It’s not as if planning and thought have never gone into an earthwork project—quite the contrary. Considerable planning and measurement have always been required. But this was performed by people, not computers, and done before and after the fact, not in real time.
Traditionally, an earthwork project, along with its project guidance and quality checking, is done in set stages. First, a surveyor comes out to the site to perform a topographic ground survey to get the lay of the land, elevations, and contouring and to locate any natural features or man-made structures. Second, armed with this information, the engineer creates topographic maps of the site as represented by existing contour elevation lines and then uses them as a basis for the site’s design and layout, including revised grading as represented by proposed contour elevation lines. Third, the surveyor takes the engineer’s plan design back out to the field and locates the points, depths, and elevations of the engineer’s planned new contours by setting a series of wooden stakes with the amount of cut and fill to be performed at each stake’s location written on its side. Ideally, these stakes serve as visual guides for the operators of the earthmoving machines, directing the location, depth, and height of the earthwork. Even under ideal operating conditions, the earthwork must be periodically measured with a weighted tape or a surveyor’s rod to determine the actual progress being made by the machine. Under realistic operating conditions, these stakes are often lost, stolen, broken, or buried. This leads to a fourth step: the resetting of grade stakes during operations—with resultant operational delays. Fifth, and finally, the surveyor returns and manually confirms that the new grades and elevations are in conformance with the engineer’s design plan.
Basic economics teaches us that it is always far more profitable and efficient to invest in capital instead of labor. Automated positioning systems, along with their supporting hardware and managing software, are such a capital investment. This investment effectively eliminates the need for manual onsite surveying, staking, measurement, and certification. It greatly reduces field waste and equipment downtime. It saves time, and time is money. Not just in the sense of reducing operating time and direct operating costs, overall project durations are shortened, allowing a contractor to complete more projects during a given construction season. On-the-job accuracy increases and overall quality of performance with it.
Does an Automated Positioning System Work?
The automated positioning reporting system (APRS) for earthmoving equipment is a direct application of the general global positioning system that has transformed the way we find our way around this world. Like many technological advances, GPS originated as a military application designed to aid in the precise positioning, locating, and tracking of ships, planes, and tanks on the battlefield. In the 1980s, the US government made GPS and its operating frequencies available for civilian use. The system consists of 24 satellites put into orbit by the Department of Defense, one for each hour of the day, or 15 degrees of longitude. Each satellite circles the Earth twice a day in a precise orbit. As it does so, it transmits its own unique time-signal data to the surface, where it is picked up by ground receivers (like those in a bulldozer). These time data are important as they tell the ground receiver how far away the satellite is. The distances result in equivalent “spheres” with a satellite at the center. The receiver is in effect on the surface of the sphere and all the other distance spheres it is in contact with. Using triangulation, information from several of these high-speed satellite signals can be used to precisely locate the receiver to an accuracy of 30 centimeters (1 foot).
Further accuracy, up to a distance of 1 centimeter, can be achieved using the ground system associated with the APRS. This application allows for the integration of earthmoving equipment of all kinds (compactors, dump trucks, bulldozers, graders, backhoes, and other excavators) with GPS. The APRS has software and hardware components. The APRS operates in reference to software-generated, three-dimensional maps of the project site showing the existing and proposed surfaces. This “map” is typically in the form of a digital terrain model (DTM). The hardware consists of automated computer controls and sensors attached to and operating the business end of the earthmoving machine (dozer blade, grading blade, excavator shovel, etc.). Controlling these operations requires both a machine interface and an operator interface that are integrated with the software map and hardware controls by means of sensors and other input devices.
With an APRS, the normal mechanical linkages that operate the hydraulic cylinders that move and position the blade or shovel are replaced by electronically controlled servo-type valves. Typically used for remote operation and control, electrohydraulic servo valves utilize an induced electrical signal to create a magnetic field that rotates a suspended armature. This armature is connected to a fixed flapper arm that provides the linkage to the rotary spools that generate pressure changes in a closed-loop hydraulic system and control the amount, pressure, and direction of fluid flow. These servo valves are in turn connected to the software control box, which is one of the electronic components of the operator interface.
