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Localization System

Modified 2019-06-28 by Rohit Suri

A fully operational Duckietown

A working localization system to localize all Autobots inside the Autolab

The first step is to build the Watchtowers

The localization system is an important part of the Autolab, as it gives the poses of all Autobots in the city to a server. In the Autolab, the Autobots are Duckiebots that have been improved to Autolab specifications (see Unit B-1 - BUILDING - Autobot specifications).

Watchtowers are an experimental feature of Duckietown, which are currently only used in Autolabs.

Modified 2019-06-28 by Rohit Suri

The localization system serves multiple purposes:

  • Automating the Autolab completely:
  • It allows the central system to discover if a Autobot moves out of the track
  • It allows more high level fleet control
  • It allows the grading of the embodied challenges for AIDO

The localization system mainly relies on AprilTags, which are a conceptually and visually similar to QR codes. Each Autobot is provided with a mounting plate with an AprilTag on top. By tracking these mounted AprilTags, the system evaluates the Autobot poses by solving an optimization problem.

To track the AprilTags, we use Watchtowers. In the spirit of Duckietown, a Watchtower uses most of the same hardware and software as a Duckiebot. A Watchtower is essentially a little tower which is about 60cm in height, and has a pi-camera at the top to look over a part of the city. The Watchtowers are spread all over the city, and by combining the field of view of each tower, it is possible to cover the whole Autolab.

Apart from AprilTags on Autobots, there are other AprilTags that are on the ground, called Ground AprilTags. The precise location for each Ground AprilTag is known in advance. We build a pose graph of all the relative poses bewteen Watchtowers and Autobots, and what they see. By running optimization on the graph, we merge the local influx of data from all agents into a global position graph of all agents, using the Ground AprilTags as global fixed references.

There are two structural elements required to have a working system:

  • The Watchtowers
  • The ground AprilTags

The localization system is designed such that Watchtowers don’t need to be at a specific height or position as long as they can view “sufficient” area of the Autolab. (TODO: specify what “sufficient” means) However, we still provide the specs of the Watchtowers so that you can produce your own Watchtowers. See hardware part in chapter Unit D-1 - BUILDING - Watchtower Hardware Preparation.

Moreover, the ground AprilTags need to follow conventions specified in the chapter Unit D-2 - BUILDING - Apriltags specifications.

Localization can be run either online or offline.

Running localization online means that the data is processed during the experiment and the results can be vizualised with some delay. The demo is found at Unit D-4 - DEMO - Localization online

Running localization offline means that the data recorded but processed only after the experiment. The demo is found at Unit D-3 - DEMO - Localization offline

While the long term objective is to only do online localization, the offline localization has proved very useful for AIDO, because it requires less computing power and does not have the network bandwidth as a bottleneck.

The software is explained in detailed in chapter Unit D-6 - SOFTWARE - Localization software

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