Actuation principles of permanent magnet synchronous planar motors:a literature survey

At Philips Applied Technologies, research is being done on the control and design of highperformance electromechanical systems. The actuators in a wafer or reticle stage used in the lithography industry are a good example of such a system. Due to increasing requirements related to the performance and accuracy of the wafer scanners used for producing IC’s, other types of electromechanical actuators have been proposed. To actuate the wafer stages in a plane, the conventional actuator is composed of linear motors placed in a H-bridge configuration. The EUV project demands the actuator to operate in a complete vacuum and therefore a so-called synchronous permanent magnet planar motor is proposed to replace the existing actuator configuration in the wafer stage. This actuator levitates and propels itself above a permanent magnet array. Because the actuator is magnetically levitated, there is no need for air or other types of bearings and the actuator can operate in vacuum. Although the servo performance of the planar motor is satisfactory in the view of the present EUV applications, the true potential of the planar motor is not used yet. This full potential will be needed in future EUV applications. The servo errors are presently dominated by components that can be attributed to various disturbances related to the particular actuation and commutation principle. Accompanying electronics can introduce disturbances in the actuator. To counteract these phenomena, a physical model of the most relevant sources for the disturbances in the servo error must be derived. The objective of this literature study is to gain insight in all physical phenomena and mechanisms playing a role in electromechanical actuators. Then, with the use of this theory the synchronous planar motor can be analysed and sources for the disturbances can be identified and resolved. This report is divided into two parts. In the first part, the electromagnetic field is analysed and the different components in electromechanical actuators are investigated. These include the electric coils and permanent magnets. The interaction between the two magnetic fields leads to the generation of forces that can be used for actuation. Modelling techniques to model the electromagnetic field and the generated forces are also discussed in this part. The second part is merely devoted to the actuation principles of the synchronous planar motor itself. The motor principle and the associated commutation principles are explained. The planar motor is based on the three-phase/four-pole actuation principle. With the appropriate distribution of the magnetic field and coil currents, the actuator forces to drive and lift the actuator can be decoupled. In this way, both components can be controlled individually. Possible sources for actuator-related disturbances are commutation errors, amplifier inaccuracies, unknown machine parameters, and the generation of additional forces and torques purely due to the actuator design and end effects of coils and permanent magnets.

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