Active Magnetic Bearings

Seminar Report 2012

ACTIVE MAGNETIC BEARINGS

CHAPTER 1

INTRODUCTION An active magnetic bearing (AMB) system is a collection of electromagnets used to suspend an object and stabilization of the system is performed by feedback control. The system is composed of a floating mechanical rotor and electromagnetic coils that provide the controlled dynamic force and thus allowing the suspended object to move in its predefined functionality. Due to this noncontact operation, AMB system has many promising advantages for high-speed, high-temperature and clean environment applications. Moreover, adjustable stiffness and damping characteristics also make the AMB suitable for elimination of vibration that presents in the system. Although the system is complex and considered an advance topic in term of its structural and control design, the advantages it offers outweigh the design complexity. Application areas of magnetic bearings are still steadily expanding because of these practically useful features. A few of the AMB applications that receive huge attentions from many research groups around the globe are the flywheel energy and storage device, turbo molecular pump, compressor, Left Ventricle Assist Device (LVAD) and artificial heart. For the LVAD and artificial hearts applications particularly, the present of any debris or dust resulted from any mechanical contact is strictly intolerable since these particles can contaminate the circulating blood that definitely will cause more hazardous effects to human. The use of magnetic bearing, as opposed to the fluid film and mechanical bearings, has offered new opportunity in this application area in which constructing both of these devices that meet the very stringent requirement is a viable option. A typical control block diagram of AMB system is as shown in Figure 1 where in order to stabilize the system, a position feedback of the rotor obtained from a few position sensors is required. The control algorithm that is designed to achieve the pre-specified performance of the system resides in the digital processor in the form of software code. Since AMB system requires a very fast response, Digital Signal Processing (DSP) based digital processor board is usually adapted as the main processor. The digital-to-analog converter (DAC) and the power amplifiers are the electronic circuits that convert the controlled signals to an appropriate level to the system.

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Other signal conditioning circuits such as noise filter and dc gain most of the time can be incorporated in the control algorithm or performed by additional circuit on the DSP board. Magnetic bearing systems are designed in a few configurations to meet various specifications for different applications. In term of the rotor position, it can be oriented in horizontal or vertical position in which in latter configuration, the effect of gravity is uncoupled from system dynamics wherein the vertical displacement is controlled separately from the other set of magnetic coils controlling horizontal position.

Bearing system configuration with horizontal rotor orientation is however is more widely used as covered in majority of the references therein. Also, some of the applications such as the artificial heart, the rotor will be in both horizontal and vertical position due to its nature of operation which depends on the position and movement of the host.

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Other than the difference in rotor position, the designs of magnetic bearings that provide the dynamic force are also made in a few configurations. As shown in [10], the conical shape magnetic bearings are used where a small angle is introduced in the bearing design such that the dynamic forces produced by the complete set of the magnetic bearings are able to control both the radial and thrust motion. With this configuration, the number of magnetic bearings used is reduced; however, a more complicated control algorithm to stabilize the system is required due to the coupling effect of motion. An artificial intelligent based control algorithm for the system is covered in however the model is linearized at an operating point such that the synthesis of the controller is feasible. Another type of arrangement of the magnetic bearings is having three magnetic bearing at each end of the rotor which requires only six magnetic bearing in total for complete system. However, the major trade-off in this configuration is the flux coupling effect between the bearings which requires a more complicated model to represent the nonlinear behavior of the system and further needs a more complicated stabilizing controller. In addition to this configuration, some systems have incorporated permanent magnets in the bearing which supply static forces to the system. The combination of this permanent magnets and active magnetic bearings form so-called hybrid magnetic bearings as have been highlighted. Another important aspect in magnetic bearing system is the method to stabilize and control the system to meet the need of the application. Both linear and nonlinear control laws have been covered in many research works and the choice of controllers usually relies on the structure of model established and the requirements of applications. Linear controllers are adapted more widely, however, nonlinear controllers promise more optimized applications. For both of these types of controller families, the power amplifiers that supply the current to the system (or voltage) exist in three classes of mode of operation. In Class-A power amplifier, the operation is performed such that a constant bias current is supplied all the magnetic coils where the value is set at the half of the allowable current value. The controlled currents are added to the bias current in one coil and subtracted from the bias current in the opposite coil that is in alignment to the direction of the force produced. This mode of operation is the most commonly used in magnetic bearing due to its good dynamic performance. For Class-B mode, a lower value of bias current is supplied to the magnetic coil but the controlled current is only added to only one side of the pair of magnetic bearing, according to the position of the rotor. DEPT. OF AUTOMOBILE

