Optical Encoder Axial Tolerance

[sheafromme]

I am using an Avago AEDM-5810-Z06 Optical Encoder and the data sheet (Link) lists a "Shaft Perpendicularity Plus Axial Displacement" value of 15 mm (.006") under the "Recommended Operating Conditions" (Pg.3). This is a tiny tolerance compared to some of the others on the sheet so I was wondering what exactly it was referring to and how it could cause issues if I'm not able to hold to it. It's strange because the optical wheel itself has a lot of play along the axis of rotation within the sensor assembly. Would I have to design a mount such that the shaft is held to a particular position? I'm a mechanical engineer by training so this is a bit peripheral for me.
Best,
-Shea

 

[3DDave]

This sets the amount the encoder is forced off center. It is large compared to the size of the encoder wheel optical measurement features.

 

[sheafromme]

@3DDave thank you for the response!
I had assumed it meant displacement along the axis of rotation, but based on what you're saying it's referring to the distance between the shaft axis and "ideal" axis location? With some bench-top tests I've noticed that the encoder sometimes misses a few counts, shifting the location of a column of a vertically scanned image upward. Do you think the axial displacement could cause that by moving the optical features on the wheel out of the path of the sensor light?
Thanks again,
-Shea

 

[IRstuff]

Generally not; axial displacement typically results in an angle error; the encoder features are much longer than the possible axial displacement.

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[sheafromme]

@IRstuff
Thank you for the input! When you say angle error are you referring to an error in the wheel angle itself or the encoder angle output? I apologize if this is relatively fundamental I am just having trouble visualizing how such a small displacement could cause such a large error.
Best,
-Shea

 

[IRstuff]

encoder output; any offset results in a cyclical error in the encoder output angles

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