Plasma Torch Height controller (not so simple voltage divider)

[Arclover]

Hi all,
I'm new here so first I will introduce myself:
I am 29 and live in the Netherlands and work as a technician E&I
Hobbies include welding, CNC and "basic" electronics

I just finished the build of my practice no-budjet CNC engraver.

Now I know I can, I want to make a CNC plasma cutting table.

The mechanical and electrical part is no problem, so I will save that for later.

The problem lies in the torch height measurement. The torch of the plasma cutter must stay at the same height during cutting, even when the plate bends because of the heat.

The trick now is to create a control system that allows the torch to follow the "curved" plate. There are special kits for sale that can do this but they are very expensive. so there is no fun in doing that.
These kits work with an arc voltage measurement, the arc voltage is a measure of the distance between the torch and the plate. If you know the distance the control system is just a small trick.

My own idea is to use a voltage divider for scaling of the open arc voltage to a 0-5v voltage.
This I can read as an analog signal into an Arduino and then use 2 outputs (up, down) to adjust the height.
The problem lies in filtering the signal and protecting my Arduino to the high peaks.
A plasma torch puts out anything from 0 to 200/300 Vdc (negative BTW) and it's very dirty. A voltage divider gets this down to the required 0 to 5 for the arduino, but I need to make sure no spikes go above this.

My knowledge of electronics is limited, two years ago I have done a basic course electronics because at work it sometimes comes in handy.

Maybe some of you already have had this problem before and could give me some helpful tips. Thanks for your help!

Greetings Arclover

 

[DJDAudio]

One option is the old school approach.

**broken link removed**

Beveled ring with high precision linear pot.

I have used them, they drop down detect metal thickness, auto adjust power, and then track the height as they go.

I can imagine the precision needed for arc to distance measurement is gong to be difficult in the 0-5v range with the resolution of the micro-controller.

 

[Arclover]

This is also an option that is easy and inexpensive to make and would work with Arduino, but it is a contact measurement, I only going to use it as plan B

But thanks DJDAudio for telling me I hadn't thought of it yet, i'm already searching for local dealer who sell linear poti's or slide poti's.

 

[tcmtech]

Most of the commercial ones I have serviced just have a set of 4 - 6 ball rollers that go on the torch head that hold the metal sheets down while the torch travels around. Its way cheaper and 100% effective.

Trying to sense the arc voltage is nearly pointless due to the large number of variables that go with the arc characteristics. The electrode burning back raises arc voltage, thicker metal raises arc voltage, and varying the travel speeds changes the arc voltage, air density, humidity and pressure change the arc voltage, and nozzle orifice erosion changes the arc voltage as well.

Basically you would be far better off and money ahead just doing it mechanically.

 

[Arclover]

you mean these:

**broken link removed**

 

[tcmtech]

Yep those or the stud mounted ones that are easier to adjust up and down either way. The torch head gets spring mounted so that it can move up and down slightly while the rollers follow the lay of the metal.

 

[tom falah]

hi guys, I'm building a plasma cutter and was wondering if anyone could give help me with the initial hight controller for the torch and also wondering if i will need a protection circuit for it, in order to protect it from the arc voltage once the arc is on?

cheers,
tom

 

[Rbeckett]

Tom,
Might want to do a search. This subject has a fairly large body of work being done by a few individuals including myself. The biggest issue is kerf crossing and tip ups resulting in a torch colission. The initial pierce height is .125, and cut height is .063 and maintaining that in a closed loop is very difficult to do without a good bit of oscilation and jitters. Good luck and take a good look at the few threads that have info, or go to one of the machining sites and look in your specific equipment manufacturers forum. Tons of info out there on how to do it. Hope this helps point you in the right direction to get started.
Wheelchair Bob