[ros-users] Closed kinematic chains in ros/gazebo

John Hsu johnhsu at willowgarage.com
Fri Oct 8 21:09:21 UTC 2010

yes, you're looking at the right place for the additional loop closure

On Fri, Oct 8, 2010 at 1:47 PM, Andrew Mor <abm760 at gmail.com> wrote:

> Thanks for the feedback.  I've been looking at urdf2gazebo.cpp to see how I
> can add in the last joint to the gazebo model, using, as you suggest, a
> gazebo extension to get the joint information.  Is the support already built
> into that code, or a previous version?  I have a plan on adding it, but may
> not be doing everything in the best manner possible.
> Looking through the gripper xacro file, there is this:
>  <joint:hinge name="${prefix}_r_parallel_root_joint">
>           <body1>${prefix}_r_parallel_link</body1>
>           <body2>${prefix}_palm_link</body2>
>           <anchor>${prefix}_palm_link</anchor>
>           <axis>0 0 -1</axis>
>           <damping>0.2</damping>
>           <anchorOffset>${0.07691+parallel_link_x_offset}
> ${-0.01-parallel_link_y_offset} 0</anchorOffset>
>       </joint:hinge>
> Is this the code that adds in the parallel link?  If so, I should be able
> to decipher what everything in there does.
> As far as the application, we have a pan/tilt platform, where the tilt axis
> controls one of the bars of a 4-bar linkage so that upper platform actually
> translates instead of tilts.  There is then another pan/tilt head on top of
> the first one to do the actual tilting.  But the 4-bar linkage is the one
> causing the difficulties.
> Thanks
> Andrew
> On Fri, Oct 8, 2010 at 2:39 PM, John Hsu <johnhsu at willowgarage.com> wrote:
>> Hi Andrew,
>> Can you provide some more detail about the robot you're simulating and
>> your application?
>> To model the closed loop chain in the PR2 gripper, I hid an additional
>> joint constraint in <gazebo> extension for each gripper so we are
>> effectively solving the dynamics of a closed 4-bar linkage in ODE.  The
>> implementation is far from pretty but that's what I had to do to get around
>> the urdf tree-structure requirement.  For urdf-2.0, we plan on adding graph
>> support <https://code.ros.org/trac/ros-pkg/ticket/4263>, but that's in
>> the discussion stage.
>> As for joint mimic, ODE internally does not support dynamic joint
>> mimicking right now, it's something on my todo list, but may take a while,
>> pending priority of the functionality.
>> Without a specific context for your simulation, I might suggest trying the
>> same trick I did for the PR2 gripper first by hiding the additional
>> "loop-closing" joint in gazebo extensions, and the physics engine can take
>> care of the implicit joint mimicking behavior due to closed loop
>> constraints.
>> Note that using the set_link_state service call for partial robot will
>> most likely disrupt dynamics of the entire of the system.
>> I cc'd the list in case someone else has more experiences with similar
>> problems.
>> John
>> On Fri, Oct 8, 2010 at 8:24 AM, Andrew Mor <abm760 at gmail.com> wrote:
>>> John,
>>> I was wondering if there was a straightforward way of constraining the
>>> orientation of a joint within the urdf framework?  I looked through the
>>> archives of the mailing list and the pr2_gripper code, and nothing is really
>>> popping out at me.  We have a simple 4 bar mechanism where the side bars are
>>> both the same size and the top is the same as the bottom, so the side links
>>> are always parallel as is the top to the bottom.  I was hoping to use the
>>> mimic tag from within my controller to set the joint_state_ position to be
>>> equal to the other joint, but position_ is read-only.  Is there another way
>>> to get around this without constructing a custom transmission?
>>> I had thought about calling the /gazebo/set_link_state service and that
>>> is still my backup plan before writing a transmission.
>>> Would this be better to send to the ros-users list?
>>> Thanks
>>> Andrew Mor
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