Re: [CAD_CAM_EDM_DRO] Ecoder resolution terms

Mina Aboul Saad wrote:

>Hi,
>
>I just need to know the difference between several encoder resolution terms.
>I read the Gecko requirement and it mentioned a min. 200 line encoder
>Searching to buy the encoders, I found the following resolution terms:
>- Count per Revolution (CPR)
>- Pulse per Revolution (PPR)
>
>Could someone please explain the difference between the 3 terms.
>
>

This is a great big mess. The only terms that you can be certain about
are cycles/rev or lines/rev, they are equivalent. When abbreviated,
however,
you have a problem. Does C mean Cycles or counts? That's when the
trouble starts.

One cycle or line on the encoder produces 4 edge transitions, so any
quadrature decoder that decodes all transitions (counts all edges, etc.)
will give 4 counts for the full cycle. So, a 1000 CYCLE/rev encoder
is a 4000 COUNT/rev encoder. COUNT and Pulse are equivalent, in
most cases. But, CPR is totally meaningless, unless you know what
tht "C" stands for.

The 200 LINE encoder mentioned above should give 800 COUNTS/Rev.
Also, in Gecko terms, this is not a hard and fast number. I suspect a
Gecko 320 could be hooked to a very small motor with a 16 CYCLE/Rev
encoder and made to perform acceptably. It wouldn't do so well on a large
motor connected to a large machine, of course.

Don't look at the encoder first. First, find the motor that is capable of
driving the machine in question with acceptable torque and speed.
Determine the belt reduction ratio, if one is needed. THEN, figure out
the revolutions of the motor/unit of linear measure, and figure out
what encoder resolution you need. For instance, if you want to describe
positions in units of .0001", it would be good to have resolution of as
close
to .00001" as possible. If a wood router only needed .005" accuracy,
than a .0005" resolution would be good enough. This is an old machine
tool "rule-of-thumb" and is not very strict, but a good guideline.

Jon