Rainer Nordmann

Active magnetic bearings for the identification and fault diagnosis in turbomachinery

Abstract Active magnetic bearings, a typical mechatronic product, have been successfully applied in industrial turbomachinery. Their main advantages are the contactless working principle, the frictionless suspension, and that they represent an active system. Therefore, the active magnetic bearings are well suited to operate contactless as actuator and sensor elements in rotating machinery. The aim of the presented projects of the Special Research Program (SFB 241) supported by the German Research Council (DFG) is to use active magnetic bearings as an identification and diagnosis tool for turbomachines. The identification and diagnosis procedures are based on frequency response functions. For this type of diagnosis using transfer functions, an accurate force measurement is crucial. The paper compares force measurement results and achievable accuracies of a radial magnetic bearing using different measurement techniques over a large operating range. Furthermore, a state-space model of the entire mechatronic system is developed to facilitate the diagnosis procedure.

Identification of Modal Parameters of an Elastic Rotor with Oil Film Bearings

Investigations of the dynamic behavior of structures have become increasingly important in the design process of mechanical systems. To gain a better understanding of the dynamic behavior of a structure, knowledge of the modal parameters is very important. The powerful method of experimental modal analysis has been used to measure modal parameters in many mechanical engineering problems. But the method has been mainly applied to nonrotating structures. This lecture presents improvements of the classical modal analysis for a successful application in rotating machinery with nonconservative effects. An example is given, investigating the modal parameters of an elastic rotor with oil film bearings.

Scanning-Based Human-Computer Interaction Using Intentional Muscle Contractions

It has already been shown in the past that it is possible to leverage tiny muscular contractions produced at will (e.g., by frowning) in order to give someone complete control over a PC [1]. The underlying interaction technique is ideal for persons with severe motor impairments who are in need for an alternative, non-standard way to operate a computer. This paper deals with a scanning-based computer application of that approach to enable its user to control the immediate environment, e.g., by making a phone call, toggling the lights, or sending particular Infra-Red (IR) remote signals. Although the software is primarily targeted at people with disabilities, it is ready --- and (in certain situations) even expected --- to be used by able-bodied individuals as well. A user study evaluating the remote control module of the system has been conducted with twelve non-impaired subjects, and the results are discussed herein.

Fault Diagnosis in a Centrifugal Pump Using Active Magnetic Bearings

The number of rotors running in active magnetic bearings (AMBs) has increased over the last few years. These systems offer a great variety of advantages compared to conventional systems. The aim of this article is to use the AMBs together with a developed built-in software for identification, fault detection, and diagnosis in a centrifugal pump. A single-stage pump representing the turbomachines is investigated. During full operation of the pump, the AMBs are used as actuators to generate defined motions respectively forces as well as very precise sensor elements for the contactless measurement of the responding displacements and forces. In the linear case, meaning small motions around an operating point, it is possible to derive compliance frequency response functions from the acquired data. Based on these functions, a model-based fault detection and diagnosis is developed which facilitates the detection of faults compared to state-of-the-art diagnostic tools which are only based on the measurement of the systems outputs, i.e., displacements. In this article, the different steps of the model-based diagnosis, which are modeling, generation of significant features, respectively symptoms, fault detection, and the diagnosis procedure itself are presented and in particular, it is shown how an exemplary fault is detected and identified.

Evaluating the Hands-Free Mouse Control System: An Initial Case Study

An initial user study evaluating the HAnds-free Mouse COntrol System (HaMCoS) will be presented. The system allows its user to fully operate a Windows® PC without using the hands. It requires a special piezo-based input sensor in order to pick up the muscular activity of a single dedicated muscle of the user. The idea is to detect intentional contractions of the monitored muscle, and to use this information for emulating a two-button mouse device. In addition, the software framework renders any keyboard input unnecessary, so HaMCoS represents a comprehensiveinput method for persons who are unable to reliably employ the hands (e.g., tetraplegics). The paper will take a closer look at the specifics of the system followed by a discussion of the experiences gained by potential end users in a first case study.

How to operate a PC without using the hands

A demo of a biosignal interface, which allows to operate a Windows® PC without using the hands, shall be given. The system -- called HaMCoS (for Hands-free Mouse Control System) -- enables its user to simulate clicks and movements of the computer mouse by issuing intentional contractions of a single muscle of choice only. Therefore, by employing HaMCoS, even a person with very severe physical disabilities can operate a PC, provided everything exclusively relies on mouse input. The framework built around the systems Main Module is optimized in this respect, since it offers a comfortable keyboard-free user interface (e.g. comprising large, easily clickable buttons).

Brain-controlled finite state machine for wheelchair navigation

This proposal is about a brain-controlled electrically powered wheelchair. The system comprises a brain-computer interface based on steady-state visual evoked potentials and a processing unit relying on a finite state machine (FSM). Results of first simulation experiments comparing two different FSMs are presented.

ANEAS - a modeling tool for nonlinear analysis of active magnetic bearing systems

Abstract Active Magnetic Bearings (AMBs) provide several advantages compared to traditional bearings, therefore they are getting more and more important in the field of rotating machinery, especially in high speed applications, in implementations where high accuracy is important or where it is of interests to use oil free support mechanisms. With the increased relevancy of AMBs, the necessity of accurate models of AMB-systems is arising. Especially for proofing the system reliability under critical conditions, a modeling methodology including a nonlinear description of the system behavior is important. The modeling tool ANEAS fulfils this demand and is presented here. It‘s capability is shown for a typical AMB system for different situations like lift up of the rotor, sudden unbalance, base excitation and a failure of one element, where only nonlinear models provide an adequate accuracy.

Active Bearing of Rotors Utilizing Robust Controlled Piezo Actuators

This paper focuses on the active vibration control of flexible rotors by piezo actuators located at the bearings. At first general relations are derived, that allow to specify the required power and dimensions of the actuators from the balance quality factor of the rotor and the stiffness of the system. Second, two control concepts, integral force feedback and robust control, are compared. The design procedure of the robust µ-synthesis controller including uncertainty modelling and selection of weighting functions is described. Experimental results obtained from a simple test rig are shown. Finally, the design is illustrated by an realistic example.Copyright

A H/sub /spl infin//-weighting scheme for PID-like motion control

For motion control systems PID- or lead-lag-controllers serve well and are industrial standard. But often flexible modes of the mechanism endanger the stability of the control loop and thereby limit the bandwidth of the loop. Tuning and augmenting the controller becomes difficult and expensive at this point. Robust control design methods, first of all the /spl mu/-synthesis, offer a straight design formalism to overcome this problem. Anyhow, the crucial point in H/sub /spl infin// and /spl mu/-synthesis is the proper choice of weighting functions. In most of the known applications of robust control, the weighting functions were shaped ad-hoc. It is often not clear to the engineer how to express the given technical specifications in terms of H/sub /spl infin//-weighting functions. In fact, theory does not give an answer to this question up to now. Therefore, in this paper a H/sub /spl infin//-weighting scheme is presented that leads to closed-loop response similar to the established one obtained by PID-control in the rigid-body range. In this way PID-like high performance motion controllers can systematically be designed in the presence of structural flexibilities. In contrast to other guidelines on weighting functions, the exact shape of the weighting functions is given as well as specific numbers of their parameters.