Decentralized Control Structures
Researchers: Martin Hast and Tore Hägglund
There is an unfortunate gap between the centralized computational approaches of multi-variable control theory and the common practice to design local control loops disregarding couplings and interaction. Today it appears that both approaches has reached a point of refinement where the gap can be reduced from both sides. This project aims to revise and improve the basic modules for decentralized control, and to develop new. The ideas to be investigated in this project are relevant not only for process control but is also of interest for general classes of multi-variable systems.
TITO control
We will develop a new module building on experiences from PID control: a TITO controller, i.e. a controller with two inputs and two outputs. To be accepted in process control, the TITO controller must be fully automatic without any parameters to be set by the user. It means that an automatic tuning procedure has to be developed. Anti-windup can e.g. be handled conveniently.
In this project, we have established collaboration with University of Córdoba in Spain. An alternative decoupling structure, inverted decoupling, has been investigated. This structure has several important advantages compared with conventional decouplers, especially when it comes to implementation of the TITO controller. Using inverted decoupling, it is possible to implement the decoupling using standard blocks in most DCS systems just by adding feedforward signals to the two PID controllers. Industrial collaboration has also been initiated in the project.
Feedforward from load disturbances
Feedforward is a powerful method to improve the performance of feedback loops. Feedforward can be made both from setpoint and measurable load disturbances. In this project, the goal is to improve both structures and design methods for feeforward control from load disturbances.
The basic idea for design of feedforward compensators is simple. The ideal compensator is formed as the dynamics between the load disturbance and the process output, divided by the dynamics between the control signal and the process output, with reversed sign. However this ideal compensator is seldom realizable. Therefore, there is a need for design methods. There are surprisingly few such methods presented in the literature, and the methods do normally not take the feedback control into account in the design.
In a first phase, new simple tuning rules for feedforward compensators have been derived. The design objective is to minimize IAE without getting any overshoot in step load disturbance responses. This work has been done in collaboration with University of Almeria in Spain.
Using a structure that decouples feedback and feedforward action, optimal design rules that minimizes ISE has been developed. The used structure simplifies tuning of the feedforward controller by allowing the controller to be tuned with respect to the open-loop system while maintaining its properties and performance when used in a closed-loop setting.
Publications
Tore Hägglund: "Signal Filtering in PID Control". In IFAC Conference on Advances in PID Control, Brescia, Italy, March 2012.
Juan Garrido, Francisco Vázquez, Fernando Morilla, Tore Hägglund: "Practical advantages of inverted decoupling". Proceedings of the Institution of Mechanical Engineers, Part I: Journal of Systems and Control, 225:7, pp. 977–992, November 2011.
José Luis Guzmán, Tore Hägglund, Karl Johan Åström, Sebastián Dormido, Manuel Berenguel, Yves Piguet: "Feedforward Control Concepts through Interactive Tools". In 18th IFAC World Congress, Milano, Italy, August 2011.
José Luis Guzmán, Tore Hägglund: "Simple tuning rules for feedforward compensators". Journal of Process Control, 21:1, pp. 92-102, January 2011.
José Luis Guzmán, Karl Johan Åström, Tore Hägglund, Sebastián Dormido, Manuel Berenguel, Yves Piguet: "Interactive Learning Module for Control Interaction Understanding". In European Control Conference, Budapest, Hungary, August 2009.
Olof Garpinger, Tore Hägglund: "A Software Tool for Robust PID Design". In Proc. 17th IFAC World Congress, Seoul, Korea, July 2008.
Alfred Theorin: "Implementation of an Autotunable Decoupling TITO Controller". Master's Thesis ISRN LUTFD2/TFRT--ISRN LUTFD2/TFRT--5800--SE--SE, Department of Automatic Control, Lund University, Sweden, July 2007.
Pontus Nordfeldt, Tore Hägglund: "Decoupler and PID Controller Design of TITO Systems". Journal of Process Control, 16:9, pp. 923-936, October 2006.
Pontus Nordfeldt: "PID control of TITO systems". Licentiate Thesis ISRN LUTFD2/TFRT--ISRN LUTFD2/TFRT--3238--SE--SE, Department of Automatic Control, Lund University, Sweden, December 2005.
Pontus Nordfeldt, Tore Hägglund: "Design of PID Controllers for Decoupled Multi-Variable Systems". In Proc. 16th IFAC World Congress, Prague, Czech Republic, July 2005.
2012-01-23