Space vectors and pseudo inverse matrix methods for the radial force control in bearingless multi-sector permanent magnet machines

Two different approaches to characterize the torque and radial force production in a Bearingless Multi-Sector Permanent Magnet (BMSPM) machine are presented in this work. The first method consists of modelling the motor in terms of torque and force production as a function of the stationary reference frame α-β currents. The current control reference signals are then evaluated adopting the Joule losses minimization as constrain by means of the pseudo inverse matrix. The second method is based on the control of the magnetic field harmonics in the airgap through the current Space Vector (SV) technique. Once the magnetic field harmonics involved in the torque and force production are determined, the SV transformation can be defined to obtain the reference current space vectors. The methods are validated by numerical simulations, Finite Element Analysis (FEA) and experimental tests. The differences in terms of two Degrees of Freedom (DOF) levitation performance and efficiency are highlighted in order to give the reader an in-depth comparison of the two methods

Rotors on Active Magnetic Bearings: Modeling and Control Techniques

In the last decades the deeper and more detailed understanding of rotating machinery dynamic behavior facilitated the study and the design of several devices aiming at friction reduction, vibration damping and control, rotational speed increase and mechanical design optimization. Among these devices a promising technology is represented by active magnetic actuators which found a great spread in rotordynamics and in high precision applications due to (a) the absence of all fatigue and tribology issues motivated by the absence of contact, (b) the small sensitivity to the operating conditions, (c) the wide possibility of tuning even during operation, (d) the predictability of the behavior. This technology can be classified as a typical mechatronic product due to its nature which involves mechanical, electrical and control aspects, merging them in a single system. The attractive potential of active magnetic suspensions motivated a considerable research effort for the past decade focused mostly on electrical actuation subsystem and control strategies. Examples of application areas are: (a) Turbomachinery, (b) Vibration isolation, (c) Machine tools and electric drives, (d) Energy storing flywheels, (e) Instruments in space and physics, (f) Non-contacting suspensions for micro-techniques, (g) Identification and test equipment in rotordynamics. This chapter illustrates the design, the modeling, the experimental tests and validation of all the subsystems of a rotors on a five-axes active magnetic suspension. The mechanical, electrical, electronic and control strategies aspects are explained with a mechatronic approach evaluating all the interactions between them. The main goals of the manuscript are: • Illustrate the design and the modeling phases of a five-axes active magnetic suspension; • Discuss the design steps and the practical implementation of a standard suspension control strategy; • Introduce an off-line technique of electrical centering of the actuators; • Illustrate the design steps and the practical implementation of an online rotor selfcentering control technique. The experimental test rig is a shaft (Weight: 5.3 kg. Length: 0.5 m) supported by two radial and one axial cylindrical active magnetic bearings and powered by an asynchronous high frequency electric motor. The chapter starts on an overview of the most common technologies used to support rotors with a deep analysis of their advantages and drawbacks with respect to active magnetic bearings. Furthermore a discussion on magnetic suspensions state of the art is carried out highlighting the research efforts directions and the goals reached in the last years. In the central sections, a detailed description of each subsystem is performed along with the modeling steps. In particular the rotor is modeled with a FE code while the actuators are considered in a linearized model. The last sections of the chapter are focused on the control strategies design and the experimental tests. An off-line technique of actuators electrical centering is explained and its advantages are described in the control design context. This strategy can be summarized as follows. Knowing that: a) each actuation axis is composed by two electromagnets; b) each electromagnet needs a current closed-loop control; c) the bandwidth of this control is depending on the mechanical airgap, then the technique allows to obtain the same value of the closed-loop bandwidth of the current control of both the electromagnets of the same actuation axis. This approach improves performance and gives more steadiness to the control behavior. The decentralized approach of the control strategy allowing the full suspensions on five axes is illustrated from the design steps to the practical implementation on the control unit. Furthermore a selfcentering technique is described and implemented on the experimental test rig: this technique uses a mobile notch filter synchronous with the rotational speed and allows the rotor to spin around its mass center. The actuators are not forced to counteract the unbalance excitation avoiding saturations. Finally, the experimental tests are carried out on the rotor to validate the suspension control, the off-line electrical centering and the selfcentering technique. The numerical and experimental results are superimposed and compared to prove the effectiveness of the modeling approach

