HI Ian, there are cutters on the market that use an outside THC controller that drives it OWN Z axis independant of MACH3 . They appear to be able to cut curragated sheet at high speed. To do that the Z MUST be very quick and fast obviously faster than teh normal Mach3 can do THC control.

SO is the speed limit a function of the stepper motor being directly pulsed without a curve or the speed of the THC to be able to respond.   With a fast Z 2tpi you can drive teh Z faster than the THC can keep up with corrections and get overshooting.

(;-)TP

Thanks Terry on all the previous items covered, following the thread closely.

I know mach3 is not up to the imposed acceleration, but did wonder about Mach4 as the scripts run much faster....  I'm still waiting for the new ESS plugin (no jokes please!!).

As posted previous, the Neuron THC and the CandCNC both directly control the Z axis so should be able to attain higher z axis motion with a very steep accel and deceleration, I'm also aware of another product that I'm eagerly awaiting to hit the market which will do the same.

What does everyone else do to turn THC on and off via gcode on the fly (I use sheetcam as I'm sure most probably do), before corners or small circles etc?

Hi Robert , You highlighed the very question I was asking. IF teh direct pulse method(on/Off) is the faster possible move of the stepper HOW is it they can use accell and deaccel with an outside Z drive and do it faster ?? .  Maybe Scurve profile ? but that would not neccesarily be quicker for the length of the average move.

The best method would be to run teh entire THC function directly from inside of the Motion Controller itself ( ESS, UC300,etc) . Just give it the reference voltage or frequency and the Ark OK signal and it handles everything internally.  There is one that can do that currently.


Another interesting investigation is to be able to Directly TALK to the THC without a COM link to the THC. That way teh THC could be  adjusted on ANY controller ON THE FLY and without a COM link. A universal fully programmable THC without teh need to create a normal Com link to talk to it.

(;-) TP

HI Ian, there are cutters on the market that use an outside THC controller that drives it OWN Z axis independant of MACH3.

Yep.

They appear to be able to cut curragated sheet at high speed. To do that the Z MUST be very quick and fast obviously faster than teh normal Mach3 can do THC control.

Yep. Though if you look at SOME vids doing corrugated, it looks to me like they've slowed the XY feed down to match the Z rather than the other way around.

SO is the speed limit a function of the stepper motor being directly pulsed without a curve or the speed of the THC to be able to respond.

Both - one is no good without the other.

With a fast Z 2tpi you can drive teh Z faster than the THC can keep up with corrections and get overshooting.

If your Z can move faster than the THC can cope then yes - you get head bounce. That will happen whether you use Mach OR a standalone.



PS - I'd love for someone here to explain to me why they think THC with an accel ramp would be beneficial.

What does everyone else do to turn THC on and off via gcode on the fly (I use sheetcam as I'm sure most probably do), before corners or small circles etc?

I've been messing a lot with this stuff lately, I have the CandCNC Mp3000-DTHC2 and I use Sheetcam rules to turn the tHC on and off all over the place as I've been messing with slowing down on corners and holes etc. I'm not convinced it makes any difference yet, need to run some tests with THC on and then slow down and see if it dives or not, I don't think i have ever had a true torch-dive yet! Normal reason for a dive for me is having the volts too low, especially on the 30A consumables as there is only 1.5mm gap to play with and a volt or three out and it will dive in on the run, sometimes you can catch it and bump the volts up but usually it catches me out.

Maybe it needs a "super anti-dive" feature whereby if it senses the THC being told to go down but the volts are not changing at all i.e. the torch is running along the plate, then it blocks further drop signals, maybe even lifts a bit until it sees a volt rise - don't know if any systems offer this but it sounds useful to me

I'm still not convinced that ZERO acceleration is needed for THC.

There's a little goodie called deadband or hysteresis in a good THC. That acts as a buffer so if you deadband was +/- 1 volt, then so long as your accel/decel is fast enough it will at least keep the voltage within the dead band. Yeah, with accel/decel you'd pass the set point a bit BUT providing the accel/decel is fast enough it would bring the Z to a stop within the dead band.

THAT'S MY THEORY  

Perhaps when Mach3 was designed the deadband factor was not part of the design, and that's the reason no accel/decel was applied to THC moves.

Fast PID control effectively has accel/decel based on error. Smaller error, then slower correction speed, bigger error, faster correction speed.

Keith.

HI Dave. Mach3 has anti corner dive built in. IF teh combined vel drops below the setting it locks out THCdown from working. You  still get the THC up function to prevent head collision. Most set it too low.

IF you set this too low teh head will dive due to teh THC trying to correct teh voltage as teh Vel slows down for a curve.  How far it will dive is a function of how slow teh Vel gets in that particular curve and where you have teh cutoff point set to.

Keith as you said.... why could you not use PID instead of bang bang control for THC?

(totally external to M3 or M4 of course)

That way accel and vel is proportional to the error and increased relative to the error time and decel used obviously once you get closer to the target voltage (D).... or PI control?

I guess in theory the intergral "I" part with a fast Z should be near zero?...

Hi Ian, Not really, IF you try to slow down with thin sheet goods you end up blowing out the Metal and the torch shuts down (;-).  SO it has to be fairly close to cut specs to even cut.

(;-) TP

Another interesting investigation is to be able to Directly TALK to the THC without a COM link to the THC. That way teh THC could be  adjusted on ANY controller ON THE FLY and without a COM link. A universal fully programmable THC without teh need to create a normal Com link to talk to it.

You have lost me there??

The best method would be to run teh entire THC function directly from inside of the Motion Controller itself ( ESS, UC300,etc) . Just give it the reference voltage or frequency and the Ark OK signal and it handles everything internally.  There is one that can do that currently.

can I ask which one? CISMO I thought did it and purelogic does something, but I'm not sure how much is shared from their THC to their motion controller?