Single axis homebuilt EMC

[garrett beaubien <garrettb77-at-yahoo.com>]

Hello everyone,

I have built a single axis servo system (with later
plans to add axes), and finally got part of it to
work.  I took Fred's advice and used the dro.c program
running as a standard LINUX process.  Thanks Fred.  I
has no limit or home switches whatsoever, so for those
dispatcher function, I just stub them out by returning
a zero, and setting the flags to appropriate values. 
My output stage does not work properly, but I am going
to look into that.  

I have a couple of questions regarding homing.  If I
unplug my amplifier, I can start up emc, and home my
axis.  Then it turns green, and by spinning the shaft
I can see the readout change.  If I plug the amp in,
and start EMC, as soon as I click home the motor runs
away.  I wonder if I have the polarity of my amp
reversed?  When it runs away, it will not home.  Is
this because it expects homing to accur slower, and
expects 2 seperate encoder "samples" to be the same? 
My Home Switch dispatcher function always sets the
flag to "home".

I was also wondering if someone who knows more about
machine tools than me can explain what I see when I
first start EMC.  As soon as I start, the Relative
position is shown as -60.5000.  What does relative
mean?  Relative to what?  The documentation says this
is relative to the workpiece.  How is this different
from absolute (which reads 0.0000).  I could
understand the difference after homing, but I don't
understand it's meaning before.  The Emc seems to
derive this first value from the initial encoder
counter value.  Is this true?  If this is the case,
what good is it?  Also, this value does not seem to
change after homing.  Should it?

After I get this thing working, would anyone be
interested if I posted a schematic and driver?  It is
pretty simple, probably has about $50 in parts, plus
$35 for surplus motor.  Should be simple to add axes,
just copy the same basic circuit.  I could easily add
some I/O, so that it could have real limit/home
switches.

Thanks in advance

Garrett. 

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