## 2 System Configuration  
The magnetic levitation wafer transfer system controls the 5-DOF motion, excluding propulsion, and the linear synchronous motor (LSM) handles the linear drive. Figure 1 illustrates the concept of the wafer transfer platform, multi-joint robot arm, and magnetic levitation transfer system. The robot arm is installed on the top, and the magnetic levitation units and propulsion system are positioned at the bottom relative to the aluminum base. On the left side of the circled diagram, the current controller, the posture controller, the isolation board, and the oscilloscope for real-time magnetic levitation control and measurement are located. To mitigate the influence of wire tension from electromagnets (EMs) and gap sensors, the wires were configured with extended lengths to ensure minimal force transmission. Consequently, the impact of wire tension can be considered negligible. In the forthcoming system operation, electric power will be provided through the wireless power supply method, allowing the effects resulting from wiring to be disregarded. To control magnetic levitation, magnetic levitation modules are placed at the four corners of the aluminum base. As shown in Figure 2a, four magnetic levitation modules generate four attractive forces (\(f_{1}\sim f_{4}\)) for vertical control and four attractive forces (\(f_{5}\sim f_{8}\)) for horizontal control. Figure 2b illustrates four gap sensors (\(c_{1}\sim c_{4}\)) for measuring the \(z\)-axis airgaps and four gap sensors (\(c_{5}\sim c_{8}\)) for the \(x\)-axis airgaps measurement. Based on the structural analysis of the base, the center of gravity (CG) of the mover can be calculated, and the 5-DOF posture can be calculated. The posture controller generates a 5-DOF force reference (\(F_{x}\), \(F_{z}\), \(M_{x}\), \(M_{y}\), \(M_{z}\)) based on the 5-DOF posture information. The generated 5-DOF force is converted into eight force (\(f_{1}\sim f_{8}\)) reference inputs and applied to each electromagnet module. The proposed magnetic levitation module, which includes \(x\)- and \(z\)-axis EMs and gap sensors, and a guide rail, is shown in Figure 2(a). To implement the \(x\)- and \(z\)-axis EMs in a single magnetic levitation module, each flux path is orthogonalized to eliminate the effects of magnetic interference. The \(x\)-axis EM generates a closed flux loop in the \(XY\) plane, and the \(z\)-axis EM produces a closed flux loop in the \(XZ\) plane. The \(z\)-axis EM was designed in a dual-coil configuration, and the \(x\)-axis EM was divided into three coils to enable a simultaneous reduction in backcore thickness, core volume, and weight. The maximum force required by the EM was determined through dynamic analysis by simulation. The experimental results demonstrated that the power consumption of the EM remained within 150 W during stationary levitation. Additionally, even after extended periods of operation, the temperature was maintained below 40 \({}^{\circ}\)C. The magnetic flux density distribution in the landing state, which is the condition for applying the maximum current of the \(z\)-axis EM, is shown in Figure 2(b). In this case, the magnetic flux generated by the \(z\)-axis current constitutes a closed loop in the \(ZX\) plane, and it can be stably driven without magnetic saturation within the maximum magnetic flux density of 1.5 T. The total mass, moment of inertia, and distances from the CG to the EM modules are presented in Table 1.  
To control the magnetic levitation, both a higher-level controller for posture control and a lower-level controller for current control of the EMs are necessary. Figure 4 illustrates the arrangement of power and signal lines for connecting the controllers and the test plant. The main power line of 180\(V_{dc}\) is supplied to the current controller, and the control power of  
Figure 1: Prototype of a magnetic levitation wafer transfer system. Figure 2: Magnetic levitation forces and airgap measurements: (**a**) Magnetic levitation forces; (**b**) Airgap measurements.