Design of wind speed sensor to help you detect air conditioner

　　In many fields such as environmental meteorology, household appliances, industrial equipment, and health care, air velocity is an important detection parameter, especially in today's society, various fans, air conditioners and other household appliances have entered homes, offices and public places in large numbers. Based on the above reasons, this paper designs a wind speed measurement circuit for measuring wind speed, which has the characteristics of low cost, easy use, high measurement accuracy, etc., and can be used in conjunction with other integrated chips such as single-chip microcomputers to become an application circuit for other systems.

　　1 Establishment of mathematical model

　　1.1 Temperature characteristics of Ptl00

　　Platinum thermal resistors are internationally recognized mature products. They are widely used due to their stable performance, good shock resistance and high precision. The following is the relationship between the Ptl00 resistance and temperature:

　　

　　Where Rt is the resistance value of the platinum thermal resistor at t℃; R0 is the resistance value of the platinum thermal resistor at 0℃; A=3.968×10-3; B=-5.847×10-7; C=-4.22×10-12. In the range of 0～100℃, the effect of B value is not obvious, and Rt and R0 are approximately linear, that is, Rt=R0×(1+At).

　　1.2 Heat balance equation of Ptl00

　　When a heated object is placed in a fluid, the heat loss of the object is mainly thermal radiation and thermal convection. When the temperature is low and the radiation heat dissipation can be ignored, the heat transfer of the object is mainly thermal convection. When the speed of the fluid increases, the heat loss of the object also increases. If the platinum resistance is heated electrically, it will reach an equilibrium temperature determined by the fluid flow rate.

　　We use platinum thermal resistors as heating objects. Since the change in temperature causes the change in the resistance of the platinum thermal resistor itself, a mathematical model of fluid velocity and bridge circuit output voltage can be established through a bridge circuit. This principle is used to measure wind speed.

　　Convective heat transfer refers to the heat transfer process that occurs when a flowing fluid flows over a stationary solid interface due to the temperature difference between the two. When air flows over a platinum thermal resistor, the amount of heat transferred per unit time is:

　　

　　Where h is the convection heat transfer coefficient; A is the convection area; △t is the temperature difference between the fluid and the interface.

　　According to the formula of heat transfer, there are Nusselt characteristic numbers and the fluid flow along the interface is all laminar flow :

　　Where uf is the velocity of the fluid; L is the interface length; vm is the average kinematic viscosity; Prm is approximately equal to 0.710 for air, and λm is the average thermal conductivity.

　　When the current heats the thermal resistor, its power is. When the heat generated by the thermal resistor per unit time W and φ are equal, the thermal resistor reaches a thermal equilibrium state.

　　

　　The following conclusion can be drawn from the above: when the thermal resistance temperature and the ambient temperature are constant, the current is proportional to the 1/4 power of the wind speed.

　　2 Circuit Working Principle

　　As shown in the circuit, the voltages at both ends of the two branches a and b are equal. According to the thermal power formula, its heat generation efficiency is about 1/10 of that of branch a. Therefore, the effect of current on branch b can be ignored when considering thermal work.

　　

　　When the wind speed is 0m/s, the ratio of the resistance of R2 and Ptl000 is designed to be less than the ratio of R1 and (Ptl00+R3), the amplifier outputs a low level, the base potential of the transistor decreases, and the collector current of the transistor Ql increases. Since the shunt ratio of the two half-bridges is about 10:1, the current of Ptl00 increases according to the shunt principle of the parallel circuit, which increases the resistance of the platinum thermal resistor and reduces the voltage at point c. Finally, the feedback circuit adjusts the potential at point c to approach point d, reaching a balanced state, and the voltage at point c is used as the output value representing the wind speed. When the wind speed increases, the convective heat dissipation increases, the temperature of Ptl00 decreases, and its resistance decreases, making the voltage at point c higher than the voltage at point d. The output voltage of the amplifier decreases, resulting in an increase in the base current of the transistor Q1. The increase in the collector current increases the resistance of Ptl00, and finally reaches a new stable equilibrium point. From the above analysis, it can be seen that as the wind speed increases, the controlled current increases, and the output voltage of terminal c increases. Due to the use of differential measurement, and the two sensing elements of the half-bridge configuration are both platinum resistors, the effect of gas temperature on the circuit measurement value can be ignored. Without the addition of other temperature compensation circuits, it can be used in a wide temperature range, which is suitable for most field measurement environments. [page]

　　3 Experimental results and error analysis

　　In order to verify the designed wind speed measurement sensor, a simple experimental verification platform was built. The experimental verification platform uses the EE66-VB5 anemometer as the standard wind speed measurement unit to compare the measurement data obtained by the designed sensor and the measurement circuit. The anemometer EE66-VB5 is a high-precision wind speed measurement sensor with a measurement range of 0-2m/s, an output voltage of 0-10V, a wind speed accuracy of ± (0.1m/s+3% of the measured value), a response time of 0.2 seconds, and an operating temperature of -10-+50°C. Due to its high accuracy and sensitivity, the experiment takes its measured value as the true value, places the anemometer and the sensor to be measured in the same environment, and compares its measured value with the measured value of the wind speed sensor composed of platinum thermal resistors at the same wind speed. The performance of the wind speed sensor composed of platinum thermal resistors is analyzed. The following are the output voltages at different wind speeds, and the measured results are shown in Table 1.

　　

　　

　　In Figure 2, due to the influence of the amplifier saturation voltage, when the input voltage is 0V, its output voltage is about 0.25V. After calculation, the standard error of the wind speed sensor designed in this experiment is 0.085, and its deviation is mainly caused by the bias of the EE66-VB5 probe and the platinum thermal resistor sampling point, as well as the instability of the wind speed of the small fan.

　　4 Conclusion

　　In summary, this paper describes the mathematical model and circuit principle of the platinum thermal wind speed sensor. And through the specific measurement, analysis and calculation of the experimental data, the error of the wind speed sensor in this experiment is obtained. The wind speed sensor designed in this experiment has strong practicality due to its simple circuit, low cost, low power consumption and high accuracy. It can measure the wind speed of household appliances such as air conditioners and fans, and can also be used in other industries such as the automotive industry to detect the air flow per unit time.