Hi Yoonsoo,
Seems like a custom transmission should work here.  You wouldn't have to
specify anything additional in URDF, except adding a transmission block for
corresponding joints.  Modeling your custom transmission after
SimpleTransmission<http://www.ros.org/doc/api/pr2_mechanism_model/html/classpr2__mechanism__model_1_1SimpleTransmission.html>or
WristTransmission<http://www.ros.org/doc/api/pr2_mechanism_model/html/classpr2__mechanism__model_1_1WristTransmission.html>should
be a good starting point.
Are you planning to run on hardware or in simulation?
John

On Wed, Oct 13, 2010 at 9:44 PM, Kim, Yoonsoo <yesarang.kim@gmail.com>wrote:

> Thanks for your kind reply, John.
>
> Let me try to explain simply how joint/actuators are coupled in my robot.
>
> There 6 groups of coupled joints per a hand and all groups of 2 joints are
> driven by 2 motors and transform matrix(2x2 or 3x2) from joint
> angle/velocity/torque to motor angle/velocity/torque can be expressed in
> terms of some ratios and vice versa.
>
> How can I express this relationship in URDF? Should I implement some
> supporting classes specific to my robot similar to PR2's SimpleTransmission,
> WriteTransmission, and PR2GripperTransmission? It may be helpful to
> understand how  the PR2-specific transmission element is designed?
>
> - Yoonsoo
>
> 2010/10/14 John Hsu <johnhsu@willowgarage.com>
>
> Hi Yoonsoo,
>> URDF does not support coupled joints.  Custom transmissions should most
>> likely be able to do what you have described.   pr2_mechanism_model<http://www.ros.org/wiki/pr2_mechanism_model>has some examples and documentation on how transmissions are used in PR2.
>> How are the joint/actuators coupled in your robot?
>> Thanks,
>> John
>>
>> On Tue, Oct 12, 2010 at 11:02 PM, Kim, Yoonsoo <yesarang.kim@gmail.com>wrote:
>>
>>> Hi,
>>>
>>> My robot is composed of various coupled joints for dexterous hands, in
>>> which M joints are driven by N motors.
>>>
>>> I tried to find out how to express coupled joints in URDF but couldn't
>>> according to URDF XML reference.
>>> What I only found was PR2-specific <transmission> element. Can this
>>> <transmission> element express coupled joints and relationship between
>>> joints and actuators?
>>>
>>> If the element can't express coupled joints, any suggestion to do that
>>> will be appreciated.
>>>
>>> TIA
>>>
>>> - Yoonsoo
>>>
>>>
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>>>
>>
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