Design of a Drilling Torque Controller for a Machining Center

In the drilling process, cutting torque control through feedrate manipulation has significant benefits in prevention of tool breakage and reduction of machine tool vibration and tool wear. In this paper, a PID torque controller was designed for real time drilling torque control in a machining centre. The drilling torque was estimated from the spindle motor current and regulated by feedrate control. The plant including the feed drive system, cutting process, and spindle system was modelled for controller design. For a certain cutting condition, the Ziegler-Nichols method was used to determine the proportional gain and derivative and integral control action times of the controller. Also, the root locus plot was used to tune the proportional gain of the controller. A simple method was also suggested to obtain the tuned controller gain instead of the Ziegler-Nichols method and the root locus plot for an arbitrary cutting condition. Experimental works have been performed to verify the performance of the proposed controller. It is shown that the gain tuning is essential for enhancement of the controller performance and the designed controller can regulate the drilling torque well at a given reference level.Nomenclature G_fc(s) transfer function of combined system of feed drive and cutting process - G_s(s) transfer function of spindle system - G_p(s) transfer function of plant - G_CL(s) transfer function of closed loop system - G_con(s) transfer function of controller - _fc1, _fc2 time constants of combined system of feed drive and cutting process - _s time constant of spindle system - K controller gain - K_c cutting process gain - K_s spindle motor current gain - K_plant plant gain - K_cr critical gain - K_p proportional gain of controller - K_{p_tune} tuned proportional gain of controller - I_s spindle motor current - I_u,v,w u, v, and w phase current of spindle motor - I_idle idling current of spindle motor - damping ratio - T oscillation period of system response - T_c cutting torque - T_m motor torque - T_idle idling torque of spindle motor - T_i integral control action time - T_d derivative control action time