Standard and Experimental Approach for Advanced Controls in Cryogenics

CERN, the European Organization for Nuclear Research, is one of the world's largest and most respected centres for scientific research. Its business is fundamental physics, finding out what the Universe is made of and how it works. At CERN, the world's largest and most complex scientific instruments are used to study the basic constituents of matter, the fundamental particles. By studying what happens when these particles collide, physicists learn about the laws of Nature. The instruments used at CERN are particle accelerators and detectors. Accelerators boost beams of particles to high energies before they are made to collide with each other or with stationary targets. Detectors observe and record the results of these collisions. The Large Hadron Collider (LHC) is the biggest and more powerful particle accelerator ever built. It is a circular particle accelerator with a circumference of 27 km, located about 100 m underground, used by physicists to study the smallest known particles, the fundamental building blocks of all things. Two beams of subatomic particles called hadrons", either protons or lead ions, travel in opposite directions inside the circular accelerator, gaining energy with every lap. The machine will be used to recreate the conditions just after the Big Bang, by colliding the two beams head-on at very high energy. Teams of physicists from around the world will analyse the particles created in the collisions usin g special detectors in a number of experiments dedicated to the LHC. The beams circulating in the LHC are accelerated by RF cavities to more than 99 % of the speed of light, thus reaching the nominal energy of 7 TeV per beam for a resulting overall nominal collision energy of 14 TeV . The beams intersect at the centre of the enormous detectors built to collect the results. The beams are bent and focused by means of a magnetic field produced by dipole and quadrupole magnets. The maximum field required in order to attain a stable beam with a sufficiently long life time reaches the peak of 8:33 T inside the dipoles, value reached by means of superconducting magnets whose Niobium-Titanium (NbTi) windings are cooled down to 1:9 K to allow the circulation of a nominal current of about 15 kA. In addition to that, also the safe and correct working of detectors and RF cavities need cryogenic temperatures. As a consequence, the operation of the accelerator requires cryogenic systems capable of reaching these extremely low temperatures. In particular, the LHC employs the world's biggest cryogenic installation with Helium as cooling uid, controlled by industrial Programmable Logic Controllers (PLCs). The realization of both installations and control systems constituted a great challenge, since for the first time cutting-edge technologies were employed on a complex large scale system. Motivations This thesis deals with the problem of mod eling and control applied to some of the cryogenic plants currently used at CERN, and exploits the experience accumulated over more than ten years on the construction, deployment and operation of the LHC cryogenic installations. A crucial point of the work lies in the mathematical modeling of the physical phenomena related to the cryogenic processes. In such systems, the superposition principle does not often hold true, because of the strong nonlinearities in the relations between causes acting on the system and their effects. In large scale installations such as those discussed in this work, the experimentation possibilities are strongly limited both by the costs and by the risks for the plants deriving from their realization. For this reason, in this thesis there will be an extensive use of simulation in the process of control design and simulation. Nonetheless, for the identication of the process under control it was exceptionally possible to carry out an experimental campaign on a Nitrogen heat exchanger. Furthermore, it is to be pointed out that in the above-described context, i.e. modeling and control applied to large scale nonlinear systems, also traditional well-established techniques may pose relevant problems for their practical application. In addition to that, new advanced techniques for modeling and control can be developed to optimize the management of the plants, with the purpose of increasing the performance of the controlled system in terms of fidelity to the desired behaviour, thus reducing undesired effects which could on the long term both increase operation costs and affect the availability of the system. Main contributions In reference to the aforementioned situation of the controls applied to large scale cryogenic installations, this thesis provides a twofold contribution: Within the frame of standard identication and control techniques, after a review of the state-of-the-art solutions proposed in literature, it shows what kind of problems these traditional approaches may pose when used in real large scale applications, and proposes a solution through the presentation of the activities carried out at CERN for parametric identification and control design of the ATLAS Nitrogen heat exchanger. In this way, a synthesis of scientific and industrial approaches is provided,in particular with reference to the implementation of the mathematical results in industrial controllers by means of Schneider PLC objects. It is worth mentioning that the heat exchanger is normally unavailable for experimentation, and that the practical work carried out on it was made possible only by an exceptional authorization granted to the author and his collaborators. This constitutes an unprecedented case study and one of the main contributions of this thesis. A novel approach to the control of cryogenic plants is proposed, including the phases of (i ) sys tem modeling through balance equations, describing mass ows and heat transfer evolution in the time domain under the assumption of spatial uniformity of the physical properties of interest, (ii ) control design and implementation, (iii )estimation of the parameters, namely communication time delays, employed by the control algorithm, and (iv ) results obtained thanks to the new approach in comparison with those of a traditional PID controller. The model is further improved in Appendix A, through the introduction of a more refined formulation of the equations taking into account variations of the physical properties both in the time and in the space domains. At the moment of closing this work, the modeling phase was still in progress and it was therefore impossible to include in this dissertation any related results. Anyway, this parts plays a central role among the contributions of the thesis, since it constitutes the starting point of further developments of the propose modeling and control approaches. Thesis overview This thesis is composed of four main parts, briefly described in the following. The first chapter introduces (i ) the basics of cryogenics, such as cryogenic uids, heat transfer theory, concepts of thermodynamics, and (ii ) cryogenic test facilities and detectors at CERN, including the ALTAS and CMS detectors, the central Helium liquefier and the Krypton calorimeter of the NA62 experiment. The second chapter gi ves an overview of the standard techniques used for system identification and control design, along with the results of the application of such techniques to the ATLAS Nitrogen heat exchanger. In particular, as far as the identification techniques are concerned, their goals and principles are illustrated, along with an overview of methods for signal post-processing, types of models, identication algorithms, and validation procedures. Subsequently, a state of the art on advanced control techniques is presented, including the Smith Predictor-based control, the Generalized Predictive Control, the Predictive Function Control, the R-S-T control. All these techniques are presented with a view to their industrial application, namely to their implementation on PLCs. In this perspective, the Advanced Automation Toolkit" from Mathworks and the MultiController Object" from the UNICOS framework are presented for (i ) model identication and validation, and (ii ) advanced control implementation, respectively, on industrial target platforms, namely Schneider PLCs. Finally, the results of the above-mentioned techniques applied to the identication and PID controller optimization in the ATLAS Nitrogen heat exchanger are presented and discussed. The third chapter presents a novel theoretical approach to modeling and control for large scale cryogenic systems, whose formulation is applied to the liquid Krypton condenser of the NA62 experiment. Firs t, a model is derived from balance equations in the form of ordinary differential equations (ODE) describing the mass flow and the heat transfer between cryogenic fluids. The modeling phase is followed by the proposal of an advanced control strategy, the Time Delay Control, whose performance obtained in simulation are presented in comparison with those of a more traditional PID-based control loop, thus showing the improvement allowed by the new approach. The implementation of the proposed solution required a further phase of analysis in order to satistically estimate the communication time delays usually encountered during the operation of the system under control. The results of these analysis are also presented. The fourth chapter presents the programming paradigm and the software tools currently used at CERN to handle the cryogenic plants. In particular, (i ) the object-based approach to industrial control, and (ii ) the UNICOS framework, developed inside the Organization in order to provide a unified platform for the management of all the cryogenic equipments, are discussed. As examples of application of the aforementioned programming paradigm,this part also provides information about the installation and commissioning of the refrigeration system for the CMS magnets, as well as about a simulation study for the virtual commissioning of the CERN central Helium liqueer. Finally, the off-line commissioning of the Helium cryogen ic plant and the operator training are discussed. In particular, a simulation environment is presented, which allows the training of the operators on large complex cryogenic systems, without any risk for the installations and any need to stop its normal operation. The thesis ends with the discussion of the conclusions drawn by the theoretical and practical work presented in the previous parts, along with an overview of the new perspectives it has opened. In particular, with regard to the future developments, a new modeling strategy for the NA62 condenser is introduced and developed in its basic aspects (Appendix A). This further improvement of the work proposed in the third part describes the phenomena occurring in the condenser through partial dierential equation, taking into account variations both in the time and in the space domains. Further investigation will be required to assess the possible benets deriving from this approach, and to check whether the additional complexity it introduces is balanced by significant improvements both in the model prediction accuracy and in the possible enhancements of the control strategy. In addition to that, further work will be devoted to the refinement of the simulation tools employed for the off-line commissioning of cryogenic plants and operator training. This task plays an important role in the frame of the future activities planned for the LHC,since the machine will be mostly used i n steady state operation for physic experiments and no time will be scheduled for operator training. In this scenario, highest importance will be given to the development of simulation tools giving the opportunity of (i ) training the operators without risks and stops for the installations, (ii ) improving the knowledge of cryogenic systems, and (iii ) saving an important time during plant re-starting by minimizing the probability of unexpected errors.