Designed Of Chilled-Mirror Dew-Point Hygrometer Based On Zigbee WSN

The dew-point temperature is an important parameter in meteorological monitoring. This article referred to the advantages of domestic and foreign chilled-mirror dew-point instruments and combined the Zigbee wireless sensor network technology, designed a chilled-mirror dew-point hygrometer based on Zigbee sensor network.To get the rough dew-point temperature by way of semiconductor laser tube irradiating mirror and silicon photocell detecting the change of intensity of reflected light. Then, on this basis, control cooling speed of the three-stage refrigerator. Mirror temperature is measured by the platinum resistance on the back of mirror, when the temperature equals to the actual dew-point temperature. This temperature is the true dew-point temperature.The dew-point hygrometer hardware design has been completed, including the photo-detector part, mainly auto power controlling circuit, the platinum resistance part, mainly four-wire platinum resistance temperature measurement circuit and the24bits analog to digital conversion circuit, the three-stage refrigerator part, mainly the three-stage refrigerator structure and H-bridge control circuit, the water-cooling part, mainly the components of water-cooling, the microcontroller part, mainly the peripheral circuit of microcontroller, the LCD serial connection circuit, the communication circuit and the JTAG interface circuit, the Zigbee node part, mainly the peripheral circuit of node chip, the communication circuit and the DBUG interface circuit.The dew-point hygrometer control algorithm design has been completed. The mode of high rate warming and slow rate cooling of mirror is achieved by Fuzzy-Digital PID composite control algorithm. Fuzzy control algorithm is used when mirror temperature deviation lager, which is no need for precise mathematical model of controlled object. Digital PID control algorithm is used when mirror temperature deviation small. The digital PID control algorithm can overcome system oscillation and reduce steady-state error. The microcontroller convert two control algorithms according to the pre-set deviation range automatically.The dew-point hygrometer software design has been completed, including the dew point detection part, mainly the subroutine of analog-to-digital conversion, the subroutine of PWM output, the subroutine of LCD driver and the subroutine of serial communication, the wireless transmission part, mainly the Z-Stack protocol workflow, the subroutine of communication with PC and the subroutine of wireless data transceiver.Experimental result shows that chilled-mirror dew-point hygrometer can meet the demand of high-precision dew-point measurement. The repeatability is better than±0.6℃, the accuracy is better than±0.4℃, and the cooling capacity is lower than-65℃(in the condition of25℃).