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

[Adolfo Rodríguez Tsouroukdissian]

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

-- 
Adolfo Rodríguez Tsouroukdissian
Senior robotics engineer
adolfo.rodriguez at pal-robotics.com
http://www.pal-robotics.com

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