Plasma and Ethercat?

Hey Guys,

I have retrofitted a lathe and two Bridgeport Interact Mills using Mesa 7i76e's.  I am happy with the 7i76e and pncconf but due to their availability (or lack thereof) I am considering doing my new 'from-scratch' plasma table build using an Ethercat interface. 

My question is this: If I go down the Ethercat path will I be able to re-use all of the good work that has been done in the plasma area or will I be doing custom builds?  More specifically can I set up a plasma system using Ethercat by modifying .ini and .cfg files?  I am willing to recompile some fork of the main line of development but I don't really want to be on the 'bleeding edge' of the development effort.

Thanks in advance and Happy Thanksgiving!

JH

 

It has been done before. Ethercat is really just a way of controlling your motors and I/O so it does not change anything or prevent you from building a plasma table.
Choose a plasma cutter with a proper CNC interface and voltage divider that does 20:1 or 30:1 division
You will need an Ethercat I/O module for inputs and outputs.
Torch voltage sensing is something the 7i76e/thcad combo does well. the THCAD has a 0-10 volt range with 500volt overvoltage protection
In an Ethercat environment, I would look for a 0-10 volt Ethercat analog input module that has overvoltage protection to 500 volts.(not hard to find)
To give QTplasmac the voltage output, you should be able to scale the voltage using the QTplasmac voltage scale settings.

Another probably more costly way would be to buy an Ethercat encoder input and attach a thcad to it.

Advanced options would be to home using the drive internals and implement torque mode homing to eliminate the need to wire home switches. This would require a custom homing module in linuxcnc. I am not sure if gantry homing would be supported.

May I ask, why would you need 500V OVP? Pick a plasma and I dare you to find one with an open circuit voltage (OCV) over 400V, this would be the voltage without current flowing. No plasma in the cutting business will sustain a 400V arc length with a current capable of cutting metal. Plasma's used to cut metal, will typically not go much beyond 200V and the powermax type of stuff 150V (these are rough numbers). Then lets say you divide it by 20 (the lower one of your dividers) that's 20V.

Mesa that is connected directly to the raw arc voltage tolerating 500V is something that makes sense, my guess the 500V withstand voltage is the resistor max rating. Note this is not OVP, its what the pcb will withstand "forever".

Then if you are thinking about example beckhoff EL3064, it has an electrical isolation of 500V, but that's from the ADC to "LAN". I can't find a definitive max input voltage but i assume its the "Dielectric strength" of 30V, I find that to be weird terminology.

There is no point to over engineer this stuff. You can have safety margins, and you should, but reading an analog voltage that normally ranges from 0-5V, or even 0-10V, does not need to withstand x 10.

You have to be a bit careful as the divider common
(+) is the table voltage which is nominally ground
but can easily have transients in the 50V or greater
range. This is because the arc start transient can have
risetimes in the fractions of a nanosecond region, making
all ground wiring look like a inductor...

Also, don't forget spikes on piercing will be much higher than cutting. Even at 40 amps, it is enough to exceed 300 volts when cut volts is around 120 volts. Not sure how much higher because that was the max voltage my thcad was calibrated to. Also when crossing a void, when a slug drops when cutting a hole, running off a sheet when severing all cause huge peaks. A thcad used for ohmic sensing Is exposed to the full torch voltageeven if calibrated fro 24volt full scale.

All things both of you said are true. But if you use the already divided voltage the requirements are much smaller.

For the ohmic sensing its even worse than electrode to work. I have scoped about +/- 3kV in one plasma during HF (this being in the MHz range).

I have seen multiple plasmas around the world having just a normal beckhoff analog input taken from the already divided arc voltage.

As far as the slug dropping or crossing the kerf, the arc voltage can spike, but it will never pass the limitiations of the plasma it self. The plasma has a maximum power it can output . Example the a maxpro200 can output 33kW, if you cut with 200A, the voltage can go only so high before the plasma hits its limit.
Its ok to put safety measures but if you compare reading the direct raw arc voltage or the divided one, these are 2 different things.
I would assume an OP amp used as a buffer rated example +/- 36V is more than enough for example divider ratio 30.

As far as the slug dropping or crossing the kerf, the arc voltage can spike, but it will never pass the limitiations of the plasma it self. The plasma has a maximum power it can output . Example the a maxpro200 can output 33kW, if you cut with 200A, the voltage can go only so high before the plasma hits its limit.

Umm no.

That assumes that the plasma output has no series inductance or parallel capacitance. This is far from the case. The problem is that the arc starts and stops at nanosecond rates, generating voltage and current spikes far in excess of the plasma power supply ratings.

The AMAX-4817 might be a better choice than beckhoff as its a seperate analog Input module with stronger isolation. Add an AMAX I/O module and its all you would need
www.advantech.com/en-au/products/cf678dc...63-97e4-27475418e86f

Nice one, have not seen that before. It's quite expensive though? And maybe a bit overkill with 8 x 16-bit ADC (for a single plasma). Would be nice to find a mixed signal one with something like16 I/O and few ADC and few PWM. Maybe one day I will have time to make one.

Maybe I misunderstand something here. I assumed we were talking about work to electrode voltage and further than that, the divided one.

So apart from the ignition or ohmic sensing the cutting itself is nothing dramatic, well at least to me its not. What do you mean by the "nanosecond" rates?

So purely from work to electrode voltage point of view the plasma does not "instantaneously" turn on the cutting power. The current is ramped up in the tens or hundreds ms time frame. This is all depending on the torch/consumable design. Same goes when you stop cutting, the arc is "extinguished" in a controlled manner.

Typically the IGBT's switch around 50kHZ, I have seen some a bit higher, some a bit lower but all that I came across switch under 100kHZ. The arc itself I see as a purely resistive load.

Now for crossing a kerf, the arc lags behind for a few degrees, depending on speed and material. When you cross a kerf, the top of the arc will touch the material before the bottom and makes the arc "jump" over the kerf. This extends the arc to a certain degree and makes the arc voltage higher (longer arc). Something similar happens when the center slug drops and the arc follows the slug for a moment, extending the arc.

Anyone can test their plasma's max output voltage during cutting by incrementing the cut height and seeing when the arc goes off. A plasma rated for example 200V cannot sustain an arc that requires 300V.

I will try to remember to scope the kerf crossing and hole cutting next time I have a chance, maybe not this year but for sure start of 25.

I'm eager to learn new things and it would be great if someone corrects me if I am wrong.