| 1. |
- Castano, Miguel, et al.
(författare)
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New methods for interaction analysis of complex processes using weighted graphs
- 2012
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Ingår i: Journal of Process Control. - : Elsevier BV. - 0959-1524 .- 1873-2771. ; 22:1, s. 280-295
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Tidskriftsartikel (refereegranskat)abstract
- The selection of the structure of a controller in large scale industry processes usually requires extensive process knowledge. The aim of this paper is to report new results on recently suggested methods for the analysis of complex processes. These methods aid the designers in comprehending a process by representing structural and functional relationships from actuators and process disturbances to measured or estimated variables. The methods are formulated in a flexible framework based on graph theory, which can also be used for closed-loop analysis. Additionally, the sensitivity of the methods to scaling and time delays are discussed and resolved. It is also proposed how filtering can be used to restrict the analysis to a frequency region of interest. The feasibility of the methods is shown by the use of three case studies. A quadruple tank process is used to exemplify the methods and their use. Then the methods are applied on a real-life process, the stock preparation plant of a pulp and paper mill. The third study case analyzes a previously published example in closed loop. It is shown that the methods can be used to take efficient decisions on decentralized and sparse control structures, as well as assessing the channel interactions in a closed-loop system.
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| 2. |
- Diehl, Stefan, et al.
(författare)
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Control of an ideal activated sludge process in wastewater treatment via an ODE–PDE model
- 2013
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Ingår i: Journal of Process Control. - : Elsevier BV. - 1873-2771 .- 0959-1524. ; 23:3, s. 359-381
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Tidskriftsartikel (refereegranskat)abstract
- The activated sludge process (ASP), found in most wastewater treatment plants, consists basically of a biological reactor followed by a sedimentation tank, which has one inlet and two outlets. The purpose of the ASP is to reduce organic material and dissolved nutrients (substrate) in the incoming wastewater by means of activated sludge (microorganisms). The major part of the discharged flow through the bottom outlet of the sedimentation tank is recirculated to the reactor, so that the biomass is reused. Only two material components are considered; the soluble substrate and the particulate sludge. The biological reactions are modelled by two nonlinear ordinary differential equations and the continuous sedimentation process by two hyperbolic partial differential equations (PDEs), which have coefficients that are discontinuous functions in space due to the inlet and outlets. In contrast to previously published modelling-control aspects of the ASP, the theory for such PDEs is utilized. It is proved that the most desired steady-state solutions can be parameterized by a natural control variable; the ratio of the recirculating volumetric flow to the input flow. This knowledge is a key ingredient in a two-variable regulator, with which the effluent dissolved nutrients concentration and the concentration profile in the sedimentation tank are controlled. Theoretical results are supported by simulations.
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| 3. |
- Garpinger, Olof, et al.
(författare)
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Performance and robustness trade-offs in PID control
- 2014
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Ingår i: Journal of Process Control. - : Elsevier BV. - 1873-2771 .- 0959-1524. ; 24:5, s. 568-577
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Tidskriftsartikel (refereegranskat)abstract
- Control design is a rich problem which requires consideration of many issues such as load disturbance attenuation, set-point tracking, robustness with respect to process variations and model uncertainty, and effects of measurement noise. The purpose of this paper is to provide insight into the trade-offs between performance and robustness explicitly. This is accomplished by introducing plots that show the trade-offs for PI and PID control. These also provide valuable understanding of design compromises used for common PI design methods.
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| 4. |
- Guzmán, José Luis, et al.
(författare)
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Simple tuning rules for feedforward compensators
- 2011
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Ingår i: Journal of Process Control. - : Elsevier BV. - 1873-2771 .- 0959-1524. ; 21:1, s. 92-102
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes new simple tuning rules for design of feedforward compensators. The problem of feedforward from load disturbances is analyzed, and it is shown that the feedback controller should be taken into account in the feedforward design process. Simple rules based on IAE minimization with restrictions on the process output overshoot and the high-frequency gain of the compensator are then derived. Several simulations are presented along the paper to show the advantages of the proposed design rules.
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| 5. |
- Hast, Martin, et al.
(författare)
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Low-order feedforward controllers: Optimal performance and practical considerations
- 2014
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Ingår i: Journal of Process Control. - : Elsevier BV. - 1873-2771 .- 0959-1524. ; 24:9, s. 1462-1471
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Tidskriftsartikel (refereegranskat)abstract
- Feedforward control from measurable disturbances can significantly improve the performance in control loops. However, tuning rules for such controllers are scarce. In this paper design rules for how to choose optimal low-order feedforward controller parameter are presented. The parameters are chosen so that the integrated squared error, when the system is subject to a step disturbance, is minimized. The approach utilizes a controller structure that decouples the feedforward and the feedback controller. The optimal controller can suffer from undesirable high-frequency noise characteristics and tuning methods for how to filter the control signal are also provided. For scenarios where perfect disturbance attenuation in theory is achievable but where noise-filtering is needed, the concept of precompensation is introduced as a way to shift the controller time-delay to compensate for the low-pass filtering.
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| 6. |
- Jacobsen, Elling W., et al.
