Improve car safety with advanced infrared sensor technology

In contemporary cars, the implementation of Occupancy Classification System (OCS) has become an increasingly standard configuration in security functions. When a collision occurs, the OCS ensures that the airbag opens in the most efficient way to protect the passenger and avoid injury. It uses some form of sensor technology to determine the height and size of the occupant. This article will discuss how the advancement of next-generation OCS technology will protect passenger safety to a greater extent.

Here we introduce the basic structure of the OCS system. The sensor system will first confirm if a passenger is occupying a seat and evaluate the physical characteristics of the passenger. This information is then transmitted to an electronic control unit (ECU) to determine what reaction to take to trigger the airbag at full speed or at a slower speed. Early OCS systems often used reasonable basic sensing mechanisms, including pressure sensors embedded inside the seat frame, which can be used to measure the occupant's weight and thereby derive a rough height data. Similar to other methods of estimating by the tension of the seat belt, this method is also inaccurate. First, the correlation between weight and height is not accurately determined. The above method also does not take into account some other possibilities, such as placing a box of items on the seat instead of occupying the seat passenger, because this technique cannot confirm whether the seat is a passenger or other inanimate item. In addition, integrating sensors inside the seat frame is also very expensive, especially if repair or replacement is required.

As children's booster car seats and rear-facing baby/infant car seats begin to appear on the market, the need for traditional technological change is further highlighted. Concerns about how to deploy airbags. Therefore, there is now a trend in the automotive industry to adopt more sophisticated classification systems, and often the most utilized is photovoltaic technology.

For any OCS solution using photodetection technology, the basic criteria are:

• It must be able to observe the area of ​​interest (ie there should be no obstruction to observe the passenger from the installed location).

• Field of view (FoV) requirements are relatively limited, usually 40o & TImes; 10o is more than enough.

• Reasonable lower frame rates from 4 Hz to 8 Hz.

• Sensors with a range of less than 1 meter can be used due to the close proximity of the target.

These do not really constitute major technical difficulties, but when the system is carefully studied at a more detailed level, the difficulties begin to surface. OCS systems implemented using optoelectronic technology have several significant disadvantages associated with the use of such a scheme if a portion of the visible spectrum is used as its sensing medium.

The problem of using OCS for visible light

When using visible light as the OCS sensing mechanism, there is of course a change in day/night illumination level, which requires the system to include active illumination in the car (this is usually placed on the roof or front bracket of the car). Hats, long hair, beards, brightly colored items, clothing, etc. can also adversely affect the optical classification process. Therefore, this sensor technology and its accuracy in judging irregular exposures is a major problem. Visible light sensing systems require considerable processing power and are equipped with relatively expensive microcontroller units (MCUs).

The use of far-infrared (FIR) technology with operating wavelengths ranging from 5 μm to 15 μm is becoming an increasingly interesting topic in automotive passenger-category applications. One of the great advantages of far-infrared imaging systems is that they better distinguish between inanimate objects and a real passenger because they detect heat radiation at the frequency of the body's heat. This method does not rely on ambient light, there is no performance difference between daytime or night work, so there is no need to actively illuminate in the car. In addition, it requires a relatively small amount of processing power. These last two points greatly reduce the complexity and cost of the entire system.

FIR performance considerations

The FIR-based OCS does not require a wide dynamic range and the sensor array needs to be able to operate between -40 ° C and 85 ° C. The requirements for optical resolution are also not too harsh, as the location of the passenger will be in a known area. However, thermal resolution is very important and needs to be in the range of ±0.25oK. It is important to consider the thermal effects of sunlight entering the vehicle. To achieve this, filters are used to filter out the visible and near-infrared (NIR) spectra so that they do not interfere with the sensor in any way. While signal processing for this type of sensing system is already simpler than visible light sensors, there is still a need to reduce overall complexity to ensure higher operating speeds and signal-to-noise ratios while reducing system cost and ease of implementation.

Through technical cooperation with Heimann Sensor GmbH, Melexis is now able to develop compact and cost-effective FIR sensor arrays that combine high-sensitivity thermopile (thermopiles) technology with cutting-edge and streamlined A combination of functions/performances such as signal processing, each FIR detection device has multiple thermopile sensor units that can be used to create a simple calorific value map in real time for the target area. Each thermopile unit in the array has its electrical signal amplifier and data converter, which avoids time multiplexing of the thermopile signal, greatly reduces signal noise, and it also provides the necessary performance without Need to deliberately use a more expensive microbolometer.

The FIR sensor array greatly simplifies the thermal imaging system and, once integrated, captures data from 64 pixels at once. It has an adjustable frame rate that is sufficient to support the performance required by even the most high-end OCS systems. Accuracy levels of ±1.5°K are maintained when operating from 0°C to 50°C. For each component of the sensing element, it has its own signal processing capability, and the noise problem that occurs in the visible light OCS system can be effectively avoided here. In addition, its interface is also easier to implement. The use of a high speed I2C compatible digital serial interface and a trigger mode synchronized with the control unit means that these devices can be used not only separately, but also together to form an array with higher imaging resolution if needed.

in conclusion

About 35,000 people die on the roads of EU member states each year, and the importance of developing a more effective car safety system cannot be underestimated. The use of airbags has played a major role in reducing the number of deaths, but it is still necessary to further reduce them. Passive occupancy recognition mechanisms are being replaced by intelligent systems to better determine who or what items are in the passenger seat to further improve the effectiveness of airbag deployment, thus minimizing the risk of harm. By implementing a more advanced system based on optoelectronic technology OCS, greater progress can be made in passenger safety. However, it is clear that sensors operating in the visible spectrum tend to degrade due to changes in illumination levels and other irregularities. While FIR sensing technology can provide equal or better performance at much lower data processing costs than visible light solutions, it can be used more by automotive manufacturers.

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