Improving Contouring Accuracy of NC/CNC Systems With Additional Velocity Feed Forward Loop

One of the factors which affects the contouring accuracy of multi-axis NC/CNC contouring systems is axes mismatch which means that the time constants and the open-loop gains of the machine servo drives are not identical. In conventional NC/CNC system each axis is separately controlled by a closed-loop velocity controller. Therefore, a position-following error which is a function of the open-loop gain and the time constant, is generated. Since the values of these parameters are different for each axis, a contouring error is produced. By adding a velocity feed forward loop to this controller, the position-following error can be reduced, or even eliminated, and consequently the contouring error is reduced.