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Robotic total stations and passive targets

How does it work and what are the advantages and drawbacks of this technology?

3D machine
                              control system with passive robotic total
                              station target
Available on:

Trimble GCS900
Topcon LPS-900

Caterpillar AccuGrade
Leica PowerGrade 3D
Carlson Carlson Grade

This article builds upon what we learned about the inner workings of a robotic total station in THIS  article.

-To briefly recap: A robot measures a vertical angle, a horizontal angle and the distance to a target. Both angles are measured by pointing the telescopic axis of the robot toward the target. The robot also transmits infrared light that gets reflected back from the target. From this reflection, the robot can determine the distance to the target.

The tracker's job is to drive the robot's on-board motors so that the robot is pointed toward the target at all times.

A tracker that utilizes passive targets sends out infrared light that is reflected back to the tracker. In one way, this is similar to the robot's distance meter, only that the distance meter either measures delay in pulses or phaseshift in modulated light to determine distance.

Depending on the tracking technology being used, the tracker either measures the incoming horizontal and vertical angles of the light being reflected back or creates an image of the reflection on an internal CCD camera.

Either way, the tracker generates correctional signals to the vertical and horizontal motors that turn the robot toward the target.

The target consists of one or more retro-reflective prisms and does not require a power source in order to operate, so we designate it 'passive'. They are relatively low cost.

Since the tracking function is dependent on a reflection, it is possible the robot can lock onto and follow any object capable of reflecting light back to its source. Particularly bothersome objects are reflective vests, car tail lights and stop signs to name a few.

Manufacturers work to alleviate the problem of false lock by employing different tactics, but the best strategy is never to lose lock in the first place.

To ensure lock, products are constantly being improved with faster servo motors, better tracking electronics and more intelligent tracking algorithms.

3D Machine Control system from Prolec
                              with relative avoidance zones

Topcon LPS-900 and passive target

But, when lock is lost, features such as dynamic search windows, -defining a small rectangle around the point where lock was lost, help minimize downtime to the 3D machine control system. Some systems can also take the speed and trajectory of the machine into account when deciding where to search for the target when lock has been lost.

Leica has a unique feature in their PowerGrade 3D where the user can add a 'home point'. This is a user-defined point the operator can bring the machine to after losing lock. The robot will then wait for the target to appear at this location.

Leica's PowerTracker and Topcon's LPS-900 both use passive targets exclusively.

Trimble's SPS-series Universal Total Stations can use passive or active targets for grade staking and data collection applications, but use active targets exclusively for 3D machine control applications.

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