RE: FPGA for PCI based servo control board

["Craig Edwards" <cedwards-at-ceinetworks.com>]

I like the CAN bus a lot.... For distributed control type architectures.
It's late tonight, so I'll just make a few quick comments..:) 

Really, the whole debate IMHO revolves around the where you want to close
the servo loops, and what software architecture you are trying to implement.
CAN bus has point to multi-point features, has high signal integrity, is
inexpensive (now), but not "native" to the standard PC platform (big
advantage for Ethernet).  The last summary I read on CANopen showed around
130uS latency (running at 1MHz clock).  This may be improved on with later
generation chips... But I suspect it will be still be higher than you might
like to close a velocity or torque servo.  On the other hand, there is a
fully defined application layer and comm profile for motion control, which
is now becoming quite popular commercially, and is a BIG advantage.  Check
out the website for Copley controls or Advanced Motion Controls for lots
more information on CANopen. The app layer specifies a PVT (position -
velocity - time) protocol that requires the control software to send segment
by segment profiles to each of the axis's for execution, relying on each
drive to insure that it complies to insure coordinated multi-axis motion.
That's why you see so many uP or DSP chips being announced with CAN
controllers in them...because they are designed to run the code to close the
servo loops, do the homing, check for faults, etc.  

Commercially, this is the direction much of the industry is going
(distributed controls).....for many good reasons.  I started this thread to
sorta "challenge" this approach, because what little I know about EMC
suggests that perhaps there are some things we could with EMC that would
allow both a lower cost and higher performance solution for small machine
control, using "low cost" PC technology to the greatest extent possible. But
personally, I'm not yet 100% convinced this is the case (thanks in part to
the many fine posting on this forum), so I'm also looking into the details
of CANopen as well.....guess I'll second the motion to open another
threads...on CAN bus...and Ethernet....:)   Need to start to study John's
post on the EMC block diagram... Perhaps there's a way to implement
something similar to CANopen "internal" to EMC.  Having an open API would
really help support all the various hardware interfaces being considered.


-Craig Edwards

-----Original Message-----
From: emc-at-nist.gov [emc-at-nist.gov] On Behalf Of John Craddock
Sent: Saturday, April 05, 2003 7:27 PM
To: Multiple recipients of list
Subject: RE: FPGA for PCI based servo control board



Let me throw in a couple of curves here.
What is wrong with the CAN bus? CAN = Controller Area Network. There are
already drivers for it in RTAI that is the real time Linux extensions used
in the current BDI. Also look at the TI DSP TMS320LF2812.
http://focus.ti.com/docs/prod/productfolder.jhtml?genericPartNumber=TMS320F2
812
Does most of what the FPGA sets out to do from what I can gather. I am
playing with an LF2407 (predecessor) at the moment. CAN bus is built in to
the chip. Can run A/C servos, BLDC servos, and steppers. 2 Axes per chip +
analogue and digital IO; programmable! What more could you want? Hope this
contributes to the debate. Kind regards John Craddock

-----Original Message-----
From: Robin Szemeti [robin-at-redpoint.org.uk]
Sent: Sunday, 6 April 2003 9:13 AM
To: Multiple recipients of list
Subject: Re: FPGA for PCI based servo control board



On Saturday 05 April 2003 23:31, John Kasunich wrote:


> Jon's board does four axes of step/dir for $200.  Additional axes cost 
> another $200.  If you need analog, Jon's solution is about $650, and 
> the STG board is $800+.  I expect that this project will cost about 
> $250 for a 4 axis EPP version.  It can provide any mix of analog and 
> digital.  Additional axes cost maybe $20 each, for the I/O parts. ISA 
> version might cost a little less, PCI and ethernet will cost a little 
> more.

hmmm .. sounds fun.  You should sell 1.25 of each at the current rates ;))
.. 
still .. (as they guy who tied helium ballons to a deck chair famously said:

 "a guy gant just sit around" ..)

Im also looking at the Gecko2002 .. thats almost *exactly* what I need .. 
except its USB and usb under linux is always a deeply painful experience
with 
much pulling of hair and frame overuns etc. .. the advantage is that it
ships 
sometime in the next 10 days ...

Right now I'm just scraping around in the bottom of my wallet to see if I
can 
find enough to order a stepper board from jon ... hmm .. maybe the kids
piggy 
bank .. wheres my hammer? ....

-- 
Robin Szemeti

Redpoint Consulting Limited
Real Solutions For A Virtual World