A PC-XT/AT based low temperature display and control system of an optical cryostat

摘要

The design and fabrication of the hardware and software of a simple and low cost A/D interface scheme through which a TC (thermocouple) and diode sensor connected to a PC-XT/AT or 8088 #mu#P kit has been described in this paper. This unit is now being used in low temperature control system of an optical cryostat using a TC as temperature sensor. For cold junction compensation, silicon diode, IN 4148 operating at constant voltage instead of the constant current has been used and the addition for the compensation process is done under the program control. The variation of thermo emf of TC with temperature is nonlinear. The process for calibration of nonlinear signal by analog means is rather difficult and it is known that all the existing sensors have either static or dynamic nonlinear response with respect to input variables. Here instead of linearizing the signal which lowers the accuracy of the measurements, a polynomial fit to the experimental points is used for the purpose of calibration. The temperature value corresponding to a thermo emf is obtained from this relation. Accuracy of measurement of this system with a 8 bit ADC over a temperature range from -196 deg C (boiling temperature of liquid nitrogen) to + 100 deg C is about 1.16 deg C. But splitting the temperature range from -196 deg C to +100 deg C in seven equal steps enables one to achieve a resolution almost equal to 0.35 deg C. Selection of different temperature ranges by software is automatic. For control a proportional control algorithm has been adopted and the set temperature remains steady at any value for about half an hour with a fluctuation of about +-0.35 deg C within the above range of temperature.