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 motion.
  
• 4th the drive status. the drive remained enable for the entire time.
  
• 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.
  

 Conclusions:

• 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 : 28dec07,29dec07,30dec07,31dec07,02jan08,03jan08,04jan08,08jan08,09jan08.
• 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 move when:
• 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).
  

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processing: x101/080116/td12fail.pro

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