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Author Topic: Another THC Hickup I have noticed...Axis acceleration  (Read 8999 times)

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Another THC Hickup I have noticed...Axis acceleration
« on: February 20, 2010, 04:11:24 PM »
It appears to me so far that the THC UP / THC DOWN inputs trigger the z axis to move up and down at her THC RATE, without acceleration.... if I put the THC RATE anywhere close the the max Z axis rate the motor stalls. Is there a way to correct this??? (If I emulate the THC inputs you can tell there is no acceleration.)



On a positive note I do have a working THC circuit for my Powermax 45, It just needs some dialing in on resistor values and i need to stop both outputs being able yo go  high at the same time. Might be a noise thing.

It is one OP-AMP for input impedance and signal inversion, a quad comparator , quad and-gate, and transistor output... resistors and a tun of jumpers all over my breadboard....LOL.
Re: Another THC Hickup I have noticed...Axis acceleration
« Reply #1 on: February 20, 2010, 06:28:24 PM »
Are you running a differential amp on input ? What voltage swing is your voltage comming in from the plasma ? 
I used a powemax 600, but had to add a 10-1 voltage divider, then a 3-1 in my diff amp circuit.
My biggest problem was filtering the massive noise on the incoming voltage.

 Is your table grouinded to the earth or grounded to the wall ground ?

If you run a very slow Z velocity setting does it loose steps ?
 
Re: Another THC Hickup I have noticed...Axis acceleration
« Reply #2 on: February 20, 2010, 10:33:51 PM »
The Powermax45  has a built in voltage divider 50:1 (its actually a 51:1 according to their schematics 100k over 2k) So it has a theoretical max of 7 volts... I'm reading up to about 5  before the torch will extinguish.
I have one input to ground via 10kokm input and a 100uF cap. The other input is the 0 to -7 volts 10k to the positive of a 1458 OpAmp as a voltage follower so the input impedance is MegOhms. Then the other half of the 1458 is a 1:1 Voltage Inverter. A voltage divider with a pot tunes the hysteresis of the output. Each side of the pot goes to a comparator, and a second voltage divider pot sets the reference voltage. the comparators go through an and gate to disable the outputs, via an input from the PC, and/or a switch. Then on to two transistors to drive LEDs and the outputs to a Gecko G540.

The Torch Lit signal from the PM45 goes straight to the G540.

The table is in a metal shed, bare ground, standing on 4 12"concrete pads, on the soil. That is it for grounding. Work clamp is to the cutting slats (surplus catwalk grid)

Maybe I'm just lucky, but there was about 1V of ac noise on the "ground" side of the input and the 100uF dropped it to 10mV. There is built in filtering inside the PM45 at the 300Volt level.

If I leave the THC speed below 28IPM It seems to work . that's probably fine for flat stuff, I'm more worried about cutting thinner materials at high speed... Is there lag in the THC UP/DOWN inputs or not... will a Low Z rate work on thin mats. with the fast federate required. I know the BRAINS cause a small delay in the output to the enable portion of the circuit, because as I posted before and no response yet, the THC enable button In Mach3 DOES NOT APPEAR TO WORK..... Well not when I install the Parallel port driver... don't install it and it functions...just not in the real world... lol   so If your using a Smoothstepper or something like it I guess your good to go. So as an alternative, I have a brain calculate x and y federates to enable the Torch Up/Down . ...Basic triangle square root of a squared plus b squared. I have yet to work the bugs out exactly how I want it to function, add a min THC speed DRO too the screen ETC. but for now it does move the torch up and down :p

There is also a weird hiccup once and a while.. If the THC had moved the axis, sometimes the jog up and down buttons loose their acceleration parameter so the motors stall and I have to reset.... So that makes me think the THC is turning acceleration off to move the axis then turning it on again for regular jogging... Or is there a separate acceleration value for the THC buried somewhere I have to set ???? That would almost make sense to have a diff acceleration. for tiny moves than regular moves/jogging.

(as a note the current Z axis is a 5/16 18TPI screw with 36000 steps per inch... might have something to do with the acceleration ???) I think Ill go try swapping pins to the y axis to try it out ... see what happens.)

Well no luck... definitely ignoring the acceleration rules though... my x and y are rack and pinion up to 1700IPM with moderate acceleration..in EMC2 or acceleration in the 300s+ if i stay under 800IPM... but that's just way too hard on the machine lol. and I cant hit those speeds with Mach and my current PC. Well anyways, I have room to reinforce things and add a little weight :p


Short answer to a simple question??? LoL .. have a good day.
Re: Another THC Hickup I have noticed...Axis acceleration
« Reply #3 on: March 04, 2010, 09:59:54 AM »
I think most of your problem is your 18tpi Z axis lead screw is waaayy to fine.   A stepper can only start and stop with no accel/decel about 2-3 turns per seconds before stalling and with the fine screw you can't get the speed you need to keep up with your fast XY.

