MATLAB: Modeling friction in translational mechanical system

Question

I am trying to add friction to a model. I am modeling a cart that is pushed by a translational source. I do have data from real measurements and I know how much friction there is and what I can expect in Torque increment on the rotational side of the system. This is the part of the model where I want to add friction:

I want to add a translational friction. But when I insert in between the mass and the itPulleyBelt though (like in the example content ssc_mechanical_system_translational_friction, but then with the mass spring removed, below you see the resulting model). Then I get very strange results which do not match my real life data….

If I add the friction in a seperate branch like in the below picture, then I do get the expected results.

Can someone explain to me why I do not get the expected results in the second example?

Answer 1

You should think of Simscape mechanical blocks as operating on the relative position/velocity between the R and C ports.

In the first case, you're saying there is friction proportional to the velocity of the belt relative to the mass rigidly attached to the belt... so it probably asks similarly to having constant static friction force applied at zero velocity.

In the second case, you're saying the friction is proportional to the relative velocity of the mass and the ground (reference)... or simply the absolute velocity of the mass.

- Sebastian