a close-loop NC systems. illustrated in figure 6.27,uses servomotors and feedback measurements to ensure that the workable is moved to the desired position.A common feedback sensor used for NC(and also for industrial robots) is the optical encoder, shown in figure 6.28. An optical encoder consists of a light source and a photo detector on either side of a disk. The disk contains slots uniformly spaced around the outside of its face. These slots allow the light source to shine through and energize the photo detector. The disk is connected,either directly or through a gear box,to a rotating shaft whose angular position and velocity are to be measured. As the shaft rotates,the slots cause the light source to be seen by the photocell as a series of flashes. The flashes are converted in to an equal number of electrical pulses. By counting the pulses and computing the frequency of the pulse train,worktable position and velocity can be determined.