With the rapid development of economic income and the automobile industry, hidden dangers also follow. Fog on the windshield of a car leads to blurry vision during driving and is one of the main reasons for traffic accidents. Nowadays, many models of cars have defogging gears. However, improper use of the HVAC system by the driver can also cause a large amount of energy consumption or accidents. It can be seen that smart defogging is not a guarantee for driving safety alone. It also saves the energy consumption of the car. This article analyzes the cause of the fogging of automobile glass and selects the appropriate sensor to realize the intelligent detection of the automobile windshield fogging controller.
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1. Detection Principle
After the moisture in the atmosphere reaches a saturated state, water droplets or mist are formed on the dust in the air, and so-called mist is formed on the glass. This is because the heavy dew point of the air is very high, and the humidity in the air becomes saturated when it meets the cold solid. It can be seen that the main cause of the fog formed on the windshield is the temperature drop of the windshield and the temperature of the dew point of the air in the cabin.
Based on this principle, we can use the MLX90614 sensor to monitor the temperature in the cabin in real-time. The MLX90614 sensor is a product customized by Melexis for space area temperature detection. The infrared-sensitive thermopile detector chip and the signal processing ASSP chip are integrated into the TO-39 package. Due to the integration of low noise amplifiers, 17-bit ADCs, and powerful DSP units, high-end thermometers can be realized. It has the characteristics of small size and low cost. It has a wide range of factory calibration temperatures, which can reach 0.5°C in the range of 0-50°C. Its single voltage 3V/5V adopts SMBUS/PWM interface. This product grade meets automotive standards. Its factory setting is SMBus output, and the resolution of the whole temperature measurement range is 0.02°C.
From the above analysis, the MLX90614 sensor can fully meet the requirements.
2. Hardware Circuit Design
On the automotive glass intelligent anti-fog controller, the hardware circuit mainly includes the power supply LDO circuit, the main controller circuit, and the sensor detection circuit based on MLX90614.
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2.1 LDO Circuit
In the automotive glass intelligent anti-fog controller, 48V is used to step down to 5V, and then through a dedicated LDO chip, the ideal voltage is reached. In this program, we use the RN5RT33AATR chip to reduce to 3.3V.
2.2 MLX90614 Sensor Drive Circuit
As the key device MLX90614 sensor in this article, it supports SMBUS and PWM interface modes. This article lists these two methods. However, we recommend the SMBUS method because it is compatible with automotive interfaces.
MLX90614 has a clamping diode, which is connected between SDA/SCL and VDD. By supplying power to the MLX90614 device, the SMBUS line does not become a load.
After the EEPROM is configured as PWM and POR, the PWM mode is free running. For PWM operating mode, the SCL pin must be high.
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MLX90614 is a system-on-chip sensor suitable for area temperature detection applications. This chip is specially designed for automotive applications, and of course, it is also suitable for applications based on area temperature detection. The intelligent anti-fog controller for automotive glass designed in this article is a typical example.
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