There are two types of positioning systems used in NC systems: (a) open loop and(b) closed loop, as shown in Figure 6.27. An open-loop system operates without verifying that the actual position achieved in the move is the same as the desired position. A closed-loop control system uses feedback measurements to confirm that the final position of the worktable is the location specified in the program. Open-loop systems cost less than closed-loop systems and are appropriate when the force resisting the actuating motion is minimal. Closed-loop systems are normally specified for machines that perform continuous path operations such as milling or turning, in which there are significant forces resisting the forward motion of the cutting tool.