 5 m, as compared with the 2-m supports at South Bulga and Beltana. These tall supports will create a much larger tunnel for the Wi-Fi transmissions, which could potentially reduce the impact of multipath interference due to reflections. Additionally, the longwall face at Broadmeadow is around 200 m wide as compared with the 250-m faces at Beltana and South Bulga. The smaller face obviously reduces the range requirements for the Wi-Fi equipment, and it was decided to proceed with a design that used two mid-face access points for a total of four infrastructure units communicating with the client onboard the shearer itself. Four access points will allow the face to be divided into lengths of 50 m, which is the nominal range of the 802.11b equipment in an indoor environment. Given that the range results from the previous project sites were better than this baseline figure of 50 m, there are reasonable grounds for expecting that the proposed combination will provide a full-face coverage solution. Despite the confidence of this expectation, the increasing reliance of the automation systems on the ubiquity of the Wi-Fi link necessitated further testing prior to the installation of the new system at Broadmeadow.

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optimal or near-optimal levels. To this end, additional research and development was undertaken in an attempt to address these issues, including the following: 1) an analysis of Fresnel zone obstruction on the longwall face; 2) an examination of the impact of reflections from the surrounding mining equipment (in particular the roof supports) on the received signal strength; 3) characterization of the extent to which multipath interference from reflections reduces the effective bandwidth; 4) proposals for modification to nominal system configuration (e.g., by altering antenna orientation and device positioning) in order to improve and optimize the link performance in terms of both coverage and bandwidth. A. Queensland Centre for Advanced Technology (QCAT) Test Facility A longwall coal mine contains an explosive atmosphere due to the presence of methane gas from the coal. Due to this potential for explosion, there are stringent restrictions on the use of electrical equipment in a coal mine, particularly near the operating face (the longwall). In order to perform testing in a similar environment to the longwall face without restricting the test equipment available, the Wi-Fi testing was first performed in an underground facility at QCAT, Brisbane, Australia. The tests sought to establish the transmission characteristics of WLAN in a similar situation to a longwall face, where a direct line-of-site link between the client and the access point was often marginal or nonexistent. An examination of the WLAN’s physical arrangement and the network behavior also revealed several areas where improvements could be sought: antenna arrangement on the shearer and/or at the gate ends, modification of system parameters to facilitate smoother handover from one access point to another, and addition of mid-face access points to improve coverage. The test site was an underground tunnel beneath the QCAT facility (Fig. 3). This undercroft region is 50 m in length, along which four access points were distributed evenly to represent a shortened version of the longwall face. The tunnel contains a variety of overhead metallic equipment for building services (cable tray, etc.), which provided a reasonable simulation of the surrounding roof supports on a longwall face. One side of the tunnel is bounded by a smooth brick wall, while the other opens out to a sloping wall of unfinished rock and earth, apart from a section approximately 15 m in length at the start of the tunnel where a series of metal shelves are installed between the earthworks and the brick wall. This varied environment provided a useful testbed for examining the performance of an underground Wi-Fi installation in a nonhazardous area. With reference to Fig. 3, the right-hand side access point (near the metal shelving) was considered to be the first, or main gate, access point, and the left-hand side access point was defined as the tail gate.

II. W I -F I T ESTING In order for automation breakthroughs to be introduced into the normal operation of longwall mines, it was clearly necessary to improve the performance of the wireless link to

B. Access Point Signal Strength Tests The first series of tests performed sought to establish the basic level of performance based on the criteria of received

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Fig. 3. Indicative plan of undercroft tunnel (nominal start of tunnel is on the right-hand side of the image).

Fig. 5. Average signal strength values for the four access points, each access point fitted with two antennas.

