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#86
04-12-2008, 05:53 PM
 gary Registered User Join Date: Apr 2005 Location: Mt. Kuring-Gai Posts: 5,443
Quote:
 Originally Posted by koputai Gary, What are the maximum and minimum steps/rev that the AN can handle (or realistically use).
Thanks for the post.

The menus allow step counts up to 9,999,999 steps. However,
when using the encoder interface, one also needs to take
into account the encoder sampling rate. Estimating your
maximum slew speed in degrees/second combined with a
knowledge of the encoder sampling rate can then allow you to
design the appropriate amount of gearing.

The following may also be helpful.

As you are aware, Argo Navis can be powered from a set of AA
batteries. The encoders are the most power hungry devices in
the system. In order to conserve power, Argo Navis powers
them on, samples them, then powers them off, thousands of
times per second. Since the encoders are sampled, there is a
maximum 'tick rate' at which the encoders can be turned and
still read reliably. In communications theory, the Nyquist
Criteria
says that one needs to sample a signal at least
twice the rate at which the signal is changing, in order to
sample it reliably.

Even when Argo Navis is powered from an external DC source
with the encoders permanently powered on, (achieved by SETUP
ENC TIMING, TON=xx, TOFF=0), the unit still samples the
encoders. In this case, the maximum sampling rate is around
16,000 samples per second. Hence the maximum slew rate
should not exceed 8000 times a second for the Nyquist
Criteria not to be violated. Working backwards, one can then
determine by how much you can gear the encoders based on
what your expected maximum slew rate is likely to be.

For a 10,000 step encoder geared 1:1, you can slew at
approx. ((16000/2)/10000) * 360 = 288 degrees per second.
The 16000/2 is owing to the Nyquist rate. Therefore you can
also work backwards and if you know in advance what your
maximum slew rate will be you can then determine the max.
number of encoder steps for one 360 degree revolution and

Anecdotally, on a colleague's 20" Dob we use quite a lot, we
have been running 56K steps on the Alt axis and 10K on the
Az axis for many years and never seen an encoder sampling
error message and this scope is purely PUSHTO.

The User Manual, in the section on SETUP ENC TIMING,
provides the sampling rates for various settings of the
SETUP ENC TIMING parameter. Using these numbers, one
can trade-off sampling rate/slew speed for battery life.

On some other GOTO scopes, rather than via the encoder port,
encoder data is supplied to Argo Navis via the serial port.
On these scopes, step counts in the millions are not
uncommon. However, for your PUSHTO configuration on a small
scope, I would recommend you aim for something around 10K to
30K steps per 360 degree resolution of the mount.

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
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au