Today while paging through some transfer functions we noticed that the low frequency transfer function from stage 1 actuators to Stage 2 CPS does not go to zero, but goes flat below ~100mHz.  This seems like a very strange thing. Last night we took a TF with all of the lockers engaged, 45kgs of extra weight in each corner of the stage 2 keel plate (45*3kgs total extra weight) and with the corner 3 CPS to AA chassis unplugged and shorted.  The attached figure shows the these transfer functions below 0.1Hz and above 0.1Hz transfer functions from the night before when the system was free and fully running

  some things to notice in the plot

  1) locking the system down reduces the low frequency signal (~20X) but there is still alot of signal there (coherence is still 1)
  2) Disconnecting the CPS controllers reduces the signal much more,  reducing the coherence to ~0
  3) We tried disconnecting various CPS sensors in both the locked and unlocked state to see if there was any sensor cross talk, with null results
  
   Very tentative conclusions, either there is pick up from the actuator or actuator drive cable directly to the cps sensor or electronics or the sensors are
actually picking up signal, flexing or motion we aren't sure.

Celine, Hugo and Rich