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Fanny Clavel

Modelling and control of a cryogenic refrigerator

Published on 18 January 2011
Thesis presented January 18, 2011

This thesis is concerned with the development of a novel control scheme on a helium refrigerator subject to high pulsed loads. Such disturbance will happen during the cooling of the superconductive magnet, used in tokamak configuration. A dynamical model of a cryogenic station, which offers a cooling capacity of 800W at 4.5K, has been produced. The modelling is based on the theoretical equations of thermodynamics, thermal physics and hydraulics and takes into account the non linear properties of helium at low temperature. Based on this model, a new control strategy has been developed for each of the two parts ofthe refrigerator: the warm compression system and the cold box. Experimental results show significant improvement with multi variable controllers as compared with the PIDs in the presence of high pulsed loads. An observer of the thermal load of the helium bath has also been developed. The model is constructed by identification using internal measures of the refrigerator. It can be used as condition monitoring tool for operators.

Cryogenic refrigerator, Predictive control, Dynamical modelling, Process control

On-line thesis.