Overview of automotive sensors
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IEEE SENSORS JOURNAL, VOL. 1, NO. 4, DECEMBER 2001
Overview of Automotive Sensors
William J. Fleming
Abstract—An up-to-date review paper on automotive sensors is
presented. Attention is focused on sensors used in production automotive systems. The primary sensor technologies in use today are
reviewed and are classified according to their three major areas
ofautomotive systems application–powertrain, chassis, and body.
This subject is extensive. As described in this paper, for use in automotive systems, there are six types of rotational motion sensors,
four types of pressure sensors, five types of position sensors, and
three types of temperature sensors. Additionally, two types of mass
air flow sensors, five types of exhaust gas oxygen sensors, one type
of engine knock sensor, four types of linear acceleration sensors,
four types of angular-rate sensors, four types of occupant comfort/convenience sensors, two types of near-distance obstacle detection sensors, four types of far-distance obstacle detection sensors,
and and ten types of emerging, state-of the-art, sensors technologies are identified.
Index Terms—Acceleration sensors, angular rate sensors,
automotive body sensors, automotive chassis sensors, automotive
powertrain sensors, obstacle detection sensors, position sensors, pressure sensors, review paper, rotational motion sensors,
state-of-the-art sensors.
I. INTRODUCTION
S
ENSORS are essential components of automotive electronic control systems. Sensors are defined as [1] “devices
that transform (or transduce) physical quantities such as
pressure or acceleration (called measurands) into output
signals (usually electrical) that serve as inputs for control
systems.” It wasn’t that long ago that the primary automotive
sensors were discrete devices used to measure oil pressure,
fuel level, coolant temperature, etc. Starting in the late 1970s,
microprocessor-based automotive engine control modules
were phased in to satisfy federal emissions regulations. These
systems required new sensors such as MAP (manifold absolute
pressure), air temperature, and exhaust-gas stoichiometric
air-fuel-ratio operating point sensors. The need for sensors is
evolving and is progressively growing. For example, in engine
control applications, the number of sensors used will increase
from approximately ten in 1995, to more than thirty in 2010,
as predicted in [2].
Automotive engineers are challenged by a multitude of
stringent requirements. For example, automotive sensors
typically must have combined/total error less than 3 % over
their entire range of operating temperature and measurand
change, including all measurement errors due to nonlinearity,
Manuscript received September 8, 2000; revised November 2, 2001. This
work was supported by Tom Vos, Director, Systems Technology, Occupant
Safety Systems, Washington, MI. The associate editor coordinating the review
of this paper and approving it for publication was Dr. Gerard L. Cote.
W. J. Fleming is with Systems Technology, TRW Occupant Safety Systems,
Washington, MI 48094 USA (e-mail: [email protected]).
Publisher Item Identifier S 1530-437X(01)11158-9.
hysteresis, temperature sensitivity and repeatability. Moreover,
even though hundreds of thousands of the sensors may be
manufactured, calibrations of each sensor must be interchangeable within 1 percent. Automotive environmental operating
requirements are also very severe, with temperatures of 40
to 125 C (engine compartment), vibration sweeps up to
10 g for 30 h, drops onto concrete floor (to simulate assembly
mishaps), electromagnetic interference and compatibility, and
so on. When purchased in high volume for automotive use, cost
is also always a major concern. Mature sensors (e.g., pressure
types) are currently sold in large-quantities (greater than one
million units annually) at a low cost of less than $3 (US) per
sensor (exact cost is dependent on application constraints and
sales volume), whereas more complex sensors (e.g., exhaust
gas oxygen, true mass intake air flow and angular rate) are
generally several times more costly. Automotive sensors
must, therefore, satisfy a difficult balance between accuracy,
robustness, manufacturability, interchangeability, and low cost.
Important automotive sensor technology developments
are micromachining and microelectromechanical systems
(MEMS). MEMS manufacturing of automotive sensors began
in 1981 with pressure sensors for engine control, continued in
the early 1990s with accelerometers to detect crash events for
air bag safety systems and in recent years has further developed
with angular-rate inertial sensors for vehicle-stability 1 chassis
systems [3]. What makes MEMS important is that it utilizes
the economy of batch processing, together with miniaturization
and integration of on-chip electronic intelligence [5]. Simply
stated, MEMS makes high-performance sensors available for
automotive applications, at the same cost as the traditional
types of limited-function sensors they replace. In other words,
to provide performance equal to today’s MEMS sensors, but
without the benefits of MEMS technology, sensors would have
to be several times more expensive if they were still made by
traditional electromechanical/discrete electronics approaches.
