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Author Topic: Plasma Application, Mach3 torch height control.  (Read 124371 times)

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Re: Plasma Application, Mach3 torch height control.
« Reply #70 on: March 25, 2010, 05:36:24 AM »
Jp - i like it, the only thing im wondering about is the polarity of your plasma voltage, my cutter is negative polarity, if yours is you need to connect your voltage divider to vdd not vss, so the arc will pull the voltage down.



Hi Matt,

As I have put a bridge rectifier, I don't think that I need to pull up the signal. I have already measured the bridge output and the voltage is positive when I plug the "black wire" to the Vss.
Furthermore I input the divider into an analog to digital converter, so I suppose that I can handle the output "logically" ...

Cheers
JP

Offline stirling

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Re: Plasma Application, Mach3 torch height control.
« Reply #71 on: March 25, 2010, 06:52:06 AM »
Agreed re: arc V being -ve. A question though about the full wave rectification of the arc voltage - why? I've seen this done on a couple of other THC interfaces. Surely any positive componant (which ideally shouldn't really be there) should be junked and not taken into consideration by the ADC. - or am I missing something?

Cheers

Ian

EDIT: the above post beat me to it... But are you sure about your polarity? a bridge rectifier can't change polarity of the arc voltage and your ADC input must be +ve.
« Last Edit: March 25, 2010, 07:03:58 AM by stirling »
Re: Plasma Application, Mach3 torch height control.
« Reply #72 on: March 25, 2010, 07:43:54 PM »
Hi,

The bridge rectifier is basically useless if you are sure of the polarity of your plasma... If not (or if you have doubt like me) then the bridge is simply protecting the output to possible reverse polarity insuring in a systematic way that the + output side will always be positive and the - always negative.

See here some more explainations : http://en.wikipedia.org/wiki/Diode_bridge





in particular check this sentence : In each case, the upper right output remains positive and lower right output negative. Since this is true whether the input is AC or DC, this circuit not only produces a DC output from an AC input, it can also provide what is sometimes called "reverse polarity protection". That is, it permits normal functioning of DC-powered equipment when batteries have been installed backwards, or when the leads (wires) from a DC power source have been reversed, and protects the equipment from potential damage caused by reverse polarity.

So whatever the polarity of you plasma is the bridge will insure that the input to the ADC is always positive.

Agreed ? (it was not so easy for me to admit it...but it's the basic  principle of a diiode bridge)

JP
Re: Plasma Application, Mach3 torch height control.
« Reply #73 on: March 26, 2010, 01:27:03 AM »
I agree with you 100%, i thought htat would work too, but it didnt for me.

My polarity is definately negative, when measure with true ground as a reference i get approximately 0v from one lead and - 1?? from the other, the bridge makesno difference to this, tried and tried, all it does is ensure that all the positive elements of both signals go one way and all the negatives go the other, if there is no positve element to either signal, ever, you cant get a positive reading.

I dont entirely understand it and im not saying you are wrong at all but all i know is for my system it made no difference to the polarity but is a good idea to protect against the wrong polarity.

Matt
Re: Plasma Application, Mach3 torch height control.
« Reply #74 on: March 26, 2010, 02:59:42 AM »
Matt,

I am surprised too, but I trust you !

I will ask this question to an electronics ingeneer I know (and specialized in high voltages) ... we probably could get more explainations on this topic.

However why do you measure with true ground as a reference ? (I understand the VSS signal being the ground)

On my plasma the ground (of the plasma box device) is not connected to any pin of the plasma torch.
Of course the metal piece to be be cut will be connected to the CNC ground... but from an electrical point of view the + and - signal of the torch are not if you isolate the metal part to be cut from the ground.

JP
« Last Edit: March 26, 2010, 03:04:46 AM by freedom2000 »

Offline stirling

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Re: Plasma Application, Mach3 torch height control.
« Reply #75 on: March 26, 2010, 05:07:10 AM »
LOL - not sure if we're agreeing or not  ;D

I think folks get mixed up with bridge rectifiers because they think they turn -ve into +ve. They don't, they basically just make sure that the CURRENT is in the same direction at the output regardless of what's going on at the input.

