how to add actuator to free joint？

[LHTLiu]

I create a free joint , however i don't know how to add actuator for free joint,

Thanks for your help. @yuvaltassa

[yuvaltassa]

This should be an issue, not a discussion.

What kind of actuator do you want? You probably want to use site transmission which lets you model rockets and also (unphysical) abstract Cartesian actuators.

[LHTLiu]

My robot is a mobile manipulator, and I want to set up a free joint type of joint for the base, that is, they are not controlled by wheels, but as a whole. I need to ask how to set up the actuator in a free joint. Is there any problem with my way of thinking?

    <!-- 平移自由度驱动器 -->
    <motor name="motor_tx" joint="base_link_free_joint" axis="1 0 0" gear="1"/>
    <motor name="motor_ty" joint="base_link_free_joint" axis="0 1 0" gear="1"/>
    <motor name="motor_tz" joint="base_link_free_joint" axis="0 0 1" gear="1"/>
    
    <!-- 旋转自由度驱动器 -->
    <motor name="motor_rx" joint="base_link_free_joint" axis="1 0 0" gear="1" ctrllimited="true" ctrlrange="-1 1"/>
    <motor name="motor_ry" joint="base_link_free_joint" axis="0 1 0" gear="1" ctrllimited="true" ctrlrange="-1 1"/>
    <motor name="motor_rz" joint="base_link_free_joint" axis="0 0 1" gear="1" ctrllimited="true" ctrlrange="-1 1"/>

but there is an error

I want to know how to add actuator to a free joint. How should I change my previous code.

Thanks~

[yuvaltassa]

replace axis="a b c" gear="1" → gear="a b c" note that this will give you forces in the global frame. For local frame use site transmission. Also note that for rotations you want (e.g. around the z axis) gear="0 0 0 0 0 1"

[LHTLiu]

replace axis="a b c" gear="1" → gear="a b c" note that this will give you forces in the global frame. For local frame use site transmission. Also note that for rotations you want (e.g. around the z axis) gear="0 0 0 0 0 1"

thanks for your suggest, this does work，Is my code below correct?

  <actuator>

    <motor name="motor_tx" joint="base_link_free_joint" gear="1 0 0 0 0 0"/>
    <motor name="motor_ty" joint="base_link_free_joint" gear="0 1 0 0 0 0"/>
    <motor name="motor_tz" joint="base_link_free_joint" gear="0 0 1 0 0 0"/>
    
    <motor name="motor_rx" joint="base_link_free_joint" gear="0 0 0 1 0 0" ctrllimited="true" ctrlrange="-1 1"/>
    <motor name="motor_ry" joint="base_link_free_joint" gear="0 0 0 0 1 0" ctrllimited="true" ctrlrange="-1 1"/>
    <motor name="motor_rz" joint="base_link_free_joint" gear="0 0 0 0 0 1" ctrllimited="true" ctrlrange="-1 1"/>

    ......

  </actuator>

Why is there no free joint in the Joint on the right side of the software? Why does the robot not move when I drag the 6 actuators I defined in Control?

thanks @yuvaltassa

[yuvaltassa]

Free joints don't have sliders because of the quaternion.
Try increasing the gear above 1 to 10 or 100