The brain of the outfit is the operator interface, with electrical cables acting as “nerve” connections to the servo valves. Its computer hardware and software take the incoming satellite positioning data, referencing the digital design files created by the design engineer to derive the machine’s position. These design files represent three-dimensional contouring plans (both existing and proposed) that can be accessed internally via data stored in a compact-flash memory card or memory stick, or they can be accessed externally from data broadcast by the site’s controlled area network (CAN). These two systems, CAN and GPS, working in tandem allow the accurate positioning of the earthmoving equipment. Operating in real time, the combined system can accurately place the dozer blade or excavator shovel in the precise location and depth needed to remove dirt and achieve the proposed design grades. The blades and shovel simultaneously move up and down with the forward and backward movement of their machines to ensure the edge is located at the required northing, easting, and elevation (X, Y, and Z coordinates). The operator always has the option of cutting into the operator box’s operations via manual override and working the machine blade directly. The operator box also has a visual and/or keyboard interface that allows the operator to observe its operations on a display screen.
So how do the equipment and its CAN interface with the GPS? This is done by means of built-in sensors. Each piece of equipment is equipped with its own GPS sensor. This consists of an antenna that receives the GPS signals and transmits the information to a receiver. In addition to the mobile sensors, there is a fixed GPS sensor combining an antenna with a receiver called the base station. This is usually located over either an established third-order benchmark or a known physical object whose location and elevation have been accurately surveyed in relationship to an established benchmark. These can include manhole rims, street curbs, and building corners. On occasion it may be acceptable to place the base station over an object whose exact elevation is unknown but that can be assigned a relative elevation value (100 feet, for example), with all the topography and proposed grade elevations recorded as elevations relative to the structure. The mobile and base sensors are in constant communication with the GPS and with each other. It is this interaction that results in the high degree of accuracy used to place the blades of the earthmoving machinery.
Positioning Systems—It’s All Done With Mirrors
GPS-derived APRS has not completely replaced the need for additional grading precision, such as that provided by lasers (or, for that matter, it hasn’t completely replaced the need for good old-fashioned site surveying done by surveyors with transits and target rods). Lasers make useful adjuncts to GPS since they allow for a polished grading effort with the construction of finely graded terrains with minimal grades. A laser positioning system also requires a base station, but this is not a unit for receiving information or data. This is a tripod set at a measured elevation above a benchmark or other fixed point that has a known elevation and location. The laser generates a one-dimensional beam that is rapidly spun about an axis on top of the tripod to effectively create a plane of laser light at a given elevation or grade. The laser itself doesn’t usually rotate, since most models utilize a laser beam projected onto a rotating head with a fixed mirror that sweeps the laser light around the tripod. These mirrors are either self-leveling or leveled manually with adjustable screws measured by bubble-level markers.
The laser light is captured by a sensor mounted on a pole for surveying or on the body of the earthmoving equipment. The height of the sensor can be adjusted until it comes in contact with the plane of laser light. Either a flashing light or an audible beep is used to indicate that the sensor is at the proper elevation. The height of the sensor either helps record the elevation of the spot of the surveyed terrain or whether the equipment has achieved design grades. The laser control systems allow for simultaneous elevating and tilting of dozer blades while they are in operation. This is made possible by the use of a digital laser receiver, similar to the sensor in the surveyor’s rod, mounted on a mast that can be raised or lowered. The mast is fixed to the equipment’s cutting edge and detects the laser beam whenever the height of the blade is set at the design elevation. Whenever the blade is not in line with the design elevation, warning information is sent to the operator’s control display. Accuracy up to 1 millimeter can be achieved with this system.
