Is there any way to control the ball joint with motor?

Discussion in 'Support requests' started by Yusuke Urakami, May 17, 2019 at 3:37 PM.

  1. I want to control the ball joint in quaternion space with motor. I made xml somthing like following.

    <worldbody>
    <bodyname="robotworld" pos="1.00 -0.162 1.2" euler="0 0 0">
    <bodyname="robotwrist_rolllink" pos="0 0 0" quat="0 0 0 1">
    <jointname="wrist_ball_joint" type="ball" pos="0.01 2.7 -0.06"/>
    </body>
    </body>
    </worldbody>
    <actuator>
    <motor ctrlrange="-2.0 2.0" gear="7.0" joint="wrist_ball_joint"/>
    </actuator>

    When I run this on Mujoco2.0 simulator, I only have one control tab to change, and when I bring this tab forward, my wrist start rotating in random direction. How I can command the torque (or position) command to actually control the orientation of this joint ?
    Or is the ball joint is meant for passive behavior?


    Thanks,
     
  2. Emo Todorov

    Emo Todorov Administrator Staff Member

    The "gear" attribute for ball joint actuators is a 3D vector specifying an axis of rotation. To actually the ball joint fully, you need to attach 3 actuators to it with orthogonal axes. The simplest choice of axes is "1 0 0", "0 1 0", "0 0 1". See gear description here:

    http://www.mujoco.org/book/XMLreference.html#general