Reaction Force-Based Position Sensing for Magnetic Levitation Platform with Exceptionally Large Hovering Distance

Round 1

Reviewer 1 Report

The authors introduce a novel sensing concept based on reaction forces for determining the position of the levitating magnet (mover) for magnetic levitation platforms (MLPs). The applied approach enables the operation in systems where the mover is completely isolated from the actuating electromagnets (EMs) of the stator while levitating at an extreme levitation height. The authors employ a dynamic model of the complete MLP, including the reaction force sensor, and implement an observer that extracts the position from the force-dependent signals. This enables to achieve active position control of the levitating mover by properly controlling the stator EM currents. Furthermore, two possible controller implementations are presented and discussed in the manuscript: a basic PID controller and a more sophisticated state-space controller that can be chosen depending on the characteristics of the MLP and the accuracy of the employed sensing method. To show the effectiveness of the proposed position sensing and control concept, a hardware demonstrator is built.

The results are original and interesting for the development of magnetic levitation platforms with large hovering distance. However, I have minor comments:

1. Fig.1 presents the 3D rendering of the magnetic levitation platform (MLP) considered in this paper, however, it would be good to indicate also “PMs” (permanent magnets) in the figure, not only EMs.

2. The manuscript very clearly describes the derivation of the MLP’s dynamic model and its validation by conducting dynamic measurements, as well as the proposed position sensing method. Moreover, the authors provide a detailed practical investigation of a MLP hardware demonstrator equipped with a (reaction) force sensor for the mover’s position sensing that proves the effectiveness of the presented and discussed model, observer, and controller design. The optimized PMs design is taken from Ref [28]. However, it remains unclear, what properties and parameters are improved in this study in comparison with the literature. It is stated in the Conclusions, that “This work presents a novel method for sensing the position of the mover in a magnetic levitation platform (MLP), where a force sensor is used to detect the reaction forces on the stator” and “With the proposed methods, a stable levitation of the mover is achieved”. Please shortly summarize the results by comparing these achievements with literature.

Author Response

Dear Reviewer, 

Thank you very much for the comments. We are happy to share our answers to your comments.

1. Thank you very much. That is a very good point. We have updated the Fig. 1 by adding PMs.
   
   
2. We thank the reviewer for this comment. To clarify this point, we added a sentence in the conclusions that is highlighted in blue: 'The proposed MLP, along with its position control via stator reaction forces, is, to our knowledge, unprecedented in the existing literature. This novelty precludes direct performance comparisons with documented studies.' 

Sincerely, 

Authors

Reviewer 2 Report

This paper suffers from only one weakness its novelty in relation to the previously published paper by the same Authors referenced as: "28. Bonetti, R.; Bortis, D.; Beglinger, L.; Kolar, J.W. Exploring the Physical Limits of Axial Magnetic Bearings Featuring Extremely 988 Large Vertical Levitation Distances. IEEE Transactions on Industry Applications 2021, 57, 6931–6943." is not explained in the introduction. Please explain what is the added value to the field under study. Moreover, the significance of this research should be underlined.

Author Response

Dear Reviewer, 

Thank you very much for your comment. Here is our answer:

• We thank the reviewer for pointing this out. To clarify this question, we added the following text in the introduction:
  'Contrasting with earlier study [28], which focuses solely on analyzing static forces and stiffness, this paper presents a comprehensive drive system for magnetic levitation platforms (MLPs). This includes integrating a force sensor for generating reaction forces, a dynamic model for controller and observer design, and dynamic measurements of the MLP, thereby affirming its operational effectiveness. This research is noteworthy because it introduces a new type of MLP that can potentially transform future manufacturing systems, as it permits a substantial air gap between the stator and the mover and allows conductive objects like robotic arms to pass through the air gap.'
  
  The added text is highlighted in blue. 

Sincerely, 

Authors

Round 2

Reviewer 2 Report

I have no further comments and suggestions for the Authors.