SENSORS are defined as devices that transduce physical

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1 1900 IEEE SENSORS JOURNAL, VOL. 8, NO. 11, NOVEMBER 2008 New Automotive Sensors A Review William J. Fleming, Life Senior Member, IEEE Invited Paper Abstract This paper focuses on the primary automotive sensor technologies used today and their related system applications. This paper describes new automotive sensors that measure position, pressure, torque, exhaust temperature, angular rate, engine oil quality, flexible fuel composition, long-range distance, short-range distance, and ambient gas concentrations. In addition, new features are described for sensors that measure linear acceleration, exhaust oxygen, comfort/convenience factors, and night vision. New automotive system applications are described for sensors that measure speed/timing, mass air flow, and occupant safety/security. Index Terms Automotive sensor applications, automotive sensors, comprehensive sensor update, review paper, road vehicle transducers, sensor technology. I. INTRODUCTION SENSORS are defined as devices that transduce physical quantities such as pressure or acceleration into electrical signals that serve as inputs for control systems [1], [2]. Engineering literature does not consistently differentiate between the terms, sensors and transducers. Whether devices are called sensors or transducers often depends on the field of application in which they are used. In the automotive field, these devices are more commonly referred to as sensors. Sensors are essential components of automotive electronic control systems. Automotive sensors must satisfy a difficult balance between accuracy, robustness, manufacturability, interchangeability, and low cost. Because of the key role sensors play in automotive systems, many advances have occurred since a prior review paper [1] was published. For example, the present paper describes 21 new types of automotive sensors, and 25 new features available in automotive sensors. In addition, 14 new automotive system applications for sensors are described. In total, therefore, 60 new developments related to automotive sensors are reviewed. The objective of this paper is to cover the most significant sensors used in present-day automotive applications. However, notwithstanding the breadth of this paper, some automotive sensors were unavoidably excluded. Decisions on which sensors to Manuscript received April 19, 2008; revised July 08, 2008; accepted July 31, Current version published October 31, The associate editor coordinating the review of this paper and approving it for publication was Dr. John Vig. The author is retired from TRW Automotive, Washington, MI USA ( WFleming@wowway.com). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /JSEN TABLE I AUTOMOTIVE SENSOR MARKET GROWTH NORTH AMERICA [3] Estimate Predicted Global automotive sensor market volumes are approximately three times larger than the values for the North American market given here. Normalized to 2002, when the average sensor cost was U.S. $6.30. exclude were based on the judgment of the author and the availability of information. 1 II. BACKGROUND There are three areas of automotive systems application for sensors, namely: powertrain, chassis, and body. Automotive control functions and associated systems for the three areas of application were previously shown in Figs. 2 4 and Tables II IV in [1]. Several new applications are reviewed in this paper. Estimates for the automotive sensor market in 2007 and a forecast for 2013 were derived from data in [3] and are given in Table I. Current luxury cars have over 100 sensors per vehicle, significantly more than the average number of 40 given in Table I. This table illustrates the dramatic growth in demand for automotive sensors. 1 Excluded automotive sensors include the following. -Brake pedal position/force sensor (force detected via a fluid pressure sensor). -Passive tire pressure sensor (no battery required) using hoop antennas to couple RF power into pressure sensors embedded in the sidewalls of rotating tires. -Fiber-optic engine in-cylinder pressure sensor based on light reflection off a diaphragm. -Side door-mounted pressure sensor (which provides wide-area side-impact crash sensing). -Multiple degree-of-freedom inertial-sensor modules for chassis monitoring that include x-y acceleration plus -angular rate sensing elements. -Fuel level detection using: (a) Hall effect sensors to measure float-arm angular position or (b) transit times of ultrasonic pulses reflected off the fuel-air surface interface. -Vehicle heading detection using magnetometer (compass) direction sensors. -Window anti-pinch/auto-reverse sensing obstacles are detected by: (a) pressure-sensitive conductive window-seal strips, or (b) electric motor load monitoring using Hall effect sensors in the motors. Since these sensors do not currently enjoy widespread production, and are based on generally straightforward operating principles, they were excluded X/$ IEEE

2 FLEMING: NEW AUTOMOTIVE SENSORS A REVIEW 1901 III. SENSOR DRIVING FACTORS Current driving factors that account for the increasing utilization of automotive sensors are given below. Needs for sensors in powertrain systems are driven by: legislation (e.g., lower emissions, improved fuel economy, and onboard diagnostic requirements), best-in-class driveability, along with the introduction of new types of alternative power sources. Chassis systems needs for sensors are driven by safety, weight reduction, multiplex compatibility, and legislation (e.g., collision avoidance stability systems and tire pressure monitoring). Body systems needs for sensors are driven by safety (e.g., advanced airbags, rollover and side crash protection), comfort, and convenience. In each application powertrain, chassis and body Moore s Law is a dominant driving factor. Moore s Law states that electronics capabilities double approximately every 18 months. Automotive electronics directly benefit and exhibit corresponding increases in computing power/memory. These increases provide greater systems demand for feedback signals, which in turn drives continually growing needs for high-performance automotive sensors. IV. NEW SENSORS AND NEW APPLICATIONS In a prior review paper [1], a total of 40, 27, and 40 sensors were listed in Tables II IV for powertrain, chassis, and body automotive systems applications. This total of 107 sensors was representative of the major applications for sensors used in automobiles seven years ago. As mentioned above, 60 new developments related to automotive sensors are reviewed here. A. Smart Sensor Operation A new feature, common to many types of automotive sensors, is smart sensor technology. This consists of electronics signal processing integrated inside the sensor which provides: Automatic gain control (e.g., to compensate for air gap variation). Conversion of internally detected time-varying waveforms into precise square-wave or digital protocol output signals. Dynamic threshold sensing which maintains zero-offset and 50% duty cycle in a square-wave output signal. Pulse-width-modulated and digital protocol output signals are clamped at specified upper and lower limits, e.g., at 0 and 5 Vdc. Ratiometric output signals are provided where output signals are normalized to the level of the supply voltage. Electronic interface with communication bus networks. Operation using two wires in place of a three-wire connection (one wire carries a digital protocol output signal superimposed on a dc power-supply loop current, and the other wire connects to the reference side of the bus network). Smart sensor features are notably incorporated into speed/ timing, pressure, and inertial acceleration/angular rate automotive sensors. B. Speed/Timing Sensors Speed/timing sensors are used to measure engine crankshaft/camshaft speeds and angles for control of spark timing and fuel injection timing. The sensors are also used for control of transmission input and output shaft speeds for electronically controlled gear shifting. In addition, high-resolution crankshaft speed sensors detect engine misfire, as evidenced by cylinder misfire-induced modulations of crankshaft speed. Another major application is wheel speed measurement of each vehicle wheel to provide inputs to antilock brake, traction control, and vehicle stability systems. There have been many improvements to speed/timing sensors, beginning with the addition of the aforementioned smart sensor features. A good example of a sensor with integrated smart sensor electronics is a giant magnetoresistive (GMR) speed/timing sensor described in [4]. This sensor, including electronics, is packaged as a device. Advances in the areas of packaging and processing have contributed to the development of greater accuracy, lower cost, and improved robustness. Further discussion of packaging and processing is beyond the scope of this paper. Variable reluctance, Hall-effect, anisotropic magneto-resistive (AMR) and GMR types of speed/timing sensors were reviewed in [1]. New applications of speed/timing sensors include the following. 1) Crankshaft Reverse-Rotation Detection: During repeated restarting of an engine in a mild hybrid electric vehicle, idle-stop engine control systems must continue to comply with emissions requirements. Excessive exhaust emissions occur as a result of time lags between the point when the engine throttle is first opened compared with the time when the engine electronic control unit (ECU) is able to determine the true crank angle and apply spark. Time lags are the result of crankshaft reverse rotations which frequently occur when an engine is shut off at the beginning of the stopping event. Speed/timing sensors with dual inline detector elements provide the required direction information. The phase angle of the sensor output signal, derived from the difference of voltage waveforms between the two detector elements, determines the direction of crankshaft rotation [5]. 2) Vibration Interference Suppression: During restarting of an idle-stop engine in a mild hybrid electric vehicle, a false engine speed signal can be generated. Vibrations in the enginestopping event can create periodic variations in the air gap of the speed/timing sensor between the sensor and the tone wheel. This creates changes in magnetic field such that the sensor may inadvertently generate an output signal based on vibration, not on rotation of the tone wheel. Because vibrations produce changes in the magnetic field, the automatic gain adjustment in the sensor s signal processor may generate excessively large gain. In this case, the voltage-crossing thresholds become too high, and the sensor s timing signal will be in error. A logic determining circuit in a sensor s signal processor detects the onset of inappropriately high levels of sensor signal gain, and it then sends a correction signal which reinitializes the voltage-crossing thresholds back to their correct levels [6]. 3) Wheel Speed Sensor New Features: Vehicle wheel speed sensors have added four important new features.

3 1902 IEEE SENSORS JOURNAL, VOL. 8, NO. 11, NOVEMBER 2008 a) Improved serviceability: Wheel speed sensors are mounted in wheel hub housings, and operate in a severe environment that includes roadway curb impacts and pot-hole-induced shocks. Consequently, these sensors are associated with a majority of antilock brake and stability system repair problems. Wheel speed sensors are made more serviceable today, no longer requiring disassembly of major components of the wheel hub/bearing components for replacement. Wherever possible, sensors today are installed in an accessible portion of the wheel-hub housing where their replacement does not require hub disassembly [7]. b) Magnetically poled encoder rings: Magnetically poled encoder-ring technology has rapidly replaced toothed tone wheels. Tone wheels required the use of a larger sensor because the sensor had to include a relatively large back-bias magnet. Magnetically poled encoder rings (with repeating north south pole patterns) produce their own magnetic field, enabling smaller magnet-free field-detection speed sensors. Sharply defined north/south poles in encoder rings produce a more desirable square-wave-like output and also yield better accuracy than tone wheels. Another key advantage is that as compared to tone wheels, encoder rings permit operation at double the range of air gaps between the sensor and the encoder ring, thereby facilitating relaxed installation tolerances and improved reliability [8]. c) Wheel rotation direction detection: Wheel rotation direction serves as an input signal for the hill holder function of an electronic parking brake system. Hill holder systems automatically apply the parking brake as soon as the car comes to a stop (as detected by the cessation of wheel rotation), or for reverse wheel rotation when the accelerator pedal is not depressed. The parking brake is electromechanically engaged and then released when the driver depresses the accelerator (detected by a pedal position sensor). The amount of brake force needed to hold a vehicle in place on a hill is determined using the dc-response of an accelerometer (functioning as an inclination sensor). Direction of wheel rotation is detected using closely spaced inline magnetic field sensing elements mounted in the head of the wheel speed sensor. Comparison of the phase angles of the leading element s speed signal, with that of a trailing element, determines direction of wheel rotation [9]. d) Self-monitored failure detection functions: An essential self-monitored failure function is the air gap between the wheel speed sensor and its encoder ring. Due to the severe on-wheel environment in which it operates, damaging air gap displacements can occur in the wheel speed sensor. Air gap is monitored by measuring magnetic field amplitudes detected by the sensing element in the sensor. Typically, three air gap operating-range conditions are monitored: i) normal air gaps (normal signal amplitude); ii) reserve range (nearly out-of-operating range); and iii) out of operating limit (loss of signal). Additional self-monitored failure functions include detection of short circuits, and intermittent open circuits. Signals corresponding to: the self-monitored functions, the wheel rotation direction, and the wheel speed signals are combined and are transmitted as a digital protocol signal from the wheel speed sensors back to an ECU on a two-wire interface. The digital protocol signals are superimposed on the Fig. 1. Inductively coupled sensor used to measure throttle plate position [11]. dc power-supply wire, and the second wire connects to the reference side of the bus network. Digital protocols transmit the operating status information via pulse-width-modulation [10] or Manchester codes. C. Position Sensors Automotive applications of position sensors include: Engine throttle plate angle. Chassis-height link-bar angle. Fuel level (float arm angle). Steering-wheel angle. Potentiometric, Hall-effect, AMR, and GMR types of position sensors were reviewed in [1]. Three new types of position sensors are described here. 1) Inductively Coupled Position Sensor: This sensor measures angular position using a multilobed single-turn conductor coil on a rotor attached to the part throttle plate, accelerator pedal, or chassis-height link bar whose angle is to be determined. An inductively coupled throttle-plate-position sensor is shown in Fig. 1. The multilobed coil on the rotor is connected to the throttle plate and is suspended next to the receive coils which consist of three or more planar coils intertwined together. The receive coils are mounted on a fixed housing. A single-loop excitation coil, also mounted on the fixed housing, encircles the receive coils and provides ac-excitation. The excitation coil generates a MHz-frequency RF field. The excitation coil s RF field inductively couples (like a transformer) to circumferential portions of the rotor multilobed coil, and induces current in the rotor s conductor. Current flowing in the radial portions of the rotor conductor lobes generates a secondary magnetic field pattern that rotates with the rotor and inductively couples to the underlying receive coils. Each of the receive coils couples with the rotor magnetic field and inductively generates its own (phase-shifted) voltage waveform as a function of rotor angle. The angle of the measured part (e.g., a throttle plate) is determined via signal processing of the magnitudes, signs, and gradients of the individually phase-shifted receive-coil voltages [11], [12]. Inductively coupled position sensors offer the following features: Noncontact operation. No magnets are required.

4 FLEMING: NEW AUTOMOTIVE SENSORS A REVIEW 1903 Fig. 2. Integrated magnetic concentrator rotary position Hall-effect sensor used to measure throttle plate, accelerator pedal, or chassis-height link-bar angle. (a) Basic configuration [14]. (b) IMC layer below magnet alters the magnetic field directions. The silicon substrate supports the IMC layer and also includes the Hall-effect sensors [13]. Low cost due to printed circuit board construction and nonresonant circuit operation. Facilitates relaxed assembly alignment tolerances. Design flexibility allows the sensor to be made into either angular or linear position-sensing configurations. 2) Integrated Magnetic Concentrator (IMC) Rotary Position Hall-Effect Sensor: This sensor measures angular position using a single bar magnet attached to the rotating part (throttle plate, accelerator pedal, or chassis-height link bar) whose angle is to be determined. As seen in Fig. 2(a), the sensor is mounted on a fixed surface underneath a magnet attached to a rotating part. The sensor consists of the following components. a) High-permeability IMC ferromagnetic layer: As illustrated in Fig. 2(b), an IMC layer (which is disk shaped) alters the direction of, otherwise, the parallel-directed magnetic field ( includes and components). The IMC changes the -field directions to perpendicular-directed field directions. The -field directions are altered as a result of a boundary condition; namely, at the transition interface between low-permeability air and the high-permeability IMC layer, the magnetic field enters perpendicularly as into the IMC. Because Hall-effect sensors respond to both the magnitude and direction of field, the use of an IMC layer to redirect the magnetic field into perpendicular -directions largely eliminates direction variability. This allows Hall-effect sensing elements in the silicon substrate below the IMC layer to respond solely to magnitudes of the -field components [13], [14]. b) Hall-effect sensing elements: Hall-effect sensing elements are mounted in the silicon substrate, in four quadrant positions, below the IMC layer. Hall sensing elements detect magnitudes of the - and -components of the magnet field. As the part (whose angle is to be measured) rotates with its bar magnet, pairs of Hall-effect sensing elements detect and generate quadrature and signal voltage waveforms [14]. c) Redundant, dual, embedded digital signal processors (DSPs): The and signals are in phase quadrature and are processed to determine a resolved angle as follows: (1) Fig. 3. Basic configuration of the dual-magnet sensor used to measure steeringwheel angle [4]. DSPs are embedded in the silicon substrate along with the Halleffect sensing elements. Dual-DSP isolated dies are used for redundancy to insure reliability [15]. The IMC rotary position sensor provides the following features: Noncontact, easy-to-install, end-of-shaft mounting. Compact size, small outline package, (excluding the magnet). Insensitivity to variations of magnetic field strength, temperature, and air gap. Absolute 360 angular position measurement. Angular accuracy bit (1024 step), and angular resolution bit (4096 step). 3) Dual-Magnet Steering-Wheel Angle Sensor: A combined optical/potentiometric type of steering-wheel angle sensor was described in [1]. A new dual-magnet type of steering-wheel angle sensor has been developed for automotive applications [4]. Automotive applications for the steering-wheel angle sensor include: vehicle electronic stability control, steerable headlights, parking assist, and road navigation. The basic configuration of the sensor is shown in Fig. 3. Measurement of steering-wheel angle is difficult because over the four or more turns of the steering wheel, i.e., over 1440 or more of rotation, the angle of rotation must be determined within 1 accuracy. The sensor in Fig. 3 uses two bar magnets, each attached to a free-running pinion gear. The pinion gears engage the large drive gear attached to the steering-wheel column. Adjacent to each pinion gear/rotating magnet, is a GMR magnetic field sensing element, mounted in a stationary sensor housing [16], [17]. The drive gear in Fig. 3 has 42 teeth, whereas one pinion gear has 14 teeth and the other pinion gear has 15 teeth. Thus, for each turn of the drive gear, the pinion gears and their embedded magnets will each turn about three times. Because of the difference in the number of pinion teeth, the two pinion gears rotate through 15 turns before their magnets realign. Therefore, as the steering wheel turns through five revolutions, one of the pinion

5 1904 IEEE SENSORS JOURNAL, VOL. 8, NO. 11, NOVEMBER 2008 gears goes through 15 turns, and the two pinions realign with each other one time. The two GMR sensing elements detect the angles of the pinion gears/magnets. The angles of each pinion gear exhibit a unique relationship to the large-gear steering-wheel angle. Steering-wheel angle is computed from the angular relationships between the pinion gear angles with respect to the large gear angle [16]. Signal processing methods used to enhance the accuracy of the steering-wheel angle measurement include [4]: Sigma-delta A/D converters. Digital filtering. Coordinate rotation digital computing (CORDIC) angle conversions. Data flow partitioning. D. Pressure Sensors Automotive applications of pressure sensors include: Engine manifold absolute pressure. Ambient barometric pressure. Evaporative fuel system leak pressure. Brake fluid pressure. Chassis adaptive suspension hydraulic pressure. Air conditioner compressor pressure. Common-rail fuel injection pressure. Microelectromechanical systems (MEMS) piezoresistor, capacitive module, polysilicon-on-steel, and fiber-optic sensors types of pressure sensors were reviewed in [1]. New types of pressure sensors are as follows. 1) MEMS Surface Mount Package Sensor: This sensor has integrated electronics and its sensing diaphragm is bulk micromachined in silicon, with an exposed area of about 1 1 mm. Four piezoresistive sensing elements are diffused into the diaphragm to detect stresses created by applied pressure. The sensor is designed for the measurement of ambient barometric pressure for engine control. The pressure sensor is one of the first to be offered in a standardized small-footprint surface-mount device package. This reduces the amount of circuit-board area taken up by the sensor and it also makes the sensor more compatible with standard electronics assembly methods. Key performance features of the sensor include [18]: Operating range: 60 to 120 kpa. Accuracy, 1%. Operating temperature, 40 C to 130 C. Small size,, surface mount package. 2) Integrated Multiparameter Tire Pressure Sensor: The United States government issued a safety standard requiring tire pressure monitoring system (TPMS) sensors on each wheel of each new car as of September 1, 2007 [19]. The standard was enacted by an Act of the U.S. Congress in reaction to sport utility vehicle (SUV) rollover-crash fatalities, and an associated tire recall. The standard is unique to the U.S. and has not been adopted by other countries. The standard requires that TPMS sensors, within 20 min, detect a 25% pressure-deflation in any or all vehicle tires. This includes the situation where all four tires deflate uniformly due to seasonal falling temperatures. The most common type of TPMS system is the direct valvemounted battery-powered type, seen in Fig. 4. The TPMS sensor Fig. 4. A MEMS multiparameter sensor is used in a direct/active type of tire pressure monitoring system. The sensor is valve-mounted inside the tire rim and is powered by a lithium battery. is attached to the valve and is positioned in a protective location inside the tire rim. This sensor: a) uses wheel motion-detection power-saver circuits that extend battery life; b) uses ultra-lowcurrent-draw integrated circuits that also extend battery life; and c) uses MEMS multiparameter pressure, temperature and acceleration sensing elements to minimize weight [20]. The RF receiver in the vehicle that receives signals from remote keyless entry (RKE) systems doubles its functionality by also receiving TPMS signals. The RKE and TPMS systems operate on the same 315-MHz frequency (United States), while utilizing different signal modulations. Integrated on a single substrate in the TPMS sensor are the following devices: a) MEMS diaphragm-type capacitive pressure sensing element; b) semiconductor temperature sensing element; c) MEMS acceleration sensing element; d) voltage sensing element; and e) signal processor and RF transmitter. The sensor, including sensing elements and electronics, has dimensions of, made possible by the use of MEMS technology [21]. The operating functions of the each sensing element are as follows. The pressure sensing element provides the desired tire pressure measurement. The temperature sensing element allows ideal-gas-law based corrections of tire pressure measurements to a standard temperature, and it also detects over-temperature conditions to provide electronics shutdown protection. The acceleration sensing element activates a battery power-saver mode when wheel movement stops. In addition, the detected sequence of gravity-induced maxima and minima output signals as a wheel begins to rotate is used to identify right-from-left tire location [22]. To distinguish front-from-rear wheels, differences in received RF signal strengths from front and rear TPMS transmitters are monitored. Because rear-wheel transmitters are more distant from the RF receiver, rear wheels are identified by their reduced signal strengths. The voltage-sensing element monitors the TPMS battery life. As indicated in Fig. 4, an integral lithium battery typically powers automotive tire pressure sensors. Various attempts to use piezoelectric bimorphs (and a tuned vibrating mass attached inside the tire) for energy harvesting, to power tire pressure sensors, have been investigated [23], [24]. Aside from the difficulty

6 FLEMING: NEW AUTOMOTIVE SENSORS A REVIEW 1905 Fig. 6. A ceramic-element capacitive sensor inside the sensor housing detects the exhaust differential backpressure across an exhaust particulate filter [29]. Fig. 5. Piezoelectric pressure sensor integrated into a diesel engine glow plug. (a) Essential elements of sensor [27]. (b) Physical appearance of sensor [28]. of maintaining bonding of the bimorph assembly on a constantly flexing tire, the generated power is considered to be insufficient to energize both the sensor and transmitter [25]. This approach is still under development. 3) In-Cylinder Combustion Piezoelectric Pressure Sensor: In-cylinder pressure measurements are used in engine combustion control systems. Through the use of a cylinder pressure feedback signal, engine control systems can better regulate cylinder fuel injection timing, exhaust gas recirculation, and turbocharger operation. For example, the control system may act to reduce combustion temperatures in cylinders in order to lower NOx emissions [26]. A fundamental problem that has impeded development of this sensor is that engine designers never wish to add more access holes in the cylinder head to accommodate combustion sensors, while existing spark-plug/glow-plug-integrated sensors up to now have not been practical. A new type of pressure sensor has been integrated into a diesel engine glow plug. The in-cylinder pressure sensor utilizes a piezoelectric ceramic sensing element mounted inside the glow plug housing. During combustion, increasing cylinder pressure applies force on the glow plug. The heating element in the glow plug functions as a pressure transfer pin. It applies the forces due to cylinder pressure onto a piezoelectric pressure sensor diaphragm inside the sensor housing. In this way, as seen in Fig. 5(a), forces due to cylinder pressure are passed along a stress transfer path and onto the piezoelectric element [27], [28]. Sensor-integrated signal processing electronics provide a real-time cylinder pressure output signal. Separate sensors monitor combustion pressures in each cylinder. 4) Diesel Exhaust Particulate-Trap Backpressure Sensor: Sensors monitor buildup of backpressure in particulate filter traps used in diesel engine exhaust emissions cleanup systems. Differential-pressure-tap upstream and downstream tubes from the trap connect to the sensor housing ports seen in Fig. 6. As particulates in diesel exhaust accumulate in a particulate filter trap, backpressure across the trap increases. When the backpressure reaches a predetermined level, as detected by the sensor, a signal is sent to the ECU to add more fuel to the engine. The resulting rich air fuel mixture causes an upstream oxidation catalyst to heat the exhaust. The increased exhaust heat then burns up the carbon particulates in the particulate filter, therein regenerating the filter by converting particulates into gas. The exhaust backpressure sensor consists of a ceramic capacitive-type pressure sensor that detects the differential pressure across its diaphragm. The sensor has high accuracy and is designed to perform with narrow pressure ranges, along with high overpressure capacity. The sensor s differential-pressure operating range is typically 0-to-34 kpa, with an overpressure capability of 690 kpa [29]. E. Temperature Sensors Operating temperature ranges for these sensors are as follows: For temperature applications in the range of 50 C to 150 C, silicon IC sensors are used. Thermistor-type sensors operate in various ranges as high as 1000 C. To measure very high temperatures over 1000 C, RTDtype sensors are commonly used. Automotive applications of temperature sensors include: Measurement of air and fluids using silicon IC sensors. Engine coolant, fuel, brake, and steering fluid levels are commonly measured using thermistors (via differences in fluid immersion/nonimmersion heat-transfer temperatures). Very high temperatures are measured in catalytic converters mounted in the exhaust system. Resistive temperature detectors (RTDs), thermistors, and silicon IC temperature sensors were reviewed in [1]. High-temperature RTD sensors are used in three new types of exhaust emissions cleanup systems, namely: Diesel particulate-trap exhaust-temperature control during the trap regeneration process [29]. Spark-ignition engine control of exhaust temperature to enhance NOx trap-catalyst performance [30]. Urea-injection diesel engine emissions control system regulation of exhaust temperature (to optimize performances

7 1906 IEEE SENSORS JOURNAL, VOL. 8, NO. 11, NOVEMBER 2008 of a particulate catalyst and an NOx selective-catalytic-reduction converter) [31]. In one study, three types of high-temperature sensors were compared for use in exhaust monitoring applications. Temperature measurement accuracy was required to be stable over an exhaust gas measuring range of 40 C to 1000 C, and after 500 h of aging at 950 C. This study showed that RTDs were superior to thermocouples or thermistors for the demanding high temperature exhaust monitoring applications [32]. F. Mass Air Flow Sensors Mass air flow sensors measure steady state and transient mass flow of air into an engine. An engine s combustion process is controlled by mixing the correct ratio of fuel to the mass, not volume, flow of intake air. Accurate mass air flow measurement permits precise metering of fuel into an engine for control of vehicle emissions, economy, performance and driveability. Mass air flow sensors were briefly described in [1]. A more comprehensive discussion of the sensors is presented here. There are three methods of measuring automotive mass air flow. 1) Engine Speed/Air Density: This method first determines volume, not mass, flow of air by treating the engine as an air pump. Volume air flow is computed as a product of cylinder displacement, times engine speed, times engine volumetric efficiency. Volumetric efficiency calibrations provide corrections for variations of engine air-pumping characteristics due to effects of: valve timing, EGR rate, engine wear, etc. To determine mass flow, the ideal gas law is used. Measurements of intake air pressure and temperature are made, and the ideal gas law is used to calculate the mass density of the air. In summary, volume air flow, times mass air density, equals mass air flow. Measurements of intake air pressure and temperature are made using a two-in-one sensor that combines two sensing elements to measure manifold air absolute pressure and temperature [33]. A thermistor measures temperature and a MEMS sensing element measures pressure. 2) Mass Air Flow Direct Measurement: Direct-measurement mass air flow sensors are based on thermal heat-loss principles. An exposed heated resistive element plus a companion, compensating, insulated element, are mounted inside the engine s throttle body and are exposed to the air flow. This sensor measures mass air flow into an engine based on convective heat losses due to air flow across the two sensing elements. The exposed heated element experiences convective-flow heat loss, while the insulated element does not. The difference in electrical heating power required to maintain both elements at the same temperature determines mass air flow. The greater the air flow, the greater the difference between current draws to the exposed and insulated resistive elements, therein providing the mass air flow measurement. This sensor cannot detect flow reversals in the intake air flow, and therefore does not measure true mass flow under these conditions. 3) True Mass Air Flow Measurement: Under certain engine operating conditions e.g., open throttle at low engine speed pulsating reversals of air flow occur. Because the flow sensor in Section IV-F2 does not detect reverse flow, it will in this case provide erroneous flow measurements. To detect flow reversal, another configuration of the mass flow sensor is used. This sensor utilizes a heat source and separate Fig. 7. Engine true mass air flow sensor. (a) Measuring principle [34]. (b) Physical appearance of sensor (cover removed) [35]. upstream and downstream thermal detection elements all fabricated on a micromachined low-thermal-mass diaphragm. The amount that the downstream sensing element is hotter or colder than the upstream sensing element indicates both the direction of airflow and its mass flow rate [34]. A true mass flow sensor and its operating principle are seen in Fig. 7. Both types of air flow sensors in Sections IV-F2 and IV-F3 include means to minimize effects of contamination. In the upstream side of the sensor s air flow channel, a channel segment is structured so that it creates air flow vortices. The vortices act on the flowing air to separate out potentially contaminating liquid droplets and solid particles entrained in the intake air flow. Contaminants are deposited on the channel segment walls before they reach the measuring element, thereby protecting the sensing element from the effects of contamination [36]. To take advantage of the best performance features of each method, vehicle manufacturers often combine different methods of air flow measurement. For example, mass airflow methods in Sections IV-F2 or IV-F3 provide more accurate steady-state air flow measurement, and one of these methods is combined with the engine speed-air density method in Section IV-F1, which provides superior transient response [37]. The combined methods, therefore, provide both steady-state accuracy and fast transient response.

8 FLEMING: NEW AUTOMOTIVE SENSORS A REVIEW 1907 Fig. 8. Magnetic encoder-ring/hall-effect sensing-element sensor used to measure steering-wheel torque [38]. The encoder ring and the stator rings are connected via the sleeve to opposite ends of the torsion bar, and twist with respect to each other when steering-wheel torque is applied. The twist angle of the encoder ring with respect to the stator rings is detected and it provides the torque signal. G. Torque Sensors Various configurations of torsion-bar twist-angle types of torque sensors were described in [1]. Twist angle due to the applied torque acting on a torsion bar is detected using one of the following approaches: a) potentiometrically (requiring sliding contacts), or by assorted noncontact means including; b) optics with varying apertures; c) magnetics with displaceable air gaps; or d) electrical eddy currents with variable shaded poles. Additionally, magnetoelastic detection methods can be used on solid (noncompliant) shafts. In this case, torque is measured by noncontact means using: a) ac-excitation to detect torsional stress-induced changes in the magnetic permeability of a shaft surface layer or b) sensing the effects due to torque-induced rotation of permanently magnetized domains in surface layers of a shaft. (As the domains rotate, they self generate a torque signal as a result of angular-dependent coupling of the magnetization to external field detectors no ac-excitation required). Applications of torque sensors of automotive interest include: Steering-wheel torque for electric power steering (EPS). Driveshaft (transmission-out) torque. Clutchshaft (engine-out) torque. Because of the more rapid development of EPS systems and the less demanding operating requirements associated with the steering column location, steering-wheel torque sensors are further developed than driveline torque sensors. New types of EPS torque sensors are described. 1) Magnetic Encoder-Ring/Hall-Effect Sensing Element Steering Wheel Torque Sensor: This EPS torque sensor, illustrated in Fig. 8, consists [38], [39] of: Two co-rotating stator rings, each with 12 intermeshed ferromagnetic teeth are both connected via a sleeve to the input end of a torsion bar that is inline with the steeringwheel column. A magnetically poled encoder-ring rotor has 12 alternating north south poles. The encoder ring is connected to the output end of the inline torsion bar. The encoder ring is concentrically located below the stator rings. When torque is applied to the steering wheel, the torsion bar experiences twist and the encoder ring is angularly displaced with respect to the stator rings. An output torque signal is generated by flux created across the stator teeth. If the north and south magnetic poles on the encoder-ring are aligned with stator teeth, flux crossing between teeth from one stator to the other (and the torque signal) is maximized. If magnet poles straddle the stator teeth, the output signal is minimized (because flux is confined within stator teeth and does not cross between teeth from one stator to the other). For example, when steering-wheel torque twists the encoder ring in one direction, the teeth in one of the stator rings may be positioned more over the encoder s north poles and these stator teeth will collect flux. The teeth of the other stator will be positioned more over the encoder s south poles and these teeth will return flux. The collected flux is detected by the Hall-effect sensing elements shown in Fig. 8. In summary the greater the steering-wheel torque, the greater the torsion bar twist angle, the greater the displacement of encoder magnetic poles with respect to stator teeth, the greater the flux crossing between teeth from one stator to the other, the greater the collected flux, and the greater the Hall-effect sensor torque output signal. Performance features of the sensor include: Accommodates a family of designs that operate with torsion bar full-range twist angles of: 8-, 4-, and 1. (The stiffest 1 torsion-bar sensor requires three times more encoder-poles/stator-teeth than the 8 design). All three torque sensor designs have the same compact dimensions of 37.5-mm outer diameter and 8-mm width. Electronic signal-processing components are mounted in a fixed housing and are not required to rotate with the steering-wheel torsion bar.

9 1908 IEEE SENSORS JOURNAL, VOL. 8, NO. 11, NOVEMBER ) Surface Acoustic Wave (SAW) Torque Sensor: SAW torque sensors can be used for driveline and steering-wheel torque measurement applications. Utilization of SAW technology enables wireless, batteryless, noncontacting measurement of the mechanical strains due to shaft torque. SAW torque sensors utilize the influence of strain on the propagation velocity of acoustic waves [40], [41]. The main elements in SAW torque sensors are: a) Coupler: A fixed-mount signal coupler with transceiver electronics in one example provides a 433-MHz RF interrogation signal. The RF signal wirelessly transmits energy to very low power-consumption SAW transducer elements on the rotating torque shaft, and it wirelessly receives return signals. b) SAW transducer: The SAW transducer elements consist of interdigital electrodes and reflective gratings that are fabricated on a quartz substrate which, in turn, is attached to a flat surface, machined on the shaft in which torque is measured. c) SAW sensing element alignment: The SAW sensing elements are aligned with the principal lines of tensile and compressive torsional strain in the shaft. These lines act along 45 angles with respect to the longitudinal axis of the shaft. d) Sensor operation: The SAW interdigital transducers are piezoelectric. In one example, 433-MHz pulsed-voltage sine waves piezoelectrically generate 433-MHz acoustic waves, which are transmitted along the quartz surface. These waves propagate in straight paths along the shaft s principle lines of strain and are reflected off gratings and travel back to the transducers that detect their return. e) Output signal: Shaft torque creates strains that physically change the spacing between transducers and reflective gratings, altering the resonant frequencies in SAW propagation-controlled resonator circuits. f) Differential measurement: A differential measurement of resonant frequencies from the two 45 oriented SAW resonators provides the torque measurement. Interfering effects of temperature and shaft bending are cancelled out in the differential measurement. g) Signal transmission: RF signals corresponding to the frequency-shifted resonant responses of the SAW sensing elements are transmitted back to the signal coupler, which includes signal processing and provides the torque signal. Key performance features of the sensor include: SAW sensors operate wirelessly and no battery is required to power the shaft-mounted sensing elements. SAW sensors are small and lightweight. The high resolution and sensitivity of the SAW sensor allows torque to be measured on a solid shaft, i.e., no torsion bar is required. 3) Magnetoelastic Torque Sensor: Magnetoelastic torque sensors, like SAW sensors, do not require a torsion bar, no battery is required to power the shaft-mounted sensing element (and no excitation is required to transmit power to the shaft), and they utilize noncontact operation. Magnetoelastic torque sensors can measure torque on a solid shaft because they respond to shaft stress (instead of strain). Note. The aforementioned ac-excitation type of magnetoelastic torque sensor is used primarily for instrumentation purposes and is not discussed here. The magnetized-domain type of magnetoelastic sensor has shown potential for automotive sensor applications and is described. An annular surface region on a shaft with magnetoelastic properties is permanently magnetized such that magnetic domains are circumferentially oriented around the outer surface of the shaft. If no torque is applied to the shaft, the circumferential magnetization field is unaffected and there is no change (no rotation) of the magnetic domains. In accordance with the magnetoelastic effect, applied shaft torque causes the magnetic domains to rotate from their initial circumferential directions towards axial directions. The rotation of domains creates an axial field component along the direction of the shaft s longitudinal axis. This axial magnetic field is detected using a dc flux-gate magnetic modulator circuit (which exhibits ultra stable operation). As shaft torque is increased, magnetic domains rotate further, strengths of the axial field components increase, and the flux-gate circuit detects a stronger magnetic field, therein providing the torque measurement signal [42]. Since the axial field reverses direction when applied torque reverses direction, the sensor output signal automatically reverses sign when applied torque is reversed. One difficulty with this sensor is that its calibration is dependent on reproducibility of the magnetoelastic properties of the shaft (sleeve, coating material, or the shaft itself). Magnetoelastic properties of materials are not specifiable or controlled by metal manufacturers. Consequently, when large numbers of magnetoelastic torque sensors are manufactured, it is difficult to maintain part-to-part variation of torque sensor calibrations within automotive interchangeability variation limits, which are typically 1%. There has been recent progress in development of the magnetized-domain type of magnetoelastic torque sensor [43]. Zero-carbon Ni-Fe maraging steel shaft material with special heat treating is used sleeves or coatings are not required. The shaft material itself functions as a magnetized surface layer [44]. To achieve interchangeability, these sensors are currently hand-sorted to insure uniform calibrations. Both driveshaft and clutchshaft torque sensors for F1 race cars are currently supplied [43]. If this sensor is to satisfy high-volume production automotive requirements, obstacles that must be overcome include: a) finding a source of magnetoelastic steel which is lower-cost than maraging steel; b) obtaining magnetic field detectors that are lower cost than flux-gate detectors; and c) developing better control of the zero-torque calibration point. H. Linear Acceleration Inertial Sensors Although the operating principles of acceleration sensors have remained the same [45], there have been many improvements, beginning with the incorporation of the smart sensor features described in Section IV-A, along with advances in

10 FLEMING: NEW AUTOMOTIVE SENSORS A REVIEW 1909 packaging and processing that have yielded smaller, more accurate, lower cost, more robust sensors. Automotive applications for linear acceleration sensors include: Vehicle stability and chassis adaptive suspension systems. Vehicle frontal, side, and rollover crash sensing. Engine knock detection (using flat-response acceleration sensors and bandpass frequency filtering). Piezoresistive MEMS, capacitive MEMS, resonant-beam MEMS and piezoelectric types of linear acceleration inertial sensors were reviewed in [1]. New automotive applications and features for acceleration sensors, not previously described, include the following. 1) Chassis Acceleration: Chassis acceleration sensors today typically offer two-axis ( - ) acceleration measurements and come in surface mount packages as small as. This minimizes the amount of circuit-board area taken up by the sensor. One chassis acceleration sensor utilizes a seismic-mass that is micromachined in silicon into the form of an elliptical-shaped plate, with tether springs integral to its body [46]. The sensor utilizes a lateral-to-substrate-displacement operating configuration. Acceleration-induced deflections of the seismic mass are detected by changes in capacitance due to lateral displacements between comb electrodes. Chassis acceleration sensors have the following features: Integrated minimal-overshoot, low-pass frequency (fourth-order Bessel) filtering of the output signal. Built-in self-monitoring failure detection and self-calibration. Accuracy, 2% of full scale (over the entire range of acceleration, temperature, and sensor-to-sensor calibration variation). Resolution, 10-mg. Electrically selectable acceleration detection ranges. Wide measurement bandwidth, dc-to-400 Hz. 2) Vehicle Crash Detection: Modern vehicles generally include five acceleration crash sensors, namely: a) a right-front and a left-front satellite crash sensor (to trigger front airbags in offset frontal crashes); b) a right-side and a left-side satellite crash sensor (to trigger side airbags and curtain airbags); and c) a central safing sensor mounted in the passenger compartment (for high reliability crash detection). Furthermore, because three-row-seat vans and SUVs have longer lateral-coverage curtain bags, these vehicles require two additional side satellite crash sensors, mounted in their rear-quarter panels. A typical satellite crash sensor utilizes a lateral-to-substrate displacement configuration, is flexure-supported, and has a rectangular-shaped seismic mass [47]. Displacement of the seismic mass is capacitancely detected using comb electrodes. The sensor is fabricated using the high aspect ratio deep-reactive ion etching (DRIE) process in silicon, as described on page 1546 of [48]. Crash-detection acceleration sensors have the following features: Integrated minimal-overshoot, low-pass (two-pole Bessel) frequency filtering of the output signal. Wide measurement bandwidth, dc-to-1000 Hz (to detect short-duration crash events). Wide dynamic measurement range, 80 db. Low noise operation, 1. Built-in self-monitoring failure detection and self-calibration. I. Angular Rate (Gyro) Inertial Sensors As in acceleration sensors, automotive angular-rate sensors also utilize MEMS technologies. Their operation is based on detection of the effects of Coriolis forces acting on various types of vibrating mechanisms. Vibrating-ring, vibrating-tine (tuning fork), and vibrating mass types of angular rate inertial sensors were reviewed in [1] and [45]. Although the operating principles of these sensors have remained the same, there have been several improvements. Prior models of vibrating-tine automotive rate sensors required a large circuit board footprint of as much as mm. To minimize the circuit-board area taken up by the sensor, considerable effort has been made to reduce sensor size. The mm sensor today comes in a mm (76% area reduction) footprint package [49]. The reduced footprint was made possible by utilizing a micromachining (etching) process to fabricate the double-ended quartz tuning fork, reducing its length to 10 mm. Smaller-footprint rate sensors have been achieved using the vibrating-ring type of sensor. Sensor footprints of 9 9mm have been realized. This was done by: a) replacing an electromagnetic actuation type of ring vibration with a capacitive electrostatic vibration actuation [50]; b) micromachining a 4-mm diameter, 100- thick, flexure-supported silicon ring; c) upgrading from analog to digital circuitry; d) using back-to-back stacking of electronics and sensing element dies; and e) utilizing surface-mount packaging. Automotive applications of angular rate sensors include the following. 1) Vehicle Electronic Stability Control (ESC): Vehicle yaw angle rate detection is a key component of ESC which is now required on all new passenger vehicles under a United States federal safety standard that phases in beginning with 2009 models [51]. This federal requirement for ESC systems created a huge demand for rate sensors. ESC systems are not mandated in other countries, but new car assessment programs (NCAPs) such as Euro NCAP, Japan NCAP, etc., additionally drive demand in these countries. 2) Active Chassis Suspension: Suspension control systems use angular-rate sensors to detect vehicle roll-rate and pitchrate. 3) Rollover-Protection Side Curtain Airbags: Vehicle rollrate sensors are a key part of a sensor suite used to trigger deployment of rollover-protection side curtain airbags. 4) Vehicle Navigation Systems: Navigation Systems use yaw-rate sensors to detect vehicle heading (yaw angle) when the autonomous dead reckoning mode of navigation is required. Yaw angle is determined by a mathematical integration of the yaw angular rate signal with respect to time. (When the system s GPS absolute position signal is unavailable, near tall buildings or inside tunnels, the system switches to an autonomous navigation mode of operation). Rollover-crash-detection rate sensors in Section IV-I3 require operating range and bandwidth several times greater

