DIY Arduino THC - Video

[EmptyNester]

I've posted a video on youtube of the user interface for the Arduino THC I'm developing for my PP50. The video is at:

This gives an overview of the interface and functionality of the THC. I'll do a video later showing it in operation.

[acourtjester]

I bet you feel like an expectant father, all the time and doctor visits are done. Now waiting for delivery time to see if it has 10 fingers and toes. Thanks for all your work we will wait for the finishing video and data.

Tom

[greygizmo]

Looks like it is coming along! The time you've spent on this will benefit the CNC plasma community - I know I'd like to use something like this sometime in the fu

[EmptyNester]

Thanks MuttonHawq.

I've implemented a number of PID's, but I'm more of a software guy than a math guy. I figured that the filtering would have to get more complex as time went on to deal with stuff like kerf crossings. I always like to start simple - but you are right, I probably need to start looking at PID's (though, that makes it more like work than fun).

I do need to do data collection on this so that I can see what the filtered voltage data and torch control signals look like. I just didn't have time last night.

I have been thinking about moving the the "Freescale Freedom" platform with a Kinetis processor because its cheaper ($12) and much faster (Kinetis, which is a 32 bit Arm processor), but it's 3.3 volts - so I thought I'd take it as far as I could with the Arduino before making that jump.

I really want to do the next pass of the hardware, but I just haven't had the time. Maybe if I get snowed in this weekend I can work on it.

[EmptyNester]

I got the boards back for OSHPark.com in less than three weeks. They look really good and I couldn't help myself and had to immediately build one. The assembled Arduino w/ display and THC shield on the left is a reasonable size.



As you can see from the picture, I did find an error that I had to run some wires for. It's for the serial port. With the wires the transmit from the Arduino works, but the receive doesn't. I had trouble with the receive wire and think I burnt out the chip. On the transmit side, the max speed of the serial port on the bench (not the noisy environment of the plasma table) is 38400. Going faster would require changing that opto isolator.

I checked out the control button wiring and that all works well.

I checked out the op-amp filter with steady voltages (I skipped the voltage divider so I could test with 3.3v and 5v) and that worked okay.

I also checked the Arc Good, Torch Up and Torch Down outputs to the CNC that are opto isolated and they all worked.

I still need to test the Torch On input from the CNC and the plasma signals for Torch On ouput/Arc Good input.

Once those last three signals have been tested, I'll be ready to start testing it on the table.

Earlier, I used the Arduino to capture voltages while starting the pilot arc. I used those voltages to play with a lightweight software filter algorithm. I have a first pass, but that's clearly going to be where all the work is.

[sportbike]

That looks nice. I will have to add OSHpark to my list. Looks like a very reasonable way to get boards made.

[SeanMurphy265]

I bet there are some THC manufacturers crapping their pants right now!

[EmptyNester]

I bet there are some THC manufacturers crapping their pants right now!

I doubt that.

There's a lot of software to do as I learn more about how the torch responds while cutting. I know kerf crossing will be an issue.

Probably the biggest "drawback" of this design is that is relies on having a CNC interface on the plasma that provides an isolated positive torch voltage (like the PP50 does).

I finished rewiring the new proto. I'm hoping to get some time soon to give it a whirl on the table.

[sportbike]

I doubt that.


Probably the biggest "drawback" of this design is that is relies on having a CNC interface on the plasma that provides an isolated positive torch voltage (like the PP50 does).

That's probably not too big of a deal. candcnc sells a "RAV-01" arc voltage divider card for $41.50.

That card could give a better signal than the PP50 does anyway??

[EmptyNester]

I've been trying to really understand how well the filtering is doing, but I kept getting corrupted data. I thought it was an issue with the terminal program I was using to capture the Arduino output. That was a little too convenient. On more thought I realized I was running into a hardware limitation.

