Backlash Detection in CNC Machines Based on Experimental Vibration Analysis

Exploiting backlash free mechanisms and gearboxes, the stiffness of servo axes in CNC machines can be improved. In fact, the rigidity of these axes will be increased, so that different types of feedforward control can be implemented. Furthermore, backlash free mechanisms facilitate designing foolproof systems without challenging with the instability and nonlinear behavior of backlash. However, any mechanical failure and looseness causes backlash which in turn may lead to severe vibrations. So, backlash detection and exploiting efficient preventive maintenance (PM) to prevent interrupt in the production line is of interest. In this paper, first a simple model for mechanical system of a CNC servo axis, and its control system will be presented. This reduced order system for speed control will be detailed. Next, the bandwidth of vibration frequencies due to backlash is estimated and behavior of a servo axis with various backlashes is simulated. Then, to encapsulate the role of backlash in different conditions, performance response of five mechanically different axes with different servo gains in various CNC machines are empirically investigated. These experimental frequencies of vibration obtained in completely different CNC machines (small, medium and heavy size) are compared with those estimated. Simulation and experimental results show that the frequency of vibration in a servo axis with backlash is not affected by the value of backlash, while the position control gain dictates this frequency. Finally, an experimental equation will be developed that estimates this frequency for various CNC machines.