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Messages - matttargett4

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61
dave,

if ive got you right you mean a transducer which will put out a 0-5v signal dependant on the current in the plasma line?

if thats what you mean it wont do what you want,

since the plasma power supply delivers a fixed current (set to whatever the knob on the front is) as the rsistance the arc feels varies the voltage must also

since V=IR

I=V/R

I is constant so an increase in resistance decreases the voltage

since the plasma is cutting the same pice of plate all thetime the defining factor for resistance is the distance the arc has to "jump" across to get to the plate, or torch to works distance so giving the arc voltage direct proportionality to the torch to work distance.

Sorry if i told you something you already know

btw you must be 0-3.3v for the pokeys

matt

62
here are the three brains i ue, i ll let you look at them to try to work it out before i tell you how it works!

matt

63
Promote and discuss your product / Cheap Plasma Digital THC
« on: March 05, 2009, 03:54:00 PM »
Hi all,

Im  looking to gauge interest on a product which i could develop a little further and bring to market but dont want to spend a load of time on it if no one is interested.

Basically the product is a $100 digital torch height control providing all the functionality of other systems, preset profiles for different materials, on screen display of torch volts, on screen adjustment of target volts etc etc.

The product would connect to any plasma cutter, probabaly those with hf start also

it could be connected through either a couple of pins on you standard parrallel port or via the modbus

im not gonna tell you it will be perfect or claim to hold it accurate to 0.25volt because it wont and i dont think it needs to but it will keep a good enough distance between the torch and the work to work on warped paltes, i think it is accurate to about 5 volts, jim colt mentioned somewhere this is good enough for air plasma and it cheap so you cant expect it to be perfect

So what do you think? would you be interested?

64
General Mach Discussion / Re: Plasma cnc
« on: March 05, 2009, 03:30:09 PM »
Hi,

i cant comment on the waterbed until i see a pic of your machine but i may be able to help with the other questions,

i dont know what post processor you are using in sheetcam but this will be where you problem is for the m01 optional stop, i think some posts have a variable in them which stops the machine to allow you to check the consumables before continuing, i think it is called "piercelimit" have a look in you post this may be the problem, the post can be edited using wordpad.

the trouble with fonts will be to do with how the cad program is exporting them, i cant remember the details but it think its something like sheetcam wont recognise splines only curves and so if your cad program exports text as splines sheetcam will not pick them up. Personallyt i use corel draw for anything with text in it and it has an option to "convert to curves" which solves the problem but the export is not very good, also check out sheetcam tng, it has the ability to put text into a part.

As for the pierce delay i found with sheetcam that the pierce delay number i put in didnt appear to correspond to seconds, my value for 6mm plate is 1500 i think, try a big value and see if it makes any difference, also if you are using a thc sheetcam may not be the problem, usually the thc uses an arc good signal and puts in its own pierce delay after the arc good signal or use a dleay in mach so the problem could also be here.

hope ive helpsed a bit not just confused you!

65
dave,

due to miss spelling i tend to refewr to them as brians, ill fetch them off the shop machine and post them up for you to have a look at..

matt

66
Dave,

just shout if you need any help

matt

67
Dave

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!!

matt

68
ok,

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!

matt


69
dave,

what do you want to know?

matt

70
Hi,

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

matt

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