All in the blink of an eye (actually, an eye-blink is exceedingly slow by comparison), a three-dimensional (3-D) grade-control system performs three bits of electronic wizardry.
First, it locates the exact three-dimensional position of a machine's work tool on the jobsite. Next, it consults an electronic, three-dimensional model of the jobsite to determine how the material beneath the tool should be shaped at that location—and how the tool should be adjusted to do the shaping. Then, it relays this information to the operator via a monitor in the cab and, more often than not, also activates the machine's hydraulics to make the required adjustments automatically.
Two types of 3-D grade-control systems are present on today's jobsites. One relies on satellites, orbiting some 12,000 miles above the earth; the other relies on a precision surveying instrument, a robotic total station. The former is usually called a "GPS system" (for Global Positioning System), and the latter an "LPS system" (for local positioning system). GPS systems presently outsell LPS systems (depending on the manufacturer you ask) by a ratio that ranges from 9:1 to around 4:1.
Each 3-D system has its niche on the construction site, and each is extremely capable. As capable as these systems are, however, they're not the best choice for every grade-control application. A rotating laser, for example, may be the simplest way to control constant grades, and a sonic tracer may be the best way to match existing grades.
Some grade-control systems, of course, have what manufacturers call "sensor independence" or "plug-and-play" capability, allowing basic control components to accommodate signals from GPS, LPS, laser and sonic inputs. Systems with multi-sensor capability include Trimble's BladePro 3D, Topcon's System Five 3D, and Leica Geosystems' Gradestar.
So, with this perspective, it's still safe to say that the use of GPS and LPS grade-control systems is expanding. Although these systems are relatively expensive, a growing number of contractors are investing in the technology and, in some instances, seeing results beyond their expectations.
To find out how these systems are being employed day-to-day on construction sites, CE visited five 3-D users. Collectively, these firms are using 3-D grade control for eleven dozers, five motor graders, an excavator and a scraper. One firm also has used a 3-D system on a milling machine.
This small sample would indicate that dozers and motor graders are the primary beneficiaries of 3-D technology. And that's true. But systems for excavators are gaining ground and may begin to rival the population of 3-D dozers, says Tom Bucklar, marketing consultant for Caterpillar, which has a joint-venture company with Trimble (Caterpillar Trimble Control Technologies). Theoretically, 3-D technology could be applied to a wide range of machines, including trimmers, reclaimers, pavers and compactors.
On a blistering hot day in late July, we caught up with Don Ahmer Jr., vice president and general manager of Read Excavating, Gilberts, Ill., at work on a new subdivision in Aurora, Ill. Ahmer told us that for 15 years he's been running small dozers with laser-guided grading systems, interfaced with the machines' hydraulics for automatic blade control. Until a year ago, one such unit was always used to finish-grade building pads that had been roughed in by larger machines.
Then he began fitting larger dozers (six so far) with GPS grade-control systems. He found that these machines could grade accurately enough, without stakes, that the laser machine was no longer required. Eliminating it—and the stakes—he says, saved $40,000 on the first job.
Like Ahmer, contractor Jim Lineberger, president of Lineberger Grading, Lancaster, S.C., has nothing against saving money on job costs. But the primary reason he invested in an LPS system for a motor grader in 1998 was to compensate for the shortage of skilled operators in the area.
"The production and accuracy we get with 3-D systems helps address the operator issue," says Lineberger. "With 3-D, the operator can be productive every minute he's on the machine, because he has the game plan right in front of him. We've found that we can accomplish as much with three machines and three people today, as we once did with six pieces and 10 people, who were pulling string lines, checking grades and setting stakes."
Although Ahmer and Lineberger decided early on to embrace 3-D grade-control technology, Kipp Cheek, P.E., superintendent for Rea Construction, Charlotte, N.C., was a skeptic. Comfortable with grade stakes and string lines, he always had one basic question when called on by Joe McNamara, vice president of Spectra I.S., a Trimble dealer in Charlotte:
"What happens when we're working late on Saturday, the surveyors have all gone home, the 3-D system fails, and there's not a grade stake in sight?"