The opto-isolators I used on the serial port can only support 38,400 baud. With 10 bits to a byte (start and stop bits are added to the 8 data bits with serial data), you get about 4000 characters per second. I was printing both unfiltered and filtered voltage in ascii every iteration of the control loop. The control loop runs every 1 millisecond. It ended up being over 10 characters per loop. So, I was trying to print over 10,000 characters/sec on a link that supports less than 4000 characters/sec. That was what was causing the corrupt data.

I have found new opto's that will support 10 Mbits, but I'll have to wait until the next hardware pass to implement them.

So that I could continue working, I spent the weekend making changes so that only one voltage is dumped per loop, and it's dumped in binary instead of ASCII. I also had to write a PC program to capture and decode it. On the plus side, It's the beginnings of a decent extended interface. (Picture below shows how it looks currently. Not all functionality implemented yet, but it can decode and capture voltage and status to a file.)



The other thing I realized is that I made a stupid mistake that could have a big impact. For the first prototype board, somebody gave me a filter cap to use on the voltage divider filter. It was similar to a ceramic capacitor. When I ordered parts for the second and third boards, I ordered electrolytic.

I've been used to using electrolytic in power supplies I'd build because they filter the voltage more by taking a charge and then releasing it as the voltage drops. That could be the reason the second and third prototypes have a slow response.

Now that I've finished the data capture app, I can test with the electrolytic and then take it off and see if the signal gets better.

[sportbike]

I would think running the loop at .01 second intervals would be plenty for torch height control. If you wanted to, you could go a little faster (.0008 or so) as well.
How many samples does the controller look at to compare and act?

[EmptyNester]

I would think running the loop at .01 second intervals would be plenty for torch height control. If you wanted to, you could go a little faster (.0008 or so) as well.
How many samples does the controller look at to compare and act?

Sorry for the delay Sportbike - just crazy at work.

I haven't used the CNC enough to know average cutting rates, but it seems that 200 inches per minute is the top speed I could find on tables recommending settings.

If you assume 200 ipm, you then have 3.33 inches/sec, which seems pretty fast when you think about the delays in signalling the steppers and getting them to start moving. It doesn't seem like its too fast when you consider that its only 0.0033 inches per millisecond. I guess it all depends on how much height distortion you want to deal with (my goal was a commerial THC demo that showed the torch stepping up and down a washboarded piece of sheet).

The other factor to figure in is the filter delay. In the Spice models, the hardware filter delay was around 15 ms. Then, there's a software delay on that. I just don't know enough about typical plasma cutting profiles to know what's appropriate. It seems that the fastest the Arduino can go is 1 millisecond. I want to run it as fast as I can because it will only slow down as I add more software capability.

I did just find a post on another board from a very sharp Hyperterm guy that said you only have to control to within 2 volts. So, I'm glad to know that and I think it's pretty reasonable to accomplish.

[Rambozo]

If you assume 200 ipm, you then have 3.33 inches/sec, which seems pretty fast when you think about the delays in signalling the steppers and getting them to start moving. It doesn't seem like its too fast when you consider that its only 0.0033 inches per millisecond. I guess it all depends on how much height distortion you want to deal with (my goal was a commerial THC demo that showed the torch stepping up and down a washboarded piece of sheet).

Because of the tip outside diameter there will always be a limit to how fast a transition the THC will be asked to deal with. The best example I saw of rapid transitions was hitting the diamonds on a piece of treadplate. You could see the torch making height adjustments going over them. At the other extreme is the corrugated roofing material that everyone seems to use for THC demos. I notice most use the sine wave type, I wonder how some of the more squarewave patterns would do.

[EmptyNester]

Because of the tip outside diameter there will always be a limit to how fast a transition the THC will be asked to deal with. The best example I saw of rapid transitions was hitting the diamonds on a piece of treadplate. You could see the torch making height adjustments going over them. At the other extreme is the corrugated roofing material that everyone seems to use for THC demos. I notice most use the sine wave type, I wonder how some of the more squarewave patterns would do.

