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8 February 2018
Hazell Bros mmGPS Komatsu grader with laser
GPS/GNSS-based 3D grade control systems have made a major impact on how heavy construction is done, but they still have limitations when it comes to achieving very tight vertical accuracies. This is true regardless of which brand you use.
While multi-constellation (GPS, GLONASS, Galileo and more recently Beidou) grade control systems are able to work in more places and under more adverse conditions, these systems are still subject to various orbital errors and environmental factors, which can degrade the vertical accuracy.
Many contractors use GNSS-based 3D grade control exclusively for earthmoving tasks and even finishing. However, the manufacturers of these systems will confirm they cannot guarantee that the vertical accuracy will be good enough for finishing.
The various GNSS-based 3D machine systems will provide a horizontal accuracy that is adequate for any earthmoving and grading task. The challenge is to find a way to improve the vertical portion of your position.
Such a system will use the GNSS signal to position the grader horizontally and the laser signal to get a very accurate vertical position, within a few millimetres.
Hazell Bros mmGPS Komatsu grader
With a conventional laser, you have the benefit of being able to use the laser you already own, but you will be limited to working within a vertical range that cannot be wider than the physical length of the laser receiver you put on the grader.
What this means: If you have a laser receiver on the grader which is physically 1m long, then you must keep your grader working within ±10cm from the height at which the laser is set up. Once benched, the laser will have a working window of around 20cm that it must stay within to remain accurate.
If your grader travels further than 20cm vertically, you will need to re-bench to maintain accuracy.
Instead of sending out a single plane of light like a standard grade laser, this laser sends out a fanned-beam that lets the operator work to high vertical precision within a 10m elevation range.
A fanned-beam laser will also let you use a ‘normal’ sized laser receiver on your grader, which is far less susceptible to failure from mechanical shock and vibration.
For most of your earthmoving tasks GNSS alone will be accurate enough, but when you need additional vertical accuracy a simple way to achieve this is to combine your GNSS 3D grade control system with a conventional, construction grade laser or a purpose-designed, fanned-beam laser.
Which of these two options you choose will be dependent on the details of your project.
The main benefit of using a laser guidance to improve vertical accuracy is that an unlimited number of machines can run off of a single laser. This not only lowers the cost of achieving high vertical accuracy, it also reduces the potential for set-up errors, because all machines are working from the same laser.
However, the benefits of the combined GNSS/laser system far outweigh the additional planning that will be necessary to ensure disruption stemming from people, other equipment, vehicles or trucks breaking the line of sight will be kept to a minimum.
Hazell Bros mmGPS Komatsu grader
This system does not make use of satellite signals for positioning and therefore works indoors or in places where there is a severe obstruction of the sky, such as directly underneath an overpass or underground.
Because the robotic total station always knows where it is in the local co-ordinate system, it will also know where the target (your grader) is at all times – simply by locking onto the target and following it.
The robotic total station feeds the 3D position of the target to the control box in your grader many times per second via a radio link.
These positions are then used by the on-board computer to determine cut/fill for where the blade is at a given moment, in the same manner as a 3D position derived from GNSS would be used.
These systems are highly accurate and have been field-proven for more than a decade.
Although outnumbered by GNSS systems in the field, the ability to work underground and indoors will ensure that these systems will be around for a long time to come.
The various positioning technologies available for 3D grade control systems each solve a particular problem and are particularly well suited for a specific application.
It is unlikely that a single positioning technology will solve all the earthmoving and grading tasks faced by your company, so it is important that you stay educated about the various solutions available.
For many contractors, the ease with which various sensor technologies can be switched around is crucial, so speak to your positioning system sales representatives to ensure you have the maximum flexibility from whatever solution you choose.
Something else to look for is a consistent operator interface between different positioning technology, so the operator screen remains unchanged and familiar no matter which type of positioning is currently being used.
That means shorter learning curves and the ability for operators to very quickly switch from one type of system to another.
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