Stable open loop poles: to cancel or not to cancel?

Stable plant poles can either be shifted by feedback, or they can be approximately canceled by a controller. While it is known that the canceling design impacts on the transients due to input disturbances, it seems little appreciated that the alternative, i.e. the shifting design, impacts on closed loop robustness. In this paper we show that shifting stable poles, rather than canceling them, increases the l₁, 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 and robustness