11 1910 IEEE SENSORS JOURNAL, VOL. 8, NO. 11, NOVEMBER 2008 Fig. 9. Essential operating components of a dual vibrating-mass angular rate sensor used in vehicle electronic stability and active suspension control systems [52]. than rate sensors used for vehicle dynamics applications in Sections IV-I1, IV-I2 and IV-I4. On the other hand, applications in Sections IV-I1 and IV I2, and in Section IV-I4 require greater accuracy than in Section IV-I3. Sensors used in Section IV-I3, therefore, are not interchangeable with the vehicle dynamics sensors. In the safety-critical applications of types in Sections IV-I1 and IV-I3, built-in-test self-monitoring failure detection is mandatory because sensor failure could result in a system anomaly. There are less stringent requirements for applications in Sections IV-I2 and IV-I4. Two new types of angular rate sensors are described. 5) Oscillating-Rotor Sensor: Comb electrodes on the periphery of a rotor electrostatically drive a center-pivoted flexuresuspended rotor into rotary oscillatory motion. If no angular rate acts perpendicular to the oscillating rotor, the disk continues its in-plane rotary oscillation. When an angular rate exists, Coriolis forces superimpose an out-of-plane tilting motion on the oscillating rotor. Rotor tilt with respect to its fixed substrate is capacitively detected. Rotor tilt angle provides the angular rate output signal i.e., the greater the angular rate acting on the sensor, the greater the rotor tilt angle, and the greater the output signal [52]. This sensor is entirely fabricated using only surface micromachining in silicon. Detail on the multiple-degree-of-freedom center-pivot flexures that suspend the sensor s oscillating and tilting rotor are given in [53]. The oscillating-rotor sensor features low cost, small size, and batch fabrication using standard micromachining processes [52]. This type of MEMS processing is limited to making smaller lighter-weight rotor masses (due to limited feature sizes). For a given applied angular rate, Coriolis forces acting on the rotors are therefore small. This type of sensor is not sensitive enough for use in the stability control applications of Section IV-I1, where high accuracy at low angular-rate inputs are required. The sensor is, however, used in the rollover and navigation applications (where higher angular rates are measured), as described in Sections IV-I3 and IV-I4. 6) Dual Vibrating-Mass Sensor: The essential operating components of this sensor are shown in Fig. 9. Comb electrodes electrostatically drive dual masses in the axis direction, in an in-plane anti-phase vibrating manner. If there is no angular rate acting on the sensor, the masses continue their vibration along the axis. When a axis angular rate input exists, Coriolis forces induce a axis lateral motion, mutually perpendicular to the directions of vibration and angular rate. By design, Coriolis force-induced axis motions of the detection-frames (coupled to the masses) oscillate at an eigenfrequency. For better detection of the angular-rate signal, the eigenfrequency is approximately 20% different from the drive frequency. The axis motions of the detection-frame masses are sensed capacitively by another set of comb electrodes, therein providing an angular rate output signal. The sensor features high accuracy, excellent signal-to-noise ratio, high reliability, surface-mount packaging and small size. More detail on this type of sensor is found in [54]. The dual vibrating-mass sensor is made using a modified DRIE process [48] which facilitates MEMS fabrication in silicon of larger, heavier, vibrating masses [52]. Although more costly, this design satisfies the more demanding high accuracy at low angular-rates requirements for use in stability control and active suspension applications, described in Sections IV-I1 and IV-I2. J. Chemical and Gas Composition Sensors Exhaust gas oxygen monitoring, spark plug-mounted in-cylinder ion-current misfire/knock combustion sensors, and exhaust gas NOx sensing types of chemical and gas composition sensors were reviewed in [1]. Updated information on the signal processing associated with the spark plug in-cylinder

12 FLEMING: NEW AUTOMOTIVE SENSORS A REVIEW 1911 combustion ion-current engine misfire/knock detection sensor is found in [55]. An update on the status of NOx exhaust gas sensors is given in [56]. New features and new types of chemical and gas composition sensors are described here. 1) Exhaust Gas Oxygen Sensors: Exhaust gas oxygen sensors have been continually improved and the following new features have been introduced. a) Pumped-channel air-reference: In planar exhaust gas zirconia oxygen sensors, a solid-state pumped-channel air-reference has replaced the previously used open-cavity access to ambient air. (Planar sensors consist of layered sheets of zirconia electrolyte bonded together into a structure that includes electrodes, heating elements, and gas diffusion channels). This prevents contamination of the electrode on the air reference side of the sensor. Oxygen is electrochemically pumped from exhaust gas (which, even for rich air-fuel ratio engine operation, includes adequate amounts of oxygen) to the sensor s reference electrode, through a zirconia solid electrolyte element. A chamber adjacent to the reference electrode, internal to the sensing element, is pumped full with oxygen. A small, 0.1 ma, bias current, with negative voltage polarity on an auxiliary exhaust electrode, supplies oxygen via oxygen-ion conduction through the zirconia channel [57]. b) Helical-swirl double-wall shroud: Certain models of zirconia oxygen gas sensors utilize a math model-designed helical-swirl double-wall shroud which covers the sensing element. The shroud s design causes exhaust gas to swirl inside the space between the shroud walls. The swirling action removes particles and droplets, protecting the sensing element from contaminants in the exhaust gas. In addition, the swirling flow inside the shroud provides longer gas residence time on the zirconia outer electrode. This promotes more complete electrochemical reactions and results in more accurate engine air-fuel ratio measurement [58]. c) Three exhaust gas oxygen sensors replace one: A decade ago engine emissions control systems would typically require only one exhaust oxygen sensor to control engine air-fuel ratio exhaust composition flowing into a catalytic converter. Current emissions systems often require two catalytic converters in series an oxidation/reduction catalyst followed by a NOx catalyst. To detect exhaust air-fuel mixture entering each converter, and to satisfy onboard diagnostics requirements, emissions systems today often use three exhaust oxygen sensors one is positioned upstream of the first converter, one is between converters, and one is downstream of the second converter [59]. 2) Oil Quality Sensors: Oil quality sensors are mounted near the bottom of the engine oil pan. Previous oil quality sensors monitored oil level, oil temperature, and oil dielectric constant (dielectric constant was used to detect ionic deterioration of the oil). Modern oil sensors possess enhanced capabilities. a) Measurement of oil viscosity: One sensor today includes an oil flow-across microacoustic sensor element. Transducers in the sensor element piezoelectrically generate 120-MHz acoustic Love shear waves in a planar quartz surface layer. (Love waves propagate longitudinally with lateral oscillating displacements). Differences in resonant frequencies of oscillator circuits controlled by Love waves propagating on a smooth quartz surface versus Love waves propagating on a grooved, micromachined, quartz surface provide continuous measurement of oil viscosity [60], [61]. A related sensor that also measures oil viscosity utilizes a small quartz tuning fork immersed in oil. The tuning fork tines are piezoelectrically driven and resonate at frequencies ranging from 26 to 32 khz. Characteristics of measured electrical impedances of the vibrating tuning fork versus frequency are analyzed to determine the oil viscosity [62]. b) Measurement of soot-in-oil: This sensor monitors oil flow between concentric tubular electrodes. The sensor measures ac conductivity of diesel engine oil, both at a low-frequency of 20 Hz and at a high-frequency of 2 MHz. Measurements are made during engine warm-up and engine cool-down cycles. In this way, it is assured that the oil temperature passes through, and is measured at, an interpolated-constant temperature of 80. When day-to-day trend data of the low-frequency oil conductivity reverse slope, this occurrence indicates that the engine oil has ionic contamination and for this reason needs to be changed [63]. On the other hand, the high-frequency measurement of the oil ac conductivity provides an indication of the concentration of soot in diesel engine oil. A computed ratio of the high-frequency ac conductivity to the low-frequency ac conductivity provides a quantitative measurement of the concentration of soot in diesel engine. When this ratio exceeds a predetermined limit, it indicates that there is too much soot and the oil should be changed [64]. A second approach to soot-in-oil measurement likewise allows oil to flow between concentric tubular electrodes in a sensor. This sensor makes mhz-to-khz scans of the electrical impedance of the oil. Changes in the relative magnitudes of various frequency components of impedance are used to determine the amount of soot in oil [65]. 3) Flexible-Fuel Composition Sensors: In the United States, corn is used to produce ethanol that is mixed with gasoline to produce E85, a blend of 85% ethanol and 15% gasoline. Because the chemical composition of ethanol includes oxygen, the greater the percent of ethanol mixed with gasoline, the more that engine air intake flow must be reduced to maintain stoichiometric combustion (correct oxygen-to-fuel mixture) for catalytic converter emissions control and good drivability. A flexible fuel composition sensor is required to measure the ethanol content of the fuel and provide an input signal to an engine control system. The flexible-fuel sensor, shown in Fig. 10, internally includes concentric tubes and fuel flows in the space between the tubes. One oscillator circuit in the sensor measures electrical capacitance i.e., dielectric constant of the fuel (which primarily determines the concentration of ethanol in the fuel). Other oscillators measure fuel conductivity and temperature (for compensation purposes). The oscillators do not utilize quartz crystal components [66]. 4) Occupant Compartment Gas Detection Sensors: Many luxury vehicles today have gas-detection sensors mounted in the air intake duct of their heating ventilation and air conditioning (HVAC) system. These sensors monitor quality of the air entering the vehicle cabin/occupant compartment. In addition, a HVAC-control humidity sensor can be mounted in the

