[Mark, Arnaud, Hugo, Cheryl, Kiwamu]

Here is our understanding of what happened on the MC1 suspension in this past Friday (alog 6591) although it is still unclear why there was a big offset in the T1 coil.

(1) Suddenly a big offset (-10 k counts ) showed up in the T1 coil (VOLTMON) which wasn't requested from the MC1 suspension model (DAC_OUTPUT_0_0).

(2) The M1 stage was pushed by ~ 10 k counts in OSEMINF_T1_INMON due to the offset. This was visible in the IMC-REFL_DC camera when I was in the control room.

(3) The DC change in the M1 stage then triggered the IOP DAC _KILL because the DC value hit the threshold of 10,000 counts.

(4) Tripping the DAC KILL somehow disabled the unknown offset too and hence it released the M1 stage with a big kick. Consequently an undamped oscillation was observed.

After this every time I untripped the watchdogs, the unknown offset persisted and kept tripping it.