Arc voltage sampling

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gamble
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Arc voltage sampling

Post by gamble » Thu Jun 09, 2016 4:54 pm

How does the CNC work different when sampling is turned on vs turned off?
Is it bad to leave it on all the time?

How often do you guys sample the voltage? Should it be done often or every sheet or every cut?
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Re: Arc voltage sampling

Post by SeanP » Thu Jun 09, 2016 5:25 pm

What I tend to do on material I have cut previously is keep a close eye on the z dro just as thc delay led goes off, if the torch drops add a volt or two depending on how much, same if it lifts as the led goes out take a volt or two off the preset volts, that's provided there is no spring back in the touch off, sheet is fairly flat and thc delay is't way too long.
Then next part as it comes off thc delay should keep to correct cut height.
If it's thick material and a slow feed rate you can also turn off thc manually, see what torch volts reading is showing as it comes off leadin and settles down, quickly add that into volts preset, switch thc button back on and away you go.
Now that works fine on my parallel port I'm not sure if that method works with ethercut,,,,try it.
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Re: Arc voltage sampling

Post by jimcolt » Fri Jun 10, 2016 12:27 pm

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.

Any questions? Jim Colt Hypertherm

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Re: Arc voltage sampling

Post by BTA Plasma » Fri Jun 10, 2016 1:23 pm

The biggest drawback to arc voltage sampling is getting a false start. If you have water on your material or you pushed it down ...ect...many other things that can affect the initial material sensing. With that in mind you may start off by dragging your material and end up with a broken torch or a released torch. I have seen various tricks to voltage sampling like minimum sample, maximum sample. But not being able to change the height on the fly makes it so you will may never be able to cut head and tail remnants from plate rolling mfg. Advantage, disadvantage

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Re: Arc voltage sampling

Post by SeanP » Fri Jun 10, 2016 1:58 pm

Interesting stuff and well explained there Jim.
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Re: Arc voltage sampling

Post by tcaudle » Sat Jun 11, 2016 5:44 pm

The single biggest issue we see is that users never go though the initial test cut steps (in the setup) so they never confirm that the touch off is working right, that it gives the correct pierce height and that the user has properly set the proper initial cut height or that the feedback voltage from the plasma is correct. If it starts out wrong and samples the voltage while its too high or too low then the result ends up that auto set does not work. It becomes more problematic when you have a whole series of short cuts. There are types of cuts and conditions that call for the THC to be temporarily disabled.
I suggest it (automatic arc volts setting) would work better if the system had a "normal" preset voltage it reads that would let it make dynamic changes within a % range. Outside of that it either makes no change or optionally stops the cut. So starting with a default preset from a stored setting or chart and letting it compensate in a rather narrow range would handle consumable wear, material density changes and even some surface contamination and only if the conditions were right , would prevent things like the sample area being in a zone with less than minimum clearance or cutting a smaller detailed object causing the settings to get too far out of whack.

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Re: Arc voltage sampling

Post by Nacs Fab » Wed Jun 15, 2016 11:26 am

Jim, since arc voltage is read from the electrode to material, will a nozzle with some use (dirty) affect the arch voltage? I had a sheet cutting last night and when I went around the outside cut, it never changed my torch height and figured that could have been the issue since I did not sand the nozzle clean prior to cutting.
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Re: Arc voltage sampling

Post by jimcolt » Wed Jun 15, 2016 11:50 am

The nozzle (worn or dirty) has little effect on arc voltage, however the electrode does. The plasma arc starts on the electrode (on the center emitter which is hafnium), goes through the nozzle orifice to the material being cut. The voltage that is monitored by the height control is measured from the electrode to the material. As the electrode is used....a pit develops as the hafnium erodes, and since the arc attachees to the center of the hafnium the arc length gets longer as the electrode wears. A longer arc = higher arc voltage......the height control then moves the torch closer to the material as the electrode wears in order to maintain the preset arc voltage.

It is not uncommon to have a 1/16" or deeper pit in the electrode, this means that with a height control that uses a "preset" voltage to control height....that the torch will run 1/16" closer to the material than it did with a new electrode. For best cut consistency in terms of dross and edge angularity...the torch should always cut at the recommended physical cut height, the voltage does not matter (it is just used as feedback to maintain cut height). On a machine with voltage preset....a good operator will slowly increase the arc voltage as the electrode wears in order to maintain the correct cut height through the useable life of the electrode. Typically, if you increase the voltage by 1 volt the torch will rise by roughly .004". So to correct for a 1/16" pit in the electrode you may need to increase arc voltage by around 15 volts between a new electrode and one that is at the end if its life.

