Voltage Scale and Auto Volts

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13 Dec 2021 23:03 #229030 by CFE
Replied by CFE on topic Voltage Scale and Auto Volts
Hi all,

I had a look with Halscope and the voltage is fluctuating about 3 volts at idle. About the same when cutting. The fluctuations seem pretty consistent in time, I don't see spikes larger than 3 volt when at idle or cutting. Can a 3 volt fluctuation trigger this behavior ? The THC treshhold is set at 1 Volt.

Do we consider setting the voltage divider on the Hypertherm at 20:1 and make the relevant adjustments ? Will there be an advantage in doing that ?

I checked the cable and shielding for the arc voltage and that looks like it is grounded as per info I have been able to find here on the forum. The THCAD has it's own power supply, separate from the motor drivers and breakout boards. I am reasonably confident that is okay, but I will try to route a different cable from the plasma cutter to the controller to see if that makes a difference. On that note, we have the THCAD in the controller box, would it be better to have the THCAD mounted in an enclosure on the back of the Hypertherm, to minimize noise on the cable feeding the arc voltage in the breakout board ?

Cheers, Peter

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14 Dec 2021 00:35 #229032 by rodw
Replied by rodw on topic Voltage Scale and Auto Volts
If you look in halscope you will probably find there is an AC ripple at 50 /60Hz depending on your  supply.
As Snowgoer mentioned, try with  the GND wire from the THCAD removed. In most countries the Neutral wire is bonded to earth at the meter box so the AC ripple comes back up the earth wire. I found it added quite a lot of noise at the THCAD with this connected to frame ground.

Just let halscope run without cutting and remove the GND and see what happens at 0 volts. It may quieten down.

There is no need to move the THCAD. I've always had mine in the cabinet with a 120 amp machine.

If you can change the divider to 30:1 on your larger machine its 300 v full scale. Otherwise 20:1 is OK with full scale of 200V which is higher than cut volts except when piercing and the THC is not looking then!

Also,  the THCAD should be powered from the mesa card. From memory there are 5v signals on the encoder for this purpose.
Its coming back to me now, thefabricator03 had similar issues to you and I think the separate power supply is the culprit! Should be an easy fix.

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14 Dec 2021 04:44 #229038 by CFE
Replied by CFE on topic Voltage Scale and Auto Volts
Hi all,
I have tried all suggestions made here (thanks for all the input !), but the gremlin is still there. As per Tommy's suggestion I have set up a configuration with lower accelerations. I have added a lowpass frequency, tried different settings for that, to no avail. I have bumped up THC treshold to 8 and 10 Volts, to no avail. Halscope shows a reasonably flat line, as least as flat as I would expect when cutting 25mm thick plate (the Hypertherm works pretty hard on that !) Disconnected the ground wires from the THCAD, that doesn't make a difference, either while cutting or on Halscope. The THCAD got power from the computer power supply, which I suggest is has a nice smooth output.

The bloody thing can just cut along nicely, and just like that the torch goes up or down very rapidly, respectively triggering the float switch or causing lost arc. I mean, if the torch lifts up, it lifts up 15, 20mm very rapidly, in the blink of an eye. I cannot imagine the THC is responsible for that. It just seems that the Z-axis gets triggered by something else than the THC, but I cannot work out what that would be.

The frustrating thing is that it just happens so randomly, I can cut 1 meter without trouble and all of a sudden the thing plays up. Just when I think I make progress the thing decides to do its own thing. Just running out of ideas.

Cheers, Peter

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14 Dec 2021 04:59 #229039 by rodw
Replied by rodw on topic Voltage Scale and Auto Volts
This is the thread I was thinking of where Stefan had similar issues.
forum.linuxcnc.org/plasma-laser/40712-th...fluctuations?start=0
Possibly ambient temp related,
Possibly noise (THCAD ground again)
Possibly becasue its not using power from the mesa card.

I really think Peter was on the money with outside environmental factors.
I'd look for the Hypertherm electrical grounding document and implement every recommendation. I did.

