Re: [CAD_CAM_EDM_DRO] Re: EDM spark control system

> A sensor (either resistor or current transformer) detects the start of the
> discharge. A delay allows the discharge to continue for some adjustable
> time. then, the electronic switch is turned off, and kept off for an
> adjustable
> time. then, the voltage is turned on again, and it waits for the next

spark

> to start.

Easy enough. I assume we are talking about microsecond delays and not
millisecond delays? Sounds like the more sparks per second we can get the
better. Any idea what the commercial machine's delay ranges are?

> The drives will probably work, but I suspect there would be a need for

much

> software change, or addition of sensors, etc. Anyway, I think these

discharges

> are in the range of hundreds of amps. The MOSFET's series resistance will
> lower the peak current, but one 47 A MOSFET may not handle it. I was

thinking

> of using about 4 similar sized units in parallel.

Oh, I wasn't suggesting one 47A MOSFET would handle it. That's just what I
have on my board now. But I can open the CAD package, rip out the stuff
that's not needed for an EDM control and drop additional MOSFETs on the
board in just a few minutes. Software changes aren't a problem as the core
of the project is already done. The user interface code and display code is
still useable as is. I'll just have to modify the menus. If the spark
delays are between 1 and 255 us, or 1 and 255 ms, I don't even have to
modify the delay code. If we need smaller than 1us resolution, I'll need to
change that code.

> Close. What you want is to increase feed when there is no discharge, and
> stop feed when the electrode shorts to the work. Within that range, you

want

> to modulate feed based on how many sparks / second there are.

If it shorts to the work, would you output a short circuit signal which
would cause the control package to back up?

How would you suggest we signal a cnc control package like EMC from the
control electronics? I can easily provde a spark rate output or other
control signals. But I've never seen an EDM run. I do have some cuts to make
in the future that will need to be done with a wire EDM. So I have a vested
interest in making it work. Or is it impractical to use EMC or other
standard CNC package to control the EDM?

> I have no idea what the
> peak current is. 100 A would be expected - and the sparks produced sure
> look like VERY short duration pulses in that range. 1000 Amps would not
> surprise me a lot! I do have the gear to measure it, but it will take a

bit of

> gear to set it up. I will need my digital storage scope to capture the
> discharge.

I also have a couple of DSO's and a good current probe. So maybe what I
should do is wire up an RC version like you did and take some measurements.
I'll have to go find some caps though. All of my caps are either smaller
aluminum electrolytics or surface mount caps. I do have several isolation
transformers to keep it all safe.

later,
Larry E.