Some like it hot, some like it cold. Is it any wonder then that individualized climate-control systems are enjoying increasing success in mid-range vehicles? While the feature is nothing new to luxury automobiles, it is still not standard in the wider automobile market, points out Thomas Conway, design and release engineer for HVAC electronics at Behr America. Together with EPCOS, the automotive supplier, based in Troy, Michigan, is working hard to help carmakers introduce these high-end features into the mass market. Their latest success: An American carmaker’s crossover SUVs for the 2007 model year will feature a new air-conditioning unit from Behr that allows passengers to select and maintain temperatures individually. EPCOS is making a major contribution to this application: The ventilation and air-conditioning unit designed and manufactured by Behr America is equipped with a temperature sensor unit from EPCOS.

New concept for the in-car sensor

A total of three sensor types are integrated into each vehicle’s air-conditioning system (see table). One is an evaporator sensor that must satisfy maximum reliability requirements and fast response times. It is subjected to 2,000 hours of water immersion tests at 80 °C. EPCOS has already been setting the highest market standard for many years with this product. Moreover, the air-conditioning system comprises four duct sensors. The in-car temperature sensor for the passenger compartment is based on a sensor concept new to both Behr and EPCOS. “Whereas existing types were used for the evaporator and duct sensors, Behr and EPCOS have jointly developed a customized design for the in-car temperature sensor,” says Holger Hegner, Product Marketing Manager for Thermistors at EPCOS in the USA.

	Figure 1: IN-CAR-ASSEMBLY
The in-car sensor assembly is mounted behind the dashboard. Behr specified a knee-shaped tube to accommodate the OEM's air-duct design.

The partners had already worked successfully together on the development and production of other air-conditioning units—including a fin probe for the evaporator of Chrysler’s Sebring and Stratus models—and can look back on a solid history of shared customers in Europe.

The development process at EPCOS focused on determining the vibration resistance, response time (below 4 s) and resistance temperature response (2.8 kΩ at 25 °C with a tolerance of ±2.5%). The product had to be adapted to make it insensitive to vibrations and moisture, among other factors. EPCOS developed the ceramic NTC (negative-temperature coefficient) sensor element on the basis of the specifications of Behr and their customer (see table).

TABLE: Thermistors in Behr’s HVAC system

All sensors are compatible with the RoHS (Restriction of Hazardous Substances) directive.

The second key problem that EPCOS had to solve was how to attach the temperature sensor unit to the dashboard. The solution: clips and sealing foam. “We had to make sure that the sensor assembly and substrate fit together perfectly,” explains Conway. The general dimensions and tolerances of the clips—position, size, shape and material—determined the thickness and tolerances allowed to the substrate manufacturer (Fig. 1). “The smallest change had consequences for the rest of the construction,” notes Conway, underscoring the need for a tightly orchestrated design process.

Partnership pays off

The special feature of the cooperation was that—as is usual in the USA—the OEM was much more actively involved in the design process with both the tier-1 and tier-2 suppliers. The close and intensive cooperation between Behr and EPCOS, in addition to the quick reaction to the OEM’s requirements, was a decisive factor in the success of the project. The regular joint meetings also provided essential support for the successful completion of a customized design—a process that ranged across the stages of design coordination, prototype fabrication and testing, tool-making, producing initial samples, release tests, and first PPAP (production part approval process) reports.

	THOMAS CONWAY
Thomas Conway, design and release engineer for HVAC electronics at Behr America: “Our design team in Michigan and the EPCOS team in Berlin have worked very effectively together.”

The dimensions and tolerances of the temperature sensor unit and its attachment were developed on a 2-D drawing and 3-D CATIA model (Fig. 2). EPCOS passed this data on to Behr’s design team, who adapted it and ran preliminary tests to ensure the proper fit. The expert team carried out air flow and temperature tests with the aim of ensuring perfect functionality. Although there were mandatory specifications, every time that something was implemented or an angle was changed, the entire design had to be adapted. Automobile design is a self-optimizing process. Thus, during the critical phase of the design process, weekly conferences and meetings to exchange information on the current status of tooling, tests and documentation were the rule, in order to stay in line with the customer’s schedule.

	FIGURE 2: 3-D MODEL OF THE IN-CAR SENSOR ASSEMBLY
The in-car sensor monitors the air temperature inside the cabin. The model on the right shows EPCOS' thermistor element positioned in the opening of the tube.

The effort was well worth it, as the results show. “Our design team in Michigan and the EPCOS team in Berlin have worked very effectively together,” reports a satisfied Conway. The prerequisite for this success was the worldwide network set up jointly by Behr and EPCOS. “We cooperate closely in matters of purchasing, development and quality assurance, both in the European design centers and in the USA,” adds Hegner of EPCOS.

Author:

John Day; freelance writer