Steering by Satellite
Imagine this…You are a bulldozer operator. Your job is to move large volumes of earth for land reclamation at a surface coal mine. This is precise work. You must make sure that the finished surface matches the exact contours of a computerized topographical map. Until now, this has involved carefully driving the machine around a series of survey stakes. Unfortunately, this is not always easy in a giant bulldozer, especially at night when it’s nearly impossible to see the stakes. So you try to cut and fill as best you can, drawing on your skills and experience to interpolate the grade between stakes, and trying hard not to knock the stakes over.
All that is rapidly changing thanks to GPS technology. A number of surface mines in the United States and other countries have recently installed GPS-based machine guidance systems with very positive results in terms of improved productivity and reduced costs.
One of the early pioneers in the use of GPS for machine guidance in excavating and land reclamation was The Falkirk Mining Company. Falkirk has been using a GPS machine guidance system on several bulldozers and other vehicles at its lignite surface mine in North Dakota for earthmoving and land reclamation since 1998. The company claims to have achieved substantial gains in productivity and cost savings since installing the GPS-based systems.
A wholly owned subsidiary of The North American Coal Company, The Falkirk Mining Company operates two pits near Underwood in central North Dakota, about 100 kilometers north of Bismarck. The Falkirk mine supplies about seven million tons of lignite annually to a 1,100 megawatt electric generating station owned and operated by the Great River Energy Cooperative of Minnesota. They are currently working two coal seams, one of them about 1 m in thickness and the other about 3-4 m at depths of 8-40 m, separated by about 6 m of clay and sandstone interburden. Overburden consists mostly of sandy shale and glacial material. The pits are about 3 km long and 60 m wide.
Reclaiming the LandThe U.S. Surface Mining Control and Reclamation Act of 1977 requires that mine operators must carefully restore the land to its pre-mining contours and conditions. Before mining operations can begin, a detailed topographic map of the site must be made. This includes measuring the depths of soil and examining their chemical and physical properties. Then the topsoil is scraped away by bulldozers, and is either immediately re-spread over reclaimed land or piled up for future use. Then the overburden (the dirt and rocks that lie above the coal seam) is removed to expose the coal seam so it can be mined. This is accomplished by a large excavating machine called a dragline (basically a tall crane with a huge bucket at the end of the cables). The overburden is hauled away and stored in spoil piles. After the coal is extracted, the overburden and topsoil are re-spread over the site to its original contours, carefully matching the topographic maps.
Land reclamation at the Falkirk mine is being accomplished contemporaneous with the mining operations in strict compliance with state and federal regulatory requirements. As the dragline works its way down the pit, a dozer fleet levels and grades the spoil piles of overburden to the approximate desired contour. A truck shovel fleet then brings in subsoil from newly uncovered land, followed by tractor-scrapers with topsoil. The land is graded to the approximate original contours, referring to topographic maps created prior to the excavations. In the case of Falkirk, the terrain is mostly flat or gently rolling prairie, used primarily for agriculture or grazing.
Pioneering GPS Machine GuidanceFalkirk started investigating the use of GPS for machine guidance in 1997. After evaluating several potential vendors, they began a demonstration project in April 1998 using the new Dozer 2000 GPS system from Leica Geosystems (Norcross, Ga.). The demo system was installed on one of the Caterpillar D11 bulldozers working primarily in overburden grading. In June, a second system was installed on a supervisor’s pickup truck for further evaluation.
The Dozer 2000 machine guidance system includes a Leica MC500 machine control ruggedized high-precision GPS receiver, radio data receiver and touch-screen computer running on specialized Dozer 2000 software developed by Carlson Software. The GPS antenna is mounted on the top of the cab, where it has a clear view of the orbiting satellites. The GPS receiver calculates its position in three dimensions, 10 times per second. The position data is corrected for local-area errors using data transmitted from a fixed differential GPS reference station. The touch-screen computer is mounted in the cab.
The computer compares the actual GPS position to the desired finished terrain, using grid files created from topographic maps. The large, bright display provides visual guidance to the operator for maneuvering the vehicle and positioning the blade to achieve the cut-and-fill values needed to match the computer model.
According to Gerry Lannoye, software engineer/analyst at The Falkirk Mining Company, who has been deeply involved in the company’s GPS program from the beginning, the system was an immediate hit with machine operators. “We took daily feedback reports from the machine operators during the trial period,” he says. “Then we worked closely with Leica Geosystems and Carlson to fine-tune the system and iron out glitches.”
Based on the success of the initial trials, additional Dozer 2000 systems were purchased over the next two years. Today, Falkirk has a total of 12 systems installed on a variety of vehicles, including eight bulldozers, one motor grader, one tractor scraper and two pickup trucks.
A fixed differential GPS (DGPS) reference station, consisting of a Leica MC1000 GPS receiver, GPS antenna and one-watt radio transmitter has been established on top of Falkirk’s coal silo to provide local-area error corrections for the satellite signals. The DGPS data is transmitted via 900 MHz radio frequencies through repeaters to the GPS “rovers” working in both pits. The mobile Dozer 2000 systems use the error correction data to fine-tune the position accuracy to 2-5 cm.
GPS-induced BenefitsLannoye reports that Falkirk has achieved important cost savings and productivity gains by reducing rehandling of overburden and subsoil. “Our goal is to put the right amount of dirt in the right place the first time,” Lannoye says. “This means handling the material once. Rehandling is very expensive, in terms of labor, machinery maintenance and life-cycle costs for the equipment.”
Using the Dozer 2000, the operators can typically bring the surface within 9-12 cm of the design grade without the use of survey stakes, even at night. There is less downtime, since the dozer operators no longer have to wait for replacement of stakes that have been knocked down or covered over. This means that surveyors are freed to pursue more high-value tasks instead of staking and re-staking the site. While the staking function is eliminated, surveyors continue to play a critical role in the mine’s operation by being responsible for doing the preliminary design work, maintaining the accuracy of the digital data on the computer and verifying the integrity of the results.
The duties of the surveyor have changed with the implementation of machine grade. In fact, the surveyor is still the best-trained candidate for control requirements. New responsibilities include the monitoring of the integrity of the entire surveying system in real-time, conducting periodic checks of the system to ensure correct processing, and managing accurate and timely data flows.
While it is harder to measure, there has been a tremendous upsurge in morale among the machine operators. “The operators love the system,” Lannoye reports. “It empowers them to do a better job. They get a better “feel” for the terrain model and can better visualize the contours. The topographic files displayed in the cab clearly show drainage, hilltops, valleys, wetlands and other terrain features. We already had the best operators in the industry and this tool enhances their performance.”
The machine operators like the touch-screen display with its large graphics and intuitive operating procedures. At a glance, the operator can see the cut-and-fill values needed to match the topographic design model.
There is a lot less friction between shifts, especially in the morning. The morning shift no longer has to worry about undoing work that had been done during the night. When the new operator climbs into the cab, he simply touches an icon on the screen. When he sees the “target” symbol in the upper right corner, he knows that the surface is within design tolerances.
Lannoye also pointed out that the system has made it easier to measure productivity. “In the past, we were unable to quantify the amount of spoil being handled by each machine,” he says. “Now we can track productivity on each machine.”