Position Tracking: Total Station
by David Kohanbash on June 13, 2014
Tracking the position of a robot is difficult to do accurately, especially when you need to track position in a variety of locations. GPS is not accurate for small movements and can have jumps as the position estimate changes. One good solution is the Leica Total Station. The Total Station can measure a robots position with an accuracy of under 1cm! The downside is that it is expensive, the Total Station can easily be $35K based on your exact settings. You can find used systems on eBay for under $10K. However if you need to accurately have an external reference of your position this is the tool for you. You can even use the Total Station to investigate crash scenes! (I recently found out that police departments use these to measure car crashes scenes for recreating the accident.)
Using the Total Station you can track a robots position at a distance of several hundred meters. There are several components to the Total Station system:
1. Robotic Total Station – This is the heart of the system that allows you to track and log the position of a target. You need the robotic version so you can automatically track a moving object. There are cases where you are surveying static points and the robotic features are not needed. Also when purchasing a robotic Total Station make sure the unit includes the software (license keys) for using the tracking features.
2. Prism – There are 2 primary types of “targets” that can be used for tracking your robot. There are stickers that you can put on a surface, or you can get a 360 degree prism to be tracked that mounts to a rod on your robot.
360 degree prism for use as a Total Station target
3. Tripod – You need a really stable tripod for the Total Station. Typically you will use a sturdy surveying tripod with metal spikes on the end that can be dug into the ground. It is important that the legs do not move at all when using the total station.
4. Tri-Bracket – Part of setting up the Total Station is making sure it is almost perfectly level. In order to do this you want a “tri-brac” that has fine controls for leveling the device. Another benefit of using a tri-brac is for easy attachment of the total station to the tripod.
5. USB cable and/or SD-card – By default the Total Station logs to an SD-card. However it can also publish data via the USB port (using a bit of a complex software protocol) that lets you log data on your computer. Logging via USB is often better as it is easier to synchronize the time to the rest of the data.
Total Station attached to tripod with a tri-brac. The bright spot in the back is the prism mounted to the robot. Image source: is here
Some random things
One big gotcha is that the total station cannot really track fast-moving objects (>0.5m or so, I am partially making up this number). Often the Total Station will try to track something that is moving fast and will return the correct angular data to the target but the range will not update or will be 0.
If something moves too fast then the Total Station can lose its lock on the prism and not be able to return the position of the prism. When this happens the Total Station will usually start to beep slowly. The newer versions will try to restore the lock on the prism by going into a search pattern to try recovering the prisms positions.
When using the sensor it is important to maintain a clear line of sight between the Total Station and the prism. If anything (or anyone) gets in the way (even briefly) the Total Station can lose its lock on the prism.
In older units you need to put the Total Station into a USB data mode to get data via the USB cable, while connected you are not able to access any other settings on the device. In the newer versions you can have the USB data mode running in the background allowing you to still access the device.
Main image and prism from: http://www.leica-geosystems.us
