• Tiedown 4 stated having problems on sat 02mar13.
• The encoder readout started jumping between the current value and 24 inches (first time around 11:00). The program was in tietrk mode at this time (use the distomats to keep the platform at a fixed height).
• Around 13:00 the mode was switched to tiepos temp (move to a fixed position and sit there).
• The jump continued intermittently.
  
• On sunday 03mar13 around 12:00 the tiedowns were turned off and disabled. For the rest of the day the operators would enable the tiedowns, move them, and then shut them off.

The plots show the tiedown status for these two days:

• sat 02mar13 td status (.ps) (.pdf)
• page 1: data 0-24 hours sat 02mar13
• top: encoder position. td 4 is in red
• the jumps are at:11,13,14,19 hours
• 2nd: velocity feedback. This is the voltage readback that is sent to the amplifier
• The bright  purple is the requested value for td4
• 3rd: amplifier speed readback. This voltage provided by the amp is proportional to the amp speed
• There are large red jumps were td 4 encoder jumped. This shows that the system tried to follow the encoder jumps
• 4th: drive status. td 4 was off (red value=0) from 14:30 to 16:15
• bottom: drive current from the amplifier. This is proportional to the motor torque
• There are large jumps in td 4 torque (red) during the encoder jumps.l\
• page 2: blowup of 14.5 to 14.7 hours  (14:30 to 14:42).
• the 5 frames are the same as page 1
• the longest encoder error lasted from 14.58 to 14.61 (.03 hrs = 108 seconds).
• The motor power was turned off just before this occurred (frame 4 status = 0).
• The computer continued to command the a/d to follow the jump (vel feedback -> -1800).
• this shows that the computer was not completely "messed up"  when the encoder failure occurred (since it was still commanding the amplifier and reading back the requested voltage). I do wonder why the cpu was commanding the td if the drive status had been set to off?
• 
  
• sun 03mar13 td status (.ps) (.pdf) 
  
• the tiedowns were turned off around 11:30. Around 13:00 emergency stop was set.
• There are encoder jumps at 10:45 and 14:00
• During these jumps there were large torque in td4

summary:

• td 4 position readback is jumping from current value to 24 inches.
• It could be:
• the encoder or encoder board
• the bus connecting the encoder board to the cpu
• the cpu
• the cpu continued to do some functions while the jumps occurred so it wasn't completely dead.
• It is important to make sure that the tiedowns are left off while this problem occurs. You don't want these large steps in position to be sent to the jack.