Sensors in Automotive HVAC Systems

Some people like their car cabins warmer, others cooler. So is it any surprise that personalized climate control systems are becoming popular in mid-range cars? While this is not new to luxury cars, it is still not standard in cars in the wider market, says Thomas Conway, product design and launch engineer for HVAC (ventilation, heating and air conditioning) electronics at Behr America. Automotive electronics supplier Behr America, based in Troy, Michigan, is working closely with EPCOS to help automakers bring these high-end features to the mass market. The latest result of their efforts: A 2007 SUV from a U.S. automaker will be equipped with a new climate control unit from Behr that allows passengers to select and maintain their own desired temperature. EPCOS made a key contribution to this application: The temperature sensors used in the ventilation and air conditioning units designed and manufactured by Behr come from EPCOS. A new concept for in-car sensors A total of three types of sensors are included in every car's air conditioning system (see Table 1). One is the sensor for the evaporator, which must meet the highest requirements for reliability and fast response time. The sensors have been subjected to immersion tests at 80°C for up to 2,000 hours. With this product, EPCOS has set the highest market standards for many years. In addition, the air conditioning system contains four duct sensors. The sensor concept used for the interior temperature sensor for the passenger compartment is also new to Behr and EPCOS. "While the evaporator and duct sensors are existing types, Behr and EPCOS jointly developed a custom design for the interior temperature sensor," says Holger Hegner, product marketing manager for thermal sensors at EPCOS in the United States . The two partners have already successfully developed and produced many other types of air conditioning components, including the fin probe for the evaporator of the Chrysler Sebring and Stratus models, which has long been well-known among European users. EPCOS's development work focused on determining vibration resistance, response time (less than 4 seconds) and impedance temperature response (2.8 kΩ at 25°C with an error of ±2.5%). The product had to be adapted so that it was not susceptible to vibration, humidity and other factors. EPCOS successfully developed a ceramic negative temperature coefficient (NTC) sensor element based on the specifications required by Behr and its users (see Table 1).

Table 1: Thermistors in Behr HVAC systems

All sensors are RoHS compliant.

The second key issue that EPCOS had to solve was how to connect the temperature sensor unit to the dashboard. The solution was to use a clamp and sealing foam. "We had to ensure a good fit between the sensor component and the mounting substrate material," Conway explained. The general dimensions and tolerances of the clamp (including position, volume, shape and material) determine the thickness and tolerance requirements that the substrate material manufacturer must meet (Figure 1). "Even small changes can have an impact on the rest of the structure," Conway said. Therefore, he particularly emphasized the importance of a tight fit during the design process.

Figure 1: In-vehicle components mounted behind the dashboard

The cooperation has yielded fruitful results

The special feature of the cooperation is that the OEM and the first and second tier suppliers are very actively involved in the design process, which is a common phenomenon in the United States. In addition to the ability to quickly respond to the OEM's requirements, the closeness and depth of the cooperation between Behr and EPCOS is also a decisive factor in the success of the project. Regular joint meetings also provide important support for the successful completion of the custom design, which spans design coordination, prototype manufacturing and testing, tool generation, production of first samples, production testing and the first production part certification process (PPAP) report.

The dimensions and tolerances of the temperature sensor components and their accessories were developed on 2D drawings and 3D CATIA models (Figure 2).

Figure 2: 3D model of the in-vehicle sensor components

EPCOS passes this data to Behr's design team, which performs adaptations and preliminary tests to ensure correct fitment. Air flow and temperature tests are then carried out by a team of experts, with the goal of ensuring perfect functionality. Although there are mandatory specifications, every time a function is added or an angle is modified, the entire design must be re-adapted. Automotive design is a self-optimizing process. Therefore, it has become customary to exchange information on the current status of processing, testing and documentation in the form of weekly meetings and special sessions at key stages of the design process in order to meet the customer's schedule.

As the results show, all the efforts have paid off. "Our design team in Michigan has worked very effectively with the EPCOS team in Berlin," Conway is very satisfied. The prerequisite for this success is the global network that Behr and EPCOS have jointly established. "The cooperation between our European design center and the US development team in purchasing, development and quality assurance is almost seamless," adds Hegner of EPCOS.