II. OBJECTIVE
MEMS-based automotive sensor technology was recently
reviewed by Eddy and Sparks [5]. Frank’s 1997 publication [6]
emphasized electronic circuits and sensor manufacture. Two
classic references on automotive sensors include: Wolber’s
1978 publication [7] and Heintz and Zabler’s 1982 publication [8]. The objective of the present paper is to provide
an up-to-date overview of current-production and emerging
state-of the-art, automotive sensor technologies.
1Stability systems, also called active handling systems, automatically minimize oversteer/understeer vehicle dynamics, which can occur during cornering
and/or hard vehicle braking or heavy acceleration on split- (split coefficient
of friction) road surfaces [4].
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III. SENSOR CLASSIFICATION
As shown in Fig. 1, the three major areas of systems application for automotive sensors are powertrain, chassis, and body.
In the present systems-classification scheme, anything that isn’t
powertrain or chassis is included as a body systems application.2
Fig. 1 also identifies the main control functions of each area of
application and the elements of the vehicle that are typically involved. The automotive industry has increasingly utilized sensors in recent years. The penetration of electronic systems and
the associated need for sensors is summarized in Table I.
Powertrain applications for sensors, shown in Table I, can be
thought of as the “1st Wave” of increased use of automotive sensors because they led the first widespread introduction of electronic sensors. Chassis applications for sensors are considered
to be the “2nd Wave” of increased use of sensors, and body applications are called the “3rd Wave.”
Automotive control functions and associated systems for
powertrain, chassis and body areas of application are shown,
respectively, in Figs. 2–4. These diagrams help to classify
the various applications for automotive sensors. Tables II–IV
provide additional detail on the types of sensors used in automotive applications.3 In these Tables, if sensors are universally
used in automotive applications, they are denoted as having a
“major” production status; if the sensors are used in just a few
automotive models, but not universally used, they’re denoted
as having “limited” production status, and some promising
sensors which are getting close to production are denoted as
having “R&D” status.
TableIIshowsthatcertaintypesofsensorspredominateinpowertrain application, namely rotational motion sensors,4 pressure,
and temperature. In North America, these three types of sensors
rank, respectively, number one, two, and four in unit sales volume
[9]. To illustrate the predominance of these sensors, there are a
total of 40 different sensors listed in Table II, of which eight are
pressure sensors, four are temperature sensors, and four are rotational motion sensors. Thus, 16 of 40 of the powertrain sensors in
Table II belong to one of these three types of sensors. New types of
recently introduced powertrain sensors, listed in Table II, include
the cylinder pressure, pedal/accelerator rotary position, and oil
quality sensors.
Table III shows that certain types of sensors also predominate
in chassis applications, namely rotational motion and pressure
(these two types were also predominate in powertrain). But, instead of temperature, inertial acceleration and angular-rate sensors round out the four types of predominant sensors. To illustrate this predominance, there are a total of 27 different sensors listed, of which four are pressure sensors, three are rotational motion sensors, five are acceleration sensors and three
are angular rate sensors. Thus, 15 of 27 of the chassis sensors in
2Body applications include occupants’ safety, security, comfort and
convenience functions. In the present classification, devices such as passive
rf-transponder ID-tags/keys, are categorized as components of communications
system, not sensors; and are therefore not be covered. Similarly, e-connected
telematics devices (wireless cell phones, e-mail, internet connection, etc.) are
likewise not covered.
3In this paper, type of sensor refers to the measurand of the sensor (i.e., the
quantity measured by the sensor).
4Rotational sensors measure shaft rotational motion (i.e., speed), as contrasted
to position sensors below that directly measure angular or linear displacements.
Fig. 1. Major areas of systems application for automotive sensors.
Table III are one of these four types of sensors. Again, new types
of sensors, currently found in chassis systems applications, include the yaw angular rate, steering wheel angular position, and
strut-displacement position sensors.
In total, there are 40 body sensors listed in Table IV. As contrasted to powertrain and chassis, Table IV shows that body sensors are very diverse and no specific types of sensors are dominant. Body sensors range from crash-sensing accelerometers, to
ultrasonic near-obstacle sensors, to infrared thermal imaging, to
millimeter-wave radar, to ambient-air electrochemical gas sensors. Once again, new types of sensors, currently found in body
systems applications, include the ultrasonic-array reversing aid,
lateral lane-departure warning, and infrared-thermal imaging
night-vision sensors.
