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Author Topic: Plasma Application, Mach3 torch height control.  (Read 154712 times)

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Re: Plasma Application, Mach3 torch height control.
« Reply #10 on: February 17, 2009, 05:15:56 AM »

i found this thread whilst looking for stuff on overburn but it hought you might appreciate my input.

Ive just this morning finished my home made dthc running entirly in mach, all i have done is made a simple voltage divider circuit with the plasma + connected to one end and 3.3v at the other since the plasma + is actually negative, this gives a 3.3v signal at 0 torch volts and 0 volts at 300 torch volts this connected to the 0-3.3vdc analogue inputs on my pokeys device, read them over the modbus into a brain which averages about 30 samples, this is then converted back to tip voltgae using a formula, compared to a value in a user dro on a custom screen i have made and triggers the relavant thc up or down command, then the usual thc perameters control the rate and the max/min moves and anti dive stuff.

It seems to work pretty well, the worst part is the pokeys samples so fast and the inherent error involved in reducing the voltage and then mathematically increasing it again mean the value for tip volts on the screen varies 3-4 volts when nothing is happening so it isnt as good as a "proper" one but it tracks the plate quite nicely if it has a bow in it which is all i am looking for and it has cost me nothing other than time, i had the pokeys device anyway and just knocked up the voltage divider using a couple of 0.25w resistors on some breadboard.

thought that may be of interest to you, i think personally all thc systems with mach will work like this in the future, i see poppabear is looking for a beta tester for such a system for an oem, i guess they think the same as me, why do i need to have a separate box with its own processing power do perform a relatively simple function when my pc is the mother of all plcs?

sorry for boring you

Re: Plasma Application, Mach3 torch height control.
« Reply #11 on: February 17, 2009, 10:22:14 AM »
It has become apparent to me that the motion controller i have only has analog inputs for the servo drives, so unfortunately i have no ability to use analog inputs at this time. I was in contact with Jim and soundlogicus about his THC and will likely order that when the time comes, it operates of a few simple I/O's and integrates well with Mach.  Im still trying to decide if I want to remove the table on my plasma and replace it with a water table etc, if i did that, i think i could get it flat enough to eleviate the need for a plasma, as the water table should reduce plate deformation etc, im gonna see how it plays out.

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Re: Plasma Application, Mach3 torch height control.
« Reply #12 on: February 18, 2009, 09:24:26 AM »
Keep us updated with that application it is good news
Re: Plasma Application, Mach3 torch height control.
« Reply #13 on: March 02, 2009, 06:34:54 AM »
Matt Would really like to know more about your DTHC.....
Thanks Dave
Re: Plasma Application, Mach3 torch height control.
« Reply #14 on: March 02, 2009, 01:45:00 PM »

what do you want to know?

Re: Plasma Application, Mach3 torch height control.
« Reply #15 on: March 03, 2009, 03:15:19 AM »
Matt I've been using Mach for sometime and never have gotten into the Brain and modbus, pokeys  thing,
But now I can see some use for it.
I have a router table that I can convert in a small amount of time to a Plasma table(wife likes me to cut wooden things, I like to cut steel) any way I have come to the conclusion that I need THC for this Beast and I am always looking for a different approach to things, not just buying whatever is out there to buy,
so with that said if you had to build from scratch would it be cheaper to go with a modbus or buy whats out there,
and if you were to build were would you get most of you componets.
Well I guess if it was easy, and cheaper, then everyone would be doing it, I guess !!
Thanks For letting me Babble as I see this could be the next great thing...I just need to understand more about pokeys, modbus and brains.
Thanks Dave
Re: Plasma Application, Mach3 torch height control.
« Reply #16 on: March 03, 2009, 08:47:51 AM »

so several questions,

is it cheaper than the systems out there?

Well i dont know much about others but candcnc is over $1000, ive spent under $100.

where did i get the components?

Pokeys comes from a chap called boris in chzech rep or somewhere, other components are just resistors and electronics components and some head scratching

how does it work?

So i connected a wire to the plasma torch and to the earth clamp of the plasma, when cutting the difference between them is 120 volts ish.

These are connected to a suitable bridge rectifier diode since the plasma puts out a square sine wave with a varying pulse width depending on current ( ithink, dont kill me if im wrong)

I cant truthfully remember which but one of the outputs from the brd is connected to samll voltage divider i made, the other is not connected since it is the negative and is just a 0v ish line.

since the plasma voltage is negative ( this had me foxed for a bit, i thought its a dc voltage, swap the wires and it will be +, no it isnt!) The voltgae divider is connected at one end to the +3.3v from the pokeys and to the plasma on the other side.

