PROCESS CONTROL STYDY OF THE BA6 COOLING TOWERS FOR THE SPS ACCELERATOR ...

181 Pages·2004·5.3 MB·English

PROCESS CONTROL STYDY OF THE
BA6 COOLING TOWERS FOR THE SPS ACCELERATOR

By Àlex Barriuso Poy
Escola Tècnica Superior d’Enginyeria
Universitat Rovira I Virgili
Tarragona, Spain

Acknowledgement

I would like to express my sincere acknowledgements to Didier Blanc and Maria
Carmen Morodo Testa, for his guidance and his excellent welcome at CERN. I would
also like to thank Michel Geoffory for his help during the fulfilment of this project, his
company, his comments and tips.
I’ve been Technical Student in the ST/CV group, I wish to thank all its
members. It has been a privilege to start my professional career in such a kind
environment. I also acknowledge the CERN for providing me all the tools needed for
the completion of this work.
I am grateful to Dr. Eduard Llobet, my eternal supervisor in the University
Rovira i Virgili, as well as, Dr. Luis Martínez Salamero for his help to go to CERN.
I thank Riichald, Juan, Sonia, Iván, Luis, Toni, Esther, Cocco, Bobo, Mirko,
Stefanos, Marcos, Albertos, Federico, Cristina, Arturo, Georgina, Eli, Giovanna,
Giuseppe, Andrea, Davide, Chiara, Rocio, Christos, Elena, Mario, Colum and Redwane.
You have done these 14 months unforgettable to me.
També voldria agrair pel seu afecte i ajut als meus pares Ignacio i Roser, el meu
germà Iñaki, als meus avis Joan i Carme i als meus oncles Jordi i Carme. Simplement,
gràcies per tot.
Per últim agrair a Gemma per la seva tendresa i suport des de sempre, quanta
paciència que has de tenir...
Report made by Àlex Barriuso Poy.
Industrial Electronics & Automatic Control Engineering student,
Escola Tècnica Superior d’Enginyeria, Universitat Rovira i Virgili, Tarragona, Spain.
Project carried out in the Controls and Electricity team of the ST/CV
(Technical Support/ Cooling and Ventilation) Design Unit,
European Centre for Nuclear Research (CERN), Geneva, Switzerland

CERN Supervisor: Didier Blanc
URV Supervisor: Eduard Llobet Valero
CONTENTS

Part A: Introduction and System Discussion.

1 Introduction 8

1.1 Project Background. 9
1.2 Project Aim and Problem Formulation. 9
1.3 Main Studied Topics during the Project Development. 10
1.4 Project Outline (Plan). 11

2 The CERN & the LHC 13

2.1 High Energy Physics Today. 14
2.2 The CERN. 15
2.3 CERN Future, the LHC. 17
2.4 The SPS. 19

3 BA6 Cooling Towers Description 20

3.1 Generalities. 21
3.2 Modes of Operation. 22
3.2.1 Normal Mode. 22
3.2.2. Local Mode – Maintenance. 22
3.2.3. Defect Mode. 22
3.3 Control Loops Description and Characteristics. 23
3.3.1. Stopgap Tank Level Regulation. 25
3.3.2. Temperature Regulation of the BA6 Cooling Towers 33
3.4 Description of the Degrade Mode. 39
3.5 Description of the Safety Mode. 40
3.6 Conclusions. 41

Part B: System Identification and Control Enhancements.

4 Stopgap Tank Discussion 42

4.1 Problem Formulation. 43
4.2 Nonlinear System. 44
4.3 Linearization. 45
4.4 Non-linearities Effects & Static Test. 49
4.5 Dynamic Test: Water Tank Filling Time f(Qe). 57
5 First Approach: Single Feedback Control 58

5.1 Assumptions. 59
5.1.1 Storage Tank - G (s) -. 59
p
5.1.2 Actuator - G (s) - 61
v
5.1.3 Disturbance - G (s) - 63
d
5.1.3 Sensor, Transducer and Transmission - G (s) - 63
s
5.2 Developing a Robust Single-Loop PI Control. 64
5.2.1 Single-Loop Proportional Gain (Model 1). 66
5.2.2 Single-Loop PI Gain (Model 2) 71
5.2.2.1 Reset (Integral Windup). 72
5.2.3 Modelling the Single Feedback for the 4 Towers. 83

6 System Identification 87

6.1 Introduction. 88
6.2 The Input-Output Data. 90
6.3 Examining the Data. 95
6.4 Estimating Models. 98
6.5 Estimation Results. 100
6.5.1. Actuator Cooling Tower 2. 100
6.5.2. Actuator Cooling Tower 3. 104
6.5.3. Tank Level Contribution. 107
6.5.4. Pressure SIG Contribution. 110
6.6 Validating Models. 113

7 System Simulation and Optimization 117

7.1 Enhancements to Single-Loop PID Control. 118
7.2 Cascade Control. 118
7.3 Building the Present Control Architecture. 125
7.4 Optimizing the PI Controllers. 129
7.4.1 Tuning Cascade Controllers 131
7.5 Definitive PI Values for a Discrete-time Control 135

Part C: Further Work.