But, Rea, along with subcontractor partners F.T. Williams Co. and Propst Construction, really wanted to win North Carolina's first design/build project, an 8.1-mile reconstruction of Interstate 77 near Charlotte. Taking some courage from William's prior good experience with LPS systems, Cheek was willing to write the use of 3-D grade control (along with a 145-day, early-completion promise) into the lump-sum bid submitted to the state.
The partnership won the bid and, consequently, Propst equipped a motor grader with an LPS system for its soil-stabilization work, and Rea fitted a similar system on a milling machine. As of January 2003, a little more than a year into the project, the job was five months ahead of even the accelerated schedule. Cheek, a skeptic turned believer, credits 3-D technology for a big part of the project's success.
Lineberger, who subsequently equipped another grader, a dozer and a scraper with 3-D grade control, once routinely figured 5 percent aggregate waste on paving jobs. Since 3-D graders have taken over spreading stone, however, he makes no such allowances. And his GPS-equipped scraper, he says, works with the pack until within a foot of grade, then works alone to prevent undercutting or overfilling, and the subsequent rework that would entail.
You could, says Murray Lodge, national sales manager for Topcon, also install a GPS system on a compactor working with scrapers at a fill site. If the compactor knows the elevation, then it serves as the check on the scrapers' progress. Or, on large cuts, he says, some users employ a GPS-equipped dozer to set reference lines for the scrapers.
Using a 3-D LPS system on the milling machine, says Rea Construction's Lori Hodge, project engineer, meant that the machine left a flat surface (as dictated by its electronic control model), instead of simply following the contour of the existing pavement by removing a constant depth of material. The net result, she says, is that every lift in the new 17-inch pavement reflected this flat surface, and rideability was never an issue. As important, says Hodge, is that asphalt yield, the amount of asphalt installed, versus the amount theoretically needed, was excellent.
According to Mark Forrest, division vice president of construction for Trimble's Geomatics and Engineering Division, a tremendous benefit of 3-D grade-control systems is the potential savings of some 90 percent on staking costs, simply because machine operators no longer need stakes for guidance. And, since you're not waiting for a survey crew to set stakes, he says, you could be on site, working, much earlier.
Machines under automated control (with the electronics driving the hydraulics) also have the capability to precisely cut complex grades, like super-elevations (banked curves), which, says Forrest, are nightmares to stake and grade manually. And even on complex grades, he says, rework typically is minimal.
In GPS systems, an antenna/receiver combination on the machine (the "rover") receives signals from satellites in the U.S. Global Positioning System (and from the Russian GLONASS system if you're using Topcon's GPS+ system). The GPS receiver uses the satellite signals to determine the machine's position, essentially its latitude, longitude and height (its XYZ coordinates). This basic GPS information is then converted by the grade-control system into local jobsite coordinates, typically expressing them as northing and easting coordinates.
But, because satellite signals can be distorted on their way to earth, another GPS antenna/receiver, known as the "base station," is positioned at a known location on the jobsite. Since the base station is surveyed in and knows exactly where it's located, it can determine any timing errors in satellite signals and calculate corrections.
The base station then broadcasts these corrections via radio to the rover. (The range and interference-blocking technology of base-station radios vary with the GPS-system manufacturer.) The rover subsequently uses the corrections to more precisely determine its location, then consults its on-board model of the jobsite for grading information at that location.
On earthmoving jobs, this electronic 3-D model of the jobsite is a "digital terrain model" (DTM), which usually is created with specialized software from project plans. On road jobs, the jobsite representation usually is in the form of "alignments" and "templates." This electronic information typically is contained on small data cards, which are placed in the grade-control system's computer onboard the machine.
Says Jerry Bickner, a principal with Laser Source, a Topcon dealer in Rolling Meadows, Ill., creating these electronic representations of the jobsite is a critical aspect of 3-D grade control. The models normally are developed in a computer program for engineering design or estimating, he says, and manufacturers of 3-D systems have supplemental software to help users generate and validate this data, as do software specialists such as Agtek. Most of his customers do their own modeling, says Bickner, but these services can be purchased.
Trimble dealer McNamara, in fact, has recently established a subsidiary business that helps 3-D users with their modeling needs. Surveying companies could do themselves a favor, he says, by offering such services to compensate for setting fewer stakes.
