In machine control we distinguish between 2D and a 3D systems:
A 2D System is based on slope, laser or ultrasonic sensor. Therefore it allows us to easily establish surfaces with up to 2 slopes. If the slope i supposed to changes we have to change the target value of the sensor.
A 3D system on the other hand, uses the position as well as height information. In the combination with a Digital Terrain model the slope can change according to the position, without the need to manually change any setting.
There are two different ways to determine the position of the working tool of the machine: GNSS based systems and Total Stations. The following blog should give an overview about the idea behind a Total Station.
A total station measures three parameters to calculate the 3D position of a target:
|1. Vertical Angle||2. Horizontal Angle||3. Distance|
Combining the measurements of the two angles with the distance we can calculate the 3D coordinates of the target. The target can be either a surface, like the wall of a house (reflectorless) or a prism (reflector). In machine control we use a prism mounted on a mast attached to the blade.Using an on-board tracking algorithm, the total station can follow the prism as the blade moves and continuously measure the two angles and the distance. With the given height of the mast we can measure the height of the blade as well as its position.
By transmitting the position information to the machine we can utilize the measurements to calculate the deviation from the blade to the Digital terrain Model. Communicating the deviations to the 2D part of the control system we can automatically steer the blade up and down. Adding a slope sensor we can as well steer the slope of the blade.
Compared to a GNSS System, using a Total Station gives a higher accuracy. While GNSS systems allow us to measure in cm-level, a total station will give us millimeter accuracy.
We need a line of sight between the total station and the prism, this can be an advantage in areas where there is insufficient or no GNSS signal (Tunnels, under trees, etc.). On the other hand, this could also be a disadvantage, making it necessary to move the station during the building process.Another point we need to consider when using a total station is that the accuracy depends on the distance between the station and the target. Typically a distance within 400m is the maximum to achieve accuracies good enough for fine grading.