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 around.
• 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 18.2
  
• 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 negative.
• 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.
  

• td tension 22:20 (.pdf). At 22.75 td12 starts to move up and the tension in td12-1 goes down.
  

Conclusions:

• 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.
  

1. PosTd line: If any of the tiedown positions read 24  or 0 inches, the tiedowns should not be re enabled.
2. 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.
   

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