Study On The Self-Heated Double Wheatstone Bridge For 2D-Wind Sensors

Compared with traditional wind sensors,MEMS wind sensors have the advantages of miniaturization,microelectronic integration,low power consumption and low cost.MEMS wind sensor has played an important role in many fields,such as agriculture,industry,traffic monitoring and national defense.Our group started to research on MEMS thermal wind sensor since 2000.A completed solution has been proposed in sensor design,fabrication,packaging,measurement/control circuits,sensor reliability.However,current version of the sensor system based on constant temperature difference mode,complex measurement/control circuits will increase the power consumption and cost of the sensor system,besides the different temperature coefficient of the ambient temperature sensor and chip temperature sensor adds the difficulty on sensor temperature compensation.In order to solve the questions above,the main work and innovations of this dissertation are described as follows:(1)This paper presents an overview of the development of MEMS wind sensor.We analysis and compare the working mode and bias/processing circuit of the sensor system.(2)A self-heating double Wheatstone bridge configuration for a 2-D wind sensor is presented.This configuration does not require ambient temperature sensor and chip temperature sensor.The double bridge configuration greatly simplifies the measurement and control circuits.By applying the constant current supply to the bridge,the terminal voltage across the bridge is utilized to measure the wind speed,the bridge output voltage is utilized to measure the wind direction,respectively.Based on thermal principles and Wheatstone bridge theory,the expressions of sensor output voltage and wind speed are established and verified by FEM simulation.The 2-D wind sensor has been fabricated directly on the ceramic substrate.The sensitivity of the sensor reaches 29.35mV/(m/s)@3.3m/s and 2.3 mV/(m/s)@23.9m/s.It shows that the measured speed varies from 0 to 40m/s with an error less than ±2m/s while the measured direction spans from 0° to 360° with an error less than ±3° after calibration.(3)The semi-empirical model has been developed for the temperature effect of the self-heating double Wheatstone bridge thermal wind sensor.The semi-empirical model has taken the thermal physical properties of the air,sensor substrate,and thermistors into account,and has been verified by COMSOL software.Temperature compensation has been performed by means of the semi-empirical model.Experimental measurements demonstrate that,when the air temperature changes from 270 K to 310K,the wind speed varies from 0 to 30 m/s with an error less than ±1.5m/s while the wind direction spans from 0° to 360° with an error less than ±4°.(4)An insulation island configuration for package has been proposed and realized.The temperature sensitive resistor is fabricated on the ceramic substrate.After dicing,the square thermal sensitive chip is connected to the outside ceramic chip with low thermal conductivity adhesive.This package method provides smooth fluid field situation and suppressed the unwanted lateral thermal loss.Measured speed varies from 0~60 m/s with an error less than ±(0.5+0.03V)m/s while the direction error less than ±3.5 ° after calibration.The self-heating double Wheatstone bridge configuration greatly simplifies the measurement and control circuits.This sensor configuration provides a wide measurement range in full temperature working condition.This work has important reference value for the development of MEMS wind sensor.