3,054 research outputs found

    Critical speeds and forced response solutions for active magnetic bearing turbomachinery, part 2

    Get PDF
    The need for better performance of turbomachinery with active magnetic bearings has necessitated a study of such systems for accurate prediction of their vibrational characteristics. A modification of existing transfer matrix methods for rotor analysis is presented to predict the response of rotor systems with active magnetic bearings. The position of the magnetic bearing sensors is taken into account and the effect of changing sensor position on the vibrational characteristics of the rotor system is studied. The modified algorithm is validated using a simpler Jeffcott model described previously. The effect of changing from a rotating unbalance excitation to a constant excitation in a single plane is also studied. A typical eight stage centrifugal compressor rotor is analyzed using the modified transfer matrix code. The results for a two mass Jeffcott model were presented previously. The results obtained by running this model with the transfer matrix method were compared with the results of the Jeffcott analysis for the purposes of verification. Also included are plots of amplitude versus frequency for the eight stage centrifugal compressor rotor. These plots demonstrate the significant influence that sensor location has on the amplitude and critical frequencies of the rotor system

    Multi - objective sliding mode control of active magnetic bearing system

    Get PDF
    Active Magnetic Bearing (AMB) system is known to inherit many nonlinearity effects due to its rotor dynamic motion and the electromagnetic actuators which make the system highly nonlinear, coupled and open-loop unstable. The major nonlinearities that are associated with AMB system are gyroscopic effect, rotor mass imbalance and nonlinear electromagnetics in which the gyroscopics and imbalance are dependent to the rotational speed of the rotor. In order to provide satisfactory system performance for a wide range of system condition, active control is thus essential. The main concern of the thesis is the modeling of the nonlinear AMB system and synthesizing a robust control method based on Sliding Mode Control (SMC) technique such that the system can achieve robust performance under various system nonlinearities. The model of the AMB system is developed based on the integration of the rotor and electromagnetic dynamics which forms nonlinear time varying state equations that represent a reasonably close description of the actual system. Based on the known bound of the system parameters and state variables, the model is restructured to become a class of uncertain system by using a deterministic approach. In formulating the control algorithm to control the system, SMC theory is adapted which involves the formulation of the sliding surface and the control law such that the state trajectories are driven to the stable sliding manifold. The surface design involves the transformation of the system into a special canonical representation such that the sliding motion can be characterized by a convex representation of the desired system performances. Optimal Linear Quadratic (LQ) characteristics and regional pole-clustering of the closed-loop poles are designed to be the objectives to be fulfilled in the surface design where the formulation is represented as a set of Linear Matrix Inequality optimization problem. For the control law design, a new continuous SMC controller is proposed in which asymptotic convergence of the system’s state trajectories in finite time is guaranteed. This is achieved by adapting the equivalent control approach with the exponential decaying boundary layer technique. The newly designed sliding surface and control law form the complete Multi-objective SMC (MO-SMC) and the proposed algorithm is applied into the nonlinear AMB in which the results show that robust system performance is achieved for various system conditions. The findings also demonstrate that the MO-SMC gives better system response than the reported ideal SMC (I-SMC) and continuous SMC (C-SMC)

    Modelling and Control of an Annular Momentum Control Device

    Get PDF
    The results of a modelling and control study for an advanced momentum storage device supported on magnetic bearings are documented. The control challenge posed by this device lies in its dynamics being such a strong function of flywheel rotational speed. At high rotational speed, this can lead to open loop instabilities, resulting in requirements for minimum and maximum control bandwidths and gains for the stabilizing controllers. Using recently developed analysis tools for systems described by complex coefficient differential equations, the closed properties of the controllers were analyzed and stability properties established. Various feedback controllers are investigated and discussed. Both translational and angular dynamics compensators are developed, and measures of system stability and robustness to plant and operational speed variations are presented

    Synchronous response modelling and control of an annular momentum control device

    Get PDF
    Research on the synchronous response modelling and control of an advanced Annular Momentun Control Device (AMCD) used to control the attitude of a spacecraft is described. For the flexible rotor AMCD, two sources of synchronous vibrations were identified. One source, which corresponds to the mass unbalance problem of rigid rotors suspended in conventional bearings, is caused by measurement errors of the rotor center of mass position. The other sources of synchronous vibrations is misalignment between the hub and flywheel masses of the AMCD. Four different control algorithms were examined. These were lead-lag compensators that mimic conventional bearing dynamics, tracking notch filters used in the feedback loop, tracking differential-notch filters, and model-based compensators. The tracking differential-notch filters were shown to have a number of advantages over more conventional approaches for both rigid-body rotor applications and flexible rotor applications such as the AMCD. Hardware implementation schemes for the tracking differential-notch filter were investigated. A simple design was developed that can be implemented with analog multipliers and low bandwidth, digital hardware

    Influence of Back-Up Bearings and Support Structure Dynamics on the Behavior of Rotors With Active Supports

    Get PDF
    This report presents a synopsis of the research work. Specific accomplishments are itemized below: (1) Experimental facilities have been developed. This includes a magnetic bearing test rig and an auxiliary bearing test rig. In addition, components have been designed, constructed, and tested for use with a rotordynamics test rig located at NASA Lewis Research Center. (2) A study of the rotordynamics of an auxiliary bearing supported T-501 engine model was performed. (3) An experimental/simulation study of auxiliary bearing rotordynamics has been performed. (4) A rotordynamical model for a magnetic bearing supported rotor system, including auxiliary bearing effects has been developed and simulation studies performed.(5) A finite element model for a foil bearing has been developed and studies of a rotor supported by foil bearings have been performed. (6) Two students affiliated with this project have graduated with M.S. degrees

