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
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