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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.<br>
<br>
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.<br>
<br>
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:
<a class="moz-txt-link-freetext" href="http://www.cs.st-andrews.ac.uk/~pn/establishing-timing-reqs-mm02.pdf">http://www.cs.st-andrews.ac.uk/~pn/establishing-timing-reqs-mm02.pdf</a><br>
<br>
If you find that you need lower jitter, I would ask on Linux RT
users mailing list: <a class="moz-txt-link-abbreviated" href="mailto:linux-rt-users@vger.kernel.org">linux-rt-users@vger.kernel.org</a><br>
<br>
Regards,<br>
-Austin<br>
<br>
On 04/10/2012 01:35 PM, Soo-Hyun Yoo wrote:
<blockquote
cite="mid:CAP1DbD5ueAeRVp=08_r=TKqHjmRrE9Eb5hAQuoXinZMqhtBjLw@mail.gmail.com"
type="cite">Hello Austin,
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>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?</div>
<div><br>
</div>
<div>Soo-Hyun Yoo<br>
<br>
<div class="gmail_quote">On Wed, Jan 11, 2012 at 11:56 AM,
Austin Hendrix <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:legotown@aol.com" target="_blank">legotown@aol.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt
0.8ex; border-left: 1px solid rgb(204, 204, 204);
padding-left: 1ex;">
This sounds nearly identical to the system we use here at
Willow Garage on the PR2.<br>
<br>
We're using a PREEMPT_RT kernel (2.6.31+, moving to 3.0)
with a 1kHz control loop in user space.<br>
<span><font color="#888888"><br>
-Austin<br>
</font></span>
<div>
<div><br>
Jonathan Hurst <<a moz-do-not-send="true"
href="mailto:jonathan.hurst@oregonstate.edu"
target="_blank">jonathan.hurst@oregonstate.edu</a>>
wrote:<br>
<br>
>Dear ROS Users,<br>
><br>
>We would like to use Robot Operating System on
ATRIAS, the bipedal<br>
>robot we are building<br>
>(<a moz-do-not-send="true"
href="http://mime.oregonstate.edu/research/drl/Site%20Pages/Atrias.html"
target="_blank">http://mime.oregonstate.edu/research/drl/Site%20Pages/Atrias.html</a>),<br>
>but we must have reliable real-time control at 1kHz
with communication<br>
>over an EtherCAT bus. We have assembled some
existing software tools<br>
>to enable this, including the EtherLab driver and
Orocos RTT to tie<br>
>things all together in real time. If you are
interested in this<br>
>discussion, please feel free to join the Email list
- it has a brief<br>
>history with one or two summaries of working
systems, but I hope we<br>
>will have much more detailed discussions, or receive
pointers to other<br>
>working systems. You can sign up for the list here:<br>
><a moz-do-not-send="true"
href="https://secure.engr.oregonstate.edu/mailman/listinfo/ethercat-ros"
target="_blank">https://secure.engr.oregonstate.edu/mailman/listinfo/ethercat-ros</a><br>
><br>
>Jonathan<br>
>--<br>
>*****************************************************************************<br>
>Jonathan W. Hurst<br>
>Assistant Professor<br>
>School of Mechanical, Industrial, and Manufacturing
Engineering<br>
>Oregon State University<br>
>204 Rogers Hall<br>
>Corvallis, OR 97331-6001<br>
><br>
><a moz-do-not-send="true"
href="mailto:jonathan.hurst@oregonstate.edu"
target="_blank">jonathan.hurst@oregonstate.edu</a><br>
>Voice: <a moz-do-not-send="true"
href="tel:%28541%29%20737-7010" value="+15417377010"
target="_blank">(541) 737-7010</a><br>
>Fax: <a moz-do-not-send="true"
href="tel:%28541%29%20737-2600" value="+15417372600"
target="_blank">(541) 737-2600</a><br>
><a moz-do-not-send="true"
href="http://mime.oregonstate.edu/research/jhurst/"
target="_blank">http://mime.oregonstate.edu/research/jhurst/</a><br>
>******************************************************************************<br>
>_______________________________________________<br>
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</div>
</blockquote>
</div>
<br>
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</blockquote>
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