Servo as a spindle motor replacement - what to look out for?

[Voicecoil]

I scoped it last night with 2 channels in differential mode and the output from the 485 were sweet (common mode noise all gone), particularly compared against the single ended measure (full of noise). Also, last night, changed the kernel frequency of the uc300 to 50khz to increase the pulse width to 10us (at 100khz kernel the pw is a rather narrow 5us).
The thing that gets me is that this was working fine. Going single ended feels an inferior move than differential driven)... unless one of the 485 outputs is starting to fail hi-z.

This was 'scoping it at the controller end or your RS485 board? What sort of signal levels were you getting?

[Doddy]

Okay, I'm half convinced that I'd got the PP+/PP- inverted (Opto in saturation?, recent cold snap?). Ignoring that I've wired it single-ended direct from the BoB and all is good again. I would look further at this but to be honest I need the space and time to look at other projects - this can wait until I need to open the cabinet again.

[Doddy]

This was 'scoping it at the controller end or your RS485 board? What sort of signal levels were you getting?

At the '485 board, which meant resting the scope on a bin, and leaning over the horizontal band saw whilst trying to set the math function up on a scope with a crap rotary encoder for the function setting. The waveforms were pretty much 5Vpk-pk though I wasn't really paying too much attention to that. Call it 2 and a bit divisions, and cramp in my calf.

[Voicecoil]

Okay, I'm half convinced that I'd got the PP+/PP- inverted (Opto in saturation?, recent cold snap?). Ignoring that I've wired it single-ended direct from the BoB and all is good again. I would look further at this but to be honest I need the space and time to look at other projects - this can wait until I need to open the cabinet again.

Glad to hear it's working again - sometimes practical considerations are far more important than reaching ideal theoretical solutions!