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Author Topic: Mach torch height control vs full external Z control  (Read 2775 times)

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Offline beefy

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Mach torch height control vs full external Z control
« on: April 27, 2015, 05:37:03 AM »
I'm on the design path again, this time to build a plasma torch height controller for Mach.

I'm going to start with letting Mach do the Z movements so my THC will simply monitor the torch volts and send the up/down signals to Machs input pins.

I've heard a certain manufacturer of a THC say Machs built in THC was too slow (????? no more details). His model fully controls the Z motor, leaving Mach to just do the XY moves during the cut.

Does anyone have any actual experience with one verses the other. What shortcomings would a THC have when it used Mach to do the Z moves. Has anyone came across this way being too slow as just mentioned.

I know Machs THC does not have acceleration / deceleration on the Z moves, and this can limit the speed at which a stepper can operate. Try to make a stepper go a certain speed instantaniously and it stalls / loses steps. Would the closed loop of a servo motor help in this regard because it can catch up to the steps commanded.

Cheers,

Keith.

Offline Davek0974

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Re: Mach torch height control vs full external Z control
« Reply #1 on: April 30, 2015, 10:06:30 AM »
There is a speed constraint related to Mach's Z inputs, but in reality it works well, for me at least. I had a little Proma THC on my small table, it could not keep up and in Proma's own words it was slow internally, add that to all the other delays in the chain and you get issues if the sheet being cut warps by more than a few mm.

I changed to one of CandCNC's excellent kits and the speed is way higher, brilliant accessory IMHO.

The only time I can see the internal control method failing is if you want to cut corrugated sheet, I do think an external control would be essential then.

I don't think the external units use accel/decel ramps though - just not enough time available, it needs to move fast and stop fast, ramping would not allow that.
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives

Offline beefy

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Re: Mach torch height control vs full external Z control
« Reply #2 on: April 30, 2015, 06:18:55 PM »
Thanks Dave,

I've downloaded the manual for the Proma and it seems quite basic. It didn't seem to have any indication of a deadband (like the SPAN in the Candcnc unit). If it hasn't got any deadband I'd see that as a problem too, it would make Mach overshoot way too easily. As a matter of fact a guy on Youtube is using the Proma and can only get his THC Correction speed to 5% (Mach3), any more and he gets instability.

The latest Proma works a little different. It takes over control of the step and direction signals. So normally the step/direction signals come from Mach, etc, but when THC is enabled the Proma sends out the step/direction signals. No doubt the motor position will be lost in the controller. The latest Candcnc system (DTHC IV) seems to do something similar but I think the Z position is continuously monitored in Mach.

Surprising that the Proma unit is slow. Even basic microcontrollers run at megahertz clock speeds so would have thought internal operational speed would have been more than adequate. I believe the Proma uses relay outputs and small relays can energise in about 3 milliseconds. I'm going to also use opto-isolators to send the signals to Mach, so the THC up/down signals will be as fast as possible. From then on it's down to whatever bottleneck Mach3 gives me. Might also put a Lava parallel port card in the PC, they are supposed to be very fast and rock solid.

I was looking at my Z axis the other day and thought to get the most out of Machs basic THC, the Z axis needs to be built as "free" as possible. No stickiness on the linear rails or leadscrew, minimum weight design, and even a weight counteract spring to make the weight of the torch/carrier very little. Might also change the stepper for a servo motor which is supposed to give somewhat better performance with Machs THC.

Will be interesting to see how it goes.

Cheers,

Keith.

Offline Davek0974

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Re: Mach torch height control vs full external Z control
« Reply #3 on: May 01, 2015, 02:16:04 AM »
The "new" Proma THC sounds like the Proma THCMC that was around as an option when i had mine, that was direct control but the price increase was not worth it. I too could never get mach's Zrate much above 5% - that was part of the issue, and yes, going higher caused the torch to saw-tooth its way along. :)

I think even the basic CandCNC units are more than adequate for the average user, they are certainly fast and can move the motor in very small steps, faster than the display update on the monitor.

Keeping the mass low sounds good as does a servo system I think, Is there a specific reason you are building for such high specs????
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives

Offline beefy

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Re: Mach torch height control vs full external Z control
« Reply #4 on: May 01, 2015, 05:25:36 AM »
It certainly sounds like the Proma has no dead band setting if the Z jackhammering starts just above 5%.

I always like to make something as good as I can just for the day when I might need it. Will also be good to see what basic Mach is capable of with THC when the ingredients are optimal. I'm designing it so all parameters can be adjusted on the fly while a cut is in progress. Torch volts, anti-dive on/off, anti-dive setting, deadband setting.

I've got a Candcnc unit, an MP3000 with DTHC v14. That's a model from about 2009 and the torch height control works for normal cutting. Toms later systems starting from DTHC2 I think, can be turned on and off in gcode without a pause in motion. They use S words instead of macros to turn the THC on and off I believe.

By building my own external THC I can send it a signal from gcode and it will "freeze" the THC commands, and I'll get no pause in motion. When I've got a spare $1K I could look into the latest Candcnc unit but I'm skint at present so will have to make my own to get the "pauseless" torch height control.

Offline Davek0974

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Re: Mach torch height control vs full external Z control
« Reply #5 on: May 01, 2015, 05:33:17 AM »
Yes, my CandCNC unit has the S commands - you can turn it on and off any time you like, so far it's been the best investment I've made after buying a Hypertherm plasma ;)

I cut lots of fine detail in stainless, small jobs but as i use every inch of the sheet, coping with warp and twist is vital as my cut height on the fine-cut consumables is only 0.5 - 0.75mm, the CandCNC kit handles this easily.
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives

Offline beefy

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Re: Mach torch height control vs full external Z control
« Reply #6 on: May 01, 2015, 06:01:00 AM »
Which THC have you got, the DTHC2 or the DTHC-IV. The latter has much higher speed and is supposed to be able to deal with corrugated sheet. It's the one that takes over the step and direction signals. I think the DTHC2 just uses Mach normal THC but not sure.

Offline Davek0974

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Re: Mach torch height control vs full external Z control
« Reply #7 on: May 01, 2015, 06:08:45 AM »
I have the latest version of the one that uses Mach control, could not afford the step up to next level at that time.
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives