In the highly competitive world of mining, like in most any venture today, the goal is high production at the lowest possible cost. One tool that brings a new level of efficiency to mining is satellite- based navigation systems.
These systems tap into the Global Positioning System (GPS) network and can accurately pinpoint the location of machines and ore bodies. They’ve already been used for a few years to survey mine sites, and now technical advances are bringing a new level of precision to the mining process itself. With the prospect of making better use of equipment while minimizing waste, experts predict that in many mines, GPS technology will soon make traditional practices obsolete.
One major source of inefficiency in conventional mining methods is translating design details such as ore location, elevation, and grade into hundreds of survey flags or wooden stakes that mark specific locations across the terrain.
This presents a host of problems. For one, surveying is tedious, costly, and prone to human error. Another is that the flags can be hard to see due to weather or darkness. And perhaps the biggest headache is that mining operations often destroy the markers, which mandates constant resurveying.
Thanks to recent advances in commercial GPS hardware and software, mine operators can now circumvent such problems. One key is the availability of GPS receivers that offer positioning accuracy to within a few centimeters. At the same time, GPS hardware has evolved to where components are rugged and portable enough to be mounted on mining machinery. Better wireless communication technology permits large amounts of data to transfer uncorrupted between a central computer and machines. And, it almost goes without saying that computer processing power has grown significantly in recent years while cost has dropped — making the systems more affordable.
The result is that instead of physically marking survey locations throughout a mine, the latest systems have an onboard computer that displays “virtual” survey markers and boundaries. They can also show details such as machine location relative to ore bodies, as well as computer models of the mine that are updated in real time as the terrain changes.
One state-of-the-art package is Caterpillar’s Computer-Aided Earthmoving System (CAES), developed by the Peoria, Ill., company in conjunction with Trimble Navigation Ltd., Sunnyvale, Calif. According to Caterpillar officials, CAES goes a long way toward alleviating problems with conventional mining practices because it transmits engineering designs directly to the working machines.
With CAES, every machine has a GPS antenna and receiver, computer and monitor, and Windowscompatible software. Designs generated with commercial site-plan software on the central engineering computer are transmitted to each machine’s onboard display via a high-speed radio network. Data can include digital terrain models, geometric displays of ore bodies, and descriptions of desired grade, slope, and other earthmoving objectives.
The system also shows topography updates as the surface changes. CAES measures and records the machines’ progress and transmits the information to the engineering management office for near realtime analysis. All this takes about one second, explains Jay Eyster, a marketing specialist with Caterpillar’s Mining & Earthmoving Technology Systems group. “But our machines don’t move at Mach 3, so for a track-type tractor, 1-sec latency is perfectly acceptable,” he explains.
Because the system accurately determines a machine’s position relative to design — without survey markers — one major benefit is fewer handling mistakes. For instance, when loading ore from a seam that lies next to a waste area, the display precisely shows the shovel location, ensuring that the operator is loading the proper material. As the machine digs, the color display changes when different grades of material are loaded.
GPS-based systems are also beneficial for other mining operations. In blasthole drill systems, for example, GPS helps the operator navigate a drill rig to the correct blasthole location without the aid of survey stakes. The system displays actual machine location and intended hole locations. Once the rig is in position and the drill is level, GPS can calculate elevation, used to determine the correct drilling depth.
In road-building applications, motor graders usually work from road design files. Beginning with a computer-generated profile for the road, the operator works the terrain until the system indicates that actual ground elevation matches the intended finished grade.
GPS can also help control the movement of haultruck fleets. With special software, a central dispatch computer monitors vehicle location and status — direction, speed, and whether it is full or empty — and determines the most efficient assignments and routes for all trucks in a mine. For instance, by having realtime access to each vehicle’s position, the software knows if several trucks are waiting at one shovel, and routes others to a different shovel to prevent bottlenecks. GPS can also tell where a load of material came from and where it was dumped, while support software keeps track of ore content and makes production calculations.
It can also enhance mine safety. A GPS-based package has the ability to notify drivers of potential collisions as they approach stationary objects or other vehicles. This could be particularly beneficial when working in bad weather or after dark. Usually, a less-costly GPS system accurate to several meters is sufficient for haul-truck applications.
Most experts agree that the declining price of computer processing power will directly impact future GPS-based systems. Officials at Trimble predict smaller, less-expensive units, while noting that software development is expanding rapidly. Developers are taking advantage of greater computer power to meld digital terrain data, CAD packages, databasemanagement software, and GPS. The speed and quantity of data that can be transmitted, and the ability to integrate this with information from other sensors will lead to even more powerful, flexible, and cost-effective systems.
Systems such as Caterpillar’s CAES already have a quick economic payback for customers as it allows them to significantly reengineer their mining processes. In metal mines, studies have shown over 5% of material is incorrectly identified and improperly handled. Even if only one truckload of ore per day is sent the wrong way, the economic return on CAES is less than three months. In coal mining, one customer has improved productivity by 30% with track-type tractors by revamping the reclamation process.
Officials at Caterpillar envision GPS-based machine systems growing to site systems. One result is that machines outfitted with these products will provide continuous survey data to the main computer, in real time, while the site is being worked. They also predict GPS will become standard equipment, especially in large mines and infrastructure projects such as reservoir and highway construction.
A bit further out on the horizon are automated machines that can navigate on their own, fully guided by systems that integrate GPS with data from motion and tilt sensors. The goal is to significantly increase productivity while making sites safer.
BASICS OF GPS
In general, a ground-based GPS unit computes its position based on radio signals it receives from several different satellites. Thus, a clear view of the sky is essential. The satellites contain highly accurate clocks, so the timing of satellite signals is precisely known. The GPS receiver calculates the distance to each satellite based on the travel time and signal speed (speed of light). It then uses these distances to calculate the receiver’s position on earth.
Different types of GPS systems offer different accuracies. For instance, the so-called autonomous mode is often used by boaters or hikers, and is considered accurate to within 15 to 100 m. It requires only one receiver and data from at least four satellites.
Real-time kinematic fixed GPS, on the other hand, is accurate within 1 to 5 cm, and is often the system of choice in mining applications. This system requires two receivers. One is a stationary, reference receiver at a known location that tracks satellites and broadcasts data over a radio link.
Thus, a survey that defines a mine’s local coordinate system is essential before GPS operations can begin. The GPS system must also be calibrated to define the relationship between the satellite and mine coordinate systems.
A second machine-mounted GPS receiver collects data from both the satellites and reference station. Based on information from the reference station, the mobile units calculate precise position in terms of the mine’s local coordinate system. For highest accuracy, mobile equipment should remain within about 10 to 15 km of the reference station.
CAES was created to lower the cost-per-ton of a mining operation. Applications include ore body and benchelevation control for wheel loaders and shovels; grade and slope determination for tractors, graders, and scrapers; and compaction and coverage for compactors.