View Single Post
Old 01-02-2009, 04:12 PM
Registered User

gary is offline
Join Date: Apr 2005
Location: Mt. Kuring-Gai
Posts: 5,444
Arrow Floating tangent arm with your Argo Navis

Originally Posted by nightstalker View Post

yes its fairly light for 2mm brass now ..behind the encoder is also drilled out extensively, the encoder cable covers the holes nicely .

I'm still not sure what you mean by fixeing the bottom end of the tangent arm ? .. wont a floating type arangement allow play to effect the pointing accuracy of the alt motion ?
Imagine for one moment that the far end of the Alt tangent arm was just totally
unattached. It then would have several degrees of freedom.

1. It could move toward or away from the side of the rocker (e.g. encoder shaft not
exactly co-axial with the Alt axis of the mount).
2. It could shift radially outward or inward from the center of the Alt axis (for example,
the bearing was not quite circular or encoder not centered).
3. It could rotate around the Alt axis.

What you want to do is ideally have an arrangement whereby '3' is not allowed
but that '1' & '2' are tolerated.

'3' is not allowed because it is in the same direction of rotation as the encoder
itself and will give rise to a positional uncertainty.

However, any geometric error that results in a pointing error residual
whereby, as a side effect, the tangent arm behaves as in either '1' or '2'
cannot be circumvented by holding the far end of the tangent arm fixed.
Instead, the problem can only be fixed by correcting the mechanical source
of the problem, say by re-centering the encoder.

The risk is that, should 1 or 2 be evident, if the far end of the tangent arm
is fixed tight, then any irregularity, say from encoder not being centered, will
result in a force being translated back up the tangent arm and thence
ultimately be applying a sideways force or moment to the encoder shaft itself.
If the force is great enough, it could damage the encoder.

Thus, use a slot carefully milled in the end of the tangent arm that slides
over a locating pin or shoulder screw. The arm will then be free to float
if '1' or '2' phenomena is evident but will not be free to rotate around the
actual axis.

Hope this textual explanation, for what is often best explained by a few
sketches, is clear. If not, I will attempt to clarify further.

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
Reply With Quote