How to form a real-time small scale network?

workaholic wrote:

Why don't you find out what the data transfer speed _is_ in some
candidate LANs?

Don't forget PC-induced latency -- without a bit of work, neither
Windows nor Linux are close to being real-time OS's.  Even with a lot of
work, neither of them are hard real time, although I could see
applications where a soft real time system may be adequate.  There _are_
hard real time environments that hijack the machine from Windows or
Linux, but those are _lots_ of work (and lots of $$$, in the case of
Windows).  Here's a test:  If you can see yourself telling your prof
"don't mind that manipulator sticking through the lab window, that's
just an occasional glitch" then a soft real time system, or a not real
time at all system, is probably adequate.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

You might consider running some drivers off of each PC that
can handle a certain time frame of instruction.  The PC's
can then chat amongst themselves with only occasional
requirements to change settings.  Given that you'll probably
have a model or two of the manipulators in the PC, they
should get deviation reports from the drivers, and issue
occasional corrections and additions to the movement
schedule.  You might also consider a 4th PC as a master that
will coordinate the actions of the other three.

Your data transfer rate is going to be largely dependent
upon what data you are going to transfer.  If it is the full
coordinate position and velocity every microsecond of each
manipulator it is going to take a lot more than if each PC
simply reported deviations and readiness for additional
commands.


Michael

I would do it all in one PC and avoid using the LAN.  There are other
choices besides Windows and Linux for operating systems.  We use QNX,
there are other choices.

Peter Nachtwey

22dce024f100@f63g2000hsf.googlegroups.com:


When it comes to inter-computer communication, I'm not sure there is a
network protocol that can claim to be real time (depending, of course, on
your definition of real time).  If your needs are critical, I'd suggest
finding some other way.

--
Scott
Reverse name to reply

wrote:

There a plenty of isochronous protocols for Ethernet and other
busses.  There are CIP Sync, PowerLink, Varan Bus,  Ethercat,
ProfiDrive and Ethernet.  Profibus DP has DP V2.

It takes a lot of effort to implement one of these protocols and can
only be justified if you are selling products with these protocols
included.

It still isn't clear  exactly how many axes of control are need in
workaholics application but one Core Duo Pentium is very powerful and
should be able to control 16 axes easily.  Why not use 2 or 3 motion
cards in the PC to off load the main CPI even more?  I don't
understand the thinking process.  workaholic will be old and gray by
the time he gets started on this project at the rate he is going.

Peter Nachtwey

I am very sorry for my absence and much thanks for all of your
advice!
It seems that it is not proper or economical for implementing a
present commerical industry protocol for such a small system. Now we
use a PCI card to control each manipulator on one PC. And it is
impossible to insert so many PCI card on one main board of the PC. We
do have to find some way to solve the communication problem.

I have heard of some information about the QNX,  can it be installed
on a PC?

f2c603596bac@l28g2000prd.googlegroups.com:


Do you need to move data in real time, or just signal events??  If the
latter, one cheat would be to use the DIO on PCI boards in each box (there
are usually some on most types of interface boards) to signal each other.


--
Scott
Reverse name to reply

On 7ÔC218ÈD5, ÏC2ÎE78ÊB128·D6, Scott Seidman <namdiestt..=
.@mindspring.com> wrote:

Yes, we have to send the data of  end position, velocity, or
acclereation of each manipulator, and we also have to consider
designing a simple data form to send and receive between devices.

88215d355aa7@i36g2000prf.googlegroups.com:


What are your time requirements for such communication?  Are they hard real
time, or are you just trying to display it on a screen or some such.


--
Scott
Reverse name to reply

Yes.  You should look for PCI motion control cards that have QNX
drivers done or look on the QNX site for a list of compatible motion
control cards.  I did a search of 'QNX motion control' and got over
14000 hits.  I did another search for 'QNX robotic' with 159,000
hits.  It looks like you will have plenty of already working
options.

Peter Nachtwey

On 7ÔC218ÈD5, ÏC2ÎE78ÊB151·D6, pnachtwey <pnacht...@gmail=
.com> wrote:

 Thanks!

     Mr Workaholic -

     Connecting so many PCI cards to a main board?  Unless I'm
     mistaken, you're looking for a "passive backplane" and the
     motherboard/BusMaster plugs into the backplane.  The instructions
     on the backplane vary some and you might have to plug this
     master into slot zero (if the PCI slots are numbered).

     Your might also look into National Instruments and their
     LabView software and PC add-on products.  They specialize
     in PC/robot automation. They make all kinds of things for
     PC instrumentation and robotics that sound like what you're
     apparently interested in doing.

     If what you're doing is rather small(not extremely complex),
     you might just look into doing it with a PIC processor or two.
     It might require building a small printed circuit (or wire
     wrap it if it's one of a kind/custom) with something on the
     order of 10 chips on it.

     2Penny  (my 2 cents worth)


workaholic wrote:

On 8ÔC23ÈD5, ÏC2ÎE710ÊB117·D6, 2Penny <lw_rog...@sbcgloba=
l.net> wrote:

Thanks! And I'll search for it.