No abstract

The successful industrial application of flexible rotors supported on active magnetic bearings (AMBs) requires careful attention not only to rotordynamic design aspects, but also to electromagnetic and feedback control design aspects. This paper describes the design, construction and modeling process for an AMB test rig which contains a 1.23m long flexible steel rotor, with a mass of 44.9 kg and two gyroscopic disks. The rotor typifies a small industrial centrifugal compressor designed to operate above 12,000 rpm and the first bending natural frequency. There are four AMBs — two AMBs at the shaft ends to support the shaft with a combined load capacity of 2600N and two additional AMBs at the mid and quarter spans to allow for the application of simulated destabilizing fluid or electromagnetic forces to the rotor. Simulated aerodynamic cross coupling stiffness values are to be applied to the rotor through these two internal AMBs with the goal of developing stabilizing robust controllers. The unique design allows multiple support and disturbance locations providing the ability to represent a variety of machine configurations, e.g., between bearing and overhung designs. The shaft transfer function in lateral movement has been developed with finite element model and then verified by experimental frequency response measurements. Models for the power amplifiers, position sensors, signal conditioning and data converter hardware were developed, verified experimentally and included in the overall system model. A PID controller was developed and tuned to levitate the rotor and enable further system characterization.

Most industrial rotors supported in active magnetic bearings (AMBs) are operated well below the first bending critical speed. Also, they are usually controlled using proportional, integral and derivative controllers, which are set up as modally uncoupled parallel and tilt rotor axes. Gyroscopic effects create mode splitting and a speed-dependent plant. Two AMBs with four axes of control must simultaneously control and stabilize the rotor/AMB system. Various analyses have been published considering this problem for different rotor/AMB configurations. There has not been a fully dimensionless analysis of these rigid rotor AMB systems. This paper will perform this analysis with a modal PD controller in terms of translation mode and tilt mode dimensionless eigenvalues and eigenvectors. The number of independent system parameters is significantly reduced. Dimensionless PD controller gains, the ratio of rotor polar to transverse moments of inertia and a dimensionless speed ratio are used to evaluate a fully general system stability rigid rotor analysis. An objective of this work is to quantify the effects of gyroscopics on rigid rotor AMB systems. These gyroscopic forces reduce the system stability margin. The paper is also intended to help provide a common framework for communication between rotating machinery designers and controls engineers

This dissertation addresses the control challenges for a practical mechatronic system subject to self-excited instability modeled as a parametric uncertainty. Achieving a balance between the conflicting requirements of performance and robustness in the face of system uncertainty is the primary objective of feedback control. Practical issues such as unstable open-loop plant dynamics, finite actuator capacity, the presence structural flexibility and suboptimal sensor/actuator placement limit the achievable performance through the use of feedback.The ROMAC Magnetic Bearing Test Rig for Rotordynamic Instability (MBTRI) is a state-of-the-art experiment designed to investigate algorithms that may affect the region of stability of a rotor-bearing system with respect to rotordynamic instability as a result of aerodynamic cross-coupled stiffness. The onset of rotordynamic instability is a significant challenge to successful design and operation of high speed rotating machinery particularly gas compressors. The unique design of the test rig includes several features of an industrial centrifugal gas compressor with a flexible rotor designed to operate above its first bending critical speed. The impellers and gas seals within compressors are the primary source of load-dependent aerodynamic cross-coupled stiffness forces which can lead to self-excited instability and serious machine damage in the absence of sufficient support damping. During the design phase of rotating machines the accurate prediction of the onset of instability is made difficult by reliance on semi-empirical dynamic models with significant uncertainty. Literature on the rotordynamic instability mechanism reveals that in the presence of optimum support damping, a maximum achievable stability threshold can be derived as a function of physical parameters of the rotor-bearing system. This presents an ideal opportunity for the exploration of optimal robust active vibration control algorithms using active ii magnetic bearings (AMBs). A notable advantage of AMBs is their ability to generate optimal support stiffness and damping characteristics. Unlike passive mechanical bearings, the support characteristics of AMBs may be modified over the operating life of the system without any major hardware changes.A general framework is presented whereby properties of the rotor-AMB system that impose fundamental limitations on achievable performance of the closed-loop system are evaluated as a nominal model of the MBTRI plant dynamic is constructed. This model was validated using system identification techniques, and augmented with uncertainty models representing the effects of variation in parameters such operating speed and the magnitude of the destabilizing stiffness. Using µ-synthesis several robust controllers were designed and implemented on the MBTRI hardware to investigate their effect on the stability threshold. The best controller established a thirty-six percent increase in the stability threshold over an existing benchmark controller. This represented an ...

A percutaneous ventricular assist device (pVAD) is an extracorporeal cardiac assist system that supports the failing ventricle in advanced stage heart failure by bypassing blood from the venous to the arterial circulation through a blood pump. The system can be implanted in a Cath lab using standard interventional techniques, and typically consists of a venous or atrial drainage cannula, the VAD (or blood pump), and an arterial perfusion cannula. Because the device allows clinicians the freedom of choosing the configuration and size of the cannulae based on the patient's body size and the size of the artery, it is extremely difficult but important to be able to predict the amount of blood flow that the device can provide before it is implanted to support the patient. In this paper, we develop a novel method that can be used to accurately predict the mean flow rate that the device can provide to the patient based on the size and configuration of the arterial cannula, the pump speed, and the patient's left atrial and mean arterial pressures. To do this, we first develop a nonlinear electric circuit model for the pVAD. This model includes a speed dependent voltage source and flow dependent resistors to simulate the pressure-flow relationship in the various cannulae in the device. We show that the flow rate through the device can be determined by solving a quadratic equation whose coefficients are scaled depending on the size and configuration of the arterial cannula. The model and prediction method were tested experimentally on a test loop supported by the TandemHeart pVAD (Cardiacassist, Inc., Pittsburgh, PA). A comparison of the predicted flow rates obtained from our method with experimental data shows that our method can predict the flow rates accurately with error indices less than 6% for all test conditions over the entire range of intended use of the device. Computer simulations of the pVAD model coupled to a cardiovascular model showed that the accuracy- of the method in estimating the mean flow rate is consistent over the normal range of operation of the device regardless of the pulsatility introduced by the cardiovascular system. This method can be used as an additional too to assist cardiologists in choosing a proper arterial cannulae configurations and sizes for pVAD patients. It can also be used as a tool to train clinical personnel to operate the device under different physiological conditions.