13 1912 IEEE SENSORS JOURNAL, VOL. 8, NO. 11, NOVEMBER 2008 Fig. 10. Flexible-fuel sensor which measures ethanol content in fuel [66]. lower portion of the instrument panel, and a windshield fogging prevention sensor can be mounted on the windshield behind the rear view mirror. Each of these sensors is described. a) Cabin air quality sensors: Cabin air quality sensors detect harmful gas fumes like carbon monoxide CO and nitrogen dioxide in the outside air drawn in by the HVAC system. When air quality is bad, i.e., when ambient air CO and/or concentrations are high, the HVAC system temporarily shuts off intake of outside air and switches to recirculation mode, routing cabin air back through the HVAC air filter [67]. The sensing element of an air quality sensor typically consists of a thin porous layer of. The sensing element is deposited on top of a silicon micromachined diaphragm. The thin Si diaphragm has reduced thermal mass that allows rapid, low power consumption, heating of the sensing element. A heating element embedded in the diaphragm raises the temperature of the sensing element to 400, where the sensor has greater gas-detection sensitivity, and is more specific to the target gases it detects. When CO adsorbs on the -surface, electrochemical surface reactions form the product gas, while also injecting (adding) electrons. This causes the element resistance to decrease. Adsorption of gas has the opposite effect on sensor resistance [68], [69]. b) Humidity sensors: Vehicle interior comfort was originally controlled using only HVAC temperature and fan adjustments. Auto manufacturers later utilized air conditioning (A/C) to extract excess humidity and to increase occupant comfort. The addition of in-cabin humidity sensors further enhances cabin comfort, by automatically regulating HVAC system operation to improve: i) comfort by automatically activating A/C when humidity is high and ii) fuel economy by turning off the A/C when it is not needed. Automotive humidity sensors commonly detect humidity-induced changes of capacitance in porous polymer films or thin layers of porous metal oxide. As humidity increases, so does the sensor capacitance. When relative humidity is low, below approximately 40%, no liquid water exists in the sensing material. Only adsorbed water molecules exist and humidity-dependent changes of sensor capacitance are due to ion transport among adsorbed water molecules in the porous sensor material [70]. When relative humidity is high, greater than 40%, liquid water can condense inside the pores of the sensing material. Electrolytic ion conduction then occurs, creating microscopic electrical shorting paths between opposite electrodes, which further increase sensor capacitance [70], [71]. Resistive-type humidity sensors function by the same principles as capacitive sensors, except that their resistance decreases with increasing humidity due to their electrode configuration. c) Windshield fogging prevention sensor: Reliance solely on a humidity sensor to prevent fogging of a windshield is not sufficient. Reliable fog sensing can be obtained using a dewpoint sensor. The dew-point sensor includes three sensing elements that independently measure: i) windshield glass temperature; ii) cabin interior temperature; and iii) cabin humidity. Window fogging is prevented, or removed, by operating the HVAC so that the cabin air dew-point temperature is maintained above the windshield glass temperature. The dew, or frost, point of cabin air is determined using electronics integral to the sensor, programmed to use pyschrometric equations to compute dew point as a function of measured values of cabin temperature and humidity [72]. K. Comfort and Convenience Sensors Dimming mirror sensors, solar radiation/twilight sensors, fluid level sensors and rain-detection comfort and convenience sensors were reviewed in [1]. While the automotive applications remain the same, there have been the following new developments. 1) Automatic Dimming Mirrors: CMOS imager sensors employ camera-on-chip technology, and are mounted on the windshield behind the rear-view mirror. Upon sensing oncoming light, a microprocessor integrated in the sensor performs object recognition. If approaching vehicle headlights or preceding vehicle tail lamps are detected, the system gradually turns off the high-beam headlights to reduce distraction to other drivers. The system also detects and ignores ambient light coming from streetlights, sign reflections, buildings and other sources. In situations where light from an approaching vehicle is immediate e.g., when another vehicle crests a hill the system then reacts automatically and quickly switches to low beam [73]. 2) Solar Radiation/Twilight Sensors: Solar radiation/twilight sensors utilize solar heat-detecting photodiodes that respond to near infrared wavelengths, plus twilight-detecting photodiodes that respond to visible wavelengths. The solar and twilight photodiodes are packaged together and mounted in a single housing atop the instrument panel, near the base of the windshield. The solar photodiodes in the sensor provide input signals for automatic HVAC temperature control systems, whereas the twilight photodiodes are used to automatically turn on headlights. 3) Multizone Infrared (IR) Sensors: Since objects emit infrared (IR) radiation as a function of their temperature, IR sensors are able to measure the surface temperature of objects or persons at a distance. Sunlight variations can cause fast temperature changes, with correspondingly rapid changes in vehicle passenger comfort. IR sensors rapidly react to passenger exposed-skin temperature changes. The sensors provide closedloop feedback to a vehicle HVAC system in order to maintain passenger-comfort desired temperatures. Dual infrared sensors

14 FLEMING: NEW AUTOMOTIVE SENSORS A REVIEW 1913 are mounted in the front face of the HVAC control panel. They independently measure body surface temperatures of the driver and passenger, and allow the HVAC to individually regulate comfort according to the body temperatures of driver and passenger [74]. Standard CMOS MEMS technologies are used to fabricate the IR sensor. Dozens of thermocouple (thermopile) junctions and associated n-well thermistors are formed in thin membranes etched in bulk silicon on each sensor. Electronics is provided via a separate integrated-circuit chip [74]. 4) Rain Sensors: Rain sensors provide feedback signals for automatic windshield wiper control. Depending on design, IR-beam optics in the sensors either refract light away from, or reflect more light back, when rain impinges on their optical path at the interface between the windshield and the outside weather. Detected changes in IR beam intensity are proportional to amount of rainfall. A capacitive type of rain/fog sensor is also in production. The capacitive sensor includes a flat substrate. One side of the substrate has surface electrodes that capacitively generate electric fields that extend (fringe) through the windshield to outside air and interact with impinging raindrops. Changes in capacitance (dielectric constant) indicate the presence of rain. The other side of the substrate includes electronic signal processing [75]. The capacitive sensor geometrically has three-times greater sensing area than the IR optical sensor and consequently detects moisture on three-times greater windshield area than the IR optical sensor. Because the capacitive sensor detects moisture on a greater windshield sensing area than its optical counterpart, it better responds to difficult-to-measure fine mist and is less affected by a dirty windshield [75]. 5) Fluid Level Sensors: Thermistors are commonly used to detect low levels in coolant, fuel, brake, and steering fluids. Differences between the self-heating temperature of the thermistor when immersed in a fluid, and not immersed, provide an output signal. Another commonly used approach to low fluid sensing uses a magnet mounted in a float. The magnet-in-float travels along a slotted keyway and rides up and down with the changes in the fluid level. A reed switch is mounted at a fixed position. As fluid level drops, the magnet-in-float descends, and when the level of the fluid drops below a predetermined point, the magnet s field actuates the reed switch, therein providing a low fluid level signal [76]. L. Occupant Safety and Security Sensors The United States enacted a safety standard that among other things includes operating requirements for advanced airbag systems [77]. The standard is unique to the U.S. and has not been adopted by other countries. The standard applies to all vehicles in the United States manufactured since model year The standard gives special attention to protection of infants in rear-facing infant seats, unbelted small children, short-stature adults, and elderly adults persons who have been disproportionately susceptible to injury by early-model airbags. Occupant safety sensors developed specifically to make airbags safer and to comply with this standard are described below. 1) Occupant Safety Sensors: The federal standard for advanced airbags offers three compliance options. The option most Fig. 11. Strain-gage sensing elements, integrated in seat-frame corner mounts detect difference between seated weights of large male passenger and small female (or child) [79]. commonly used by automakers utilizes occupant-sensing technologies to statically classify occupants (weight, size, position) and when necessary to suppress airbag inflation if an at-risk occupant is detected in a seat [77]. The following sensors are used for occupant static classification purposes in advanced airbag control systems. a) Seated weight sensors: Seated weight sensors measure occupant s seated weight to distinguish small children from adults in the right-front passenger seat. When a lighter-weight passenger is detected, the airbag system is adjusted to provide a softer bag deployment, or no deployment at all if a child or empty seat is detected. There are two main types of seated weight sensors used in production vehicles today. Seat cushion-embedded, fluid-filled bladder with pressure sensor readout [78]. Strain-gage sensing elements, integrated in seat-base corner mounts [79], as shown in Fig. 11. b) Seatbelt tension sensors: Seatbelt tension sensors detect the apparent added weight of a tightly belted child restraint seat. For example, a large toddler in heavy child seat, buckled-in with high seatbelt tension, might be mistaken for a small adult female. Even though they both may have the same apparent seated weight, the airbag cannot deploy on the child, but must deploy on the adult female. Inputs from the seatbelt tension sensor allow belt tension to be factored out of the seated weight measurement, therein avoiding an inappropriate airbag deployment on a child. Seatbelt tension sensors are typically mounted at the seatbelt buckle-anchor locations. The sensors often consist of a magnet in a spring-loaded assembly, where belt-tension-induced displacement of the magnet is sensed using a Hall-effect sensor [80]. c) Seatbelt buckle status sensors: Seatbelt buckle status sensors are used to detect whether or not an occupant s seatbelt is buckled. This input is, for example, used by airbag systems, which have dual-level bag inflation rates. When an occupant s seatbelt is buckled, a less aggressive bag inflation rate is used because the seatbelt is already restraining the occupant. Buckle status sensors consist of a magnetic circuit internal to the buckle

