Stability Limits of a PD Controller for a Flywheel Supported on Rigid Rotor and Magnetic Bearings

Magnetic bearings have long offered the potential for significant turbomachinery system improvements due to their oil-free, non-contact, low loss nature and their ability to actively control shaft dynamic motion. However, end-users and many designers are hesitant to apply this technology. There are two basic stumbling blocks: active magnetic bearings (AMBs) have little overload capacity, and failure of any portion of the AMB system could result in catastrophic damage to the machine. To cope with both of these problems, a secondary back-up bearing must be included in the system. This paper describes a new full scale, high speed test rig which has the capability to test a variety of back-up bearings at speeds of up to 35,000 RPM, and bearing loads of up to 6.7 kN. Preliminary data for two novel back-up bearings are presented as a demonstration of the test rig’s capabilities.

In this study, the system consists of 5-DOF magnetic bearing and a built-in motor/generator. The magnetic bearing is of hybrid type, with passive axial magnetic bearing and active radial magnetic bearing. For the passive magnetic bearing (PMB), a pair of ring-type Halbach arrays of permanent magnets are arranged vertically to support the weight of rotor and flywheel. For the active magnetic bearing (AMB), a set of ring-type Halbach array is placed on the rotor side, which will correspond to a coil set on the stator side. The AMB can produce both attraction and repulsion forces on the radial direction, depending on the direction of the coil currents. Furthermore, fabrication of ring-type permanent magnets will be discussed, and it mainly consists of arc-shaped permanent magnet and bonding them with adhesives, so it probably causes the existence of eccentricity. On other hand, a polygon structure with cuboid permanent magnets is considered and equivalent to a ring-type permanent magnet when the analysis of magnetic force is conducted. When driven by an active feedback control system operating in conjunction with a position sensor, the actuator can produce appropriate magnetic force to balance the radial forces which are caused by the PMB and to achieve stable levitation. In the AMB, the differential winding mode is applied, which can result in a linear force-current relation. In order to achieve higher reliability, the variation of rotating speed will be considered in the dynamics of the system, because it is very important and related to resonance and natural frequency. At last, the controller is designed by integral sliding mode control (ISMC) to overcome the effects of uncertainty and to achieve good steady-state accuracy. Moreover, numerical simulation results verify the effectiveness of the controller.

The blower with Active Magnetic Bearing (AMB) has the advantage of high speed, high reliability, small size, good adaptability. Power amplifier is an important part of the magnetic bearing system, directly determines the magnetic bearing system performance. The three-level PWM signal generation algorithm is analyses in this paper, and the IR2113 gate driven circuits with a negative bias is designed, and build a three-level magnetic bearing power amplifier. The gate driven signal of circuit is test, and entire power amplifier tracking step signal and sinusoidal signal also be test, the test results indicate that the three-level power amplifier meet the design requirements. At last, the power amplifier application on the blowers with AMB successful. Introduction Active Magnetic Bearings (AMB) using electromagnetic force to suspend rotor, with no friction loss, no lubrication, high-speed, non-polluting characteristics.The blower with AMB has the advantage of high speed, high reliability, small size, good adaptability Power amplifier is an important part of the overall magnetic bearing system, directly determines the magnetic bearing system performance . Depending on the realization of the principle, magnetic bearings power amplifier divided into linear power amplifier and switching power amplifier. With respect to the linear power amplifier, switching power amplifier has the advantage of low power consumption and high conversion efficiency; occupy a dominant position in large power amplifiers. Switching power amplifier can be divided into two-level amplifier switching amplifier and three-level switching amplifier depending on the switching signal. In two-level switching amplifier, the magnitude of current ripple is related to the DC voltage. Increases the DC voltage to improve the dynamic characteristics of the system, it will increase the electromagnetic coil current ripple at the same time. The current ripple of three-level switching power amplifier is not only smaller than the two-level power amplifier, but also nothing affected by the DC voltage , and therefore more suitable for high-power active magnetic bearings. In this paper, using FPGA chip to generate three-level switching signals, half-bridge circuit using IGBT modules and IGBT gate drive circuit is designed based on the drive requirements. The Three-level PWM Signal Generation Algorithm: D2 L Drive circuit FPGA Current sensor feedback AD circuit A D1 Vref S1 S2 I Reference signal Q1

