Design of automobile glass anti-fog controller based on MLX90614

Introduction
With the rapid development of economic income and automobile industry, hidden dangers also come along. One of the main causes of traffic accidents caused by fogging of car windshields is blurred vision of car drivers during driving. Nowadays, many models of cars have defogger gears. However, improper use of HVAC system by drivers can also cause a lot of energy consumption or accidents. It can be seen that intelligent defogger is not only a guarantee of driving safety, but also saves energy consumption of cars. This article analyzes the causes of fogging of car glass and selects appropriate sensors to realize intelligent detection of car windshield fogging controller.

1. Detection principle
When the moisture in the atmosphere reaches saturation, it forms water droplets or mist attached to dust in the air, and forms so-called fog attached to the glass. This is because the dew point of heavy air is very high, and the humidity in the air forms a saturated state after encountering cold solids. It can be seen that the main reason for the fog on the windshield is that the temperature of the windshield drops to the temperature of the dew point of the air in the car. Based on

this principle, we can use the MLX90614 sensor to monitor the temperature in the car in real time. The MLX90614 sensor is a product customized by Melexis for temperature detection in space areas. The infrared-sensitive thermopile detector chip and signal processing ASSP (application-specific integrated circuit) chip are integrated in the TO-39 package. The high-precision thermometer is realized due to the integration of low-noise amplifiers, 17-bit ADCs and powerful DSP units. It has the characteristics of small size and low cost, wide factory calibration temperature range, and the accuracy can reach 0.5℃ in the range of 0-50℃. It has a single voltage of 3V/5V and uses an SMBUS/PWM interface. The product grade meets the automotive standard. The factory setting is SMBus output, and the resolution in the entire temperature measurement range is 0.02°C.



Figure 1. MLX90614 sensor


Figure 2. MLX90614 accuracy table

From the above analysis, the use of MLX90614 sensor can fully meet the requirements.

2. Hardware circuit design
In the intelligent anti-fog controller for automotive glass, the hardware circuit mainly includes power supply LDO circuit, main controller circuit, and detection circuit based on MLX90614 sensor.


Figure 3. System composition block diagram

2.1. LDO circuit
In the intelligent anti-fog controller for automotive glass, 48V is stepped down to 5V, and then passes through a dedicated LDO chip to reach the ideal voltage. In this solution, we use the integrated RN5RT33AATR chip to reduce it to 3.3V.


Figure 4. LDO filter circuit

2.2. MLX90614 sensor driver circuit
As the key device in this article, the MLX90614 sensor supports SMBUS and PWM interface methods. Both methods are listed in this article. However, we recommend the SMBUS method because it is compatible with the automotive interface. [page]

Figure 5. MLX90614 PWM output driver circuit

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


Figure 6. MLX90614 SMbus output driver circuit

After the EEPROM is configured as PWM and POR, the PWM mode is free running. For PWM working mode, the SCL pin must be high level.

2.3. MCU circuit design
In this solution, we selected the cost-effective Renesas microcontroller as the MCU, its model is R5F101AG. The main controller mainly completes the drive and communication with MLX90614, data analysis and processing.


Figure 7. Main controller 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); //display temperature 
  Temh=wreadtemp(); 
  displayw(Temh); 
  delay(10000); 
} }
4. Conclusion
MLX90614 is a system-on-chip sensor suitable for regional temperature detection applications. This chip is specially designed for automotive applications, and of course it is also suitable for applications based on regional temperature detection. The automotive glass intelligent anti-fog controller designed in this article is a typical example.