Hardware to Software—What Makes the System Tick
Many of the leaders in the field of automated positioning systems started off as suppliers of GPS surveying equipment and/or developers of design software. A pioneer in the application of GPS hardware and software to the job site, Trimble provides a family of GPS and laser grade-control software and associated hardware systems. Operating in a CAN environment, each software package provides an increasing level of control and operating sophistication and can be tailored to the work being performed. Furthermore, each software system is fully upgradeable allowing for cost-effective increases in earthmoving capabilities, beginning with the basic Trimble GCS3000 single-control laser guidance system, which uses a laser receiver to control the lift of the machine blade. Though intended for use on dozers, it can also be used on other machines. This simple-to-operate system is useful for small construction projects, such as the grading of housing lots, small subdivisions, and sports fields. Next in complexity and capability is the Trimble GCS400 grade-control system, capable of dual controls for both the lift and tilt of the machine blade. It is also laser-guided by an additional AS400 slope sensor. The GCS400 can operate in linked mode, which unites the two controls and displays them as a single control for greater ease, accuracy, and consistency of operation. At the next level is the Trimble GCS500 grade-control system, which includes cross-slope control and is designed to be used on motor graders for fine grading. With an RS400 rotational sensor, the operator can calculate the cross slope of the blade, choosing which side of the blade is being controlled and switching sides for return passes. More complicated earthwork projects can be performed with this system, including road slopes, ditches, and embankments. The company’s most advanced laser control system is the Trimble GCS600 grade-control system, which utilizes the hardware of the GCS500 system but can be augmented by an ST300 sonic tracer to provide elevation control. This sonic tracer allows for string-line, previous-pass, or curb and gutter tracing. The combination of controls makes this system suitable for projects with very tight tolerances, such as finish-grade work.
Trimble’s most advanced grade-control system, the GCS900, utilizes GPS and laser guidance to accurately position the machine blade in real time as it moves across the job site. Not only can it be used by dozers; the GCS900 can be used by excavators, backhoes, motor graders, and scrapers. The multiple positioning sensors compute the exact position of the bucket or blade many times per second. Use of CAN-based information broadcasting allows the system to be moved from machine to machine as needed. This allows for economical purchase of this advanced system in only quantities needed for key pieces of equipment (there is no need to buy it for your entire equipment fleet). Information displays combining the CAN data with the positioning of the blade, bucket, and machine itself show surfaces, design grades, and alignments to the operator. This information is displayed in multiple modes: plan, profile, cross-section, or text. Continuous computations allow for real-time updates of cut-and-fill elevations. A heads-up display utilizing light bars provides additional guidance (up/down to define grade and left/right to define alignment) to the operator. The system’s flexibility allows its use in all phases of earthmoving, from groundbreaking mass excavation to certified finished grading.
In addition to its own internally developed GPS grade-control software systems, Trimble has been involved in a joint venture with Caterpillar: Caterpillar Trimble Controls Technologies LLC. According to a corporate spokesperson, the goal of the venture is to “develop the next generation of advanced electronic guidance and control products for earthmoving machines in the construction, mining, and waste industries. The joint venture develops machine-control products that use site design information combined with accurate positioning technology to automatically control dozer blades and other machine tools. This leading-edge machine-control technology combines historical Trimble positioning technology with capability gained through its acquisition of Spectra Precision. The joint venture is the exclusive supplier to Trimble and Caterpillar, which will each market, distribute, service, and support the products using both companies’ independent distribution channels. Caterpillar offers products as a factory-installed option, while Trimble continues to address the aftermarket with products for earthmoving machines from Caterpillar and other equipment manufacturers.”
Topcon Positioning Systems Inc. produces the Topcon 3D-GPS software package, which combines an effective machine-control platform with state-of-the-art GPS technology to create an automated operating system that allows the grading of any digital design surface to within a few centimeters of accuracy. By utilizing GPS-satellite control and a three-dimensional grade-control interface, the need for traditional survey checks for field accuracy is eliminated. The software allows for visual displays of virtual grading-plan previews as well as multiple-axis cross-section alignments showing slopes along the current path of the earthmoving equipment. These in turn provide the equipment operator with real-time location of the machine and its relation to the site design’s surface elevation and grade slope and the difference between the machine blade’s current position and slope during operations. These guidance displays are available to the operator whether the machine is working in manual or automatic mode. The software runs a complete GPS-guidance hardware system, Topcon’s 3DXi GPS+, which includes four 360-degree-capable-reception, CAN-based tilt sensors that each measure the tilt angles from the machine’s cab, boom, stick, bucket, or blade. Two GPS+ antennas, a GPS+ receiver, and a GX-60 control box providing the operator with visual displays of the machine’s location and its bucket’s position complete the hardware ensemble.
