ESAB Ultrarex UXD-P 2500 retrofit with Linuxcnc and Proma THC

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09 Feb 2020 11:13 #156862 by alex.statie
Hello!

Could you please tell me what would be the advantage of the Proma thc instead of using an arduino linked to the plasma’s voltage divider that sends the up/down signals to the 7i77 GPIO pins?

Is there any advantage besides noise insulation? I am trying to figure out if the cost of the Proma is worth it.

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09 Feb 2020 12:16 #156870 by rodw
Given you are using a Mesa 7i77, I would go with a Mesa THCAD (USD $69) attached to an encoder input and let the Plasmac config do the THC function internally within Linuxcnc. Even Tommy the staunchest supporter of the Proma used a THCAD for his last build.
The following user(s) said Thank You: tommylight

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09 Feb 2020 15:11 #156879 by tommylight
The following user(s) said Thank You: rodw

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09 Feb 2020 20:45 #156906 by alexandrub

tommylight wrote: As Rod said, you can add an encoder to the existing DC motor and use a servo drive for Z axis.


I agree this, i can adapt an encoder but where to find an analogic servo driver? I suposse i cant find a new one on the market. If i wrong, please give me an advice.
I attach a picture with the motor .
Attachments:

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10 Feb 2020 17:39 #156978 by Todd Zuercher
A servo drive (or amp) for an ordinary brushed DC motor isn't difficult to find new replacements for. (And inexpensive used ones are plentiful as well.) Figuring out which one is appropriate for your motor's specs is the toughest part.

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10 Feb 2020 18:07 #156980 by tommylight
The machine has a drive for that motor that you can use, it is located in a metal box behind the Z axis metal box, with plenty of other electronics inside, so you will have to find the drive, find the control inputs for it, wire a shielded cable from that input to the Mesa 7i77. Also wire the new encoder that you can mount to the side tacho or above it, also to the 7i77 encoder inputs.
Make sure you check the schematics for the drive to see if it uses +-10V or +5V and dir pins.
The THC is controlled through an RS232 originally, so i did not bother with it, just wired a drive to a stepper motor that i put on top of the existing motor after removing the tacho and using a coupling.

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11 Feb 2020 05:01 - 11 Feb 2020 05:02 #157012 by alexandrub
Instead of encoder, can i use just the mesa or proma thc as ecoder? I touch of the metal sheet, lift up the torch a time...20ms for example and then start the cutting with thc control.
Last edit: 11 Feb 2020 05:02 by alexandrub. Reason: I forgot a question

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11 Feb 2020 05:27 #157019 by rodw

alexandrub wrote: Instead of encoder, can i use just the mesa or proma thc as ecoder? I touch of the metal sheet, lift up the torch a time...20ms for example and then start the cutting with thc control.


I wondered about that too. Its just the initial touch off that needs accuracy. Once you are cutting you could use a PID to manage the PWM signals with arc voltage as the feedback signal I'm sure. Its possible you could monitor the tacho until it stalls to set the material height. I did skim the manual for the motor and I think the tacho just outputs a DC voltage based on the speed. You may be able to use a 7i76e which has some analog voltage inputs to measure this voltage but it might need amplifying to get a good full scale reading.

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11 Feb 2020 05:30 #157020 by rodw
Actually, you might be better off monitoring the Tacho with a THCAD if you have enough encoder inputs. It would be much more precise with the voltage measurements....

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11 Feb 2020 10:13 #157037 by tommylight
Warning, tacho outputs positive and negative voltage, so wiring it to Mesa inputs will result in damage to inputs.
Velocity mode servo can not be used without feedback for precise control.
It can be used for THC positioning but it requires using AD converters complicating things a lot.
The cheapest solution in this case is using an encoder, even one with very low count works as the motor is geared.
The easiest solution is to add a stepper motor and drive.

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