In NC machine system, the instructions are picked from the punched card or tape by the reading Unit. Processors send electric pulses which actuate the motors and slides to govern the movement of the spindle or tables. A control system is necessary to check the actual output movement with the desired value as given in the input. This system is known as feedback system.
Open loop System
When a NC system does not have any feedback arrangement, it is known as open loop system. Actual output movement is not directly measured and checked with the desired movement.
Closed loop system
In this system, the final movements (spindle speed and slide displacement) are compared and balanced with the values given through the input commands. The movements of spindle and slides are measured by feedback devices like velocity transducers and position transducers and sent back to control unit for comparing. If the feedback values does not match with the input values, the difference is corrected. This system is adopted where highest positional accuracy is required.