IV. CURRENT-PRODUCT SENSOR TECHNOLOGIES
Table II through IV list 40, 27, and 40 sensors; respectively,
for powertrain, chassis and body automotive systems applications. This gives a total of 107 sensors (which still isn’t all inclusive). These 107 sensors are thought to be representative of
most of the major applications for sensors used in automobiles.5
Coverage of all details, pertaining to all automotive sensors, is
beyond the scope and size constraints of this paper. Attention
is, therefore, focused on sensors used in automotive production
systems (i.e., sensors used for instrumentation, or less significant applications, are omitted).
The approach used in this review will consist of ranking and describing sensor types, approximately in order, according to sales
volume and revenue. Additionally, a given type of sensor often
5It’s noted that in Table I of Frank’s publication [6], a list of automotive sensor
applications was independently developed and Frank similarly obtained a total
of just over 100 types of automotive sensors.
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IEEE SENSORS JOURNAL, VOL. 1, NO. 4, DECEMBER 2001
TABLE I
DRIVING FACTORS LEADING TO INCREASED USE OF SENSORS (NORTH AMERICAN AUTOMOTIVE MARKET)
Fig. 2. Powertrain systems, control functions and applications (Simplified
diagram).
can be made utilizing any of several different kinds of technologies.6 For example, rotational motion is a type of sensor which is
Fig. 3. Chassis systems, control functions and applications (Simplified
diagram).
6In this paper, different technologies refer to different operating principles.
Discussions of sensor manufacturing technologies and/or design configurations
are not addressed.
made using any one of the following technologies/operating principles: variable reluctance, Hall effect, magnetoresistance, and so
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TABLE II
SENSORS USED IN POWERTRAIN APPLICATIONS
Fig. 4.
Body systems, control functions and applications (Simplified diagram).
on.Becauseautomotiveapplicationsoften arespecifictodifferent
sensor technologies, applications of sensors will therefore be described after all sensor technologies are first covered. References
for additional information on each type of automotive sensor and
for each kind of technology will also be provided.
A. Rotational Motion Sensors
Rotational motion sensors measure shaft rotational motion
(they also detect reference points such as those created by the
absence of one tone-wheel tooth). In North America, rotational
motion sensors have the most unit sales and also the highest
dollar sales (gross sales revenue), which makes them number one
in the present categorization scheme. In 1999, they had slightly
more than 20 percent of the gross sales revenue of all automotive
sensors, with unit sales of 89 million sensors [3], [9].
1) Variable Reluctance: These sensors—also called inductive types—are electromagnetic devices which produce a pulsetrain-like voltage-output signal governed by the time-varying
fluctuations of magnetic flux created by rotating motion of mechanical parts. As gear teeth, slots, or magnetized poles, rotate
with a shaft and pass by a sensor; flux variations are generated
in the sensor’s magnetic circuit (which includes a bias magnet).
Via Faraday’s law, the sensor generates voltage variations in its
sensing coil which correspond to the derivative of magnetic flux
with respect to time. Variable reluctance sensors feature low
cost, small-to-moderate size, self-generated signals, and good
temperature stability. On the other hand, disadvantages include
loss of signal at zero speed, variable signal strength and signal
phase which are dependent on shaft speed (which typically limit
rotational measurement repeatability to about 0.1 degree), and
operation generally limited to sensor air gaps no greater than
about 2 mm. For additional information on this sensor, see [10]
and [11, pages 194–201].
2) Wiegand Effect: Wiegand effect sensors are based on the
interaction of an applied magnetic field with a sensing element
that consists of a magnetic-alloy wire having a radial-gradient
magnetization that varies from the wire’s core to its periphery
[12]. When the strength of the field in the magnetic-circuit of
the sensor exceeds a threshold value, the magnetization state
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TABLE III
SENSORS USED IN CHASSIS APPLICATIONS
in the Wiegand wire element rapidly switches polarity, thereby
self-generating a voltage pulse, detected by a pickup coil.
Wiegand sensors feature: self-generated signal and a high-level
voltage-pulse signal (at low rotation speeds). Disadvantages
include spikelike-signal output and high-volume manufacturability/cost issues.
