Detection and compensation for stiction in multi-loop control systems

  • Author / Creator
    Alemohammad, Mahdi
  • Unsatisfactory performance of a control system may have different root causes, of which diagnosis and control have been subjects of interest. Numerous approaches have been used to identify the source of the oscillatory behavior of control systems. This work will focus on the nonlinearities introduced by process equipment, more specifically, static friction (stiction) in control valves. Using shape-based stiction detection methods and surrogate testing for time series, a new detection method is proposed for systems containing one or more sticky valves. Performance of this method is validated by both simulation and industrial data. The existence of stiction in a control valve may lead to oscillations in all loops of the process. In this work, frequency analysis of multi-loop processes oscillating due to stiction will be presented. Derivation of a general mathematical representation of the condition, under which oscillations occur in a multi-loop system because of stiction, is the contribution of the proposed analysis. The proposed condition for occurrence of oscillations provides a compensation framework for this problem. In this scheme, given dynamics of the system and severity of stiction, the appropriate tuning for the controller will be found which reduces or removes oscillations from the system. An alternative compensation algorithm will also be proposed, which aims removal of oscillations from systems for which the previously proposed approach cannot permanently remove undesirable oscillations. Achieving a non-oscillatory output without making the valve stem to move more aggressively, is the main characteristic of this algorithm.

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  • Type of Item
  • Degree
    Master of Science
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    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.