International Chemnitz Manufacturing Colloquium (ICMC 2010) September 29th-30th, Chemnitz
When high positioning and contouring accuracy is demanded in parallel kinematic machining the application of closed loop position controllers is as advisable as for common machine tools. However the tracking errors which are inversely proportional to the controller gains are limited by the stability criterion due to mechanical and controller properties. Within a constant motion the velocity gain is represented by the constant ratio of actual velocity and actual tracking error. In case of machining with perpendicular arranged machine axes the machine accuracy is found to be unaffected by the tracking errors when the velocity gains of all axes are set to the same value. Parallel kinematic machines feature a different behavior. Although the basic correlation of velocity gain and tracking error is still valid in machine coordinates - where they originate - the skew machine axes transform the actual tracking error and the actual velocity differently to the machining space - where they act as geometric deviations. Two major features are observed even with constant velocity gains in the machine drive controllers, an inhomogeneity and a gain flip effect that depends on the motion direction. Both effects are studied with simulations and experimental setups.