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Author Topic: Mach4 can do Arc voltage sampling for a CNC plasma table?  (Read 742 times)

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Mach4 can do Arc voltage sampling for a CNC plasma table?
« on: June 07, 2020, 01:31:14 AM »
Hello everyone, it's been a long time since I wrote anything in this forum, today I have something to ask.

I have two plasma cutting tables, one of them 100% operational with Mach3 and an independent THC. I am still manufacturing the other table, I want to control it with Mach4, I am going to use the analog THC function "and I have a question about it.

Is it possible to use the method called "Arc voltage sampling" to set the correct plasma cutting height? Has anyone done it or knows how to do it?

For those who do not know what I am talking about, in this link there is a video of Lincoln Electric with a very clear explanation of this https://youtu.be/-OT8UWz69s0 and here is an article written by Jim Colt from Hypertherm in other forum https://www.plasmaspider.com/viewtopic.php?t=20551, where he explains very clearly how this method works.

Quote
Arc voltage sampling is an accurate method of setting the torch height using a physical height input instead of dialing in a specific arc voltage.
Not many height control systems use this....though they all should! Arc voltage sampling was first used about 25 years ago on high end ($100k plus) industrial cnc plasma cutting machines as a way to make height control systems easier to use and more accurate...critical to good cut quality and long consumable life.

Here's how a conventional (non sampling) plasma height control works:

-Operator hits start button...z axis moves torch toward material to be cut.
-Initial Height Sensing method (torque sensing, ohmic sensing, stall force sensing, limit switch sensing (also known as "floating head"), inductive sensing and more) locates the surface of the material.....then the torch retracts to the suggested pierce height.
-Torch fires.....stays at the pierce height until pierce delay times out (suggested by torch manufacturer for material being cut)....then indexes down to an operator set "cut height" (from the torch cut charts).
-x and y motion starts....and when the machine accelerates to near programmed cut speed....the arc voltage feedback height control takes over to maintain a constant distance between torch and material. This feedback circuit reads the actual voltage between the torch electrode and the material.....and compares that voltage to a voltage set by the operator (from the torch cut charts).....if the read voltage is higher than the operator set voltage....then the z axis motor drives the torch closer to the plate until the voltages both match (actual voltage vs pre-set).

The problem with this method is that a lot of things affect the voltage reading.....if you change cut speed you need a different voltage to maintain the correct physical cut height, if the cut air pressure changes....you need a different voltage, as the consumables wear you have to set a different voltage, and more.

A system that uses "Voltage Sampling" is easier to set and use....as the operator does not have to set the arc voltage. Here's how it works with the differences in bold:

-Operator hits start button...z axis moves torch toward material to be cut.
-Initial Height Sensing method used is ohmic plate contact...because it more accurately locates the material surface as compared to any other method. It electrically locates the surface of the material.....then the torch retracts to the suggested pierce height.
-Torch fires.....stays at the pierce height until pierce delay times out (suggested by torch manufacturer for material being cut)....then indexes down to an operator set "cut height" (from the torch cut charts).
-x and y motion starts....and when the machine accelerates to near programmed cut speed....the actual arc voltage is read by the THC electronics.....which "lock onto this voltage (does not have to be set by operator) and the arc voltage feedback height control takes over (using the sample voltage) to maintain a constant distance between torch and material. This feedback circuit reads the actual voltage between the torch electrode and the material.....and compares that to the sample voltage. If the read voltage is higher than the sample voltage....then the z axis motor drives the torch closer to the plate until the voltages both match (actual voltage vs sampled).

The advantage of this....the operator only sets cut height (physical height) and the electronics locks onto the voltage sampled at that height. This calibrates the height control voltage / height relationship before every cut cycle.....and compensates for speed changes, air pressure changes and consumable wear...maintaining constant height. Ohmic contact is required with this based on its accuracy at setting the correct physical pierce height and cut height...as it uses a very light touch without deflecting the material as the other Initial Height Sensing methods (torque sensing, stall force sensing, limit switch sensing (also known as "floating head"), inductive sensing and more) do.

- Jim Colt Hypertherm
Re: Mach4 can do Arc voltage sampling for a CNC plasma table?
« Reply #1 on: June 07, 2020, 07:25:54 AM »
Hi,
there are two means of THC.

Mach4 has introduced a script based THC, one of the first Windows based CBC solutions to do so. Because Windows is
not a realtime system in the past the communication delays have all but precluded a script THC solution. Given that
Mach4 is faster it can do a fairly slow control loop without a great deal of hardware.

If you demand really high dynamics and robust anti-dive features then you need to get a genuine realtime THC solution.
At the current time only two control boards support realtime THC, the Hicon from Vital Systems and the ESS from Warp9TD.
The ESS supports the TMC torch height control system specifically and probably has the most up-to-date anti-dive algorithms.

Craig
My wife left with my best friend...
     and I miss him!
Re: Mach4 can do Arc voltage sampling for a CNC plasma table?
« Reply #2 on: June 07, 2020, 03:54:41 PM »
Hi,
the script based Mach4 THC solution samples the arc votlage approximately 100 times a second.
The Nyquist Sampling theorem tells us the theoretical maximum control bandwidth is half, ie 50Hz.
A Control Engineer will tell you that the 'discrimination' is rubbish at 50Hz and only starts looking like
a modest performing control loop around 5-10Hz.

Realtime THC controllers sample the arc voltage thousands of times per second.

Craig
My wife left with my best friend...
     and I miss him!
Re: Mach4 can do Arc voltage sampling for a CNC plasma table?
« Reply #3 on: June 17, 2020, 03:46:32 PM »
the Hicon from Vital Systems and the ESS from Warp9TD.
The ESS supports the TMC torch height control system specifically and probably has the most up-to-date anti-dive algorithms.

