The electronic feedback circuit is then set to try and hold the average postion of the laser beam at the center of the photodetector.
But then (as in the static mode) when the cantilever moves up a bump on the sample surface, the laser will move even farther back on the photodetector. The feedback circuit then kicks in turning up the top electromagnet. This rotates the whole tripod assembly restoring the laser's average postion to the center of the photodetector.
The net result is that, as the cantilever moves across the sample, it will only tap the sample at the very bottom of its swings. For this reason this is sometimes called the "tapping mode." Tapping has the huge advantage of allowing the tip to climb up abrupt slopes without breaking off.
But "tapping mode" can still damage samples (and cantilever points) and it turns out that there is an even better way of operating the AFM:
