[ros-users] [Orocos-Dev] Control system design question.

[Herman Bruyninckx]

On Fri, 17 Sep 2010, Stuart Glaser wrote:

> 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.

Our research involves mostly (on-line, "soft" realtime sensor-based) use
cases that _need_ realtime generation of trajectories. That means that (i)
they can only be short (which is not a problem in the mentioned use cases),
and (ii) they can only be very simple in the geometric, dynamic,...
constraints that they satisfy. So, the discussion of 'realtime' vs
'trajectories' is not a simple one, but depends on many, many contextual
factors; one should not generalize, but state the context as explicit as
possible.

> 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.

Probably :-) This is consistent with my remarks above: there are _a lot_ of
different types of trajectories, and (among many others) the realtime
properties of each type can be very different.

> 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.

I don't think so: ROS node code is extremely difficult to reuse in a
non-ROS software framework.

Herman

> 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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>>
>>
>
>
>
> -- 
> Stuart Glaser
> sglaser -at- willowgarage -dot- com
> www.willowgarage.com
> -- 
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>

--
   K.U.Leuven, Mechanical Eng., Mechatronics & Robotics Research Group
     <http://people.mech.kuleuven.be/~bruyninc> Tel: +32 16 328056
   EURON Coordinator (European Robotics Research Network) <http://www.euron.org>
   Open Realtime Control Services <http://www.orocos.org>
   Associate Editor JOSER <http://www.joser.org>, IJRR <http://www.ijrr.org>