Influences of Feedback and Servo Drive Systems on the Performance of Ultra Precision Machine Axes

The mechanical design of ultra-precision machine tools and respectively the mechanical behavior of precision axes are very well understood today. Detailed investigations on precision axes behavior, dimensioning of bearings and drives and overall machine concepts have built a broad basis for designing very stiff and accurate state of the art machine tools. Enhancements to further increase the achievable form accuracy and surface quality of a machined optical part and at the same time decrease cycle times and error sensitivity can only be accomplished by innovative control and drive strategies. In contrast to mechanical aspects, feedback and servo drive systems and their interactional behavior within a complex machine setup have not been sufficiently analyzed yet. This applies especially to ultra-precision machining. However the mechanical performance of an ultra-precision axis strongly depends on the control loop parameters and components. At Fraunhofer IPT a test bench has been developed to analyze CNC machine controls, servo drives and encoder systems with regard to an evaluation of their application in ultra-precision applications for high accuracy motion control. Investigating on all components applied in closed loop controls, their individual performance and simultaneously mutual disturbances and limitations within a complex system setup can be identified. Focusing on hardware structures, software modules and data processing structures, an overall statement concerning all aspects of modern closed loop control systems is targeted. The test bench has been configured as an ultra-precision lathe to later validate the measured results by diamond turning an optical part. The setup uses two air bearing ironless linear drives and an air bearing spindle. A modular mounting grid allows for the flexible integration of external metrology such as laser interferometers, laser vibrometers as well as capacitive or acceleration sensors. Thus, a complete investigation on all aspects of precision motion control can be guaranteed. This paper will present the results of the analysis of feedback and servo drive systems and will give a close inside on the mutual influences of linear scales and analog and digital servo drives for applications in ultra-precision machining. The performed measurements include the analysis of the position accuracy and repeatability (step response test) as well as the determination of the dynamic frequency characteristics (stiffness/ compliance) of an air bearing axis. With respect to the aforementioned measurements the tests have been performed under the variation of linear scales (vendor, pitch, signal, sampling frequency, etc.) and servo drives (vendor, switching or linear amplifiers, PWM frequency, control architecture, DC bus voltage, etc.). The paper will give a summary on all results of the analyzed topics, carefully chosen regarding their relevance to ultra-precision machining. The results will include both, a comparison of individual components and an analysis of mutual influences within the whole control loop. Further, additional measurements of electrical characteristics such as stability of DC bus voltage, motor current ripple and noise, etc. will be presented. Finally, a short outlook to future research work concerning the analysis of CNC controls will be given.