CNC Interface and Torch Voltage Capture

[EmptyNester]

I started lookign at the CNC interface.

When the CNC/manual switch is in the top position (CNC/Track Torch 4T) the torch doesn't trigger with lines 1 and 2 shorting on the CNC connector. That only works when the switch is in the bottom position (Manual Torch 2T).

So, it works fine in the manual mode, but I wondered if there is something about the CNC mode that I'm missing?

I also did a capture of the voltage on the CNC output port. Here's what I got.



I captured the voltage on pins 4 and 6. I didn't change the voltage divide jumper, but with a peak voltage of a little over 12 volts, it looks like the jumper is already in the 1/16 position. The X axis shows the sample number. The samples are 20 ms. apart.

From the graph, I'm guessing that the intial full voltage is prior to the arc igniting. I would guess that the voltage drops on ignition and then recovers as the torch stabilizes. Does that seem to make sense?

[SeanMurphy265]

pins 1 and 2 are the relay
I have pins 5 and 7 wired for the full arc voltage

pins 4 and 6 should read the divided arc voltage.

Check 5 and 7 to see if they read the full arc voltage.

Which THC are you running?

[EmptyNester]

Thanks Sean.

I understand the pin out. I'm actually trying to understand the behavior of the voltage output so that I can write my own THC software. The graph shows the noise on the output with the divided voltage.

I do plan on looking at the noise on the undivided voltage to compare noise levels. However, I'd prefer to use the 1/16th voltage over raw voltage if the noise is comparable.

I still don't get what the front panel switch does though.

[Rambozo]

From what I understand, in 2T the arc is on whenever the switch lines connected, while in 4T the arc is started with one pulse then terminated with another. I don't have one to play with so I'm not sure if the actual switch line is brought high or low. I look forward to your experiments with an arduino THC. Because the voltage is an analog signal, I have always wondered if it would be simple to design an analog servo circuit to do THC. The noise levels are really something, I didn't expect that, but I guess it shouldn't surprise me with anything arcing.

[SeanMurphy265]

Which plasma cutter are you running?
If your running a multi purpose unit look at Zalkys thread at cnczone.com. From what I have read the multi-purpose units are the most noisy units for THC. The guy at CandCNC.com actually bought Zalky's unit to do some testing.

[EmptyNester]

Which plasma cutter are you running?
If your running a multi purpose unit look at Zalkys thread at cnczone.com. From what I have read the multi-purpose units are the most noisy units for THC. The guy at CandCNC.com actually bought Zalky's unit to do some testing.

I'm using a PP50.

This was a quick/informal test. I didn't do anything with respect to shielding, ferrites, etc. I just wanted to see some waveforms.

[EmptyNester]

I ended up with a voltage divider of 100k and 2.53k. This gave me a divided voltage reading of 1.2V while the full output voltage was at 50V.

The voltage reading I used was a meter on the output. The difference between the meter and the scope is the scope is about 0.4 volts higher. I trust the meter more than the scope (scope is a DSO201 and I used two different meters). The graph below shows the full voltage with my divider and using a 100 mf filter capacitor.



I still don't understand why I'm not seeing a negative voltage on the output of the CNC port since the cutting voltage is negative.

I did connect both my scope and my meter between the CNC port outputs and the ground on an Arduino board (running on A/C). I check both the positive and negative torch voltage outputs (one, then the other) against the Arduino ground.

When powering up the Arduino, the meter reading jumped on very quickly (too fast to read), but then settled at around 84 milivolts. The scope traces for both are the similar.



I'm not sure what's going on. I'll talk to my consulting electric engineer tomorrow to see if he can shed any light on what I'm seeing. Any thoughts anyone else has would be appreciated.

[EmptyNester]

Just found this article on how a THC works. It had a good listing of challenges.

http://www.fabricatingandmetalworkin...pplications-2/

The software itself shouldn't be hard to do, figuring out how it should work is the challenge. The article lists "Levels of Complexity". A brief summary is:
1) cutting speed changes: going slower gives a wider kerf and can cause torch dive
2) Gas pressure flucuations: pressure changes can cause the torch to go up or down
3) Kerf crossing: crossing a kerf can cause dive
4) Timing issues: is the torch is active at the end of the cut while the system decelerates the torch will dive
5) Electrode wear: as the electrode wears the tip will get closer to the metal

My initial thoughts on these are:

1) Not real sure about this one
2) I think this is best addressed by the compressor and regulator
3) kerf crossing is easy to adjust for by looking at the rate of change of voltage. I would expect kerf crossing to cause a large immediate jump in voltage versus warped metal giving a slower change in voltage
4) I would think this is handled by Mach
5) It would be easy to track tip changes and cutting time and have some compensation factor

Anyone have any other thoughts?