15 1914 IEEE SENSORS JOURNAL, VOL. 8, NO. 11, NOVEMBER 2008 that includes a magnet, a Hall-effect sensor, and a circuit-completion buckle latch member. Full engagement of the buckle s latch into the tongue s latch window completes the magnetic circuit [81]. d) Seat position sensors: Seat position sensors are used to detect whether a driver s seat is positioned far forward which indicates that a short-stature person is driving the vehicle. When the driver s seat is far forward, typically in 80%-to-100% of full forward travel, this indicates that the driver is close to the steering-wheel airbag. When a driver is seated this far forward, and a moderate-severity crash occurs, bag deployment can be suppressed because: i) at this close distance the steering wheel itself is protecting the driver and ii) there is a risk that the driver s close proximity could result in unintended injury by the deploying airbag. Seat position sensors typically consist of a magnetic circuit that includes a magnet, a Hall-effect sensor, and a circuit-completion vane member. A ferromagnetic steel vane attached to the bottom of the driver s seat acts as the circuit-completion member. The vane completes the magnetic circuit when the seat is positioned far forward [82]. 2) Intrusion-Detection Security Sensors: Two-way remote keyless entry (RKE) and antitheft systems both utilize sensors to detect unauthorized intrusion into vehicles. Intrusion sensors are mounted inside the vehicle cabin. Types of sensors most commonly used for intrusion detection are: Shock/vibration/motion, where low-frequency interior vibrations, vehicle swaying, or vehicle bouncing are detected. Glass breakage [83]. In some cases, microphones detect the breakage and neural-network pattern recognition of the detected acoustic energy spectrum is utilized. Ultrasonic doppler motion detection utilizes signal processing that ignores stationary objects [84]. Passive far-ir body-heat detection [83]. M. Distance Sensors Distance sensors monitor areas surrounding a vehicle, and are designed to detect dangerous obstacles such as other vehicles on paths of potential collision. Distance sensors are categorized as: a) long range sensors which look forward at distances of approximately m and b) short range sensors which look in all directions around the vehicle at distances of approximately 0 30 m. Advances in sensor technology and new automotive applications are presented here. 1) Long Range Distance Sensors: Adaptive cruise control (ACC) systems require long-range distance sensors which use either 77-GHz (a government-regulated frequency) millimeterwave radar or near-infrared laser radar. Instead of simply maintaining vehicle speed, ACC maintains distance from the car in front. If a vehicle cuts in front, the subject vehicle automatically slows down and maintains a safe separation distance. If a lead vehicle speeds away, then the subject vehicle automatically resumes to its own set speed. Mutual interference among multiple vehicle radar beams is suppressed, for example, by synchronizing the modulation of the radar s transmit carrier frequency with its receiver tuning frequency, thereby distinguishing its own received signal from those of other radars. Pseudorandom modulation of the carrier frequency is one method used to suppress mutual interference. Another application for long-range distance sensors is forward collision warning. Studies have shown that, 60% of rear-end collisions could be avoided if drivers had an extra half second to react, and there is 90% avoidance with a full one second reaction time [85]. ACC distance sensors provide the necessary reaction time. The four types of sensors, described here were selected because each one is currently used in one or more production vehicles. Pros and cons corresponding to the various types of long-range distance sensors are summarized as follows. Three sensor types are millimeter-wave radars each featuring all-electronic (no moving parts) scanning operation, along with the ability to penetrate inclement weather. The pulsed doppler type of radar features a GaAs Monolithic Microwave Integrated Circuit (MMIC) design providing very fast radar scan update rates. The FM/CW radar claims to be the smallest and lightest automotive radar sensor in production today. Monopulse radar also features MMIC technology along with complete range and azimuthal (horizontal) angle information derived from each received pulse. Laser radar, the fourth type of long-range distance sensor, features great accuracy and very low cost (said to be 1/3 the cost of mm-wave radar), but laser radar cannot penetrate heavy fog, rain, or snow. a) Pulsed doppler radar: Pulsed doppler radar transmits pulses (bursts) of continuous-wave signals which upon reflecting from a moving target additionally include a doppler frequency shift as a means for discriminating moving from fixed targets [112]. Gallium-arsenide MMIC circuits provide fast switching of three transmit/receive beams which sequentially scan the right-side, center, and left-side azimuthal areas of the roadway [87]. The output signal of pulsed-doppler radar provides range, range closing rate, and azimuthal location of targets. Target range is derived from pulse transit time, range closing rate is derived from the doppler frequency shift in the received pulse, and target azimuthal angle is derived from knowledge of which one of the three beams, or combination of beams, detected the target. Pulsed doppler radar is used in certain luxury European and North American vehicles. b) FM/CW radar: Frequency-modulated/continuous-wave radar directly measures range and closing speed. Beat frequencies, the differences between transmit and doppler-shifted received signal frequencies, are computed. To extract vehicle range, sums of the beat frequencies are formed (doppler shifts cancel out when beat frequencies are summed). On the other hand, differences between beat frequencies indicates range-closing rate (range components cancel out when differences are computed) [86]. In this way, both target range and closing rate are simultaneously measured. One configuration of this radar transmits a 10 wide flood beam. Three 3 wide received beam directions are electronically switched in the receive antenna, therein providing azimuthal scanning of the roadway [85]. In another configuration of the radar, instead of just switching receive beams, the combined transmit-and-receive beams are simultaneously switched among the three or four azimuthal directions [88]. This latter type of

16 FLEMING: NEW AUTOMOTIVE SENSORS A REVIEW 1915 Fig. 12. Types of automotive long-range distance sensors currently found in production vehicles. distance sensor is the smallest,, automotive radar sensor in production today. FM/CW radars are found in certain European and North American luxury vehicles. c) Monopulse radar: Range and azimuthal angle information is obtained from single pulses that are transmitted and cover a wide forward area. Sum and differences of detected phases of wavefronts of reflected pulses are detected by dual side-by-side receive antenna elements in the radar [89], [113]. Target range is derived from pulse transit time of the sum signal, range closing rate is derived by tracking target range data versus time, and target azimuthal angle is derived from the phase difference of the received pulse wavefront as detected by the side-by-side receive antenna elements. When a target is straight ahead, the dual antennas will simultaneously detect the received pulse wavefront, and the azimuthal phase angle is zero. If the target is located to the right of the vehicle, then the detected phase of the right-hand receive antenna will be leading, and the azimuthal phase angle will have a positive value, and vice versa if the target is on the left side. Monopulse radar is found in certain luxury European vehicles and in heavy trucks in both North America and Europe. d) Laser radar: In addition to millimeter-wave radar, 850-nm-wavelength laser radar is another long-range distance automotive sensor. (Laser radar is also called lidar, acronym light + radar). Transit times of laser pulses (from laser, to target, and back, divided by two), times the speed of light, determine distances. Electromechanically driven mirrors scan the laser beam in two directions: i) in the azimuthal plane and ii) in the elevation plane [90]. Target range is derived from pulse transit time, range closing rate is derived by tracking target range data versus time, and target azimuthal angle is derived from knowledge of the direction the beam was pointed when it detected the target. In rotating-mirror types of scanning ACC systems, laser diodes generate the laser beam [91]. Laser radar is found in certain luxury and mid-price Japanese and North American vehicles. Examples of each of the four types of long-range distance sensors are seen in Fig ) Short Range Distance Sensors: Seven automotive system applications use short-range distance sensors, namely: Blind spot detection uses radar, or camera vision, to monitor side rear-quarter areas outside a vehicle s side mirrors fields of view. When cameras are used, image recognition algorithms detect shapes of vehicles. Lane departure warning uses cameras, plus vision processing and lane recognition algorithms, to detect vehicle departure from road lanes. Forward collision warning with pre-impact brake assist uses short-range radar, and in some systems camera vision with vehicle recognition algorithms, to detect rapid closing rates with respect to slower-speed, stopped vehicles, or pedestrians, ahead. Pre-Safing uses radar to detect imminent collisions. Pre-safing distance sensor information is used to pretension motorized seatbelts, pre-arm airbags, apply prebraking, and move seats and windows into more protective positions. In Europe and Japan, it can also deploy pedestrian protective devices such as external air bags or raised hoods. Backup/reversing obstacle detection uses ultrasonic sensors, radar, or camera vision, and combinations of the sensors, some with object recognition algorithms. Backup obstacle detection warns drivers of potential backup collision objects. Backover crashes in the United States currently cause over 180 fatalities annually (mostly children, often occurring in the driver s driveway). The problem has worsened due to vision obscuration associated with increasingly popular vans and SUVs. Parking assist generally uses ultrasonic sensors, but certain luxury vehicles use radar or camera vision. Self-parking systems require the added availability of certain vehicle subsystems such as: electrically actuated steering, electrohydraulically actuated braking, wheel-speed sensing, and steering-wheel angle sensing. These subsystems are already included in other vehicle systems such as EPS and ESC, and are simply shared with the parking assist system. Stop-and-go/low-speed ACC utilizes radar and camera-vision sensor fusion, together with object recognition algorithms. This system takes control of the vehicle during stop-and-go driving. The system automates driving so the driver can do other things (read, download , etc.), while the car automatically negotiates stop-and-go traffic. Automotive system applications that use short-range distance sensors, and respective sensing areas, are illustrated in Fig. 13. In Table II, eight distance sensor technologies are categorized in terms of short- and long-range automotive applications. This table also identifies areas around the vehicle that are monitored in each application. Pros and cons corresponding to short-range distance sensors are summarized as follows. Radar short-range sensors

17 1916 IEEE SENSORS JOURNAL, VOL. 8, NO. 11, NOVEMBER 2008 Fig. 13. Automotive system applications that use short-range distance sensors [92]. TABLE II DISTANCE SENSOR AUTOMOTIVE APPLICATIONS Sense areas and application uses are based on published literature and the judgment of the author. Also utilizes vehicle dynamics sensor inputs (braking, deceleration, etc.) Various types of short-range radars, sometimes together with camera vision, detect rapid closing rates of slower-speed vehicles with respect to nearby slow or stopped vehicles or pedestrians ahead. Various types of long-range radars detect rapid closing rates of faster-speed vehicles with respect to more distant slow or stopped vehicles ahead. typically operate at a frequency of 24-GHz which, compared to long-range 77-GHz frequency, allows for wider beamwidths and broader road coverage, as required for short-range operation. Ultra-wideband (UWB) radars feature extremely fast measurement update rates and close-range high resolution that allows separate tracking of multiple approaching targets. Multibeam-forming radars feature near real-time broad-area coverage of blind spots. Laser radars feature great accuracy, fast update rates, and very low cost, but they can t penetrate heavy fog, rain, or snow. Camera vision has good lateral object size resolution, but its range measurement accuracy is poor, and it also does not penetrate heavy fog, rain, or snow. Ultrasonic sensors have slow measurement update rates and are susceptible to errors caused by inclement weather (including high wind), but are very low cost. a) Ultra-wideband radar: Short-range automotive UWB radars currently operate in Europe and the United States at the same regulation-allowed center frequency of 24 GHz, with bandwidths of 7 GHz (U.S.) and 5 GHz (EU). This will be the case for the foreseeable future in the U.S. However, in Europe, starting in year 2013, the UWB center frequency will likely be moved, from 24 to 79 GHz. UWB sensors transmit very short pulses and, therefore, require large ultra wide bandwidths. Although UWB sensors transmit wide bandwidth signals, interference effects are mitigated because [92], [93]:

