We commissioned some soft dithering loops for locking their DC positions to fixed spots on the ETMs. This is similar to what is used at LLO.
Specifically, after we increased power to 20 W, we used PIT4/PIT5 and YAW4/YAW5 to dither ETMX/Y PIT and YAW, respectively. We then demodulated the A2L response in DARM control and servoed the signal back to X/Y SOFT to set their DC locking point.
To make the control unconditionally stable, we turned off the FM3 (integrator) and FM4 (z=0.01Hz, p=0.1Hz) filters in the C/DSOFT control filter bank so that those filters approached DC as a constant (flat for < 0.1 Hz) . Meanwhile, the dithering loops are just simple integrators with 1/f shape in the freq band of interest. (Also there is another integrator in the top mass suspension). Such an 1/f - constant crossover guaranteed loop stabilities for small dithering loop gain. The crossover frequency is around a few mHz for now, which seemed fine.
By engaging the dithering loop the A2L noise is also improved significantly.
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More details:
The dithering frequencies are
| PIT4 | PIT5 | YAW4 | YAW5 | |
| freq (Hz) | 20.131 | 20.789 | 22.347 | 21.9 |
| amp (Ct) | 300 | 300 | 300 | 300 |
The control filter setups are attached to this log. Note that we changed the BP filters in DEMOD_SIG to FM6 (though many of them had the same names as the original BP filters), which was a 4th order cheby1 filters with width of 0.06 Hz.
We also created a guardian state SOFT_DITHER for engaging the dithering loops. However, we have not yet connected this node to the main guardian and will test it tomorrow.
The SOFT_DITHER guardian state has been tested out. For now it happens after CHARD_BLEND and before LOWNOISE_ASC. The crossover from dither to QPD happens at around 10 mHz.
We also moved the zeroing of CLK gain and ADS ctrl history to the Down state instead of prep for locking state, so that they should be turned off more timely.