Identification of magnetically levitated machines

Magnetically levitated machines use magnetic levitation to maintain the separation between the bearing races. They offer various benefits compared with other types of bearings: no contact between the bearing races, the absence of lubrication, and suitability for high-speed applications. Magnetically levitated machines can be divided into two groups: active magnetic bearing (AMB) and bearingless machines. An active magnetic bearing is the traditional magnetic bearing. AMBs use electromagnets to maintain the rotor in a stable position. Bearingless machines are similar to AMB machines, but they use the same air gap for the generation of the torque-producing and levitation flux. Magnetically levitated systems are unstable, complex, and nonlinear multi-input multi-output systems. Thus, they require feedback control for stable operation. Further, accurate modeling is essential for the robust control of these systems. In this doctoral dissertation, the system identification aspect of magnetically levitated systems is considered. In general, system identification refers to construction of mathematical models of systems by measuring input-output data during an identification experiment. System identification can assist in the modeling, and more accurate models can be built with real data. In this dissertation, different excitation signals; pseudorandom binary sequence (PRBS), chirp, stepped sine, and multisine are first applied to the AMB system identification for single-input single-output and multi-input multi-output cases. Next, the online identification of an AMB rotor–bearing system with a sliding discrete Fourier transform with the direct and indirect identification is shown. Then, the identification methods used for AMB machines are applied to bearingless machines. The effects of noise and delay on the linearized plant identification accuracy based on nonlinear simulation models are examined. Finally, the AMB rotor–bearing system identification with the PRBS for a multi-input multi-output system is presented. The doctoral dissertation provides results for the identification of a magnetically levitated system. Several laboratory test rigs were used to obtain the results. It is shown that the methods used for the AMB system identification can be applied to bearingless machines

Design and control of a permanent magnet bearingless machine

The overall efficiency of high-speed applications can be improved by applying direct drive motor technology. Operating in the high-speed region is a demanding task for the traditional bearing technology. With active magnetic bearings, the rotor can be supported by the magnetic force. As the shaft is rotating in the air, polluting oil lubrication is not needed, and in practice, the rotor system is maintenance free. However, the magnetic bearing construction increases the rotor length, which has an adverse effect on the dynamical behavior of the rotor. Bearingless motor technology combines the levitating force capability of the magnetic bearing with the traditional electrical motor. This integrated structure enables a shorter machine length than with the active magnetic bearings. Compared with the traditional electrical machine design flow, additional parameters must be taken into account when incorporating the bearingless feature into a motor system. It is important to analyze the interaction of the generated torque and the levitating force. The main objective is to minimize this interaction so that the control of the bearingless machine is more straightforward. The rotor controlled by bearingless motors constitutes a multi-input multi-output system. The system includes cross-couplings between the rotor and the motor units. This issue must be taken account in the control of the bearingless machine. This doctoral dissertation addresses issues related to the design of a bearingless machine. The main focus is on how to minimize the interaction between torque and levitation force generation. A model-based control approach is adopted to control the bearingless machine by taking into account the cross-couplings. The model is validated by a system identification approach, and the controllers are tested experimentally in the bearingless machine

Model-Based Levitation Control of A 100 kW Bearingless Electric Motor

The use of magnetically levitated rotors for various applications, especially in pumps and compressors, has seen an unprecedented rise in the last few years. Bearingless motors combine levitation and torque production capabilities. They offer more compact footprint and require less power electronics compared to more traditional active magnetic bearing supported motors. A lot of significance has been attached to reducing cost, complexity and broadening applicability of the magnetically levitated rotors. Hence, the levitation control of rotors in such bearingless machines has become quite an interesting topic of research. Digital control strategies need to be adopted for proper levitation control of rotors. Furthermore, it has to be kept in mind that these rotors cannot afford to have too many oscillations under different environmental conditions because oscillations can eventually lead to instability and heavy losses. This thesis presents a state-of-the-art model-based digital control of the levitation of a 100 kW bearingless electric motor where the point-mass of the rotor is considered. This motor has a rated speed of 22000 rpm. The entire bearingless motor system is converted into state-space models by taking into account the bearingless machine's nominal operating points and conditions. Then, a model-based controller with Pincer's conditions, coupled with an estimator with Kalman filtering, integral action and state-command path, is implemented and tested for the levitation control. FEM derived Simulink model of the bearingless motor is tested to verify the proposed control strategies. The closed-loop poles and zeroes, step responses of the closed-loop system and the frequency responses are also recorded from the simulations. In the end, the control of the rotor is investigated with five different combinations involving controller, estimator, integrator and state-command path. Comparisons are conducted on the the proposed control strategies and conclusions are drawn based on the findings