 

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14 Dec 2021 11:06 #229047 by tommylight
Can you cut manually with that plasma?
Might be Z axis drive interference.
Run the machine with torch off in LinuxCNC, and start the torch manualy see if that happens.
If it does, check Z axis wiring and shielding.

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14 Dec 2021 11:11 #229049 by snowgoer540
Do you run the default P I D values for the THC in the Parameters tab?

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15 Dec 2021 04:25 #229094 by phillc54
You could possibly try plotting axis.z.eoffset and joint.3.motor-pos-cmd (assuming joint 3 is your axis z) probably with both having a gain of 50/div.
You would need to use the Roll Run Mode and see if either or both of these have any value while your Z axis is doing it's odd thing.

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20 Dec 2021 05:22 #229539 by CFE
Replied by CFE on topic Voltage Scale and Auto Volts
Hi all,

I have redone the wiring and shielding on the THCAD, changed the frequency divider to F/64 and changed the voltage divider on the plasma cutter to 20:1, along with the voltage scale and offset adjustments. I have cut 12mm plate at 65A no problem, but again got in trouble cutting 25mm plate on 125A, 610mm/min.

I had a play with the THC Speed setting, and it improved when I set that at 1. Setting it at zero basically locks the THC. I have set PID-I at 1000 after a few trials and that made a minor improvement. But every now and then the gremlin still rears its ugly head, and the torch just dives or lifts up, albeit much slower than before, I suppose due to the Speed (PID-P) set at 1.

I wonder if it is possible to set the Speed (PID-P) somewhere at a fraction, for instance at 0.5 to see what effect that has ? The current interface only allows for multiples of 1. Is there a value in a config file which I can manually set ? Or a way to change it in the interface ?

Cheers, Peter

 

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20 Dec 2021 05:54 #229542 by phillc54
You can do the following to achieve this but you are really only masking the issue.

In the [DISPLAY] section of the ini file add a new variable:
USER_COMMAND_FILE = custom_code.py

Create a new file in the config directory named custom_code.py

Have the following in the custom_code.py file:
self.w.pid_p_gain.setDecimals(1)
self.w.pid_p_gain.setSingleStep(0.1)

The second line is optional and is only used to step 0.1 at a time with the arrow buttons.

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20 Dec 2021 23:39 #229580 by CFE
Replied by CFE on topic Voltage Scale and Auto Volts
Hi Phil,

Thank you for that. I might be masking an issue, but setting the Speed (PID-I) at 0.1 enabled me to see what is happening.

I am cutting 25mm steel at 125A, feedrate 610mm/min. Voltage hovers around 110 Volts (Hypertherm 125 manaual says it should be 169, but it cuts alright at this voltage, don't know why). I pierce the material first to remove dross. Then start cutting, and at some point the Z-axis slowly starts moving up (it used to do this rapidly with Speed (PID-I) at 10. I can see the voltage steadily drop to 100 Volts, but the THC is not correcting, the Z-axis just keeps lifting the torch up until of course it loses arc.

The configuration I use for cutting thick plate has all of everybody's recommendations, lowered accelerations as per Tommy's suggestion etc. I have also rechecked all the wiring, moved the THCAD away from power sources to avoid interference etc. Parameter settings are 4 seconds for THC delay (for the lower feedrate, gives the arc time to stabilize), THC Threshold 1 Volt, Speed (PID-P) 0.1 (to see what happens), VAD Threshold 80%, Void Override 80%, PID-I and PID-D both 0.

I am quite confident we can rule out Z-axis interference, I have used this configuration successfully to cut 12mm and 16mm steel without any hiccups, the THC worked just fine.

I do not know if other people cut much 25mm plate and maybe have suggestions ? Or are we the odd duck out here ? Mind you, if we disable the THC the machine cuts 25mm plate no worries, but the plate has to be exactly level on the bed. It would be just nice if we could find out why this gremlin occurs.

Cheers, Peter

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