Design and Manufacture of Automotive Pressure Sensors - AN762

Design and manufacturing methodologies for automotive pressure sensors to create robust sensors for this demanding environment. Material property selection and circuit design principles are covered. Cost sensitive designs that meet the range of automotive specifications and requirements are discussed. Analyzing defective units and manufacturing fallout as a tool to improving the design and manufacturing process. Among the more challenging tasks undertaken by sensor designers is the design and manufacture of reliable, robust piezoresistive sensors for the automotive market. Through years of trial and error, these designers have developed practices and techniques that have proven useful again and again for automotive applications. To enlighten and inform those who work with this technology (which addresses the leading volume user for piezoresistive sensors), the following discussion discloses several tricks of the trade. As a rule, 80% of the final cost for a product is established in the first 20% of the design effort. Thus, understanding the necessary design practices beforehand can minimize cost and improve your success in designing robust PRT pressure sensors for the demanding automotive environment. If you've never designed such sensors before, the first task is to open the hood of a recent-model vehicle (your own, perhaps) and have a good look at the engine compartment. That nest of wires, hoses, connectors, and engine is one of the most severe environments a sensor can have. Consider the thermal shock to an engine-mounted sensor operating above 100°C and sprayed suddenly with slushy, salty water at 70mph. Other properties of this environment are just as severe: Temperature: -40°C to +125°C (to +150°C for engine-mounted sensors) Vibration: Severe to extreme Sensing Media: Fuels (mixed blends), oil, brake fluid, transmission fluid, noxious vapors, etc. External Media: Water and saltwater spray, oils, brake fluids, solvents, fuels, etc. Thermal Shock: High-temperature operation plus frequent cold-water splashing EMI: 2MHz to 2GHz radiated exposure at 200 volts per meter Sensors located within the passenger compartment have much less exposure to fluids and solvents but are subject to low temperatures, water, and EMI: Temperature: -40°C to +85°C Vibration: Reduced levels Sensing Media: Air typically External Media: Water spray Thermal Shock: Reduced rate of change EMI: 2MHz to 2GHz radiated exposure at 200 volts per meter For either environment, the sensor should have the following: Reliability: 100% operational, every time, all the time Lifetime: 10 to 15 years, sometimes expressed in terms of mileage (100,000 150, 000) as well Low Cost: Automotive is among the most cost-sensitive of markets