[ros-users] [Orocos-users] Joint controller manager

Jonathan Bohren jonathan.bohren at gmail.com
Fri Feb 1 17:08:13 UTC 2013

On Thu, Jan 31, 2013 at 6:55 AM, Adolfo Rodríguez Tsouroukdissian <
adolfo.rodriguez at pal-robotics.com> wrote:

> All,
> I'd like to bring up the subject of having controllers running at lower
> rates than the controller_manager. I can think of two possible solutions:
> 1. Frequency divider:
> - Individual controller configurations can specify an optional parameter
> that states the desired frequency, or that work should be done only '1 out
> of n' manager cycles.
> - Pros: Easy to implement. It is specific to a particular controller
> implementation, hence does not need to be part of the controller_manager
> library. See [1] for an example.
> - Cons: Desired controller update period is subject to an error of up to
> one controller manager control period.
> 2. Individual/grouped controller updating:
> - The controller manager allows the option of updating single controllers,
> or groups of them. Triggering these updates is not handled by the manager.
> - Pros: Desired controller update period is unaffected by the controller
> manager's update period.
> - Cons: This solution requires significant changes to the existing
> controller_manager codebase:
>   - Extending the API to allow not only batch update()s, but also updates
> to a subset of the running controllers. Resolving which controllers to
> update should be a constant-time operation.
>   - Add protection against concurrent access to data (eg. simultaneous
> read() and update()), possibly with lock-free data structures.
>   - Because of the above point, the chosen concurrency handling mechanism
> might render reusing the existing standard hardware interfaces impossible
> (lock-free structures usually expose data through accessors, not raw
> pointers).
> At the moment I'm leaning towards the frequency divider as it is
> sufficient for our use cases. I wonder if someone has a use case that does
> not fit well with this approach.
> [1]
> https://github.com/willowgarage/ros_controllers/blob/master/joint_state_controller/src/joint_state_controller.cpp#L78

So this also seems like it's a decision between serial (1) or parallel (2)
computation of different components of the final output signal. Is that

On Fri, Feb 1, 2013 at 3:48 AM, Carles Lopez <carles.lopez at pal-robotics.com>

> My two cents: maybe allowing single controllers to return the expected
> time they want to be updated given their own internal state. With this
> information control manager will adapt its own rate depending on the
> earliest deadline of the controllers it manages.

I like this bottom-up strategy, since it would also help debug
timing-related problems since the system will know at what rates each
components needs to run.


Jonathan Bohren
PhD Student
Dynamical Systems and Control Laboratory
Laboratory for Computational Sensing and Robotics
The Johns Hopkins University

(707) 520-4736
jbo at jhu.edu
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