Today's latest vehicles are equipped with many sensors. Thesecomponents convert physical input variables into electrical signalsneeded by the engine management's ECU, and by the safety andconvenience systems for open- and functions.
Besides subjective considerations, many modern motor-vehicle buyersselect a car brand based on the criteria of fuel efficiency, emissions,safety, and luxury or convenience. These motivational factors for thepurchasing process and legislative requirements are boosting demand forintelligent sensors for automotive applications.
Today's automotive systems register and process many sensorial inputvariables, such as acceleration, pressure, temperature, engine speed,rotation rate, angle, force, distance, fluid levels and chemicalcompositions (air or oil quality). Modern sensors that offer thepossibility of integrating sensor elements and signal-evaluationelectronics on a chip, and transmitting this information wirelesslygive significant potential for improvement.
The technologies being used today correlate directly to the use ofpressure sensors in cars. The installation location for the sensorswith their prevailing temperatures, pressures and environmental mediadetermines which sensor element and package technologies are selected.
For applications in the area of pressure sensors, we can assume thatfor pressures >10bar, the sensor is operated in a liquid or thepressure is transmitted to or led to the sensor using a liquid. Forapplications <10bar, such as the measurement of air (under) pressurein the induction tract (manifoldabsolute pressure or MAP) or themeasurement of the barometricabsolute pressure (BAP), gel coatings aresufficient, with respect to the package.
Due to the rising number of road accidents involving a side impactand an increased number of injured passenger-car occupants, theguidelines for performing side-impact crash tests have been revised.This has resulted in more stringent requirements for detecting such aside impact. The established target is for the sensor system toreliably detect the severity of the collision in an extremely shortperiod.
Side-impact tests were originally performed with an obstacle of asize that hits both the vehicle doors and the B-pillar in parallel withthe side of the vehicle (ECE-R95,96/27/EG,Euro NCAP, IIHS and FMVSS214). This meant that the impact impulse was transmitted directlytothe vehicle's B-pillar. Thus, an acceleration sensor positioned therecan detect this impact with sufficient speed to transmit the data fortriggering the safety systems. But given the growing number of sportutility vehicles (SUVs) on the road and their higher construction,SUVsfigure in a different type of accident during side-impact collisions.
The conventional side-impact test does not sufficiently cover thisscenario. In collisions with SUVs, more and more cases involve only avehicle door and not the entire side of the vehicle. Consequently, thisaccident scenario has led to a revised guideline for side-impact tests(FMVSS 214 NPRM).
|Figure1. The MAP application measures the pressure in the air induction tractand the air volume that results from this value.|
In cases like these, the acceleration sensors used for this task are ata disadvantage. From their preferred installation in the vehicle'sB-pillar, they can't sense an impact on the vehicle door until after adelay.
Another approach for detecting a side impact is made possible byusing a pressure sensor. Since side impact deforms the vehicle door,pressure rises within the door cavity. This pressure impulse can bemeasured by pressure sensors located in the door cavity.
For one thing, this makes it possible to significantly decrease theamount of time required to reliably make the decision to activate thesafety systems. Moreover, the characteristic of the output signal forpressure sensors makes it significantly easier to distinguish between agenuine collision and irrelevant impulses.
An additional advantage for pressure sensors used to detect sideimpacts is that the entire door serves as a sensing element, due to theconstant distribution of pressure in the space inside the door.Consequently, the output signal is independent of the particular pointon the door at which the impact occurs—it solely depends on the forceof the impact. This brings another positioning advantage of the sensorwithin the door and the technology used for installation: the pressuresensor's output is largely independent of these factors.
Current systems for side-impact protection consist of a combinationof pressure and acceleration sensors to be able to use the advantagesof both sensor characteristics.
Measure air pressure
To better control the combustion process, car makers are increasinglyusing sensors that permit more precise measurement of the physicalparameters.
This example describes an already well-established application inengine management. The sensors from the MAP application measure thepressure in the air induction tract and the air volume that resultsfrom this value. This sensor information and the data from the BAPsensor provide crucial information for the mixture preparation and thusserve to minimize emissions.
The market for MAP and BAP sensors is mature. In 2005, worldwidedemand for MAP sensors amounted to approximately 37 million units.Worldwide demand for BAP sensors is approximately 19 million units. Theaverage annual growth rate for this market is estimated at 6 percent.
A significantly higher rate of market growth is anticipated for tirepressure monitoring system (TPMS) applications, due to thelegislationpassed in the United States after many fatal accidents caused byblown-out tires. In Europe, this application is also gainingsignificance. Due to the convenience and safety benefits it provides,it is currently making its way into the mid-size car category.
|Figure2. In the MAP process, structuring processes are also applied to theunderside (bulk micromachining) to achieve the implementations|
Besides tire pressure, the sensor measures the tire's temperatureand the sensor module's battery voltage. An acceleration sensormonitors the system and reports physical motion from the resting state.This is important to reactivate the system from an energy-saving stateif the vehicle has not moved for a long period of time. For thisapplication, the batteries are required to last longer than 10 years.The role of the receiver is played by a central reception unit that canalso receive and process the signals from the familiar remote keylessentry application.
TPMS technology used differs from surfacemicromachining—themanufacturing process is not only applied to the chip surface. In thisprocess, structuring processes are also applied to the underside (bulkmicromachining) to achieve the implementations.
The advantage of this technology is that it is particularly robustwhen it comes to withstanding the influence of potentially aggressivemedia. In this case, via the pressure inlet, the medium comes incontact with the robust silicon diaphragm rather than with the sensorelectronics.
In car applications, the development and mastery of surface and bulkmicromachining for highly integrated pressure sensors show potentialfor implementing complex systems that require very little space. Wehave shown that this type of pressure sensor is alreadywell-established within the car and has become indispensable for futureapplications. Future trends and demand for the development of sensorsfor automotive applications are driven by requirements of highreliability, low system costs, harsh operating conditions, smallfootprint and high precision.
Michael Wycisk is SeniorManager, Application Engineering, Sense and Control, InfineonTechnologies AG.