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Class-B mode, however, inherits poor bearing stiffness and poor robustness against system vibration which make it suitable for low performance applications. Then, when there is no bias current used, where only the controlled current is supplied to the magnetic bearing, it is categorized as Class-C power amplifier. In this mode, only nonlinear controllers work due to its singularity error at initial point of operation and severe nonlinearity that resulted when the control force is zero. AMB system is considered an advance m power consumption which is favorable in most that requires four pairs of electromagnetic coils to tem in which a successful design depends heavily on the mathematical models that represents the system behaviour at design stage. The performance of the system can be accessed through computer simulation which is more cost effective and practical towards constructing the actual physical system. However, under these various AMB configurations, magnetic coils arrangement as well as different types of mode of power amplifiers, modeling the AMB system is in fact a very challenging task. Many of early works in AMB modeling involve the derivation of linear or linearized models which operate at certain operating condition. This procedure is performed in order to accommodate a linear dynamic controller for stabilization of the AMB system. The disadvantage of this approach is the model is valid at a very small operating condition and the system performance will degrade as the model of the physical system is perturbed from this operating point. However, in order to maximize the performance of the system, the derived model needs to cover wider operational ranges that further forces the system into its nonlinear regime. In order to achieve this, a more sophisticated mathematical model that can describe the behaviour of the system within this boundary is required. In this paper, two nonlinear mathematical models of a horizontal shaft AMB system are derived in which the gyroscopic effect and mass imbalance are also considered. The two AMB models are developed based on the system with voltages as the inputs and the system with currents as the inputs. The derived model will be presented in a state variable form that is suitable for the design of a class of robust controller.

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CHAPTER 2

ACTIVE MAGNETIC BEARINGS (AMB)

Fig. 1: Magnetic bearing A magnetic bearing is a bearing which supports a load using magnetic levitation. Magnetic bearings support moving machinery without physical contact, for example, they can levitate a rotating shaft and permit relative motion without friction or wear. In active magnetic bearings (AMB) a stable equilibrium is achieved by means of one or morecontrol loops. The use of control loop for maintaining the gap between the shaft and bearing differentiate the active magnetic bearings (AMB) from passive ones. They are inservice in such industrial applications as electric power generation, petroleum refining, machine tool operation and natural gas pipelines

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CHAPTER 3

BASIC OPERATION

Fig. 2: Typical AMB system diagram The typical AMB system diagram is illustrated in above figure. Besides the controller, the general control system also includes the sensor, A/D and D/A conversion and power amplifier. The rotor’s displacement along one of the axes is detected by the position sensors and converted into signals of standard voltage. Then compared with the setting value, the error signal enters the controller. After A/D conversion, the controller processes this digital signal according to a given regulating rule (control arithmetic) and generates a signal of current setting. After D/A conversion, this current signal enters the power amplifier, whose function is to maintain the current value in the electric magnet winding at the current level set by the controller. Therefore, if the rotor leaves its center position, the control system will change the electromagnet current in order to change its attraction force and, respectively, draws the rotor back to its balance position.

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PRINCIPLE OF AMB

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CHAPTER 4

IMPROVED MACHINE PERFORMANCE USING ACTIVE CONTROL TECHNOLOGY Active Magnetic Bearings (AMBs) with their control system inherently offer the possibility of continuously recording bearing forces and rotor displacements. This allows online monitoring of critical process parameters and early detection of incipient faults, such that reliability is increased. Furthermore, AMBs can actively influence a machine's behavior. Thereby, operation can be adjusted and optimized according to process changes.

