[multibody] Should support joint-level dry friction / stiction?

[EricCousineau-TRI]

Is your feature request related to a problem? Please describe. Relates #6527

Per f2f w/ Ale, we most likely don't support joint-level dry friction, aside from damping (viscous friction). Seems confirmed by not seeing something relevant in current API, and per message here: https://github.com/RobotLocomotion/drake/blob/2467945ffa9b9afe3879bbbe1a975a40e5b57759/multibody/parsing/detail_urdf_parser.cc#L405-L407

Specs appear to support it:

• http://wiki.ros.org/urdf/XML/joint#line-71 - //joint/dynamics/@friction
• http://sdformat.org/spec?ver=1.9&elem=joint#dynamics_frictionhttp://sdformat.org/spec?ver=1.9&elem=joint#dynamics_friction - //joint/axis/dynamics/@friction
• https://mujoco.readthedocs.io/en/latest/XMLreference.html#body-j-joint - I think it's the //body/joint/@frictionloss attribute?

Describe the solution you'd like Would be nice to at least simple dry friction in MultibodyPlant for both continuous-time and discrete-time plants / solvers.

Ideally, where viable, it would be nice to have Stribeck static / dynamic friction (only continuous-time?).

Describe alternatives you've considered As a workaround, I can implement my own simple dry stiction model as a separate applied generalized force that I will then feed that into the MbP. If want some level of accuracy, then I can use an implicit integrator.

Additional context This is for doing basic system-id, trying to estimate stiction at the joints.

\cc @AdityaBhat-TRI

[EricCousineau-TRI]

In the FWIW category, using a sigmoid like hyperbolic tangent seems "better" than arctan, at least for making the continuous-time integrator "happy":

def arctanh(y):
    u = -(y + 1) / (y - 1)
    x = np.log(u) / 2
    return x

def regularizer_tanh(s, *, m=0.95):
    """m: Desired value for r(1)"""
    c = arctanh(m)
    value = np.tanh(c * s)
    return value

Still (slowly) doing basic sys id and very simple simulation experiments on compensation.

[RussTedrake]

fwiw -- i just ran into this, too. Specifically I was hoping to have joint stiction (via the urdf/joint friction element) in SAP, and found the same message in the urdf parser.

[EricCousineau-TRI]

From having tried out control with stiction + damping compensation on the plant, these terms do seem critical.

Is it possible to have these also as parameters in the plant? \cc @joemasterjohn

[nepfaff]

Supporting joint-level dry friction would be very useful for more accurate manipulator modelling