[Ethercat-ros] RT configuration, ROS+Orocos+EtherCAT

Johnathan Van Why jrvanwhy at gmail.com
Tue Feb 5 12:47:47 PST 2013

On Tue, Feb 5, 2013 at 8:27 PM, Lambert, Michel
<JasonMichel.Lambert at cnrc-nrc.gc.ca> wrote:
> Hello,
> We are currently putting together a system for doing haptics/teleoperation.
> We would like to use ROS as the high-level framework interacting with a
> real-time environment through Orocos. EtherCAT would be used for I/O. I was
> happy to come across this list because it seems you have tried a few
> configurations. Looking at the two last archives it looks like you switched
> from using PREEMPT_RT to Xenomai as your real-time framework. Can you
> elaborate on the reasons for doing this?

We experienced unexpectedly high latencies with RT_PREEMPT -- roughly
an average of one >30 ms spike each minute. We do not understand the
source of these latencies -- for hardware compatibility reasons, we
would love to solve this issue. Fortunately, Orocos abstracts away
most interfacing, so it was not unreasonably difficult to make the
switch (dealing with SOEM for EtherCAT was the majority of the work).

> Also, have you looked at EtherLAB
> as a means to do real-time communication?

Yes, we have. Prior to our switch to our current
Orocos/Xenomai/SOEM-based RT stack, we utilized EtherLAB and RTAI for
realtime EtherCAT communications. We switched away from RTAI since
RTAI appeared inactive.

As part of our switch, the sequencing of our EtherCAT frames has
changed. The EtherLabs Master does not support waiting for a returned
packet -- you need to just try to sleep for the correct amount of
time. This was a consideration when we decided to change from
EtherLabs to SOEM. A more important reason was that, for us, the RTDM
DC functions in the EtherLabs Master API did not work under Xenomai.
Since we need the DC functionality of EtherCAT, this was a
"deal-breaker" for us.

I hope this helps,
Johnathan Van Why
Dynamic Robotics Laboratory
Oregon State University

> Cheers,
> Jason Michel Lambert
> Agent de Recherche/Research Officer
> Centre de Technologies de Fabrication Aérospatiale/
> Aerospace Manufacturing Technology Center
> Conseil National de Recherche du Canada/
> National Research Council Canada
> 5145 Avenue Decelles
> Campus de l'Université de Montréal
> Montréal, Quebec, H3T-2B2
> Tel: 514-283-9175
> Fax: 514-283-9604
> <mailto:michel.lambert at nrc-cnrc.gc.ca>
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