lightning breaks td 12 encoder board
A lightning strike at 18:06 blew out
td 12 encoder interface board as well as loadcell 2 (not sure if it is
the load cell or the amp). The velocity feedback of the DtoA output
also went bad at the lightning strike. The tiedowns had been
disabled prior to the lightning strike. Around 22:30 the tiedowns were
re-enabled. td 12 encoder was reading 24 inches so the td control tried
to release tension on td 12. The moved td12 all the way up. The
distomat monitoring saw the platform rise to td 4 and 8 were pulled all
the way down to 22 inches. This gave the maximum tilt to the platform.
The plots show the failure
- td position 1800 (.pdf):
- top: td 12 position jumped to 24 inches at 18.15 hours.
- 2nd: around 18.2 the velocity feedback for td12 started to jump
- 3rd: the speed monitor readback from td12 also started to jump
at 18.2 even though the motor was off.
- 4th: all 3 tiedowns remained faulted because the emergency stop
switch had shut off the power.
- 5th: the drive current also started to jump for td 12 around
- td position 22:30 (.pdf):
- top: the tiedown position for td 12 read 24 all the time. td4,8
started to increase (pull down) starting around 22.77 hours. This was
caused by the tiedown tracking trying to keep the average height of the
platform constant. During the period td 12 position was going towards 1
inch (all the way up).
- 2nd: the velocity feedback for td12 remained at the max
- 3rd: the motor speed shows that td4,8 were pulling down while
td12 was moving up (to a smaller value) around 22.75 hours.
- td tension 18:00 (.pdf): At
18.1 hours ld cell td12 load cell readings went to 0. At 18.2 load cell
12-1 started to read again. ldcell td 12-2 remained at 0. The change in
td tension is being caused by temperature changes and the motion of the
azimuth/dome. The tiedowns are not moving.
- The tiedowns should not have been re enabled at 22:30. This
caused the tiedowns to tilt the platform and wreck the pointing.
- After a lightning storm where the tiedowns have been disabled,
the following checks should be made before re enabling the tiedowns.
The lines refer to the tiedown display image below.
- PosTd line: If any of
the tiedown positions read 24 or 0 inches, the tiedowns should
not be re enabled.
- TdTime line: The
tiedown time for each of the tiedowns should be incrementing and be
close (within 1 second) of the correct time. If a tiedown's time is
wrong, that probably means that the tiedown communications is broken,
or that the td little star computer is having trouble.