Advanced Search

Hello,

Let me start off with the purpose of this thread. I've never built a plasma CNC and I've never used a THC. The purpose of this thread is partially to confirm some of the research I've done and partially to probe as a bit of a thought experiment, as from what I can see, things seem to be in a transitional state between a few control schemes. I don't know if what I've identified is valid, or if there's a reason behind it. Maybe it doesn't matter, but either way, I thought it might be fun to design a system (At a high level) from scratch and ask "what if?".


Let's take a step back and first talk about closed loop, older analogue controls and the like as that's quite important. (Or at least, my understanding. I'm basing some of this on an old Cincinnati Sabre 500 VMC with Fanuc controls I researched a while ago as I was looking to purchase it - It turns out that a 4t machine is a logistical nightmare, but the control findings were interesting.)

In ye-olde-days, you had motor drives with an analogue velocity signal. The drive didn't know what the position was, nor did it care. The motion control managed the position, and you had a full closed-loop setup. This was, I suspect, because back in those days, sharing digital signals over multiple devices was a pain, so it was easier to keep the digital stuff inside the motion controller and to have a fully analogue motor driver with analogue signals. An interesting part of this was the spindle motor orientation used for the tool changer - The spindle drive had a motor orientation board which was connected to the encoder. This meant that when the motion controller stopped the spindle, the spindle drive would automatically park it in a specific orientation. The motion controller didn't control this, it just told the drive to sort it out and the responsibilities were clear.

Next we have our standard step/direction drives, with stepper motors operating in a fully open loop. The motion controller assumes that when it commands a movement, that the motor and driver will keep up and hopes for the best. We know about the limitations, so let's not dwell.

After this, we have the hybrid closed-loop setups with step/direction signalling controlling either a "closed loop" stepper or an AC servo. (Which is closed-loop by nature) Some would say that these are not true closed-loop systems, but as long as there's a fault signal being fed back into the motion controller, it really is closed-loop by definition. Some servo drives allow a second machine encoder for increased accuracy.

Most of the more expensive Centroid offerings include ways to close the loop right back to the motion controller, but I believe this is more for compatibility with older hardware. (See above) I don't believe there's any benefit in a new setup as long as the motor drive can report a fault back to the motion controller; they are both closed-loop, just with different responsibility splits.


I wanted to take that detour about exactly what closed loop means because I think it applies to THC. There are many approaches, but I think it's also worth considering where the respective responsibilities should live between each component, and I think a good test is whether or not the system could, at least in theory, support buffered motion planning. I know that doesn't apply to LinuxCNC, but I think it's a good benchmark for responsibility splits either way.


As with motor drivers, there are many ways of approaching THC:
- You have "fully" stand-alone systems such as the Proma unit, which intercept the Z-axis motor control and over-ride it mid-cut.
- You have fully closed-loop systems where the THC is implemented within the motion controller in software with a hardware interface such as the Mesa THCAD.

I think most people would say that software solutions are superior as they can look ahead, implementing features such as anti-diving. However, from my research, I'd argue that the stand-alone systems are not actually standalone; there's a lot of confusion regarding responsibility, as both parts in these standalone systems control Z axis position. I'd also argue that a purely software solution isn't ideal when you start getting into Ethercat and certainly doesn't allow for offloading via buffered motion planning. I would propose that there's another way.


Fully stand-alone THC, with "fully" no longer in quotation marks. Consider the following statement: The motion controller does not care about the Z-axis position for a CNC plasma machine at all. It cares about the target arc voltage, it cares about anti-dive, it cares about the initial torch height - But it doesn't directly care about the torch height. What if the motion controller told the torch height controller about the target arc voltage, when to start/stop the torch, what initial torch height and dwell to use when starting, and when anti-dive should be considered? What if the THC handled probing and piercing itself, if it fully and directly controlled the Z axis, including the limit switches? What if the THC told the motion controller when it was good to start the motion, rather than just when the ARC is okay?

This is the step which I think is missing. You could wrap all of this up into an Ethercat device and it's a lot closer to being able to handle buffered motion. (Although it's not perfect, as the X/Y motor drivers would need to wait until the all-clear from the THC before they start their motion.) It could handle all things plasma, including gas and current control (E.g. Hypertherm's RS485 interface), it could report back all of the diagnostics (Such as current arc voltage, current torch height relative to the probe position, information about the torch tip. I think this would also be much closer aligned to the modern Fibre laser setups with how their focus control works.


I'm curious about people's thoughts. Have I made any mistakes or misunderstood anything? Is it nonsense or even just a waste of time? I'm also hoping my explanation on closed loop helped others understand what it really means, as it took quite a lot of research to get my head around what exactly it meant and why "proper" CNC machines used to have velocity control.

Hi
I have developed a method to center punch locations for drilling holes on my PlasmaCNC setup.I use CAD software (Fusion 360) to locate where I want holes drilled in my finished cut parts. I use fusion to show the location and size of the holes and then use the snap function to locate the centers. I do this with the circle sketch function snapped to the center then draw a 0.003” dot there. After saving in DXF format and importing into SheetCam I change the layer of the dots to separate the center punching from the normal cutting operations. I then have a tool in SheetCam that has the perimeter for the center punching operations. This has the pierce and cut height set at the same height and lower the Amps some for this operation. After planning the whole operation I select the center punching functions into on G-code and then the cutting function into another G-code. SheetCam will ID the center punching as a inside cut and you can make one G-code where it will do the 2 operations in the same G-code. QTPlasmaC seems to have a quicker response for this operation then I had with other controller software, but both produce a very nice dimple in the metal.Attached are images of the center punches and a video of the operation.  I have used this method on different thickness metal with thin metal using the fine cut nozzle. I have used it to create a template for hole marking by center punching a thin plate to transfer the locations on another project surface.

drive.google.com/file/d/1EtZ_7zD49-VbqYf...sVL/view?usp=sharing
       

Try omitting that line with scalemax, right now i can not recall i ever used it.
Just add # in front of the line, save, test.

