[ros-users] Flexible & remote transmission

Thu Dec 16 00:14:25 UTC 2010

I went ahead and checked it into the sandbox:
https://code.ros.org/svn/wg-ros-pkg/branches/trunk_cturtle/sandbox/pr2_spring_transmission_example
John

On Wed, Dec 15, 2010 at 4:08 PM, John Hsu <johnhsu at willowgarage.com> wrote:

> Hi Jeroen,
>
> Thanks for the updated description, though I think I might understand the
> gist of the problem, can you attach a diagram as well?
>
> To briefly describe what I think you are modeling here, the motor output
> has it's own degree of freedom and an encoder attached to the motor output
> position, independent of the leg joint's degree of freedom.   In addition,
> the motor dof and the leg joint dof are coupled by a spring-like link
> mechanism?
>
> One way to do this is to model the spring dynamics in transmission.  To do
> this, you can model the two joints independently, and specify a transmission
> that applies the right amount of torque on the leg joint based on joint
> position differences between motor output joint and leg joint.  In the
> attached package, I created a simple urdf with two joints
> (motor_output_joint and leg_joint).  leg_joint is connected to
> motor_output_joint via something called a SpringTransmission.  Basically,
> motor_output_joint can potentially be controlled by one of the mechanism
> controllers (e.g. Joint Effort Controller<http://www.ros.org/wiki/robot_mechanism_controllers/JointEffortController>,
> Joint Velocity Controller or
> <http://www.ros.org/wiki/robot_mechanism_controllers/JointVelocityController>Joint
> Position Controller<http://www.ros.org/wiki/robot_mechanism_controllers/JointPositionController>)
> while the position difference between motor_output_joint and leg_joint
> dictates the amount of torque to be applied to the leg_joint.
> Does that sound about right?
>
> You can see the spring joint in action by:
>
> roslaunch pr2_spring_transmission_example spring_link.launch
>
> and in a separate terminal:
>
> rostopic pub /test_controller/command std_msgs/Float64 0.5
>
> John
>
>
>
>
> On Wed, Dec 15, 2010 at 4:02 AM, Jeroen Willems <jjpa.willems at gmail.com>wrote:
>
>> To problem can be simplified to a motor which is connected through a ~20cm
>> axis with a joint. Both, the motor and joint, are having position encoders.
>> We assume that the main dynamics of the axis (the transmission) can be
>> modeled as a linear spring ( T_spring = k * (theta_joint - theta_motor) ).
>>
>> This problem is probably also due the fact I'm have problems understanding
>> the mechnism_model. I'm used to that the controller is getting a joint
>> error, it outputs a control signal to the robot (torque applied to the motor
>> for instance). So the mechanism_model is somewhat confusing to me because
>> you put the effort directly on the joints and then via a transmission the
>> effort on the actuator is determined. If I get it correctly.
>>
>> I'm using an hexapod so imagine that if you eventually want to jump, you
>> want to use the kinetic energy of the springs.  When landing you also want
>> to use the
>> springs to reduce the impact on the robot by obtaining a part of the
>> energy.
>>
>> Jeroen
>>
>>
>>
>> On Tue, Dec 14, 2010 at 23:39, Stuart Glaser <sglaser at willowgarage.com>wrote:
>>
>>> Hi Jeroen,
>>>
>>> Could you describe what you're trying to model in more detail?  In
>>> particular, where are the encoders, motors, springs, gears, and joints
>>> in relation to each other?
>>>
>>> -Stu
>>>
>>> On Tue, Dec 14, 2010 at 4:37 AM, Jeroen Willems <jjpa.willems at gmail.com>
>>> wrote:
>>> > Okay I'm looking into the flexibility between the actuator and the
>>> joint by
>>> > adding a custom transmission but I'm having some difficulties.
>>> >
>>> > I've looked into the existing transmissions and those are pretty
>>> > straightforward (except for the pr2_gripper).
>>> > I want to add the (linear, rotational) flexibility, I will neglect the
>>> > damping for simplicity.
>>> > I'm used to use the equations of motion:
>>> > Load:  Jl theta_L_ddot + k ( theta_L - theta_M ) = 0
>>> > Motor: Jm theta_M_ddot - k ( theta_L - theta_M ) = u
>>> >
>>> > I don't get it how to implement this in the transmission. I was
>>> thinking of
>>> > a little bit of rewriting as in the attachment but it did not influence
>>> > anything at all.
>>> > Would be great if someone has an idea, solution or suggestion on how
>>> > implementing this flexibility.
>>> >
>>> > Thanks in advance
>>> >
>>> > Jeroen
>>> >
>>> > On Sat, Nov 20, 2010 at 21:07, Stuart Glaser <sglaser at willowgarage.com
>>> >
>>> > wrote:
>>> >>
>>> >> Hi Joeren,
>>> >>
>>> >> For both of these, the solution is to write a transmission.  Both
>>> >> issues require a specific translation between actuators and joints,
>>> >> and the best way to encode that translation is in a transmission.
>>> >>
>>> >> -Stu
>>> >>
>>> >> On Fri, Nov 19, 2010 at 1:36 AM, Jeroen Willems <
>>> jjpa.willems at gmail.com>
>>> >> wrote:
>>> >> > Hi All,
>>> >> >
>>> >> > I'm currently modeling a hexapod in Gazebo but I'm currently having
>>> some
>>> >> > difficulties.
>>> >> >
>>> >> > I have basically 2 questions, I hope that this is not a problem for
>>> one
>>> >> > post
>>> >> > but both have to do with the transmission:
>>> >> >
>>> >> > 1) Between the actuators and the joints are torsion springs. I
>>> haven't
>>> >> > found
>>> >> > something that incorporates flexibility between actuators and
>>> joints.
>>> >> > Is there some solution or workaround to obtain this or should I try
>>> an
>>> >> > alternative solution like writing a custom transmission?
>>> >> >
>>> >> > 2) As you can imagine I have 6 legs consisting of three links,
>>> basically
>>> >> > an
>>> >> > elbow manipulator.
>>> >> > The last two links can be regarded as in a 2D-plane. The third link
>>> is
>>> >> > remotely driven.
>>> >> > This means that when rotating the second link the third link does
>>> not
>>> >> > change
>>> >> > the rotation with respect to the body.
>>> >> > So in a 'standard' situation the third link rotates with the second
>>> link
>>> >> > so
>>> >> > the angle between those two does not change.
>>> >> > I hope this is a bit clear.
>>> >> > So again is there some solution or workaround?
>>> >> >
>>> >> > Thanks!
>>> >> >
>>> >> > Jeroen
>>> >> >
>>> >> > _______________________________________________
>>> >> > ros-users mailing list
>>> >> > ros-users at code.ros.org
>>> >> > https://code.ros.org/mailman/listinfo/ros-users
>>> >> >
>>> >> >
>>> >>
>>> >>
>>> >>
>>> >> --
>>> >> Stuart Glaser
>>> >> sglaser -at- willowgarage -dot- com
>>> >> www.willowgarage.com
>>> >> _______________________________________________
>>> >> ros-users mailing list
>>> >> ros-users at code.ros.org
>>> >> https://code.ros.org/mailman/listinfo/ros-users
>>> >
>>> >
>>> > _______________________________________________
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>>> >
>>> >
>>>
>>>
>>>
>>> --
>>> Stuart Glaser
>>> sglaser -at- willowgarage -dot- com
>>> www.willowgarage.com
>>> _______________________________________________
>>> ros-users mailing list
>>> ros-users at code.ros.org
>>> https://code.ros.org/mailman/listinfo/ros-users
>>>
>>
>>
>
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