(författare)
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Robustification as a tool in modeling biochemical reaction networks
- 2011
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Ingår i: Journal of Process Control. - : Elsevier BV. - 0959-1524 .- 1873-2771. ; 21:10, s. 1517-1525
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Tidskriftsartikel (refereegranskat)abstract
- Biological functions have evolved to become robust against a multitude of perturbations such as gene mutations, intracellular noise and changes in the physical and chemical environment. This robustness should be reflected in models of the underlying biochemical networks, and robustness analysis has consequently been established as an important tool for model validation in systems biology. However, while robustness analysis can be used to invalidate a given model, it does not support the postulation of model modifications that can serve to improve the robustness. Herein we propose a method for this purpose, based on computing the sensitivity of the robustness with respect to generic dynamic perturbations applied to the individual network edges. To quantify robustness we compute the smallest simultaneous change in the activity of the network nodes that induces a bifurcation in the network, resulting in a qualitative change in the network behavior. The proposed method can be used to identify network interactions with the most significant impact on the overall robustness of a given function. By considering the impact of adding new nodes and edges, the method can also be used to postulate the presence of important unmodeled components and interactions. The focus here is on biological functions related to bistable switches and sustained oscillations, and the proposed methodology is demonstrated through application to metabolic oscillations in white blood cells and bistable switching in MAPK signal transduction. The classical Goodwin model of oscillations in a simple gene regulatory network is used for illustration throughout the paper.
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| 7. |
- Kirtania, Kawnish, et al.
(författare)
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A novel dead time compensator for stable processes with long dead times
- 2012
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Ingår i: Journal of Process Control. - : Elsevier BV. - 0959-1524 .- 1873-2771. ; 22:3, s. 612-625
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a new and simplified approach for the design of dead time compensators for processes with long dead times. The approach is based on a modified structure of the Smith predictor that allows the user to isolate the disturbance and set-point responses and thereby, provide a two-degrees-of- freedom control scheme. The proposed structure is easy to analyze and tune. Using an estimation of the dead time and process model of the plant, the proposed compensator is left with two tuning parameters that determine the closed-loop performance and robustness. The performance of the proposed compensator is compared with the most recent dead time compensator appeared in the literature. The method is evaluated on two simulated processes and a computer-interfaced pilot-scale two tank heating system to demonstrate the practicality and utility of the proposed scheme. © 2012 Elsevier Ltd. All rights reserved.
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| 8. |
- Lindholm, Anna, et al.
(författare)
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Minimization of economical losses due to utility disturbances in the process industry
- 2013
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Ingår i: Journal of Process Control. - : Elsevier BV. - 1873-2771 .- 0959-1524. ; 23:5, s. 767-777
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Tidskriftsartikel (refereegranskat)abstract
- A process industrial site may consist of several production areas, some producing intermediate products for further refinement in other areas, and some producing end products. The areas may share the same utilities, such as steam and cooling water, which means that the areas could be connected both by the flow of products through the site and by the use of the same utilities. Management of utility disturbances thus becomes an interesting topic. In this paper, a simple approach for modeling utilities is suggested and used to formulate a mixed-integer quadratic program (MIQP) that aims at minimizing the total economic loss at the site, due to utility disturbances. The optimization problem is reformulated as an ordinary quadratic program (QP), where auxiliary variables are utilized to avoid the use of integer variables. For suitable choices of the optimization weights, the solutions to the MIQP and the QP are in many cases equal. Two examples are given, where one is a small example inspired by a real site at the specialty chemicals company Perstorp, and the second is a larger problem that aims to show the advantage of the QP formulation when the number of areas, and thus the number of integer variables, becomes large.
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| 9. |
- Maggio, Martina, et al.
(författare)
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The PID+p controller structure and its contextual autotuning
- 2012
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Ingår i: Journal of Process Control. - : Elsevier BV. - 1873-2771 .- 0959-1524. ; 22:7, s. 1237-1245
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Tidskriftsartikel (refereegranskat)abstract
- A previous paper addressed the "PI+p" controller (a PI augmented with an additional, stable pole) and demonstrated its advantages over standard PIs, particularly if tuned with a combined model-relay-based method along the "contextual" approach proposed in another recent work. This manuscript extends the scenario to the PID+p structure, with obvious meaning, and applies the contextual approach to its autotuning. Benchmark tests and a quite realistic simulation example support the proposal.
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| 10. |
- Rodríguez, C., et al.
(författare)
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Generalized feedforward tuning rules for non-realizable delay inversion
- 2013
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Ingår i: Journal of Process Control. - : Elsevier BV. - 1873-2771 .- 0959-1524. ; 23:9, s. 1241-1250
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes the problem of load disturbance rejection using feedforward for the case when perfect compensation is not realizable due to processes delays inversion. First, a new generalized open-loop design tuning rule for feedforward compensators is proposed. Simple and intuitive guidelines are given for aggressive, moderate, and conservative performance results. Moreover, a switching feedforward compensator is proposed to be used when oscillations are not allowed in the process output. Simulation results show important improvements in disturbance rejection, reference tracking error and control effort.
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