Re: [CAD_CAM_EDM_DRO] Encoder/Servo question

Thanks for the info, Les. I think I'll pass on the book for the time being,
though, unless you've got one you want to sell cheap. :-) It's just too big a
chunk out of everything else.

Bill

On Sat Jul 17 03:39:34 2004 Leslie Watts wrote:

>As for the encoder resolution question...Most controls
>do something called a 4X interpolation on encoder lines.
>This is a way to use the leading and trailing edges of the
>two quadrature channels to effectively multiply the line count by 4.
>So a 500 line encoder actually effectively generates 2000 per rev
>in a control system. This is done by the device reading it.
>Just a bit of extra info.
>
>Now for your example of a 1 turn per degree worm drive...
>
>Generally servo systems for machine tools work well if ten
>pulses (not lines) represent about the desired level of repeatability.
>This is not a hard and fast rule, but I use it in many applications.
>A simple PID servo loop (without feedforward)requires an error for movement.
>Very high pulse rates can be used, but this creates some problems.
>If the machine structure is not capable of that kind of repeatability
>there is no use in creating the extra problems.
>
>One pulse per 0.0001 is commonly used on small mills and generally
>gives .001" repeatability or better if the structure is capable.
>Note repeatability and accuracy are not the same thing.
>
>On a rotary table I would want about that equivalent on the outer
>radii. So lets say the diameter is 10". One degree would have an arc length
>of 0.0873 at the outside of the table. At a 10,000 pulse per inch pitch
>that would require 872 pulses per worm revolution and would be obtained
>with about a 200 line encoder! For things closer to the table center the
>resolution would be better. A 100 line encoder would give the same results
>for a 5" diameter table.
>
>It might be nice to have the encoder on the table axis rather than the worm,
>but that would require about an 80,000 line encoder! (10xPIx10000/4)
>
>As far as the best tooth profile I would have to defer to higher authority
>:).
>I know the pattern is an involute profile- but only along the worm
>centerline.
>High pressure angles are often used (25 degrees plus). The worm on
>centerline
>looks a bit like a rack so the involute shape is a trapezoid in that case.
>There is a huge amount of information out there though. Nice thick
>engineering
>books are written on just low backlash worm gear design. Fun reading I'm
>sure!
>
>Ah here's a link...
>http://www.hermespenton.com/asp/bookdetails.asp?key=3056
>
>Buy this book then tell me the answers. :)
>
>Les
>
>Leslie M.Watts
>L M Watts Furniture
>Tiger Georgia
>
>Main page:
>http://www.lmwatts.com
>Engineering:
>http://www.lmwatts.com/shop.html
>Cnc surplus for sale:
>http://www.lmwatts.com/forsale.html
>Carved signs:
>http://www.lmwatts.com/signwp.html
>
>It's on the, "todo", list. At the moment, though, I'm just trying to put
>together basic info on things. I finally saved up enough pennies to get a
>7X14"
>Minilathe, and hopefully, will soon be looking at a Minimill. Ya gotta
>start
>somewhere. I'll be trying to do a manual rotary first, and once the bugs
>are
>worked through, a servo job. That being said, I'm trying to figure out some
>basic ballpark stuff, like what degree of accuracy I can get out of one,
>hence
>the encoder line count vs % of rotation question. Assuming a 360:1
>reduction,
>a 1000 line encoder would theoretically give you one thousandth of a degree
>accuracy, yes?
>
>On another tack, there's the question of what thread to use. How would a
>28/29/29.5/30 degree. (whichever), acme thread compare to the 40 degree gear
>type thread for the worm/wormwheel? Seems like it should have better torque
>transmission, but whether they would be smoother acting or even more or less
>suitable for cnc-ing, one over the other, is something I haven't seen
>discussed
>in this context.
>
>Bill


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