NASA Tech Briefs, March 2008

Topics covered include: WRATS Integrated Data Acquisition System; Breadboard Signal Processor for Arraying DSN Antennas; Digital Receiver Phase Meter; Split-Block Waveguide Polarization Twist for 220 to 325 GHz; Nano-Multiplication-Region Avalanche Photodiodes and Arrays; Tailored Asymmetry for Enhanced Coupling to WGM Resonators; Disabling CNT Electronic Devices by Use of Electron Beams; Conical Bearingless Motor/Generators; Integrated Force Method for Indeterminate Structures; Carbon-Nanotube-Based Electrodes for Biomedical Applications; Compact Directional Microwave Antenna for Localized Heating; Using Hyperspectral Imagery to Identify Turfgrass Stresses; Shaping Diffraction-Grating Grooves to Optimize Efficiency; Low-Light-Shift Cesium Fountain without Mechanical Shutters; Magnetic Compensation for Second-Order Doppler Shift in LITS; Nanostructures Exploit Hybrid-Polariton Resonances; Microfluidics, Chromatography, and Atomic-Force Microscopy; Model of Image Artifacts from Dust Particles; Pattern-Recognition System for Approaching a Known Target; Orchestrator Telemetry Processing Pipeline; Scheme for Quantum Computing Immune to Decoherence; Spin-Stabilized Microsatellites with Solar Concentrators; Phase Calibration of Antenna Arrays Aimed at Spacecraft; Ring Bus Architecture for a Solid-State Recorder; and Image Compression Algorithm Altered to Improve Stereo Ranging

Control System Commissioning of Fully Levitated Bearingless Machine

The bearingless permanent magnet synchronous motor (BPMSM) is a compact motor structure that combines the motoring and bearing functions based on well-designed integrated windings for generating both torque and magnetic suspension force. In order to achieve a successful high-performance control design for the BPMSM, an adequate model of the rotor dynamics is essential. This paper proposes simplified multiple-input and multiple-output (MIMO) control approaches, namely the pole placement and the linear-quadratic regulator (LQR), that allow to carry out identification experiments in full levitation. Additionally, the stability of the MIMO levitation controller is verified with the rotation tests. Compared with other recently published works, the novelty of this paper is to experimentally demonstrate that a stable fully levitated five-degrees-of-freedom (5-DOF) operation of a bearingless machine can be achieved by the proposed approach, and thereby, options for commissioning of such a system are obtained

Development of Noncontact Mechanical Testing Devices using Magnetic Levitation Mechanism

高知工科大学doctoral thesi

Power-Sharing Control in Bearingless Multi-Sector and Multi-Three-Phase Permanent Magnet Machines

This paper deals with the power-sharing control of bearingless multi-sector and multi-three-phase permanent magnet machines. The proposed control strategy allows to distribute the power flows among the three-phase inverters supplying the machine during bearingless operation of the drive. The control technique is based on the extension of the vector space decomposition modeling approach. The components producing the electromagnetic torque, i.e. the q-axis currents, are controlled independently from the d-axis ones, also with the aim of managing the power flows among the three-phase systems. Conversely, the d-axis currents are exploited for the generation of the radial forces needed to levitate the rotor, while considering the compensation of the forces caused by the q-axis currents in case of unbalanced power sharing strategy. The validity of the proposed method is confirmed by simulations and experimental tests on a prototyped bearingless multi-sector permanent magnet synchronous machine. The proposed approach is a contribution to the development of advanced control systems employing multiphase drives in the field of bearingless and multiport applications

Radial force control for triple three-phase sectored SPM machines. Part II: Open winding fault tolerant control

A new advanced fault tolerant control technique for a triple three-phase Surface Permanent Magnet (SPM) machine is investigated in this paper. The machine has a nine-phase winding arranged in three sectors and supplied by three different Voltage Source Inverters (VSIs). The proposed current control technique is firstly exploited to avoid the radial force appearance in case of open winding of one machine sector. Then, the radial force fault tolerant control is improved to compensate for a bearing fault or another source of radial force in this open winding condition. Finite element simulations are used to validate the two proposed control techniques. Finally, advantages and drawbacks of the solution are highlighted