VREP definition of friction

[kleinash]

Hi,

I was wondering if there was a good way to look at how to vary the parameters for materials using ODE:

Friction: a friction value. Two colliding objects will have a combined friction value of value1*value2. This does not correspond to the real friction coefficient.
Maximum contacts: the maximum number of contact points to generate. Two colliding objects will have a combined maximum contact value of (value1+value2)/2.
Soft ERP: the error reduction parameter of the contact normal, this is useful to make surfaces soft. Two colliding objects will have a combined soft ERP value of (value1+value2)/2.
Soft CFM: the constraint force mixing parameter of the contact normal, this is useful to make surfaces soft. Two colliding objects will have a combined soft CFM value of (value1+value2)/2.
Linear damping: a linear movement damping value, that adds linear drag, and that can increase stability.
Angular damping: an angular movement damping value, that adds angular drag, and that can increase stability.


The definition for ODE (http://www.ode.org/ode-latest-userguide.html#sec_3_11_1) gives two options for what that could possibly mean. So if I was to vary the friction in VREP what would be the best range of values? Do they lie between 0-1?

Same question for Soft ERP and Soft CFM.

Thx

[coppelia]

Hello,

the V-REP friction, ERP and CFM for the ODE engine is directly transmitted to the ODE engine, and has the same range a and meaning. This is (as far as I know) not limited to the 0-1 range.

Cheers

[kleinash]

Thanks, but there is a distinction on the ODE description, so I am still a bit unclear:

ODE's friction models are approximations to the friction cone, for reasons of efficiency. There are currently two approximations to chose from:

- The meaning of mu is changed so that it specifies the maximum friction (tangential) force that can be present at a contact, in either of the tangential friction directions. This is rather non physical because it is independent of the normal force, but it can be useful and it is the computationally cheapest option. Note that in this case mu is a force limit an must be chosen appropriate to the simulation.
- The friction cone is approximated by a friction pyramid aligned with the first and second friction directions . A further approximation is made: first ODE computes the normal forces assuming that all the contacts are frictionless. Then it computes the maximum limits fm for the friction (tangential) forces from

fm = mu * | fN |

and then proceeds to solve for the entire system with these fixed limits (in a manner similar to approximation 1 above). This differs from a true friction pyramid in that the "effective" mu is not quite fixed. This approximation is easier to use as mu is a unit-less ratio the same as the normal Coloumb friction coefficient, and thus can be set to a constant value around 1.0 without regard for the specific simulation.

[coppelia]

In the current V-REP, we are not exposing all ODE parameters in the GUI, so you have limited ways of modifying them. We'll work on that within 1-2 weeks, its true that it could be useful in some situations.

Cheers

[kleinash]

For a little more clarity from my side. ODE seems to have to ways to work out friction. We are just wondering which one VRep uses?