Lathe C axis homing

I was thinking that 1:1.45 overdrive of the spindle could be an issue, but it just doesn't make sense that it would also misbehave without encoder feedback, when it's just an open loop system.

The motor is rated 5.5kW. It's on the slower side but the torque should be plenty I'd think. I've spent the last night rolling around thinking of this and I'm pretty sure it must be something to do with step scaling, accel values or something like that. Maybe there is too many steps? 10k per revolution and it's causing issues for unknown (to me) reason. I will try to test that next and see if that makes any difference.

Can you plot (with halscope) the C axis commanded and feedback positions?

That would help diagnose the issue.

I have these snippets right now. The first is the original behaviour and the next one is after a full day of tweaking the PID values and acceleration. In both the axis moves very slowly.   

I guess my question now would be: Why are there relatively huge steps in the commanded position?

The drive is at 10000 PPR and around 1.45:1 spindle:motor overdrive, from my calculations 6k rpm spindle speed is around the limit.

I finally checked your drive manual.  Your drive can handle 4mhz pulse input when using differential signals.

If your drive is set to the default input of 10,000ppr (parameter P0.22), and you want ~4100rpm (motor) for 6000rpm (spindle), that means your maximum pulse frequency will be ~680khz.

Tuning or inertia issues aside, it also results in a fairly coarse positioning resolution of 19 pulses per degree.

You could increase the drive positioning resolution by a factor of 4 without running in to the drive's (or Mesa's) input frequency limit.

However...unless I'm looking at the wrong manual - or you have an unlisted motor - the 5.5kw servo is advertised as having a max speed of 2000rpm.

Do you have a different motor/drive than is listed in the manual?  If not, I'm not sure that motor is going to do what you want in spindle mode.

PCW wrote:

I guess my question now would be: Why are there relatively huge steps in the commanded position?
 

That's exactly the issue I can't get rid of. This is just with G0Cxx commands. I'm suspecting if I zoomed in the second plot enough it would look the same just at higher frequency. For example when homing it acts fine but that's probably because the commanded position is instantly much different and the PID loop takes care of reaching it. But here the commanded position supposed to trail ahead by some degree to let the PID do it's thing I suppose and it just doesn't seem to work with the joint either reaching the position and output going to 0 and then falling behind and shooting up, rinse and repeat... If I lower the P ratio to where it trails just behind, the commanded position slows down aswell eventually leading to coarse steps like before.

I finally checked your drive manual.  Your drive can handle 4mhz pulse input when using differential signals.

If your drive is set to the default input of 10,000ppr (parameter P0.22), and you want ~4100rpm (motor) for 6000rpm (spindle), that means your maximum pulse frequency will be ~680khz.

Tuning or inertia issues aside, it also results in a fairly coarse positioning resolution of 19 pulses per degree.

You could increase the drive positioning resolution by a factor of 4 without running in to the drive's (or Mesa's) input frequency limit. 

I'm aware that the motor is not exactly right for this, however it was already on there before me. It's the same one from the manual with 1500rpm rated speed, making for a pretty slow spindle. The 4MHz value is what I had in mind when figuring out the pulse widths and so on, I admit though I forgot about the motor rating, but for now in spindle mode I only had it in the hundreds range and in axis mode it's even slower. I will try increasing the resolution and see, thanks.