Fig. 4. Average signal strength values for the four access points, each access point employing a single antenna only.

signal strength. A laptop computer with a Cisco Aironet client adaptor card was used for the testing, in which repeated signal strength levels from each of the four access points were logged along the tunnel at measured distances. In addition, signal strength and quality values were logged based on the signal received at the client adaptor itself. The data gathered from individual access points are plotted in Figs. 4 and 5, which show the mean RSSI values plotted against position along the tunnel of the moving client (simulating the shearer movement across the longwall face). Note that for all plots, the 0-m position corresponds to the right-hand side of the tunnel (the “main gate”), as shown in Fig. 3. Fig. 4 shows the results of testing with only a single antenna attached for each access point, while Fig. 5 exhibits the two-antenna case. Tests were performed for both of these cases in order to investigate the effect of Cisco’s multipath mitigation scheme, which uses spatial diversity based on antenna selection. The most striking aspect of the results is the markedly superior signal strength values received from the fourth access point (positioned at the far left in Fig. 3), at least over the final third of the tunnel. The superior signal strength results obtained from the “tail gate” access point seem to clearly correspond with this unit having the fewest obstructions from surrounding metallic surfaces of any of the access points. While there is a short section of brick wall on the earthworks side of this unit, this stops well before it is likely to interfere with the central Fresnel zone of the access point when considered with reference to any point along the tunnel path. Fresnel zones are regions of constructive

interference that are inherent in any electromagnetic radiation between two devices. The central Fresnel zone is the most critical, and a site must keep at least 60% of this first zone clear to avoid severe signal degradation. The equation for calculating the maximum radius of the central Fresnel zone is given as  d (1) r = 17.32 × 4f where r is the radius of the first zone (in meters) at a point equidistant between the two devices that are separated by distance d (in kilometers). Using the 802.11b operating frequency of 2.4 GHz and assuming the client is at a distance of 50 m or 0.05 km from a given access point yield a maximum radius (at a point half way between the client and the access point) for the first Fresnel zone of 1.25 m. In the case of the main gate access point, the metal shelving would certainly obscure a large segment of this first zone, even considering that the zone radius for communications with a point half way along the face (25 m) would be only 880 mm. The tail gate access point has no such interference and hence can maintain a reasonably high signal level out to the center of the face. At this point, the signal is likely compromised by two large pillars (not shown in Fig. 3) that are also likely to be responsible for the relatively poor signal strength performance of the “mid-face” access points. C. Gate Road and Localized Equipment Tests Following on from these tests, it was decided to examine how far reasonable communications could be maintained with the wireless equipment on the longwall by a client in the longwall gate road. The gate road runs orthogonally to the longwall face, so testing was performed by leaving the undercroft tunnel and walking directly away from the exit. These results are shown in Fig. 6, which show that there is a surprising communications range even when around the corner from the nearest access point and certainly well clear of any line-of-sight. These results may prove useful in the future for the automation project, since the area immediately around the gate road corner is home to

HARGRAVE et al.: OPTIMIZING WIRELESS LAN FOR LONGWALL COAL MINE AUTOMATION

Fig. 6.

Average signal strength versus “gate road” position.

several instruments used by the project, and the Wi-Fi coverage could provide an easier means of communication. The gate road simulation tests showed that a reasonable communications link was possible based solely on signal reflections. In order to test this aspect of the system, some simple measurements were made of signal strength values in the presence of deliberately interfering reflective surfaces. The client was placed around the corner from an access point, and various metallic structures were introduced into the intervening space. As expected, the signal strength varied significantly depending on the arrangement of the reflectors but without any noticeable correlation with particular orientations. This result is unsurprising given the highly complex nature of the multipath reflections that contribute to the total received power at the client; however, the result does emphasize the unreliable nature of Wi-Fi communications that rely on reflected paths for data transmission. A related experiment was also performed, where a metallic structure was brought directly into the nearfield region of an access point antenna in order to simulate the undesirable cluttering of the Wi-Fi equipment, which can occur at times on an operating longwall. While the signal strength from the access point remained reasonably high, the information throughput of the link was significantly degraded. This important difference between signal strength and link quality was further investigated in the context of overall WLAN coverage. D. Client Signal Strength and Quality The final series of tests performed prior to the Broadmeadow installation involved retracing the undercroft tunnel to acquire the signal strength and quality data for the Cisco Aironet client card itself. Unlike the results based on individual access point signal strength, these tests provide a metric for the overall WLAN coverage across the face. In order to get some indication of the system behavior, the average signal strength and quality results were fitted to a general trend line using a first-order leastsquares calculation (Figs. 7 and 8). The results revealed some interesting contrasts with the data obtained from the access points. Most importantly, despite the better individual signal

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Fig. 7. Average signal strength values for the client with trend line.