8 The Feedforward Control 138

8.1 Introduction 139
8.2 Feedforward Control Design Criteria. 140
8.3 Feedforward and Feedback are Complementary. 141
8.4 Is Feedforward Control Necessary? 143
8.5 Feedforward Controller Design. 144
8.6 Feedforward-Feedback Control Results. 146
8.7 Further Implementation of a Feeedforward Control. 150
9 The Digital RST Controller 152

9.1 Introduction to Computer Control. 153
9.2 Structure of Digital Controllers. 155
9.3 Digital Controller Design Methods. 158
9.4 Controller Design Procedure. 162
9.5 Controller Development. 163

10 Conclusions. 169

References 171
Appendix A: Keywords. 176
Appendix B: System Schema. 178
Part A: Introduction and System Discussion

CHAPTER 1: INTRODUCTION
1. Introduction Process Control Study of the BA6 Cooling Towers for the SPS accelerator

1.1. PROJECT BACKGROUND.
This Project was carried out at CERN (European Center for Nuclear Research)
in the framework of the Controls and Electricity team of the ST-CV Design Unit. This
team is responsible for the control systems' engineering of the CERN cooling and
ventilation infrastructure. This includes the conception, design, implementation,
validation, evolution and dismantling of the CV control systems, which are built by
using industrial standard control hardware and software.
1.2. PROJECT AIM AND PROBLEM FORMULATION.

The target of this project is to develop a tool to simulate and improve the present
control system for the BA6 cooling towers which are a strategic part for the cooling of
the SPS accelerator.
The cooling system is based on a closed primary circuit around the SPS
accelerator that brings mineralized water by two ways, one with chilled water, and the
other one with warm water. The warm water outcomes from a heat exchanger with a
secondary circuit which transport demineralized water, this secondary circuit is in
charge directly of the magnets cooling. The warm mineralized water from the heat
exchanger with the secondary circuit, when finish its stretch, comes in a storage tank
that must remain at a constant level, in its turn, this stopgap tank feeds four cooling
towers assembled each one with a ventilator in charge of the water refrigeration; and
thus, the cooled water comes in the primary closed circuit again.
To carry out this study, an accurate model of the dynamic system has to be
obtained. This is the most important part of the project since, with a properly modeled
system, further work can be done. Each element that forms the system has been
modeled separately by two ways; firstly the elements were created using mathematical
modeling principles, and secondly, a more accurate modelization was achieved by using
the data extracted from a supervisory SCADA system and the System Identification
toolbox of Matlab®.
Once each element that shapes the system is identified, it is implemented the
present control architecture in order to check if the whole identified system matches
with the real one. Thus, the present parameters for the PID controllers that form the
system can be optimized.
Àlex Barriuso Poy, Universitat Rovira i Virgili Page 9/182
1. Introduction Process Control Study of the BA6 Cooling Towers for the SPS accelerator

Finally, after optimizing the present control system, new proposals for the kind
of control architecture and for the type of controllers are studied, simulated and
compared with the actual behaviour of the plant for a further upgrade of the BA6
cooling towers control system.

1.3. MAIN STUDIED TOPICS DURING THE PROJECT DEVELOPMENT.

During the modelization and study of the control system the following “tools”
were used:
• System Identification Toolbox of Matlab©: This toolbox allows us to find out
mathematical models of dynamics systems based on the observed input/output
data extracted from the supervisory SCADA system. These mathematical
models can be used later to create the overall system block diagram. This
toolbox was created by Lennart Ljung and has been chosen over another tools,
as WINPIM of Adaptech®, due to the variety of possibilities that provides
during the identification, and because its facility of use.
• Cascade Control Architecture: This is the present control architecture
implemented on the BA6 cooling towers for the controls. It is one of the most
successful methods for enhancing single-loop control performance. It can
improve the performance of control strategies, reducing both the maximum
deviation and the integral error for disturbance responses. Cascade control uses
an additional, “secondary” measured process input variable that has the
important characteristic that it indicates the occurrence of the key disturbance.
The important feature in the cascade structure is the way in which the controllers
are connected.
• Feedforward Control Architecture: This is a first proposal for a further upgrade
on the present control system. Feedforward uses the measurement of an input
disturbance to the plant as additional information for enhancing single-loop PID
control performances. This measurement provides an “early warning” that the
controlled variable will be upset some time in the future. With this warning the
feedforward controller has the opportunity to adjust the manipulated variable
before the controlled variable deviates from its set point.
Àlex Barriuso Poy, Universitat Rovira i Virgili Page 10/182