Development and Test of Software Method to Achieve Constant-Temperature-Difference Control for Thermal Wind Sensor*

WANG Fang，QIN Ming

（Key Laboratory of MEMS of Ministry of Education，Southeast University，Nanjing 210096，China）

Development and Test of Software Method to Achieve Constant-Temperature-Difference Control for Thermal Wind Sensor*

WANG Fang，QIN Ming*

（Key Laboratory of MEMS of Ministry of Education，Southeast University，Nanjing 210096，China）

A software method for controlling constant temperature difference（CTD）mode for the thermal wind sensor has been presented.As control unit，MCU is utilized to sample ambient temperature and chip temperature and adjust the power for output in order to maintain the state of CTD.The experiment shows that by means of the software method for control，the thermal wind sensor could almost maintain the output constant during the temperature range from-20℃to 40℃.Besides，it can measure the wind speed during 0～20 m/s with relative error less than 8%which has a response speed of less than 15 seconds.

MEMS；thermal wind sensor；CTD mode；MCU control

EEACC：11222doi：10.3969/j.issn.1004-1699.2016.10.002

A thermal wind sensor which usually consists of heaters and temperature sensitive elements measures wind speed by monitoring the change of thermal field induced by an airflow that passes along the chip［1-3］.The thermal field is generated by heaters that work on constant-temperature-difference（CTD）mode or constant-power（CP）mode.Nowadays，the CTD mode is utilized widely.By making use of an additional resistive temperature sensor，the average temperature of the thermal wind sensor is kept a constant temperature above the ambient temperature ensuring the output is independent of the ambient temperature so that it can solve the problem of temperature drift and has a large measurement range theoretically［4-5］.

On CTD mode，the wind sensor system has to sample chip temperature Thand ambient temperature Taconsecutively and calculate the difference between the chip temperature and the ambient temperature［6］and compareΔT with the defaultΔT0in order to adjust the heating power for a CTD state.However，the temperature drift is a key factor affecting the performance of the wind sensor system［6］.The temperaturecompensation methods for CTD mode are divided into three categories.One is to improve the temperature compensation circuit［7-10］.In 2010，Christoph Sosna and his partners proposed a circuit with two Wheatstone bridges which could maintain the temperature difference between the chip and the environment and has a preferable performance［7］.The second method is to revise the structure and the fabrication of the wind sensor chip which is a fundamental method to improve the property of the wind sensor system but is not enough for improvement，such as manufacturing some pendent structures［9-13］.And the third is to use some modeling methods to compensate the output.This method is effective but has a complex procedure and needs too much compute［14］.In this paper，another method to realize CTD control has been proposed.MCU，which was utilized as the main unit，aided by peripheral circuits can achieve the purpose of CTD control，for it has reduced as many components as possible which is an important factor attributing to the temperature drift.Besides，it allows the temperature sensors including chip temperature sensor and ambient temperature sensor have different resistances and different temperature coefficient of resistance（TCR）favorable for manufacture.

1　Principle and System Scheme of Thermal Wind Sensor System

A software method has been developed to make the sensor work on CTD mode and then complete one dimensional wind velocity measurement.The main procedure is presented below：①Sampling the signals from chip temperature sensor and environment temperature sensor；②Calculating the amount of regulation according to the difference between sampleΔT and defaultΔT0，and theΔT is equal to the difference between chip temperature and ambient temperature；③Outputting the heating power.

All these functions above are controlled by MCU.And in order to fulfill these functions，the system is designed from two parts-hardware module and software module.

1.1Hardware module

The hardware module is divided into five sections covering power section，heating control section，MCU processing section and the thermal wind sensor chip.The schematic diagram is as follows.

Fig.1　Schematic diagram of thermal wind sensor system which works on CTD mode controlled by software

The thermal wind sensor chip is a one dimensional wind speed sensor chip containing two metal resistors，of which one called Ramonitors the ambient temperature and the other called Rhis heated to form thermal field and also senses the chip temperature.In order to avoid the self-heating influence of Ra，the current through Rashould be small enough.The resistance and TCR of Raand Rhcan be measured through experiment.

1.1.1Information collection section

In this paper，a subtraction circuit has been applied as a signal processing circuit in consideration of the level mismatching between the output of resistive temperature sensors and the sampling scale of MCU.Through the circuit，the difference between the power voltage and the sample signal is served as the input to MCU for post compute.The diagram is showed in Fig.2.

1.1.2Heating control section

For thermal wind sensor system in this paper，the heating control circuit regulates the heating power by changing current through the heater Rh.The heating circuit is a constant current source circuit capable of amplifying the current outputted from MCU with the purpose of providing larger power for Rh.As is show in Fig.2，the amplifying factor can be adjusted by changing the ratio between resistors R10and R11.Besides，with the current configured by MCU and the voltage sampled by information collection section，the temperature of the heater can be obtained.

1.2Software module

The main part of software module is coding the control program for MCU.The software flow chart is showed in Fig.3.

Fig.2　Interface circuit of thermal wind sensor system which includes three parts-simplified sensor structure diagram（No.1），information collection section（No.2）and heating control section（No.3）.

