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Robotic total stations and active targets

What is an active 3D grade control target and could you benefit from using one?

3D grade
                              control system with active robotic total
                              station target
Available on:

Trimble GCS900

Topcon LPS-900
Caterpillar AccuGrade

Leica PowerGrade 3D

Carlson Carlson Grade

As we explained in the article about robotic total stations, the robot provides the 3D grade control system with a 3D position of the target several times per second. The job of the robot's tracker is to always keep the robot pointed toward the target.

It is beneficial to learn about how passive targets work before proceeding to read this article.

An active target serves two functions: It reflects light back to the robot's distance meter so that the distance between the target and the robot can be determined, and it emits infrared light that the robot's tracker uses to align itself to.

The latter is done by determining the horizontal and vertical angles of the incoming light and then driving the robot's motors until these angles are nil, -meaning the telescopic axis is pointed straight at the target.

Alternatively, an internal CCD camera can be used to create an image of the target based on the light emitted and then determine the position of this image relative to the optical center of the robot.

The purpose is the same: To generate a signal to the motors, driving them to align the robot toward the target.

As an interesting side note, the 3rd generation of 3D grade control robots with active targets actually have the capability of using both passive and active targets.

While used in grade control applications, only active targets are used. But, when used for grade checking and stake-out, the robot can track passive targets or active targets.


Active target on a grader and a Trimble robotic total station

In these application, however, the active portion of the target serves as an identifier only and the actual tracking is done to the prism.

These active targets require power, necessitating cabling to the target that is not needed when using passive targets. Additionally, active targets cost significantly more than passive targets and are arguably less robust simply by the nature of their construction and the fact that they must have electronics inside and connectors outside.

Should you desire an active backsight, this must also be powered, -usually with batteries.

Active targets have two advantages over passive targets, however. The active target emits a target ID, ensuring that the robot only locks onto this target. This means several machines can be working in close proximity without their respective robots ever getting confused about which target to lock onto and track. Currently, Trimble offers 16 separate IDs on the machine targets for their SPS-series Universal Total Station.

Additionally, use of active targets makes false target lock much less likely. Because the system does not rely upon a reflection only, it becomes much easier to distinguish the target from its surroundings. This helps reduce downtime for the 3D grade control system.

Trimble is currently the only manufacturer offering active target technology.

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