Automated mine scanning

Mine surveying with robots and laser scanners produce 3D (three-dimensional models).

When James Jobling-Purser combined a laser scanner with a robotic vehicle for scanning mines he probably wouldn't have dreamt that what started out as a university project would see him scanning mines in Mexico and South Africa, where his company 3D MSI and Francois Stroh from Leica Geosystems were invited to scan the world's deepest mine.

"I did my undergraduate degree in Mineral Surveying and Resource Management at the Camborne School of Mines," he explains. "Part of that degree was a thesis at the end of our final year, and my thesis was about the development of a low-cost laser scanner for mining applications using standard off-the-shelf components. The conclusion of the thesis put forward the idea of a roving unit, which led onto postgraduate research looking at the practicalities of developing such a system."

Funding was acquired for a PhD in 2003 to take the undergraduate work a step further by developing a remote vehicle to be used in conjunction with the scanning instrumentation. This work was carried out under Dr Patrick Foster at the Camborne School of Mines.

The robot produced combines state-of-the-art laser scanning instrumentation with wireless communications and robotic technology in the form of a remote surveying vehicle (RSV), to make data capture much faster and improve the range and coverage of measurements, as well as safeguarding the survey personnel.

However, it started with fairly humble beginnings. "I took the Lego Mindstorms kit, and I fitted the theodolite with the motor drive systems from Lego Mindstorms and rigged up a tooth belt drive to give movement in the vertical domain and in the horizontal, which allowed us to drive the instrument remotely. There were laser scanners available back in 2002 and there were robotic instruments but I didn't have any at my disposal.

"Very quickly the project evolved and I actually looked at making the whole process remote, eliminating the tripod from the traditional surveying process and mounting the instrument on a vehicle. This allowed us to access hazardous areas that may previously have been inaccessible, and also sped up the whole surveying process by eliminating set-up time.

"I also designed a self-levelling system for the RSV that allowed the instrument to remain level while carrying out surveys," he says. "It uses gyroscopic stabilisers that were developed for gun stabilisation platforms on ships. Kenyon Laboratories (KenLab) then went on to develop them as camera stabilisation platforms for aerospace. The movie industry also very quickly adopted them and used them for large video cameras in helicopters."

These gyroscopes are of the spinning mass variety rather than piezo-ceramic, fibre optic or digital gyroscopes like those associated with the Segway.

"It levels the instrument to about 98 per cent and then we have a self-levelling tri-bracket, which is the mount the instrument is attached to, which allows us to level that last 2 per cent, or certainly within the tolerances of the compensator on the instrument. It was relatively quick - it wasn't real-time but the vehicle would stop and it would go through a function of tracking the spirit level bubble and then levelling itself."

Initial research

Initial research for this was carried out on Lego Mindstorms system using the light sensor to track a spirit-level bubble, and once the bubble had been calibrated all the Mindstorms device had to do was perform a series of movements on the motor that would cause the spirit-level bubble to go back and forth. When it went past a certain point on the light source the motor stopped and it was level. "Very quickly we realised that using a total station and electronic distance measuring device was inadequate for the kind of applications we were looking for as it was quite slow and a bit clunky," explains Jobling-Purser. "We immediately changed to laser scanners which are incredibly rapid. One of the Faro photon scanners can gather 975,000 points a second so you are talking a phenomenally fast rate of data capture. That enabled us to capture much more detailed scene rapidly."

Several local companies were involved in the design and manufacturing elements of the prototype, with the robot being manufactured entirely within 15 miles of the Cornwall Campus. James has developed RSV with the support of 3D Laser Mapping, a company specialising in laser scanning solutions. As well as providing the laser scanning device, which forms part of the robot, 3D Laser Mapping has provided the project with advice and support, and software to aid the research and development behind the project.

The machine is not kinematic, being essentially a stop-and-go system. "We go into a mine and do a reference of its location using four targets that have been coordinated with GPS or a total station," he says." That scan is essentially locked in space and then we can move the vehicle forward and conduct another scan with overlapping geometry and so on, like a daisy chain around the mine."

Some errors can however creep into this, so you get a compound error over time as overlapping scans do not match perfectly and tests have shown that the system is accurate to about 40mm over 250m, which is a vector error.

Jobling-Purser believes that there will be a need to improve this in the future, but mines cannot currently be blasted to those tolerances anyway. "In time, we may be able to eliminate traditional surveying but at the moment the way I see surveying is that it's split into two categories. There is precision surveying and then there is what I call detailing.

"Precision surveying is within five seconds of arc and that will put you on target, so if you are developing a drive or a mine going forward, you take a bearing of the line or the centre of your heading, and any inaccuracies will occur in the blasting around that but essentially your heading is on the money.

"Then there is detailing which just involves picking up the outlying structure of the tunnel. Traditionally, that has been done in 2D and more recently 3D, but only as a line which can be represented as a DXF line. In time, if we can get this 3D model as accurate as a total station there will be no need to conduct conventional surveying." This will speed up the whole surveying process.

"In the not so distant future we will see laser scanners and total stations becoming one instrument," predicts Jobling-Purser. "However, I don't think it's fair to compare traditional surveying with the type of 3D surveying we are doing. The cost of the instruments is very high and the data set is so large and complex, getting simple information from it needs quite a bit of work. We are looking at this as a completely different form of mine surveying."

The future of data

"The way we see things is to have the data set at the head of a pyramid, which will be managed and upgraded by the surveyors within the mining operation, so they ensure that the levels of accuracy are upheld and that the data is produced regularly," he explains.

"In the next tier down on the pyramid we are looking at how several different disciplines in the mining industry could benefit from the use of the three dimensional data. For example, geotechnical analysis can be done from the laser scan data using software like Split-FX. Then we are looking at mine development. So for example if a mine buys a new truck or plant to go into the operation, we can actually take that laser scan data and we can build up a 3D model of how the machine is going to fit within the working environment."

Health and safety may require that a drive be made a little bit wider or a little bit needs to be taken off a corner or the equipment might need to be changed for that particular task. "That will then improve the cost benefits because you aren't buying a machine and then seeing if it will go in there, you are doing everything onto the CAD and using digital technology before you even make the choice as to what equipment you are going to buy. We see that as being a major benefit to mining companies."

The other tier on the pyramid is in updating old mine plans and old workings where operations have been taken over by new companies wanting to develop the mine. "We can come in and 'carpet bomb' the site, surveying the whole operation in 3D and delivering a state of the art plan that will allow the operation to be optimised. I think that the speed of the vehicle really comes into its own then as we can survey approximately 1.5km of workings a day with a potential turnaround of 2-3 days per days surveying, depending on the clients deliverable. "One of the jobs I did last year was in an operation which had previously taken 18 months to survey using traditional techniques. The information also didn't give the client exactly what they wanted as they wanted volumes for the existing stopes and workings so they could accurately evaluate the mine and tunnels and the resource. We went in and surveyed all of the workings, about 2.2 km of tunnels, in three and a half days. So, what had previously taken 18 months we did in 3.5 days, and given them information on the size of their stopes and voids." 

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