Position control of brushless AC servo motors

[mackeym]

I would like to use AC brushless servo motors for a CNC lathe retrofit. However, i've read some things here and there that basically say accurate position and low speed control aren't strong points for this type of motor. Does anyone here have 1st hand experience with position and low speed control of AC brushless motors. Also can these motors hold in a position without moving even if some force is applied.

I'm now under the impression that brushed DC servo motors are the way to go (for good accuracy <0.001"). Speed is not a big issue with me...i'll take accuracy over speed.

[Al_The_Man]

AC servo's have just about replaced DC for most commercial systems, as for accuracy at low rpm etc, alot depends on how they are commutated and controlled and how many poles etc, e.g. sinusoidal or stepped sinusoidal have equivalent performance or better (lower inertia) than DC brushed.
if you have (BLDC), Brushless DC, they can display some cogging at low speed, but even these will can be very accurate if any kind of reduction is used, then cogging is not usually apparent.
Al.

[mackeym]

ok, thanks. sounds like they work well if a nice drive (digital sinusoidal) and high gear reduction are used. One other question; do they have holding power. In other words, if they are told to remain at a given position is there significant resistance to moving the shaft?

[Al_The_Man]

I have found them easily comparable to DC Brushed, using torque (current) mode amplifier and encoder on the motor.
Al.