Recommend a microcontroller or PLC for welding/plasma cutting?

Note sure if sci.engr.control is dead or not.
I am messing with my welding machine -- former 3 phase Hobart CyberTig
-- by changing it to use a PCTI SCR firing controller. Without tiring you guys with details, it will also have a switchable voltage range, 0-90VDC and 0-300VDC (the latter for plasma cutting).
I have gotten to the point where basic stick welding works fine.
Now I am facing a task of making a real control system for it. I could do it with relays, but that is cumbersome and hard to reconfigure.
I am now considering some programmable control, since it makes making changes considerably easier. I am a computer programmer, so I know the basics of programming. I had some suggestions in the past that I shoudl consider the microcontroller route, and now I agree.
It would be good to get something that could handle maybe a dozen inputs, have a dozen or so outputs, and is programmable via serial port.
Not sure what exactly I need, PLC or microcontroller or whatever, but it needs to have relay outputs and discrete and non discrete inputs and outputs.
I would greatly prefer something that can be i nterfaced to through Linux, since that's what I have, but I could use Windows software also.
I have 12VDC and 120 VAC power inside the welder.
Any suggestions? Used stuff is fine with me. I saw some Omron Sysmacs sold on ebay for not too much, anyone knows if they are any good?
thanks
i
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Ignoramus is right!

You didn't say exactly what you want to control, but I would suggest that, whatever you go with, you don't use the power inside the welder or mount the gear anywhere near the welder.. If you do, each time you strike an arc, you'll have to replace your entire control system (especially if it's an Omron Sysmac ;-)
Cameron:-)
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On Wed, 24 May 2006 09:27:02 +1000, Cameron Dorrough

Well, I am definitely looking for something relatively hardened. I already have a SCR firing system, it looks like a PLC, and it seems to survive both HF and striking arcs.
i
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Ignoramus8797 wrote:

I'm answering from it. Compared to sci.electronics.design or rec.crafts.metalworking it's dead, if by "dead" you mean nearly all the posts are on topic and apolitical.

I can't help you much with off-the-shelf hardware, but I can give you some questions that you'll need to answer sooner or later, basically to expand on Cameron's "what do you need" question:
* Are you just going to be sequencing things, or are you going to be doing continuous control? Continuous control is much more fun (harder).
* Are you going to be depending much on feedback, IOW are you going to be monitoring some continuous quantity from the welder and using it to switch things on or off?
If you're OK with embedded programming then an option to consider is a single-board computer in the welder (properly shielded -- that'll be an adventure) that runs everything. Embedded Linux seems to be maturing, and you can't beat the licensing.
--

Tim Wescott
Wescott Design Services
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By dead I mean a group where a simple on topic question cannot be answered due to lack of posters.

Well, most continuous control would fall under sequencing. Think TIG pulsing, adjusting of max voltage after an arc is struck (you may have seen my references to two inch long arcs), etc.
The SCR firing system by PCTI already does fine control of phase angle to match desired current and voltage.

Yes, for instance I would switch high frequency off when circuit voltage is below some limit.

Well, I am very interested in it -- as long as it can read analog inputs and drive relays. Do you know of any such working systems?
I am very comfortable with linux, I always had linux on my home computer since 1995.
I have no experience with embedded programming, but I have some amount of common sense.
So... Do you know where I can start looking regarding embedded linux in control applications?
i
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Ignoramus8797 wrote:

"Linux for Embedded and Real-Time Applications", by Doug Abbot. I haven't read the whole thing, but where I've poked my nose in it looks good.
Linux with the RTAI extensions is being used for machine tool control, including the motion control. It should be stout enough for what you're trying to do.
If you're a desktop programmer the two biggest things you'll have to wrap your brain around are the idea of real-time tasks (and staying out of their way), and working in an environment where the system simply cannot stop for the intervention of a user or a sysop. If you're a smart desktop programmer you'll figure it out.
--

Tim Wescott
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My concern is that there does not seem to be "design kits" available as such computers. Mostly it is either talk, or big lots. I like some of the kits by CUBLOC, looks like they have a lot of what I may need, in industrial hardened packages.
i
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Ignoramus8797 wrote:

Start subscribing to "Circuit Cellar" and "Servo" magazines. You're firmly in Circuit Cellar's target market, and on the top end of Servo's. They have ads for just that sort of thing, including some pretty nice looking ARM boards already targeted with Linux for around $150 to $200.
There are also PC-104 stacks out there available in onesie-twosie. They can be a bit spendy, but as bare boards go they're just exactly what you'd want for your application.
--

Tim Wescott
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Thanks Tim. I already got something last night:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&itemv21603358
I hope that it will do what I want. I am now reading manuals for it.
i
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