Brief Slow stable open-loop poles: to cancel or not to cancel

  • Authors:

  • Richard H. Middleton;Stefan F. Graebe

  • Affiliations:

  • Department of Electrical and Computer Engineering, The University of Newcastle, University Drive, Callaghan NSW 2308, Australia;OMV Aktiengesellschaft, MRS-TO/Optimization & Automation, Mannswoertherstr. 28, A-2320 Schwechat, Austria

  • Venue:
  • Automatica (Journal of IFAC)
  • Year:
  • 1999

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Abstract

Stable plant poles can either be shifted by feedback, or they can be approximately cancelled by a controller. While it is known that the cancelling design impacts on the transients due to input disturbances, here we show that the alternative, i.e., the shifting design, impacts on closed-loop robustness. In this paper we consider control loops with integral action and show that shifting slow stable real poles, rather than cancelling them, increases the l"1, H"~, and the point-wise in frequency norms of the complementary sensitivity. Thus, the choice to either cancel or shift any one stable pole emerges as a design issue that requires a deliberate trade-off between input disturbance response versus output disturbance response and robustness.