This page has some notes on
how things actually work and what we may need to do to make things work
Tracking into the prelimits:
When tracking into the pre limits the little stars are
moving to stop mode. The tiedowns will move from stop mode to moving
mode if a new command is sent. When inside the prelimit, only positions
that move away from the limit are acted on. The PI loop is not used
to hold the tiedowns at a fixed position inside the limit. Instead, a fixed
dac value is output that should keep the tiedowns stationary. I guess
this was chosen so that encoder failures would not continue to drive the
system toward the final limit.
Tracking into the
The drawback with this technique is that everytime
we change a dac or analog output signal conditioning board, we need to
make sure that this zero velocity offset is set correctly (it changes).
If it is off by a little then the tiedown will drift into the final limit
(at either the top or the bottom). If this happens then you need to go
down to the tiedowns, bypass the final limit switch, and drive the
tiedowns out of the final limit (after fixing the original problem). I'm
not sure which method is better in practice.
Another problem is the tietrk commands interaction
with the tiedowns running into the prelimit. When the tiedowns
stop, i don't think the tietrk mode is sending another command to move
again, it is just sending tracking positions. The operator has to manually
execute the tietrk command (or some other command) that tells the tiedowns
to move to tracking mode (i need to verify that this is really true).
This is becoming more of a problem since the platform
is low and during the summer days we can go into the prelimits.
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