Effects of transmission stiffness variations on the dynamic accuracy consistency of CNC feed drive systems

The fluctuation of CNC machine's dynamic accuracy, due to different configurations in whole work space, have great effects on the improvements of working accuracy and effectiveness. The concept of dynamic accuracy consistency is defined, and a method using sensitivity of close loop transfer function to study it is presented. A two-mass mechanical model is constructed and an asymptote approximation approach is presented to avoid analytical difficulties. The obtained sensitivity functions show significant differences of semi close loop and close loop control. On this basis, spectrum features of commands relate to typical dynamic errors are studied and used to find the effects of transmission stiffness variations on the change of tracking error, residue vibration overshoot, and circular radius error. It's concluded that the main factor in the effects is ratio of command frequency spectrum width to close loop band width, i.e. there's significant change only when the two are close. Other main factors are control structure (semi close loop or close loop), and the ratio of close loop band width to mechanical resonance frequency. A series of simulation experiments verified the achieved conclusion.