Grading and Excavation Contractor | Conquering the Complexity of Excavator Bucket Controls

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Remember Mike Mulligan and his shiny red steam shovel, Mary Anne?

By George Leposky

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Adventures in the Field

Generations have grown up reading Mike Mulligan and His Steam Shovel, a children’s book by Virginia Lee Burton published in 1939. It tells how, in a single day, Mike and Mary Anne dug the basement for the new Popperville town hall with “four corners … neat and square [and] four walls … straight down.”

Central to Mike and Mary Anne’s accuracy was the intimate touch-and-feel relationship that a skilled heavy-equipment operator develops with his digging machine.

Today, increasing numbers of excavator operators no longer rely solely on that relationship, augmenting it with electronic systems that consist of sensors linked to computers, lasers, and global positioning systems. These technologies tell an operator where and how deep to dig. In a few cases, they even automate the digging.

In this electronics revolution, bulldozers, graders, scrapers, milling machines, and pavers are further along in using technology to locate a specific point in space. Excavators have lagged behind due to their flexibility and broader range of movement, explains Pat Smith, president of Mikrofyn Positioning Products Inc. Smith’s firm, based in Jacksonville, AR, is the North American headquarters for Mikrofyn A/S products made in Odense, Denmark.

“The excavator is the most geometrically complicated machine for machine control applications,” Smith says. “An excavator moves dirt with the teeth on the edge of its bucket. The excavator’s bucket connects to the stick, which connects to the boom, which is attached to the tractor. There’s a great deal of articulation in those joints. Then the tractor pivots 360 degrees on its undercarriage, and the undercarriage also moves across the ground like a bulldozer.”

An electronic bucket control system for an excavator is an aftermarket product, installed by dealers who specialize in this technology. So far, no excavator manufacturer provides electronic bucket controls installed as original equipment at the factory, though Caterpillar Inc. plans to begin doing so soon (see sidebar.)

A Hierarchy of Control
Excavator bucket control systems range from simple and inexpensive to complex and costly. The most basic systems involve use of a site reference. The operator touches the bucket to a fixed reference point (an existing curb, for example, or a stake of known height driven into the ground), enters the grade he wants, and digs down until a detector signals him that the bucket’s cutting edge has reached the desired depth below the reference point.

The next level of complexity relies on laser grade control systems that employ a transmitter mounted on a tripod on the job site and a sensor mounted on the machine. The transmitter generates a straight beam of light in a pentaprism that rotates at 600 to 1,000 rpm, creating a plane of light over the job site. The machine-mounted sensor detects the laser signal to determine its reference point and whether the bucket’s position with respect to the reference point is above, on, or below the desired grade. Lasers used for machine control and grade checking operate with laser light in the invisible portion of the spectrum. Pipe lasers and lasers used for alignment tasks in building interiors produce a visible light beam.

Construction lasers are diode lasers operating at a wavelength of 635 to 680 nanometers and a maximum power of 5 milliwatts. They are powered by alkaline batteries, rechargeable battery packs, or connection to an external power source such as a truck battery. “The rechargeables are about 35% less efficient than the alkalines,” says Murray Lodge, national sales manager for Topcon Positioning Systems Inc. of Livermore, CA. “Alkaline batteries have an operating life of 40 to 120 hours, while rechargeables are limited to 25 to 80 hours.”

The most primitive application of laser-based guidance technology employs a single photocell sensor, which can be handheld or mounted on the machine or on a mast attached to the machine’s stick. As the operator passes the sensor through the laser beam, the photocell captures the laser beam’s plane of light and finds its center, which becomes the reference point.
The detector for such a system costs less than $2,000, Smith says, but “it’s a crude and awkward way to get a measurement for an excavator out of a laser because the bucket pivots on the bottom of the stick. Thus, the operator doesn’t get good information on the bucket’s orientation or the actual elevation of the teeth.”

A more sophisticated type of system consists of sensors that measure angle changes on the bucket, stick, and boom, and incorporate a laser detector in the boom sensor so a laser beam can be used as a reference anywhere on the job site. A laser receiver provides a constant vertical reference relative to the desired grade, eliminating the need to touch the fixed reference point each time the excavator moves.

The operator sets the desired excavation depth on the control box—which really is a small computer—in the cab. When he approaches the finished elevation, a light comes on to tell him that he’s close. When he reaches grade, a different light tells him to stop. If he digs too deep, a warning light comes on. Such models typically have a numerical display as well (though many operators prefer to watch the lights rather than read the numbers), and a series of horns for operators who like an audible guidance system.

Contractors using a quick-disconnect attachment can mount the bucket sensor there instead of on the bucket, allowing the operator to change buckets without having to move the bucket sensor to the new bucket. The new bucket’s measurements will be pre-stored in the control box so the operator can maintain the correct grade and depth.

Dual-Slope Sensing
Still another refinement is slope sensors to correct for the orientation of the machine’s position. “Stick your hands out and lean to the right. That’s single. Now lean forward. That’s dual,” Lodge explains. “If you’re excavating for sewer utilities, there will be a single constant slope along the trench bottom. You would dial in the percentage of slope in a single axis.”

Mikrofyn and Topcon offer a choice of single-plane or dual-plane slope lasers. Mikrofyn combines pitch (forward and back), roll (left-right), and heading sensors into a single package. The pitch and roll sensors compensate for the machine’s undercarriage not sitting on level ground and enable accurate grade control 360 degrees around the machine. The heading sensor is used to establish the alignment of one axis when digging a compound grade. This is the same principle as using a dual-slope rotating laser.

