From the brute force search for the 3.4 Hz peak it turned out that the peak was dominating PRCL and visible in the PRM suspension signals.

I compared the M1/M3 offloading of PRM adn SRM, that should be nominally the same. I discovered that PRM M1 path has a gain of -0.02, the double of the gain in SRM M1 (-0.01). The offloading filters and the M1 damping filter are the same.

So tried reducing the M1 gain from -0.02 to -0.01. This had a clear and repetible effect on the 3.4 Hz peak. The lower offload gain reduced the peak a lot in PRCL, PRM and in many ASC signals (CHARD_P, CHARD_Y, CSOFT_Y and DSOFT_Y).

The first plot shows the PRCL_IN1 RMS reducing when changing the PRM M1 gain: there is an evident reduction since ther 3.4 Hz peak is dominating the PRCL motion in normal conditions.

The second plot shows how much the peak is reduced by changing the M1 gain: red and green are with high gain (-0.02) and blue with reduced gain (-0.01).

We shoudl measure and retune the PRM M1 and M3 crossover: there is room for further improvement.