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Dynamic Design for Improved Control |
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Introduction
In the design of Mechatronic Systems for Precision Positioning the design team must consider many different aspects. Multi-disciplinary trade-off is often required and each discipline can influence the system performance. In the conceptual stage of development the following four steps can be identified,
• Assessment of the customer requirements to translate them to accuracy targets
• Identification of disturbances and quantitative estimation of their magnitude and frequency content
• Determination of the required feedback controller power to handle disturbances and achieve the required accuracy
• Design the system elements, using the multi-disciplinary knowledge, such that factors limiting the controller performance are avoided
Limitations may be present in all disciplines. Here the attention will be put on designing the dynamics of the mechanical modules so as to allow for higher performance of the feedback system.
Description of the system
As an example of a mechanical module a wafer stage for a semiconductor lithography machine will be used. An early example of such a module, ( ASML, 1985 ), is shown in figure 1.a and a schematic of the system in figure 1.b. The wafer is supported on a mirror block that is used for interferometric measurement for the XY motion. For vertical motion an actuator assembly supports this mirror. For motion in the X-direction a linear motor is used and the total system is supported by an air-bearing foot. The air-bearing floats over a granite base and allows for XY horizontal motion.
This system must position the wafer over a stroke of about 250 mm with position stability better than 50 nm. Disturbances from floor vibrations and reaction forces in the frame call for a higher feedback control performance, requiring a higher crossover frequency of the Open Loop frequency transfer. In the original design the dynamics of the stage on the air-bearings and the internal dynamics are seriously limiting the performance. The transfer function from driving force to measured displacement is shown in figure 2.a.
In this example the dynamics can be modeled using a 6 Degree of Freedom model. Using that model adaptations of the dynamics can be analyzed. A general rule is to drive linear slides in line with the Centre of Gravity. Applying this rule did not give the desired improvement. Based on understanding also the internal dynamics the design can be altered to arrive at an improved behavior as demonstrated in figure 2.b.
The methodology applied here has proven to be applicable in many cases and has formed the basis for a large number of successful designs.
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