Error in kinematic model description

Benchmark & Challenges
1

Hi,

I just wanted to note that the description of the kinematic vehicle model is slightly incorrect.
It is true, that the kinematic vehicle model does not consider the tire slip, this is what differentiates it from the dynamical one. However that is not true that Beta is zero.
A comparison should be made based on for which point of the model the states are expressed. The car has got an angular velocity around the rear axis of the vehicle (In the kinematic model). So, if the states are expressed for the rear axis of the vehicle, it is true that that specific point only has a velocity component parallel to the vehicle. However if it is expressed in the CoG, as it is depicted in the figures, the velocity has got a perpendicular component as well and hence Beta is not equal to zero.
As I have seen, the state equations of the vehicle model are written for the rear axis. So yes, it is true that at that point there is no perpendicular component and hence Beta is zero. However, all the figures are drawn as if the states were expressed in the CoG, where it is not true.

The tire slip has nothing to do with angle Beta. The tireslip could be depicted at the wheels in a manner, that the velocity of an individual wheel does not point exactly in the direction of the wheel. (It is usually denoted by alpha_f and alpha_r for the front and rear wheels respectively.)

If after all the vehicle model is implemented correctly in the code, so the calculated states are considered as the state at the rear axis and not in the CoG than it is only a minor mistake. However, both in the vehicle model documentation and in the introduction to commonRoad-io pdf it is written incorrectly which might be confusing for someone.

Best regards,
Feri

2

Hi Feri,
Thanks for your post. We will look into it : )

3

Hello Feri,

you are right, the description was not clear on that. Prof. Althoff updated the documentation and clarified that the reference point of the model is the center of the rear axle.

Best, Moritz

4

Hi,

it is now clear that the positions s_x and s_y refer to the centre of the rear axle. I would like to know if that is also the case for the single track (dynamic) and the multi-body model, because the figure says otherwise.
If however the positions refer to the centre of mass, speaking of “state variables shared with the kinematic single-track model” (Vehicle models documentation p. 7) would be confusing to me, and I don’t know if the switch to the KS model at low speeds is really justified, because it would suddenly change the physical meaning of the first two state variables.
I now need to know to which point of the vehicle exactly the position state variables refer to, in order to interpret them correctly and create correct animations to illustrate results.

Best regards,
Tristan

5

Hi Tristan,

thank you for your post! The dynamic single-track and the multi-body model use the center of mass as the reference point.
We are currently revising the vehicle models (and the documentation) in CommonRoad and are also looking into the issue you addressed when switching the model at low speeds. We’ll get back to you once we have taken a closer look at that.

Best regards,
Gerald