Benjamin Bradu

Modeling, Simulation and Control of Large Scale Cryogenic Systems

Abstract This paper presents a dynamic simulator for large scale cryogenic systems using helium refrigerators and controlled by Programmable Logic Controllers (PLC) for the European Organization for Nuclear Research (CERN). The process is modeled by a set of linear differential and algebraic equations and the control policy is based on a hierarchical multilevel and multilayer framework control. First simulation results carried out on the refrigerator used in the Compact Muon Solenoid (CMS) experiment are presented. It is worth to mention that CMS is a particle detector used in the future CERN accelerator (the LHC) where a superconducting magnet of 225 tons, the largest ever built, must be maintained at 4.5K (-268.7°C). The model of this cryogenic plant is composed of 4126 equations whereof 287 differential-algebraic equations. The work objectives of this simulator are threefold: first, to provide a tool to train the operators, second to validate new control strategies before their implementation and, third, to improve our knowledge about large scale complex cryogenic systems. In order to respect the real system architecture, the simulator is composed of different modules sharing data.

An expert knowledge based methodology for online detection of signal oscillations

The CERNs accelerator complex and its experiments rely on the proper functioning of a multitude of heterogeneous industrial control systems. Over 600 industrial control systems with more than 40 million sensors, actuators and control objects store more than 100 terabytes of data per year (the volume of generated data is much more). This paper describes a mathematical approach to monitor online a multitude of sensors/actuators and automatically detect signals oscillations. In order to achieve it the presented method combines both expert knowledge and spectrum analysis. Some results, obtained by the application of this analysis to the CERN cryogenics system, are presented showing multiple plant-wide oscillations. Finally the paper briefly describes the deployment of Spark and Hadoop platform into the CERN industrial environment to deal with huge datasets and to spread the computational load of the analysis across multiple hosts.

Beam screen cryogenic control improvements for the LHC run 2

This paper presents the improvements made on the cryogenic control system for the LHC beam screens. The regulation objective is to maintain an acceptable temperature range around 20 K which simultaneously ensures a good LHC beam vacuum and limits cryogenic heat loads. In total, through the 27 km of the LHC machine, there are 485 regulation loops affected by beam disturbances. Due to the increase of the LHC performance during Run 2, standard PID controllers cannot keeps the temperature transients of the beam screens within desired limits. Several alternative control techniques have been studied and validated using dynamic simulation and then deployed on the LHC cryogenic control system in 2015. The main contribution is the addition of a feed-forward control in order to compensate the beam effects on the beam screen temperature based on the main beam parameters of the machine in real time.

Overview of different control strategies for a typical cryogenic warm compressor station at CERN

Helium cryogenic systems are extensively used at CERN under several configurations for accelerators and detectors. The Warm Compressor Station (WCS) is the primary component of the helium cryogenic systems. The basic controls structure mainly depends on the bypass, charge and discharge valves configuration ensuring the nominal flow and compression ratio. This paper presents three studied methods for the WCS process control systems covering all transient and operational requirements: the proportional-integral-derivative (PID) control approach, the Fuzzy Logic Control approach (FLC) and the Internal Model Control approach (IMC). The paper emphasizes on simulation results of the different control strategies using Ecosimpro software associated to the CERN CryoLib library. Advantages and limitations of each method are presented.