3) Hall Effect: Hall sensors produce a voltage signal that
corresponds one-to-one with the fluctuations of magnetic
flux created by rotating motion of mechanical parts. As
tone-wheel gear teeth rotate past a Hall sensor (and its integral
bias-magnet); magnetic flux variations are generated similar to
those for the variable reluctance sensor, but instead of detecting
the time-derivative of flux, the Hall sensor detects the flux
level itself. Hall sensors are semiconductor active devices and
therefore require a bias current. The Hall voltage output signal
is linearly proportional to the transverse component of the flux
density passing through the sensing element. In order to (a)
cancel out the common-mode dc voltage component associated
with the average flux level and (b) to double the output signal,
pairs of Hall elements are mounted in a differential mode,
side-by-side, parallel to the direction of tooth travel. For effective differential operation, spacing between sensing elements is
matched to the pitch between tone-wheel teeth.
Hall sensors are made using bipolar semiconductor technology which allows their fabrication directly on the same
chip along with microelectronic signal-processing circuitry.
Functions such as amplification, temperature compensation,
TABLE IV
SENSORS USED IN BODY APPLICATIONS
signal conditioning, etc., can be economically added. Hall
sensors feature low cost, small size, operation to zero speed,
excellent linearity, and rotational measurement repeatability in
the neighborhood of 0.05 . On the other hand, disadvantages
include maximum operating temperature of about 175 C, air
gap operation limited to no greater than about 2.5 mm, and
sensitivity to external pressure acting on the sensor package.
Additional information on this sensor is found in [11, pages
201–204] and [13, pages 73–148].
4) Magnetoresistor: Magnetoresistor devices exhibit a
change of resistance, proportional to magnetic flux density. The
resistance change is based on Lorentz force, where geometric
patterns of narrow, uniformly spaced, conductive shorting
stripes are deposited, perpendicular to current flow direction,
on thin layers of high-carrier-mobility semiconductors (InSb
or InAs). As current flows in the presence of an orthogonal
external magnetic field, Hall-fields and internal shorting by
FLEMING: OVERVIEW OF AUTOMOTIVE SENSORS
the conductive stripes cause the conduction current to follow
more tortuous (more zig-zag), higher-resistance, paths; thereby
creating a resistive output signal. Magnetoresistor sensors are
likewise amenable to fabrication of microelectronic signal-processing integrated circuitry directly on the same chip with the
sensing element. The sensor features operation to zero speed,
rotation-direction sense, excellent rotational-measurement
repeatability in the neighborhood of 0.025 , air gap operation
up to 3 mm and outstanding temperature stability (maximum
operating temperature of 200 C). On the other hand, disadvantages include medium size, medium cost, and the active-device
bias current requirement. Additional information on this sensor
is found in [13] pages 151–171 and [15].
5) AMR
Magnetoresistive: AMR
anisotropic
magnetoresistive sensors generate changes of resistance
as an external magnetic field is rotated with respect to their
magnetized thin film (typically consisting of magnetized
NiFe permalloy). The sensor primarily responds to field
orientation/direction, rather than field strength. Typically,
four AMR sense elements, deposited on a common substrate,
are connected in a Wheatstone signal-detection bridge
arrangement. AMR sensors are also amenable to fabrication
of integrated circuitry directly on the same chip. The sensor
similarly features operation to zero speed, rotation-direction
sense, excellent rotational-measurement repeatability, air gap
operation up to 3 mm, and maximum operating temperature of
200 C. Disadvantages include medium size, medium cost,
and the active-device bias current requirement. Additional
information on this sensor is found in [14].
6) GMR Magnetoresistive: GMR giant magnetoresistive
sensors utilize ferromagnetic/nonmagnetic layered structures
made up of atomically thin films, in the range of 2-to-5-nm
thickness. The GMR effect is quantum mechanical in nature.
The reason GMR sensors are called “giant” is because (at very
low temperatures) they exhibit sensitivities to variations of applied magnetic field which are up to 20 times greater than those
for AMR sensors. At room temperature, the GMR sensitivity
advantage diminishes, but is still three to six times greater than
that for AMR sensors. Although GMR and AMR sensors have
different operating mechanisms, the two sensors function similarly; i.e., both respond primarily to field orientation/direction
rather than to field strength. GMR sensors again are amenable
to fabrication of integrated circuitry directly on the same
chip. The sensor similarly features operation to zero speed,
rotation direction sense, excellent rotational-measurement
repeatability, extended air gap operation up to 3.5 mm, and
a maximum operating temperature of 150 C. Disadvantages
likewise include: medium size, medium cost, the active-device
bias current requirement, and need for tightly controlled limits
on its bias point. Additional information on this sensor is found
in [13, pages 175–196] and [14].
