Study On MEMS Thermal Wind Sensors Based On The Temperature-balanced Mode

As basic and critical devices for meteorological monitoring and early warning,wind speed and direction sensors are indispensable in various fields,such as highways,high-speed railways,wind power generation,smart grids,and ship navigation.However,the traditional mechanical wind cup and ultrasonic wind sensors have large volume,high cost,and large power consumption,which cannot meet the huge demand for the development of the Internet of Things(IOT).Micro-electromechanical Systems(MEMS)wind sensors have been the advanced technology direction for its advantages of miniaturization,mass production and low power consumption,etc.Key laboratory of MEMS of Ministry of Education in Southeast University has been studying MEMS wind sensors since 2000 and has developed complete solutions.Nonetheless,the current product adopts constant temperature difference(CTD)mode,where an external ambient temperature sensor is required,leading to heavy workloads for temperature drift compensation.Moreover,wind sensors are desired to work outdoors for a long time,which further results in the stability degradation of resistances on sensor chips.In this paper,we propose a thermal wind sensor based on the Temperature-Balanced(TB)mode.Theoretical model,sensor construction optimization,fabrication process,circuit system implementation,and temperature drift compensation of the presented thermal wind sensor are investigated.The main contribution and innovations are described as follows:(1)A thermal wind speed and direction sensor based on the Temperature-Balanced(TB)mode is proposed and implemented,which improves the measurement accuracy.By considering the influence of airflow,sensor chip,ceramic substrate and packaging adhesive,a theoretical model is developed as a function of wind speed and wind direction.A standard MEMS technology process is applied to deposit a Pt metal film on the ceramic substrate,after which the chip the chip is self-packaged for protection.Thus,a highperformance two-dimensional wind speed and direction sensor based on the TemperatureBalanced mode is fabricated.The Temperature-Balanced control mode of the proposed sensor is realized by the measurement and control system integrating hardware circuits and software algorithms.Experimental results at room temperature indicate that the measurement range of the proposed sensor reaches 0?40 m/s.The maximum measurement error of low wind speed(0?4 m/s)is less than ±0.2 m/s,while that of high wind speed(4?40 m/s)is about ±0.5 m/s.Additionally,over the full direction measurement range of 0?360o,the maximum wind direction measurement error is less than ±2o.(2)The temperature effect model of MEMS wind sensor based on the TemperatureBalanced mode is established and the temperature relationship of the measurement and control circuits is analyzed.Hence,the temperature drift of the proposed sensor is effectively restrained through temperature drift compensation.The temperature effect model of the sensor is established by examining thermophysical properties of the air and the thermal conductivity of the substrate material as a function of the ambient temperature,and the Finite Element Modeling(FEM)simulation is performed for verification.The wind sensor is tested in a wind tunnel with an adjustable ambient temperature to obtain the temperature effect coefficient and realize temperature drift compensation.The experimental results show that the compensated sensor has a maximum wind speed error of less than ±1 m/s and a maximum direction error of less than ±2o in the range from 273 K to 303 K.(3)A compact MEMS thermal wind sensor with dual-polysilicon layers is explored,which significantly reduces the sensor chip size and thus improves the integration degree.The designed sensor employs dual polysilicon layers,where four heaters and four thermistors are placed vertically at the same position on the sensor chip.This method ensures that the temperature of the heating element and the temperature measuring element are approximately equal in the Temperature-Balanced mode.Compared with the above sensor,the chip size has been reduced by 43.5%.Experimental results demonstrate that within the measurement range of 0?40 m/s,the wind speed measurement error of the sensor is less than1.5%,and the wind direction measurement error is less than ±2o.This dissertation systematically studies the wind speed and direction sensor based on the Temperature-Balanced mode,which provides essential reference value for the applications of MEMS wind sensor.