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Sensors support breakthrough in mineshaft inspection

06 March 2013

Regular mineshaft inspection is mandatory in every country that plays host to a mining industry, but inspection procedures result in large revenue losses due to losses of production; moreover, these inspections are necessarily performed in a potentially dangerous and high-risk environment.

Promising substantial savings and improvements in safety, Canada-based Sight Power Inc is working with Shaft Sinkers Holdings and Parsec in South Africa to develop an innovative automated mineshaft inspection technology, based on mobile laser scanning and high-resolution imagery.

Mineshafts accommodate the hoisting systems that provide access to a mine’s working areas, providing transport for miners and materials and serving as a conduit for ventilation and cooling. They also provide an emergency exit. Hence the importance of mineshaft visual inspections, a process that involves regular weekly closures for periods of between four and twelve hours at a time.

Apart from the operational downsides of the inspection, the potential for human error is high, not just due to the lack of light, but because performing an inspection is arduous and fatiguing. Sight Power’s Sergey Reznichenko takes up the story:

“The current shaft inspection technique is largely the same as that employed from the dawn of mining – pure visual observation while descending a shaft. There have been attempts to modernise the process, but these have been complementary to the visual method and have never been accepted and fully implemented.”

Mineshafts are essentially cylinders, the diameters of which range between 5 and 20m and whose depths are anywhere between 100 and 3,000m. The shaft contains a multitude of constructional elements, including vertical elevator guides, pipework; steel beams; fixing brackets and cables, and a lining commonly constructed from reinforced concrete or cast-iron ‘tubbing’ rings. In order to check all of these constructional elements, the conventional procedure is to lower inspection engineers in the shaft elevator at slow speed - certainly less than 1m/s.

A quicker alternative
Sight Power and its partners, Shaft Sinkers and Parsec, have developed a fully-automated shaft inspection technology which they have dubbed Mobile Shaft Scanning System (MS3). This comprises a laser scanning device that is attached to a conveyance that moves up and down the shaft at a significantly higher average speed than that of conventional manual methods. A realistic 3D image of the entire shaft with all its internal infrastructures is subsequently rendered and imported into a software package for analysis.

MS3 is based on LiDAR (Light Detection and Ranging) technology and high-resolution imagery. It combines three core elements - lasers, a global positioning system (GPS), and inertial navigation systems (INS) – in a single platform, enabling a laser scanned position to be determined to a high degree of accuracy. But this arrangement does pose a problem, as Sight Power’s CEO and CTO, Borys Vorobyov explains.

“LiDAR systems are ideal for mineshaft inspection, because the typical distance from the scanning device to objects within the shaft barrel is only 3-4 meters. However, the fact that GPS signals are not available underground presented a significant challenge in terms of obtaining the highly-accurate geo-reference position data required.”

Typically, LiDAR devices are mounted on a mobile platform, with a GPS providing accurate geographical information and an INS recording the precise orientation (pitch, roll and yaw). But in the absence of a GPS, a variety of positional aids are required to assist the INS used by the MS3 system, including inertial sensors such as accelerometers and inclinometers.

The development team also had to design a much more sophisticated system capable of achieving measurements to within 5mm accuracy (compared with 20-40mm for common terrestrial systems). To fulfil these requirements, the MS3 hardware designer team opted for Sherborne Sensors’ DSIC inclinometers and A323 accelerometers. Borys Vorobyov again:

“We compared products from a number of suppliers and the consensus was that Sherborne Sensors provided the best match for accuracy, price and reputation. It can often be the case with these types of sensors that they don’t perform as expected. However, we checked each unit separately before connecting them up to the MS3 system and they have proven to be very reliable.”

During the laboratory test phase the inertial sensors assisted in mapping and characterising the horizontal test set-up performance, as well as indicating any flex, twist and tilt in the MS3 mechanical housing system. They are also being employed to characterise and understand the mineshaft environment, so that the Sight Power team can refine the MS3 system and its design for optimal accuracy and reliability.

The gravity referenced servo inclinometers and accelerometers offer high dynamic range and seamless integration into the MS3 data acquisition system. They are also temperature compensated - a key factor as temperature varies significantly with depth in a mineshaft.

Sherborne's DSIC high-precision inclinometers will be used to measure the amount of roll and pitch of the carriers to give an indication of whether or not their movement down the shaft is adequately controlled. The MS3 system carrier movement varies from normal speed to very slow during the inspection cycle, and the accelerometers will measure their performance with regard to vibration and acceleration across this speed range.

Once distance and location information is accurately determined, the laser scanner yields direct 3D measurements, creating a ‘point cloud’ of data that can be interpreted by the MS3 software in order to build a digital spatial 3D model. The initial model thus provides an accurate view of current shaft conditions, with subsequent scans being compared against it to determine any changes and their severity.

MS3 will also have the ability to take digital images and combine them with the LiDAR data to create a more realistic 3D rendition. When analysing these 3D images, engineers will be able to determine and highlight areas of concerns – such as ground movements, cracks in the shaft lining, as well as misalignment and deflections of pipes and guides. According to Sight Power, MS3 has the potential to reduce production stoppage times by at least 50 per cent annually, as Sergey Reznichenko explains:

“MS3 will improve mine safety, with a reduction in man-hours spent underground in hostile environments and the early detection of defects. It will also mean increased availability of hoisting time and consequently a significant increase in a mine’s revenue and profitability. Crucially, engineers will be able to detect defects at a much earlier stage, cutting shaft maintenance repair costs and downtime and enabling engineers to address any issues before they become a major safety hazard and operational issue.”

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