Plasma Application, Mach3 torch height control.

[stirling]

Well I was going to retire from this topic but as you've asked...  . What I did right at the start was to model this in SPICE. The problem there is that SPICE (as far as I'm aware anyway) absolutely requires a properly earthed circuit. So the results I was getting were of course quite different. I was a little slow on the uptake with JP's insistance that none of his circuit is in fact earthed at all as I'm not used to working "un earthed".

It's not for me to say how other people go about things but sooner or later IMHO the plasma table/work will or at least should be earthed. At this point, it seems to me that JP's Vss becomes "TipV" and Vdd floates 5V above "TipV" with the scaled output somewhere between these two - all below and with respect to, true earth. Though it will most likely still work, I'll leave it to others to decide what their views are on the method.

[freedom2000]

Hi,


Any time you are using a voltage transformer, you are working "un earthed"



Any time you connect an electrical device on sector (110VAC or 220 VAC) neither the phase nor the neutral is connected to earth.

However these devices are safe... Only the body is connected to earth to protect you against leakage between sector and you (earth)

Of course the plasma "clamp" will be connected to earth on the CNC, it will be difficult not to do it !

Thus the Torch "clamp" V+ will be connected de facto to earth

And my schema will still work ... provided that neither Vss nor Vdd touch the earth (which is easy to achieve --> put it in a plastic box 
And even if you touch the earth with Vss... it would be the same as touching the metal part with the torch tip...
My electronics should survive if the plasma does.
In those conditions voltage difference between Vdd-Vss will be 5V (in blue)  when voltage difference between torch clamp and torch tip will be 300V (in red)



JP

[matttargett4]

I think im in the same camp as stirling there, there is no avoiding it is negative with respect to true earth and so has to be inverted

[freedom2000]

Just to go on a bit !

You are absolutely right when saying that Vss (and Vdd) will float (or rather sink) under the earth level. But who cares ?

The only NEEDED condition is also that any signal entering in (or going out from) the µcontroler is NOT connected both to earth and Vss (or Vdd). And this is achieved safely as these connections are optical ones (via the optocoupler). These signals are thus fully isolated from the plasma side. You can go on using your PC without risking an "explosion"  

Note also that I have on purpose (for clarity) suppressed the diode bridge as it is useless when you know where the + and - sides at the plasma level are.
(If not or if you want an "universal" system just put the diode birdge).

Is this schema safe ? The answer is YES (if not please go on explaining why)
I proposed to put the PCB into a plastic box --> this is NOT a joke : we must do it
And by the way it is exactly how electronic devices not grouded work. Let's look at a dremel : it is NOT grounded, and to be safe it is surrounded by a plastic body...

JP

[freedom2000]

I think im in the same camp as stirling there, there is no avoiding it is negative with respect to true earth and so has to be inverted

Hi Matt,

Why do you say this ?

VClamp - VTorch = 0 - (- 300) = 300 V --> there is no trick here.

If you put the "black" on the tip and the "red" on the clamp you WILL measure a positive voltage

f you connect the clamp to the earth (even if it is not the case electrically speaking on my Plasma) then you will still measure a positive voltage between clamp and tip (clamp - tip = 300V)

Right or not ?
JP

[ex apprentice]

Hi Matt, Scot & JP

Where does it go from here, I must say and I am sure I speak for a lot of other readers whats next? I have really enjoyed reading the posts.
Is this topic closed or will it continue to run so close yet so far.
This in my mind is what its all about, you gentlemen are stars and it would be of great service to all us lesser mortals to continue to a conclusion
hopefully a cost effective home brew of a THC system that challenges the commercial options.
I am sure I am not alone in trying to build things for myself (even if it takes longer) If money was not an issue we would all go out and buy the things we want
and life would be simple, but that's not what Mach is all about for the majority of people
So come on gents and please continue your excellent debate and hopefully we can all learn as we read

Thank you for getting this far

Ex Apprentice

[freedom2000]

Hi,

I have not given up :-)

I just have to finish the PIC code and upgrade my CNC with my plasma torch.
I just need time.. not enough to do all I want to do   

JP

[ex apprentice]

Hi JP

That's good news, as I said I am sure loads of people have read these posts and really enjoyed the content
It would be soul destroying for quite a few of us if this subject just died a death with no outcome
Having a mechanical leaning (but learning the electronics side at a very steep curve - I find this very interesting)
couple that with the fact that it has the potential to be such a good system
Cant wait to read more
I fully understand the time thing my customers just seem to get in the way of my hobby

Thanks again

Ex Apprentice

[matttargett4]

I have also not given up!

Summers are very busy for me, im a farmer! Add in that just after i last posted i got offered an old esab magic eye machine for cheap, which i bought and then sold my home made contraption and you understand my absence, its a weak excuse i know! The stuff still goes round in my head buut i just dont get to note it down.

Heres where i am with it.

Im trying to build a new machine to improve cut quality, speed, accuracy etc etc, i have bought a larger transformer type plasma and everything i read tells me my results will only be as good as my thc. As you may have read ive always been of the opinion that using your pc for the processing seems obvious to me, the trouble is you have to get the signal there and get it back again, im sure its quite possible and could be very fast and accurate but we have to be able to integrate it with mach and make it quick and easy and cheap. Invarialy if you want to get it into mach you appear to have to use modbus, this gets accused of being slow which personally i dont buy but for me i think where it falls down is the modbus devices availabe with analogue inputs lack flexibility, fo instance we ended up on much debate about if the voltage signal from the plasma needed to be inverted, if you were able to use a different device with more flexible analogue inputs then maybe it wouldnt. The other thing that worries me with any mach based system is that the only ability we have to adjust the z on the go is simple up or down with no adjustable speed, thats ok but for best results surely a servo responce is required?? My new z axis also has a straihtforward dc window wiper motor on it as it was only ever meant for gas cutting, but this is what the big boys use, so im thinking that in order to make the system as cheap and simple and effective as i can im going to have to sell my soul a little, im thinking i will use a pic or plc connected to the dc motor with a motor controller, much like a servo drive and with an encoder so speed will be infinately variable using a pwn about from the processor allowinf a servo response,. this will integrate with mach as if there were no z in mach, mach has the x and y co ordinates correct and wishes to start the cut it will send a signal tot he plc and the plc will control the z axis move down to the metal, touch off, fire the torch and when the plasma gives the ok to move signal it wil move, the plc then monitors voltage and thc.

It may sound complex but i think it might work and be easier and cheaper!

matt

[Tekno]

Hi Guys
for cur read from plasma. must be 35/1 redictor or coverter noise filter pcb,because  140to350volt  verynoise cur out
If it is noclear ithe analog input voltage PIC is blocked . dc analog voltage 0 to 4.500mlv max (must be very clean)

if from Lpt1 the contact  runs very fast  up down or cur ok ,from modbus 100mls delay mode  contact  very slow process
makes the continuous problem