Carbon dioxide sensor Better air quality in the vehicle, without restricting the range

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FAULHABER Drive Systems

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With a special CO₂ sensor coupled with a microcontroller for signal processing, the air conditioning can be efficiently controlled in an electric car. How exactly this works and what advantages the sensor system offers.

Edgar Schäfer is a Field Application Engineer Automotive at Rutronik.

The limited range and lifespan of batteries is one of the biggest technical challenges when using electric vehicles. One way to increase this is to improve the efficiency of the entire vehicle. However, there are large consumers in the automotive environment that run counter to this. One of these is the air conditioning system. The electric drive generates less power loss than an internal combustion engine, and therefore also less waste heat that can be used to heat the interior. In electric vehicles, additional electrical heating is required to achieve or maintain the desired temperatures.

Problem: Stale air in the vehicle

One way to increase the efficiency of the climate system is to reuse the air in the interior (recirculated air). In the winter, the already warmed air is heated again, in the summer the air-conditioned air is cooled again and led into the interior. Since a smaller temperature difference has to be bridged, less energy is needed.

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A major disadvantage of this method is that no fresh supply air is brought into the interior. If the spent air is not renewed, the CO₂ level rises and the quality of the indoor air deteriorates. This can lead to headaches, fatigue and an unpleasant feeling while driving. It is important to ventilate the indoor air regularly. Ventilation measures are required from a CO₂ content of 1,000 ppm.

Goal: Better air quality inside the vehicle

CO₂ sensors can provide a solution by being integrated into the vehicle's climate system. They monitor the air quality in the vehicle and can issue a warning or directly mix in fresh air at correspondingly high CO₂ levels in order to maintain a healthy air quality. The choice of the right sensor for the application in electric vehicles depends on various factors, such as the size of the vehicle, the desired measurement range and the type of measurement. The dimensions, performance and cost of the sensors are also crucial.

A CO₂ sensor, the XENSIV PAS by Infineon, with a small form factor of 14.0 mm x 13.8 mm x 7.5 mm, reduces the space required by more than 75 percent compared to conventional CO₂ sensors. At the same time, it provides precise CO₂ measurement based on MEMS technology.

A microcontroller for signal processing

The CO₂ sensor XENSIV PAS integrates on a printed circuit board a photoacoustic transducer with detector, infrared source and optical filter, a microcontroller for signal processing and algorithms, and a MOSFET chip for operating the infrared source. The integrated microcontroller performs ppm calculations as well as advanced compensation and configuration algorithms. The result is the actual CO₂ content, not just a correlation.

In addition, various configuration options such as measuring frequency or basic calibration and interfaces (UART, serial I²C, PWM interface are available. The measuring range for CO₂ extends from 0 to 32,000 ppm. The accuracy is ±30 ppm or ±3 percent (0 to 8,000 ppm) of the measured value. The manufacturer guarantees this for the measuring range from 400 to 5,000 ppm. This is sufficient for this application, as the typical atmosphere has a CO₂ content of 400 ppm, and the value in the vehicle cabin is usually higher.

First SMD-capable CO₂ sensor

Additional benefits for the users of the sensors lie in the production process. According to their own reports, Infineon offers the first SMD-capable CO₂ sensor (SMD package, available on tape & reel), which meets the international JEDEC standard for lead-free surface mounting in the reflow process and is designed for easy assembly and system integration even at high volumes.

The CO₂ sensor XENSIV PAS also stands out due to its high flexibility thanks to various configuration options and plug-and-play. In addition, an evaluation kit is available, consisting of the CO₂ sensor and a microcontroller from the PSoC 4100S family for data evaluation. Thanks to its properties and capabilities, air conditioning systems in electric vehicles can be controlled with the CO₂ sensor. On the one hand, it ensures optimal air quality and thus also contributes to the safety of the passengers. On the other hand, it increases efficiency and thus the range and battery life. (heh)

Background: The PAS method

The PAS method is based on the photoacoustic effect. Gas molecules absorb light of a certain wavelength and thereby expand. For carbon dioxide, it is the wavelength 4.2 μm. Light pulses are emitted in rapid succession via an infrared source. Only light with a wavelength of 4.2 µm enters the sensor chamber through an optical filter that is specifically tuned to CO₂ molecules.

The CO₂ molecules in the sensor chamber absorb the energy. Rapid heating and cooling lead to thermal expansion and contraction. These processes generate a change in pressure, which is detected by the highly sensitive MEMS detector. The higher the CO₂ concentration in the chamber, the stronger the signal. The signal processing is done by an integrated microcontroller, which outputs the result in real-time as ppm (parts per million). For accurate result output, the acoustic detector is optimized for low frequencies and the absorption chamber is shielded from external noises.