Welding -- what to do about HUGE 2" arcs

[Glen Walpert]

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.

 

[Ignoramus16643]

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.

Glen, thanks. I have decided to settle down on one of the Comfile
controllers. It is programmable both in BASIC as well as ladder
logic. It can also have serial or ethernet interface. What this
hopefully means is that I could set my laptop on top of the
welder and use it for debugging of the BASIC programs, or send
commands (though, surely, the laptop will not be part of the final
product).

Regarding isolation: right now, perhaps wrongly, I have negative side
grounded. That makes some things a little simplier for me. PCTI power
supply is designed to work this way, I think. To change that (unground
negative side), I would need to switch to using current and voltage
transducers. I already have a voltage transducer, but no current
transducer yet.

i

 

[GD]

When I stick weld with my tig I use the foot control still to control
the amperage. That way you could also shut the curent off when you want
to stop. It is also great when you get to the edge of the meterial and
need less amperage or more if you want more penatration.
Gary

 

[Glen Walpert]

Regarding isolation: right now, perhaps wrongly, I have negative side
grounded. That makes some things a little simplier for me. PCTI power
supply is designed to work this way, I think. To change that (unground
negative side), I would need to switch to using current and voltage
transducers. I already have a voltage transducer, but no current
transducer yet.

The grounded negative side is a safety issue. You will never see this
done in any commercial welder. Standard safety practices include
connecting the welder frame to safety ground, and connecting all metal
workbenches used for welding to safety ground. If you want to use
straight polarity (electrode negative) or AC (electrode sometimes
negative) then you have an immediate problem when you connect the work
lead to a grounded workpiece, as full welder output current will
travel through the safety ground system back to the grounded negative
bus, likely starting a fire and certainly melting the safety ground
wiring unless it is sized way above code requirements. Even if you
only ever weld DCEP you would immediatly have a problem if the work
connection falls off while you are welding, with welding current again
returning through safety ground. You should definitely fix this, and
in the meantime make sure nothing you weld on is grounded, and treat
everything you weld on as electrically hot while you are welding.

 

[Ignoramus5457]

The grounded negative side is a safety issue. You will never see this
done in any commercial welder. Standard safety practices include
connecting the welder frame to safety ground, and connecting all metal
workbenches used for welding to safety ground. If you want to use
straight polarity (electrode negative) or AC (electrode sometimes
negative) then you have an immediate problem when you connect the work
lead to a grounded workpiece, as full welder output current will
travel through the safety ground system back to the grounded negative
bus, likely starting a fire and certainly melting the safety ground
wiring unless it is sized way above code requirements. Even if you
only ever weld DCEP you would immediatly have a problem if the work
connection falls off while you are welding, with welding current again
returning through safety ground. You should definitely fix this, and
in the meantime make sure nothing you weld on is grounded, and treat
everything you weld on as electrically hot while you are welding.

Thank you. To resolve this, I likely need to install current and
voltage transducers, to "untie" myself from the ground. Right?

i

 

[Glen Walpert]

Thank you. To resolve this, I likely need to install current and
voltage transducers, to "untie" myself from the ground. Right?

Correct, you need isolation for both measured variables. If the PCTI
controller current limit and voltage limit set points signals are also
referenced to the negative bus, then you need to "untie" or isolate
these control signals from ground also. You could either buy isolated
transducers, or build your own.

 

[Ignoramus5457]

Correct, you need isolation for both measured variables. If the PCTI
controller current limit and voltage limit set points signals are also
referenced to the negative bus,

they are.

then you need to "untie" or isolate these control signals from
ground also. You could either buy isolated transducers, or build
your own.

Thank you. I will definitely do it. Tying negative bus to the ground
was a temporary measure, just to get things going. Instead of using
transducers, can I isolate the signals that I have, using a
optocoupler or some such?

i

 

[Glen Walpert]

On Thu, 25 May 2006 16:12:58 GMT, Ignoramus5457

they are.


Thank you. I will definitely do it. Tying negative bus to the ground
was a temporary measure, just to get things going. Instead of using
transducers, can I isolate the signals that I have, using a
optocoupler or some such?

An optocoupler by itself will not provode an accurate transfer of your
signals across the isolation barrier, although it can be done with
voltage to frequency -> optocoupler -> frequency to voltage for
example. You would be better off buying an isolation amplifier such
as the TI ISO124:
http://focus.ti.com/docs/prod/folders/print/iso124.html
There are lots of other iso amps but this is the cheapest suitable
part I know of. An eBay deal on a relatively expensive 3-port iso amp
might be even better since it transfers power across the isolation gap
too; a plain iso amp like the 124 needs separate isolated power on
both sides.

 

[Ignoramus5457]

On Thu, 25 May 2006 16:12:58 GMT, Ignoramus5457



An optocoupler by itself will not provode an accurate transfer of your
signals across the isolation barrier, although it can be done with
voltage to frequency -> optocoupler -> frequency to voltage for
example. You would be better off buying an isolation amplifier such
as the TI ISO124:
http://focus.ti.com/docs/prod/folders/print/iso124.html
There are lots of other iso amps but this is the cheapest suitable
part I know of. An eBay deal on a relatively expensive 3-port iso amp
might be even better since it transfers power across the isolation gap
too; a plain iso amp like the 124 needs separate isolated power on
both sides.

Got it Glen. So, the item I should look for is called "isolation
amplifier", right? I will also call LEM USA tomorrow. I am wrapping up
voltage switch contactor work today.

i

 

[Ignoramus32746]

On Thu, 25 May 2006 16:12:58 GMT, Ignoramus5457



An optocoupler by itself will not provode an accurate transfer of your
signals across the isolation barrier, although it can be done with
voltage to frequency -> optocoupler -> frequency to voltage for
example. You would be better off buying an isolation amplifier such
as the TI ISO124:
http://focus.ti.com/docs/prod/folders/print/iso124.html
There are lots of other iso amps but this is the cheapest suitable
part I know of. An eBay deal on a relatively expensive 3-port iso amp
might be even better since it transfers power across the isolation gap
too; a plain iso amp like the 124 needs separate isolated power on
both sides.

Glen, that item is not available anywhere, earliest it ships is
August. Do you think that AD629 would be suitable? I have one right at
home:

http://www.analog.com/UploadedFiles/Data_Sheets/225410897AD629_a.pdf

the PDF for it even shows a sample connection to a current shunt.

i

 

[Glen Walpert]

On Fri, 26 May 2006 16:26:47 GMT, Ignoramus32746

....

Glen, that item is not available anywhere, earliest it ships is
August. Do you think that AD629 would be suitable? I have one right at
home:

http://www.analog.com/UploadedFiles/Data_Sheets/225410897AD629_a.pdf

the PDF for it even shows a sample connection to a current shunt.

Yes, that looks like it should work fine. Analog Devices also makes
isolation amps which are actually available now, but they are
relatively pricey, especially compared to parts you already have.

 

[Ignoramus32746]

On Fri, 26 May 2006 16:26:47 GMT, Ignoramus32746

...

Yes, that looks like it should work fine. Analog Devices also makes
isolation amps which are actually available now, but they are
relatively pricey, especially compared to parts you already have.

That's great Glen. I will look for it. Should be easy to do, I think.

i