Frequency modulation atomic force microscopy, introduced by Albrecht, Grütter, Horne and Rugar in 1991,[REF] is a mode of nc-AFM where the change in resonant frequency of the sensor is tracked directly, by always exciting the sensor on resonance.
Amplitude modulation can fail if the non-conservative forces (damping) change during the experiment, as this changes the amplitude of the resonance peak itself, which will be interpreted as a change in resonant frequency.