Automotive Applications: Major uses for variable reluctance
sensors include engine crankshaft and camshaft rotational control of spark timing, fuel injection timing and engine speed measurement, and for control of transmission input and output shaft
speeds for electronically controlled gear shifting. Another major
application for variable reluctance sensors is wheel speed, on all
four wheels (for the ABS antilock brake system, traction control
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and vehicle stability). Wiegand effect sensors find application
in aftermarket high-performance ignition systems. More stringent, OBD onboard diagnostic engine misfire detection requirements, newly enacted by California and federal regulators, have
necessitated higher-accuracy crankshaft angular-measurements
to detect the absence of individual cylinder firing torques (i.e.,
misfire) and this has spurred the introduction of the higher-performance magnetoresistor, AMR and GMR types of sensors.
Another important application for higher-performance sensors
which operate to zero speed, is the measurement of wheel rotation in vehicle navigation systems.
B. Pressure Sensors
Pressure sensors have some very diverse automotive applications. They measure pressures ranging from 10 kPa-vacuum (for
OBD evaporative fuel leak detection), to 180 MPa (for diesel
common-rail fuel pressure systems). This is a 18 000:1 variation
in full-scale pressure range measurement requirements! Clearly,
a sensor technology used in the 10-kPa application won’t be robust enough for the 180-MPa fuel-pressure application. Consequently, there exist several different pressure sensor technologies. Pressure sensors have the second greatest unit sales and
the sixth highest gross sales revenue, which makes them number
two in the present categorization scheme.7 In 1999, in North
America, pressure sensors accounted for 9 % of all automotive
sensors sales revenue, with unit sales of 78 million sensors [3],
[9].
1) Piezoresistive Micromachined: Pressure sensing elements are batch fabricated, a thousand or more per wafer, using
a “bond and etchback” process. Silicon diaphragms are micromachined using electrochemical etching and a silicon-to-silicon
bonding process forms a vacuum reference chamber [16]. Over
the past two decades, sensor die sizes have shrunk and wafer
diameters have increased—both factors have helped to lower
the cost of micromachined pressure sensors—see [5] page
1752. Piezoresistive strain-sense elements are implanted in appropriate areas of an etched silicon diaphragm where strains are
most sensitive to applied pressure. The strain-sense elements
are electrically connected into a Wheatstone bridge circuit,
thereby providing a means of detecting pressure acting on the
diaphragm. Modern sensors feature on-chip digital electronics
which provide signal conditioning, programmable calibration
of span and offset, built-in compensation for nonlinearity and
temperature effects, ratiometric output signal, high accuracy
over a wide temperature range and nearly identical part-to-part
interchangeability.
2) Capacitive Touch-Mode Micromachined: In applications
where zero-pressure range measurement is not required and
where low power consumption is an advantage; capacitive
“touch-mode” micromachined pressure sensors are used. In this
case an extended, more flexible, silicon diaphragm is fabricated.
Increasing pressure, acting on the outside surface of the flexible
diaphragm, progressively deflects the diaphragm downwards,
progressively flattening it against a dielectric/insulating layer
7Despite having unit sales nearly as great as rotational motion sensors, pressure sensor sales revenue is less than half that of rotational motion sensors. This
is a tribute to the remarkable cost reductions made possible by the prevalent use
of IC micromachining technology in the manufacture of pressure sensors.
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deposited above a base electrode. This geometric, progressive,
flattening produces a linear increase in capacitance which is
insensitive to the interfering effects of temperature [17].
3) Capacitive Ceramic-Module: In very harsh automotive
applications—such as hydraulic fluids (brake, power steering,
suspension, etc.)—capacitive ceramic-module configurations,
also called capsules, are utilized [18]. This sensor basically
consists of a diaphragm and a much thicker substrate which
has a shallow cavity aligned under the diaphragm. Adjacent
surfaces of the diaphragm and substrate are electroded using a
guard ring geometry (which eliminates the influence of stray
capacitance). The two pieces are bonded together to form
a vacuum reference chamber. Increased hydraulic pressure,
acting on the outside surface of the diaphragm, deflects the
diaphragm closer to the underlying substrate and this produces
an increase in capacitance. To insure EMI noise immunity, a
high-level, binary, pulse-width-modulated output signal is provided by custom IC electronics, integrally built into the sensor
package. [It’s noted that this is possible with any capacitive
sensor].
