Rods "Spaceship" Scratch built Plasma Cutter build

rodw wrote:
I kind of think that the offset adjustments are stacking up before the hardware can fully apply the adjustment.

If that is the case then you need to calculate how much movement one offset count would cause, calculate the maximum amount of movement per period from your maximum velocity then limit the number of counts the THC can output per period to suit this maximum movement per period.

Cheers, Phill.

I am not sure this going to quite do it. I will actually be surprised if the Z axis ever gets up to full velocity given the short distances involved and the short time periods we are looking at. If this is actually a case of inadequate acceleration then we need to significantly increase the power we are putting into our Z axis so we can accelerate fast enough. This may be part of the reason all the high dollar commercial THCs actually use servo motors to drive them. I have an old Animatics Smart Motor here that I could try but it is so under powered I doubt it will be an improvement. So my money is still on a delay some where that is causing the mechanics to overshoot the correct position.

I forgot about the .ini values for velocity and acceleration being twice what is actually applied under EO when the stock .5 AV ratio is used. I just assumed that your adjustments were the actual values used. I still believe that slowing down the acceleration will help or cure the problem. After all skipping PID update times is just doing the same thing by a different method.

John

Well i tried that component (after fixing some errors). It did not break anything but neither gave a significant improvement. Then I tried slowing down the velocity and acceleration settings as John suggested and it did make a difference by reducing the twitching but the amplitude of the oscillations increaed. Visually not as nervous. The added amplitude may be able to be tuned out.

Phill, thanks for the idea. Its not quite like that. What I mean is the adjustments might become "stale". The external offsets respects the machine velocity and acceleration. so if an offset takes more than one servo period to execute everything is OK, but another offset might be calculated which due to voltage signal hysteresis is more than required (overlaid on the last). I might look at the idea and extend my component to include a limit as the datatype used in the limit components are floats and we need S32. Its probably a better method than just blindly scaling.

Well, here is a slower version. I started to change the pid settings but had to go so I think this will be it for today. I think the range is about 2 volts which is not too bad really. Thats a range of about 0.27mm based on my earlier sampling work, about +- 0.13mm

Rod

I tried looking the source code and no surprise I couldn't find the code where the eoffset.counts is processed. As I recall the EO is handled like a jog wheel. This means that only the change in counts each servo period is applied to the position command. So if the error count is constant the motion commanded position will not change. I hope someone with better coding skills than I have can confirm or correct how the EO works.

I have trouble reading your arc voltage well because of the scale but on the screen shot it appears to be holding at +/- 5 volts. I think that is an acceptable number if the cut quality is ok. I usually display my arc voltage at 10v per division so I can count the tic marks on the screen. I understand that with the TD torch your arc voltage may be over 100v.

I have some ramblings here about how my system responds that may be helpful or not. Assuming a linear arc voltage to distance relationship a 1volt change in arc voltage is equal to .000946 inches of arc length, to make things simple lets call that .001". When using the HT Duramax torch they say you need to hold the torch height to +/- .005" of the set point height which works out to about +/- 5 volts in my case. When cutting long straight cuts and cuts without the corner lock becoming active I usually have the arc voltage vary by less than +/- 1 volt. This is on 2mm HRS which is never flat when being cut! The Z axis acceleration on my system right now is 55ips^s (.145g). This works out to distance traveled the first servo period of a correction of .0000275", not vary far. So to correct for a 1 volt error in arc voltage it will take about .006 seconds ( 6 servo periods) reaching a velocity of .332ips, well within the step gen limits I use. Now with 10:1 micro stepping and a .2"/revolution lead screw the resolution is .0001" per step so 10 steps to correct for that same 1 volt error in arc voltage. Now I am not sure why my Z axis wants oscillate at 10hz (.1s period) which is quite slow compared to the movement that can be achieved, can this be the current ripple on the plasma power supply output?.

John

I think Rod can make his plasma cutter working like magic. He is a good c programmer, so i do not worry about his program skills.

At the time i build my plasma component. I did it step by step. It takes time. But in the end. It's powerfull !!