In the paper, the investigator offered a method of reducing the amplitude of vibrations of turbo machinery rotors with passive and active magnetic bearings in resonances and resonance zones corresponding to one of the critical speeds ranging from zero to operating rotational ones. The method was based on the ability to vary the nonlinear force characteristic and the damping properties of active magnetic bearings and passive magnetic bearings of a novel design by changing the electric parameters of electromagnet circuits. The offered design of a passive magnetic bearing with permanent ring magnets and a control winding enabled a short-time change of rotor bearing stiffness. Such rotor bearings can realise the method of detuning from critical speeds during rotor speed-up and run-down. Earlier, this method was feasible only when active magnetic bearings were used. However, a complete magnetic suspension of a rotor with the employment of two radial passive bearings and one axial active magnetic bearing was advantageous as compared to three active magnetic bearings. The feasibility of the detuning method had been substantiated by study results. The study presented the results of numerical analysis, which simulated the process of transition through resonances of the initial system with a significant reduction of vibration amplitudes. The nonlinear system of differential magnetic-mechanical equations was solved using the 5th-order Runge-Kutta method with a validation for the duality of solutions. The results were shown as 3-dimensional spectra of displacement of bearing points and amplitude-frequency responses. These results had been confirmed by experimental data obtained for a laboratory setup that implemented a combined passive-active magnetic suspension of a rotor. The overall total relative error of measurements was maintained at less than 0.5%. The discrepancy between the experimental and design data for amplitudes was within 2-3%, and for resonance frequencies, it was less than 0.2%. Hence, these studies substantiated the feasibility of passive magnetic bearings with controlled stiffness. They also confirmed the possibility of using passive-active magnetic suspensions for lightweight rotors (e.g., expanders and compressors) with the implementation of the suggested method of detuning from resonance modes.

Active magnetic bearings, which are open-loop and unstable, require a feedback control system to ensure stable operation of the rotating machines that they support. Proportional-integral-derivative (PID) controllers are widely used in field applications of these bearings for this purpose. PID controllers are designed to work effectively within the linear region of operation of the rotating machines. Due to the inherent nonlinearity of the active magnetic bearings, large unbalance forces that may occur in these machines result in nonlinear vibration responses. Therefore, the PID controller’s effectiveness to control the vibration of the rotating machines is considerably reduced when the unbalance forces in these machines become large. Other control strategies, such as the fuzzy logic and the sliding mode control schemes, are more apt to deal with the nonlinear responses of the rotating machines supported by active magnetic bearings. The present work proposes an integrated fuzzy bang-bang relay controller for a rigid rotor mounted on active magnetic bearings. The effectiveness of this controller to suppress rotor vibrations is examined numerically. Performance comparison of this controller with the conventional fuzzy logic and PD controllers are made for different initial conditions, rotor imbalance magnitudes, and rotor angular speeds. At extreme operating conditions due to large rotor unbalance forces, where the magnetic bearings are highly nonlinear, the proposed integrated fuzzy bang-bang relay controller proved to be more superior over the conventional fuzzy logic and PD controllers.

Electrodynamic Bearings (EDBs) are a kind of passive magnetic bearings that exploits the interaction between the induced eddy currents in a conductor and a magnetic field to provide re-storing forces. They have been regarded as an appealing alternative to Active Magnetic Bearings (AMBs), having the ability to provide positive stiffness passively without introducing negative stiff-ness in any direction. Compared to AMBs, EDBs present advantages such as lower cost and higher reliability due to simplicity of configurations. One of the most interesting features of EDBs is the possibility to obtain stable levitation using standard conductive materials at room temperature, requiring no control systems, power electronics or sensors. Thus EDBs could be suitable solutions for high–speed rotating machinery such as flywheels, small size compressors, centrifuges and vac-cum pumps. Despite these promising characteristics of EDBs, applications are still limited because of instability issues. The main problem is that the effect of the rotating damping force in EDBs causes unstable behavior of the rotor. In existing solutions, stabilization is achieved mainly by introducing non-rotating damping to the rotor with passive ways. Although stable levitation is possible, the effectiveness of the existing methods is still limited. A hybrid solution has been proposed in this thesis, where EDBs are combined with active magnetic dampers (AMDs). Using similar magnetic actuators as those used in classical active magnetic bearings (AMBs), non–rotating damping forces are applied on the rotor supported by EDBs to obtain stable operation. This system is designed to exploit the high reliability of EDBs, overcoming the stability problem by means of controllable AMDs. It results in increased global system reliability. In case of AMBs failure, the EDBs are able to guarantee a stable levitation down to a certain speed considered safe for touch–down. During the operation speed range, the AMDs provide non–rotating damping to stabilize the rotor. This non–rotating damping can be easily tuned during rotor operation phase. At low speeds when the EDB forces are not sufficient to support the rotor, the active magnetic actuators work as AMBs to guarantee stable levitation of the rotor in a wide speed range. Besides, the EDB−AMD configuration also allows characterizeing in dynamic condition, which opens the possibility to establish damping strategy that can in perspective be implemented by totally passive means, such as eddy currents, elastomeric mounts. The combination of EDB and AMD forces are studied both analytically and experimentally. An analytical model of the system, as well as a test rig, has been built. Simulations and experi-mental tests validate the model and characterize the system. The effectiveness of the proposed solution is confirmed. The control strategy of AMDs and stabilizing alternatives of EDBs are dis-cussed consequently. The combination of EDB and AMD can be exploited to investigate easily dif-ferent damping strategies.

