Parallel an d Serial Kine m atics / Metrology
Direct and Indirect Metrology Non-contact sensors are usedto obtain the most accurate position values possible for position servo-control systems. Two-plate capacitive sensorsinstalled directly on the movingplatform and measuring on the axis of motion offer the best performance. Resolution and repeatability can attain 0.1 nanometer in such systems. Indirect metrology—measuring strain at some point in the drivetrain—cannot be used in sys-tems with the highest accuracy requirements. Fig. 50 a. Working principle of a stacked XY piezo stage (serial kinematics). Advantages: Modular, simple design. Disadvantages compared with parallelkinematics: More inertia, higher center of gravity, moving cables (can causefriction and hysteresis). Integrated parallel metrology and active trajectory control (automatic off-axis error correction) are not possible. Fig. 50 b. Working principle of a nested XY piezo stage (serial kinematics).Lower center of gravity and somewhat better dynamics compared withstacked system, but retains all the other disadvantages of a stacked system, including asymmetric dynamic behavior of X and Y axes. Fig. 50 c. Working principle of a monolithic XY Z parallel kinematics piezo stage. Allactuators act directly on the central platform. Integrated parallel metrology keeps allmotion in all controlled degrees of freedom inside the servo-loop. The position of the central, moving platform is measured directly with capacitive sensors, permitting all deviations from the prescribed trajectory to be corrected in real-time. This feature, called active trajectory control, is not possible with serial metrology. © PI 1998-2005. Subject to change w/o notice. Cat 118 05/09.17 4-44