scope encoders and precise mode


Mark Copper
 

This is silly but it bothers me that I don't understand.

Working with Servo I

Scope encoders set to "Precise mode" (see photo)

It was my understanding that in precise mode the controller would bring the motor and scope to agreement.

But today under the numbers tab, "SAxis" is constantly about 2 arcminutes ahead of "ScopeAl". Why is that?

Here are photos of my screen about 12 minutes apart:
   https://drive.google.com/file/d/1pNTUVQUL7IXGP-YGl2GgDavuiOMtiys1/view?usp=share_link
   https://drive.google.com/file/d/1pd-gY0PFwkhqo3mG2bfFKc5lnBTfqKzt/view?usp=share_link
   https://drive.google.com/file/d/1pbvvkdvYKONLbba3_u1nxlTmbM05GCZo/view?usp=share_link

Thanks.

Mark


Don W
 

Hi Mark,

Dan may have a more definitive answer.  But you are looking south.  I don't know how you aligned your mount to the sky and what your latitude is at the mount.  But if your mount is not perfectly level (tilted by 2 arc-min) then that may be the source of your "error".  But see that the SAxisError is reported as zero.  Mount encoder accuracy for 4K tics is 5.4 arc-min.  For 10K encoders the resolution is 2.16 arc-min (about your error).

Don W


Dan Gray
 

Did you read the help file in SiTechExe/Config/ChangeConfig/Encoders?
Here's the snippet about precise mode:
2. Precise Mode:  
If selected, then as soon as there is a dis-agreement between the motor location and the encoder location that exceeds the Supervisory Threshold,  then the motor location is updated to the encoder location, and the scope will re-center.  Use the Precise mode if you have the low resolution encoders, but you're not doing photography (the "Precise" word is used loosely!).

Make sure that you set up the "Supervisory Threshold" to have a value of about 1.5 to 2 times the resolution of the encoder.
As an example, if you have 10,000 tick (2500 line) USDigital encoders, then you have about 2 arc minute resolution.  You should set up your "Supervisory Threshold to at least 3 arc minutes.




On Thu, Nov 17, 2022 at 6:20 PM Don W <westergren@...> wrote:
Hi Mark,

Dan may have a more definitive answer.  But you are looking south.  I don't know how you aligned your mount to the sky and what your latitude is at the mount.  But if your mount is not perfectly level (tilted by 2 arc-min) then that may be the source of your "error".  But see that the SAxisError is reported as zero.  Mount encoder accuracy for 4K tics is 5.4 arc-min.  For 10K encoders the resolution is 2.16 arc-min (about your error).

Don W