Tuning Cartographer is unfortunately really difficult. The system has many parameters many of which affect each other. This tuning guide tries to explain a principled approach on concrete examples.
Cartographer can be seen as two separate, but related systems. The first one is local SLAM (sometimes also called frontend). Its job is build a locally consistent set of submaps and tie them together, but it will drift over time. Most of its options can be found in trajectory_builder_2d.lua for 2D and trajectory_builder_3d.lua for 3D.
The other system is global SLAM (sometimes called the backend). It runs in background threads and its main job is to find loop closure constraints. It does that by scan-matching scans against submaps. It also incorporates other sensor data to get a higher level view and identify the most consistent global solution. In 3D, it also tries to find the direction of gravity. Most of its options can be found in pose_graph.lua
On a higher abstraction, the job of local SLAM is to generate good submaps and the job of global SLAM is to tie them most consistently together.
Tuning local SLAM¶
At our starting configuration, we see some slipping pretty early in the bag. The backpack passed over a ramp in the Deutsches Museum which violates the 2D assumption of a flat floor. It is visible in the laser scan data that contradicting information is passed to the SLAM. But the slipping also indicates that we trust the point cloud matching too much and disregard the other sensors quite strongly. Our aim is to improve the situation through tuning.
If we only look at this particular submap, that the error is fully contained in one submap. We also see that over time, global SLAM figures out that something weird happened and partially corrects for it. The broken submap is broken forever though.
Since the problem here is slippage inside a submap, it is a local SLAM issue. So let’s turn off global SLAM to not mess with our tuning.
POSE_GRAPH.optimize_every_n_nodes = 0
Correct size of submaps¶
Local SLAM drifts over time, only loop closure can fix this drift.
Submaps must be small enough so that the drift inside them is below the resolution, so that they are locally correct.
On the other hand, they should be large enough to be being distinct for loop closure to work properly.
The size of submaps is configured through
Looking at the individual submaps for this example they already fit the two constraints rather well, so we assume this parameter is well tuned.
The choice of scan matchers¶
The idea behind local SLAM is to use sensor data of other sensors besides the range finder to predict where the next scan should be inserted into the submap.
CeresScanMatcher takes this as prior and finds the best spot where the scan match fits the submap.
It does this by interpolating the submap and sub-pixel aligning the scan.
This is fast, but cannot fix errors that are significantly larger than the resolution of the submaps.
If your sensor setup and timing is reasonable, using only the
CeresScanMatcher is usually the best choice to make.
If you do not have other sensors or you do not trust them, Cartographer also provides a
It uses an approach similar to how scans are matched against submaps in loop closure, but instead it matches against the current submap.
The best match is then used as prior for the
This scan matcher is very expensive and will essentially override any signal from other sensors but the range finder, but it is robust in feature rich environments.
Tuning the correlative scan matcher¶
In our case, the scan matcher can freely move the match forward and backwards without impacting the score.
We’d like to penalize this situation by making the scan matcher pay more for deviating from the prior that it got.
The two parameters controlling this are
The higher, the more expensive it is to move the result away from the prior, or in other words: scan matching has to generate a higher score in another position to be accepted.
For instructional purposes, let’s make deviating from the prior really expensive:
TRAJECTORY_BUILDER_2D.ceres_scan_matcher.translation_weight = 1e3
This allows the optimizer to pretty liberally overwrite the scan matcher results.
This results in poses close to the prior, but inconsistent with the depth sensor and clearly broken.
Experimenting with this value yields a better result at
Here, the scan matcher used rotation to still slightly mess up the result though.
4e2 leaves us with a reasonable result.
The default configuration and the above tuning steps are focused on quality. Only after we have achieved good quality, we can further consider special cases.
By low latency, we mean that an optimized local pose becomes available shortly after sensor input was received, usually within a second, and that global optimization has no backlog. Low latency is required for online algorithms, such as robot localization. Local SLAM, which operates in the foreground, directly affects latency. Global SLAM builds up a queue of background tasks. When global SLAM cannot keep up the queue, drift can accumulate indefinitely, so global SLAM should be tuned to work in real time.
There are many options to tune the different components for speed, and we list them ordered from the recommended, straightforward ones to the those that are more intrusive. It is recommended to only explore one option at a time, starting with the first. Configuration parameters are documented in the Cartographer documentation.
To tune global SLAM for lower latency, we reduce its computational load until is consistently keeps up with real-time input. Below this threshold, we do not reduce it further, but try to achieve the best possible quality. To reduce global SLAM latency, we can
MAP_BUILDER.num_background_threadsup to the number of cores
- for the adaptive voxel filter(s), decrease
- decrease search windows sizes,
To tune local SLAM for lower latency, we can
- for the adaptive voxel filter(s), decrease
max_range(especially if data is noisy)
Note that larger voxels will slightly increase scan matching scores as a side effect, so score thresholds should be increased accordingly.
Pure Localization in a Given Map¶
Pure localization is different from mapping. First, we expect a lower latency of both local and global SLAM. Second, global SLAM will usually find a very large number of inter constraints between the frozen trajectory that serves as a map and the current trajectory.
To tune for pure localization, we should first enable
TRAJECTORY_BUILDER.pure_localization = true and
POSE_GRAPH.optimize_every_n_nodes to receive frequent results.
With these settings, global SLAM will usually be too slow and cannot keep up.
As a next step, we strongly decrease
to compensate for the large number of constraints.
We then tune for lower latency as explained above until the system reliably works in real time.
To make sure that we did not overtune for this particular issue, we need to run the configuration against other collected data.
In this case, the new parameters did reveal slipping, for example at the beginning of
b2-2016-04-05-14-44-52.bag, so we had to lower the
This setting is worse for the case we wanted to fix, but no longer slips.
Before checking them in, we normalize all weights, since they only have relative meaning.
The result of this tuning was PR 428.
In general, always try to tune for a platform, not a particular bag.