Adding smarts to the MPCNC

[Hillsdale]

Moving this here from another less appropriate thread.

@Hillsdale

I’ve also been excited to follow your progress on the closed loop motors - I’d been wondering the same thing. I’ve used expensive brushless motors with encoders in the past (maxongroup.com), both with hand-rolled PID loops and hardware controllers and was excited to see that there are now consumer level cost effective options that appear to just work.

For closing the loop on the X and Y axis I feel like it winds up more valuable to track absolute position rather than motor position (removing any worries about belt stretching, slipping, etc). There are really expensive linear encoders (at least, I assume they’re expensive because they’ve got no listed price) such as Serie 3 - Fagor Automation , but I’m not sure what the cheap equivalent looks like. From my prior life I’m wondering if it could be as simple as a sticker with an encoding pattern along the rail and a cheap optical sensor to count dots, but obviously there’s a lot of development to get from concept to implementation…

I pointed to a couple of really inexpensive options, including the Broadcom aedr chipset family for optical reading and an off the shelf optical strip:

AEDR-8300-1Q0

Generic Linear Encoder 180 LPI 3400mm 13mm

and I wound up ordering the first one, set into a small board, for $2.50 off AliExpress, and a similar strip for around $10/piece (actually bought a 3 pack of 1200mm for $28.50, but they go up 7000mm).

Steve noted:

@stevempotter

This would be a good pretty cheap option for closed loop feedback. Even better if you can find one that has a finer pitch. 180 lpi is 141 microns per line. And super delux if you can find one that can tell its absolute position without homing – A non-repeating pattern on the tape. If you do find, put a post in my Adding Smarts to CNC thread.

I really wanted absolute positioning as well, but couldn’t find a good solution. Broadcom has an absolute positioning chipset (link below) but the signalling seemed more difficult and more importantly the absolute encoding tops out at only 52mm at 318lpi. Broadcom sells a tape strip double that length (a bit longer for 180lpi) but I didn’t see any option or availability of one long enough to be useful to us.

AR35-L80E; AR35-L80S

They also support what they call pseudo-absolute positioning in the AEDR line - I need to reread the docs to understand its limits, but it does let you use a second row in the grating at various binary multiples to create an absolute position window. Unfortunately I didn’t find any off-the-shelf tape that seemed to already be in that format, and there are places that will custom print the grating inexpensively in volume but small quantities are very steep.

Regarding the lines-per-inch, I ordered my samples at 180dpi, mostly because the Broadcom chips that support that resolution work at 5v while anything higher resolution was at 2.8 or 3.3v and I didn’t feel like adding another transformer for now. 318lpi is the highest I found for reflective sensors, though there may be other vendors. For transmissive sensors (where the light shines through the strip to the receiver) you can go up to 2000lpi but that seemed mechanically more difficult and delicate. I’ve been thinking about Y for now and assuming the strip would mount directly to the rail of a lowrider (out of the way of the rollers) or if it doesn’t like the curve on a piece of square steel tubing running parallel to the rail. The encoder wants to be about 1mm from the strip, so a small roller to keep it off the strip and a spring to keep it on the strip should be enough.

No idea how it likes the presence of wood dust.

Will update when parts arrive and if I get a chance to play with them. I have a feeling it will take a while just to show something on a signal analyzer but maybe I’ll get lucky.