[ros-users] Hard Realtime requirements and ROS/OROCOS

Ruben Smits ruben.smits at mech.kuleuven.be
Sat May 15 09:29:22 UTC 2010

On Friday 14 May 2010 20:06:08 Stefan Kohlbrecher wrote:
> Hi everyone,
> right now, I'm evaluating the ROS-OROCOS integration because my group
> has hard realtime requirements for some control parts of our system.
> The integration works fine so far,  but there are still some things
> that could be easier to do (Automatic timestamp
> conversion/synchronization and generation of OROCOS types from ROS
> msgs for example). 

This is part of our PR2 Beta Program project.

> From what I gather, on PR2 a RT-PREEMPT Kernel with
> a corresponding SCHED_FIFO Thread is used for realtime control. While
> this seems to work pretty well, one is bound to using RT-preemption
> instead of the dual kernel approach OROCOS permits one to use.
> So, without going into much detail, it seems like using ROS only is
> possible, but it might lead to higher latencies in special cases and
> isn't as flexible in the choice of the underlying RTOS. OROCOS on the
> other hand is more flexible in choice of RTOS, but has some more
> overhead (especially in terms of implementation) when integrating it
> with ROS nodes.

The goal of the Orocos RTT ROS integration is to remove this overhead. Since 
their is a good semantic mapping between the interface primitives of both 
frameworks (RTT Ports <-> ROS Topics, RTT(2.0)Operations <-> ROS Services, RTT 
Properties <-> ROS Parameters) I believe the interface of a realtime Orocos 
process can be made available in the ROS ecosystem without compromising the 
realtimeness of the Orocos process and vice versa make the interface of a ROS 
node available in an Orocos process.

> Of course, the best of both worlds would be the best thing to have, so
> are there plans for a even tighter integration of both (like the
> aforementioned automatic generation of types)? Does anyone have
> further insights/experience/tips regarding hard realtime using ROS
> with or without OROCOS? Is there some roadmap or cooperation between
> developers regarding this topic?

I'll be working on this tight integration the coming year together with ROS 
and Orocos developers. Any suggestions and comments are welcome.


> Thanks in advance for every answer,
> regards,
> Stefan
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