Experimental UFM implementation

A practical procedure for tuning the frequency starts with acquisition of a spectrum of the non-linear response. With amplitude modulation applied, the ultrasonic vibration frequency is swept over a wide interval centred at the nominal resonance frequency of the piezoelectric transducer. From this spectrum, one finds the frequencies at which a non-linear response can 

The amplitude modulation frequency should generally be in the range 0.5-3 kHz. Below 0.5 kHz the feedback usually modifies the ultrasonically induced normal deflection. Above 3 kHz the characteristic time constants and delays of the cantilever response can become comparable with the amplitude modulation period. 

The maximum ultrasonic amplitude must be chosen so that the average threshold amplitude occurs at roughly three-quarters of the reference period. 

The lateral deflection signal should also be checked. If the lateral response is high or unstable, it is quite likely that the normal deflection is influenced, even though it might look stable. 

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