Bernd Riemann

Bio: Bernd Riemann is an academic researcher from Technische Universität Darmstadt. The author has contributed to research in topics: Control theory & Rotor (electric). The author has an hindex of 6, co-authored 10 publications receiving 84 citations.

Real gyroscopic uncertainties in robust control of flexible rotors

Abstract: Control laws for flexible high-speed rotors need to account for gyroscopic effects, resulting in a dependency of linear plant models on the rotational speed of the rotor. When these variations are captured by uncertainties in a robust control manner, it is common to employ unstructured or structured complex model perturbations. In order to reduce design-induced conservatism, the synthesis setup presented in this paper deploys a real parametric uncertainty to account for speed-dependent terms of the plant model. The resulting Linear Fractional Transformation (LFT) decomposition of the system can be tackled appropriately using mixed μ synthesis techniques. Suitable approaches for explicit treatment of such problems include (D,G)-K and μ-K algorithms, modifications of the latter being suggested in this paper. To demonstrate potential improvements achievable when using real parametric uncertainties for active vibration control of flexible high-speed rotors via mixed μ synthesis, the methodology is applied with respect to a particular test rig exposed to severe gyroscopic effects. At the hand of this system, efficient performance measures are suggested, leading to promising results both in simulations and validating experiments.

Robust control of flexible high-speed rotors via mixed uncertainties

Abstract: Active vibration control for flexible high-speed rotors tends to be a particularly challenging problem due to the influence of gyroscopic terms, resulting in the need for speed-dependent system models. This paper addresses robust control of such systems, using Linear Fractional Transformations (LFTs) for decoupling the system model into speed-dependent and -independent components in LFT feedback. Based on the resulting LFT decomposition, the speed-dependent terms are efficiently reduced in order and considered uncertain with respect to the rotational speed of the shaft. The resulting perturbations are augmented by complex, additive uncertainties and explicitly used for control synthesis. Defining semi-modal performance measures, the perturbed open-loop systems are well-suited for mixed μ synthesis techniques. In particular, (D,G)-K and μ-K algorithm, both enabling explicit treatment of mixed perturbations, are investigated in approaching the robust vibration attenuation problem across the range of operating speeds.

State Feedback in the Context of a Gyroscopic Rotor using a Disturbance Observer

Abstract: In this article, a Disturbance Observer is utilized to realize state feedback in the context of a rotor test rig. The rotor is actively supported by means of piezoelectric stack actuators and subject to unbalance excitation and gyroscopic effect. The presence of gyroscopic effect leads to a dependence of the system dynamics on rotary frequency of the shaft. Due to unknown disturbances in the form of unbalance excitation and system deviation due to gyroscopic effect, ordinary linear time invariant observers fail to observe the system states accurately for gyroscopic rotors, possibly leading to significant control performance reduction. To overcome this problem, a Disturbance Observer is applied to the problem. It is shown that the gyroscopic effect can be approximated accurately by an additive term in the state space equation and can thus be treated as an additional disturbance in Disturbance Observer design. Due to high steady state estimation accuracy, the presence of the Disturbance Observer does not affect steady sta te control performance and thus, controller and observer design are decoupled regarding control performance. Due to this fact, the controller can be designed prior to the observer despite system deviation due to gyroscopic effect and disturbances. However, since the separation principle does not hold for the system, stability proof has to be carried out by consideration of the entire closed loop system for all rotational frequencies within the operating range. A Linear Quadratic Regulator is used as a controller for the sake of simplicity. However, the proposed observer structure is applicable to arbitrary state space controllers. The resulting controller- observer combination is validated in simulation and experiment. Index Terms—Active Vibration Control, Disturbance Observer, Gyroscopic Effect, Rotordynamics, State Space Control

Linear Quadratic Regulation of a Rotating Shaft being Subject to Gyroscopic Effect

