Making Sense of Automotive Pressure Sensors

Whether the application is in the engine, tire, or passenger compartment (see sidebar "Side Airbag Pressure Sensors"), the appropriate piezoresistive automotive pressure sensor (Figure 1) can deliver cost-effective reliability and long operational life. The operative word is appropriate. To select the right sensor, you need to understand the functions it performs, the conditions in which it operates, and the design attributes that make it unique.

Figure 1. Silicon wafer containing piezoresistive pressure sensors for low pressure differential pressure measurement

Designers use automotive piezoresistive pressure sensors primarily in three application areas: engine optimization, emission control, and safety enhancement. In the first category, the devices enable you to achieve optimal engine conditions by determining manifold intake air pressure (MAP) and barometric air pressure (BAP) for the engine control unit.

To achieve greater emission control, sensors serve as a key component of the evaporative emission control system, which protects the environment from hydrocarbon emissions from the fuel tank. They also help to clean soot particles from exhaust gases by triggering the regeneration of filters in diesel engines. For safety enhancement, sensors monitor tire pressure and warn the driver when a dangerous loss of pressure is detected (see sidebar "Tire Pressure Sensors") and help to deploy air bags in side-impact situations. These devices even make driving more comfortable by controlling inflatable air bolsters in dynamic seats.

Operating Conditions
Across the board, automotive pressure sensors operate in extremely hostile environments. Engine-mounted sensors function in temperatures ranging from –40°C to 150°C. The same sensors, along with tire pressure monitoring devices, are exposed to fuel, oil, and brake and transmission fluids, all of which can clog the sensors and threaten their performance. At the same time, salt water and solvents attack the very structure of the devices. In these applications, electromagnetic interference (EMI) ranges from 2 MHz to 2 GHz of exposure at 200 V/m. Pressure sensors in the passenger compartment are exposed to similar EMI and to low temperatures.

While these conditions pose formidable challenges, they are not insurmountable. Sound design protects the sensors and delivers reliability and long operational life.

Basic requirements in the design and manufacturing of pressure sensors for automotive applications ensure optimal performance in the range of temperature, vibration, media, shock, and electromagnetic conditions the sensor must endure to perform vital functions (Figure 2). Harsh environments require enhanced protection of the sensor's electrical structures against the pressurized medium.

Figure 2. Typical requirements for an automotive pressure sensor
Minimum operating temperature –40°C
Maximum operating temperature
   Passenger compartment 85°C
   Chassis/wheel 125°C
   Under the hood >140°C
Static acceleration >2000 g (for TPMS applications)
Dynamic acceleration (shock) >5000 g (for TPMS applications)
Vibration 20 Hz–2 kHz (over extended time period)
Chemical resistance To all kinds of liquids present in an automobile environment, including the workshop: fuels, oils, brake fluids, cleaners, waters, salt water

To this end, the piezoresistive bridge on the micromachined silicon membrane typically is not exposed directly to the environment in which it operates.

Figure 3. An ultra-small piezoresistive absolute pressure sensor die, covered by a silicone-gel coating (the light blue area at the bottom right of the structure) to protect the piezo-resistive structures and the micro-machined silicon membrane of the airbag pressure sensor from harsh environmental conditions
Instead, protective silicone-gel coatings, or caps, form a barrier between the sensor's front side and harsh environments, such as those involved in MAP and BAP applications (Figure 3). Field shields and noble metals are also used to protect the sensor's electrical structures against pressurized media. The idea is to protect the sensor's front side from the medium and leave only the silicon backside exposed to the harsh environment. This approach is used widely in tire pressure monitors and MAP sensors.

Choosing a Sensor
There are three overall key attributes to look for when choosing a pressure sensor: reliability, lifetime, and cost. The sensor should work 100% of the time. It should operate reliably for 10–15 years, or 150,000–250,000 miles. Finally, given the automotive industry's cost consciousness, the sensor must be economical.

