According to Pierre, the GAIN PIEZO in ACTUATOR module controls also the saturation of PIEZO ELEC signal and should not be small value. Pierre suggested to set this gain as 7 and adjust the servo gain with INPUT ATTENUATOR in SIGNAL module. I set the GAIN PIEZO as 7 and the INPUT ATTENUATOR as 0.2 for 1/f^4 filter. UGF is 13 kHz and phase margin is 60 deg (Fig. 1).

Fig. 2 shows the correction signal (CH1) and error signal (CH2). The correction signal does not saturate even at 2.36V (4.72V for PIEZO ELEC) and FC gets more stable. However, FC still often unlocks even without saturation.