[Rambozo]

Good article. As to the kerf getting wider as the torch slows, it would be real cool to give the CNC control over the plasma power so that compensations could be made. Not sure about programming Mach 3 to do that, but I think it could be done. Possibly even the smoothing in software or hardware, to average the voltage, and remove noise, would take care of the kerf crossing. As to electrode wear, you could do a voltage re-calibrate when the torch measures the plate height. Once the cut starts, you will know the voltage at the desired height, so you would just take that as the following voltage.

[EmptyNester]

I have a 16 character by 2 line display that came with the Arduino Mega 2560 I bought on eBay.

I've been thinking about what type of capabilities I'd want on a THC and how a menu system would work on the small 16x2 dispaly. Assuming the use of four push buttons (<Up>, <Down>, <Menu/Select>, <Cancel>), I came up with the following:



When operating, you can adjust your target voltage.

The menu system would be disabled when the torch is on. To configure it when the torch is off, you'll want to be able to:

• adjust target voltage
• calibrate the voltage reading so the display is accurate
• adjust the anti-dive setting (which I expect would be to damping very fast voltage jumps when going over existing cuts)
• test the different interface lines
• display software version info

I expect the anti-dive would detemine when you're crossing an earlier cut by a faster than normal voltage change. It would then "dampen" the response if it thought that was the case.

Does that seem like a complete list?

[SeanMurphy265]

I do have a question about voltage. For THC to work it reads the voltage and makes the adjustment up or down to get the correct voltage.
1. How do you know the correct voltage for the optimal cut height?
2. How would the voltage read if the torch is too high or too low? Will the voltage read more or less than the correct voltage if the torch is too high or low?

[sportbike]

I've seen a few posts across the internet regarding the output of the voltage circuits on the CNC connection being sketchy and sometimes dropping off midway through a cut. Not sure if it is really an issue, just something to keep an eye on.

I had kind of forgotten about Arduino. I have another potential use for it for a speed controller on a small clutch Dyno I hope to someday get completed.

I have seen a few THC applications that just end as well. Seems weird.

[EmptyNester]

So, I used a 100K ohm and a 2.2K ohm for a voltage divider.

If I metered the raw output, it was around 140 volts. Based on the voltage divider it should read 3.014 on the o-scope. It looks like I'm getting about half of that. But I guess that's due to the resistors in the CNC interface.



I guess the thing I don't understand is that I thought the torch voltage is negative. But I'm reading a positive voltage. Is that because the signs are reversed on the CNC port pin-out description?

[everlastsupport]

I started lookign at the CNC interface.

When the CNC/manual switch is in the top position (CNC/Track Torch 4T) the torch doesn't trigger with lines 1 and 2 shorting on the CNC connector. That only works when the switch is in the bottom position (Manual Torch 2T).

So, it works fine in the manual mode, but I wondered if there is something about the CNC mode that I'm missing?

I also did a capture of the voltage on the CNC output port. Here's what I got.



I captured the voltage on pins 4 and 6. I didn't change the voltage divide jumper, but with a peak voltage of a little over 12 volts, it looks like the jumper is already in the 1/16 position. The X axis shows the sample number. The samples are 20 ms. apart.

From the graph, I'm guessing that the intial full voltage is prior to the arc igniting. I would guess that the voltage drops on ignition and then recovers as the torch stabilizes. Does that seem to make sense?

Some controllers flip a relay on and off (2T) , some pulse (4T), so the 2T and 4T can handle either.

On the graph, was that just firing the torch and pilot arc, but not cutting anything?

Most PIC and AVR chips have A/D if needed and you can add a cap and resister and time the charge. Never played with the pre-made (arduino) boards. But PICs are so cheap, internal OSC, low power, I would lean that way and all the free compilers.

[EmptyNester]

Some controllers flip a relay on and off (2T) , some pulse (4T), so the 2T and 4T can handle either.

On the graph, was that just firing the torch and pilot arc, but not cutting anything?

Most PIC and AVR chips have A/D if needed and you can add a cap and resister and time the charge. Never played with the pre-made (arduino) boards. But PICs are so cheap, internal OSC, low power, I would lean that way and all the free compilers.

Thanks for the info. I didn't know that 4T required pulsing. I'd rather stick with 2T so a cable break will kill the torch.

I have access to a lot of different embedded development platforms and tools. I figured Arduino would be good because it is:
- cheap processors
- has plenty of digital and analog I/O
- cheap prototype boards
- cheap displays (notice the theme)
- readily available
- well documented
- has a decent tool chain (though no debugger)
- has good support forums

I'm currently using a Mega256 board and a 2 line 16 character display.

I haven't yet done the voltage divider/filter/inverter yet. I wanted to decide what input voltage to use first (based on signal to noise ratio).

