[ros-users] How to express coupled joints in URDF

[Kim, Yoonsoo]

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 at 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 at 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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