Система регулювання температури для кріостату занурюваного типу

Abstract

Описана автоматизована система регулювання для кріостата занурюваного типу, яка дозволяє вимірювати температуру з точністю 0.001 К, змінювати її по наперед заданому алгоритму та стабілізувати з точністю ± 0,002 К в діапазоні 80 – 480 K. Система побудована з використанням сучасної елементної бази.

Описана автоматизированная система регулирования для криостата погружного типа, которая позволяет измерять температуру с точностью 0.001 К, изменять ее по наперёд заданному алгоритму и стабилизировать с точностью ± 0,002 К в диапазоне 80 – 480 K. Система построена с использованием современной элементной базы.

The main difficulty of investigations in the vicinity of phase transitions associated with high system requirements for the temperature stabilization. Due to the limited range of observed phenomena (~0.1-0.5K) [1], the algorithm of temperature stabilization should provide accuracy better than 0,01 ÷ 0.001K without "over-regulation." This requirement means the choice of automatic control law parameters according to the following requirement - in the process of temperature stabilization, the system is not passed through the phase transition. Note also substantial dependence of physical parameters (permittivity, electrical conductivity, etc.) from the rate of temperature change, especially in the case of semiconductor materials [2]. Most thermostats used in experimental practice [3] do not meet the above requirements, so the study of critical phenomena in ferroelectrics - semiconductors need the development of non-standard equipment. This paper describes an automated control system for immersion type cryostat, which allows measuring the temperature Т with an accuracy of 0.001 K, change it according to prescribed algorithm and stabilize the T with an accuracy of ± 0,002 K in the range of 80 - 480 K. The system is built with the use of modern element base. Controller works as follows: Input signal from the PT100 [4] thermoresistor through ultra-low noise amplifier is supplied to the input of 24 bit analog-to-digital converter. The resulting digital code corresponds to the value of intensified voltage drop on PT100. For its conversion into the temperature the spreadsheet method is used. In the computer шы stored the calibration table according to R → T with the increments of 1 K. The temperature, which falls in the interval between two points of the table, is determined by spline-based interpolation that passes through the ten neighboring points of calibration table (five points above and the five points below of the coincidence interval). The resulting value of T is compared with the required temperature. Using standard PID (proportional-integral-differential) algorithm the necessary power of heater is determined. The code, corresponding to a given power, through the USB bus is fed to the digital-to-analog converter. DAC output voltage is determine the current on heater. For the output stage, the standard current amplifier is used. Computer based systems for measuring and stabilization of temperature provides the following benefits: o Temperature sensor replacement require only changing of the calibration table; o Using statistical analysis of the measurements data, exist a possibility for the significant increase of accuracy (noise compensation and "emissions"); o Temperature stabilization algorithm can be modified according to the objectives; o PID regulator parameters can be determined automatically (Autotuning PID or Adaptive PID).