Collision management and ODE parameters

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Hi everyone !

I am currently building a spherical robot with masses moving inside and I have a few questions about collisions management and ODE parameters for constraints.

First, is it possible in Gazebo to specify with which elements a given link should be able to collide ? The idea is that I have multiple masses moving inside the spherical robot but I want to consider collision of these masses only with the outer shell and not between each other (the masses). So each mass is a link and the shell is another one.

Another thing, I would like to have a spring behaviour when the masses collide this shell, so I wanted to tweak the ERP and CFM parameters. I have some questions with respect to these.

First, what is the difference between the ERP and CFM specified in the <world><physics><ode> of the sdf file and the ones specified in the <link> under <soft_cfm> and <soft_erp> tags.

Second, what is the meaning of of <kp> and <kd> tags in the <collision><contact> properties of a link, since, if I understand the ODE documentation, kd and kp are fully described by CFM and ERP parameters (so it seems to be redundant here).

And last, what is the difference between describing these cfm and erp parameters for a spring behaviour compared to the <surface><bounce> options (I'm not sure how bounce acts here).

Thanks in advance for your enlightenments.

Best regards.