See attached pic - whatever potential diff you feed in and whatever way round it is you allways get the MOST positive side to the top output and the MOST negetive side to the bottom and therefore the CURRENT is allways in the same direction. BUT positive NEVER changes to negetive or visa virsa.
Re: Plasma Application, Mach3 torch height control.
« Reply #76 on: March 26, 2010, 03:42:01 PM »
We may not be agreeing but we are provoking interesting discussion!!

The reason i always test with respect to ground ground is because the meter measures potential difference, put it between you plasma + and - and you get 1?? volts, meausre one with respect to ground and you get approx 0v, the other gives - 1??. When you feed it into an ic unless it has a voltgae comparator and uses 2 wires for each analogue input, it will compare it to the -ve side of its supply voltage, which probably is on or near ground, so unless you want to float your whole power supply ground at the same as one side of the plasma thats what it will be compared to.

Sorry if i cant explain what i mean very effectively, i know what i mean i just cant get it out!

Matt

Offline poppabear

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Re: Plasma Application, Mach3 torch height control.
« Reply #77 on: March 27, 2010, 04:52:32 AM »
This is very interesting stuff, a low cost THC would be great!!!

   If you guys want, I can put alot of this stuff into a Plugin, and make an installer that would install it, a NEW THC screen set if someone wants to make one, the xml, a Flash Help file/how to doc, any needed VB macros, through a msi installer.

   I dont do any electronics nor do I fool with THC other than to tune a machine, so I dont really know that much about the inner workings of the THC process.
So, if you would tell me what you want, and what you want it to do, it can feed stuff to Screen DROS, Labels, tickers, LEDs what ever, through the Modbus, or pokeys, or what ever, it doesnt really matter.....

scott
Commercial Mach3 & Mach 4, Design/Build/Retrofit CNC and Industrial machines.
http://www.ss-systems-llc.com/
Re: Plasma Application, Mach3 torch height control.
« Reply #78 on: March 27, 2010, 08:21:07 AM »
Scott, Long time, no speak, too long!

Im interested in if you can come up with a way for mach to use a pid loop or some kind of proportional response to an analogue voltage input, maybe instead of just using a straight forward threshold for up or down you could adjust thc rate as well in order to make a proportional response, seems crazy that the torch is going to move up and down at the same rate if the voltage is a long way off as it would if it was nearly correct. Any thoughts??

The other thing im wondering is do you know of another way to get a straight forward analogue voltage into mach other than through the modbus?? People have criticised the system for being too slow because of the rate at which the modbus and or brains work, if maybe we could get the signal into mach using the pp or the serial port we could do better??

Just thinking as i type im wondering if we could come up with a screenset with all the required coding behind it to allow users to tailor their own solution, i made myself a normal mach screenset with an extra thc tab where i can see all thats going on, in addition perhaps we could have a thc setup tab where you specify where your input is coming from, you can adjust the proportional response to suit your system, you can set target voltage and use the look up table like others use, and then all the coding is behind it so that someone can simply download the plugin, instal it, provide an analogue or frequency input, tune the proportional response and they will have a working system, which is made for them and them only, they supply any hardware required to interface to their plasma etc maybe with some pointers as to how to do it and all thats needed is the plugin. Maybe an option for those who wish to use a plc to just use thc up and down and then use the screenset to give an on screen display of the arc voltage in mach.


What does anybody think??

Matt
Re: Plasma Application, Mach3 torch height control.
« Reply #79 on: March 27, 2010, 08:47:35 AM »
Great idea Matt !

Regarding the PID I think it's easy to do the job into the PIC, but would need a "speed signal" to be output for Mach3 to get the result information in addition to the UP/DOWN pins

For the I/O to/from Mach3 of "bytes values" an easy way would be to use serial COM (at least with a PIC it's easy to do). Don't know yet if Mach3 supports serial COMs but I suppose that yes !

JP