# Decentralized Control Structures for Process Control

**Researchers:** Tore Hägglund

This project aims to revise, improve, and develop new basic control structures for decentralized control used in the regulatory control layer in process control. However, the ideas to be investigated in this project are relevant in other application areas as well.

### Low-order Feedforward Controllers

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 feedforward 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 for feedforward controllers.

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. This collaboration continues, and University of Brescia has also been involved in the project where e.g. performance indices for feedforward control have been developed.

Using a structure that decouples feedback and feedforward action, optimal design rules that minimizes ISE have 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. The structure also enables independent re-tuning of both the feedback and the feedforward controller. Work has also been done concerning characterization of optimal low-order feedforward controllers and practical considerations for implementation.

PID controllers are often implemented with set-point weighting to improve the response to changes in the reference. By using convex optimization techniques the parameters for the set-point weights can be found efficiently, fast as well as be guaranteed to be globally optimal. By solving an optimization problem to find the optimal set-point weights for a large batch of processes, tuning rules have been found that minimizes IAE. The same optimization framework and formulations can also been used to tune feedforward controllers from measurable disturbances.

### Ratio Control

In ratio control, the control objective is to keep the ratio between two signals, normally flow measurements, at a desired value in spite of variations in the setpoints, load disturbances, and possible control signal saturations. It is also desirable to keep the ratio also in situations when one of the controllers is switched to local setpoint or manual control. Ratio control is a very common problem in process control. It is estimated that about 15% of all controllers in a process control plant are used for ratio control.

The industry standard today is to use either a parallell ratio station or a series ration station. These methods can only handle a few of the disturbances mentioned above.

In the project we have derived a new method, the Tracking Ratio Station, that handles all disturbances. It has been field tested in a paper mill and is able to track the ratio during setpoint changes, load disturbances in both loops, saturations in both loops, and also the situation when one of the controllers is switched to local setpoint or manual control.

### Feedforward Mid-Ranging Control

Mid-ranging control is a control strategy that is used when there are more than one manipulated variable available to control a process variable. Mid-ranging control handles the redundancy by coordinating the roles of the different manipulated variables. The most common approach is to introduce valve position controllers (VPC) that control the steady-state position of manipulated variables.There are, unfortunately, some severe drawbacks with the VPC approach that makes it unsuitable for many industrial applications.

In this project a new approach to mid-ranging control is investigated. The new strategy uses feedforward control to obtain desired steady-state values of the manipulated variables. The approach avoids the drawbacks associated with the VPC approach. The project is sponsored by PiiA-Vinnova and is performed in collaboration with ABB.

### Publications

**Ángeles Hoyo, José Carlos Moreno, José Luis Guzmán, Tore Hägglund: ** "Robust QFT-based PID controller for a feedforward control scheme". IFAC-PapersOnLine, pp. 262–267, 2018.

**Tore Hägglund, José Luis Guzmán:** "Development of basic process control structures". *IFAC-PapersOnLine,* ** 51:**4, pp. 775–780, 2018.

**Max Veronesi, Josè Luis Guzman, Antonio Visioli, Tore Hägglund:** "Closed-loop tuning rules for feedforward compensator gains". *IFAC-PapersOnLine,* ** 50:**1, pp. 7523–7528, 2017.

**Tore Hägglund:** "The Tracking Ratio Station". *Control Engineering Practice,* ** 69**, pp. 122–130, 2017.

**José Luis Guzmán, Tore Hägglund, Max Veronesi, Antonio Visioli:** "Performance indices for feedforward control". *Journal of Process Control,* ** 26**, pp. 26–34, 2015.

**Martin Hast, Tore Hägglund:** "Low-order feedforward controllers: Optimal performance and practical considerations". *Journal of Process Control,* ** 24:**9, pp. 1462–1471, 2014.

**Carlos Rodríguez, José Luis Guzmán, Manuel Berenguel, Tore Hägglund:** "Optimal feedforward compensators for systems with right-half plane zeros". *Journal of Process Control,* ** 24:**4, pp. 368–374, 2014.

**Tore Hägglund:** "A Unified Discussion on Signal Filtering in PID Control". *Control Engineering Practice,* ** 21:**8, pp. 994–1006, 2013.

**José Luis Guzmán, Tore Hägglund, Antonio Visioli:** "Feedforward compensation for PID control loops". 2012.

**Tore Hägglund:** "Signal Filtering in PID Control". In *IFAC Conference on Advances in PID Control*, Brescia, Italy, March 2012.

**Martin Hast, Tore Hägglund:** "Design of Optimal Low-Order Feedforward Controllers for Disturbance Rejection". In *17th Nordic Process Control Workshop, 2012*, Kgs Lyngby, Denmark, January 2012.

**Martin Hast, Tore Hägglund:** "Design of Optimal Low-Order Feedforward Controllers". 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, 2011.

**José Luis Guzmán, Tore Hägglund:** "Simple tuning rules for feedforward compensators". *Journal of Process Control,* ** 21:**1, pp. 92–102, 2011.

**Pontus Nordfeldt, Tore Hägglund:** "Decoupler and PID controller design of TITO systems". *Journal of Process Control,* ** 16:**9, pp. 923–936, 2006.

**Pontus Nordfeldt:** "PID control of TITO systems". Licentiate Thesis Department of Automatic Control, Lund University, Sweden, March 2005.

**Pontus Nordfeldt, Tore Hägglund:** "Design of PID controllers for decoupled multi-variable systems". 2005.