Excavator-Specific Control

Sept. 1, 2008

As the use of Global Navigation Satellite Systems (GNSS) to survey and prepare work sites as well as precisely guide earthmoving operations becomes more commonplace, the technology is predictably increasing in sophistication to keep up with contractors’ challenges. One area that leading construction GNSS providers are focusing on is the development of systems specifically for excavators, arguably machines with the greatest total range of motion on a work site.

The excavator’s bucket can operate toward and away from the operator cab as well as up and down. And even though a backhoe is equipped with buckets on the front and back, the excavator can swing in a 360-degree arc. This total range of motion creates quite a challenge for GNSS providers in delivering precise location data at any given time with so many moving parts working in conjunction. Configuring an excavator for GNSS control is a much more challenging endeavor than on a grader or dozer.

A dozer has a single-slope (tilt) sensor on the blade to measure the cross-slope of the cutting edge. Like a dozer, a grader has a slope sensor for blade tilt, in addition to a rotation sensor at the rotation swivel. A third sensor, known as a mainfall sensor, is mounted on the mainframe of a grader and provides slope measurement in the direction of machine travel for adjustment of the blade up or down. These sensors work together to maintain cross-slope while allowing for the rotation of the grader blade.

In a class by itself, an excavator has a swinging body and a two-piece articulating element between the machine body and the earthmoving attachment, giving it a wider range of motion and requiring more sensors to account for swing, cross-slope, and long slope as well as to pinpoint the bucket location. A typical excavator control system must make four measurements: cab to boom, boom to stick, stick to bucket hinge, and bucket hinge to bucket teeth. The control box accounts for boom-and-stick articulation (and thus the distance between the bucket and cab at any given moment), bucket tilt, and machine body levelness.

Tilt sensors on the stick, boom, and bucket continuously calculate the distance from the cab to the bucket teeth. The excavator’s 2D system comprises these non-GNSS sensors. The rover station serves as the machine’s 3D GNSS sensor and pinpoints the location of the bucket teeth relative to the slope. The rover does this while the machine body rotates to a different digging position by also taking into account the machine body’s position relative to the longitudinal slope.

Several contractors recently shared the significant productivity and accuracy benefits they have been receiving from their excavator-compatible GNSS. Their systems have handled widely varying and challenging projects, from preparing a site for 2010 Olympic hockey rinks to the foundation and utility work for an industrial processing facility.

Managing Difficult Bedrock, Foundation Work
Preparing the site for two full-size Olympic ice hockey rinks in Vancouver, BC, for the February 2010 XXI Olympic Winter Games involves digging in hard glacial till, notes Corey Reutlinger, president of Norex Civil Contractors Inc. of Abbotsford, BC. Glacial till consists of the finely compressed accumulations of unsorted, unstratified mixtures of clay, silt, sand, gravel, and boulders that were formed when the glaciers moved over southwestern British Columbia. As if these conditions weren’t difficult enough, Vancouver is a part of the Pacific coastal earthquake fault line. “To meet rigorous building codes, the seismic building footings need to be deeper, broader and much larger,” says Reutlinger. “Plus, they must be tied together strategically for vertical and lateral stresses.” To handle such precise work, Norex was the first contractor in British Columbia to use GNSS machine control on excavators.

In 2006, Norex purchased two Trimble GCS900 Grade Control Systems for its Caterpillar 330C and 345C hydraulic excavator. The Trimble GCS900 Grade Control System-which also can be used on dozers, scrapers, graders, and compactors-uses the manufacturer’s dual antenna configuration with solid-state angle sensors to measure the exact 3D position and orientation of the bucket. An onboard CB430 computer determines the position of each tip of the bucket and compares these positions to a design elevation. Inside the cab, the operator sees what bucket movement is required to achieve grade by viewing mounted light bars.

“We’re able to make straight, vertical cuts and achieve accurate alignment where the footings end both horizontally and vertically, because we have a technological advantage,” Reutlinger says. He reports that in 2007, within the first few months of work on the campus of the University of British Columbia, Norex was about a month ahead of schedule because there was no need for staking or surveying the site. “We’ve been able to complete the project in less than half the time than if we had to stake everything the conventional way,” he adds. “This efficiency is helping us meet the tough deadlines on our part of the Olympic Games preparations.”

