[ros-users] Control system design question.

Stuart Glaser sglaser at willowgarage.com
Fri Sep 17 22:06:37 UTC 2010


If the robot reaches the end of the trajectories, it will hold at the
final point.  If you want to feed trajectories continuously, then you
must feed them quickly enough or make the pieces long enough so that
the robot never runs out of trajectory.

-Stu

On Fri, Sep 17, 2010 at 2:52 PM, Konrad Banachowicz <konradb3 at gmail.com> wrote:
> I have an other concern, what will happen if processing of trajectory take
> more time than remaining to first point of new trajectory.
>
> Pozdrawiam
> Konrad Banachowicz
>
>
> 2010/9/17 Stuart Glaser <sglaser at willowgarage.com>
>>
>> Hi,
>>
>> On Fri, Sep 17, 2010 at 12:53 PM, Konrad Banachowicz <konradb3 at gmail.com>
>> wrote:
>> > yes RT and non-RT work execute in separated threads but what about
>> > shared
>> > data ? as far as i know there is some locking involved ?
>>
>> The buffer used in the JointSplineTrajectoryController is called a
>> RealtimeBox.  It currently uses a mutex (locked for a guaranteed
>> maximum number of instructions on a system that supports priority
>> inheritance), though it can certainly switch to a lock-free buffer.
>>
>> > Communication between RT and non-RT threads is complex problem, but
>> > OROCOS
>> > solves most problems. it use lock-free buffers for communication between
>> > components. Currently in ROS joint_trajectory_action is whole non-RT and
>> > JointSplineTrajectoryControlle is partialy RT. In my solution all non-RT
>> > work are moved into joint_trajectory_action. This make clear border
>> > between
>> > RT and non-RT part.
>>
>> Sounds like you've moved the non-realtime parts of the
>> JointSplineTrajectoryController into your version of the
>> joint_trajectory_action.  That sounds like a reasonable solution.
>>
>> -Stu
>>
>> >
>> > Pozdrawiam
>> > Konrad Banachowicz
>> >
>> >
>> > 2010/9/17 Stuart Glaser <sglaser at willowgarage.com>
>> >>
>> >> Hi Konrad,
>> >>
>> >> There are no issues with breaking realtime constraints because the
>> >> unsafe code is called from a non-realtime thread.
>> >>
>> >> You're correct that the JointSplineTrajectoryController code performs
>> >> several operation which are not realtime safe.  These operations are
>> >> performed not in the realtime loop but rather in the ROS subscriber
>> >> callback, which is called from a non-realtime thread.
>> >>
>> >> Your proposed solution is therefore quite similar to what already
>> >> exists: the non-realtime portion of the
>> >> JointSplineTrajectoryController reorders the joints and composes
>> >> trajectories, and then sends it to the realtime portion which just
>> >> performs interpolation.
>> >>
>> >> There are a few faults with this system, and I'm interested in
>> >> discussing solutions which address them:
>> >> 1. Trajectories cannot be sent from within realtime--they only come
>> >> from outside of realtime.
>> >> 2. The current design has a great deal of complexity, particularly in
>> >> taking care when transferring the trajectory to realtime, composing
>> >> trajectories, and responding with a result to the action.
>> >>
>> >> There's probably more.
>> >>
>> >> That said, this interface allows us to comfortably separate the motion
>> >> planning pipeline from the realtime loop.  Once implemented,  you can
>> >> run much of the code that was developed to move the PR2 on your own
>> >> robot.
>> >>
>> >> Lastly, you may want to consider examining the
>> >> JointTrajectoryActionController which combines the
>> >> joint_trajectory_action and the JointSplineTrajectoryController.
>> >>
>> >> -Stu
>> >>
>> >> On Fri, Sep 17, 2010 at 3:12 AM, Konrad Banachowicz
>> >> <konradb3 at gmail.com>
>> >> wrote:
>> >> > Hi,
>> >> > I'm working on integration of orocos RT-controller with ROS
>> >> > manipulation
>> >> > pipeline.
>> >> > I already done joint_state_publisher and oro_action_server. Next on
>> >> > my
>> >> > list
>> >> > are joint_trajectory_action and JointSplineTrajectoryController.
>> >> > I have some concerns about real-time behaviours of those, and way of
>> >> > implementation in orocos.
>> >> >
>> >> > joint_trajectory_action :
>> >> > It receive goal containing trajectory (variable size, unbounded
>> >> > size),
>> >> > then
>> >> > check constraints and send to JointSplineTrajectoryController.
>> >> >
>> >> > JointSplineTrajectoryController:
>> >> > It loop generating interpolated position for joint regulator.
>> >> > When it receive new trajectory it reorder joints in received
>> >> > trajectory
>> >> > (memory allocation, iterating on unknown size array) and try to
>> >> > compose
>> >> > current trajectory with newly received (memory allocation, iterating
>> >> > on
>> >> > unknown size array).
>> >> > It have to by done between two sequential interpolations (in my
>> >> > system
>> >> > 1ms).
>> >> >
>> >> > My solution :
>> >> > Data transmited between  joint_trajectory_action and
>> >> > JointSplineTrajectoryController contains only single point (constant
>> >> > size).
>> >> >
>> >> > joint_trajectory_action do joint reordering and compose trajectories
>> >> > and
>> >> > is
>> >> > feeding JointSplineTrajectoryController with trajectory points.
>> >> >
>> >> > JointSplineTrajectoryController interpolate between two points (have
>> >> > one
>> >> > point buffer)
>> >> >
>> >> > This solution should work with trajectories when time between points
>> >> > is
>> >> > >>
>> >> > 1ms.
>> >> >
>> >> > What do you think about doing this this way ?
>> >> > What is alternative solutions ?
>> >> > What do you think about re-usability of this solution in yours
>> >> > systems ?
>> >> >
>> >> > Pozdrawiam
>> >> > Konrad Banachowicz
>> >> >
>> >> > _______________________________________________
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>> >> > ros-users at code.ros.org
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>> >> >
>> >> >
>> >>
>> >>
>> >>
>> >> --
>> >> Stuart Glaser
>> >> sglaser -at- willowgarage -dot- com
>> >> www.willowgarage.com
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>> >
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>>
>>
>>
>> --
>> Stuart Glaser
>> sglaser -at- willowgarage -dot- com
>> www.willowgarage.com
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-- 
Stuart Glaser
sglaser -at- willowgarage -dot- com
www.willowgarage.com



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