The article brings a brief revision of the two-degree-of-freedom (2-DoF) internal model control (IMC) and the 2-DoF Smith-Predictor-based (SP) control of unstable systems. It shows that the first important reason for distinguishing between these approaches is the limitations of the control action. However, it also reminds that, in addition to the seemingly lucrative dynamics of transients, the proposed approaches can conceal a tricky behavior with a structural instability, which may manifest itself only after a longer period of time. Instead, as one of possible reliable alternatives, two-step IMC and filtered Smith predictor (FSP) design are applied to unstable first-order time-delayed (UFOTD) systems. Firstly, the 2-DoF P controller yielding a double real dominant closed loop pole is applied. Only then the 2-DoF IMC or FSP controllers are designed, providing slightly slower, but more robust transients. These remain stable even in the long run, while also showing increased robustness.

中文翻译：

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基于2DOF IMC和Smith预估器的稳定一阶时滞不稳定工厂控制

本文对不稳定系统的两自由度（2-DoF）内部模型控制（IMC）和基于2-DoF Smith-Predictor（SP）的控制进行了简要修订。它表明，区分这些方法的第一个重要原因是控制动作的局限性。然而，它也提醒我们，除了看似有利可图的瞬态动力学，所提出的方法还可以掩盖具有结构不稳定的棘手行为，这种行为只有在较长一段时间后才会显现出来。相反，作为可能的可靠替代方法之一，将两步式IMC和滤波的Smith预估器（FSP）设计应用于不稳定的一阶时间延迟（UFOTD）系统。首先，应用产生双实数主导闭环极点的2-DoF P控制器。只有这样，才能设计2自由度IMC或FSP控制器，以提供稍慢但更稳定的瞬态。即使从长远来看，它们仍然保持稳定，同时还显示出更高的鲁棒性。