16jan08 td12 stops moving
On 16jan08 at 18:32 ast tiedown 12
stopped moving. It remained stationary until around 5am 17jan08.
The plots show what
the tiedowns were doing(.ps) (.pdf):
- top: tiedown position versus time. The black trace is td12. The
dashed vertical line shows where the problem occurred. the tiedowns
were being commanded to move every few minutes when the distomats saw a
change in the platform height.
- 2nd: the velocity feedback of the D/A. At 18.54 hrs the d/a
ramped up for td12 from 0 to the maximum voltage. This should have
caused td 12 to move at maximum velocity. It remained stationary.
- 3rd : the amplifier speed monitor. I blew up the plot around the
failure and it showed that the feedback from the motor resolver also
showed that the motor was not turning. This rules out a problem with
the encoder since both the encoder and the motor resolver showed no
- 4th the drive status. the drive remained enable for the entire
- bottom: the drive current (torque) used to move the motor. Around
18.0 the current started to increase for td12 when a move was requested
(but td12 would still move). Finally at 18.54 the current hit the max
allowed and td12 no longer moved.
- td 12 stopped moving around 18.54 on 16jan08. It didn't restart
until 5am the next morning.
- Prior to the failure the current needed to move the td was
increasing (though the td continued to move).
- The encoder and motor resolver agree with the measured motion so
it is not an encoder error.
- The motor remained enabled the whole time.
- The current (torque) remained constant at 5. This is the maximum
value that the current for td 12 every gets to.
- The current for td4,8 will occasionally increase to 7 or 8.
- Looking back through the previous month i found:
- 16dec07 td12 current would go up to 8.
- The first failure of this kind occurred on 23dec07 where the
current for td12 remained at 5.
- Other failures occurred on :
- The failures occur at nite when the tension is high. They
tend to occur at the same time range. They do not occur at the
same tiedown position.
- It looks like the current limit adjustment for td12 has been set
too low. It was probably modified sometime after 16dec07. \
- As it gets colder and the dome is at high za opposite the
tiedown, the tiedown needs more torque to move the td. When the needed
torque exceeds the available torque (limited by the torque limit
adjustment), td12 sits at the same spot until the temperature warms up
(or the dome swings away).
- The problem has probably gotten worse since we now use the
tiedowns to compensate for the 20kips of lead that used to be in corner
12 and corner 4.
- We need to verify that all of the tiedowns have enough torque to
- The dome is at 180 degrees from the td and at high za
- The temperature is cold
- The tiedown tries to move from a stationary position (to
overcome static friction).