4) Piezoresistive Polysilicon-on-Steel: When extreme high
pressure is measured—such as diesel-engine common-rail
fuel pressure (up to 180-MPa)—polysilicon-on-steel sensor
configurations are utilized [19]. A stainless-steel cylinder,
has a closed end which is thinned down to create a stiff
diaphragm. Increased hydraulic pressure, acting on the inside
surface of the diaphragm, deflects the diaphragm. Polysilicon
pressure-sensing elements are vapor deposited on the outside
(protected side) of the steel diaphragm. Strain sensing elements
are electrically connected in a Wheatstone bridge circuit,
thereby providing a means of detecting pressure acting on the
diaphragm.
Automotive Applications: Piezoresistive micromachined
sensors are extensively used to measure engine manifold
pressure (absolute and barometric), turbo-boost pressure,
and evaporative fuel leak pressure. Capacitive touch-mode
micromachined sensors are used to measure tire pressure inside
the rotating wheel and engine oil pressure (two applications
where accurate indication of the zero point isn’t required).
Capacitive ceramic-module sensors, are used to measure
brake fluid pressure (for cruise control disengagement and
ABS braking regulation), suspension hydraulic pressure, and
A/C compressor pressure. Piezoresistive polysilicon-on-steel
sensors are used to measure common-rail FI (fuel injection)
pressure, and vehicle suspension dynamic-control hydraulic
pressure.
C. Angular and Linear Position Sensors
Position sensors measure linear displacements ranging from
less than one micron (a typical full-scale sensing-element movement inside a MEMS sensor) to over 200 mm (the stroke/travel
of a strut in an active suspension system). This is a 200 000:1
variation in full-scale displacement range. An example of an angular-position application is the measurement over four complete revolutions, with a 1-degree measurement accuracy requirement, of steering-wheel angular position. Position sensors
have the third greatest unit sales and the third highest gross sales
IEEE SENSORS JOURNAL, VOL. 1, NO. 4, DECEMBER 2001
revenue, which makes them number three in the present categorization scheme. In 1999, in North America, position sensors
accounted for about 18 % of all automotive sensors sales revenue, with unit sales of 48-million sensors [3], [9].
1) Potentiometric: Potentiometric sensors utilize the property that the resistance of an appropriately made film, or screenprinted track, varies linearly with length. The wiper(s) can be either linearly or angularly displaced by the part whose position is
to measured. The use of multiple, redundant, wipers and tracks
provides improved sensor reliability [20], [21].
2) Hall Effect : In an appropriate magnetic circuit, Hall
sensor voltage
varies as
; where
is the angle
between flux density acting on the sensor and bias current
applied to the sensor. Typically, two Hall sensing elements are
mounted in quadrature (geometrically oriented 90 from each
other). The two Hall elements each provide output signals; one
,” and the other as “
2
”.
varying as “
The output signal is derived from the inverse tangent of
,” the ratio of the quadrature element signals.
“
This provides a linear indication of the angular position of
(attached to the shaft), thereby
the magnet creating field
determining the angular position of the shaft [22]. Hall sensors are also used for linear position measurements, where
magnet “head-on” and “slide-by” movements detect linear
position—see [13, pages 99–103].
3) AMR Anisotropic Magnetoresistive: This sensor was
previously described in part 5 of Section A. The sensor exhibits
changes of resistance as an external magnetic field rotates
with respect to its sensing-elements. Two sets of four sensing
elements are typically used, each set is physically mounted (i.e.,
mechanically) offset from each other by a 45 angle. This 45
offset again produces a quadrature 90 electrical phase angle
difference. The two sets of sensing elements are connected in
Wheatstone bridge signal-detection IC circuits. Both bridge
circuits respond to the orientation of the external magnetic field
(not its field strength). In a manner akin to the Hall sensor,
output signals from the two AMR-sensor bridge circuits are
2 ” and
obtained; but in this case, the signals vary as, “
2 .” From these signals, the inverse tangent of their ratio
“
similarly produces a linear measure of the angular position,
“2 ,” of a magnet (attached to a shaft). Here, the electrical
angle goes through two cycles, as angular position of the
shaft/magnet traverses one 360-degree revolution. Further
information on AMR position sensors is found in [23].
4) Optical Encoder: For a steering-wheel angle sensor application, a slotted-aperture optical-encoder sensor is combined
with a gear-reduction-driven potentiometric sensor [24]. The
potentiometric sensor provides a continuous measurement of
steering-wheel angle over a four-turn lock-to-lock turn range,
but with less accuracy than the optical encoder. The encoder,
with two offset bands of 90 aperture slots each, is accurate to
within 1-degree accuracy, but it can’t determine the absolute
position of the steering wheel. Whenever the vehicle starts up,
the sensor’s encoder “learns” the true center (or zero) absolute
position of the steering wheel by starting with the position indicated by the potentiometer and then refining the calibration
based on a period of straight-road driving (as detected by vehicle yaw angular-rate sensors like those described below).
