Low Velocity Friction Compensation for Machine Tool Feed Drives

One of the most prominent machining errors in end-milling is the quadrant glitch. In cutting a circular profile, quadrant glitches appear at ninety degree intervals around the circle when the X-Y bed of the machining center makes a zero velocity crossing, i.e. when an axis reverses direction. These errors are attributable to static friction (stiction), a nonlinear disturbance. To find the control input which eliminates stiction-induced errors, repetitive control, a subclass of learning control, is employed. Experiments are conducted on the X-Y bed of a CNC milling machine. The repetitive controller is implemented digitally in the position servo loop. The repetitive controller undergoes successive passes in tracking a profile containing a zero velocity crossing. Experimental data indicates that near-perfect tracking of the trajectory can be achieved in twelve cycles or less. The nonlinear differential inputs generated by the repetitive controller to overcome the stiction are used as feedforward inputs in two proposed controller architectures. Experimental end-milling of circular pieces have verified that the architectures are capable of removing the quadrant glitches, as measured by a coordinate measuring machine.