# Decoupling Control of Multi-DOF Supporting System of MLDSB
## 1 Introduction  
The high-pressure desalination pump is the core power equipment that provides the reverse osmosis pressure and the flow in the reverse osmosis desalination process. As the \"heart\" of the reverse osmosis desalination project, it directly affects the energy consumption and operational reliability of the reverse osmosis desalination process. The reverse osmosis desalination process is shown in Figure 1.  
With the rapid development of the social economy and the continuous increase of population, the lack of freshwater resources has become more and more serious, forcing seawater desalination projects to gradually develop in the direction of high power and large-scale [1]. The bearing system is a key core component to ensure the supporting performance and operational stability of the seawater desalination high-pressure pump, which directly affects the operation status, maintenance period, and service life. At present, the high-pressure pumps of medium and large seawater desalination projects are all multi-stage centrifugal pumps, which are supported by sliding bearings that use seawater as the lubricating medium. However, due to the low viscosity and special physical and chemical properties of seawater, it is difficult to form the lubricating film of seawater-lubricated friction bearings, and the supporting capacity and stiffness of liquid film are poor, which has become one of the common problems in this field [2]. To sum up, based on former research and experience, the theory of electromagnetic levitation was introduced into the seawater lubricated bearing to form a magnetic liquid double suspension bearing (MLDSB), which is suitable for a seawater desalination high-pressure pump. The MLDSB Experimental Table shown in Figure 2, includes the axial loading rig, radial loading rig, radial unit, composite unit, and coupling, etc. The MLDSB is mainly composed of a stator, coil, rotor, magnetic sleeve, and other components. The stator has eight circumferential magnetic poles. The coil that is arranged on the magnetic pole can make the magnetic pole produce electromagnetic force after  
Figure 1: Process of reverse osmosis seawater desalination. Figure 2: Magnetic liquid double suspension bearing (MLDSB) experimental table. electrifying, and the oil between the supporting cavity and magnetic conducting sleeve provide hydrostatic force [3]. The structure of the radial unit is shown in Figures 3 and 4. Due to the coupling between each magnetic pole and between the hydrostatic system and the electromagnetic system, the stability and reliability of the MLDSB can be reduced greatly [4]. As one of the most popular research topics, multi-DOF coupling characteristics of electromagnetic bearings have been studied deeply by researchers in many countries obtaining valuable results. Focusing on the problem of electromagnetic force coupling between two DOF of a four-pole permanent magnet offset radial magnetic bearing, the causes of coupling were studied, and the feedforward decoupling control strategy and the decoupling algorithm of the feedforward compensator for the permanent magnet offset radial magnetic bearing was reviewed in the literature [5]. The comparison between the electromagnetic force and the control current before and after decoupling shows that the proposed feedforward compensation decoupling algorithm was feasible. The literature [6] proposed a feedforward decoupling control strategy of radial four degrees of freedom for the rigid rotor system of the electromagnetic bearing. The simulation and experimental results showed that the proposed feedforward decoupling controller can decouple the original radial coupling of the rigid rotor system of the electromagnetic bearing from a four-DOF system to a four single-DOF system. The literature [7] proposed a method based on inverse system decoupling that improved the two-DOF control. The results showed that the control algorithm can make the flywheel rotor suspend more stable and effectively suppress its vibration.