Modeling and control of a helium closed cycle calibrating cryostat

A calibrating cryostat serves as a thermal medium for measuring characteristics of extremely low temperature transducers. An isothermal block in the cryostat is required to promote temperature uniformity. Efficient transducer calibration demands that step temperature changes result in temperature equality throughout the isothermal block, in minimum time. This work improves the time response of a calibrating cryostat. Step changes in temperature and resultant response of thermal equilibrium of the isothermal block was extensively examined for the purpose of improving the design of the controller in the thermal loop system.

Analysis of a cryostat control system starts with development of a mathematical model. The model permits adjustment of system parameters to identify control improvement candidates. Digital computer simulation of the model indicated faster cryostat thermal response than experimental results. Differences between simulated and measured response were determined to be caused by omission of thermal loads in the model. The thermal capacitance of the model was increased until it was in agreement with the system's response to a step change. The corrected model was then employed in the controller design.

An improvement in the response time of the system was obtained by tuning the controller using classical control techniques. A significant reduction in the response time of the isothermal block was obtained by modifying the control loop of the system to include the temperature of the isothermal block. This was accomplished by feed forward compensation of the controller's temperature setting. The observed temperature difference between the desired control setting and a thermometer mounted inside the isothermal block was multiplied by a gain factor and added to the desired control setting. This value was then input to the controller as the set point temperature, which will equal the desired setting when the observed temperature difference is zero. This compensates for the thermal delay between the control element and the desired control point.