So when the plasma is doing nothing, or at 0v the voltage in the middle of the divider is nearly 3.3v, dependant on the resistor values you used, mine is about 3.0. When the plasma is fired the torch voltage goes to -120 so pulling the voltage in the middle of the divider down, i designed it so at -300 torch volts (thats the max voltage of the transformmer in the plasma) the output from the divider was 0v, so as not to kill the pokeys by applying a negative voltage.

as the voltage varies on the torch the divider voltage goes up and down.

So what is the pokeys?

the pokeys is a keyboard emulator, but for what im interested in its a usb device you install a plugin for in mach and provides 50 something inputs and outputs and three or four analog inputs which can easily be read into mach using the modbus feature, i know nothing about modbus but all you have to do is install the plugin and enable it, then in the brain editor select a new input from the modbus address of the pin and it will bring in value of 0 -1035, 0v =0 3.3v = 1035, ( ithink thats 10 bit?)

How does mach use the value?

I have a simple spice circuit simulator in which i modelled my voltage divider, its literally 2 0.25w resistors, i think one is approx 1.7k and the other 178k but dont quote that.

By testing different values in the simulator i was able to work out a simple equation which connects the modbus value (the o-1035 one) to the voltage of the torch  (it needs a x - in it to take out the negative voltage thing)

using a node in the brain with this formula in it will produce you a torch voltage in mach on the screen in whicheve dro you want!

From here i did some investigation, there are already dros in mach alloted for torch voltage, desired torch volatge etc etc, to find what these were i opened the candcnc screenset in screen editor and found all the dro numbers, button scripts etc and made my own screen with all of these in it.

Mach already has all of the logic for dthc built in so there is no need to use anything else, my brain simply compares my value read in (in the process described above) with the value i have typed into dro 1036 i think, but its whatever is in the candcnc screenset for preset volts and moves up if the voltage is too low or down if its too high, the thc controls are there in the brain editor under thc up and thc dwn, and mach does the rest, if all criteria are met, arc ok signal is on and thc is turned on it will adjust the torch position.

Bingo youve got dthc

ive added a few other things, candcnc make big play about tip saver and other things which basically amonut to anti dive, so i have put in an upper and lower threshold on the torch voltage outside of which the thc is disabled, so if you croos a kerf or something it doesnt go mad, or atleast leesens the amount it can go mad by.

I have also been looking at how to do this with no pokeys, i think if i was to put a simple analogue to digital converter on my analogue voltage and reading in the pulses across the parrallel port using the spindle speed input or something which will measure frequency, this may also give the advantage of a wider working band, the limitation with my device is that i am taking a perhaps 10v variation and reducing it to perhaps 0.5v variation and then mathematically multiplying it back up, so there is error but in fairness to it i can make a 200mm long cut with one end of the material raised by 50mm and the cut height is pretty constant and when im hundreds in pocket i cant complain.

I genuinely think your right, there is a merket for this, all you need to do is to make a little pcb with the voltage divider and adc on it and a mach plugin to install all the brain etc and you have yourself a dthc unit that will connect to any plasma (mine isnt hf start) and to any parallel port to make fully funtional dthc that shows you tip volts on screen, preset volts etc, etc, for cheap, its the future but im not the right person to do it so i guess ill let someone else make a fortune from it!!

if ive confused you dont wory ive confused myself, if you have any questions shout!


Re: Plasma Application, Mach3 torch height control.
« Reply #17 on: March 03, 2009, 10:54:49 AM »
Thanks Matt that was great, and no I'm not the one to make a fortune from something like this either, I just want a usable DTHC and I Think you have a great solution, that I would like to try. I'm sure I will be asking more Q's in the future as I pick up a few parts and understand more about the brain and pokeys.hope you don't mind.
Re: Plasma Application, Mach3 torch height control.
« Reply #18 on: March 03, 2009, 11:52:43 AM »

Not at all ask anything you want, the only way i learnt cnc was by reading what others had done so i feel that i should hand on anything i have managed to find, im like you, im not really interested in being able to control my torch to the nearest 1/4 volt but did find it frustrating when making long cuts and a slight warp in the plate made a mess of a part and am happy my system works well enough for me, others may rip it to pieces but it works and its cheap and the time i spent on doing it is what makes cnc a hobby!!

Re: Plasma Application, Mach3 torch height control.
« Reply #19 on: March 04, 2009, 03:35:53 AM »
Just ordered a pokeys, and yes just like you I have read allot about what other people have done and that is the best way to learn, or try new things.
I'm going back to your post and to watch the brain videos again,to learn and  understand this as much as I can before it shows up( plus my TD 39 plasma cutter quit, it's in for repair) so can't do much cutting until it comes back  anyways