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5) Magnetostrictive Pulse Transit Time: Magnetostrictivepulse transit-time sensors are used to make long, 200-mm,
linear-position measurements. A donut-shaped magnet is
attached to and travels with, a displacement-varying element
of a suspension strut. A fixed metal rod, concentric to the
center axis of a strut, serves as both a magnetostrictive medium
and as an acoustic waveguide. A current pulse is applied
through the entire length of the rod. When the pulse passes
the magnet (attached to the strut), an acoustic pulse is created
in the rod due to the interaction of the magnet’s field with
the applied current in the magnetostrictive rod (i.e., the direct
magnetostrictive effect). An acoustic wave is launched back
up the rod. When the wave reaches the top end of the rod, the
magnetic permeability of the rod material is modulated by
the interaction of the acoustic wave with an applied field of
a bias magnet (i.e., the inverse magnetostrictive effect). This
permeability change creates a voltage pulse in the sense coil
circuit and the measured transit time between initiation of
the current pulse and the detection of the return-wave voltage
pulse, determines the magnet position (i.e., the displacement of
the suspension strut) [25].
Automotive Applications: Because of their mature state
of development and low cost; potentiometric sensors are extensively used to measure fuel-float level, accelerator pedal angle,
and transmission gear position. Due to the harsh environment
of the engine and the high number of lifetime dither cycles,
noncontact Hall sensors are used to measure throttle angle,
EGR valve position, and wheel-to-chassis height (via a 2-bar,
linear-to-rotary displacement linkage). AMR position sensors
are used in the same applications as for potentiometric and
Hall sensors, however, these are sensors of choice when larger
air gaps and/or higher-limit maximum operating temperatures
must be accommodated. Hall sensors are also used in seat belt
buckles for high-reliability detection of proper buckle engagement8 —i.e., proper linear positions of latch and tongue parts
inside the buckle [26]. Because optical sensors can be susceptible to contamination by dirt/oil, they are used in applications
that provide environmentally protected mounting locations.
A good example is the optical-encoder steering-wheel angle
sensor used in vehicle stability enhancement systems, which
is mounted on the steering column, near the IP (instrument
panel). In active suspension systems, the stroke/position of
a strut is accurately measured over an extended-length using
magnetostrictive-pulse transit-time sensors.
D. Temperature Sensors
Temperature sensors have the fourth greatest unit sales and
the seventh highest gross sales revenue, which makes them
number four in the present categorization scheme. In 1999, in
North America, temperature sensors accounted for about 5 %
of all automotive sensors sales revenue, with unit sales of 39
million sensors [3], [9]. Temperature sensor technologies, in
general use today, are listed below.
1) Silicon IC: Use of single-crystal silicon permits on-chip
fabrication of IC (integrated circuit) enhancements. However,
8This signal is an input to a system that controls the rate of inflation of air
bag deployments, based upon whether or not an occupant’s seat belt is buckled
(together with other input signals).
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the use of IC processes also restricts the operation of siliconbased temperature sensors to an upper limit of about 150 C.
Two types of silicon sensors are in general use: (a) spreading
resistance based on bulk charge conduction [13] pages 65–70
and (b) pn-junction voltage difference [27].
2) Thermistor: Ceramic-oxide compositions are manufactured to exhibit NTC or PTC (negative, or positive, temperature coefficient) resistance characteristics, where resistance of
the sensors decrease, or increase, several orders of magnitude
as temperature is increased [28].
3) RTD Resistive Temperature Detector: In RTD high-temperature sensors, a platinum-film sensing element is printed and
then embedded inside an alumina-ceramic layered structure.
The resistance of the platinum element linearly increases as
temperature is increased [29].
Automotive Applications: In the temperature range of 50
to 150 C, silicon sensors are used for measurement and control of air, gases and fluids. Thermistor-type sensors operate in
various ranges between 55 to 1000 C. Thermistors are used
for engine coolant temperature measurement [28] and are also
commonly used as level sensors to monitor coolant, fuel, lubricant, brake and steering fluids (where differences between
the sensor’s self-heating temperatures when immersed and not
immersed, in a fluid provide the output signal). To measure
very high temperature, over 1050 C, as required by OBD regulations for catalyst overheat monitoring; both thermistor-type
sensor and RTD-type sensors are utilized. To satisfy OBD requirements, these sensors must respond to 0-to-1000 C step
changes of temperature within 10 s.
