# Design of a Compact Planar Magnetic Levitation System with Wrench-Current Decoupling Enhancement
## 6 Discussion
### Levitation Experiment Results  
The experiment results demonstrated both precision motion and long-range motion. The 2-D linear interpolation could enhance the wrench-current transformation. The effectiveness of the interpolation was tested by moving the mover to the location with no data available in the lookup table. For high-precision motion, the mover was controlled to move with steps of 10 and 20 \(\upmu\)m for translation and 0.5 and 1 mrad for roll and pitch. The precision of the system was limited by the motion sensor.  
To prove the concept of long-range motion, the mover was moved within the horizontal area of 20 mm \(\times\) 20 mm with a vertical motion range from 18 mm to 26 mm. The rotation range was \(\pm\)10 degrees for both roll and pitch. A large overshoot could be observed in large-step motion. Even though the multidimensional linear interpolation could improve the levitation as discussed, the real-time solution to decouple the wrench command was still important.  
From the experiment results, there were two main problems that significantly deteriorated the levitation performance: undesired yaw motion and system lag. In the experiments, the untethered mover was suspended in the air with five controllable degrees of freedom. Imperfect control efforts could result in an undesired yaw motion, especially, during the large motion. Unwanted rotation could cause oscillations in other axes.  
Figure 18: The disc magnet mover levitated at 26 mm and rotated with pitch angle \(\theta\) of 10 degrees.  
Next, the authors believe that the reason for the large spikes in both precision and long-range motion might be the limitation of the old controller hardware. Prior to the experiment, large spikes occurred frequently when too many features were implemented. Therefore, the linear interpolation was limited to 2-D with a ramp command feature to reduce the computational load in this work.  
By comparing with other previous works, the proposed planar maglev prototype had several advantages and room for improvement. First, for the range of motion, the system could move with a long range, especially in the vertical direction with a 26 mm levitation gap, compared to other works with the disc magnet [22] and Halbach array [21]. A solution for unlimited translation is needed since the horizontal range was relatively small and yaw motion was not controlled. Next, the fine-step motion of 10 and 20 \(\upmu\)m demonstrated an improvement of this system compared to other works with disc magnets [23; 30]. The precision motion quality can still be improved for better performance such as that of the Halbach array [17].  
From the discussions above, there are plenty of research opportunities for the design and levitation performance of the proposed planar maglev system. For educational purposes, a cost-effective planar maglev system using inexpensive sensors and a controller can be built as found in [33]. For precision motion, with the most recent review in [34], the challenges lie in solutions for effective fine-level sensing, long-range extension, and extreme performance such as high-speed motion. In high-speed motion or long-time use, the coils must consume a lot of currents and heat up quickly. The generated heat will affect levitation performance. The analysis of the thermal model [35] and the design of the cooling system are important. The eddy current damping, produced by array cover materials such as aluminum, could improve the stability even though it affects the speed. However, the selected material must be optimized to avoid an excessive damping force on the mover [36]. Lastly, advanced control can be applied for complicated tasks, for example, iterative control for higher precision in repetitive motion [19; 37], or a study on transient responses with external disturbances and the design of a robust controller to improve levitation performance in real-world applications.