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  • 1. Bask, Michael
    Dynamic threshold generators for robust fault detection2005Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Detection of faults, such as clogged valves, broken bearings or biased sensors, has been brought more and more into focus during the last few decades. There are two main reasons why faults are important to detect at an early stage. Firstly, faults in safety critical applications, such as aircraft, nuclear reactors, cars and trains, may create risks of personal injuries. Secondly, faults in the manufacturing or process industry, e.g. flotation processes and steel plants, may cause decrease in quality or interruptions of production. A fault detection algorithm consists of two parts, the residual generator, which generates a residual, and the residual evaluator, which compares the residual, or a function of it, with a threshold to determine if a fault is present. The residual generation contains a process model and the residual can be described as a filtered difference between the measured and estimated process outputs. When no fault is present, the residual will be nonzero due to residual disturbances, i.e. measurement disturbances, process disturbances and model uncertainties. Therefore, the residual evaluation must be robust against these disturbances to avoid false alarms. Due to the model uncertainties, the residual is affected by the known input signals, which are, in general, time varying. To achieve a threshold that is as tight to the residual as possible, the threshold should also depend on the known input signals. To make this possible, parametric uncertainty in the process model is considered in this thesis. The dynamic threshold generator is introduced, a dynamic system whose output is the threshold and the inputs are the known process inputs. A dynamic threshold generator is developed for full-state measurement systems, assuming that the residual disturbances are constant and unknown but bounded. This dynamic threshold generator is then generalized to non-full state measurement systems with time-varying but bounded residual disturbances. Both generators depend on the unknown upper bounds of the residual disturbances. These upper bounds are replaced by design parameters, which are determined by minimizing the threshold for a set of fault free data. A nonlinear optimization solution is discussed. It is also shown that the residual generator state vector can always be parameterized such that the designing of the parameters can be done by linear optimization. A part of the generalized dynamic threshold generator is a system whose impulse response is an upper bound to another impulse response. Automatic methods to find realizable upper bounds are derived. To validate the methods in this thesis, two applications have been considered, detection of clogging in the valves of a flotation process and detection of faults in the compressor inlet temperature sensor of a jet engine.

  • 2. Bask, Michael
    On external model controller of vibrations2001Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    A promising way to increase the suppression of vibration in mechanical systems is to apply active damping instead of passive damping, which is most widespread today. An active damper system consists of an actuator, a controller and at least one sensor, usually an accelerometer. The sensor measures the vibration and sends the measurements further to the controller in order to calculate the necessary damping force to be fed back to the mechanical system. A number of questions arises regarding active damper design, namely how to construct the active damper, what kind of sensors to use and where to place them, how to model the vibration and the mechanical system itself, and, last but not least, how to design the controller. This licentiate thesis deals with the controller design issue. The most widely used controller in active vibration control is the Filtered-X Least Mean Square Controller and it is briefly reviewed in this thesis. The main focus of the thesis is however on another structure, the External Model Controller (EMC). The EMC has much more degrees of freedom than the Filtered-X Controller but lacks performance and robust stability analysis as well reliable design methods. The four papers included in the thesis contribute to bridging this gap.

  • 3. Bask, Michael
    et al.
    Johansson, Andreas
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Linear optimization of parameters in dynamic threshold generators2005In: Proceedings of 16th IFAC World Congress: Prague, Czech Republic, July 3 - 8, 2005, Oxford: Elsevier, 2005Conference paper (Refereed)
    Abstract [en]

    A model-based fault detection algorithm for linear systems with uncertain parameters is treated. An error system, bilinear in the uncertainties, generates the residual. The residual is compared to a threshold, which is generated by a linear system with the unknown uncertainty upper bounds as parameters. These unknown uncertainty upper bounds can be substituted by design parameters and this article suggests an algorithm to choose design parameter values such that the threshold is larger than the residual when no fault is present. This parameter design algorithm is applied to a sensor fault detection algorithm for a jet engine.

  • 4. Bask, Michael
    et al.
    Johansson, Andreas
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Model-based supervision of valves in a flotation process2003In: 42nd IEEE International Conference on Decision and Control, Piscataway, NJ: IEEE Communications Society, 2003, p. 744-749Conference paper (Refereed)
    Abstract [en]

    An observer based approach for detecting clogging in valves in flotation processes is investigated. Integral action and linear feedback applied to a nonlinear process model constitutes the observer for which local stability is shown. The integral terms give estimates of the clogging in the valves of the process and this estimate is compared to a constant threshold. Experiments on real data from Boliden's flotation series at the Boliden Area Concentrator, Sweden, show no false alarm during any of the two working conditions, PI control and LQ control. It is also shown that cloggings, simulated by manipulating measurement data, are promptly detected. The observer based fault detection algorithm is compared to an algorithm based on parameter estimation and advantages of the two approaches are highlighted.

  • 5. Bask, Michael
    et al.
    Johansson, Andreas
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Realizable upper bounds for the modulus of impulse responses2005In: SICE 2005 proceedings: SICE annual conference in Okayama : August 8-10, 2005 Okayama University, Japan, Tokyo: Society of Instrument and Control Engineers , 2005, p. 2045-2050Conference paper (Refereed)
    Abstract [en]

    This article treats the problem of finding a linear system whose impulse response is an upper bound for the modulus of the impulse response of another given system. These upper bounds are required for a newly developed fault detection algorithm1). Three different methods to calculate a realizable upper bound for an impulse response, which contains multiple real poles and distinct complex poles, are presented. The triangular inequality and linear optimization are used in the first and second method, respectively. In the third method, the original impulse response is used combined with time-delays. The upper bounds are calculated for a fictitious impulse response and compared with its modulus.