Trimble Navigation Ltd. of Sunnyvale, CA, uses a patented slope sensor in its Spectra Precision Laser machine control products, says Arthur J. Taylor, Trimble’s segment manager for three-dimensional machine control products. Taylor says Trimble refers to its excavator bucket control products as “indicate systems” because they tell the machine operator where to adjust the bucket.

These sophisticated multi-sensor laser systems cost $12,000 to $20,000, depending on accessories.

Three-Dimensional Guidance
Even the best laser-based systems are two-dimensional. To achieve three-dimensional bucket guidance in the real world (referred to as real-time kinematic or RTK), the makers of bucket control systems have turned to the satellite-based global positioning system (GPS) as a source of locational information.

GPS is a generic term that may refer to the NAVSTAR system owned by the US Department of Defense and/or the GLONASS system owned by the Russian Federation. Each has at least 24 operational satellites in orbit. At any time, from any point on the globe, at least five satellites per system are within a direct line of sight.

Each of these satellites emits a radio signal that transmits its location in orbit. Users with special GPS receivers can pick up these signals and, through triangulation, find the receiver’s location on Earth. The more satellites a receiver can detect at once, the more precise the locational determination. Sensors on an excavator then establish the bucket’s precise location in relation to the receiver.

Leica Geosystems, which has a US headquarters in Norcross, GA, and world headquarters in Heerbrugg, Switzerland, is preparing to release a GPS-based bucket control product this year as an add-on to its MC200 Digger two-dimensional system.

“We put two GPS antennas on the back of an excavator, one on each corner, to receive signals from the satellites,” says Reynolds Boyd, machine automation product marketing manager in the survey and engineering division of Leica Geosystems. “The position of those two antennas takes the pitch, roll, and yaw [sideways slippage] of the machine into account in conjunction with the MC200 data, which provides information on the position of the bucket teeth.”

These additional parameters allow the operator to work on a digital terrain model (DTM) loaded into the onboard computer. “The DTM works like a wire mesh frame overlaid onto the desired contours,” Boyd says. “The system compares the GPS positioning to what the operator has in the DTM, and gives him the cut or fill level.”

A GPS-based control system enables “stakeless excavation,” says Smith. “In a traditional type of operation, an engineer would stake the job, and those stakes would be the reference points the contractor would use to find his position on the face of the earth to put in pipe, a drainage ditch, or whatever. With GPS, you take the DTM out of the engineer’s computer and load it into the machine control computer. The DTM tells you the desired elevations and positions, where this pipeline is going in, and at what depth. The GPS plus the sensor system on the machine can guide the machine to the cutting spot and then to the appropriate depth.”

GPS control systems can cost $18,000 to $24,000 for a base station that sends signals to the machines, and then $55,000 to $60,000 to equip a machine.

Excavator Automation
Beyond guidance, automated systems for excavators integrate the control system into the machine’s hydraulics. The computer displays the bucket’s position relative to the planned grade, and also signals the hydraulics to move up or down to achieve that grade. Topcon sells an automated excavator control system that costs more than $30,000.

Topcon’s TS-5 Touch Series allows the operator to run the excavator manually until it gets close to grade. Then the operator can put the machine in automatic mode to cut accurately to the design grade.

“With an automated excavator,” Lodge says, “multiple hydraulic components have to be synchronized. Even though considerable expertise is required for setup, it is still popular with many contractors. Topcon is the only manufacturer to offer an automated excavator system. The biggest market for this system has been in Europe, where excavators are used for almost every construction task. In the US, where a wider range of heavy equipment is used for site work, there are fewer than 50 such systems.”

Other manufacturers, however, are more reserved on their vision of excavator automation. At Trimble, Taylor says, “We assist the operator; we do not take control of the machine.
“Looking at full automation, a factory is more likely to be automated as it makes a large number of the same item. Automation reduces human error and increases throughput/productivity. The same does not apply to a construction project. Each project is a one-off, a custom. As a result automation is a significantly harder task and there is less incentive.”

Taylor says full automation for excavators is “not likely to happen in the short to medium term,” although he notes that Trimble already provides automatic control of elevation on other machine types—dozers and graders, for example—and is likely to offer this for the excavator at some point in the future.”
“To get precision automated control on an excavator with all its moving parts will be very tough,” says Smith at Mikrofyn. “Doing it is possible, but there doesn’t seem to be much of a demand for it, and it will be a really expensive system.”

“From the contractor’s perspective, the cost to automate an excavator is pretty high,” says Boyd at Leica Geosystems. “I’m not sure what you get back in return. It may save money and time, but the market is not for mass excavation; it’s for small-detail jobs that don’t bring in enough money to automate. If the market starts demanding it, I’m sure we would develop such a system.”

“I’m not going to rule out automatics eventually, but not now,” says Thomas E. Bucklar, North American regional manager for machine control and guidance products. “Excavators have a complex control system, with wider range-of-motion and safety issues.

“Machine control and guidance technologies will be integrated into excavators in the near future to raise the bar on supportability while driving the productivity gains,” he says. “The initial products will focus on guidance technologies as opposed to full automatic control as you see on dozers and motor graders.”

George Leposky is a science and technology writer based in Miami, FL.

GEC - July/August 2006

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