Design of Automobile Glass Anti-fog Controller Based on MLX90614

introduction

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With the rapid development of economic income and the automobile industry, hidden dangers have followed. One of the main causes of traffic accidents caused by blurring of the car driver's vision during driving due to fogging of the windshield of the car. Today, many models have demisting gears. However, improper timing of the driver's use of the HVAC system can also cause a lot of energy consumption or accidents. It can be seen that intelligent defogging is not a guarantee of driving safety alone, and also saves energy consumption of the car. This paper analyzes the causes of fogging of automotive glass and selects appropriate sensors to realize intelligent detection of windshield fogging controllers.

1, detection principle

After the moisture in the atmosphere reaches saturation, water or mist is formed on the dust in the air, and the so-called fog is formed on the glass. This is because the air has a high dew point and the humidity in the air is cold. The solid forms a saturated state. It can be seen that the main cause of the fog formed on the windshield is the temperature of the windshield and the temperature of the air dew point in the cabin.

Based on this principle, we can monitor the temperature inside the cabin in real time through the MLX90614 sensor. The MLX90614 sensor is custom-designed by Melexis for space zone temperature sensing. The TO-39 package integrates an infrared sensitive thermopile detector chip and signal processing ASSP (Application Specific Integrated Circuit) chip. Thanks to the integrated low noise amplifier, 17-bit ADC and powerful DSP unit, high-precision thermometers are realized. It has the characteristics of small body and low cost. It has a wide range of factory calibration temperature. It can reach 0.5°C in the range of 0-50°C, single voltage 3V/5V, SMBUS/PWM interface, and the product grade meets the vehicle standard. Set to SMBus output with a resolution of 0.02 °C over the entire temperature measurement range.

Figure 1. MLX90614 sensor

Figure 2. MLX90614 precision table

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 MLX90614 sensor detection circuit.

Figure 3. System block diagram

2.1, LDO circuit

In the automotive glass intelligent anti-fog controller, the voltage is reduced from 48V to 5V, and the dedicated LDO chip is used to achieve the desired voltage. In this scenario, we used the RN5RT33AATR chip to drop to 3.3V.

Figure 4. LDO filter circuit

2.2, MLX90614 sensor drive circuit

As the key device in this article, the MLX90614 sensor supports SMBUS and PWM interface modes. This article lists both methods. But we recommend the SMBUS method because it is compatible with the car interface.

Figure 5. PWM output drive circuit of MLX90614

The MLX90614 has a clamping diode that is connected between SDA/SCL and VDD to provide power to the MLX90614 device without the SMBUS line becoming a load.

Figure 6. SMbus output driver circuit for the MLX90614

After the EEPROM is configured for PWM, POR, the PWM mode is free running. For the PWM mode of operation, the SCL pin must be high.

2.3, MCU circuit design

In this scheme, we chose the Renesas MCU with high cost performance as the MCU, and its model is R5F101AG. The main controller mainly completes the drive and communication, data analysis and processing with the MLX90614.

Figure 7. Master circuit

3, software design

In terms of software design, it mainly includes initialization, MLX90614 configuration, MLX90614 data reading and temperature calculation.

Void main()

{ uint Tem; // temperature variable uint Temh;

SCK=1;

SDA=1;

Delay(4);

SCK=0;

Delay(1000);

SCK=1;

Screen_initial();

While(1) {

Tem=readtemp(); //Read temperature

Display(Tem); //Show temperature

Temh=wreadtemp();

Displayw(Temh);

Delay(10000);

} }

4 Conclusion

The MLX90614 is a system-on-chip sensor for area temperature sensing applications. This chip is specifically designed for automotive applications and is also suitable for applications based on zone temperature sensing. The automotive glass intelligent anti-fog controller designed in this paper is a typical example.

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