The system made an initial impact in subgrade preparation and increasingly is being used for milling and paving. Just as Millimeter GPS+ has been used for fine grading, contractors are beginning to use it for “fine milling” — and achieving accuracies within a quarter-inch, in contrast to the tenth-of-a foot precision inherent in conventional machine control.
The role of GNSS
Conventional GNSS machine control uses satellite signals alone. Such a system uses a rugged antenna mounted to a shock-absorbing, vibration-damping pole along with a GNSS receiver box mounted in a secure location on the machine. Satellites send positioning data to another antenna/receiver combination at a stationary base station. The base station then sends a three-dimensional position and 3-D corrections via radio to the mobile or machine control receiver.
Positioning data is also sent to the machine. The stationary base and machine work together to provide real-time kinetic (RTK) position information, revealing the machine’s three-dimensional location on the site. Software compares the machine’s position to the design grade, which was determined using site plans, at a given location. The system also provides visual guidance for machine operators by displaying a site model on an in-cab color monitor, or it automatically adjusts the needed elevation and desired cross-slope of the blade as the operator guides the machine forward.
Millimeter GPS+ combines GNSS and laser. In addition to a GNSS base and rover, the system uses a PZL-1 Lazer Zone transmitter and a PZS-MC machine-control sensor or PZS-1 rover sensor that gets integrated with the contractor’s GNSS receiver. The PZL-1 transmitter sends out a wall of laser light 33 feet tall and up to 2,000 feet in diameter.
The contractor can link up to four transmitters for a total reach of 8,000 horizontal feet and 132 vertical feet. The PZL-1 transmitter can operate multiple machines equipped to accept its signals. The GNSS component of the system plots the location of the machine while the laser component guides the grader to position and elevate the blade precisely. The system “knows” the three-dimensional position of the laser transmitter and the three-dimensional position of the machine and is then able to calculate the vertical angle from the laser up to the sensor on the machine and provide a vertical correction.
The roughly 12,000-foot runway was divided into quadrants of some 6,000 feet in length and 200 feet wide from the centerline. The 200-foot widths were subdivided into two adjacent 100-foot-wide subsections to accommodate eight passes by the 12-½ foot-wide Roadtecs. With the entire job now accurately localized for GPS at its midpoint with a single control point file, Intercounty figured that mounting the PZL-1s on 15-foot-tall towers would provide the lasers with continuous coverage. But nonstop 40-mph-plus winds would not allow continuous rotation of the self-leveling Topcon PZL-1 lasers at that height.
So Intercounty’s Topcon dealer, Cleary Machinery Co., Inc. of South Bound Brook, NJ lowered the PZL-1s to their standard 2-meter height on tripods. Intercounty had five milling machine rovers and three survey rovers, all corrected by a single base station despite being located thousands of feet apart. A three-dimensional site model developed by Mesh Consulting, Eagleville, PA, was loaded into the Roadtecs’ machine-control systems.
The original plan was to fine-mill the surface to within three-quarters of an inch of the specified elevation in a single pass using three machines deployed in a staggered formation. The machine making the “virgin cut” would have Millimeter GPS+ controlling both sides of the drum. Each trailing machine would “joint match” on one side using the RX-900’s hydromation system, while utilizing Millimeter GPS+ on the other side. Four PZL-1s were spaced apart by 750 feet, affording a pass length of 3,000 feet before the machines would have to “square up” and return to the original starting point.
Equipment blockages and weather caused the Roadtecs to cut to within 0.02 foot with laser reception and within 0.05 foot without it. These factors, coupled with severe machine vibration caused by the deep cut in hard material, made precisely milling to the specified elevation in one pass difficult. But the situation turned out to be a blessing in disguise, as it forced Intercounty’s milling department to devise an alternative process that proved to be even more efficient than the original plan.
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