Reliability is the result of a number of sensor characteristics. Electrical output, accuracy, operating environment, and mechanical coupling are important considerations in choosing an automotive pressure sensor. Perhaps the most important, though, are stability and repeatability. A sensor's temperature and pressure responses should be predictable. A supplier can ensure the repeatability of its sensors with several tests and rigorous qualification routines. Such benchmarking includes cycling temperature in increments from low to high and vice versa and checking for material degradation during a high-temperature soak test, further enhanced by applying constant and/or changing pressure and mechanical stress. Because conventional automotive qualification standards, such as AEC-Q100, are tailored to qualify CMOS circuits and established standards for pressure sensor qualification are still missing, a lot of experience is required to perform such qualification tests.

To some extent, long operational life relies on packaging and assembly, which ironically often cost more than the actual sensor. The packaging cost, size, and ability to survive the rigors of the road are critical. Extensive exposure to gasoline and high pressures makes it imperative that the sensor manufacturer uses the proper materials and mounting.

As in most mass-production industries, change is the major concern in high-volume automobile manufacturing. Cost reduction is the reason for most production changes. Pressure from automobile manufacturers to keep unit prices low poses a challenge for MEMS sensor suppliers that can affect you, the customer. It can take up to four years to bring a pressure sensor to market, and you need to be aware that only a few of the most well-established MEMS suppliers may offer the pressure sensors that meet your needs. As demand grows, there may be a scarcity of some types of sensors.

Side Airbag Pressure Sensors
Optimal side-impact protection is a challenging task, despite the usually much lower collision speeds. The lateral space available to absorb the collision energy in the car body is much lower, and there is much less distance between the airbag and the passenger. Literally, every single inch counts to save lives and prevent serious injuries.

In the event of a serious side impact, pressure sensors help gain precious reaction time by measuring the steep and quick increase of pressure within the cavities of passenger car doors. Even before the accelerometers attached to the airbag control unit receive a signal indicating a heavy impact, the pressure sensors have determined that the door cavities have been compressed by an accident. This early detection gives the airbag control unit additional time to run sophisticated algorithms to determine the airbag-deployment strategy that will deliver optimal passenger protection.

Hostile conditions in the door cavity complicate the application. Summer and winter temperatures easily generate temperature differences of more than 100°C, and the relatively closed, but not completely hermetic, nature of the door allows vapor and condensation to collect in the space. Add the possibility of freezing humidity—spiced with salt, spray, and other hostile substances—and vibrations and shocks, and the challenges become significant. Rigorous qualification test procedures, including mechanical and chemical tests and sophisticated self-diagnostic functions, prevent adverse consequences in case of a rare malfunction despite the entire testing rigor.

Tire Pressure Sensors
The tire pressure monitoring system (TPMS) is a relatively old application of pressure sensors. Initially, it was used primarily in high-end and high-performance vehicles because of its high cost. Recent U.S. regulations have boosted this application by recognizing that underinflated tires can cause accidents, and the National Highway Traffic Safety Administration has mandated that auto manufacturers install a TPMS in all cars and light trucks by the 2008 model year.

Two TPMS types are available. The first uses indirect measurement, which relies on slight changes in wheel diameter in response to pressure loss. The system detects and computes the changes based on wheel speed measurements provided by the antilock braking system. No pressure sensors are required. However, this technology cannot detect simultaneous pressure loss in all four tires and, therefore, is insufficient to meet the requirements of the U.S. mandate. Consequently, the second type of system, based on direct measurement, must be used. In this instance, an electronic device containing a pressure sensor, motion switch, and wireless transmitter is installed in the tire's valve stem or wheel to make the measurement. The sensor sends information on inflation pressure levels by radio signals to a receiving unit inside the vehicle.

Both TPMS types have a dashboard indicator to alert drivers that the pressure level in one or more tires has dropped below 25% of the recommended pressure. The direct method is considered to be more accurate than the indirect approach, but one industry expert notes that the advantage of greater accuracy is offset by a cost of $65 to $80 per vehicle. A market-friendly price for a system consisting of a pressure sensor, electronics, and a wireless transmitter is $12–$15 per wheel. High-volume manufacturing should reduce the price to below $10.

Because a typical tire pressure sensor system is battery powered, it operates at voltages from below 2.5 V to 3.6 V, with a low-voltage alert if the voltage drops below 2.5 V. The sensor's range is 4.5 to 8 bar absolute pressure, with a temperature range of –40°C to 125°C.