On a system with voltage sampling you do not preset a voltage, rather, you accurately sense the location of the surface of the material (ohmic sensing)..then when the torch moves accurately to the physical cut height and starts moving....the electronics sense the voltage at the correct height and lock on that voltage. With this type of sensing the height control recalibrates on every cut cycle...and automatically compensates for electrode pit depth, maintain the best height for consistent edge angularity.

Don't sand or file nozzles and electrodes, if you feel the need to clean them use a piece of scotchbright and shine them. Filing and sanding will affect the accurate dimensions that these parts are manufactured to...and all bets on cut quality will be off!

Jim Colt Hypertherm

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Re: Arc voltage sampling

Post by gamble » Wed Jun 15, 2016 1:11 pm

I thought hypertherm electrodes had tungsten in the cneter, not hafnium?
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Re: Arc voltage sampling

Post by SeanP » Wed Jun 15, 2016 3:41 pm

Recently I have been using a little brass suede shoe cleaning wire brush to clean up consumables and shield, it works really well.
jimcolt wrote: Don't sand or file nozzles and electrodes, if you feel the need to clean them use a piece of scotchbright and shine them. Filing and sanding will affect the accurate dimensions that these parts are manufactured to...and all bets on cut quality will be off!

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Re: Arc voltage sampling

Post by jimcolt » Tue Jun 21, 2016 4:10 pm

A tungsten electrode would last seconds, not hours using air (or oxygen) as the plasma gas. They are hafnium....and hafnium works well with nitrogen as well. We do produce tungsten electrodes for our systems that use non oxidizing gases at 130 amps through 800 amps.

Jim Colt

gamble wrote:I thought hypertherm electrodes had tungsten in the cneter, not hafnium?

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Re: Arc voltage sampling

Post by gamble » Wed Jun 22, 2016 9:33 am

AH ok.
I can't for the life of me figure out why my torch is crashing. Last night I was cutting some .060 5052 with sampling on and the torch decided to dive into the metal. Every time I don't have sampling on the same thing happens. New consumables as well. How can I test to be sure there isn't a hiccup in the PM45?
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Re: Arc voltage sampling

Post by jimcolt » Wed Jun 22, 2016 1:57 pm

Can you tell when it is diving? In corners? Kerf crossing? Straight cuts? Right after it starts moving?

All the Plasma does in regards to height control.....is to supply the actual arc voltage divided to 1/50th. This voltage is used by the height control electronics to determine approximate height.

If the cut speed slows (as it will on every sharp corner)...the cnc control has to "freeze" the height control so the torch will not dive.....this is because when the cut speed slows....the arc produces a wider kerf, the arc gets longer to reach the metal (longer arc equals higher arc voltage)....and the height control moves the torch closer to stay at the voltage that was determined during sampling. If diving in corners and on small holes and features....then maybe the setting for height freeze is incorrect.

If the torch dives at the beginning of a cut.....then there may be an incorrect setting for "arc Voltage Control Delay" . This delay is to keep the torch from diving when the torch accelerates from zero to the cut speed at the beginning of the cut.

If diving on a straight cut where speed changes are unlikely....I would suspect a poor connection between the work lead and the material being cut. (loose connection, dirty table slats, poor connection between the aluminum and the slats.....or maybe a loose connection to the divided arc voltage at the plasma cutter.

Voltage based height control is dependent on speed , on amperage and on proper settings.

Jim Colt Hypertherm

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Re: Arc voltage sampling

Post by gamble » Tue Jun 28, 2016 1:04 am

Jim,
I was playing around tonight with it. I was cutting 25 (or supposed to) of the same piece. With sampling arc voltage on it was set to 117v, after a few cuts it dropped to 105, then 95 then would crash. I set the voltage to 111 with sampling off and got it to cut a few small parts (6 pierces, 1 minute each part) and then it would still crash. All the same parts nested together. Mostly when it first started but not always. All parts were the exact same, so I would think if it was in a corner it would of done it a few parts before it.
So doesn't matter if sampling is on or off it will still crash. With sampling off it will crash every time within 2 -3 minutes of cutting.
Also the pierce height was set to .160 and cut height at .6. Many times tonight it would go to the pierce height and the pilot arc would go on but never go into cutting mode. Sometimes i'd lift up on the sheet to bring that closer and it would cut and at one point I lifted that sheet to touch the shield and it still didn't go into cut mode.
Curious to know if maybe my PM45 has some internal issue that is causing some of this. I just wrote flashcut a big email with a lot of the issues I was having tonight
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