  • 6. Bask, Michael
    et al.
    Johansson, Andreas
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Robust time-varying thresholds for supervision of valves in a flotation process2004In: 2004 43rd IEEE Conference on Decision and Control: Nassau, Bahamas, 14 - 17 December 2004, Piscataway, NJ: IEEE Communications Society, 2004, p. 4305-4310Conference paper (Refereed)
    Abstract [en]

    Robust thresholds for observer-based residuals are developed for the purpose of detecting clogging in the valves of a flotation process. The observer in the residual generator is a linear model of the flotation process extended with integrators and corrected with a linear feedback term. The integral states in the observer constitute the residual. Contribution to the residual comes not only from the faults but also from uncertainties in measurements, estimates and working point To avoid false alarms generated by these uncertainties, robust time-varying thresholds depending on the uncertainties are derived. Experiments on a froth flotation process with four cascade coupled tanks are carried out successfully. The data was provided by Boliden Area Concentrator in Boliden, Sweden.

  • 7. Bask, Michael
    et al.
    Medvedev, Alexander
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Analysis of least-squares state estimators for a harmonic oscillator2000In: Proceedings of the 39th IEEE Conference on Decision and Control: December 12 - 15, 2000, Sydney Convention and Exhibition Centre, Sydney, Australia, Piscataway, NJ: IEEE Communications Society, 2000, Vol. 4, p. 3819-3824Conference paper (Refereed)
    Abstract [en]

    The concept of least-squares observer is revisited. Robustness properties of this class of observers with respect to norm-bounded measurement noise are investigated and shown to be very much dependent on the operator chosen for the observer implementation. For the case of a harmonic oscillator, an explicit observer parameterization in terms of the implementation operator and the oscillator frequency is obtained, and observer's existence conditions are proven and analyzed

  • 8. Bask, Michael
    et al.
    Medvedev, Alexander
    External model controller for active vibration isolation1998Conference paper (Refereed)
  • 9. Bask, Michael
    et al.
    Medvedev, Alexander
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    External model repetitive controller for active vibration isolation1999In: Proceedings of Active 99: International Symposium on Active Control of Sound and Vibration : an international INCE symposium, Fort Lauderdale, Florida, USA, 1999 December 02-04 / [ed] Scott Douglas, Washington, DC: Institute of noise control engineering , 1999, p. 177-188Conference paper (Refereed)
    Abstract [en]

    This paper deals with the External Model Repetitive Controller, a structure that combines the classical repetitive disturbance model (in the form of a time delay with a unit feedback) with a feedback of a disturbance estimate. The latter is often termed the External Model Controller and has a broad spectrum of technical applications, e.g., active vibration control. By analyzing the controller performance under the assumption of a disturbance period modeling error, it is shown that an unacceptable performance degradation might occur even for very small values of the modeling error

  • 10.
    Johansson, Andreas
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Bask, Michael
    Dynamic threshold generators for fault detection in uncertain systems2005In: Proceedings of the 16th IFAC World Congress: Prague, Czech Republic, July 3 - 8, 2005, Oxford: Elsevier, 2005, p. 460-465Conference paper (Refereed)
    Abstract [en]

    The problem of developing robust thresholds for fault detection is addressed. An inequality for the solution of a linear system with uncertain parameters is provided and is shown to be a valuable tool for developing dynamic threshold generators for fault detection. Such threshold generators are desirable for achieving robustness against model uncertainty in combination with sensitivity to small faults.The usefulness of the inequality is illustrated by developing an algorithm for detection of clogging in the valves of a flotation process. Simulations with measurement data show that the algorithm detects faults without generating false alarms.

  • 11.
    Johansson, Andreas
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Bask, Michael
    Dynamic threshold generators for fault detection in uncertain systems2005In: Proceedings of the 16th IFAC World Congress: Prague, Czech Republic, July 3 - 8, 2005, Oxford: Elsevier, 2005Conference paper (Refereed)
  • 12.
    Johansson, Andreas
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Signals and Systems.
    Bask, Michael
    Norlander, Torbjörn
    Dynamic threshold generators for robust fault detection in linear systems with parameter uncertainty2006In: Automatica, ISSN 0005-1098, E-ISSN 1873-2836, Vol. 42, no 7, p. 1095-1106Article in journal (Refereed)
    Abstract [en]

    The problem of developing robust thresholds for fault detection is addressed. An inequality for the solution of a linear system with uncertain parameters is provided and is shown to be a valuable tool for developing dynamic threshold generators for fault detection. Such threshold generators are desirable for achieving robustness against model uncertainty in combination with sensitivity to small faults. The usefulness of the inequality is illustrated by developing an algorithm for detection of sensor faults in a turbofan engine. The proposed algorithm consists of a state observer with integral action. A dynamic threshold generator is derived under the assumption of parametric uncertainty in the process model. Successful simulations with measurement data show that the algorithm is capable of detecting faults without generating false alarms.

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