4.1 Identification and Control AMB systems are often used to control structural resonance frequencies coming from the rotor or from elastic supports. The resonance frequencies may vary significantly with the rotational speed. Controller design for AMB systems is therefore important for the system performance. Controller design requires a plant model. Identification, i.e. modeling based on dynamic measurements, is a fast way for obtaining such a model. Both controller design and identification are topics of current research.

Fig. 3: Controller design for AMB system

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CHAPTER 5

SYSTEM COMPONENTS 5.1 Magnetic Bearing The radial and axial magnetic bearings are located in the generator. In order to reduce the range of products, magnetic bearings for generator rotor and turbocompressor rotor are designed as the unified size according to the generator rotor load in operation condition. The radial bearing radial gap is 0.15mm considering the gap of 0.4mm between the compressor stator and blades in order to protect the compressor.

5.2 Position Sensor The rotor displacements in radial and axial are monitored by the position sensors, which are of induction type. The sensor consists of sensitive elements located on the stator and an acting element located on the rotor in front of the sensitive elements. The sensitive element is an annular magnetic circuit with 24 poles, of which each 6 poles are grouped to detect the radial displacements in X and Y directions. In such design, a kind of 2/3 redundancy working mode for sensor signals can be easily realized. The acting element is an extension made of the laminated ferromagnetic steel, which is fixed on turbomachine shaft. Windings around the stator perimeter are distributed in order to average and smooth the measure value. This kind of sensor has good sensitivity of no less than 10mV/μm and resolution of at least 1μm. Its cut-off frequency is enough so high (>5k Hz) that the phase lag at operation frequency can be neglected. The voltage signal after the sensor modulator can be transferred more than 200m without obvious attenuation.

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position sensors

5.3 Controller The controllers, as well as all its peripheral equipment, including A/D, D/A, network card, etc., is standard industry type, usually selected as high speed Digital Signal Processing (DSP) computer, which has good stability and excellent hard real-time interrupt processing capability. For example, the new DSP product of TI 6713 has powerful floating-point operation of 1350 MFLOPS and can be adopted as the ideal micro processor of the controller. The A/D converter has 10 channels with 500kS/s rate and 16bit precision, while the D/A converter have 5 channels with 1MS/s rate and 14bit precision. 1. The controller shall have the following functions: 2. Receive information about displacement, rotation speed and angular position of the machine rotor from the sensor converters; 3. Receive the control commands from the operation computer to change some parameters of the AMB control system; 4. Generate and release the current control signals in coil windings according to the specified algorithms and control commands; 5. Diagnose the states of the elements of the AMB system and transmit this information to the operator computer via networks; 6. Release signals about alarm and emergency protection. DEPT. OF AUTOMOBILE

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5.4 Host Computer The operating and monitoring computer (host) lies on the high level control channel, whose type is standard PXI industry computer and its operation system is universal MS Windows. The typical configuration of the host computer can select the NI with 2.3GHz Pentium 4 CPU. The communication between controller and host computer is based on industry network. The main functions of the host computer are listed in the follows. 1. Establish and change the control algorithms or rules of the AMB. 2. Start up and stop the AMB control system. 3. Receive information about the states of AMB components and display this information by different graphical means on the monitor. 4. Diagnose controller state and make decision. 5. Log and print information about the state of the AMB control system components. 6. Send process information to the Instrument and Control (I&C) system of reactor plant.

5.6 Power Amplifier The power amplifier receives the control signal in analog voltage from the controller and keeps the current in the magnet winding according to this voltage signal. Generally speaking, power amplifier is a kind of controlled constant-current source to the inductive reactance. As the power of single amplifier unit is about 4.5kVA (300V, 15A), switch amplifier is the best type considering the losses and efficiency. In order to reduce the drawback of switch amplifier of sharp oscillation impulsion at stable operation state, special method is selected to realize a relative smoothly current, such as three-state voltage level, two H-bridge connecting in series, high switch frequency of 60k Hz and so on. The phase lag is less than 3° at 200 Hz to achieve good dynamic characteristics.

