Servomechanisms

are devices that can position themselves precisely based on a continuing feedback loop. They receive an instruction to move, and then move in that direction until they realize they've reached it or gone past it, in which case they correct themselves. It's extremely simple, but it's the basic principle on which most mechanical automation has been founded. Control by the numbers was impossible until these systems were developed, and an entire industry developed to support them. The military was an early aficionado.