Re: [CAD_CAM_EDM_DRO] Linear vs. Rotary Enc.

About backlash.. . . don't linear encoders deal with backlash (with regards
to leadscrew lash and all other sources of mechanical slop) much more
accurately than rotary's, being that they are located at the end of the
mechanical system, essentially? About EMC's control loop. . does EMC have a
way to define an acceleration time constant? Would that effectively deal
with this problem of the servos searching for position, if you were to
lengthen the time they take to decelerate at the end of a movement in one
direction?


-John

----- Original Message -----
From: "Jon Elson" <elson@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Tuesday, August 10, 2004 8:11 PM
Subject: Re: [CAD_CAM_EDM_DRO] Linear vs. Rotary Enc.


> John D. Bussema wrote:
>
> >Howdy. I have a few questions about encoders. . . the main one being: Is
it possible to use linear and rotary encoders interchangeably? If both give
the same quadrature feedback. . . is there even a difference besides the
obvious mechanical differences (and price)? For that matter, I guess I could
ask the same of laser linear encoders. Has anyone tried to use linears on a
small machine tool? I would sure like to see some pictures, because it seems
like mounting them might be a little tricky.
> >
> >
> The biggest difference is that very high resolution is easily achieved
with
> inexpensive shaft encoders. For instance I have 1000 cycle/rev encoders
> on all 3 axes of my mill, direct coupled to the leadscrews. This gives
4000
> counts/rev in quadrature. So, with the 5 TPI screws on X and Y, I get
> 20,000 counts/inch, or .00005"/count. On the Z axis, with a 10 TPI
> ballscrew,
> I get 40,000 counts/Inch, or .000025"/count.
>
> These are Sumtak encoders, and I paid $75 each for them some years ago,
> but similar units are often available on eBay for $30 - 50. Try to get
> these
> resolutions with a linear encoder, and you are almost certainly talking
> thousands
> of $ for an analog scale and an interpolator box. It is generally
> considered
> necessary to have an encoder resolution 10 x better than the "system
> resolution"
> which is 0.0001" on my EMC system. or the resolution you are programming
to
> (would be more like 0.001" for me). I'm somewhere in the middle of those.
>
> The typical linear DRO scales are good to .0002", generally (really 50
uM).
>
> The other big gotcha is backlash between the motor and the encoder. With
a
> shaft encoder, this is no problem. With an Acme screw and a linear
encoder,
> you will have a dithering disaster, as the motor pounds from one end to
> the other
> of the backlash, trying to control the encoder position.
>
> Jon
>
>
>
>
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