RE: [CAD_CAM_EDM_DRO] digital protractor for faceting machine

Carbon pot’s tend to be very non linear, unless you buy expensive robotics
ones. If you feed this into a digital system you are then limited by the
analogue to digital resolution (no of bits) of that system. You also find
the loading of the circuit your pot drives will effect linearity being worse
at about 75% of the swing, something so many engineers seem to miss. Then if
you want 360 degrees you use two pots at 90 or 180 degrees to each other so
there is no dead space at the track ends, we call these sine/cosine pots,
and the price tag goes up again as does the complexity. Really I think this
is 1960’s control technology and old hat now but such systems to still crop
up.

An encoder as Brian has suggested is already digitised and has no linearity
issue (so small you’ll probably never detect it).

Your then down to two types of encoder, incremental rely on finding an index
position, and most have an index pulse which you could set to zero degrees.
You system must then count each step as it comes from the encoder. This are
available from about £20 for a Hewlet Packard HEDS logic level type encoder,
to a couple of hundred for a professional CNC encoder.

There are also Absolute position encoders which are starting to become more
common, these always give an output, could be digital, or serial, that gives
the exact position, if you switch the power off, they know where they are
when you switch back on. They tend to be a couple of hundred pounds each,
but I expect prices to start to drop as many manufacturers have moved into
this area.

If you want a simple circuit for the incremental ones, you can often get
little counter LCD modules quite cheap. There are standard chips that will
convert the encoder output to pulses and direction. You put the direction
into the up/down input of the counter, and pulses into the count input, then
use the index pulse to zero the counter. If you pick a 3600 count encoder
you will get 1/tenth of a degree steps. Haven’t used this method for quite
sometime but we used to use it for positional monitoring a few years back
and it worked quite well.



Anyhow that should give you some ideas to get you going.





Dr. Mark Vaughan Ph'D. B.Eng. M0VAU

Managing Director

Vaughan Industries Ltd, reg in UK no 2561068

Water Care Technology Ltd, reg in UK no 4129351

Addr Unit3, Sydney House, Blackwater, Truro, Cornwall, TR4 8HH, UK.
Phone/Fax 44 1872 561288

_____

From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of BRIAN FOLEY
Sent: 04 March 2007 11:21
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] digital protractor for faceting machine



Hi, having faceted a few things, i might be able to help. read up on
encoders, the nice part of encoders is no matter how many times you go
around it still knows where you started from. i am not strong on encoders
but that will be the best to try. cul brian f.

----- Original Message -----
From: "ol3_m3" <ol3_m3@yahoo. <mailto:ol3_m3%40yahoo.com> com>
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DRO@yahoogroups.com>
Sent: Saturday, March 03, 2007 2:21 PM
Subject: [CAD_CAM_EDM_DRO] digital protractor for faceting machine

> I am a new member here and am trying to find information of how to
> build a digital protractor for a faceting machine. Had information
> form a site that used a carbon trim pot, but the site has gone black.
> from a search of this site I also see information on rotary encoders.
>
> I need to know where I can get information of circuit s and such. also
> any tutorials on electronics.
>
> Thanks
>
> O M
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