In my opinion, the Vital Systems torch height control system is leaps and bounds better than the ESS. In fact, I heard the guys at Swift Cut automation integrate with their controllers for some of their plasma machines. The anti-dive on the Vital Systems THC is very sophisticated. Just watch it move up and down this piece in the video.

Code: [Select]
https://www.youtube.com/watch?v=rNXNdOduXRo
   
Re: Mach4 can do Arc voltage sampling for a CNC plasma table?
« Reply #4 on: June 18, 2020, 12:52:20 PM »
Thank you guys for fast response. but I still have the question of whether it is possible to use the method called "Arc voltage sampling" to set the correct plasma cutting height?

It is possible tho do this with the mach4 script based THC or with an external THC?
Re: Mach4 can do Arc voltage sampling for a CNC plasma table?
« Reply #5 on: June 18, 2020, 10:21:06 PM »
I'm sure that is how most torch height controllers work - it is on the two external ones I own (TMC3in1 and a price cnc THC). They sample the cutting voltage either directly from the plasma cutter (if the plasma cutter can supply the required reduced voltage) or through a module that reduces the voltage to a level that the height controller can read.
Steve
Re: Mach4 can do Arc voltage sampling for a CNC plasma table?
« Reply #6 on: June 18, 2020, 10:39:55 PM »
Hi,
as Wallerwang has posted the THC controllers sample the arc voltage and present an Up/Down signal to the motion controller.

Quote
It is possible tho do this with the mach4 script based THC or with an external THC?

Mach4 cannot read an analogue voltage, you must have some other device to do that. For instance the PoKeys 57CNC has several
analogue input channels. Therefore if you had a suitable voltage divider and signal conditioning circuit the PoKeys could sample the
voltage and the numeric value would be communicated to Mach whereon it could do its script based THC thing.

Alternatively you could use a THC module like a TMC or a Proma and that would sample the arc voltage, with or without a voltage divider,
and compare that to the target voltage (that you program into the module) which would produce an Up/Down signal which Mach
could then operate on.

The last alternative, and the best method is to use a TMC module in conjunction with an ESS or a Proma module in conjunction with a Hicon Integra.
These last two solutions are genuine realtime.

Craig
My wife left with my best friend...
     and I miss him!
Re: Mach4 can do Arc voltage sampling for a CNC plasma table?
« Reply #7 on: June 19, 2020, 05:09:47 PM »
Thank you guys for the help.

Recently I built a new CNC plasma table, this one have a CSMIO motion controller form CS-LAB, this motion controller have analog input on it (0-10 V), I send to this port the voltge from my plasma source (Powermax65) divided 50:1 by the CPC port of the plasma source.

It is works well, but I have to change the "target voltage" every time I need to cut diferent tickness metal. (see attached image)

I hear about the "arc sample voltage" option in some machines, it is why I'm looking if it possible to have this opption in Mach4 with my actual setup or know that I have to change in my machine to have it.

With this option I do not have to set a "target voltage" every time it changes thickness, since this is done automatically.
Re: Mach4 can do Arc voltage sampling for a CNC plasma table?
« Reply #8 on: June 19, 2020, 09:04:09 PM »
Hi,

Quote
I hear about the "arc sample voltage" option in some machines, it is why I'm looking if it possible to have this opption in Mach4 with my actual setup or know that I have to change in my machine to have it.

That is exactly what the analogue input of the CSMIO is doing, its sampling the arc voltage and reporting to Mach. Mach4 or the CSMIO
do not know the thickness of the material unless you tell it.

If you did indeed tell Mach4 the thickness of the material it would be a simple matter of having Mach calculate the appropriate target voltage,
or alternatively look up a table for the appropriate voltage for a given thickness and then have Mach automatically update the target voltage register.

Craig
My wife left with my best friend...
     and I miss him!
Re: Mach4 can do Arc voltage sampling for a CNC plasma table?
« Reply #9 on: June 19, 2020, 10:46:21 PM »
Hi Johnny
I don’t think you fully understand the plasma cutting process yet. The automatic arc voltage sampling you are looking at is the last thing you would want it to do because you would get poor quality cuts because more factors need to change other than auto adjusting voltage for different material thickness.
You need to be able to set your material thickness in your cam software (I recommend SheetCam) because a lot of different aspects of machine setup effect cut quality other than just cut voltage.
As material thickness changes also so does things such as your kerf width, cutting speed, cutting voltage to get a clean cut and on and on. None of this can be set just by measuring the cutting voltage. I can cut a lot of different material at the same voltage but it won’t look as good as it could as well as the same material thickness at different voltages and again not all will look good. You need to have some manual input to get the best you can.
The arc voltage sampling has been done for you by the plasma cutters manufacturer and is in the machine manual (it is in my Hypertherm one) - you set your required voltage from the manual into your cam software for each material set up.
In SheetCam you setup tools for each material type, thickness, consumables used and quality required and you can get very good results with some effort.
I can tell you from my experience that getting height control to work is not that hard - getting it not to work when you don’t want it to is the key to success. You don’t want height control to work at the start of a cut, while your machine is accelerating or decelerating, while cutting small holes, going over pre-cut lines, going around sharp bends and at the end of cuts - all these things will make your torch height control crash your torch.
Your success with this will be in your cam software and it having the ability to turn on and off height control depending on the features you are cutting and also having a height control that can be disabled in Gcode by an output activated during the run.
I hope this makes sense as it is at the end of the day for me and I’m a bit tired.
Steve