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Drive safely with sensor-equipped autos

Posted: 01 Dec 2006 ?? ?Print Version ?Bookmark and Share

Keywords:micromechanical pressure sensor in car? pressure sensor for car? tire pressure monitoring system application? automotive sensor technology? vehicle sensor technology?

Today's latest vehicles are equipped with many sensors. These components convert physical input variables into electrical signals needed by the engine management's ECU, and by the safety and convenience systems for open- and closed-loop control functions.

Besides subjective considerations, many modern motor-vehicle buyers select a car brand based on the criteria of fuel efficiency, emissions, safety, and luxury or convenience. These motivational factors for the purchasing process and legislative requirements are boosting demand for intelligent sensors for automotive applications.

Today's automotive systems register and process many sensorial input variables, such as acceleration, pressure, temperature, engine speed, rotation rate, angle, force, distance, fluid levels and chemical compositions (air or oil quality). Modern sensors that offer the possibility of integrating sensor elements and signal-evaluation electronics on a chip, and transmitting this information wirelessly give significant potential for improvement.

Pressure sensors
The technologies being used today correlate directly to the use of pressure sensors in cars. The installation location for the sensors with their prevailing temperatures, pressures and environmental media determines which sensor element and package technologies are selected.

For applications in the area of pressure sensors, we can assume that for pressures >10bar, the sensor is operated in a liquid or the pressure is transmitted to or led to the sensor using a liquid. For applications Due to the rising number of road accidents involving a side impact and an increased number of injured passenger-car occupants, the guidelines for performing side-impact crash tests have been revised. This has resulted in more stringent requirements for detecting such a side impact. The established target is for the sensor system to reliably detect the severity of the collision in an extremely short period.

Side-impact tests were originally performed with an obstacle of a size that hits both the vehicle doors and the B-pillar in parallel with the side of the vehicle (ECE-R95, 96/27/EG, Euro NCAP, IIHS und FMVSS 214). This meant that the impact impulse was transmitted directly to the vehicle's B-pillar. Thus, an acceleration sensor positioned there can detect this impact with sufficient speed to transmit the data for triggering the safety systems. But given the growing number of sport utility vehicles (SUVs) on the road and their higher construction, SUVs figure in a different type of accident during side-impact collisions.

The conventional side-impact test does not sufficiently cover this scenario. In collisions with SUVs, more and more cases involve only a vehicle door and not the entire side of the vehicle. Consequently, this accident scenario has led to a revised guideline for side-impact tests (FMVSS 214 NPRM).

MAP application measures the pressure in the air induction tract and the air volume that results from this value.

In cases like these, the acceleration sensors used for this task are at a disadvantage. From their preferred installation in the vehicle's B-pillar, they can't sense an impact on the vehicle door until after a delay.

Another approach for detecting a side impact is made possible by using a pressure sensor. Since side impact deforms the vehicle door, pressure rises within the door cavity. This pressure impulse can be measured by pressure sensors located in the door cavity.

For one thing, this makes it possible to significantly decrease the amount of time required to reliably make the decision to activate the safety systems. Moreover, the characteristic of the output signal for pressure sensors makes it significantly easier to distinguish between a genuine collision and irrelevant impulses.

An additional advantage for pressure sensors used to detect side impacts is that the entire door serves as a sensing element, due to the constant distribution of pressure in the space inside the door. Consequently, the output signal is independent of the particular point on the door at which the impact occursit solely depends on the force of the impact. This brings another positioning advantage of the sensor within the door and the technology used for installation: the pressure sensor's output is largely independent of these factors.

Current systems for side-impact protection consist of a combination of pressure and acceleration sensors to be able to use the advantages of both sensor characteristics.

Measure air pressure
To better control the combustion process, car makers are increasingly using sensors that permit more precise measurement of the physical parameters.

This example describes an already well-established application in engine management. The sensors from the MAP application measure the pressure in the air induction tract and the air volume that results from this value. This sensor information and the data from the BAP sensor provide crucial information for the mixture preparation and thus serve to minimize emissions.

The market for MAP and BAP sensors is mature. In 2005, worldwide demand for MAP sensors amounted to approximately 37 million units. Worldwide demand for BAP sensors is approximately 19 million units. The average annual growth rate for this market is estimated at 6 percent.

A significantly higher rate of market growth is anticipated for tire pressure monitoring system (TPMS) applications, due to the legislation passed in the United States after many fatal accidents caused by blown-out tires. In Europe, this application is also gaining significance. Due to the convenience and safety benefits it provides, it is currently making its way into the mid-size car category.

In this process, structuring processes are also applied to the underside (bulk micromachining) to achieve the implementations.

Besides tire pressure, the sensor measures the tire's temperature and the sensor module's battery voltage. An acceleration sensor monitors the system and reports physical motion from the resting state. This is important to reactivate the system from an energy-saving state if the vehicle has not moved for a long period of time. For this application, the batteries are required to last longer than 10 years. The role of the receiver is played by a central reception unit that can also receive and process the signals from the familiar remote keyless entry application.

Bulk micromachining

TPMS technology used differs from surface micromachiningthe manufacturing process is not only applied to the chip surface. In this process, structuring processes are also applied to the underside (bulk micromachining) to achieve the implementations.

The advantage of this technology is that it is particularly robust when it comes to withstanding the influence of potentially aggressive media. In this case, via the pressure inlet, the medium comes in contact with the robust silicon diaphragm rather than with the sensor electronics.

In car applications, the development and mastery of surface and bulk micromachining for highly integrated pressure sensors show potential for implementing complex systems that require very little space. We have shown that this type of pressure sensor is already well-established within the car and has become indispensable for future applications. Future trends and demand for the development of sensors for automotive applications are driven by requirements of high reliability, low system costs, harsh operating conditions, small footprint and high precision.

- Michael Wycisk
Senior Manager, Application Engineering, Sense and Control
Infineon Technologies AG

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