25jun05 td12 trouble

        On saturday 25jun05 around noon, lightning struck tower 12. For the next 24 hours the encoder reading for td12 said that the jack was at 14 inches. On sunday 26jun05 at 18:00 a visual inspection showed that td 12 had released (the jack was at 2 inches). The plots show the tiedown positions from the encoder (.ps )  (.pdf):

•  Fig 1 (sat 25jun05). The top plot has the encoder positions (black td12, red td4, green td8).  The blue dashed line at the bottom is the up directional limit switch for td12 (5--> at upper limit of 2 inches, 0 --> not at upper limit). The dashed red line shows the amplifier speed feedback. Non zero --> that the jack was moving.  The bottom plot shows the distomat positions for td12 corner (black) and the average height of the platform (blue). Td12 corner height can change as the az,za moves. The average height can change if the tiedowns move or if the temperature changes.
• he tiedowns were moved to the stow position around 12 noon (since a lightening storm was approaching).
• at 14.5 it looks like someone enabled the tiedowns for awhile. Td4 and td8 moved but td12 did not (according to the encoder). The amplifier showed some motion of td12 and the limit switch came on temporarily. The td12 corner shot up then back down. It looks like td12 released and then came back.
• At around 17 hours it looks like the tiedowns were re enabled. The limit switch for td12 became active and stays active for the next 14 hours. The td12 corner shot up so td12 probably did release. The encoder remained at 14 inches.
• Fig 2 (sun 26jun05).
• At 8 am it looks like someone re enabled the tiedowns. td4 and 8 moved but td12 (encoder) remained at 14 inched.  The td12 limit switch went off and the td12 corner came down.  So td12 probably went back up to where it was supposed to be.
• at 12:30 pm the jacks were moved again. Td12 limit switch went on and td12 corner shot up. So td12 probably released again.
• 18:00  luis murray goes down to td12 and finds it released (2 inches). He brings it back to 8 inches and shuts off the tiedowns. The tiedowns remain in this position until  9:30 am on 27jun05 (monday).

• 25jun05 _14:30  probably a 1/2 hour when td12 was moving around
• 25jun05 _18:00 to 26jun05_8:00 td12 is released
• 26jun05_8:00 to 12:30 td12 not released
• 26jun05_12:30 to 18:00 td12 is released
• 26jun05_18:00 to 27jun05 9:30 tiedowns off with average height around 8 inches.

    The lightning strike damaged part of the little star and it's program. Some of the problems could have been:

• The bad encoder reading probably came from how the little star reads the encoder. If the error bit is set on the encoder read, an encoder error flag is set and the encoder reading is ignored. The constant value of 13 was because the encoder was never being read correctly. 13 was probably the last valid value read (before the lightening strike.
• The releasing of the tiedown. The little star program needs to hit a watchdog timer every few seconds, or the tiedown will release automatically. For some reason it was probably not hitting the timer.
• Most other things on the little start continued to function. The communications remained ok, and a number of the readings looked reasonable.

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