The advantages of using a digital temperature controller in a miniature Stirling cryogenic refrigerator for infrared imagers

Abstract

Modern Infra-Red (IR) night-vision thermal imagers for reconnaissance, surveillance, recognition and targeting rely mostly on Stirling-cycle cryogenic refrigerators thanks to their high thermodynamic efficiency. Traditionally, rotary cryogenic refrigerators comprised analog temperature controllers for controlling the cold-tip temperature. These controllers usually consist operational amplifiers, comparators, resistors and capacitors. The fine-tuning of the pre-set cold-tip temperature is achieved by setting a potentiometer to a certain resistance. It is known that potentiometers are affected by environmental temperature variations, continuous exposure to extreme temperatures, and aging. Another aspect of using a potentiometer is the difficulty for the customer to change the pre-set cold tip temperature, particularly with the RICOR On-Board (patented) controllers. Even without the use of potentiometers, the accuracy and stability of the analog components are not sufficient for the increasing requirements of advanced IR detectors at various environmental temperatures, loads, and input voltages. Moreover, manufacturers of cryogenic refrigerators could improve the reliability and traceability of their products by adding various functions to the controllers. A digital temperature controller that is based on a highly integrated flash MCU could serve both goals: improve the accuracy of the cold-tip temperature, and provide with extra features aimed at improving the functionality and reliability of the refrigerators. This paper describes the various functions and advantages of an integral (on-board) digital temperature controller that was developed in RICOR Vacuum and Cryogenic Systems.