I use Ditron magnetic scales all the time. Installed my first ones in 2019 on my 5 axis machine. Since the I have used installed about 20 of them. Not a single failure yet.

Be sure to specify that you want 1um resolution. Reader head model DMR2000. And also specify that you want the index pulse version (z signal) otherwise you get only A and B.

Yes. They are differential output

Henk

We've got plenty of trees, but the city white fellas cos playing as black fellas wanna stop everyone walking into where the trees are, unless you're a black fella.
The real black fellas are good fellas.

Now before everyone gets their knickas in a knot black fella isn't an insult, it doesn't matter if ya a black fella or white fella what matters is to be a good fella.

There's large swathes of land here that are off limits unless you're Aboriginal.

Hi,
only one 7c81, you tip has done the trick. Thank you!
but the next problem occured
The error changed to:
/intos.hal:88: parameter or pin 'hm2_7c81.0.7i77.0.1.analogout0-scalemax' not found

so checked this line in the old, working hal, without the 7i78, and its exactly the same. What does change here when i use 2 daughterboards?
Any Ideas?

EDIT:
i have also tried to change the channel number to 0:
hm2_7c81.0.7i77.0.0.analogout0-scalemax'
and 2 and 3

Es siet die 5V fuhr die Mesa nicht OK zu sein, haben sie ein andere 5V strom wers... (power supply, forgot how that is written in German, sorry) zum testen, oder ein DVM die 5V zu messen? Es soll zwishen 4.75 und 5.25V sein.

unknown wrote:

I'm thinking of selling up, clearing the garage and start carving pipes..... .I don't even smoke.

I'm thinking of leaving everything and heading for woods to live there!
The issue is, we have no woods left, it's all been cut down, except for some bushes, and i can not hide behind those!
Painfully true, i am surrounded by huge mountains that have no more trees, at all.
But i do smoke, since i do not drink nor gamble nor ....

jackdavies wrote:

The Y axis has dual motors, should I use a scale on each side or just one?
Any recommendations of mesa boards to use here? 7i95T + 7i84U or 7i96 + 7i85
Whats the process for setting this up in LCNC?
I don’t believe these scales are differential signal, should I look for that?

1. You should, the expense does not justify having missed steps on one side and ending up with a miss aligned gantry
2. For step/dir drives, 7i95T should be enough if you do not need a lot of IO. So a 7i84 would be required on both combos if you do need more IO.
3. a bit of editing will be needed either way, plenty of info and help here once you get to that point
4. differential are better for noise immunity, but Mesa works with both differential and single ended. They should be incremental TTL, as far as i know. Absolute type would require other boards/config/ depending on type of comms.

I'm thinking of selling up, clearing the garage and start carving pipes..... .I don't even smoke.

Hallo,
der Ping mit 200ms Latenz scheint mir recht hoch. Hast du dir die Sachen die PCW im 2. post hier forum.linuxcnc.org/27-driver-boards/5284...nishing-read?start=0 vorschlägt schon alle erledigt?
Ich nehme an die Karte hängt am ethernet port und nicht an einem usb2ethernet adapter oder dergleichen?

You are welcomed, always.

How many 7c81 are in use?
for one only, delete the line containing 1
addf hm2_7c81.0.read
addf hm2_7c81.1.read
Pretty sure this should only be used if you have 2 of 7c81, 0 and 1 there are enumeration for 2 of 7c81 not the add on cards, they do have their own enumeration after the model number, example:
hm2_7c81.0.7i77.0.0.input-07
First 7c81 that has the first 7i77 attached, first is always 0 with Mesa and LinuxCNC enumeration.
Had a look at your hal, there is more editing required, so it might be easier to make a new config with a single 7c81.

pgf wrote:

... designing arbitrarily complex digital "circuits" in a text file.

That is a nice way to explain it, thank you.
-
-GRBL is simple and quite easy to setup and use, once you figure out the quirks.
-FluidNC is simple and easy to setup and use and runs on an ESP32, magnificent!
-Klipper is complicated in every way possible, but quite easy to setup after spending weeks of reading, it is very good at what it does, can be modified a lot, but the huge overhead (hundreds of required packages) makes it not very reliable (works for months without issues then suddenly goes on hiatus), with very little ability for troubleshooting
-Marlin is not easy to setup but luckily there are ready made configs/images, easy to use, runs even on 8Bit micros, works "stand alone", and frankly it is amazing how much functionality is crammed into it.
-LinuxCNC is very dependent on what you need from it, very easy when using parallel port, easy enough when using most Mesa cards, all thanks to the insane amount of work put in by members of this forum and developers, but can be complicated as much as you are willing to ask from it. I can go on for days...
-
To better understand the flexibility of LinuxCNC, just pick one of the projects started on this forum, like QtPyVCP Probe Basic (or Basic Probe, i always mix those), QtPlasmaC, QtDragon, PlasmaC, MonoKrom, etc. Some of them started just by someone asking here and inside a week there were actual working GUI's being developed, search for "Brender".
I have to stop...

Hi,
thanks for your fast reply.
if you read in the hal, there is written :
addf hm2_7c81.0.read
addf hm2_7c81.1.read
for both cards
.0 is the 7i77 and .1 is the 7i78
I interpreted the documentation that it should be done like this.
Any advice how it is configured correctly?