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How does the AFM work?

AFM Feedback

This video was captured real time from the computer screen, while the AFM was scanning the surface of a butterfly wing (Morpho). In this sequence the effect of setting the feedback speed (integral gain) is visble. Starting with a "normal" value, the microscope is operated in constant deflection mode, i.e. the feedback loop tries to adjust the z-height such, that the deflection stays constant (00:00 .. 00:05). Nevertheless a slight error in the deflection signal can be picked up, due to limited response time in the feedback loop. Increasing the gain (00:5..00:07) does not improve the situation, since the system starts to oscillate. Reducing the gain (at 00:11 and 00:15) and finally setting it to zero (00:21) sets the instrument in constant height mode, shifting the information from the height channel to the deflection channel.