Nanometer accuracy: development of high-precision motion systems

April 29, 2020

Electrical engineer Ioannis Proimadis defends his PhD (digitally) on April 30th. 

Computers, mobile phones and washing machines; they all contain integrated circuits. To produce them, complex machines are used in which planar motors play a crucial role. Electrical engineer Ioannis Proimadis improved the accuracy significantly of a specific type of those high-precision motion systems, where the moving parts are free floating bodies with no physical connection to the environment. He defends his PhD (digitally) on April 30th. 

Planar motors are mechanical devices whose moving part can move and rotate in the three-dimensional space. A state-of-the-art application of such motors are wafer scanners; complex machines used in the semiconductor industry to produce integrated circuits by lithography. The expectations for the modern wafer stages include both nanometer accuracy and high-speed motion profiles.

To achieve these goals, modern wafer stages rely on the use of magnetically-levitated planar motors. Magnets are fixed, while the coils are attached to the mover. The coils are powered via cables, which have to move together with the mover.

Freely floating body

Proimadis studied a specific type of motor, a moving-magnet planar motor. Such a motor prototype has been developed at TU/e. In this magnetically levitated motor, the mover is a freely floating body with no physical connection to the environment, thereby suppressing the effect of environmental disturbances on the mover.

However, several challenging dynamic phenomena are present in this motor, such as the intrinsically position-dependent electromagnetic interaction. Additionally, the magnetic field creates forces on the mover, which are not evenly distributed along its surface. As a result, mechanical deformation can be exerted on the mover, severely hindering the attained positioning accuracy.

High-fidelity model

As a first step towards coping with these effects, Proimadis developed a high-fidelity model to describe the fundamental electromagnetic and mechanical principles of the planar motor, by employing the so-called Linear Parameter Varying (LPV) modeling framework. As a result, the developed model can be utilized to investigate the properties of the motor dynamics and moreover to design model-based controllers.

Proimadis utilized a data-driven modeling approach to experimentally verify the dynamic behavior of the planar motor. Moreover, he developed a model update method, which adjusts the dynamic characteristics of the analytic LPV model, by making use of the experimental evidence.

Increased accuracy by 50%

Using the developed model, control techniques that achieve robustness against position-dependent and uncertain dynamics were implemented. Moreover, various disturbances that deteriorate the positioning accuracy have been suppressed via a Machine Learning compensation scheme. Experimental results showed that this scheme increased accuracy by 50%. Finally, by exploiting the existence of multiple actuators, Proimadis proposed and experimentally verified an active control method of the deformations, which properly shapes the force distribution on the moving magnet plate.

Proimadis’ research is an important step towards high-precision moving-magnet planar motors. This project will therefore be continued at the TU/e to further advance the modeling and control techniques for this type of motor.

Title of PhD-thesis: Nanometer-accurate motion control of moving-magnet planar motors. Promotors: Roland Tóth and Hans Butler. Other main parties involved: NWO, ASML, Philips, TNO, Prodrive Technologies, Tecnotion and SKF.