It's funny, but I had never thought about how the diameter of the tip would be a factor. Sometimes you get your head so deep in the software, you ignore everything else.

I would expect the square wave type wouldn't be nearly as good. What do you think the top speed is for a home CNC Ram?

[Rambozo]

It's funny, but I had never thought about how the diameter of the tip would be a factor. Sometimes you get your head so deep in the software, you ignore everything else.

I would expect the square wave type wouldn't be nearly as good. What do you think the top speed is for a home CNC Ram?

Not without a 5 axis or better machine. http://www.youtube.com/watch?v=q42zN2iFD5E

As far as speeds, I would say with good motors you can probably expect rapids around 200 inches a minute or better. A lot depends on the mass of the unit as far as motor tuning. For actual cutting speeds, I expect that to be limited by the output of the plasma cutter. Maybe in 16ga. or lighter you will hit full speeds, but it all goes down from there.

[EmptyNester]

I'm currently using an Atmega2560 that runs at 16 Mhz.

I've done a lot of "old school" microprocessor stuff that's needed to be developed for speed. I figured this should be able to handle software developed for readability and extensibility (since I plan on open sourcing it when it works). So, while there's not a ton of processing in the loop, it's definitely not written for speed. (I used C++ classes to handle the different modes of operation.)

I had expected the op-amp butterworth filter to take care of everything, but it had a 100 hz cutoff (based on a recommendation from a "consultant"). After looking at some of the 2 pole butterworth filters I found a calculator that said for a 35 hz cutoff to use a 470 ohm and 10 uF. I can solder those on the existing board and just tie them into another analog input. So, I'll give that a try this weekend.

I had done some averaging over different ranges from 10 to 50 or 60. The problem with the rolling average is that to get a cleaner signal you ended up adding too much delay for high speed cutting.

Based on earlier discussions with Rambozo - I've been hoping to get sufficient performance for over 100 ipm.

All the voltage captures I do only start after the torch on signal. I know when Arc good is, but the voltage is still on the initial climb when that happens. I've assumed that the initial drop in voltage is when the arc transfers. AFter that there's a brief jump up. I don't know if that's after the pierce completes and before movement starts. I haven't been able to make sense of the timing.

The "cruise control" does work well though. You can set it in that mode and once it starts cutting, just hit the "select" button and it uses the current voltage for the control voltage. That was a great idea Ram.


Rambozo - assuming I'm just doing 18 guage mild steel as the thinnest, any idea on how what the maximum height change over a distance would be? It seemed like the 3/4" over 4" was pretty significant. I didn't seem to have any problem with that, though I was only cutting at 60 ipm.

[Rambozo]

I've seen one video where there was a pretty thin part and it sorta curled up from the heat and the heat sinking of the slats. When the torch finished I would say the edge was probably close to 2 inches above the slats. Now while I'm sure a lot of that was bad settings, the THC running that table had no problem in maintaining that cut. Of course there is that corrugated roofing material that everyone likes to demo. I think the PlasmaCAM demo has some that looks to be 3 or 4 inches tall, with a peak to peak of around 6 inches.

[EmptyNester]

I've seen one video where there was a pretty thin part and it sorta curled up from the heat and the heat sinking of the slats. When the torch finished I would say the edge was probably close to 2 inches above the slats. Now while I'm sure a lot of that was bad settings, the THC running that table had no problem in maintaining that cut. Of course there is that corrugated roofing material that everyone likes to demo. I think the PlasmaCAM demo has some that looks to be 3 or 4 inches tall, with a peak to peak of around 6 inches.

Any idea what kind of distance the torch travelled for a 2" height gain?

[Rambozo]

Of course I can't find the example I was looking for, but here is another that illustrates the control required.

[sportbike]

I just looked at it again and noticed the torch crash and material drag at the end.
I assume that is because the voltage gets out of range because the cut is finished.