Under the background of intelligent new manufacturing, rotating machinery is developing towards the direction of high speed, high precision, intelligence and automation. Bearing, as the key component of high-speed precision rotor, has higher and higher requirements. As a high performance electromechanical integrated bearing, active magnetic bearings(AMBS) have a wide application prospect. In this paper, based on Simplorer and Ansoft Maxwell, the converter field-circuit coupling joint simulation of active radial magnetic bearing is carried out, including the structural design of active radial magnetic bearing and the establishment of the finite element model of active radial magnetic bearing in Maxwell. The mathematical model of single-degree-of-freedom of active radial magnetic bearing is established and the PID controller is designed. Power amplifier circuit is established in Simplorer, active radial magnetic bearing control program C module is created, and active radial electromagnetic bearing finite element model created in Maxwell is imported into Simplorer for joint simulation. Finally, the simulation results of magnetic force line, magnetic density, current stiffness and displacement stiffness of active radial magnetic bearing based on PID controller are given. Research shows that applying control strategy based on Simplorer field-circuit coupling joint simulation method combining software and hardware features, the simulation results more close to reality, the whole design of the active magnetic bearing system is verified.

The newly developed pump is a magnetically levitated centrifugal blood pump in which active and passive magnetic bearings are integrated to construct a durable ventricular assist device. The developed maglev centrifugal pump consists of an active magnetic bearing, a passive magnetic bearing, a levitated impeller, and a motor stator. The impeller is set between the active magnetic bearing and the motor stator. The active magnetic bearing uses four electromagnets to control the tilt and the axial position of the impeller. The radial movement of the levitated impeller is restricted with the passive stability dependent upon the top stator and the passive permanent magnetic bearing to reduce the energy consumption and the control system complexity. The top stator was designed based upon a magnetic field analysis to develop the maglev pump with sufficient passive stability in the radial direction. By implementing this analysis design, the oscillating amplitude of the impeller in the radial direction was cut in half when compared with the simple shape stator. This study concluded that the newly developed maglev centrifugal pump displayed excellent levitation performance and sufficient pump performance as a ventricular assist device.

The paper deals with rapid prototyping of active magnetic bearings. Modern trends in aviation and possibilities of magnetic suspension using are characterized. Geometric analysis of active axial magnetic bearing are also described. Designing input parameters are defined and calculation results are presented. MES model and MES analysis of designed magnetic bearing are also shown. Active magnetic bearing control system with PD controller are presented as well as its time characteristics.

Recently, there has been an increasing need for development of active magnetic bearing (AMB) systems of smaller size and lower energy consumption than ever, as AMBs seek for applications in compact, portable rotating machines such as hard disk spindle and artificial heart blood pump. Among others, the three-pole AMB configuration turns out to be more profitable in terms of compactness in design and low power loss than the conventional four- or eight-pole AMB. However, one of the inherent drawbacks in controller design of three-pole AMB is the strong coupling in magnetic flux between magnetic poles. It leads to the strongly nonlinear system behavior, when the equation of motion is formulated in the conventional Cartesian coordinates. In this paper, we propose use of the redundant bearing coordinates to describe the three-pole AMB system behavior, so that the potential difference controllers can be easily designed, based on the decoupled linearized control model for each pole. The proposed method is applied to control of a five-axis AMB system, which consists of a three-pole radial AMB for stabilization in the radial direction and a ring-type permanent magnet bearing for levitation in the axial and tilt directions. It is shown that simplistic equation of motion for the five-axis AMB system can be derived in the proposed redundant bearing coordinates and the resulting control equations become completely decoupled. Experiments are also carried out with the five-axis disk-type AMB system equipped with Hall diodes for measurement of the radial displacement and it is confirmed that the proposed control scheme succeeds in the run-up test.

The paper considers a complete magnetic suspension of a small-sized horizontal rotor in passive radial and active axial magnetic bearings. It describes experimental studies of the dynamics of a model rotor in a combined passive-active magnetic suspension. The studies were performed using the developed specialized software and hardware complex for measuring and analyzing the dynamic characteristics of rotors with magnetic suspension systems. Its software includes special algorithms of the mathematical data processing for measuring and evaluating the main parameters of the signals characterizing the vibration displacement of the rotor points. The results are presented in the form of vibrograms of rotor support section motion in the radial direction, spectrograms of these signals and amplitude-frequency characteristics. The work shows how the proposed experimental analysis of the vibration state of a rotor system with magnetic bearings allows modifying the rotor design to reduce the vibration amplitudes at rotation speed values close to the critical ones.

