vel_steering_offset might be wrong

[facontidavide]

Hi, I was taking a look at this code:

https://github.com/ros-controls/ros_controllers/blob/melodic-devel/four_wheel_steering_controller/src/four_wheel_steering_controller.cpp#L386-L398

And I am puzzled by this line:

double vel_steering_offset = (curr_cmd_twist.ang*wheel_steering_y_offset_)/wheel_radius_;

I am quite positive that this is wrong, since you should use the velocity of the steering angle, instead of curr_cmd_twist.ang.

As a reference, the 4ws seems to be conceptually correct

double angular_speed_cmd = curr_cmd_4ws.lin * (tan(curr_cmd_4ws.front_steering)-tan(curr_cmd_4ws.rear_steering))/wheel_base_;
double vel_steering_offset = (angular_speed_cmd*wheel_steering_y_offset_)/wheel_radius_;

Am I missing something?

[bmagyar]

FYI @vincentrou

[vincentrou]

I am not sure to understand your concern @facontidavide . Both vel_steering_offset are based on current command (one from the twist and the other from the 4ws). By the way thanks for the refactoring.

[facontidavide]

mathematically speaking what is the purpose of vel_steering_offset?

If it is to compensate for the rotation of the Z axis and the corresponding translation at the wheel level, the the former formula is wrong and the latter is correct.

Think about the case in which curr_cmd_twist.ang is a constant: vel_steering_offset should be probably proportional to the gradient of that value, not the value itself.

Both vel_steering_offset are based on current command

This doesn't mean it is correct.