Encoder Count

[Billiam]

I'm buying some servos for an old VMC retrofit.

Automation Direct's servos have 2500 count encoder (10,000ppr quadrature). Seems plenty for my 5mm lead ballscrews.

But then I see Yasakawa Sigma 2 motors with 131,072 ppr encoders, Yaskawa Sigma 5 motors with 20-bit 1 million count encoders, and Fanuc motors with 1M and above.

So why use such high count encoders? They'd have to be electronically geared down anyway to get fast rapids due to the limitations of the encoder pulse frequency from even high end MC cards, and I'm guessing that even at low speed, a couple pulses wouldn't even be enough to get the machine to budge due to static friction.

Thanks for any info!

[One of Many]

The new types of encoders are serial, not quadrature ABZ, so the MC system needs to have the capability to interface them. Yes, many are geared on the axis screw or internally so they only turn one full revolution or less over the entire run length of that axis, which places most of them in a absolution encoder class. Many are programmable from their max resolution too. Primarily used on high end applications. Not your typical retrofit on the cheap!

General rule of thumb is to select a resolution at least .5-10x finer than your displayed value. This will give you at least +/- one count of the encoder or remain within the tolerance of the axis repeatability.

DC