For years, paving equipment manufacturers have been telling us that stringless paving technology is close at hand. But freeing paving machines from the string lines that guide them has been much more difficult than expected. All of the paving equipment manufacturers are working hard to develop this technology. Contractors, too, are anxious to move into this new realm. Setting up and checking string lines is expensive, especially with the current emphasis on smooth-riding pavements. Early attempts to introduce stringless pavers produced poor results, so equipment manufacturers have been careful not to market equipment that isn't fully up to the task.

There are two schools of thought on how to guide paving equipment without strings: Global Positioning Systems (GPS) and Robotic Total Station technology. GPS uses the U.S. government's NavStar satellite system to provide location information. A machine-mounted antenna receives signals from a “constellation” of satellites and sends them to a GPS receiver, which calculates the location of the antenna. A GPS unit must receive at least five satellite signals to provide the most accurate real-time position information. But there are problems: specifically, elevation (grade) information is insufficiently accurate to guide paving equipment (grade errors are typically in the range of ±1 inch) and the satellite signals are very weak, so trees, buildings, mountains, bridges, and other obstructions can block the signal. Ongoing research and development is attempting to find ways to use GPS effectively for paver guidance—including the addition of laser technology to make up for weaknesses with GPS.

Stringless paving technology has been dreamed about for a long time. The first entrant in the U.S. market uses three robotic total stations to guide the direction of travel and produce pavement within a hundredth of a foot of proper elevations.
Stringless paving technology has been dreamed about for a long time. The first entrant in the U.S. market uses three robotic total stations to guide the direction of travel and produce pavement within a hundredth of a foot of proper elevations.

Robotic total stations is the other position measurement method adapted for guiding paving machines. This technology has been available since 1999. Robotic stations have been used to guide elevations for 3-D laser screed heads for several years—but not to move the machines into position. Robotic stations combine laser, infrared, and radio telemetry technology and use basic geometry to determine location and direction.

Leica Geosystems, Heerbrugg, Switzerland, teaming up with GOMACO, Ida Grove, Iowa, is the first to market paving equipment that is completely free from string line guidance systems. They are currently co-marketing systems for stringless trimmers, paving machines, and curb-and-gutter slip-forming pavers. Their system depends entirely on robotic station technology. Because of the high-risk nature of the paving business, research and development have been long and careful, and mostly conducted in Europe. But several projects have now been successfully completed in the United States, and the manufacturers have a high level of confidence in the system

How it works

This graphic shows the equipment added to an existing model paver to convert it to stringless technology. Two robotic total stations are shown in front of the machine, but a third one is positioned at the back (not shown).
This graphic shows the equipment added to an existing model paver to convert it to stringless technology. Two robotic total stations are shown in front of the machine, but a third one is positioned at the back (not shown).

Karl Soar, product manager for Pavement Engineering Systems at Leica, says that three robotic stations positioned along the paving path are required for guiding highway paving machines. One robotic station guides the front of the paver, one guides the back, and the third checks the fresh pavement. This third unit is necessary because of variations in concrete such as slump, age, and other factors (string-line paving systems require the same independent checking, but the pavement is checked manually). Only one robotic station is needed for trimmers or curb-and-gutter machines.

In addition to the robotic stations (sometimes referred to as “guns”), slope sensors, radios, and the Leica machine computer complete the system hardware. GOMACO provides a 3-D Upgrade Kit for all its machines equipped with digital controllers, and it can also retrofit older machines with digital controllers, making this technology available to all of its customers.

The robotic stations automatically follow the machine-mounted targets (also referred to as prisms) and radio their X-Y-Z position coordinates back to the computer. The rugged, weatherproof computer or “system control center” that is mounted on the machine receives information from the gun and sends commands to the paving machine controller. Software at the heart of the system makes information input easy and also provides the control for the paving process. And finally the controller unit interfaces with the GOMACO machine's digital electronic system, regulating the hydraulics to guide movement.

Following the process from plans to pavement

Millstone Bangert is using a trimmer/placer to place base at its St. Louis airport project. Trimmers require only one robotic station to guide them.
Millstone Bangert is using a trimmer/placer to place base at its St. Louis airport project. Trimmers require only one robotic station to guide them.

Leica's machine-guidance system for slipform pavers (LMGS-S) is the software that makes everything possible. Soar says that the process starts with loading the plans and survey or control point information into the computer. Most projects today are designed using computer-assisted drawing (CAD), so the plans can be downloaded directly into the computer. Some projects, however, are still drawn by hand—these layout points must be digitized before loading. Once loaded into the machine's computer, plans are referred to as the project model. Soar recommends that to complete this first step, both the owner's representative and the paving contractor should verify the accuracy of both the model and control point information.

On the jobsite, robotic stations can be set up either in front of or behind the paver—typically wherever there is less interference with trucks, personnel, and other equipment. The stations can guide the machine up to 450 feet from the paver, but 300 feet is best because at that range grade errors can be limited to as little as ±2 mm (less than 1/100 foot).

The prisms are mounted on masts attached to the paving machine, typically 12 feet above the mold (screed) and perpendicular to it. They provide the necessary crossfall, mainfall, and steering direction information. The computer compares this information to the design model and directs changes in elevation and direction to the controller, which adjusts the crossfall (elevation and pitch of the mold), main-fall (changes in topography), and the paver's direction of travel so that actual conditions match the model. This process takes place six to eight times per second, and the adjustments to the mold elevation and steering are in thousandths of a foot. The controller can adjust each end of the mold up or down independently. Independent quality assurance is crucial, and, as a final check, the third robotic station occasionally checks the surface elevations and position of the fresh pavement and compares it to the model. If the pavement is out of spec, adjustments are made automatically.

The paver moves forward at 3 to 4 feet per minute. As it passes the robotic stations, one is moved ahead without stopping the paver. Workers move the third station while the other two continue to guide the paver. In its new position the third gun is orientated on the project by shooting a minimum of two control points (that must be located very accurately by surveyors). Then it's automatically aimed at a machine prism and takes control. By leapfrogging the robotic stations, paving can continue nonstop.