| Radiometer is the measurement system which is sensitive to temperature,and the stability of the operating temperature affects the stability of radiometer system and the accuracy of measurement directly.In order to improve the temperature drift and ensure the stability of operating temperature of radiometer,it is necessary to develop miniature temperature control system which can keep the ambient temperature of the radiometer constant.In this thesis,the temperature control system for radiometer is studied.The temperature measurement circuit,the feedback control loop,the driving circuit of the thermoelectric cooling and the structure of the box are designed.Compared with the traditional temperature control system,the volume of the temperature control system designed in this thesis is smaller and the cost is lower.The main work of this thesis includes the following aspects.The temperature measurement circuit is the basis of system,and the temperature measurement circuit which has high precision and good linearity is essential to system.The temperature sensor and the measurement method are the key to the temperature measurement circuit.Considering the accuracy of the measurement,the Platinum resistance is selected and the constant current source is adopted for the measurement by four-wire to reduce the error.Finally,the accuracy rating of the constant current source is 1.5% by fitting the test data in MATLAB.The closed-loop control circuit is utilized as the control circuit,and the feedback signal is the output of the temperature measurement circuit.The output of the control circuit is the difference of the feedback signal and the reference signal,which is the input of the driving circuit of the thermoelectric cooling to achieve the quantitative control of the thermoelectric cooling.Besides,The H bridge is used as the driving circuit of the thermoelectric cooling.The direction and value of the current of the thermoelectric cooling can be changed by the H bridge so as to change the heating/cooling mode of the thermoelectric cooling and the amount of heating/cooling.In addition,the Pulse Width Modulation(PWM)wave generation circuit is also designed which can generate the PWM wave with frequency adjustable and duty cycle adjustable from 0-100% to control the H bridge.When the duty ratio is less than 50%,the thermoelectric cooling is heated,and when the duty ratio is more than 50%,the thermoelectric cooling is cooled.The polyurethane foam which has good insulation property is utilized as the material of the box.Besides,in order to ensure the cooling effect of the hot side of the thermoelectric cooling,the method of air forcing convection cooling is selected.The experimental results show that: when the magnification times are set to 15,the accuracy rating of the temperature control is 0.4oC and the temperature stability is 0.2oC in the heating process.The accuracy rating of the temperature control is 0.6oC and the temperature stability is 0.3oC in the cooling process.Finally,the temperature experiment of the W-band radiometer is implemented using the designed temperature control system.The results show that the brightness temperature of the W-band radiometer decreases from 56.9K to 12.9K during the turning-on process.The variation range of the output voltage is only 10 mV when the radiometer is stable,and the equivalent brightness temperature is 1.7K.Obviously,the temperature control system designed in this thesis greatly improves the temperature drift of the radiometer and the stability of radiometer.In order to verify and improve the analog control circuit,a digital PID controller is also designed based on the driving circuit of the thermoelectric cooling.The final measurements show that in the heating process,the temperature stability is 0.25 oC and the accuracy rating of the temperature is 0.2oC.In the cooling process,the temperature stability is 0.24 oC and the accuracy of the temperature is 0.44 oC.The results can meet the requirements of the system. |
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