[ros-users] RT ROS + EtherCAT

[Austin Hendrix]

We have about 50us of jitter in our realtime loop, and it isn't a 
problem for us, but our robot is also statically stable.

We're currently using the 3.0.6-rt17 kernel, which is a bit old at this 
point, but it has proven more stable than the previous PR2 kernel. My 
configuration is set up with a minimal number of drivers built into the 
kernel, and with a large number of USB device drivers built as modules. 
Since we use NFS on the PR2, there are a number of NFS-related things 
enabled; and probably a few other obscure things we use that most other 
users don't need.

Before you worry too much about the remaining jitter in your system, I 
would try to determine how much jitter is acceptable. This seems to have 
some ideas on where to start in that process: 
http://www.cs.st-andrews.ac.uk/~pn/establishing-timing-reqs-mm02.pdf

If you find that you need lower jitter, I would ask on Linux RT users 
mailing list: linux-rt-users at vger.kernel.org

Regards,
-Austin

On 04/10/2012 01:35 PM, Soo-Hyun Yoo wrote:
> Hello Austin,
>
> Would you be willing to share your PREEMPT_RT kernel configuration 
> file for the PR2? I have tried disabling all unnecessary (in my 
> judgment, which could be wrong!) features for kernel v3.0.25 patched 
> with PREEMPT_RT v44, but the EtherLab Master node still suffers from a 
> jitter of around 60 us.
>
> Does that sounds reasonable? The high jitter could just be due to our 
> use of an Intel Atom processor compared to the PR2's Core i7's, but I 
> don't know how (if at all) that would affect the jitter. What kind of 
> jitter do you get on the PR2?
>
> Soo-Hyun Yoo
>
> On Wed, Jan 11, 2012 at 11:56 AM, Austin Hendrix <legotown at aol.com 
> <mailto:legotown at aol.com>> wrote:
>
>     This sounds nearly identical to the system we use here at Willow
>     Garage on the PR2.
>
>     We're using a PREEMPT_RT kernel (2.6.31+, moving to 3.0) with a
>     1kHz control loop in user space.
>
>     -Austin
>
>     Jonathan Hurst <jonathan.hurst at oregonstate.edu
>     <mailto:jonathan.hurst at oregonstate.edu>> wrote:
>
>     >Dear ROS Users,
>     >
>     >We would like to use Robot Operating System on ATRIAS, the bipedal
>     >robot we are building
>     >(http://mime.oregonstate.edu/research/drl/Site%20Pages/Atrias.html),
>     >but we must have reliable real-time control at 1kHz with
>     communication
>     >over an EtherCAT bus.  We have assembled some existing software tools
>     >to enable this, including the EtherLab driver and Orocos RTT to tie
>     >things all together in real time.  If you are interested in this
>     >discussion, please feel free to join the Email list - it has a brief
>     >history with one or two summaries of working systems, but I hope we
>     >will have much more detailed discussions, or receive pointers to
>     other
>     >working systems.  You can sign up for the list here:
>     >https://secure.engr.oregonstate.edu/mailman/listinfo/ethercat-ros
>     >
>     >Jonathan
>     >--
>     >*****************************************************************************
>     >Jonathan W. Hurst
>     >Assistant Professor
>     >School of Mechanical, Industrial, and Manufacturing Engineering
>     >Oregon State University
>     >204 Rogers Hall
>     >Corvallis, OR 97331-6001
>     >
>     >jonathan.hurst at oregonstate.edu
>     <mailto:jonathan.hurst at oregonstate.edu>
>     >Voice: (541) 737-7010 <tel:%28541%29%20737-7010>
>     >Fax: (541) 737-2600 <tel:%28541%29%20737-2600>
>     >http://mime.oregonstate.edu/research/jhurst/
>     >******************************************************************************
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