    Application of Active Magnetic Force Actuator for Control of Flexible Rotor System Vibrations

    Get PDF
    The purpose of this work was to develop and experimentally demonstrate a novel approach to minimize lateral vibrations of flexible rotor. The applied feed forward control approach employed magnetic force actuator to inject a specially designed force to counteract the rotor unbalance force. By specific selection of frequency and phase as functions of the rotor running speed and rotor natural frequency, the proposed simplified injection waveform has been shown to be effective both in reducing the rotor\u27s vibrations and for hardware implementation. A model of the test rig was constructed using the finite element (FE) method and was validated using experimental data. The effectiveness of the proposed current injection was numerically simulated with FE model and experimentally validated using a residual unbalance force. It was noticed that at a selected constant running speed, just below the first rotor critical speed, the rotor vibrations were reduced approximately by 90 . The method was also implemented during the speed ramp test, which passes through the first critical speed. In this test the proposed force injection also reduced vibrations at various rotor speeds. These results agree well with the results of simulatio

    Culture, Efficacy, and Outcome Expectancy in Teacher Preparation;How Do the Beliefs of Pre-Service Interns, Mentor Teachers, University Supervisors and Teacher Educators Compare?

    Get PDF
    Researchers agree that teachers\u27 attitudes and efficacy beliefs play a significant role in student achievement (Armor et al., 1976 Hoy & Spero, 2005 Szabo & Mokhtari, 2004 Woolfolk & Hoy, 1990). Teachers with high self-efficacy: 1) believe they can positively influence students\u27 learning and achievement despite environmental conditions (Armor et al., 1976 Ashton & Webb, 1986 Gibson & Dembo, 1984) and 2) assume accountability for student learning (Gibson & Dembo, 1984 Ross, 1998 Siwatu, 2007). On the contrary, teachers with low teacher efficacy have minimal expectations for and fewer interactions with minority students. They are also more likely to feel teacher burnout and abandon the profession (Betoret, 2006 Friedman, 2004 Guerra, Attar, & Weissberg, 1997). Various studies have confirmed that even after successfully completing multicultural courses and field experiences, interns\u27 negative beliefs and low outcome expectancies for minority students remained (Garmon, 1996, 2004 Easter, Schultz, Neyhart & Reck, 1999). Although little is known about how teaching efficacy develops, it is possible that programs incorporating efficacy-building opportunities assist to create and maintain a pool of quality teachers in culturally diverse schools (Clark & Wegener, 2009 Garcia, 2004 Milner, 2005 Santoro & Allard, 2005 Tucker et al., 2005 Siwatu, 2007 Swearingen, 2009). The purpose of this study will be to examine patterns in culturally responsive teaching self efficacy and outcome expectancies between interns, mentor teachers, university supervisors, and teacher educators in an urban teacher education progra

    Culture, Efficacy, and Outcome Expectancy in Teacher Preparation;How Do the Beliefs of Pre-Service Interns, Mentor Teachers, University Supervisors and Teacher Educators Compare?

    Get PDF
    Researchers agree that teachers\u27 attitudes and efficacy beliefs play a significant role in student achievement (Armor et al., 1976 Hoy & Spero, 2005 Szabo & Mokhtari, 2004 Woolfolk & Hoy, 1990). Teachers with high self-efficacy: 1) believe they can positively influence students\u27 learning and achievement despite environmental conditions (Armor et al., 1976 Ashton & Webb, 1986 Gibson & Dembo, 1984) and 2) assume accountability for student learning (Gibson & Dembo, 1984 Ross, 1998 Siwatu, 2007). On the contrary, teachers with low teacher efficacy have minimal expectations for and fewer interactions with minority students. They are also more likely to feel teacher burnout and abandon the profession (Betoret, 2006 Friedman, 2004 Guerra, Attar, & Weissberg, 1997). Various studies have confirmed that even after successfully completing multicultural courses and field experiences, interns\u27 negative beliefs and low outcome expectancies for minority students remained (Garmon, 1996, 2004 Easter, Schultz, Neyhart & Reck, 1999). Although little is known about how teaching efficacy develops, it is possible that programs incorporating efficacy-building opportunities assist to create and maintain a pool of quality teachers in culturally diverse schools (Clark & Wegener, 2009 Garcia, 2004 Milner, 2005 Santoro & Allard, 2005 Tucker et al., 2005 Siwatu, 2007 Swearingen, 2009). The purpose of this study will be to examine patterns in culturally responsive teaching self efficacy and outcome expectancies between interns, mentor teachers, university supervisors, and teacher educators in an urban teacher education progra

    Electromechanical Co-Simulation for Ball Screw Feed Drive System

    Get PDF
    Ball screw feed drive system is the most widely used linear drive system in the field of industrial automation. The continuous search for efficiency puts forward higher requests to the machine tool for high speed and high acceleration, which makes the feed drive system of lightweight-designed and large-size machine tools more likely to produce vibration during high-speed and high-acceleration feed operation. Electromechanical co-simulation for ball screw feed drive dynamics is an important technique for solving vibration problems occurring in the feed motion. This chapter elaborates on this technology from three aspects: modeling and simulation of dynamic characteristics of ball screw feed drive, modeling and simulation of servo control system, and the electromechanical co-simulation of ball screw feed drive system. In this chapter, the basic theoretical models, the establishment of simulation models and the comparison between simulation and experiment results of ball screw feed drive system are comprehensively introduced to provide technical references for readers
    corecore