Abstract: A Linear Quadratic Regulator and a Kalman Filter are designed for a rotor test rig being subject to unbalance excitation and gyroscopic effect. Rotor vibration is controlled by means of two piezoelectric stack actuators installed at one of the two supports of the rotor. The presence of gyroscopic effect leads to an undesirable dependence of the system dynamics on rotational frequency of the shaft. As a result, there is a need for high robustness and furthermore, the separation principle does not hold. Due to the latter aspect, controller and observer design become a coupled problem in the case of the rig. In a first step, the number of free design parameters of the controller-observer combination is reduced to a manageable number of 5. Subsequently, these parameters are determined by means of a genetic optimization algorithm on the basis of a Finite Element model of the test rig. It is shown, that it is possible in this way to determine a controller-observer combination leading to robust stability and excellent performance in the whole operating range which contains two unbalance induced resonances. Control performance is validated in simulation as well as experiments at the test rig.

Study of Nonsingular Fast Terminal Sliding-Mode Fault-Tolerant Control

Abstract: This paper studies fault-tolerant control (FTC) designs based on nonsingular terminal sliding-mode control and nonsingular fast terminal sliding-mode control (NFTSMC). The proposed active FTC laws are shown to be able to achieve fault-tolerant objectives and maintain stabilization performance even when some of the actuators fail to operate. In comparison to existing sliding-mode control (SMC) fault-tolerant designs, the proposed schemes not only can retain the advantages of traditional SMC, including fast response, easy implementation, and robustness to disturbances/uncertainties, but also make the system states reach the control objective point in a finite amount of time. Moreover, they also resolve the potential singularity phenomena in traditional terminal and faster terminal SMC designs; meanwhile, the proposed NFTSMC fault-tolerant scheme also possesses the benefit of faster state convergence speed of NFTSMC. Finally, the proposed analytical results are also applied to the attitude control of a spacecraft. Simulation results demonstrate the benefits of the proposed schemes.

Hydrodynamic lubrication: Experiment with ‘Floating’ drops

Abstract: This article gives the principle of hydrodynamic lubrication and also presents the new phenomenon of levitating drops over liquid film flow, which is explained using hydrodynamic lubrication theory.

Regelungstechnik. Einführung in die Methoden und ihre Anwendung

Abstract: Professor Otto Föllinger (1924–1999) leitete seit 1965 den Lehrstuhl für Regelungsund Steuerungssysteme an der Universität Karlsruhe. Er entwickelte das Karlsruher Institut zu einem Leuchtturm der akademischen regelungstechnischen Ausbildung, der auf den ganzen deutschen Sprachraum ausstrahlte. Neben Föllingers zahlreichen monographisch angelegten Lehrwerken erschien seine umfangreiche „Einführung in die Methoden der Regelungstechnik und ihre Anwendung“ seit 1972 wiederholt in durchgesehenen und verbesserten Auflagen. Nun haben die Professoren Ulrich Konigorski (TU Darmstadt), Boris Lohmann (TU München), Günter Roppenecker (Uni Erlangen-Nürnberg), Ansgar Trächtler (Uni Paderborn), die zwischen 1983 und 1991 an Föllingers Karlsruher Institut promoviert worden waren, die „Regelungstechnik“ ihres gemeinsamen Lehrers erneut überarbeitet und als „11., völlig neu bearbeitete Auflage“ drucken lassen. Im Vorwort der Neuauflage, die imSeptember 2013 erschien, nehmen die Bearbeiter Bezug auf die 6. Auflage, die Otto Föllinger 1990 veröffentlichen ließ, und skizzieren die ihnen nennenswerten Änderungen. Bei der Vorbereitung dieser Buchbesprechung hat der Berichterstatter deshalb die 6. und die 11. Auflage parallel aufgeschlagen und abschnittsweise verglichen. Alle Abschnitte der 6. Auflage, die sich mit Hardwareund Softwarerealisierungen, wie sie Ende der 1980er Jahre aktuell waren, befassten, konnten entfallen. Statt der früherenAngaben von Struktogrammenundnumerischen Auswertungenfindet der Leser nunHinweise auf die Simulationsmöglichkeiten mit MATLAB. Die Anzahl der Kapitel wurde von 17 auf 11 reduziert, über deren Inhalt nun berichtet wird: