Mary Kasarda

Cracked shaft detection and diagnostics: A literature review

Cracks in shafts have long been identified as factors limiting the safe and reliable operation of turbomachines. They can sometimes result in catastrophic failure of equipment (rotor bursts) and, more often, in costly process upsets, repairs and premature scrapping and replacement of equipment. Cracked shafts still pose a significant and real threat to equipment in spite of the great advances made in the areas of metallurgy, manufacturing and design. In the past two decades, much research and many resources have gone into developing various on-line and off-line diagnostic techniques to effectively detect cracks before they cause serious damage. Because of the enormous amount of ongoing research in this area (more than 500 technical papers have been published in English alone in the past 30 years), there is a real need to periodically condense and summarize the information. This paper reviews literature on cracked shaft detection and diagnostics published after 1990.

Damage detection of a rotating cracked shaft using an active magnetic bearing as a force actuator - analysis and experimental verification

The active health monitoring of rotordynamic systems in the presence of breathing shaft cracks is considered in this work. The shaft is assumed to be supported by conventional bearings, and the active magnetic bearing (AMB) is used in a midshaft or outboard location as an actuator to apply specified, time-dependent forcing on the system. These forces, if properly chosen, induce a combination resonance that can be used to identify the magnitude of the time-dependent stiffness arising from the breathing mode of the shaft crack. The technique is verified experimentally on a high speed test rotor with a healthy and a cracked shaft.

The effect of actuator and sensor placement on the active control of rotor unbalance

Vibration and Acoustic Labs Mechanical Engineering Virginia Tech, Blacksburg, VA 24061-0238

A multipoint measurement technique for the enhancement of force measurement with active magnetic bearings

Active magnetic bearings (AMBs) have the unique capability to act concurrently as support bearings and load cells for measuring shaft forces. Current state-of-the-art methods for force measurement rely on models with limited accuracy due to effects which are difficult to characterize such as fringing, leakage, and variations in material properties. In addition, these effects may be a function of actual air gaps which are difficult to determine in a dynamic operating environment. This paper discusses a new force measurement methodology that inherently accounts for these types of effects and other system uncertainties by utilizing multiple sets of current pairs in opposing actuators, in conjunction with a calculation algorithm, to accurately determine the force applied by the AMB. This new multipoint methodology allows for the determination of bearing forces from information on basic actuator geometry and control currents only, with no knowledge of actual operating air gaps required. The inherent nature of the methodology accounts for model uncertainties such as fringing, leakage, and other system unknowns. Initial static experimental test results are presented demonstrating 3% error in measuring the nominal determined bearing load, and a variation in calculated forces of less than 5% in most cases (8% in one case) when the location of the rotor within the bearing stator is modified. For the analogous conventional single-point measurements, the results show 15% error and 23% variation.

Gender Classification of Walkers via Underfloor Accelerometer Measurements

The ability to classify the gender of occupants in a building has far-reaching applications including security and retail sales. The authors demonstrate the success of machine learning techniques for gender classification. High-sensitivity accelerometers mounted noninvasively beneath an actual building floor provide the input for these machine learning methods. While other approaches using gait measurements, such as vision systems and wearable sensors, provide the potential for gender classification, they each face limitations. These limitations include an invasion of privacy, occupant compliance, required line of sight, and/or high sensor density. Underfloor mounted accelerometers overcome these limitations. The authors utilize the highly-instrumented Goodwin Hall smart building on the Virginia Tech campus to measure vibrations of the walking surface caused by walkers. In this paper, the gait of 15 individual walkers was recorded as they, alone, walked down the instrumented hallway. Fourteen accelerometers, mounted underneath the walking surface, recorded walking trials with the placement of the sensors unknown to the walker. This paper studies bagged decision trees, boosted decision trees, support vector machines, and neural networks as the machine learning techniques for their ability to classify gender. A tenfold-cross-validation method is used to comment on the validity of the algorithms ability to generalize to new walkers. This paper demonstrates that a gender classification accuracy of 88% is achievable using the underfloor vibration data from the Virginia Tech Goodwin Hall by using decision tree approaches.

Concurrent use of magnetic bearings for rotor support and force sensing for the nondestructive evaluation of manufacturing processes

Active magnetic bearings are a proven technology in turbomachinery applications and they offer considerable promise for improving the performance of manufacturing processes. The Active Magnetic Bearing (AMB) is a feedback mechanism that supports a spinning shaft by levitating it in a magnetic field. AMBs have significantly higher surface speed capability than rolling element bearings and they eliminate the potential for product contamination by eliminating the requirement for bearing lubrication. In addition, one of the most promising capabilities for manufacturing applications is the ability of the AMB to act concurrently as both a support bearing and non-invasive force sensor. The feedback nature of the AMB allows for its use as a load cell to continuously measure shaft forces necessary for levitation based on information about the magnetic flux density in the air gaps. This measurement capability may be exploited to improve the process control of such products as textile fibers and photographic films where changes in shaft loads may indicate changes in product quality. This paper discusses the operation of AMBs and their potential benefits in manufacturing equipment along with results from research addressing accurate AMB force sensing performance in field applications. Specifically, results from the development of enhanced AMB measurement algorithms to better account for magnetic fringing and leakage effects to improve the accuracy of this technique are presented. Results from the development of a new on-line calibration procedure for robust in-situ calibration of AMBs in a field application such as a manufacturing plant scenario are also presented including results of Magnetic Finite Element Analysis (MFEA) verification of the procedure.

Special session — Attracting and supporting military veterans in engineering programs

The Post-9/11 GI Bill overhauled the educational benefits available to military veterans. Additionally, the Yellow Ribbon Program makes private institutions more affordable to veterans. Consequently, more veterans are seeking undergraduate degrees, and many schools are seeing significant numbers of veterans for the first time. The National Science Foundation (NSF) has recognized that veteran engineers may address shortages in the engineering workforce and has funded projects to develop models for helping veterans make the transition from active duty to successful student. This special session provides a forum for several awardees to describe their work and to engage a larger audience in discussions about engineering education for veterans. The goals for this session are to raise awareness in the engineering education community about the issues facing veteran engineering students, identify issues commonly encountered as veterans transition from service to the classroom, promote the dissemination of results from NSF-supported efforts to support veterans in engineering programs, provide a forum for sharing best practices related to the successful transition of a veteran from the military to engineering programs, and establish relationships between schools with the shared interest of serving military veteran students.

Magnetic Bearings for Non-Destructive Health Monitoring of Rotating Machinery Supported in Conventional Bearings

This paper describes initial results from a project expanding the field of rotor health monitoring by using Active Magnetic Bearings (AMBs) as actuators for applying a variety of known force inputs to a spinning rotor in order to monitor and evaluate response signals resulting from these inputs on-line. Similar to modal analysis and other nondestructive evaluation (NDE) techniques which apply input signals to static structures in order to monitor responses; this approach allows for the measurement of both input and output response in a rotating system for evaluation. However, unlike these techniques, the new procedure allows for multiple forms of force input signals to be applied to a rotating structure. This technique is being developed for use on rotating equipment supported in conventional bearings where an AMB actuator is added to a system for improved health monitoring. This paper presents initial results from this project including a demonstration of the system identification capability of the procedure during the commissioning of a test rig, and a summary of a technique developed for identifying breathing-cracks in rotors using the new technique.

Enhancements to AMB Force Measurement Procedures for Application to a Rocket Thrust Measurement System

Active Magnetic Bearings (AMBs) can be used concurrently as support bearings and as load cells for the measurement of support forces. This paper discusses the preliminary design for a test rig to simulate a full-scale rocket thrust measurement system utilizing AMBs and procedures that have been developed to enhance the use of AMBs in this application. These enhancements include the development of a model of the effect of fringing on force measurements and an on-line calibration procedure. The fringing effect model is based on actuator geometry and has been verified through finite element analyses. On-line calibration procedures for a magnetic thrust bearing arrangement have also been developed and verified through finite element analyses. Since the rocket thrust measurement system is not rotating, planar magnetic bearings will also be used for vertical support and will utilize the same calibration procedure, resulting in force and torque measurements in all six degrees of freedom.Copyright

A System Identification Technique Using Bias Current Perturbation for Determining the Effective Rotor Origin of Active Magnetic Bearings

Locating the effective rotor origin of an active magnetic bearing (AMB) is an important step toward accurate characterization of the bearing air gaps for field tuning, performance analyses, and some shaft force measurement techniques. Specifically, application of current-based force measurement techniques to AMBs requires accurate modeling of air gaps in order to predict dynamic forces with accuracy. This paper discusses the application of a system identification technique that employs perturbation of the bias current and allows the user to establish the location of the effective rotor origin, an important step in characterizing the actual bearing gap. The technique analyzes the AMB systems response to the perturbation of bias currents in conjunction with a magnetic circuit model to infer the center position. The effective rotor origin identification technique developed here does not require additional hardware and is suitable for use in the general class of AMBs in field applications. For our purposes, the effective rotor origin of an electro-magnet biased magnetic bearing is defined as the unique rotor location for which a magnetic circuit based force model of the bearing is satisfied for zero position offset of the rotor along each control axis. Note that the effective rotor origin referred to here is the radial origin.