Sign of /velocity topic


currently I am playing around with the settings of the ekf on my ROSbot 2.0 (mbed firmware v0.9.0).
I noticed that the ‘msgs_conversion’ node converts the topics /mpu9250 and /pose, which are published by the ‘serial_node’, to /imu and /odom/wheel respectively.
But the twist part of the /odom/wheel topic stays zero. Wouldn’t it be useful to incorporate the /velocity topic into it, which is also published by the ‘serial_node’?

And as to my question: I noticed that the /velocity topic shows only positive values for x, regardless of forward/backward movement, when usually one would expect a negative velocity when moving back.
Is this intended behaviour or some trait of the motors/sensors? Is it changable?

Thanks and best regards.

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