What is the accuracy of servo position encoders

[men8ifr]

My understanding is servos use a motor position encoder, but what is the accuracy of that? Is it a patterned optical disk attached to the shaft?

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[joeavaerage]

Hi,
the simplest optical encoders are a transparent disc with many dark lines around its perimeter. The presence or absence of a line is detected
by an optical senor and infra-red LED.

There are usually three distinct channels, A and B and an index. The A and B channels each have an identical number of lines but one comes on slightly after the other,
they are said to be in quadrature. The reason is that it means that depending on which signal leads will determine the direction of rotation.

The Index channel has just one line and 'blinks' just once in every rotation, and is used to synchronize the computer monitoring the A and B channels.

Once upon a time it was common to have encoders with say 512 or 1024 lines around the periphirary of the disc. Nowadays the entry level norm is 2500 lines.
As short as ten years ago such an encoder would have been very expensive, not so today.

With 2500 lines when you combine both A and B channels you get 10,000 (2500 X 4) distinct 'counts' Thus you could have the servo drive to 'a position 5643 counts clockwise
from the index mark'. Your servo has a potential resolution of 1/10,000^th of a turn or 0.036 ⁰ or 2.16 minutes of arc.
Older servos with fewer lines had correspondingly less resolution. As a practical matter to get within 2 or 3 counts of the perfect position is more achieveable....
so about 1/10 ^th of a degree....still very very good. These encoders are called incremental encoders, you need the index signal to give some reference
position.

More recent advances are called absolute encoders, they use a Grey Code arrangement of lines and don't need an index mark. They are said to have 16 bit, 17 bit
and 20 bit resolution. 20 bit resolution will result in 1/1,000,000 ^th of a turn resolution!!! Even more recent and expensive are 24 bit absolute where the encoder
counts a whole number of turns and then still has 17 bit resolution within one turn, very exotic!

If you provide some details about the servos you are looking at we could give you a pretty good estimate of what you might expect in practice.
The theoretical best is the (number of lines X 4)= counts per revolution. So the angular (best) resolution is 360 /counts. You should get close to this
unless using electronic gearing within the servo drive.

Craig