In many separation processes, filtration performance degrades over time due to retained particles blocking the flow through the filter membrane. A novel bearingless spinfilter extends the long-term performance by self-cleaning effects. The filter rotor is magnetically levitated and actuated by two self-bearing motors inside a hermetically sealed housing, which eliminates the need for bearings and rotary sealings, that both lead to process fluid contamination. Both bearingless motors have integrated electronics and independently control the levitation of the spinfilter rotor. A first prototype is designed and the concept is validated by the separation of a yeast cell culture. Special focus is placed on the internal rotary seal between the feed and filtrate regions, that is inevitably created when the filter membrane is in motion. Any leakage flow through the seal leads to filtrate impurities, which is minimized in this article with an embedded impeller as a pressure compensation method. A constant filtrate flux of $1750 \,\mathrm{L}\mathrm{h}^{-1}\text{m}^{-2}$ and a filtrate purity of $75 \%$ was achieved in a first series of tests.

This work introduces a novel sensing concept based on reaction forces for determining the position of the levitating magnet (mover) for magnetic levitation platforms (MLPs). Besides being effective in conventional magnetic bearings, the applied approach enables operation in systems where the mover is completely isolated from the actuating electromagnets (EMs) of the stator (e.g., located inside a sealed process chamber) while levitating at an extreme levitation height. To achieve active position control of the levitating mover by properly controlling the stator’s EM currents, it is necessary to 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, given that the position is not directly tied to the measured forces. Furthermore, two possible controller implementations are discussed in detail: 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 employing a 207 mm outer-diameter (characteristic dimension, CD) stator with permanent magnets, a set of electromagnets, and a commercial multi-axis force sensor is built, where a 0.36 kg mover is stably levitated at an extreme air gap of 104 mm.

This work provides an analytical method, based on the Amprian model of permanent magnets (PM), for a fast calculation of the magnetic flux density in three-dimensional space applying Biot-Savart law, and the calculation of the forces using Lorentzs law. The applied approach enables the characterization regarding forces, torques, and stiffnesses of the levitating PM for any arbitrary position in space. Furthermore, it permits the extension of the investigation to any shape and configuration of ironless magnetic bearings (MB). In order to demonstrate the simple use of the analytical model, in this paper, the dimensions of an ironless axial MB employing PMs are optimized with a multi-objective Pareto analysis which reveals the physical limits concerning maximum achievable levitation height with respect to given constraints on, e.g., the required force and tilting stiffnesses, and the MB robustness defined by the maximum allowable payload on the levitating magnet. Moreover, the optimized axial MB and a corresponding test bench are realized to validate the proposed model with experimental results. For sake of completeness, it should be mentioned that in a later stage the optimized MB can also be scaled with simple scaling laws if the demanded specifications, e.g., concerning desired maximum levitation height or payload capability would have changed.