A laser, which is basically an amplified beam of light, can only work in a plane. A beam of light can not be curved, and very few roads run in a straight line for any great distance. Roads come with banks, turns and curves. Lasers can not handle banks, turns and curves.
Joe Cohoon, marketing manager, Spectra Precision, explains, "Lasers work on a plane of light that is flat, or has a single dual slope, but roads follow the terrain, and have vertical curves."
Dennis Louton, machine control development manager, AGL, Jacksonville, Ark., adds "Lasers create a flat plane as a reference. You can slope that plane, for instance we build a laser that can do a two-to-one slope, but the problem is that roads have a lot of vertical and horizontal curves. When you induce those two factors into it the laser is not a real useful instrument in building a road.
"There are some uses, such as setting up grade hubs, or in the control of some of the equipment. You can use them when you have a long straight run, but usually the contractor does not want to slow down long enough to utilize them, this will interrupt his flow of work. It is kind of tough to make a laser work on a road." Would a road contractor want to invest in the technology? "They probably own a laser because of the other jobs they have been on. Airport jobs for instance. Resurfacing an airport is a real good application. Some larger parking lots also are good applications, " answers Louton.
There is technology available today that is more useful for a road builder. "A more useful technology, that is currently available, is ultra sonic," states Louton.
A sonic sensor device can be mounted to pavers or graders to improve their performance. These devices bounce sound waves off of objects, such as a stringline, or curb. When the sound pulse returns to the sensor the distance to the reference point on the stringline is calculated using the speed of the sound. In addition to the sonic devices, a new laser tracer developed by Spectra Precision can be modified for use for machine control. Cohoon explains, "A roadbuilder can use either a sonic tracer or laser tracers. A laser tracer is placed on the grader to control the elevation of the blade. It shoots a thin line of laser light down to the string line, curb lip or a previous pass.
"For example, when the light is going down to the string line there is a camera in the laser tracer that looks down and sees where the beam has been broken by the string. This controls the elevation of the blade. The cross slope is determine by a sensor on the blade. There also is a sensor that determines whether the machine is going up or down a hill and a sensor that determines if the machine is leaning from one side to the other side. All these affect the cross slope of the blade."
Cohoon continues, "This is a little different from what most people think of when they think of lasers. Usually they think of a transmitter on a tripod that shoots out a beam of light. We took the beam of light and turned it into a device that's used to control motor graders."
Lasers also have limited applications in the construction of bridges, because almost all bridges have a curvature to them. "You are dealing with a non-linear function so it is hard to use a laser in that application, except as a reference point when the distance is known," states Louton.
However, some recent advances promise to improve the use of lasers in machine control for road construction.
Conquering the x-, y- and z-axis
The problem has been to provide three-dimensional control. However, systems which can do this are being introduced to the industry. In a press conference at the recent World of Concrete show in Orlando, Fla., Topcon Laser Systems introduced a three-dimensional machine control system.
"What has been talked about for years is now a reality," said Chuck Haas, machine control product manager, Topcon. "We've spent several years developing the 3-D system and are now ready to let the industry know that three-dimensional automatic grade control is a reality."
According to Haas three key components had to be developed; a new tracking laser, the Topcon GRT-2000, a laser receiver/reflector, the Topcon LS-BX, and software that translates the engineering plan for use by the system.
"The major benefit of this system is the elimination of grade staking and checking. The tracking laser transmits the grade information directly to the equipment, providing automatic grade control," states Haas. According to Topcon the system is controlled by the actual engineering plan that is downloaded into the 3-D software on a field computer connected to the tracking laser. As the 3-D equipped grading equipment begins work, a laser beam from the tracking laser locks onto the receiver/reflector laser and tracks the equipment. A separate laser is reflected back from the receiver/reflector laser to the tracking laser, allowing it to accurately locate the equipment on the jobsite.
The grade data for that point is immediately transmitted to the sensor via the laser beam. Grade is then automatically controlled.
"We call the laser beam a virtual stringline, because if you could trace its path it would run parallel to the stringline typically used for highway grade control," said Haas.
"The applications for this system are unlimited," states Mike Forrest, machine control sales manager, Topcon. "Obviously we've had our eye on highway construction as the immediate high-benefit user, but this system will have a tremendous impact on all types of site preparation work."
According to Forrest support material for the system is being completed and the product should be commercially introduced in mid-1998.
Spectra Precision also has been working on a system for three-dimensional control of equipment. Cohoon explains, "One of our sister companies, who we merged with last September makes robotics total stations. This station shoots a light beam to a sensor mounted on a grader's blade. The information is radio linked to the control box in the cab of the machine. This tells the operator where he is in all three dimensions. This technology is quite new. We have some systems in use in Europe but it is still being field tested in the U.S. We plan to release a finished product this year."
Other technological advances may influence the use of lasers in road construction. "There will be some changes. GPS will change a lot of what we are doing in the future, but it is really hard to say where GPS technology is going. New products are becoming available such as the robotic total control station. With the smarter instruments we may have a different style of controls," says Louton.
As a tool for construction, lasers are definitely here to stay. New advances in technology will solidify the laser's place in construction and make it a more valuable tool for the road builder.