AQS汽车外空气质量传感器应用指南

Automotive Air Quality SensorsUsing MiCS Air Quality SensorsThis application note describes the appropriate integration of a MiCS Air Quality Sensor in a modern automobile. MiCS is the AQS brand of e2v technologies.Important note : Reproduction and distribution of this document is restricted by e2v. The following specifications are subject to change to accommodate continuous improvement.The AQS functionThe Air Quality Sensor is based on two semiconductor gas sensors that detect pollution peaks in the traffic. It calculates an “Air Quality level” that is sent to the CPU of the HVAC. The HVAC can then open and close the recirculation flap so as to minimize the pollution in the cabin.Location in vehicleThe AQS module must be exposed to a source of external air at all times. The location must be chosen so as to maximize air renewal; dead spaces must be avoided. The best location is in the air intake BEFORE (upstream) the filter. When the flap is closed, it can create a “pocket” of stagnant air; for that reason, the AQS module should not be too “deep in the duct”.Preferably, the filtering membrane should be facing downward. If this is not possible, it should be vertical, but should never be facing upward, to prevent accumulation of dustIf the AQS module is attached with a bayonet to the air intake wall with its body “in the cabin”, the fixation should be sufficiently air tight so that no air from the cabin is pulled in front of the sensor head.HVAC unitAir inlet without AQSAirinlet with AQS DetailedviewFirst possibility : bayonet attachmentSecond possibility : attachment with plastic snap-in clipElectrical integrationConnector reference : Tyco C-967642 coding “B”Recommended control panel interface:Note on warm-up time:The Air Quality Sensor is turned on when the car is ignited. It takes approximately 20 seconds for the two sensitive elements to reach their chemical balance. During that time their resistances rise sharply. The output signal is exploitable only when the sensor resistances are sufficiently stable. The Air Quality Sensor output is therefore “valid” only after 20 seconds.The Air Quality Sensor outputs 4 Air Quality levels indicating a “rate of change of the pollution”:Î Fresh air : no pollutionÎ Level 1 : small pollutionÎ Level 2 : medium pollutionÎ Level 3 : big pollutionA typical way to manage the air inlet door closing is:If external temperature > 6°C:¾Close door if Air Quality level is Î 1, 2 or 3¾Open the door when the AQS level is Î “fresh air”If external temperature <= 6°C:¾Close door only if Air Quaity level is Î 3¾Open the door when the AQS level is Î “fresh air” or after 2 minutes(this will prevent too frequent closing and fogging of the windshield). Note that in cars equipped with an in-cabin Relative Humidity sensor, the Air Quality Sensor “recommendation” is typically overridden by the temperature and humidity conditions that can lead to flash windshield fogging. Similarly, when starting the car, under certain temperature conditions, the Air Quality Sensor signal will be ignored to favour a faster heating or cooling of the cabin.The MiCS dual gas sensor detects the level of pollutions due to both gasoline and diesel exhausts. The sensing resistances vary in a fully reversible way in the presence of those gases. The resistance values are typically in the range of 1kOhhms to 1MOhms.Each chip is dedicated to one gas family. The chip referred to as “CO sensor” detects a broad family of reducing gases such as CO, Toluene, Benzene, Pentane, and most common hydrocarbons.The chip referred to as “NO2 sensor” detects mostly the NO x molecules that are more present in diesel exhausts.The CO sensor signal therefore varies significantly in the presence of pollution from motorbikes and gasoline vehicles. The NO2 sensor signal therefore varies significantly in the presence of pollution from diesel vehicles such as diesel cars, buses and trucks.t in sAbsolute pollution detection means that the sensor assembly outputs the same value for the same gas concentration detected, regardless of: temperature, humidity, air flow, sensor production dispersion, age of the sensor, presence of other gases.However each of those variables has an effect on the sensor signal and therefore requires adequate compensation. Such compensation is costly and not fully accurate. The following paragraph provides some information on the effect of each factor as well as a discussion on potential solutions to reduce that effect:- Temperature: The sensitivity of semiconductor sensors decreases significantly with temperature, something of the order of 50% for a change of 20°C.- Humidity: relative humidity changes between 5% and 95% have a small effect on the sensor resistance and sensitivity.