[everlastsupport]

Thanks for the info. I didn't know that 4T required pulsing. I'd rather stick with 2T so a cable break will kill the torch.

I have access to a lot of different embedded development platforms and tools. I figured Arduino would be good because it is:
- cheap processors
- has plenty of digital and analog I/O
- cheap prototype boards
- cheap displays (notice the theme)
- readily available
- well documented
- has a decent tool chain (though no debugger)
- has good support forums

I'm currently using a Mega256 board and a 2 line 16 character display.

I haven't yet done the voltage divider/filter/inverter yet. I wanted to decide what input voltage to use first (based on signal to noise ratio).

If it is 10 volts. Making the voltage divider easy. So the theme I take it was cheap. hahaha.

[EmptyNester]

I talked to my consulting electrical engineer. He agreed that it's probably an isolated supply. He thought that the a/c wave form I saw between grounds was probably just noise and so low a current it wouldn't be an issue.

I got the voltage divider set up. The calculated values weren't giving me the expected voltages. I believe that it's due to the existing resistors in the CNC interface. So, I went with the "scientific empirical testing model". I just kept trying different resistor combinations until I got the voltage I wanted

I also decided I'm going to add a couple zener diodes to protect the Arduino from overvoltage.

Once I get it all hooked up and know it works - I'll publish a schematic.

I'm not going to do anything with the torch height control loop yet. Since I don't have my table finished, it won't do me any good.

I may start working on some other aspects of the software (calibration, test mode, etc).

I haven't decided if I'd write a PC app to talk to it. I went to the trouble of isolating the power supplies for the BOB. If I do a serial connection and don't isolate the serial port, it would make the BOB isolation a wasted effort. I'll probaby wait until I know Mach better to understand if there's value in interfacing the Arduino with it via a serial port.

Just spent the day working on the gantry rail adjustment so that I can move the overall project along.

If the THC works, I'm curious what level of interest there would be in others duplicating it. If I can get it to work, I'll make all the info available, but I'm doing it specifically for the PP50. At this point I'm not real interested in making it a general solution.

[Rambozo]

Since you are working with Arduino for the THC, here is something interesting I stumbled across that might interest you.
As an alternative to Mach 3, a complete open source Arduino based CNC, GRBL. I can really see using this for some smaller projects that have been on the back burner for a long time.

http://dank.bengler.no/-/page/show/5470_grbl

http://www.shapeoko.com/wiki/index.php/Grbl

And a slick little driver board to go with it.
http://blog.makezine.com/arduino/grbl/

[everlastsupport]

Since you are working with Arduino for the THC, here is something interesting I stumbled across that might interest you.
As an alternative to Mach 3, a complete open source Arduino based CNC, GRBL. I can really see using this for some smaller projects that have been on the back burner for a long time.

http://dank.bengler.no/-/page/show/5470_grbl

http://www.shapeoko.com/wiki/index.php/Grbl

And a slick little driver board to go with it.
http://blog.makezine.com/arduino/grbl/

I have a mill on the way with servos, but this is a pretty slick controller. Wonder if there has been a servo hack for it.

Assuming small scale based on the spark fun motors? The Shapeoko mod one could add a double throw switch and turn it on and off, not sure if that would gain anything.

Ram, do you have any links to full blow system plans for the GRBL. Looks cheap to build. $70 controller, not sure what an Uno (is) or cost but my guess would be about $100 or so for both, then motors and start building.

Could be a fun projects for my son and I and we have spare laptops so maybe something portable. I have a PP50 on the way soon, waiting on it now.

[EmptyNester]

Since you are working with Arduino for the THC, here is something interesting I stumbled across that might interest you.
As an alternative to Mach 3, a complete open source Arduino based CNC, GRBL. I can really see using this for some smaller projects that have been on the back burner for a long time.

http://dank.bengler.no/-/page/show/5470_grbl

http://www.shapeoko.com/wiki/index.php/Grbl

And a slick little driver board to go with it.
http://blog.makezine.com/arduino/grbl/

Ram,

Thanks for the links - those are pretty cool and I hadn't seen the shapeoko or grbl before (and I've done a lot of searching for Arduino/CNC info).

Its a shame the Grbl board can't handle bigger motors or 4 axis. I wouldn't mind replacing the individual motor controllers with an Arduino for the CNC.

I had looked at the Linux based open source CNC software but decided to go the traditional path. I've had projects before where I've taken on too much - then couldn't get to the point that it was functional. Since the THC is optional, that seemed like a good starting point. After it's working and dialed in, I may start looking at other software and control systems.

On the topic of Arduino projects, I think a 3D printer would be cool. I'll be curious to see if I can make one from steel with the plasma table when its done.

Rege