As noted, a 3-D local positioning system is based on a robotic total station (RTS), a surveying instrument that has the intelligence to track a machine and to determine its XYZ coordinates. The RTS tracks the machine by following a prism or a light beam (called an "active target") on the machine. If the line of sight between the RTS and machine is interrupted (by a passing truck, for instance), the RTS typically has the capability to quickly search for its target and reestablish contact.
In Trimble and Leica Geosystems 3-D LPS systems, the electronic jobsite model is located in a computer onboard the machine, and the RTS sends positioning information to the machine via radio. In the Topcon 3-D LPS system, the model is integrated with the RTS, and grade-control information is sent to the machine via laser. Each manufacturer claims advantages for its particular system. This is a technical aspect to investigate if you're in the market for an LPS system.
Each type of 3-D grade-control system has its pros and cons. On the plus side for GPS systems is that any number of machines can work off one base station, versus having to use a separate robotic total station with each LPS machine. Also, with GPS, long-range control is feasible, perhaps up to 6 miles, depending on the configuration of the base station's radio, although accuracy may degrade with distance. (An LPS system typically is limited to a practical working radius of 1,000 to 1,500 feet.) And, GPS operation is not hampered by line-of-site disruptions.
Although some would say that vertical-accuracy estimates normally stated for GPS systems are too conservative, the range usually claimed is ± 3 centimeters (± 0.10 foot) or, says Trimble's Forrest, about the height of a golf ball. For most rough-grading projects, this level of accuracy is acceptable.
On the other hand, among the advantages of LPS systems is greater vertical accuracy, on the order of ± 5 millimeters (± 0.02 foot), or a bit less than ¼ inch. Also, the LPS system can track the machine inside buildings and in areas where overhead obstructions (tunnels, overpasses, tall buildings and trees, for instance) may block satellite signals. Plus, the robotic total station is a valuable surveying instrument that can be used elsewhere on the site.
Nearly all 3-D grade-control systems for dozers and motor graders are sold with an interface that allows the electronics to automatically drive the hydraulics. Such machines as trimmers and milling machines also would use this interface, but scrapers might or might not. (If not, the system is called an "indicate-only" system.) Excavators, by virtue of the work they do, don't use the interface.
If a dozer or a motor grader with 3-D control is hogging dirt and not close to finish grade, the operator switches off the interface and uses the system in its indicate-only mode. If, however, the operator changes the "offset" by temporarily giving the system a new target elevation close to the machine's present elevation, then the interface can be switched on and material automatically peeled off thin layers. An operator might do this, say, to clean up the site.
Even when using a 3-D grade-control system, you still need the surveyor to locate jobsite control points and to stake points that may have liability ramifications. But for the most part, since a machine under 3-D control always knows where it is and what it's supposed to do, a lot fewer stakes are required.
"On a 100-lot subdivision, we might have in excess of 2,000 stakes and pay $120,000 to $130,000 in total surveying costs," says Tom DiGiampaolo, operations manager for Southgate Constructors, Charlottle, N.C. "But restaking is the real killer, especially if you have to pay travel time for the survey crew."
DiGiampaolo estimates that the operator on the company's Cat D6M was grading two building pads per day following stakes. After installing an automated GPS system on the machine, he says, the operator began cutting six pads per day, minimum, with no rework.
Working on a site with virtually no stakes can be disconcerting at first, says Brad Heavner, vice president and operations manager for Scurry Construction, Cornelius, N.C. But, says owner Bill Scurry, the company has gained confidence in the grade-control ability of its GPS-equipped Cat D6M dozer and Deere 450 excavator. A recent 100-acre, site-development job entailed moving one million cubic yards of dirt, he says, and essentially no stakes were used in the cut and fill areas.
Based on averaged dealer prices, expect to pay from $85,000 to $110,000 for a 3-D control system, complete with hydraulic interface, depending on options. This includes the base station in a GPS system, and the robotic total station in an LPS system. Each LPS machine, of course, will need its own RTS, but subsequent GPS machines on the same site won't need the $22,000 to $25,000 base station.
Finally, here are a few basic questions to consider if you're in the market for 3-D grade control. Are you comfortable with the dealer's technical knowledge and support capability? Do you have a source for developing good digital terrain models? Are you limited on the size of machine that can be equipped with the hydraulic interface? And, what features are built into a GPS system to minimize work disruption if satellite signals are degraded by tall trees or by poor satellite alignment?