  You should try a 5 tpi screw  or even coarser and a microstep drive.


  So you find EMC much faster than mach3 ?
Re: Another THC Hickup I have noticed...Axis acceleration
« Reply #4 on: March 04, 2010, 10:34:56 AM »
It is DEFINITELY Mach 3 ignoring acceleration values when the THC UP/DOWN are used...
I swapped the pins to my y axis that is rack and pinion, about 2080 steps per inch on a Gecko 540(10 microsteps) and a 387OzIn motor, capable of over 1700IPM and accelerating well over 100IPS/S I can't run it that hard or it will shake the machine to pieces... but it still managed to stall a couple of times.

I agree when I make a more permanent Z-axis at a lower step count it "might" work better, but the fact of the mater is the acceleration rules are not being applied... if that's the case then why do we bother tuning the motors for top performance in the first place???



As for EMC... (the PC I'm using can't run faster than 25KHz In Mach3 = 800IPM Max) yes the pulse train is way faster / more stable... but its running in Linux and isn't exactly easy to configure past a very basic setup(they are improving the configuration utility but it's not quite there yet). It can be customized, but you have to spend some time reading up on how the hardware abstraction layers and .INI files are configured. I think I read, but forgot to bookmark that there is a routine someone wrote to slave and properly home a two motor axis... In Mach you click a radio button and a checkbox... DONE...
EMC though...lol

I really like the look and functionality of Mach3,  but if I figure out how to Modify one of their sample THC setups to my machine before I can get Mach3 to work for me, I'll be using EMC instead. Maybe Mach4 will work properly??
« Last Edit: March 04, 2010, 10:37:32 AM by Pandinus »

Offline stirling

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Re: Another THC Hickup I have noticed...Axis acceleration
« Reply #5 on: March 04, 2010, 12:50:53 PM »
well there's a lot here, but one thing I can bring to the party is that Mach does NOT apply acceleration to the Z axis under THC up and down moves. If you think about - it can't - because the Z would not be able to react fast enough to follow the arc voltage. i.e. if a system looks like it needs accel with THC it's just not quick enough. On my system - rack and pinion with motor tuning set at around 42 steps per mm (1077 steps / inch) I have to turn THC rate down to a tiny 2.5 otherwise Z pogos up and down overshooting like crazy.

Cheers

Ian

EDIT: Just to add a little more reasoning - was in a bit of a rush last night.
OK suppose there was acceleration on Z. Also suppose the THC gives a few Z ups and then some Z downs. When Mach gets the Z downs it would have to DEcelerate (still going up) and then stop and ONLY then could start on the Z down. But that Z down was *ages* ago - way to late. So with THC, not only does Z have to be able to *instantly* move up or down, but it has to be able to change direction *on a dime*. i.e. full speed one way must be able to change *instantly* to full speed the other way. This of course means that the speed of your Z should be whatever the max speed of Z can be on your system (or less) WITHOUT needing Mach's *soft* accel to get it there.
« Last Edit: March 05, 2010, 04:01:41 AM by stirling »

Offline hoyospetrola

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Re: Another THC Hickup I have noticed...Axis acceleration
« Reply #6 on: June 21, 2011, 05:20:02 PM »
At last!!!!!!!!!!!, I found out why my torch doesn´t work properly!!!!!. I have noticed that trouble, after moving the Z axis with thc up or down buttons Z axis looses many steps, and works so noisy but with your jog buttons it runs very fast and smooth, I´m also using the commands JogOn for moving my Z axis within one script and once you push these buttons it runs nuts. How should I set up my stepper amplifier to react without acceleration?

Thanks everybody for such a useful forum.
Is better keep quiet and looks like an Idiot rather than speak and clear any doubt

Offline BR549

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Re: Another THC Hickup I have noticed...Axis acceleration
« Reply #7 on: June 21, 2011, 06:23:43 PM »
THAT is NOT a quirk, that is the way it was designed and has been for many years.  That is what the THC RATE setting is "for" to the get the accel/vel rates DOWN below the stalling point of the motors. (;-)

Use the THC Rate to TUNE the motors when in THC MODE. 

Also as stated a FINE Z feedscrew is NOT the way to go with steppers. Steppers run best at lower RPMs

Just a thought, (;-) TP

Offline hoyospetrola

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Re: Another THC Hickup I have noticed...Axis acceleration
« Reply #8 on: June 22, 2011, 08:31:22 AM »
Thanks a million for your quick replay, Can you please tell me where can I find the THC acceleration rates controls?
Is better keep quiet and looks like an Idiot rather than speak and clear any doubt

Offline BR549

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Re: Another THC Hickup I have noticed...Axis acceleration
« Reply #9 on: June 22, 2011, 09:28:52 AM »
It should be somewhere on your screen set. I don't know what screenset you are running so can't even make a guess from here.

The value is a percentage of the rapid Z feedrate.

(;-) TP