18 FLEMING: NEW AUTOMOTIVE SENSORS A REVIEW 1917 Fig. 14. An automotive 24-GHz UWB distance sensor (exploded view). This short-range radar sensor is currently available in the United States and Europe and is approximately the size of a deck of playing cards [92]. By regulation, total radiated power from automotive radars is limited to a few tens of milliwatts. The low level of radiated power is spread over a wide bandwidth, resulting in extremely low radiated spectral power density. An automotive 24-GHz UWB distance sensor is shown in Fig. 14. UWB radar is used in short-range applications because it features: i) fast measurement update rates (typically 100 updates per second); ii) wide field of view (80 at 24 GHz); and iii) close-range high resolution which allows separate tracking of multiple targets. The use of pulse modulation provides extremely fast update rates which are also facilitated by rangegated processing of the pulse signals [94]. Short-range UWB radar sensors are also easier to conceal than, for example, laser sensors. This is because radar sensors can be integrally housed in a vehicle bumper, behind plastic fascia, giving them a concealment edge over laser radar and cameras. Certain luxury cars today have as many as seven radars on-board a single long-range forward-viewing pulsed-doppler radar and six short-range UWB pulse-modulated radars. The radars operate on a high-speed common-bus network. Two radars provide forward-viewing short-range coverage for forward collision warning and pre-safing. The other four short-range radars are mounted near each of the four corners of the vehicle for parking assist and blind spot detection [95]. b) Multibeam-forming radar: For rapid scanning of shortrange wide areas, certain automotive radars employ electronic forming of eight or more narrow beams. Multiple beams are electronically generated via switching among ports in a Butler matrix using computer-controlled excitation of planar-patch antennas. (A Butler matrix divides input power into output ports with equal amplitudes and with linear phase taper. Electronic beam scanning can be realized when a Butler matrix is used as a feed circuit for antennas) [96]. Alternatively, multiple beams are obtained by transmitting electronically switched FM/CW beams through phase-array antenna elements using digital beam forming [97]. c) Laser radar: Laser radar emits narrow, pulsed, 850-nm-wavelength IR beams. Short-range laser beams are scanned over a wide area of horizontal and vertical directions. Transit times of individual pulses determine distances to reflecting targets. The laser beam is scanned using an electromechanically driven mirror. A trifocal lens provides a variable azimuthal coverage pattern; namely, forward scanning, widening to short-range side-angle scanning. Throughout the entire azimuthal range, the beam also scans over a -elevation range. This scanning pattern facilitates near object detection. The side-angle beams detect vehicles turning off the roadway at sharp angles and/or cutting-in-front vehicles [98]. d) Camera vision: Vehicle camera-vision serves either of two applications: i) scene viewing, e.g., as used in vehiclebackup camera displays and ii) scene understanding machine vision; e.g., when no one actually sees the video, as in lane departure warning. For use in automotive scene understanding applications, camera vision must have 120-dB dynamic brightness adaptability (to allow the camera to produce clear images in all lighting conditions). On-vehicle cameras also have to work reliably in a harsh environment with operating temperatures ranging from Cto85 C, and work with longer lifetimes than for consumer-oriented cameras [99]. Space around the windshield rearview mirror where these cameras are mounted is valuable, so another key factor is reduced camera size. As evident in Table II, camera vision has more distance sensing applications than other sensor technologies. Although camera vision has good lateral resolution (i.e., good size of object resolution), its range measurement accuracy is poor. On the other hand, radar has excellent range accuracy, but its lateral resolution is limited. Consequently, radar and camera vision technologies are often combined, using sensor fusion, to reliably detect both the range and size of objects [100]. e) Ultrasonic sensors: Vehicle reversing and parking aids commonly use low-cost short-range ultrasonic sensors. The sensors operate at frequencies in the neighborhood of 50 khz. They simultaneously transmit and receive short ultrasonic pulses by means of a piezoelectric membrane element. A single sensor lacks sufficient beamwidth. Therefore, full coverage of a vehicle s backup lateral area requires acquisition of signals from typically four sensors. Signal processing circuitry is integrated in the sensor. Ultrasonic sensors have a detection range of about 2.5 m. There are development efforts to extend the range to 4.0 m. The sensors are mounted in the vehicle bumper fascia and have the appearance of a linear array of four circular dimples [100]. N. Night Vision Sensors Night vision sensors view the road and roadside nighttime scene ahead. To assist the driver, these systems project video of the scene onto heads-up or instrument-panel mounted displays. Two different technologies are used for night vision: a) far-infrared (FIR) sensors that detect long-wavelength IR warm-body thermal radiation and b) near-infrared (NIR) sensors that project

19 1918 IEEE SENSORS JOURNAL, VOL. 8, NO. 11, NOVEMBER 2008 regardless of temperature. A NIR camera uses plain glass optics and is lower cost than a FIR camera. When it is warm outside, with temperature above about 32, NIR vision stays clear, while FIR displays may turn fuzzy gray. NIR illumination is created by special headlamps. CCD or CMOS cameras image the roadway illuminated by the NIR light and their output signals are processed to provide continuous display of the nighttime scene [105]. NIR illuminators are typically mounted in vehicle headlight clusters. The NIR camera is mounted inside the windshield behind the rearview mirror. To avoid blinding between oncoming vehicles, NIR systems use random pulse-modulation of the NIR illumination and synchronous detection cameras. Fig. 15. Two types of automotive night vision technology are used. (a) FIR warm body-detect camera [106]. (b) NIR illuminator and camera [100]. shorter-wavelength nonvisible IR illumination to provide daytime-like images of the roadway. The two types of night vision sensors are seen in Fig. 15. Both types of night vision sensing technologies are in current use on production luxury vehicles in Europe, Japan, and North America. Advances in night vision sensor technology are described. 1) FIR Thermal-Radiation Sensors: FIR thermal camera vision is passive i.e., nonradiating. This sensor detects nonvisible, 8 14, long-ir wavelengths, emitted by warm-body objects. Since no illumination source is required, a FIR system does not blind oncoming night vision-equipped cars, and extra electronics are not needed to prevent blinding. Because FIR detection accentuates warm bodies of humans and animals; pedestrians, for example, are detected at greater distances than with the NIR type of night vision. There has also been continued improvement of FIR image quality [101]. Focal plane arrays are used to detect the image in FIR night vision cameras. (Note. Bolometric elements change electrical resistance or capacitance upon exposure to FIR radiation. Materials such as barium-strontium-titanate [102] or vanadium oxide [103] exhibit useful bolometric performance). A typical focal plane array includes bolometric elements. Signals from bolometric elements are processed using on-chip electronics in the camera to provide a continuous display of the nighttime scene. An example of an advanced FIR night vision system that additionally includes image processing (to recognize pedestrians at night) is described in [104]. 2) NIR Illumination Sensors: NIR camera vision detects nonvisible 0.78-to-1.0 short-wavelength IR illumination. These wavelengths are slightly longer than visible wavelengths. NIR sensors provide a driver-friendly night scene display, visibly showing road markers and reflective signs. It provides images that the driver is used to seeing, and detects all objects O. Future Automotive Sensors Needs Beyond obvious needs of being smaller, lower cost, and better integrated into system networks; there are additionally the following future needs for automotive sensors. The introduction of advanced engine and alternative power-source control system technologies, to satisfy more demanding fuel economy and emissions standards, will require new sensors to provide monitoring of combustion processes. This will necessitate the development of high-temperature (greater than 400 ) sensors that measure power-source internal pressures, temperatures, and, NOx, and likely gas concentrations. There is a continuing need for powertrain clutchshaft (engine-out) and driveshaft (transmission-out) torque sensors. As described in Section IV-G, progress has been made, but there is currently no practical torque sensor available for the difficult clutchshaft and driveshaft applications. As evident in Table II, there are several applications for radar and camera distance sensors. Exemplary new technologies currently under development to serve these applications are: (a) SiGe BiCMOS radar technology that functions to 100 GHz [107] and (b) highly integrated low-cost detector-array cameras with high-performance image processors [108]. Because numerous nonautomotive applications also exist for these technologies, nonautomotive needs will drive these two developments to maturity irrespective of automotive driving factors. P. Automotive Sensors Technology Forecast Sensor technologies forecast to find new automotive applications in the future are as follows. Within 5-to-10 years, sensors that operate at temperatures above 400 will provide new means of monitoring on-engine combustion processes [109]. Within 10 years, sensors will have evolved from: becoming wireless, then batteryless and wireless, and ultimately becoming energy harvesting and batteryless and wireless [110]. Ten-to-twenty years from now, carbon nanotubes may serve as sensing elements in nanoelectromechanical sensors which will provide new means of measuring temperature, fluid flow, chemical gas concentrations, pressure, and strain [111].

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22 FLEMING: NEW AUTOMOTIVE SENSORS A REVIEW 1921 [97] K. Natsume, Y. Miyake, K. Hoshino, and C. Yamano, Compact high-resolution millimeter-wave radar for front-obstacle detection, presented at the 2006 SAE World Congr., Detroit, MI, Apr. 3, 2006, Paper [98] Y. Teguri, Laser sensor for low-speed cruise control, presented at the SAE Convergence Int. Congr., Detroit, MI, Oct. 18, 2004, Paper [99] T. Costlow, Adding foresight, AEI Auto. Eng. Int. (SAE), pp , Apr [100] W. Uhler and P. Knoll, Surround sensors Enablers for predictive safety systems, presented at the SAE Convergence Int. Congr., Detroit, MI, Oct. 16, 2006, Paper [101] B. Terre, J. Kostrzewa, J. Kallhammer, and T. Hoglund, Infrared camera systems and methods, U.S. Patent , Mar. 4, [102] H. Beratan and C. Hanson, Monolithic thermal detector with pyroelectric film and method, U.S. Patent , Feb. 11, [103] Y. Zhao, M. Mao, R. Horowitz, A. Majumdar, J. Varesi, P. Norton, and J. Kitching, Optomechanical uncooled infrared imaging system: Design, microfabrication, and performance, IEEE J. Microelectromech. Syst., vol. 11, no. 2, pp , Apr [104] H. Shuldiner, New Autoliv night vision to boost sales, protect pedestrians, WARD S AutoWorld, Mar. 5, [Online]. Available: [105] M. Holz and E. Weidel, Device for improving visibility in vehicles, U.S. Patent , Mar. 21, [106] R. Allan, Far IR promotes safer nighttime driving, Nov. 30, [Online]. Available: [107] C. Johnson, Silicon-germanium ICs eye low-cost radar systems, Electron. Eng. Times, Jan. 9, [Online]. Available: [108] J. Day, Vision quest, Auto Electronics, Jan. 1, [Online]. Available: [109] C. Wu, C. Zorman, and M. Mehregany, Fabrication and testing of bulk micromachined silicon carbide piezoelectric pressure sensors for high temperature applications, IEEE Sensors J., vol. 6, no. 2, pp , Apr [110] N. Mokhoff, Sensors get their field of dreams, Electron. Eng. Times, Aug. 29, [Online]. Available: [111] B. Mahar, C. Laslau, R. Yip, and Y. Sun, Development of carbon nanotube-based sensors A review, IEEE Sensors J., vol. 7, no. 2, pp , Feb [112] M. Skolnik, Pulse doppler radar, in Introduction to Radar Systems. New York: McGraw-Hill, 1980, pp [113] M. Skolnik, Monopulse tracking radar, in Introduction to Radar Systems. New York: McGraw-Hill, 1980, pp William J. Fleming (S 63 M 64 SM 78 LSM 08) received the B.E.E. degree in electrical engineering from the University of Detroit, Detroit, MI, and the M.S.E. and Ph.D. degrees in electrical engineering from the University of Michigan, Ann Arbor. Retired, served as a Technical Specialist at TRW Automotive Inc., Occupant Safety Systems, where he developed new safety-restraint sensor-related products and did studies involving risk-analysis and new technology. Prior to this, he developed new types of sensors for use in automotive control systems at TRW Automotive Electronics Group. Before that, he was involved in sensor development for automotive engine control systems at General Motors Research Laboratories. From 1995 to 2007, he served as an Instructor of the seminar, Sensors and Actuators Powertrain, Chassis and Body Applications, offered by the Society of Automotive Engineers. Approximately 1500 automotive industry personnel attended the seminars. He has published more than 30 papers. Dr. Fleming was the recipient of the Society of Automotive Engineers Vincent Bendix, and Forest R. McFarland Awards. He also received the Avant Garde Award, and the Long-Term Leadership Award from the IEEE Vehicular Technology Society.