Grading is made easier with GPS

Originally developed by the US government for military purposes, the benefits of GPS technology are now being realised by the construction industry. Equipped with GPS receivers, a machine can now determine its exact position on a map and automatically excavate to the required grade.

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By  Colin Foreman Published  May 28, 2005

|~|74prodbody.jpg|~||~|The novelist Alfred Doblin once remarked: “The world is made of sugar and dirt.” For many earthmoving contractors, the sweet taste of success is being found with the help of automated grade control technologies, which provide not only precise, accurate results, but also a productivityenhancing competitive edge. These days it is all about machine control. Machine control extends from basic systems that rely on stringlines and lasers, to highly precise three-dimensional GPS (Global Positioning Systems). Most of the systems are fitted into construction equipment as an after-market add-on, although some manufacturers — like Caterpillar — are now offering integrated systems at the time of manufacture. lasers There are three major categories of systems available. The first are laser-based display systems; these are generally easy-to-use modular systems that are flexible and configured to the machine and site requirements. Used for machine guidance, these laser-based systems are found in a range of earthmoving applications on machines that include dozers, backhoes, scrapers, skid steers and excavators. The systems are simple to use and understand. Some contractors start out with a digital linear laser receiver mounted on the front of the machine on top of a heavy-duty mast. Perched above the operator’s compartment, a number of the receivers provide 360 degree operation. The receiver is used as a stand-alone battery-powered display system for guidance. With the receiver, the operator has additional valuable information needed to perform grade work faster and more accurately. The second option are laser-based grade control systems that add proportional valve controls to the machine’s hydraulics, and a user interface to upgrade the laser-based display system to automatic. If these changes are combined with an electric mast option, the laser receiver can be raised or lowered from within the operator’s cab, saving time and effort. The signals from the laser receiver are used to control a proportional hydraulic valve for blade correction, allowing operators to grade faster and more accurately. This type of system is best suited for larger site preparation work. Laser-based grade control systems have numerous benefits, including reduced stakeout requirements, improved material yields, faster job cycles, eliminating communication errors, rework and idle machine time. The third category, 3D grade control, is probably the most revolutionary change in earthmoving to have come about. The 3D system measures the X-Y and Z co-ordinates of the machine blade, and compares the data to the pre-loaded digital terrain model. The design elevation and cross-slope for the current position are then calculated, and the system automatically moves the blade to the correct cut or fill position elevation and slope via the machine’s hydraulics. Grade and slope information, along with the blade’s position, is shown on the cab display. The GPS grade control systems is precise to 30 mm and enables operators to perform bulk earthmoving in a stakeless environment, using either automatic or manual blade control. When combined with an advanced tracking sensor, the grading accuracy can narrow to ±5mm, which provides precise finished grade work. There are two major categories of 3D grade control systems: GPS Grade Control, and Advanced Tracking Sensor (ATS) Robotic Total Stations. GPS Grade Control puts design surfaces, grades and alignments inside the cab and enables operators to perform bulk earthworks and mass excavation in a stakeless environment. The GPS approach takes advantage of a network of satellites encircling our planet. GPS antennae are mounted on both sides of the machine’s blade. The GPS receiver on the machine computes the exact position of the GPS antennae many times per second. Advanced Tracking Sensor Robotic Total Stations are used for precise finished grade work. The ATS robotic total station automatically tracks a target, which is mounted on the blade of the machine. The system continuously measures the target’s position and transmits the data to the in-cab computer, which then determines the desired elevation and slope for that position. One particularly beneficial feature of the ATS instrument is its ‘search intelligence’ capability. This means that if the sight line between the machine and its target is interrupted, the ATS is able to relocate the target quickly and automatically. Both 3D systems provide the equipment operator with all the details of the automatic grade control system right at their fingertips. An on-board computer determines the exact position of each blade tip; it then compares these positions to the design elevation, and computes the cut or fill to grade. This information is displayed on both the screen in plan view, cross-section view, or text and a lightbar display. The lightbar display — used for manual operation — helps to guide the operator up/down for grade, and right/left for a defined alignment. In automatic operation, the cut/fill data is used to drive the valves for automatic blade control. Advanced technology is taking design data from the office and placing it right into the machine cab, allowing operators to grade complex designs such as vertical curves, transitions, super-elevated curves and complex site designs — all without having to use stakes, stringlines or paper layouts. Since the data is there ready in the field, the site foreman or operator can quickly set the new grade or pad elevation right in the operator’s compartment, without having to wait for the project’s surveyors to set or reposition grade stakes. Additionally, the precision afforded by these technologies can provide material savings, better job estimates, reduced rework, and when necessary, extend the working day into the night. As the construction industry becomes more demanding and competitive than ever before, contractors are increasingly turning to sophisticated machine control systems for significant process and productivity advantages, reduced costs and greater efficiency. Earthmoving grade control technologies, which have been widely used in the US, Europe and Australia for many years, are now rapidly being embraced by local contractors in the Middle East as the benefits and advantages of these technologies become apparent to them. In the UAE, grade control technologies were used on several major projects. The Dubai Autodrome project, for example, used the Trimble BladePro 3D system with ATS robotic total station to achieve improved accuracy and time-saving productivity. Other projects include The Burj Dubai, where a Trimble Sitevision GPS excavator system was employed; and Festival City, where the BladePro 3D system was used.||**||

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