Non-integral dead time in computer-controlled systems causes a zero to be created on the real axis in the z-plane, the position of which can be outside the unit circle or near the ringing pole position (z = -1). Generalized predictive control (GPC), under certain circumstances, uses an inverse plant model to place closed-loop poles on open loop zeros. Non-minimum phase systems and some systems with fractional dead time can have zeros that are outside the unit circle or near the ringing pole. GPC, under certain circumstances, would place closed-loop poles in positions that result in undesirable ringing or instability in the closed-loop system. An analysis into the effect of the GPC control variables, plant gain and fractional dead time on the closed-loop poles positions is presented.
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