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Reports until 20:49, Friday 16 March 2012
LHO General
robert.schofield@LIGO.ORG - posted 20:49, Friday 16 March 2012 (2457)
Fans in aLIGO I/O chassis show up on channels through power supply ripple

Rich Abbott reminded me of the fans in the I/O chasis, so I looked for coherence between a magnetometer (which detects the fan rotation frequency) and channels passing through the A to D in one of the chassis. I found coherence for all channels examined. Figure 1 shows coherence between an OSEM channel and  my magnetometer channel and also shows that I was able to repeatedly move the coherence peak back and forth in frequency by covering/uncovering the fan ports (which changes the rotation frequency). We have had problems with fans in electronics for a long time – I found that the fans in coil drivers showed up in AS_Q, using the fan port covering technique, before S1 (here). My recollection was that, in this previous case, the coupling was through power supply ripple so Richard, Filberto and I tried powering the fans of the aLIGO chassis with a separate supply. Coherence was no longer evident. While a separate supply would mitigate this coupling, I do like Rich’s idea to eliminate all fans with vacuum suction cooling.

Robert S., Rich A., Richard M., Filberto C.

Figure: When there is no port covering, the right hand fan produces the blue peaks at about 570 Hz, the left hand fan produces the blue peaks at about 562 Hz. I adjusted the port covering to put both peaks at about 547 Hz (red). I went back and forth between the two states several times (the multiple red and blue traces).

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