Contact model

Discussion in 'Priority support' started by George Zhao, Feb 16, 2019.

  1. While simulating the walking of humanoid robots, the foot would always bounce back when it touches ground. And the contact forces that Mujoco returns to us are thus difficult to use. Is there anyway, or any material, such as materials like rubber, that we can apply to the bottom of the foot so as to eliminate the bounces?
     
  2. Emo Todorov

    Emo Todorov Administrator Staff Member

    The contact model has many parameters that can be adjusted. In your case, it sounds like you need more damping. This can be done by setting solref, for example:

    <geom solref="0.02 1.2" .../>

    The first number is the time constant, the second number is the damping ratio. If you increase the damping ratio above 1 (which is the default) that will make contacts more damped. See the documentation for more details:

    http://www.mujoco.org/book/modeling.html#CSolver
     
  3. I have tried to increase the damping ration, resulting the contact force and position in z axis of the left foot as following:
    [​IMG] [​IMG]
    it seems that, the response the contact force Fz getting more stiffness, but the bouncing back still existing. Is there any other paresmeter settings that can be applied to make the contact impacting energy totally absorbed.
     
    Last edited: Feb 17, 2019
  4. Emo Todorov

    Emo Todorov Administrator Staff Member

    you can also play with solimp. increasing the first two values makes constraints stronger. smaller time steps can also help.

    but in general, mujoco uses a continuous-time formulation of physics. if you are looking for infinite contact force in the limit of timestep going to zero, that cannot happen for any parameter settings.