Thanks for the reply Sterling! The unit I'm getting is an older machine, only paid $2500 for it so a "couple grand" for a THC is kinda out of the books for now! I'll keep open for ideas, maybe come up with a solution yet! Thanks again, Brian

I forgot about the hysteresis setting in the THC too, I had it set for +/-1v but having just made some straight cuts without THC it seems the voltage does not settle at a fixd point for any length of time and +/-1v is not a wide enough envelope for it.

I now have it set at +/-2v which should give it some leeway hopefully, out of the box it comes set at +/-4v which seems a bit too wide.

A faster loop response seems to be the only way of getting full control though, but at least I'm getting a handle on how it all works together now.

@Dave - I was just attempting to get you to do the numbers so that you can actually predict what will happen. Every feedback loop needs smarts to cope with the delay. First approach should be to reduce the delay as much as poss but after that the longer the delay the more smarts are needed.

Back to the maths though for your system. That 5 in THC rate means max Z speed under THC will be 5% of what you have in motor tuning. So in your case 55mm/min. At (say) 3300mm/min feed just as an example, that means the max slope you could even hope for - all other things being ideal - is just under 1 degree.

@Brian - no worries.

Interesting, not sure I could figure out how to calculate that

The Z axis motor in motor tuning is set at 1187.8mm/min so with the 5% THC rate it's running at 55.89mm/min.

I think that gives me a z axis travel of 0.93mm per second, and with the other axes possibly moving at 55mm/sec (3300mm/min) I can now see that I have a height change possibility of 0.93mm over a distance of 55mm.

I think that equates to a warp factor of 5.28mm halfway along a sheet of steel 625mm wide.

What I don't know is is that bad, useless, reasonable etc?

5mm of warp seems quite believable on a plate that size.

If I am correct on all of my assumptions above, maybe I have been testing it on too extreme of an incline??

I think that gives me a z axis travel of 0.93mm per second, and with the other axes possibly moving at 55mm/sec (3300mm/min) I can now see that I have a height change possibility of 0.93mm over a distance of 55mm.

I think that equates to a warp factor of 5.28mm halfway along a sheet of steel 625mm wide.

Correct - or to put it another way - a slope of 0.967977387 degrees i.e. just under 1 degree 

What I don't know is is that bad, useless, reasonable etc?

5mm of warp seems quite believable on a plate that size.

If I am correct on all of my assumptions above, maybe I have been testing it on too extreme of an incline??

If you can cut what you need to cut - then your THC is great - if you can't it isn't... 

Good stuff, at least I'm on the right track.

I spent a fair bit of time playing this afternoon, using up scrap and doing repeat straight cuts with varying settings.

I think I have managed to find a set of figures that allow it to follow a a warp of 8mm across 620mm quite closely and at speeds from 900 to 3300mm/min so I think it will function ok if I keep an eye on warp limits etc

It's better than it was at least.

THC Rate is now at 9% and Hysteresis is +\-2v