5.7 Ambs for High-Temperature Applications AMBs could be attractive for gas turbines in airplanes and power production. Problems to be investigated include design, materials, sensors, control, backup bearings, and

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other topics relevant in the context of high-temperature operation. We participate in a research project of the EU addressing these new applications.

CHAPTER 6

HIGH TEMPERATURE MAGNETIC BEARINGS Synchrony has developed a line of high temperature magnetic bearings for aerospace and defense applications. The bearings are specifically designed for use in next generation gas turbine engines, where high temperatures and high rotational speeds preclude the use of industrial magnetic bearings.

Fig. 4: High temperature magnetic bearings

6.1 Advanced Features of the High Temperature Magnetic Bearings Include: 1.

High temperature coils and magnets reduce cooling requirements and improve efficiency and power density at temperatures to 1000°F

2.

Redundant magnets, signal processors, sensing, and amplification ensure continuous operation

3.

Integrated, high temperature sensors reduce the size and improve the performance

4.

Compact, light-weight magnetic structure increases power density

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Advanced, magnetic flux control algorithms improve the performance and dynamic stability

6.

Inertial Balance System automatically and adaptively compensates for unbalance, reducing vibration and power consumption

7.

These magnetic bearings are used with Synchrony's compact digital controllers to provide a total solution for high power density, rotating machinery.

6.2 Self-Sensing Magnetic Bearing The self-sensing (sensorless) magnetic bearing is a special kind of magnetic bearing, which needs no external position sensors. The position information is deduced from the air gap dependent properties of the electromagnets. The main advantage is the reduction of the manufacturing costs. Furthermore, self-sensing bearings have a number of features that make them interesting for solving technical problems. The absence of the position sensor simplifies the construction, the assembly, and the maintenance of the magnetic bearing system. Additionally, it allows a more compact design of the rotor, which increases its natural frequencies. Two different concepts for self-sensing bearings have been developed and realized at the ICMB.

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Thermal Network of the AMB

Thermal Network The model has been built up in two steps. First a simplified coil model (subsection 5.2.2) has been developed and validated. Secondly a model including coils, iron core, housing and rotor (subsection 5.2.3) has been developed and validated. The block diagram of figure 5.2 shows the three main blocks in the model structure. The first one reads the values from a file containing all the parameters of the AMB and build the T and G matrices. At this stage, the material temperatures are defined equal to the ambient temperature. The second block calculates the temperature from the given matrices. Since some parameters and the thermal radiation conductances are temperature dependent, the calculated matrices T and G of the first block must be redefined. The second block proceeds by iterations and tries to converge to equilibrium. The convergence of the system is ensured by a factor allowing to tune the closed loop gain. The third block is an optimization loop, which compares the model predictions with measurements and tunes the polynomial coefficients of the material properties accordingly. The measurement conditions, i.e: coil currents and ambient temperature, are given to the model as well as the value of the parameter to fit. A temperature computation is done and compared to the measurement data. The function fmincon of Matlab tries, using a mediumscale algorithm (line search), to find an optimum parameter value reducing the error between the measured and the calculated temperatures. Failures Related to High Temperature In order to be able to perform diagnosis, the failures to detect have to be known. Failures can occur in different locations of the magnetic bearing and some of them take place suddenly whereas other ones rise slowly in function of time. The consequences on the AMB behaviour is different for each case, going from small changes compensated by the controller to critical faults that suddenly stop the AMB from working.

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In order to have an overview of the encountered cases shows a summary of the failures related to high temperature. Diagnosis implies to use sensors, and obviously at high

temperature, the available sensor choice is very limited. Major points are considered; the coils aspect, the magnetic material properties, the position sensors and the rotor.

DOF high temperature AMB.

