Slide 7 of 16
Notes:
1) Vertical gradients in magnetic force cause a shift in the cantilever’s resonany frequency
2) frequency modulation: the feedback loop modulates the drive frequency to keep the cantilever’s phase lag at 90 degrees relative to the drive, corresponding to resonance.
3) ampltitude detection problems: ambient laser light (around or through the cantilever then reflect off sample) interferes with laser light reflecting from the cantilever.
4) setpoint: desired voltage for the feedback loop. Compare to the RMS amplitude voltage to calculate the change needed in the piezo position. This changes to keep the amplitude voltage close to the setpoint voltage so the vibration amplitude remains constant.
5) set the drive frequency so it corresponds to a 10% decrease in the vibration amplitude.
6) Height setpoint value = low force.
7) You can get quantitativer information on force gradients from the frequency modulation data
