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are the turtlebot covariance matrices right?

asked 2013-11-30 12:06:47 -0600

dan gravatar image

I am settting up covariance matrices and don't understand some of the entries used for turtlebot. from: /opt/ros/groovy/stacks/turtlebot_create/create_node/src/create_node/covariances.py

first, we have :

> ODOM_POSE_COVARIANCE = [1e-3, 0, 0, 0, 0, 0, 
>                         0, 1e-3, 0, 0, 0, 0,
>                         0, 0, 1e6, 0, 0, 0,
>                         0, 0, 0, 1e6, 0, 0,
>                         0, 0, 0, 0, 1e6, 0,
>                         0, 0, 0, 0, 0, 1e3]

Why is the covariance on z axis (last row) rotation = 1e3? This is large considering the gyro accuracy. Seems like it should be more like 1e-3.

next, we have the covariance used when the robot is stopped, so we know some parameters much more accurately:

ODOM_POSE_COVARIANCE2 = [1e-9, 0, 0, 0, 0, 0, 
                         0, 1e-3, 1e-9, 0, 0, 0,
                         0, 0, 1e6, 0, 0, 0,
                         0, 0, 0, 1e6, 0, 0,
                         0, 0, 0, 0, 1e6, 0,
                         0, 0, 0, 0, 0, 1e-9]

I don't understand the entries here for covariance y (second row). Seems like the diagonal entry should be the same as x (1e-9 instead of 1e-3) and I don't understand why there is an off-diagonal nonzero (but close, at 1e-9) entry for y-z. Are those axes coupled somehow?

Next, we have the twist ones:

ODOM_TWIST_COVARIANCE = [1e-3, 0, 0, 0, 0, 0, 
                         0, 1e-3, 0, 0, 0, 0,
                         0, 0, 1e6, 0, 0, 0,
                         0, 0, 0, 1e6, 0, 0,
                         0, 0, 0, 0, 1e6, 0,
                         0, 0, 0, 0, 0, 1e3]

Since odom twist is reported in the child (robot, typically base_footprint) frame, for a turtlebot, there is no y or z velocity, only x. So why does the y velocity covariance (second row) = 1e-3 instead of 1e6? And again, why is the z rotation (last row) covariance (1e3) so large?

Finally, for twist when stopped:

ODOM_TWIST_COVARIANCE2 = [1e-9, 0, 0, 0, 0, 0, 
                          0, 1e-3, 1e-9, 0, 0, 0,
                          0, 0, 1e6, 0, 0, 0,
                          0, 0, 0, 1e6, 0, 0,
                          0, 0, 0, 0, 1e6, 0,
                          0, 0, 0, 0, 0, 1e-9]

Again, I don't understand why the y twist covariance (2nd row) is 1e-3 instead of 1e6 and also why there is an off-diagonal entry (1e-9) y-z coupling instead of just 0 there.

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answered 2018-06-28 12:03:41 -0600

Mark Rose gravatar image

Hmmm... I don't know enough about the Turtlebot to know whether those variances make sense, but last matrix is definitely screwy for two reasons:

  1. The matrix should be symmetric (A = A'): if there is an upper-diagonal, non-zero value, it should have an equal counterpart in the lower diagonal.
  2. It seems strange that the Y variance – the diagonal entries are variances, not covariances – would be larger than the X variance when turning in place.

My guess is that the ODOM_TWIST_COVARANCE2 matrix is corrupted, and the Y variance should be 1e-9 to match the X variance, with no non-zero, off-diagonal entries.

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Asked: 2013-11-30 12:06:47 -0600

Seen: 315 times

Last updated: Jun 28 '18