Welcome to the third Automating your World column, a series of advice articles by Position Partners’ technical experts. In this series of articles, we look at the various aspects of machine control solutions.
Look at any job site and you can immediately tell whether or not the contractor is taking advantage of 3D grade control:
The ubiquitous masts sticking up from the blades of the machines give them away instantly.
They are on graders, dozers, scrapers and excavators and are now a common sight on Australian job sites.
But why is it that some machines have a single mast and others have two? Is there a difference between the systems? Is one better than the other? How do I choose which set-up is right for me?
Before looking closer at the various options, let’s recap what a 3D grade control system does: A data model of the job site resides in the on-machine computer. A GNSS (Global Navigation Satellite System) antenna determines the position of the cutting edge of the blade several times per second and feeds this information to the computer.
The difference between where your edge is (as measured by the GNSS antenna) and where it should be (as determined by the data model in the on-machine computer) is your cut/fill for the particular position where the blade is currently located.
The cut/fill information can be fed to the hydraulic system in order to drive the blade to design elevation in real time.
We must know the 3D position of the entire cutting edge of the blade, not just a point along the edge. Most graders are therefore set up with a rotation sensor, blade slope sensor and a mainfall slope sensor.
When all the dimensions of the machine is known to the computer, these sensors combine with the position from a single GNSS antenna to give us the exact 3D position of the entire cutting edge.
For many applications, a single-antenna system is a good solution. The drawback with this configuration is that the machine doesn’t instantaneously know the direction in which it travels.
A single 3D point is not capable of giving the system an orientation. The on-board computer therefore makes certain assumptions (such as the machine must be going forward when it is grading; no back dragging) in its algorithms.
An example of where a single-antenna system falls short would be when a dozer is used for finishing a steep slope.
If the slope and conditions are such that the tracks are slipping, the machine will be ‘crabbing’, meaning the direction in which its nose is pointing is different to the direction the machine is actually travelling. This could lead the system to query the data model in a spot that is slightly off horizontally, resulting in an elevation error.
Enter the twin-antenna system.
In the above example, we could query the data model in the correct spot if we just knew where the machine was pointing. By putting two GNSS antennas on the same mast we get two 3D positions and therefore know the heading of the machine in real time.
If the machine is fitted with a PAT (power-angle-tilt) blade, we can add a rotation sensor and the system will know both the heading of the machine and the orientation of the blade, ensuring correct cut/fill calculations.
The twin-antenna system also excels when installed on a motor grader. In addition to the system always knowing the direction of travel, a twin-antenna lets the operator roll the blade to suit the material and current conditions.
Other systems either require the operator to keep the blade fixed (so that the mast and antenna are always perpendicular to the mainfall of the machine) or must have a slope sensor installed to measure the roll. A twin-antenna system overcomes both these problems.
It is also possible to dispense with the rotation, blade slope and mainfall sensors on a motor grader and replace them with an additional GNSS mast and antenna.
Such a system is referred to as a dual-mast system and consists of two masts, each with a single GNSS antenna on top. The masts are mounted close to each blade corner.
Because the system gets a 3D point for each corner, it will instantly know the 3D position of the whole cutting edge and its direction of travel.
Note that I said direction of travel of the blade, not the machine. With a dual-mast system there is no way to measure the relationship between the blade and the machine.
The system must make assumptions as to the direction of the machine by having the software use empirical data to chart the path of the machine. For most grading applications on near-level surfaces this is usually not a problem.
A dual-mast system can let an operator articulate the machine and side-shift the whole A-frame to work very steep slopes.
However, one must be careful of how much the blade is sloped.
Because we deal with satellite signals, a very steep blade slope means the GNSS antennas will be pointing towards the horizon and not straight up. This can affect their ability to receive the satellite signals.
A drawback of the dual-mast system is that it is difficult to switch from using the GNSS antennas to using other positioning sensors.
Laser grade control, sonic grade control and 3D systems using robotic total stations all rely on the mainfall, blade slope and rotation sensors ‘normally’ found on a motor grader.
A dual-mast system cannot easily switch over to other poisoning methods unless the above sensors are already installed on the machine.
On dual-mast machines, they are normally not. If they were, there would no need to install two masts and have two antennas, since a single mast and antenna would be sufficient.
On excavators, though, a dual-mast system is the most practical solution.
This machine is fundamentally different from other earthmoving equipment in that it doesn’t work when it is travelling. It works only when the tracks are still.
It does, however, rotate. Because of this, two separate masts with one antenna on top of each are mounted on the back of the machine body. When the tail swings, the system knows it instantly because it works with two 3D positions at all times.
Be sure to ask your sales representative which antenna configuration is right for your application.
And because some manufacturers only supply one or two of the above configurations be sure to ask why the sales representative is offering you a particular configuration over another.
Getting the right system for you application will ensure you the most return on your investment.
Grade well, grade quickly.