Novelist Alfred Doblin once remarked, “The world is made of sugar and dirt.” Many dirt-moving contractors find the sweet taste of success with the help of automated grade-control technologies, which provide precise, accurate results and a productivity-enhancing competitive edge. It's all about machine control.
Machine control runs the gamut from basic systems that rely on stringlines and lasers, to highly precise 3-D global positioning systems (GPS). Most systems are added to construction equipment as an aftermarket feature, although some manufacturers are starting to offer integrated systems.
There are three major categories of systems available to contractors:1. Laser-based display systems
These are generally easy-to-use, modular systems that can be configured to machine and site needs. Used for machine guidance and machine control, they are found in a range of construction and earthmoving applications and machines, including dozers, scrapers, motor graders, skid-steer loaders, and excavators.
With a portable, tripod-mounted laser transmitter, 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 atop 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 laser receiver, the operator has additional, valuable information needed to perform grade work faster and more accurately.2. Laser-based grade-control systems
Add proportional valve controls to the machine's hydraulics and a user interface, and the laser-based display system is up-graded to automatic. If these changes are combined with an electric mast option, the receiver can be raised or lowered from within the cab, saving the operator time and effort. Signals from the receiver are used to control a proportional hydraulic valve for blade correction, allowing operators to grade faster and more accurately.
This type of grade-control system is well-suited for contractors and machine control operators who require the flexibility to customize the system to fit the specific needs and dimensions of each job-site. Laser-based grade-control systems can reduce stakeout requirements, improve material yields, and speed job cycles. This can lead to lower costs, fewer communication errors, and less rework and idle machine time.3. 3-D grade control
The introduction of 3-D grade control is one of the most significant changes in earthmoving. The system measures the X, Y, and Z coordinates of the machine blade and compares the data to the preloaded 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, slope, and blade position are shown on the cab display.
The GPS grade-control systems can be precise to 0.1 feet and can enable operators to perform bulk earthmoving in a stake-less environment, using automatic or manual blade control. When combined with an advanced tracking sensor, the grading accuracy can narrow to ±0.02 feet, which provides precise finished-grade work.
There are two major categories of 3-D grade-control systems:
GPS grade control: This system puts design surfaces, grades, and alignments inside the cab and enables operators to perform bulk earthworks and mass excavation in a stake-less environment. The GPS approach takes advantage of a network of satellites encircling our planet. GPS antennas are mounted on both sides of the machine's blade. The GPS receiver computes the exact position of the GPS antennas many times per second.
Advanced tracking sensor (ATS) robotic total station: For precise finished-grade work, the ATS robotic total station automatically tracks a target that is mounted on the blade of the machine. It continuously measures the target's position and transmits the data to the in-cab computer, which determines the desired elevation and slope. A beneficial feature of the ATS instrument is that it has “search intelligence”—if the sight line between the machine and its target is interrupted, the ATS automatically relocates the target.
Both 3-D systems put all the details of the automatic grade-control system right at the operator's fingertips. An onboard computer determines the exact position of each blade tip. It compares these positions to the design elevation and computes the cut or fill to grade. This information is displayed on the screen in plan view, cross-section view, or as text. The cut/fill data is passed to light-bars, which for manual operation guide the operator up/down for grade and right/left of a defined alignment. In automatic operation, the cut/fill data is used to drive the valves for automatic blade control.
Earthmoving is required on nearly every construction job. Advanced technology is taking design data from the office and putting them right into the machine cab, allowing operators to grade complex designs—vertical curves, transitions, super-elevated curves, and complex site designs—without stakes, string-lines, or paper layouts.
Because the data are there in the field, the site foreman or operator can quickly set the new grade or pad elevation, without waiting for grade stakes to be set or repositioned. Additionally, the precision afforded by these technologies can provide material savings, better job estimates, reduced rework, and, when necessary, extend the work day into the night.
With the construction market becoming more demanding and competitive than before, contractors are increasingly turning to sophisticated machine control systems for significant process and productivity advantages, reduced costs, and greater efficiency.
— Jeff Winke is a Milwaukee-based construction-industry writer.
The next step: laser augmentation
Laser augmentation for GPS-based 3-D grade-control systems provides another way to achieving precise vertical grade for jobs with tight tolerances. Contractors have the option of using a robotic total station 3-D system or adding laser augmentation to their GPS-based system.
Laser augmentation can achieve vertical accuracy from 0.01 to 0.02 feet, which should satisfy the most stringent finish-grading requirements. With the subbase and base being more precise it allows for the more accurate placement of the overlay.
The system requires a laser transmitter, which is a standard rotating laser typically set on a tripod. The best systems do not require the placement of the laser transmitter on a known design-plan mark, but allow the transmitter to essentially define itself as the benchmark base. The setup for a flat laser plane is that simple.
Unfortunately, not all jobsites are flat. To optimize the laser-augmentation system, a sloping laser plane needs to be set by tilting the tripod-mounted laser transmitter to properly orient it. It is critical to define the orientation in order to benchmark the system.
A critical technical issue is to make certain the laser plane is precisely aligned and benched, or grading errors will occur. Some grade lasers include both grade-match mode and automatic axis alignment to make setup for single and dual slope grade work easy.
Laser augmentation offers its greatest use on motor graders, although there are contractors who use it exclusively on dozers, and some are finding advantages when it is used on excavators.