The movement of the STM tip is synchronized with the position of pixels drawn on the computer screen. The actual piezo movement is too small to see, but we′ll continue to indicate it by glow.
The X piezo (at the right) cycles on and off to move the tip side-to-side. This creates an X scan line across the sample and the screen.
The Y piezo (below) steps steps up its voltage after each X-scan line is completed. This starts the tip (and screen pixels) on a new scan line.
The feedback loop moves the Z piezo to hold the electron tunneling current constant. Its movement is very subtle (tenths of a nanometer) but changes in this piezo voltage are used to determine the color of each screen pixel.