In 1957, Gordon Gould patented the laser—an acronym for light amplification by stimulated emission of radiation. A laser is a crystal rod with a full mirror on one end and a half-silvered mirror on the other. High energy is added to the rod by shining a light into the side so as to cause some electrons in the crystal to rise to a higher energy state. When they drop back to the lower state, they emit a photon of light. Photons that travel the length of the rod are reflected from end to end by the mirrors and stimulate other electrons to emit photons of the exact same wave length. When this beam of light finally escapes through the un-silvered part of the mirror on one end, it is a highly focused, intense beam of light that will travel long distances without degrading.
Today, nearly everyone in the United States has seen a laser in action. In the construction industry, innovators have come up with hundreds of applications and as the price of lasers decreases, the applications continue to multiply. Today, lasers lend themselves to greater efficiency in nearly any sort of work involving measurement, level, and elevation. Laser guidance or machine control for grading and excavation equipment, once cost-effective only for large excavation contractors, is now priced as low as $700 for a simple magnetic receiver that can be used with nearly any kind of grading or excavating equipment. Getting into automation, where the laser is actually hooked into the hydraulics of the machine and controls its operation, is much more expensive.
Much of the laser technology comes from companies that started out as manufacturers of optical surveying equipment. Lasers have dramatically changed that business, and today GPS is taking it to a new level. GPS, the Global Positioning System, was established by the U.S. military during the 1980s. The first real test of the system was during the Gulf War in 1990 where it was extremely successful at tracking movement of equipment
and personnel. Today there are more than 40 satellites orbiting the earth at 12,600 miles, each emitting a continuous radio signal. Each “constellation” of satellites is positioned so that at least four are always in view from every spot on earth. Today there are U.S., European, and Russian satellites—receivers are rapidly being updated to be able to receive signals from any of these. Basically, what a GPS receiver does is measure how long it takes the radio signal to travel from each satellite and uses these measurements to determine its location on the surface of the earth.
For construction, the development of GPS means that benchmarks can be established anywhere, with accuracy in the x-y direction within millimeters of actual. For many applications, such as grading and even paving operations, GPS can be used with no other guidance. The elevation reading (z-direction) from a GPS signal is typically, however, accurate to about 0.05 inch. “On finished stone or concrete pavement,” says Matt Scothorn of Leica, “you might want to get the accuracy of a laser, using what we call TPS—a total robotic station.” The applications of GPS in construction are exploding with everything from tracking equipment location to recovering stolen equipment (see box).
Controlling excavators
“When you use a laser with an excavator,” says Murray Lodge of Topcon, “all you are really trying to do is keep the teeth of the bucket a certain distance below the laser beam.” But while that concept is simple enough, there are many ways it is accomplished.
The basic, low-end laser receiver mounts magnetically to the dipper arm of the excavator (or to any digging or grading machine). It picks up the plane established by a rotating laser that is set up on a tripod somewhere nearby on the site. Bright LEDs on the receiver indicate with red arrows if the excavator's cutting edge is too high or too low and with green arrows when at finished grade. Receivers come in different lengths, from about 6 inches to about 16 inches. The longer ones allow the operator to have a better feel for where he is in relation to the desired grade and help to avoid over-excavation. Some receivers automatically move up and down a rod on the dipper stick of the excavator to keep the laser in range. For the lower end receivers, the dipper arm must be close to perpendicular to the ground when it is reading to grade; higher end receivers automatically compensate for dipper angles up to 30 degrees. A remote display can be mounted inside the cab to make it easier to see in bright light or unusual situations, and an audible tone can be used to tell when the operator is at or near grade.
To establish grade for a simple excavator receiver, the laser is first set up at some height above the benchmark. “First you cut a portion of the footing to the desired grade,” says Lodge. “You set the bucket flat in the bottom of the excavation, set your boom perpendicular, and then move the receiver up and down until you get a solid on-grade light. So at that point you know where the finished grade for the bottom of that footing is, and you can dig the rest of the footing to that grade. It's a simple indicate type of system where the operator watches the light to maintain the elevation in the bottom of the trench or footing.”
For those who want to move up a few levels, though, there are much more advanced systems. With these high-end systems, angle sensors or tilt sensors are mounted on each part of the excavator, including roll and pitch sensors on the cab. Knowing the pin-to-pin length of the boom and dipper stick and the dimensions of the cab and the bucket, these systems use a bit of geometry to provide the operator with a graphical display of the position and angle of the bucket in real time. When the profile of the excavation has been entered into the system in advance and the system has been calibrated to grade with a rotating laser, the operator can clearly see what has been dug and what remains. Nearby obstructions, overhead wires, or pipes below the excavation can also be entered into the system to alert the operator if he gets too close to something. Some systems include a weight sensor that determines the total weight of material that has been cut. The cost of this type of system starts at less than $11,000.
“This equipment empowers operators, because they are no longer dependent on a grade checker to work with them,” says Scothorn. “It allows them to speed up the operation because they don't have to wait for someone to come and check the grade. They can do it themselves and keep an expensive piece of equipment moving. It also really shines in blind cuts, like underwater or in really deep excavations where the operator can't see the bottom. I always say that anyone who has to dig can use a system like this.”
Some manufacturers have taken this one step even further and tied the control system into the excavator's hydraulics for automatic control. Once the excavation gets to within a certain distance of finished grade, about 6 inches or so, the automatic controls kick in and finish the cut. “The operator controls the stick, and the automatic controls the boom and the bucket,” says Lodge. “He determines how fast he wants to pull back, but it's all tied into the hydraulics so it will maintain the design grade whether it's flat or sloping. The display shows the operator the entire profile view.”
