IMU-only configuration is not something I would recommend. It would be helpful if you could provide a sample sensor input message as well as a sample EKF output message after the robot has moved for some time, but without that information, I can only guess.

If you are just fusing linear acceleration and rotational velocity, then the double-integration of acceleration data into pose data is going to make your covariance explode. The correlation between linear and rotational variables in the state transition function will cause odd effects in that case.

IMU-only configuration is not something I would recommend. It would be helpful if you could provide a sample sensor input message as well as a sample EKF output message after the robot has moved for some time, but without that information, I can only guess.

If you are just fusing linear acceleration and rotational velocity, then the double-integration of acceleration data into pose data is going to make your covariance explode. The correlation between linear and rotational variables in the state transition function will cause odd effects in that case.

EDIT: sorry, I think I understand what you are saying better. You are expecting the robot to continue in a straight line but rotate "in place" along that line, correct?

Assuming that is correct, the kinematic model that the EKF uses boils down to the unicycle model in 2D, so if you have some fixed linear velocity and introduce a rotational velocity, I would expect the robot to move along an arc. Apologies if this is not what your point was.