Fig. 8. Average signal quality values for the client with trend line.

strength performance of the fourth access point, the overall RSSI at the client shows a general decline in signal strength as the face is traversed, apart from a small region directly around the tail gate access point. In addition to the signal strength results for the client, it was also possible to log the signal quality values for the link between the client and the network backbone connecting the wired LAN side of the access points. In order to obtain some valid signal quality metrics, a high-bandwidth data stream was established between the wired network and the wireless client. The resulting quality information is particularly valuable for characterizing the behavior of the WLAN installation and shows that despite the decrease in signal strength as the client moves toward the tail gate, there is a marginal increase in signal quality. This second measure of network performance provides a clue as to the nature of the high signal strength values measured near the main gate in Fig. 7. It appears that the higher signal strength values actually result from the large number of multipath reflections received at the client from the surrounding

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metallic structures at the main gate; however, the intersymbol interference caused by these reflections has actually degraded the link performance. E. Future Work The results obtained from the QCAT testing provide a great deal of information that is directly applicable to future Wi-Fi installations in underground mines and in particular to the case of the new automation project at Broadmeadow colliery. As part of the commissioning process for this mine site, an opportunity may be provided for testing the Wi-Fi infrastructure prior to the commencement of mining. It is anticipated that a similar series of tests to those performed at QCAT could be carried out onsite at this time. Several strategies for improving the WLAN performance will be examined during this phase of the project. III. C ONCLUSION The usual rules governing Wi-Fi installations naturally continue to apply in an underground environment, as the results obtained have indicated. However, due to the unusually high degree of multipath interference that is prevalent in the underground, there are some areas of the installation that require particular care. To somewhat mitigate the effect of multipath interference, the use of Wi-Fi equipment that supports a diversity antenna system is recommended. The effects of multipath can be extremely localized (especially in the 2.4-GHz range of 802.11b), so the spatial variation between the two antennas can often result in significant differences in the integrity of the received signals. If the system is able to select a particular antenna for use based on better signal quality, the link integrity can often be maintained despite the high incidence of multipath interference. A second criterion for link optimization lies in the necessity to maintain a clear near-field region around each antenna in the system. Due to the fact that almost all the structures on a longwall are metallic, it is difficult to avoid some interference due to near-field reflections; however, the dramatic impact on signal quality that can result from such a situation has been noted and brought to the attention of the mine engineers for consideration in the design of mounting arrangements. At the Broadmeadow mine, for example, a pair of specialized brackets has been proposed to ensure that the antennas mounted on the shearer have sufficient clearance. Improving the link characteristics by maintaining a substantially clear first Fresnel zone is a more difficult proposition on a longwall face. The seam height (and hence the height of the metallic roof supports) can vary from less than a meter to over 5 m between different mines. In the case of thin seam mining, it is difficult to avoid substantial occlusion to the central zone, and inevitably, the link will have to rely on reflections to achieve any reasonable range. The multipath interference that results from this use of reflections will have some detrimental affect on the signal quality despite the use of antenna diversity. The typical behavior for 802.11b Wi-Fi in such an environment is to maintain the link up to the point when the DSSS encoding can no longer differentiate symbols at even the lowest data rate: at this point, the link will fail abruptly.

