At BLUEsat UNSW, the Off-World Robotics Software Team uses the Robotics Operating System (ROS) as the basis for much of our code. The modularity, communications framework, and existing packages it provides are all factors in this, but another key benefit is its “transform” system. The transform library, which is often referred to as “tf”, provides a standardised way of managing the position of your robot[s] in a ROS system, as well as the moving components of said robots and features in the environment. This article is the first of a series that will introduce you to ROS tf.
The tf system operates by providing a tree of co-ordinate systems, called “frames”. Each frame is an independent co-ordinate system, with an origin that can move relative to its parent in the tree. A common example of this is localising a robot to its environment. To do this you would need at least two frames: one frame for your robot’s local area – lets call it “map”, and another for your robot itself – lets call it “base_link”. In order for ROS tf to localise your robot on the map we need to publish a transform between the “map” and “base_link” frames. Such a transform will often be published by a SLAM package such as hector_slam or an EKF package such as that provided in the ROS navigation packages.
For every link in the tree we need to provide ROS with a “transform”, that defines the relative position of the two frames. This is where ROS’s modularity kicks in, as you are not restricted to a single node publishing all of your transforms. Instead, as with normal ROS topics, you can have many nodes providing different transforms! This means that you could have one node publishing your robot’s position in your map co-ordinate system, and another node handling the rotation of your rover’s arm. ROS tf then puts these together for you, so that you could, for example, calculate the location for the end of your arm in the map co-ordinate system.
Transforms are time stamped which means that nodes can deal with temporary data loss or lag and do accurate mapping calculations. It also means that they can be recorded with rosbag for future playback. However the time-stamping can also create some issues, which I shall talk about later in the article.
The Justification for ROS TF
So why is this useful? Say I have a LIDAR on a gimbal, and I need to know the positions of its point cloud relative to my rover’s centre of mass. But the LIDAR only publishes a plain of point information relative to its own position. Furthermore the angle of the gimbal is controlled by a separate system, to the one publishing the LIDAR data. Sound familiar?
In a traditional solution the code that reads the LIDAR data, must be aware of the position of the gimbal and manually transform its output to its desired co-ordinate system. This means that the gimbal must know to provide that data, and your system must have a way of syncing the timing of the inputs.
In ROS all of this work is done for you: the LIDAR driver publishes a header indicating that it is in a co-ordinate system who’s parent is the top of the gimbal, and the instant the data was recorded at. The system responsible for controlling the gimbal publishes a transform, indicating its position at that instant. And any system that needs the two pieces of data in a common co-ordinate system, such as that of the base of the rover, can simply run the data through a standard filter provided by ROS to get the information it needs. The video below shows our original BLUEtongue 1.0 Mars Rover doing exactly that.
The Transform Tree
As mentioned previously ROS transforms exist in a tree structure. This has some important implications for how we can structure our graph. The first of these is that a transform can only have one parent. So ROS tf won’t solve more complex graphs. This is especially relevant if you are using something like ROS’s Joint State Publisher to publish the position of joints on your robot as ROS won’t do calculations for multi-link joints. You’ll need to do that yourself.
It also means that if one link in your tree fails you won’t be able to do transforms across that gap as there is no redundancy. However, the system is reasonably resilient. You can do a transform between any two connected points, even if the rest of the graph is not fully connected.
As well as resilience, the tf tree does offer several advantages. As it’s a tree, each node only needs to know about the two co-ordinate frames it is publishing between; dramatically reducing the complexity of any publisher. This is especially useful in a distributed system with many moving parts, or even a ROS environment with multiple robots! Furthermore if you follow ROS conventions for naming your frames you can easily combine 3rd party tools, such as one of the many ROS packages used for localisation or calculating the position of your robots joints, without having to modify them in any way.
The Timing Problem
I’d be amiss if I didn’t mention that the ROS tf system is not without its issues and difficulties. Foremost of these is ensuring that you have the transforms you need when you need them. Normally any data you want to transform will have a standard ROS header as part of its message. This header not only identifies the frame, but also the time the data was received. Lets look at our example with the LIDAR gimbal to see why this is important. In that scenario, by the time the LIDAR data is ready to publish on our ROS network, the gimbal would have moved on. However, we want to do our transforms using the position at the time the data was collected. The timestamp allows us to do this.
But, unsurprisingly, ROS only keeps a limited buffer of transforms in memory. This can cause problems if one of your transform publishers suffers from lag, or your data takes a long time to process before a transformation is applied. Usually your system will need to be able to cope with dropping frames of data, although there are other ways to handle this that I will discuss later in the series.
Well that’s it for now. I hope the first part of this guide to the ROS tf system has proven useful to you! Keep an eye on our blog for my next article, where I’ll be looking at tools and libraries provided by ROS that take advantage of the tf system. In the meantime you may want to take a look at our guide to ROS debugging, which introduces some of the tools we are going to be looking at, or if you are feeling impatient you can skip ahead and look at the official tutorials. If you are interested in learning more about the robotics we do at BLUEsat UNSW you could also take a look at the Off-World Robotics page.