Re: Welding -- what to do about HUGE 2" arcs

On Tue, 23 May 2006 14:40:39 GMT, Ignoramus8797
<ignoramus8797@xxxxxxxxxxxxxxxxxxx> wrote:

On Tue, 23 May 2006 14:36:36 GMT, Glen Walpert <gwalpert@xxxxxxxxxx> wrote:
That is an excellent idea. In this case he is using a standard
controller which wants a DC voltage limit signal, which can be reduced
when current is above some threshold, in the same analog computer
which implements droop and whatever other effects he wants to try.

Glen, do you think that a microcontroller based control solution would
let me kill many birds with one stone? to do droop, tig timing, plasma
cutting timing, etc etc?

If so, any suggestions for something simple? I could spend some bucks
(not too much) if it saves me time and wiring efforts.

Sure, a microcontroller could do this job fairly easily. You need
something with 2 A/D inputs, 2 D/A outputs (12 bits would be
adequate), a bit of digital I/O for the front panel interface. I'm
sure you could find something suitable from one of the many single
board computer manufacturers. Or use an old laptop computer with an
appropriate analog I/O module such as made by Measurement Computing
Inc, National Instruments, etc - most of these have a few digital I/O
thrown in, and a laptop computer would make a pretty good front panel.
The job is made easier since you do not need to make any "front panel"
adjustments while you are welding. You would probably want the
control routine to be driven by a timed non-maskable interrupt; check
voltage and current from A/D, compute new controller voltage and
current limit setpoints, output new values to D/A, *toggle the
watch-dog timer output*, return. Where an external hardware watchdog
timer will clamp both set-points to zero if the control loop stops.
You probably could get away with 100 updates/sec or so.

But this does not let you off the hook for some analog circuit design.
You need to get those voltage and current signals properly scaled,
level shifted and protected before you can connect then to an A/D
converter. This job is made a tad more difficult by the fact that the
welder output is completely isolated from ground except for the
arbitrary connection of either lead to a grounded workpiece, while
your controller must be grounded. By the time you have filtered,
protected and scaled these signals to a ground-referenced 0 to 4.096
volt (for example) signal, you are about 90% of the way to a simple
analog computer, which would be easier to make extremely reliable in
an electrically noisy environment. (A laptop or anything with a HDD
would be the least reliable IMO.)

What is the required signal range for your controller voltage and
current limit setpoints, and what is the reference for these signals?
If the reference for these signals is not connected to ground you have
additional isolation concerns.
.

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