d435i realsense and robot_localization

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Hi perhaps you want to try SLAM with D435i provided by the intel realsense wiki. This will provide much more accurate odometry than you can ever get from purely IMU based localization as the IMU is not the best quality and requires double integration to get position, which will drift after a few seconds.

If for some reason you still want to use the IMU to fuse with other odometry sources you might have (wheel, lidar, etc), then you can use the imu_filter_madgwick package to fuse the Realsense gyroscope and accelerometer values to a single IMU sensor message, which will be much easier to integrate with robot_localization. The filter also does some post processing to smoothen the values, which is better for real life performance.