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):
The timeline looks like:
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
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
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).
On monday 27jun05 the little star computer was replaced
as well as the amplifier for the motor. On Tues. 28jun05 the parameters
were set correctly and things started to work.
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
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.