The rink project is a sign of things to come for Norex. “Because we can complete large projects more quickly, the machine-control systems have helped us bid on and complete more projects,” says Reutlinger. “We wouldn’t be able to bid that volume of projects without it.”

Another recent challenging project that utilized the Trimble GCS900 Grade Control System was a 609-home, 235-acre development on and adjacent to Scott Air Force Base in Belleville, Ill., located about 20 miles east of St. Louis. In three phases, the homes for US Air Force personnel and their families are being constructed by private contractors. The two-family and single-family dwellings will not be the drab, functional units normally associated with military housing.

Randle Construction Inc. of Freeburg, IL, is responsible for completing all of the concrete footings and foundations. “After we won the bid, we started to look closely at cost-saving ways and how we could do things more productively,” says Ron Randle, owner of Randle Construction. “We had a 20-month schedule [2006-2007] given to us, so we needed to figure out how to get it all done in that time frame.”

Home sizes vary according to the resident’s rank. All the buildings are being built of a quality expected to last at least 50 years. For its part, Randle Construction was dealing with 36 different foundation plans. There are 72 different layouts, in fact, because each plan is also flipped to provide a different living configuration.

Randle was looking for timesaving advantages for both the foundation layouts on the building pads and for digging the foundation configurations. “This would have been a tremendous amount of staking, manpower, and time had we not acquired technology to help,” Randle says. To handle this daunting challenge, Randle had the Trimble GCS900 Grade Control System installed on a Caterpillar 308C mini-hydraulic excavator-a solution that is unique to the local market.

With the savings in labor alone, Randle expects to pay for the system well before the 20-month project is completed. “Foundations that now take four hours to dig with the [GNSS-based] grade control would easily have taken a full day using traditional methods,” says Randle. “In this market, if you’re digging with a backhoe you need a laborer in the hole with a story board to check the grades. With this system on the mini-excavator, I don’t need that extra laborer monitoring the grade-besides, the operator doesn’t need anyone babysitting her work.”

Prior to digging, workers use a Trimble SPS880 Site Positioning System Smart GPS Antenna and a handheld Trimble TSC2 Controller to virtually stake out sites. The SPS880 integrates the receiver, antenna, radio, radio antenna and battery into one unit that interfaces with the TSC2 Controller. The combination provides Randle with an all-on-the-pole solution for conducting stakeout and grade checking. “We’re completing stakeout on a building pad in about 30 to 35 minutes, which used to take us three to three and a half hours to complete,” Randle reports.

“With the rover, a base station and [GNSS] grade control, we’ll be able to bid extremely competitively on new projects-and different types of projects we wouldn’t ordinarily consider. I know that once we’re finished with this project, we plan to use the excavator system to perform small utility jobs and other tasks that we’ve never before been competitive on.

“I’m sold on the technology; it’s already saved us a lot of money and it will become more and more significant for future work,” Randle continues. “For me, there are three big benefits: accuracy of placement, the faster speed in which work can be completed and the significant labor savings. We’re learning a lot on this Scott Air Force Base housing project that will help us be even more productive in the future.”

Underwater Grade-Checking Challenges
Grading and excavation projects in and around bodies of water would be extremely difficult in many cases without the use of GNSS. One such case was a recent dredging project at Smith Mountain Lake, located in Smith Mountain Lake State Park, Huddleston, VA. The lake’s currents had washed away much of the sand lining the bottom of the lake near the only beach on the lake. The water near the beach had a greater depth than what was preferable, so officials with the Virginia Department of Conservation and Recreation wanted to dredge undesirable silt from below the waterline, replacing it with sand from a local quarry and rebuilding the underwater elevation. A turbidity curtain would also be constructed offshore in order to prevent a recurrence of the erosion.

Mark Womble, project manager for contractor Virginia Carolina Paving of South Boston, VA, indicated that checking the grade on the project without having GNSS mounted on an excavator would have been extremely difficult.

“It wasn’t just a matter of having someone go out and physically check the grade,” says Womble. “You had to have a boat and it would have been a real problem without the GNSS.” Indeed, a surveyor that actually did the as-built survey drove stakes into the lakebed and mounted the rover onto the stakes to read the grade.