In order to overcome some of the problems associated with multipath interference, it has been proposed that the more recent Wi-Fi standards 802.11g/a should be investigated. Both systems make use of orthogonal frequency-division multiplexing (OFDM), which should show more resistance to multipath interference than DSSS. Some of the issues to be considered, however, include the fact that 802.11a operates in the 5-GHz band and, hence, has a much reduced range than the 2.4-GHz b and g standards. Part of the standard defining the implementation of 802.11a also calls for the antennas of each unit to be captive, thus preventing any illegal downstream amplification of the effective radiated power. Unfortunately, due to the explosive nature of the coal mine, the actual Wi-Fi units are contained within a flameproof enclosure, and antenna connections are only permitted after the antenna cable has passed through a barrier to prevent any possible transmission of a spark to the outside environment. Given these issues with 802.11a, it has been proposed that 802.11g be investigated for use for future installations in the automation project. 802.11g operates in the 2.4-GHz band used by 802.11b and is backward compatible with the earlier standard, making a staged implementation feasible (an attractive feature considering the difficulty of obtaining physical access to our equipment once it is underground). The higher maximum data rate of this standard (54 Mb/s) is an added incentive as the bandwidth requirements of the field equipment increase. However, one caveat for the use of this technology is worth noting, that is, the system will swap from an OFDM encoding scheme back to the DSSS system if the link quality drops to a point where higher data rates (20 Mb/s and up) are no longer sustainable. At this point (which is typically when resistance to multipath interference is most needed), the system will essentially exhibit the same characteristics as have been described in this paper for the existing 802.11b system. For this reason, the other optimization strategies suggested herein remain of crucial importance if the Wi-Fi system is to achieve its full potential in the automation system. A PPENDIX A DDENDUM Since presenting this paper at the 2005 IAS conference, the wireless network described herein has been installed and commissioned at the Broadmeadow longwall coal mine. A survey of the longwall with a wireless client demonstrated that the entire face was covered with good signal strength and quality across the face. More importantly, the network has proven to be very successful during the actual operation of the mining equipment, providing full-face coverage for the various automation systems including some closed-loop components that rely on a continuous network connection. The good results from this installation have been used to aid the design process for the communications infrastructure on future longwalls both at Broadmeadow and other sites. ACKNOWLEDGMENT The authors would like to thank J. P. Thompson for providing assistance with the hardware setup.

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R EFERENCES [1] C. O. Hargrave, R. J. McPhee, J. R. Ralston, D. W. Hainsworth, and D. C. Reid, “Wireless Ethernet for longwall coal mine automation,” in Proc. CAMI, Calgary, AB, Canada, Sep. 8–10, 2003, CD-ROM. [2] IEEE Standard 802.11, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, p. 150, 1999. [3] Cisco Systems Text Part Number OL-6383-03, Cisco 7920 Wireless IP Phone Design and Deployment Guide, p. 25, Cisco Systems. [4] IEEE Standard 802.11, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, p. 213, 1999.

Chad O. Hargrave received the Bachelor of Engineering degree in communications and electronics and the Bachelor of Arts (with Honors) degree from the University of Queensland, Brisbane, Australia, in 1994 and 1999, respectively. He is currently working toward the Masters degree in engineering at the University of Queensland. In 1999, he was with Stork Electrical, Brisbane, working on various commercial and industrial projects with special emphasis on design and programming work for industrial automation systems. Since 2001, he has been with the Commonwealth Scientific and Industrial Research Organisation (CSIRO), Queensland Centre for Advanced Technologies (QCAT), Brisbane, where he is currently a Senior Electrical Engineer and working with the Mining Automation Group. His primary research interests include the development and application of radar and laser technologies to problems in the mining industry and the development of wireless networking solutions for mining-related applications.

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Jonathon C. Ralston received the Bachelor of Engineering (with Honors) degree in electronics and computing and the Doctorate of Philosophy degree in signal processing from Queensland University of Technology, Brisbane, Australia, in 1991 and 1996, respectively. In 1996, he was with the Mining Automation Group, Commonwealth Scientific and Industrial Research Organisation (CSIRO). He is currently a Senior Research Engineer with CSIRO, Queensland Centre for Advanced Technologies (QCAT), Brisbane. He is a Principal Hardware and Software Architect responsible for the design, implementation, and deployment of sensing, guidance, teleoperation, communications and visualization, signal processing, and control technologies for the automation of range of mining equipment and processes. His primary research interests are in the areas of ground penetrating radar and signal processing.

David W. Hainsworth received the Bachelor of Engineering (with First Class Honors) degree in communications and electronics and the Doctorate of Philosophy degree in signal processing from the University of Queensland, Brisbane, Australia, in 1971 and 1976, respectively. He is currently a Senior Principal Research Engineer with Exploration and Mining, Commonwealth Scientific and Industrial Research Organisation (CSIRO), Queensland Centre for Advanced Technologies (QCAT), Brisbane. His research activities include remote control and automation of mine equipment, highwall mining guidance, underground communications, coal thickness measurement techniques, and teleoperation methods. He is currently working on research and development projects relating to rapid roadway development and longwall automation.