Special Issue: CoDIT’17 Proc IMechE Part I: J Systems and Control Engineering 1–14 Ó IMechE 2018 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0959651818755957 journals.sagepub.com/home/pii Limits of a simplified controller design based on integral plus dead time models Mikula ´s ˇ Huba and Igor Be ´lai Abstract This article presents design and evaluation of filtered proportional–integral controllers and filtered Smith predictor– inspired constrained dead time compensators. Both are based on the integral plus dead time and on the first-order time delayed plant models. They are compared as for tuning simplicity, robustness and noise attenuation. Such a comparison, which presents a robustness test regarding the importance of the internal plant feedback approximation, may be carried out by performance measures built on deviations of the input and output transient responses from their ideal shapes. When combined with integral of absolute error measures of both solution types with the disturbance responses set as nearly equivalent, we can see that the filtered Smith predictor setpoint responses may be significantly faster than the fil- tered proportional–integral controller responses, more robust and, using higher-order filters, also sufficiently smooth. Furthermore, tuning of the possibly higher-order filters for filtered Smith predictor is simpler. Its overall design is more transparent and straightforward with respect to the control constraints, where the filtered Smith predictor requires some additional anti-windup measures. Keywords Filtered proportional–integral control, dead time compensator, filtered Smith predictor, constrained control, controller tuning Date received: 5 July 2017; accepted: 7 December 2017 Introduction Proportional–integral (PI) control represents the most widespread control structure used for systems with non- measured disturbances. 1 It is still being developed both from the point of view of tuning (as, for example, dif- ferent robust approaches) 2 and differently modified structures such as PI+Clegg integrator (CI) reset con- troller, 3 filtered proportional–integral (FPI) control 4 or the latest data-driven approaches 5 adding new func- tionality. The PI controller design could also be trans- formed into an output feedback control procedure. 6,7 The aims are to automatize the controller design as much as possible by simultaneously obtaining reliable results with an improved performance. A reliable PI application frequently requires that the problems of integrator windup 8,9 are solved. The inte- grator windup may directly be avoided by different model-based approaches, such as the state observers extended by states corresponding to disturbances, 10–12 the internal model control (dealing primarily with output disturbances 13 ), the disturbance observer (DOB)-based control proposing different solutions for input disturbances 14 and so on. In the literature, the Smith predictor (SP) 15 has rep- resented one of the most frequently used control struc- tures for systems with long dead time. Despite its 60th anniversary, and probably due to the still existing potential to improve the closed-loop performance accompanied with several tricky properties, this struc- ture even now attracts attention of the research com- munity. 16–22 However, in the original version, it was proposed just for the stable plants. In the following decades, an extremely high number of contributions appeared. These dealt with the robustness aspects and extension to integral and unstable time delayed sys- tems. 23–31 Since the PI control may be derived from a model-based control by a simplified substitution FEI STU Bratislava, Bratislava, Slovakia Corresponding author: Mikula ´s ˇ Huba, FEI STU Bratislava, Ilkovic ˇova 3, 812 19 Bratislava, Slovakia. Email: mikulas.huba@stuba.sk