The current automobile development direction and situation force the automobile air conditioner to develop in the direction of standardization and to the direction of all season compartment climate control. In the modern automobile automatic air conditioning system, the most important two aspects are temperature gradient control and relative humidity control. The air conditioning ECU detects the signal in the car according to the external climate conditions and temperature and humidity sensors, and automatically turns on the heating, refrigeration or dehumidification device, so that the temperature and humidity in the car can be maintained within a more comfortable range of the human body, and can automatically defrost according to the temperature difference inside and outside the car, so as to avoid the reduction of visibility caused by windshield fogging.
In the development of automotive air conditioning automatic system, China started late, and the research and development of air conditioning sensors lags behind developed countries. Now the full-automatic air conditioning sensors used by domestic brand cars rely on imports, which restricts the development level of China’s automotive industry to a certain extent. In view of the current situation, an integrated design idea is proposed, and an integrated sensor combined with forced air flow scheme is adopted. This design concept helps to improve China’s innovation level in this field.
1. Design idea
The relative humidity of the environment is changed by the temperature of the environment, so the temperature at a certain point must be measured at the same time when measuring the relative humidity. Considering the sensitivity, frequency response characteristics, linear range, stability and accuracy of the vehicle environment and sensors, the current type integrated temperature sensor is adopted as the temperature sensor, and the IH3605 integrated humidity sensor is adopted as the humidity sensor. In order to ensure the authenticity and reliability of the measured data and prevent condensation and ash collection on the sensor surface, the air on the sensor surface shall have a certain fluidity and must be equipped with a fan for forced flow. The motor of the fan shall be brushless DC motor, so as to avoid electromagnetic interference, and the three units shall be designed as a whole. So as to achieve the characteristics of compact structure, small volume, accurate measurement and high reliability. The sensor structure is shown in Figure 1.
The humidity and temperature control principle of automobile air conditioner is shown in Figure 2. As the moisture emitted by the human body increases the humidity in the compartment, the humidity measurement unit will detect the change of 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 compartment to flow through the surface of the cooler. After cooling, cooling and dehumidification, it is heated to a certain temperature, so as to achieve the effect of defogging and maintain a more comfortable environment in the carriage; The temperature measurement unit is used by the air conditioning controller to measure the temperature change in the compartment, control the heating solenoid valve to open the circulating water heating time and heating time.
2. Integrated humidity sensor
IH3605 humidity sensor is a capacitive integrated humidity sensor. It makes the capacitive humidity sensor and conditioning circuit on the ceramic substrate together. The water vapor in the air enters the dielectric layer through the protective polyacetic acid layer and the porous platinum layer to change the dielectric constant, so as to change the capacitance. Through the conversion circuit of capacitance / frequency and frequency / voltage, the capacitance change is converted into DC voltage output. At 25 ℃, the output DC voltage uo has a linear relationship with the relative humidity Ho:
3. Integrated temperature sensor
Such sensors are divided into: current type integrated temperature sensor, voltage type integrated temperature sensor, frequency type integrated temperature sensor and digital type integrated temperature sensor. The design scheme adopts a voltage type integrated temperature sensor. The main feature of this sensor is that the current and voltage are relatively low, generally at 50 μ Below a. Therefore, the self heating of the device is very small. In the integrated design, 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 the same as those of T3
4. Fan drive circuit design
As shown in Figure 4, U2 is a special driving circuit for US91 double wound resistance brushless DC motor. It has unique recovery circuit and anti-interference suppression ability to control its electromagnetic radiation within an acceptable range. 12V power supply is directly connected from M + and m-terminals.
The design scheme makes full use of the advantages of integration. It has the advantages of small distributed capacitance, strong anti electromagnetic radiation ability, low cost, small volume and compact structure. Through the test, it has the advantages of stable performance, good output signal linearity, high precision level, accurate measurement, convenient debugging and calibration, can form product consistency, can work continuously in the environment of – 50-100 ℃, and the RH range can reach ± 0.5% accuracy when the temperature is 20% – 90%. It is especially suitable for the environmental measurement requirements of automotive full-automatic air conditioning. Development and production can be implemented to replace the current imported products.
Responsible editor: GT