This thesis is motivated by one of the largest markets: aviation. This market, in which competition and investments are extraordinary, requires constant technical improvements in order to increase its competitiveness. Jet engines have an important influence on fuel consumption and servicing of airplanes, thus on the transportation cost. The present work was supported by the European Community [9]. This thesis investigates the use of active magnetic bearings for jet engines. It is expected that magnetic bearings could considerably reduce losses and service intervals in jet engines. The present work concentrates on the design and characterization an active magnetic bearing for applications at high temperature. The report begins with an introduction locating the accomplished work into the current economic and technical context. The advantages of such system are given, as well as the scientific contribution of the research which has been undertaken. An introduction to magnetic bearings is given, so that the key elements of the dimensioning of such a system are comprehensible for readers of all horizons. The maximum force produced by magnetic bearings is mainly limited by the heating related to the losses in the coils. A thermal model for high temperature magnetic bearings has been implemented. An experimental part allowed the validation of this model for a wide temperature range. The construction of a magnetic bearing requires various types of materials; soft magnetic materials, electrical conductors, electrical insulators as well as several fixation materials. Tests have been carried out and a catalogue lists the materials available for high temperature magnetic bearings. Position sensors are usually used in magnetic bearings. Eddy current position sensors have been developed. They were realized with coils printed on a ceramics substrate by using thick-film technology. Measurements done at high temperature show the great characteristics of these sensors. Problems of silver migration between the wires welding have been encountered during tests carried out at high temperature. Various solutions have been tested with the aim of avoiding the silver migration. The materials exposed at high temperature have unfortunately sometimes a limited lifespan. Studies of failures related to an exposure at high temperature have been done. Some failures are detectable by the magnetic bearing. An example of detection of possible short-circuits in the actuator coils is presented. Finally a prototype of an active magnetic bearing system with five degrees of freedom has been built. A furnace especially developed, makes it possible to create environments at high temperature. The characteristics of the active magnetic bearing have been measured during levitation at ambient temperatures from 25°C up to 550°C.

ABSTRACTThe purpose of this paper is to study the surge control effects of centrifugal compressors with active magnetic bearings (AMBs) under several operating conditions. First, the working principle of the magnetic thrust bearing is introduced, and the tracking performance of the rotor under the low frequency is verified by experiments. Then, the Greitzer model of centrifugal compressor is established. The relationship between tip clearance and the performance of compressor is derived, and the proportional-integral-derivative surge controller is designed based on the mass flow feedback. Finally, the surge control platform of the centrifugal compressor with active magnetic bearings is set up, and the surge control experiment of the magnetic bearing system is carried out. The influence of different speeds and different throttle valve openings on surge control performance is analyzed. The experimental results illustrate that speed has little effect on surge control, and the different throttle valve opening sizes have a certain influence on surge. The surge controller designed in this paper has a good control effect on different throttle valve opening sizes. Thus, the magnetic thrust bearing is effective in surge control by adjusting the tip clearance of the compressor.

Within a subproject of the RAPHAEL-Program, which is part of the 6th EURATOM Framework Program supervised by the European Commission it was investigated whether the use of a Hybrid Magnetic Bearing Concept (HMBC) will be beneficial for a blower application. As in the RAPHAEL program the subproject “Component Development” deals with R&D on components of High Temperature Reactor Technology (HTR), a major focus is on safety- and reliability-related issues. That implies special requirements for the support of high speed rotating shafts in HTR-Applications that only can be satisfied by using Active Magnetic Bearings (AMB). Regarding safety and competitiveness, AMBs are considered key components for the support of rotating HTR-components due to their technical features. AMBs are characterized by an electromagnetic actuator that is generating the bearing force depending on the clearance between stator and rotor, in which the rotor is levitated. Therefore an active control of the coil current is necessary. Furthermore, Touch Down Bearings (TDB) are needed to avoid damages in case of an emergency shut down or in case of energy supply losses. This contribution provides an internal insight on the advantages of a Hybrid Magnetic Bearing Concept that is characterized by a completely Active Magnetic Bearing-supported vertical arranged rotor and an additional permanent magnetic Radial Bearing. One benefit of the HMBC is an additional radial guidance of the shaft that may reduce the loads while dropping into the Touch Down Bearings e.g. in case of energy supply losses of the AMBs. Reduced loads on the TDBs will increase their life cycle and the availability of the AMB supported component. The Scope of this R&D-Project, which will be described more detailed in this contribution, includes the analytical modeling and simulation of the dynamic behavior of the Hybrid Magnetic Bearing System, the modification of the completely AMB-supported test facility FLP500 with a radial PMB and the experimental tests and validation of the analytical models to provide recommendations for the investigated blower application as an HTR-component. Furthermore, the effects occurring during the modification of the test facility and the approach that was necessary to solve unexpected problems will be described.

Magnetic Static Force Evaluation of Active Magnetic Bearing for a Hemocompatibility Assessment Device: Numerical and Experimental Approach