Re: [CAD_CAM_EDM_DRO] Re: EMC stg2mod.o and problem with homing

Asim Khan wrote:

>>Of course. The actual position is never what you ask for, just
>>
>>
>within some
>
>
>>acceptable error bound.
>>
>>
>>
>but in case of joging the axis in manual or running the machine in
>mdi mode the actual position error i can see on position counters in
>the range of +/- .0001 mm. then why the homing positions get offsets
>of some greater values?
>
>

OH! So it is ONLY when homing that you have these large differences?
Now, that is real strange. I suppose it may have something to do with the
fact that it registers the index pulse from the encoder on the fly, then
has to decelerate to a safe stop. You might try turning the homing velocity
down and see if the divergence is smaller. But, actually, why should it
matter? What is important is that the homed position be repeatable.
You can always do a X0 Y0 Z0 in MDI mode to get to zero. If that proves
to be repeatable, then I would not worry about it.

If it is NOT repeatable, then there may be a problem in the STG driver
routines. One thing you do have to do is calculate a homing velocity such
that only one encoder tick can happen during each servo update cycle.
If you have 25,000 counts/inch, and the update cycle is 1000 Hz, then you
can only move at 1 inch every 25 seconds when homing, with the
STG card (I think) the way the code is written. I think the STG hardware
allows the encoder count to be frozen or set to zero, depending on the
setting of a control bit, when the encoder count is seen. But, I don't
think it works that way in the software. I think the driver looks for the
index bit seen flag to be set, and then either sets the counter to zero, or
just reads the counter as soon as it noticed the flag. You might look at
the code to see how the index is searched for.

Jon