But for grade checking and machine guidance during the project, Virginia Carolina Paving used a Caterpillar trackhoe outfitted with a new Topcon X-63 GNSS that is specially designed for excavators. An advantage to the contractor was the fact that a recent drought had lowered the lake level below normal so that the trackhoe could be driven into the water. The machine’s roughly 50-foot-long boom handled the placement of the sand. “The sensor was at the end of the boom, so any time the operator placed the bucket out there, wherever it was, the screen in the cab told him whether or not he had to cut or place fill.”

This was the first project in which Virginia Carolina Paving Equipment used the X-63 indicate 3D GPS+ system. The system is compatible with both GPS and GLONASS satellites. The operator has access to plan, profile or section views, which display real-time movement of the bucket, stick, boom and machine. A color cut-fill indicator provides instant grade verification and an elevation reference tool allows the operator to control grade on the left, right or middle of the bucket. New CAN-based tilt sensors are said to mount in any location on the machine. The system is compatible with GR-3, legacy, HiPer+ and HiPer Lite+ receivers.

Virginia Carolina Paving purchased the X-63 system in late 2007, and it began paying dividends immediately. “We actually had looked somewhat into a [GNSS] system for an excavator and a dozer before we had this project,” says Womble. “Specifically because this project was under water, it compounded our need and we actively pursued the X-63 then. You just wouldn’t be able to use a laser system under water.”

In December 2007, Virginia Carolina Paving began work on the beach. The trackhoe was outfitted with spill kits in case the machine leaked any oil. Then the trackhoe was driven out into the water to excavate the silt. The trackhoe turned around and loaded an articulated dump truck that had backed down to the shoreline. “From the waterline to the farthest point to where the trackhoe had to reach was about 70 feet,” says Womble. “When the operator took the excavator out into the water, he would stop when the water got up over the tracks but before it got over the cab-the controls had to stay dry.”

Over the next two weeks, Womble had sand hauled in and dumped it on the beach and had it pushed down to the waterline with a dozer, which pushed the sand row by row along the 100-yard-plus beach shoreline. Using the X-6 location data as a guide, the trackhoe picked up sand from the beach and placed it in spots where large volumes of material were needed as fill. After all of the sand had been placed, the excavator operator dragged the bucket over the newly placed material to level it out, keeping it within the specified grade and elevation tolerances. “If there were any areas that were low or high, they’d fill it or cut it as needed,” says Womble.

This process enabled the contractor to finish the project in about 60 days, compared with a schedule of 90 days.

Womble says that the X-63 system worked so well that the owner and engineer were satisfied with the as-built survey model that he built with Topcon software. “On most projects, we’ll have a progress meeting with the owner and engineer a couple of times a month,” Womble notes. “Part of the system we purchased has a handheld field monitor, and when we met them for the meetings on this project, I just grabbed the field monitor and they’d walk around the site with me and I’d take spot checks. We submitted a couple of draft grade elevations to see if they liked the format, and they did.”

Complex Underground Utility Excavations
BWC Excavating, Solon, IA, efficiently handled recent site work for an Archer Daniels Midland (ADM) natural biodegradable plastics plant in Clinton, IA, with the help of a Caterpillar AccuGrade GNSS-enabled machine-control system mounted on a Caterpillar 345 excavator. The excavation project consisted of takeoffs that had to be made down to bedrock for multiple building foundations because the buildings will be equipped with heavy processing equipment.

Metabolix, a company based in Cambridge, MA, is partnering with ADM to form a joint venture called Telles to produce “bioplastics” for the trademarked Mirel plastics line. The bioplastics processing facility will take sugars fermented from corn to produce 110 million pounds of bioplastic resin per year. The facility was scheduled for startup in late 2008.

The practice of using GNSS for site preparation and machine control is fairly new to BWC Excavating, which outfitted two Caterpillar D6 dozers with the AccuGrade system in April 2007. Soon afterward, the company was approached by its Caterpillar dealer about the benefits it would receive from a GNSS-capable excavator and purchased the 345 machine in July 2007.

The system, which is used exclusively on Caterpillar equipment, has two GNSS receivers mounted on the rear of the machine, as well as one pitch sensor and three angle sensors mounted on the boom, stick, and bucket for pinpoint bucket positioning. The MS990C receivers can receive signals from both US GPS satellites and Russian GLONASS satellites. The communications radio is mounted on the cab and receives real-time compact measurement record (CMR) data from the GPS base station radio.

