Brushed DC motor closed loop speed control

[Martinn]

Did some more measurements.
Lowest reasonable feeds for a milling machine are in the order of 5-10 mm/min. Given a 4 mm leadscrew pitch that results in about 2.5 rpm (about 1% of the max 240 rpm at the output of the gearbox).
With the current setup I can get down to about 16 rpm, which is the lowest value that is reasonably stable. This gives 64 mm/min, which is well above of what I'd like to see (around 10 mm/min). BTW I added lowpass filtering in the current path, which gets rid of the oscillations.

Played a bit with the 30A8. Tried out different modes - current, voltage, IR compensation.
And I have to say this thing just... works. Out of the box with default values performance is already decent. For IR compensation I tried different compensation values (try to find R8 on the PCB) and you quickly find the one that is just right.
While there is not much to adjust overall, what they have as defaults is clearly very well designed.
So what did I get? With IR tuned and loop gain quite a bit below oscillation, lowest usable speed seems about 12 s/rev (after gearbox) or 5 rpm, resulting in 20 mm/min feed. Not quite as low as I'd like, but 3x lower than the previous analog implementation.
It looks like I have to come up with a mechanically sound way to integrate the amplifier to the feed unit. It is many times too large to fit into the original housing, so I'll have to 3d print something.