Design of automotive air conditioning integrated system using IH3605 humidity sensor

The current development direction and status of automobiles force automobile air conditioners to develop in the direction of standardization and all-season cabin climate control. In modern automobile automatic air conditioning systems, the most important aspects are temperature gradient control and relative humidity control. The air conditioning ECU automatically turns on the heating, cooling or dehumidification device based on the external climate conditions and the temperature and humidity sensors to detect the signals in the cabin, so that the temperature and humidity in the car cabin can be maintained within a comfortable range for the human body, and it can automatically defrost according to the temperature difference between the inside and outside of the car to avoid fogging of the windshield and reduced visibility.

my country started late in developing the automatic air conditioning system for automobiles, and the research and development of air conditioning sensors lags behind developed countries. The fully automatic air conditioning sensors used by domestic brand cars are now imported, which to a certain extent restricts the development level of my country's automobile industry. In view of the current situation, an integrated design concept is proposed, and an integrated sensor combined with a forced airflow solution is adopted. This design concept helps to improve my country's innovation level in this field.

1. Design Concept

The relative humidity of the environment changes with the temperature of the environment, so when measuring the relative humidity of a certain point, the temperature of that point must be measured at the same time. Taking into account the requirements of the automotive environment and the sensitivity, frequency response characteristics, linear range, stability and accuracy of the sensor, the temperature sensor uses a current-type integrated temperature sensor, and the humidity sensor uses an IH3605 integrated humidity sensor. In order to ensure the authenticity and reliability of the measured data and prevent condensation and dust accumulation on the sensor surface, the air on the sensor surface should have a certain fluidity and must be equipped with a fan to force flow. The fan motor should be a brushless DC motor to avoid electromagnetic interference, and the three units should be designed as an integrated whole. This achieves the characteristics of compact structure, small size, accurate measurement, and high reliability. The sensor structure is shown in Figure 1.

The humidity and temperature control principle of automobile air conditioning is shown in Figure 2. As the moisture emitted by the human body causes the humidity in the car to increase, the humidity measurement unit will detect the change in relative humidity. When it exceeds a certain value, the air conditioning controller starts the refrigeration compressor, and then the circulating fan forces the air in the car to flow through the cooler surface. After cooling and dehumidification, it is heated to a certain temperature to achieve the effect of demisting and maintain a more comfortable environment in the car; the temperature measurement unit is used by the air conditioning controller to measure the temperature change in the car and control the heating solenoid valve to open the circulating water heating timing and heating time.

2. Integrated humidity sensor

The IH3605 humidity sensor is a capacitive integrated humidity sensor. It manufactures the capacitive humidity sensor and the conditioning circuit together on a ceramic substrate. The water vapor in the air enters the dielectric layer through the protective polyester layer and the porous platinum layer, changing the dielectric constant of the dielectric, thereby changing the capacitance. The capacitance change is converted into a DC voltage output through the capacitance/frequency and frequency/voltage conversion circuits. At 25°C, the output DC voltage Uo is linearly related to the relative humidity Ho:

3. Integrated temperature sensor

This type of sensor is divided into: current type integrated temperature sensor, voltage type integrated temperature sensor, frequency type integrated temperature sensor, digital type integrated temperature sensor. This design adopts a voltage type integrated temperature sensor. The main feature of this sensor is that the current and voltage are relatively low, generally below 50μA. Therefore, the self-heating of the device is very small. When the integrated design is used, the unit will not affect the humidity measurement unit, and can directly become the input signal of the air conditioning control system, as shown in Figure 3.

In Figure 3, the base-emitter junction voltage and base-emitter junction area of ​​T5 are similar to those of T3,

4. Fan drive circuit design

As shown in Figure 4, U2 is a dedicated drive circuit for US91 dual-winding brushless DC motors. It has a unique recovery circuit and anti-interference suppression capability, which keeps its electromagnetic radiation within an acceptable range. The 12V power supply is directly connected to the M+ and M- terminals.

5. Conclusion

This design scheme makes full use of the advantages of integration. It has small distributed capacitance, strong anti-electromagnetic radiation ability, low cost, small size, compact structure. Tests have shown that it has stable performance, good output signal linearity, high precision, accurate measurement, convenient debugging and calibration, and can form product consistency. It can work continuously in an environment of -50-100℃, and the RH range can reach an accuracy of ±0.5% when the temperature is 20%-90%. It is particularly suitable for the environmental measurement requirements of fully automatic air conditioners in automobiles. It can be developed and produced to replace the current imported products.