The photodetector (and other electronics) can create an electrical signal which is sent to the ring electrode surrounding the base of the piezoelectric tube, causing it to shrink upward.
If the laser moves to the upper quadrants of the photodetector, the voltage to the piezoelectric will steadily increase.
If the laser is centered on the detector, the voltage remains constant.
If the laser strikes the lower quadrants of the detector, the signal steadily decreases.