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CHAPTER 7

ADVANTAGES OF AMB 1. Elimination of leaks, flash, contaminants 2. Direct-drive, direct-coupled machines (no gearbox) 3. Accurate, dynamic control of rotor position & stability 4. Improved rotor dynamic performance through more compact designs, shorter shafts and bearing spans and the possibility of multiple bearing systems. 5. Absence of mechanical contact between shaft and bearing. 6. Very low wear rate. 7. Low power dissipation in the bearing. 8. Absence of lubricants. 9. High speeds of rotation. 10. Possibility to adjust position between the shaft and the bearing. 11. Unbalance compensation. 12. Ability to work in a broad spectrum of temperatures, in vacuum, in aggressive surroundings, etc. 13. Low vibration.

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CHAPTER 8

DISADVANTAGES OF AMB AMBs have also several significant disadvantages. 1. One of the most crucial is very high complexity, which results in relatively high probability of faults or failures. 2. Properties of the AMBs are completely dependent on the quality of the control system. Because AMBs are inherently unstable, they are not able to work correctly without permanent feedback (e.g. information about displacements measured by sensors). 3. A failure occurred in the loop can have fatal consequences for the process (e.g. machined part) or for the equipment (e.g. spindle, tool, etc.). System component faults can be classified (according to [3]) as external or internal to the magnetic bearing control system. A fault is considered to be external when either it manifests itself as or its effect can be replicated by external disturbance acting on the system. External faults include •

Rotor impact,

•

Rotor mass loss,

•

Base motion,

•

Rotor deformations

•

Sudden changes in loading

•

Rotor rub

•

Cracked rotor

External faults usually cause abnormal rotor vibrations, and can be treated by sufficient control force and suitable controller design . Changes in control system can include simple adjustment of controller parameters, adjustment or adaptation of the control algorithm. Therefore the development of models of the hovering body (rotor, beam or lumped mass) is necessary.

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CHAPTER 9

APPLICATIONS Magnetic bearing advantages include very low and predictable friction, ability to run without lubrication and in a vacuum. Magnetic bearings are commonly used in watt-hour meters by electric utilities to measure home power consumption. Magnetic bearings are also used in high-precision instruments and to support equipment in a vacuum, for example in flywheel energy storage systems. A flywheel in a vacuum has very low windage losses, but conventional bearings usually fail quickly in a vacuum due to poor lubrication. Magnetic bearings are also used to support maglev trains in order to get low noise and smooth ride by eliminating physical contact surfaces. Disadvantages include high cost, and relatively large size. Magnetic bearings allow contact-free levitation. This offers a number of interesting advantages. Magnetic bearings do not require lubrication, they allow high circumferential speeds at high loads, they do not suffer friction or wear, and therefore they offer a virtually unlimited lifetime while no maintenance is needed. Furthermore, the bearing force can be modulated, either for compensating unbalance forces, or for deliberately exciting vibrations. Because of these advantages, they are used in an increasing number of commercial high-performance applications in the domain of rotating machinery. These include ultra-high vacuum pumps, canned pipeline compressors and expanders, high-speed milling and grinding spindles, flywheels for energy storage, gyroscopes for space navigation, spinning spindles, and others.

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CHAPTER 10

CONCLUSION Limitations in active magnetic bearings arise from two reasons: the state of the actual technology in design and material, and from basic physical relations. the paper has given a survey on such limitations, giving a brief theoretical background, showing examples and pointing to actual data. they may help to make substantiated design decisions. The various issues are summarized subsequently Further research appears to be indicated in developing insight and outlook at the boundaries of the field of magnetic bearings. a systematic comparison of amb performance with that of classical bearings needs consistent data. the joint operation of a magnetic bearing with a roller bearing under emergency situations, in load sharing or in touch down contacts, needs further experiments and design efforts. the operation at supercritical speeds, passing many elastic rotor and structure frequencies needs more research on the control design. the advanced information processing within the bearing system, extending the smartness of the rotating machinery, will be a promising research area. the potential and limitations of high temperature super-conductors, as an extension or an alternative to amb's, is not yet sufficiently known.

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CHAPTER 11

REFERENCES 1. www.wikipedia.org 2. www.nasa.org 3. www.ieee.org

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