Who Knows Servos?

[Sparky_NY]

As to servo position error, for the DIY systems I put together, I used Galil Motion cards, that sit in a PC slot, these are about the closest you can get to a Industrial designed system.
As to PID tuning, you can achieve around 20 pulse error. The Minimum resolution encoders used are 1200 pulse/rev which translates to 1200x4 = 4800 pulses/rev when using the quadrature count.
This error is only meaningful when you then compare this to something Fanuc calls your 'Least Input Increment' or what you desire to be the smallest positioning value, e.g. 2 μm for example.

Positioning error is one parameter, following error is another and the parameter tuning is normally to optimize. (position error is static without movement, following error is while the axis is in motion)

Optimizing following error is what ensures your circles don't end up being ovals.

[MaxHeadRoom]

With the Galil Motion cards, tuning the PID loop for optimum, (~20 pulses) guarantees minimum/acceptable following error.
When 0v-10v analogue Torque mode drives used

[skunkworks]

Sure - On the emco lathe - each encoder count is .0000196" (these are pwm amps - the actual input is up/down pwm)

LOL - I just looked at the K&T - the lowest resolution axis is also around .0000194" per encoder count. (+/-10v drives)

The motion/hal layer of linuxcnc can compete with any industrial control. (the user interface? to each his own.. )

I have a video of the k&t (multi-ton machine) positioning to within .0001"



sam

[Noitoen]

Used to do some work at a plant with a few turret lathe type cnc's. All the axis except for the spindle were hydraulic actuated with glass scale feedback. The control card operated with step/dir signals and would give "excessive following error" if the hydraulic actuator for some reason didn't keep up the pace. I was amazed how precise the system was with only one hydraulic pump controlling various axis at the same time with precision.