Fig.3　Software flow chart of CTD mode control.

1.2.1Temperature calculation

The resistances of Raand Rhare dependent on the environment temperature.TCR is a coefficient reflecting this phenomenon.The relation of TCR，resistance and temperature can be presented as follow：

WhereR1andR2are values of the same resistor at different environment temperatures ofT1andT2respectively.Andα1is TCR of the resistor showing the change of resistance induced by ambient temperature.For metal material，α1is positive.During normal temperature range，the relationship between resistance and temperature is linear.Owing to this linear relationship，MCU can calculate current temperature of the chip and the environment on the basis of the formula below.

WhereUis the voltage on the heater or the temperature sensor at the temperature ofT2，and I is corresponding current.R1is the resistance at the temperature ofT1.

1.2.2CTD control

In order to obtain accurate chip temperatureThand ambient temperatureTa，a filter has been used and the sample data have been averaged.After getting accurateThandTa，the regulating factoradjis concluded by comparing default value with the calculated differenceΔT.The factor adj can be computed as follows：

Where k is a factor to control the response speed.It may been set different values at different situations in case of current overshoot of the circuit.

2　Experiment and Results

Measurement of resistance and TCR of the heater and the temperature sensor has been conducted and the heater is 99.158Ω of which TCR is 0.003 08/℃at room temperature.For the temperature sensor，the resistance is 2.3 kΩand its TCR is 0.002 51/℃.

To test the operation of the thermal wind sensor，wind speed experiment and static property experiment have been performed.

Wind speed measurement was done with the wind sensor in a wind tunnel capable of setting the flow rate of air，which has then been measured with the thermal wind sensor.

This experiment has been repeated three times and the results are showed in Fig.4 and Fig.5.These figures display that the presented wind sensor system is working well during the wind range from 0 m/s to 20 m/s with accuracy of less than 0.5 m/s.Its relative error is less than 8%when wind speed is lower than 10 m/s and 4%at a wind speed higher than 10 m/s.

Fig.4　Relationship between wind speed tested by the thermal wind sensor and wind speed set by the wind tunnel

Fig.5　Relative error of the output of the thermal wind sensor at different wind speed

Fig.6 shows the result of response speed experiment，which illustrates that the output reaches a stable value in 15 seconds after wind speed changed.The figure also shows that there is inconformity between different wind curves，which is induced by experiment methods.For the wind speed was changed gradually by manual operation，the time that flow rate of air in the wind tunnel need to change and reach steady state can not be the same in multiple operations and then the difference of response speed between different wind curves appeared.This phenomenon can be promoted in future experiment by adjusting experimental operation.

Fig.6　Output of the thermal wind sensor changes with time when wind in the environment changes

The test about static property of the wind sensor system has been done in the climate chamber.With the help of the climate chamber，the air temperature was varied from-20℃ to 40℃.The result showed that the static output is almost independent on the air temperature.The deviation observed here is in the range of 0.4 m/s caused by the climate chamber which has air flow when changing the air temperature in it（Fig.7）.

Fig.7　The relationship between the output of the thermal wind sensor and the ambient temperature when there is no wind in the environment

3　Conclusion

The software method for CTD control of the thermal wind sensor has been developed in this paper.Experiments show that this method can maintain the CTD mode of the wind sensor which can reduce the temperature drift effectively.This wind sensor system can also monitor wind speed from 0 m/s to 20 m/s in less than 20 seconds with accuracy of less than 0.5 m/s.This method has less limit to the material of temperature sensitive resistors and also reduces the scale of the peripheral circuit.The wind sensor can have greater performance by improving the algorithm and test methods.

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王芳（1991-），女，硕士研究生，东南大学MEMS教育部重点实验室，主要从事传感器系统信号检测与控制方面的研究；

秦明（1967-），男，博士生导师，教授，博士，东南大学MEMS教育部重点实验室，从事CMOS兼容MEMS热式风速风向传感器、CMOS集成湿度传感器、CMOS气压传感器等项目的研究，mqin@seu.edu.cn。

热式风速计恒温差控制的软件实现及其性能测试*

王芳，秦明*

（东南大学MEMS教育部重点实验室，南京210096）

介绍了一种利用软件实现热式风速风向传感器恒温差模式（CTD）控制的方案。传感器系统采用单片机作为主控单元，对芯片温度和环境温度进行采样，并以此为依据调节输出功率，从而实现恒温差状态的控制。实验表明，采用这种软件控制方案，在无风状态下，在-20℃到40℃的环境温度变化范围内，热式风速风向传感器能够基本维持输出信号恒定不变。除此之外，该风速风向传感器系统还可以实现对0～20 m/s风速范围内的风速测量，最大测量误差不超过8%，并且具有低于15 s的响应速度。

MEMS；热式风速计；CTD模式；单片机控制

TN409

A

1004-1699（2016）10-1478-05

2016-05-16修改日期：2016-06-25

项目来源：国家高技术研究发展计划项目（2013AA041106）