Hybrid modelling technique applied to digital feed drives

The hybrid modelling technique considering distributed load, explicit damping factors and measured non-linear effects has been shown to effectively represent the dynamics of CNC machine tool feed drives. The resonant frequencies produced by the mechanical elements of the machine were simulated for an analogue feed drive with DC motors and belt-pulley transmission system. The rapid increase in the use of digital control systems and permanent magnet synchronous AC motors for modern feed drives requires an update on this modelling technique. This paper presents an investigation into the application of the technique for the digital feed drive of a vertical machining centre equipped with a Siemens Sinumerik 840D controller. The drive utilises a direct transmission system and the field oriented control technique in the current control loop. The discrete method of approach to the current control loop is used to represent the digital states of the current, torque and speed control. The model is developed in MATLAB/SIMULINK employing the Power System (Simpower) Blockset. Model validation in the time and frequency domain is performed using the deterministic and pseudo random binary signals from the controller's built-in routines. The results are shown to provide a sufficient solution into the representation of the feed drive dynamic behaviour. The investigation is part of a wider study to develop an active vibration system for CNC machine tools and to develop a machine tool drive model, useful for the design, diagnosis, optimisation and condition monitoring purposes.