Inside the cab, light bars visually guide the operator by indicating adjustments to bucket tip elevation and left-right bucket tip positioning. The top vertical light bar provides cut-fill guidance based on elevation of the bucket tip. The horizontal light bar indicates the left or right alignment of the bucket tip relative to the design plan. The bottom vertical light bar shows the operator what forward-backward bucket movement is necessary to achieve grade.

The precision that the system provides also boosted the productivity of BWC Excavating during the excavation phase of the project, says Bart Seger, general manager for the contractor. “We did a lot of underground piping-lots of process piping,” he says. “We laid storm-sewer pipe and dug all of the foundation footings on four buildings. A lot of times, we had to go down to the rock base due to the size of the buildings” (many of which are several stories tall and required concrete pads for large motors as well).

“There was a certain amount of rock we’d have to pull up to get to a firm base-usually anywhere from 2 to 3 feet,” he continues. “The concrete contractor would form the footings on top of the rock, and then we would backfill. Because there was so much area where there were motor mounts and tank foundations, it was easier to do it that way.”

The way ADM’s building engineers oriented the buildings on northerly and easterly lines, combined with BWC Excavating’s use of GNSS machine control, made the underground utility work extremely productive, Seger points out. “For the storm sewer, [the GNSS] cuts down a lot on laying down lines because the system keeps you on your northerly and easterly lines,” he says. “Also, we don’t have to be down there with a stake and laser all the time. When you come to a manhole or something, all that is laid out with the machine while you’re in the machine digging; you don’t have to worry about stopping and making sure you’re not running over your marks or digging up your marks. There are no offset lines to worry about.”

On this project, the owner did not make a CAD survey file available for transfer to the AccuGrade system via a memory flash card. However, like other GNSS surveying/machine control systems on the market, AccuGrade allows the contractor to gather its own survey data and load the data into the system for machine control. The latter scenario was the case for BWC Excavating at the ADM project. Seger says that for much of the trench excavating for underground pipe, someone would enter the northerly and easterly starting points, the slope, the elevation, and the northerly and easterly endpoints directly into the excavator’s GNSS control monitor.

“You can drive to your starting point, set your bucket down and put starting point here, drive to your finish point, put the bucket down and take the elevation readings,” Seger says, “and then you can just offset the elevation you want to dig to or put a slope in or do whatever you want to do on the monitor of the 345 itself. Or, I could stake out points on the handheld and then export the file to the 345.”

Besides the accuracy within one-tenth of a foot as well as the efficiency that the GNSS provided BWC Excavating, the site preparation work was necessary only once, Seger points out. “Another thing nice is that, on a project like this, there are several different contractors working at one time-ironworkers and carpenters and electricians. It’s very nice not have to lay out offset lines, because they don’t last. When we’d be on a break, somebody would run over them.”

New System Unveiled at ConExpo
In March 2008, Leica Geosystems unveiled its new Leica PowerDigger 3D GNSS excavator guidance system, which includes a choice of single or dual GNSS (i.e., GPS and GLONASS satellite compatibility) at ConExpo-Con/Agg. The company has designed the system to use its PowerBox and PowerAntenna sensors, which are each capable of precisely locating the exact 3D position of the excavator bucket relative to the design surface. The dual GNSS configurations are provided for maximum speed and flexibility for projects with frequent moves, while single GNSS configurations are included to provide a cost-effective entry to three-dimensional excavating.

Rich Calvird, product-marketing manager for Leica Geosystems, points out that the PowerAntenna sensor is designed to be mounted on either the excavator or on a range pole. In the single configuration in which the PowerAntenna sensor is mounted on the range pole, the sensor interfaces with main system using Bluetooth technology. This configuration allows the contractor to be doubly productive, using the system for both machine control and as-built surveying, for example.

The PowerDigger 3D system also maintains the X-Functionality interface capability of the system’s DigSmart predecessor. This feature allows continuous updating of an as-built survey, meaning that the actual data can be recorded while the machine is working, and the contractor can see the progress of a project more continuously, says Calvird.