Understanding Smart Sensors - Nomads.usp

The Next Phase of Sensing Systems 315Communication between the sensors and the tools is essential, an area in whichthe IEEE 1451 standards should help.14.3 Future System RequirementsThe unprecedented capability that semiconductor technology will provideraises other questions. How much technology will we really need? Who is goingto apply that technology to smart sensors? The answers to those questions andseveral others that could be asked come from examples that are present today.The extensive use of manifold absolute pressure sensors in vehicle emissioncontrol systems resulted from two factors: (1) government legislationforced a change in the way automotive engines were controlled, and (2)semiconductor-based sensors proved to be more cost effective and more reliablethan the mechanical versions that were originally used to define the control system.Legislation and customer demand drove the development of technology.In invasive blood pressure measurements, disposable semiconductorbasedsensors initially proved to be lower cost than resterilizing and periodicallyrecalibrating expensive mechanical units. As the replacement of mechanicalunits was proceeding at an as-predicted rate, the threat of the human immunodeficiencyvirus (HIV) accelerated the conversion process. Cost effectivenessand consumer demand drove this second sensor application.More recently, government legislation mandated passive restraint systemsfor drivers of passenger vehicles. The manufacturers’ choices were automaticseat belts or airbag systems. Customers quickly accepted the airbag system afternumerous reports of users walking away from accidents that previously wouldhave been classified as fatal crashes. As a result, an application that was opposedby vehicle manufacturers in the 1970s quickly became a selling feature in the1990s. Buyers perceived an airbag system, relative to the cost of a vehicle, as areason to buy one vehicle over another or, in some cases, justification for buyinga new vehicle because their old vehicle just was not as safe.The mechanical crash sensors initially used in these systems did not have aself-test feature and required calibration for each vehicle body style.Semiconductor-based accelerometers have allowed the number of crash sensorsto be reduced from as many as five to only one or two, depending on the system’sredundancy requirements. Furthermore, the system can be tuned to meetthe requirements of a particular vehicle style after the sensor is in the module byprogramming the EEPROM in the microcontroller. Several techniques havebeen developed by sensor manufacturers that allow the accelerometer to be activatedfor self-testing each time the vehicle is started. The ability to perform the