Automating your World: Unlocking the data model mystery – or how does a 3D system know how much to cut?

Welcome to the second 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.

Figure 1

One of the questions I am asked most often is: “How does the 3D system know how much to cut?”

Many prospective 3D grade control buyers quickly grasp the concept that a GNSS (Global Navigation Satellite System) receiver mounted on the blade of a motor grader can give you the position of the cutting edge of the blade in real time.

However, how the computer in the cab knows where grade is and how it gets that information deserves an explanation.

Before we proceed, though, let’s review what the 3D grade control system actually does.

In short, an on-board computer in the cab has a digital copy of the site plans stored in its memory and a GNSS receiver combined with various on-board sensors tell the computer the position of the cutting edge of the blade.

The computer then makes a determination of how far the cutting edge is from grade and sends a signal to the hydraulics to drive the blade to grade. This comparison is done several times per second, creating a real-time 3D grade control system.

When we use a 3D grade control system we are essentially bypassing the traditional process of grade staking.
Instead of giving site plans to the surveyor, who uses these to set stakes and then having the machine operator visually read those stakes, we are now putting the site plans on a screen directly in front of the machine operator.

In order for the system to have useful information to work with, the site plans have to be prepared in a format that the on-machine computer can understand.

A site model, or DTM (Digital Terrain Model) needs to be created – either from paper plans or from the engineer’s CAD (Computer Aided Design) file.

When starting with 2D printed plans (Fig 1), information on the paper will have to be manually transferred to a computer in an office environment.

Co-ordinates, along with templates and alignments must then be turned into digitised linework (Fig 2). There are a number of software packages on the market that can do this, so check with your machine control system sales representative for what he recommends.

Once in the office computer, this linework can be further transformed into a DTM (Digital Terrain Model) through a process often referred to as “tinning”. TIN stands for Triangular Irregular Network and is the description of a mathematical method of creating surfaces from a set of point

Lines are drawn between three close points to create a surface between them. This process is then repeated using adjacent points until the whole site has been turned into a 3D surface, comprised in turn of tiny, triangular surfaces (Fig 3).
On some projects the engineer can make a CAD file of the site available. If this can be obtained, a DTM can sometimes be generated more easily.

However, it is important to remember that the engineer’s CAD file shouldn’t go directly in the machine; rather it needs to be “stripped down” to only the relevant grade information.

Manufacturers of 3D grade control solutions often use proprietary formats (types of DTMs) in their on-machine computers.
This is because DTMs created for purposes other than grade control often are large and difficult to use in a real-time application.

So, in order for the system to run well, the on-machine computer will often require a specific format and one that differs from those of other manufacturers.

This is normally not an issue, as most manufacturers supply conversion software with their 3D grade control system, which will convert an industry-standard DTM to the specific format required in their on-machine computer.

Additionally, these software packages also contain a data model viewer that is very helpful in controlling the quality of the data you put into your 3D system.

A viewer allows you to look at the completed model from all angles, spin it around and zoom in and out. If there is an error in the data, it is usually easy to spot as it will show up as a “wild” grade break.

While a machine operator can learn the basics of a 3D grade control system in about a day, learning how to prepare the data models can take a bit longer.

If you are doing it in-house, you’d want to give the task to someone with CAD experience. The trick in learning how to make a good data model is knowing which pieces of information the machine wants.

When considering the purchase of a 3D system, be sure to ask your supplier about the data preparation. Your supplier should be able to give you a good understanding of the data flow as it relates to that particular system and let you know which options are available for training on how to prepare the data models.

They should also tell you if the conversion software comes with the system or if it has to be purchased separately.

The data is a very important part of your 3D grade control system. It need not be difficult, but it helps to be educated about the process so you can better implement the technology in your company.

Grade well, grade quickly.

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