Manuel, Eleonora

We plotted the transfer function of the measured actuator (plant) and fitted it with a Matlab script based on the zpk function.
We used two simple poles at 40kHz, a gain of 0.16 in DC, and a delay of 2.3e-6. see the first plot

We plot the measured open loop TF obtained using a sr560 set with a first order low pass filter at 100Hz, a gain of 200.
We fitted it as the product of the modeled plant TF and a filter TF.
The filter that best fits the data is a first order low pass at 100Hz  with a gain of 240. see the second plot
The UGF is at 3.8kHz with a phase margin of 76deg.

We verify that the loop becomes unstable for a gain of 2000 (as observed experimentally).
Indeed, the UGF becomes 23kHz and the phase margin 7deg. See the third plot.

We will use this model to design a better filter in order to have more gain at low frequencies, compatibly with the possible configurations of the sr560.