The Pokeys57CNC has a non-zero volt output when Mach4 outputs a S0 (zero RPM) G-Code.
In my case the offset was 71mV. My servo drive has a setting where I can compensate for that. So that went well. BUT: there is a temperature drift on the 0-10V Pokeys57CNC output as well. It is in the mV/°C range and it does affect the output.
For zero RPM, my servo drive has a dead zone programming in increments of 10mV around 0V, so it is usefull. But the true RPM will also fluctuate with that temperature drift.
Now, because we are in the analog world here, with a servo drive set up as a speed controller, we can expect some deviations, which is fine.
Since for threading operations Pokey57CNC is synchronizing with the real RPM it gets through the Ultra High Speed encoders there is no issue, other than cosmetics on your DRO Mach4 screen where there is some mismatch (if all is well in the single percent range).
I did initially set up the servo drive as a Step/Dir device and there the speed was 100% accurate. The dynamics of the drive were also superior and much more stable than the analog speed control setup. I was very happy with that. Until I tried to do some G32 and G76 threading. It is a no go. After consulting Polabs, they confirmed Step/Dir and threading combination are not supported. So I was forced to use the servo drive in analog speed mode. Very unfortunate.
Now, in analog speed controller mode, my drive has some stability problems.
Not the usual ones. The setup on a timing belt driven lathe is easy as long as you use low gains and high timing constants to avoid harsh oscillations.
In this case it seems to nicely rev up to it's set point and then starts oscillating very slowly over and under that setpoint. The frequency is low, some 0.5Hz to 2Hz and faster at RPMs higher than 500RPM. No PI setting seems to solve this. I know a lot about PI and PID controllers but somehow I still am missing something here. The voltage on the Pokeys output seems to be rock solid with a fast multimeter but I will connect a scope to it tomorrow to see how it really behaves and if there is a correlation with the RPM swing that I can see.
Hope this helps anyone working with POkeys57CNC and analog 0-10V setup.