- Air flow: Air flow influences the sensor resistance value. This effect is negligible when using a relative algorithm but needs to be compensated otherwise. Some suppliers argue that a temperature compensation cancels that effect but our experience is that it is not fully the case.- Sensor production dispersion: When hundreds of thousands of sensors are produced, the “clean air” resistance is spread over a factor 10, typically 10k to 100k ohms. Similarly, the sensitivities are spread over a factor 2 to 3.- Semiconductor sensors are not fully stable: typically, over 6000 hours of ON-time over a period of 10 years, the “clean air” resistance can increase by a factor 2 to 5 and the sensitivity can increase or decrease depending on the environment. This is no problem for a relative system that makes use of the “auto-adaptation” principle(patented by Denso in 1981).- Finally, and probably most importantly, a lot of gases are present in the traffic and their proportions vary significantly from cases to cases. A sensor that is well calibrated for CO might not actually respond well in the presence of a strong pollution containing a majority of benzene, heptane and pentane. Therefore, calibrating with only one gas is a self defeating purpose. A multi gas calibration with a mixture that is more representative of the “traffic” could be envisioned but it would be even more costly and would still not in most cases provide theexpected absolute response.Relative treatment of the sensor signal allows to get rid of all obstacles mentioned above, while providing a signal that is close to the perception of the passengers. Indeed the human nose is “relative” in nature. A human being will detect 10ppm of ammonia if the air is very clean, but will not detect this same concentration in a heavy traffic pollution. Therefore, it is appropriate to control the incoming air that will be breathed by the passengers with a system based on a relative signal treatment because it will adequately minimize the discomfort of the passengers while not over activating the flap.A good Air Quality Sensor has the following attributes:¾Fast response to close the flap as soon as external pollution is significantly increasing¾Detection of both types of pollutionNO2 type … mostly from diesel enginesHydrocarbon types (& CO) … mostly from gasoline engines¾Good management of tunnels to avoid erroneous flap re-openings¾Homogeneous function on a large number of sensor¾Function valid for at least 6000 hours in vehicleThis paragraph intends to provide some basic information on a typical Air Quality Project schedule:- customer chooses a location of the AQS in the car- customer adapts HVAC unit or car to fit the AQS mechanically- customer designs the ECU software to read the signals of the AQS and generate the air inlet door closing- customer orders samples from e2v to perform road test to assess the function in the car with the support of e2v - agreement to freeze AQS software / function- customer can receive series parts 12 weeks after POThe standard AQS that is described in this application note has following features:¾12V power supply¾ PWM output¾ IP56 (watrertight)The AQS modules can be delivered with other features:¾Non watertight module to save on costs if the module body is inside the cabin¾Analog outputs. A solution with no microcontroller for significantly lower cost¾LIN output (more expensive)¾5V regulated power supply to save some electronics¾Specific connectors according to customer requirementsAll these options can require new tooling and therefore can create supplementary investment and generate a longer lead time.e 2v t e c h n o l o g i e sA P P L I C A T I O N N O T ECONFIDENTIAL Page 11 of 11 Rev A ReferencesEvery effort has been made to ensure the accuracy of this document at the time of printing. In accordance with the company’s policy of continued product improvement, e2v reserves the right to make product changes without notice.As these products may be used by the client in circumstances beyond the knowledge and control of e2v, e2v cannot give any warranty as to the direct relevance of this application note to a particular application. It is the clients’ responsibility to carry out the necessary tests to determine the usefulness of the products and to ensure their safe operation in said application.The data provided herein is for information purposes only. It does not constitute a specification, an offer for sale, or a guarantee of the feasibility and safety of a particular application. e2v accepts no liability for any losses, injury or damage resulting from the use of an e2v product in unverified operating conditions. Direct AQS customersTier OnesOEMsMiCS products are or will be on cars of the following brands。