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Part of the problem is that IMU data lacks separate frame_id fields for orientation, angular velocity, and linear acceleration, so we have to make some assumptions.

Let's say your IMU's frame_id was set to the same value as your EKF base_link_frame. In that event, the filter would assume that all orientations are being reported in the EKF's world_frame parameter value, e.g., odom or map. If your IMU is rotated w.r.t. the base_link frame and the data is given in a frame like imu_link, then the filter will rotate the orientation data according to that base_link->imu_link transform, and then fuse it.

If you always want the orientation data to start at 0, turn on the _relative parameter for your IMU.