# Robot Localization: Mirrored Results

Hey there. I want to use robot_localization to fuse my sensor data from optical encoders with the onboard IMU (a BNO-055). I've looked through most of the documentation around robot_localization, but adapting from the examples seems to give me funny results. I'm currently only interested in local odometry (the odom -> base_link transform).

First, let me explain the setup of the robot. The robot is a simple differential driven robot with two motorized wheels at the back and a caster wheel in the front. The motors are hooked up to optical encoders, the data of which is sent to diff_drive. diff_drive computes odometry by integrating the difference in encoders, as is common with encoder-based odometry. Judging from RViz, this odometry is fairly accurate by itself (no weird drifts, 180 degree turns line up). I haven't confirmed if the actual distances line up, but the scale does at least.

As mentioned, an IMU is also present. This is the BNO-055, with this node as a ROS integration. If I position the robot such that the front of the robot is pointing to the magnetic north, the X axis of the IMU points west, while the Y axis points south. Since robot_localization expects IMU data in an ENU frame (as per REP-103), I've set up TF to rotate the imu frame by 180 degrees. I _think_ this is correct, but let me know if I should be manually editing IMU data or rotating the frame differently (or not at all!).

The TF frames are the typical frames: odom -> base_link -> imu. The imu transform is a simple static publisher:  rosrun tf2_ros static_transform_publisher 0 0 0 3.14 0 0 base_link imu 

Given this above setup, I've created the following config for robot_localization:

frequency: 10

# We're a 2D robot.
two_d_mode: true

# Set up frames.
odom_frame: odom
world_frame: odom
publish_tf: true

# Our odometry is on /odom. We only track the x and theta velocity, since other values are derived from those.
# We cannot move in the y direction, but the documentation recommends to add the value so that it is integrated.
odom0: /odom
odom0_config: [false, false, false,
false, false, false,
true, true, false,
false, false, true,
false, false, false]
odom0_differential: true

# Our IMU is on /imu/data. We track all of it.
imu0: /imu/data
imu0_config: [false, false, false,
true,  true,  true,
false, false, false,
true,  true,  true,
true, true, true]
imu0_differential: false
imu0_relative: true


As far as I understand, this should be correct for the setup. However, I get very interesting results.

With the 180deg imu rotation, I get the following (green is the fused odometry, red is the raw encoder odometry):

If I leave out the rotation, simply attaching imu to base_link directly, I get a nearly identical result:

As you can see, in both images the output from robot_localization (green) is mirrored/rotated with respect to the raw odometry (red). I was expecting the green odometry to largely overlay the red odometry ...

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I just stumbled onto this question which has a similar setup to mine. Tom recommends to only integrate the IMU yaw values in that answer.

With that, the config becomes:

imu0_config: [false, false, false,
false, false, true,
false, false, false,
false, false, true,
false,  false, false]


If I do that, the drifting issues of the localization disappear. However, the fused odometry still seems to be mirrored or rotated with respect to the original data.

Here's a screenshot of how it looks when you only fuse the yaw data of the IMU.

( 2019-12-17 05:01:58 -0500 )edit

Please attach your images directly to the question. I've given you sufficient karma for that.

( 2019-12-17 09:20:02 -0500 )edit

Thanks for the images and config information

Is your imu really providing absolute yaw information?

It kind of looks like the imu and odom are providing opposite direction yaw change. For a clock-wise rotation of the robot, are both odom and imu reporting a positive change in yaw?

( 2019-12-17 15:47:19 -0500 )edit

Please attach your images directly to the question. I've given you sufficient karma for that.

( 2019-12-18 13:45:15 -0500 )edit

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@johnconn's comment lead me to the answer: Turns out that my encoder and IMU had different thoughts about which rotation direction would lead to an increase in yaw. If anyone else sees weird rotating/mirroring patterns, consider checking if both go in the same direction.

more

To add to this: ROS uses a righthanded system, so positive (and negative) rotation directions are unambiguously defined. See also REP 103: Standard Units of Measure and Coordinate Conventions - Rotation Representation.

( 2019-12-18 13:44:30 -0500 )edit

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Last updated: Dec 18 '19