Plasma Questions

Hello

My present THC uses SBC very similar to an Arduino. It uses an Atmel Mega 8 micro processor. So I am sure that the Arduino will be more than good enough as a low cost development platform. Of course one should only run fully compiled code outside of the Arduino operating system for a production machine.

I originally bought my THCad card to use with an Arduino to make my self a new THC before switching to the Linuxcnc route. The main problem was I couldn't figure out how to get real time machine velocity information out of my present controller. When the manufacture finally admitted that it couldn't be done without access to the source code I gave up this approach and went with Linuxcnc.

Yes, the stand alone THC for machine Z axis control makes for very simple G code and a very simple Post Processor.

John

The 7I87 has been used for THC applications, its not quite as rugged as the THCAD
(more capacitance from input to output because of the isolated power supply transformer)
but has better resolution.

It also should be possible to get min/max and average readings for THC sampling use with a 7I87
(it already samples all inputs at about 50 KHz each but currently just presents the average values
to the host)

I agree a good AFE (Analog Front End) need to be designed including Voltage divider.
but the resolution and speed are good.
the other possible option can be, design an ADC compatible with Mesa serial port

Great to see so many people get involved. Here is the journal article I found.
app.aws.org/wj/supplement/WJ_1992_08_s277.pdf
It took me a few hours of methodical study before I understood how simple the final algorithm really is. It outlines the rationale behind their choice of 40 msec sample period (so that they can account for the 3 phase ripple.) on a single phase machine, the ripple period should be 100-120 depending if you have 50 or 60 Hz but I doubt you'd want to get down so quick.

PCW wrote: The 7I87 has been used for THC applications, its not quite as rugged as the THCAD
(more capacitance from input to output because of the isolated power supply transformer)
but has better resolution.

I don't think the lack of robustness would be an issue in a modern plasma that does not use HF start and includes an internal voltage divider. Mine has a blow back start design and a configurable 16:1 or 50:1 voltage divider.

PCW wrote: It also should be possible to get min/max and average readings for THC sampling use with a 7I87
(it already samples all inputs at about 50 KHz each but currently just presents the average values
to the host)

It would be awesome if Mesa could provide software drivers that presented LCNC pins for min/max and average volts for a multiple of the given frequency. Another alternative would be to leave the sample period where it is and allow the number of samples included in the min/max and averages to be defined. either way, we need to get the min value every 40 ms.

A couple of other observations.
There is already a complete config for Plasma with no Z axis stuff in it for Independent THC. Tomylight reposted a link to it recently in response to a question of mine.

The independent THC I have also requires connection to the X & Y pulse generator signals. I assumed this is so it can monitor torch velocity to solve the problem islander had with his arduino.

Keep the discussion going guys.

I forgot to mention there is no real difference between an Arduino compiled binary and code output from the Atmel compiler. Remembering that the feature of the Atmel chip used by Arduino is that they support in circuit programming. eg. You download new firmware when the circuit is powered up. You can take an Arduino binary and download it using the Atmel programmer. I've done this several times. You can also program an Atmel chip in an Arduino and transplant it into a non-Arduino circuit and it will function accordingly. Yes there may be some overhead in the libraries but when you get to interruprts and Analog sampling frequency, there is not the overheads you might think. I would not use floating point anywhere though!

rodw

I think everyone that has considered the THC problem has looked at that AWS article. From what I have been able to tell unless you are cutting quite thick plate the arc source point movement get swamped by the accumulated system errors and filtering. This is a good intellectual exercise, I was advised by HT guru to not worry about it, a working solution is much simpler.

Just monitoring the the step output of the controller only provides the information to solve the speed problem on slow down at the end of the cut. It has no way of knowing what the target speed is during acceleration. Of course you can just use a fixed time at initial cut height or make a bunch of assumptions based on recent motions to get around this. Now if the THC can accept an analog or digital representation of the Feed Rate then it can work. With LCNC this is no problem, but with most closed commercial system and MACH your out of luck.

John