The Australian firm H&S has taken the concept of GPS-driven equipment positioning to the next level by applying a degree of artificial intelligence to allow for more efficient mining operations. Unlike the relatively simple grading work associated with commercial and residential site development projects, mining has to be more focused on the optimum areas that may contain precious ores. The company’s MP3 grade-control system serves as a guide to the earthmoving equipment while acting as an assistant to the mine geologists who need to locate ore and avoid waste in both open-pit and underground mining. The software’s analysis tools include univariate and bivariate statistical analysis, data quality analysis, and modeling tools for conditional simulations and decision optimization. It can handle the optimization and graphic visualization of single or multiple geological domains. Stockpile management and production scheduling are made possible by the software’s scenario-optimization tools.
Caterpillar provides its own integrated laser and GPS grade-control system. Its Accugrade GPS 3D system uses GPS-satellite information to determine blade positions with a high degree of accuracy. The system compares the actual location of the blade with the elevation and slope of the designed surface at the blade’s corresponding location, measures the relative difference between the two, and submits this information to the operator on a display screen in his cab. A complete list of required information (blade elevation, required cut/fill, visual display of the blade position versus the design surface, etc.) is provided in an easy-to-read graphical format. The display’s light bars are adjusted by the operator to guide the machine and ensure consistency and accuracy of the grading. The Accugrade Laser 2D system can be used on its own or in conjunction with the Accugrade GPS 3D system to provide a highly accurate grade-elevation reference and machine-control guidance. A base station projects a laser signal across the job site; the signal is in turn captured by a digital laser receiver on the machine as it operates. The control system automatically makes adjustments to the elevation and tilt of the machine’s blade as it moves across the site. The operator merely steers the machine, moving it back and forth while the blade adjusts itself.
Leica Geosystems publishes GIS DataPRO software, which enables direct communication and data transfer between Leica GPS and many industry-leading GIS software suites. It is a Windows-based application that allows for GPS post-processing, editing, and data export in the ESRI shapefile format. It makes efficient integration of fieldwork to CAD and GIS functions possible. GIS DataPRO supports both single- and dual-frequency GPS and can post-process both submeter and centimeter positions, depending on the equipment used and the method of observation. Report generation and documentation are two of its many functions. The software is also Internet-enabled and can automatically retrieve GPS reference data from FTP servers maintained by continuously operating reference stations (CORSs) and the Cooperative CORS Network. Data import and file transfers can be accomplished by removable flash memory, serial-cable input, or wireless Bluetooth communication. All field data can be exported to AutoCAD (DWG and DXF), Microstation (DGN), MapInfo (MIF), and 3D shapefile formats allowing for compatibility with multiple software systems.Future Is Now—Intelligent Earthwork Systems The next and possibly final stage of development for automatic positioning systems is the intelligent earthwork system (IES). In theory, IES could do away with the human operator completely, with fully automated systems not just adjusting the elevation and tilt of the machine blade as it moves, but actually moving and steering the machine across the job site in accordance with existing topography and the designed grading plan. Naturally, nobody is going to trust a computer to safely maneuver itself without human oversight, so a flesh-and-blood operator will always need to be in the loop to pull the plug if necessary on a machine that is about to run over someone or ram into another piece of expensive equipment. There have been some advances in this direction, with the operating box also controlling the machine’s movement, transmission, steering, braking, etc. Further advances in artificial intelligence and robotics will be needed before such a system becomes practical. As the saying goes, they’re still working the bugs out.