Research On Performance Enhancement Technology Of MEMS Thermal Wind Sensors

Traditional wind speed detection methods are mostly mechanical,with large volume,need to be fixed,and movable parts are easy to wear and other defects.Thermal wind sensors based on MEMS can avoid these problems.At present,researches on MEMS thermal wind sensor are mostly focused on the improvement of chip structure and encapsulation.The purpose is always to optimize the range,sensitivity,response speed and power consumption.On the basis of many previous studies,solutions to improve the performance of the MEMS thermal wind sensors are proposed.Meanwhile a more detailed study on the start-up time of the thermal wind sensor,which is illustrated in the way of software simulation,experimental test and mathematical derivation is carried out.Firstly,starting from the complete anemometer system,this article elaborated the structure,principle and encapsulation of the sensor chip used,and illustrated the control,amplification,detection and communication circuit for this sensor chip.Through wind tunnel experiments on the complete wind speed sensor system,the main performance characteristics are summarized.At the same time,three problems were found during the experiments,namely the heat loss output voltage-wind speed curve is abnormal,the sensitivity of the output curve at high wind speed is reduced,and the startup time is uncertain.To solve the first problem,locating the reason to the environmental temperature diode.Experiments prove that the environmental temperature diode will be affected by the thermal field of the sensor chip when working,thus triggering the abnormal curve of heating voltage and wind speed.After replacing it with an external temperature diode,problems of long stabilization time and hysteresis also appeared.Considering the importance of the location and encapsulation of the environmental temperature sensors,the chip is optimized,and the position of the chip body and the environment temperature sensor is rearranged,eliminating the mutual interference and unnecessary heat loss of the thermal field as far as possible,and ensuring the good contact between the sensing element and the air.After these steps,problems are solved.The output-wind speed curve is smooth and the repeatability is good.High measurement accuracy of wind speed and direction is achieved,and the error is controlled within 4%and 4°,respectively.The time required for the entire system to be stable after powered on is around 15s.Aiming at the second problem,a sensor sensitivity enhancement circuit based on the current-voltage conversion characteristic of MOSFET is designed.Firstly,theoretical model on the output of the MEMS thermal wind speed sensors is studied.From the perspective of heat transfer and fluid mechanics,two kinds of temperature measurement methods,heat loss mode and thermal temperature difference mode,are deduced.The output curve characteristics of the two methods are summarized,which are linearly related to the square root and the fourth power root of wind speed respectively.On this basis,combined with the current characteristics in saturation region of the MOSFET,a square circuit based on MOSFET is proposed.Then this circuit was simulated and experimented.Simulation and experiment results show that the circuit can greatly improve the sensitivity of the output voltage.The sensitivity of the output voltage in the measurement mode of temperature difference and heat loss is 45.44mv/ms^-1 and 171.88mv/ms^-1,respectively.The nonlinear error is reduced from 3.15%to2.77%.Finally,the start-up time which is one of the significant parameters of thermal wind sensors,is studied in detail.It has been explained thoroughly from the perspectives of parameter definition,experiments,finite element simulation and mathematical derivation.The research results show that the start-up time of chip.1 is about 15s,while the start-up time of chip.2 embedded in ceramic substrate and having thermal insulation structure is about 5s.Both of them can meet practical requirements.In addition,you can also determine the startup time of sensors with different chip structures by modifying parameters in the mathematical derivation results,so that it can be optimized as much as possible during early research and design.Through the above research,the performance parameters such as repeatability and sensitivity of MEMS wind speed sensor have been optimized,and the start-up time parameters are determined,which has practical significance for the subsequent chip optimization design.