encoder question

[sml2010]

Hi to all

Can you please tell me what is difference between accuracy and resolution of a encoder?
I saw a Heidenhain encoder with below specification :

http://sml2010.persiangig.com/encoder%202.JPG

Position per revolution is 268435456(28bit),
1- Does it mean this encoder have 268435456 line per revolution?
2- Does line per revolution is equal with resolution?
3- What is incremental signals?

Please sea below picture :

http://sml2010.persiangig.com/encoder.JPG

Please tell me :

4- what is Incremental signals and Absolute position values?
5- What is difference between signal periods per revolution with position per revolution?


Thank you for your answer.

[Al_The_Man]

First off Heidenhain use both 5v incremental signals and 1v p/p sine cosine output encoders.
The 90° (quadrature) incremental/differential simply uses two detected pulses with a 90° phase shift between them, the pulse count of one of these pulses is the basic resolution of the encoder, the other pulse indicates direction, controllers and drives either utilize the basic count or use a way of increasing the resolution of the encoder by using 2 edges of one pulse or all four edges of the two pulses.
The resulting axis resolution will depend on how many of these detected pulses occur for a given distance of travel, i.e. how much gearing etc.
The name differential comes from each pulse and its complement transmitted by the RS422 standard.

All encoders start off as sine cosine, the incremental type just used the basic pulse/rev count.
The alternative is to use the sine/cosine signal and use the Arc-tangent function to, in theory detect infinite degree of resolution, in practice the result of this function is to produce a absolute digital number, typical resolutions is of the order of 100,000 pulses.
The optical detection method used by encoders is another story, read Moiré effect.

Phew
Al.