E. Other Sensors
1) Mass Air Flow: MAF mass air flow sensors are fourth
highest in gross sales revenue. On high-performance engines,
sensors based on a thermal heat-loss principle, including a
hot-wire element (plus a companion compensating hot-wire
element), are mounted in a bypass channel of the air intake to
measure mass air flow into an engine [30]. This type of sensor
measures true mass provided there’s no pulsating reversal of
air flow. Under certain operating conditions, pulsating reversal
of air flow does occur; in which case, another configuration
of the thermal flow sensor is used. This type utilizes a heat
source and dual upstream and downstream thermal flow-detection elements (which are fabricated on a micromachined
low-thermal-mass diaphragm) [31].
2) Exhaust Gas: EGO exhaust gas oxygen sensors have
the fifth greatest unit sales and the second highest gross sales
revenue. Their high sales revenue reflects the higher costs
of oxygen sensors which are about three times higher per
sensor than, for example, rotational motion sensors. For use in
closed-loop three-way catalytic-converter emissions control of
engines, three types of exhaust gas oxygen sensors are currently
utilized.
(zirconium-dioxide)
i) Exhaust
gas-heated
ZrO
solid-electrolyte sensors electrochemically self-generate a voltage output signal which exhibits a step-like
transition at the stoichiometric point (which is the
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ii)
iii)
iv)
v)
chemically correct air-to-fuel mixture ratio)–see [11,
pages 208–213] and [32].
Electrically heated titanium-dioxide sensors self-generate a resistive output signal which also makes a step
transition at the stoichiometric ratio [33].
Electrically heated, planar, low-thermal-mass, ZrO sensors feature fast light off–i.e., they become operational
within 5 to 10 s from the ignition-on time of engine
start-up [34], [35].
For use in lean A/F airfuel ratio control of engine emissions, two additional types of exhaust gas sensors are utilized.
Electrically
heated,
low-thermal-mass,
planar,
dual-chamber ZrO sensors utilize oxygen-pump
electrochemical-titration operating principles to measure A/F-ratio over a wide range [34].
Dual-chamber ZrO sensors, similar to the wide-A/F
sensor of (iv), but where the first chamber removes
(pumps out) exhaust oxygen, leaving NOx (oxides of
nitrogen) and the second chamber dissociates NOx into
N and O . In this sensor, the amount of electrochemical
pump current in the second chamber is indicative of the
exhaust-gas NOx concentration [36].
3) Engine Knock: To obtain maximum power, high-performance engines are run at their borderline limit of incipient
knock. This is done using closed-loop control of spark timing
based on knock sensor feedback. Cylinder-head vibrations in
the frequency range of 4-to-8 kHz, excited by engine knock, are
detected using broadband-resonant vibration sensors mounted
on the engine cylinder-head. Vibration/knock sensors consist of
piezoelectric sensing elements in spring-mass sensor packages
[37].
4) Linear Acceleration: Linear-acceleration inertial sensors
have the sixth greatest unit sales and the fifth highest gross sales
revenue. Acceleration sensors are used as inputs for chassis
applications such as: adaptive suspension, vehicle stability
and ABS braking systems; as well as inputs for body-systems
frontal, side and rollover crash-sensing applications. Reference
[38] gives an excellent review of all types of micromachined
inertial sensors, including automotive micromachined-based
MEMS accelerometers. Three main types of automotive
acceleration sensors employ MEMS technology, they are
i) piezoresistive MEMS sensors which incorporate silicon
piezoresistors in suspension beams to detect the acceleration-induced movement of a micromachined proof mass
[38, pages 1641–1642];
ii) capacitive MEMS sensors which incorporate micromachined electrodes to both sense and detect the acceleration-induced movement of a micro-beam (or plate) proof
masses [38, pages 1642–1644];
iii) resonant-beam MEMS sensors which utilize the principle that a vibrating member will shift its resonant frequency proportional to the (inertial) force exerted on the
member [38, page 1644] and [39].
The above MEMS types of acceleration sensors include
features such as: instrumentation-grade performance at traditional-sensor low cost, robustness, electronically selectable
output scales, self testability/diagnostics, on-chip signal conditioning, and multiplex/bus network connectivity. In some
chassis applications, however, due to the harsh operating
environment; traditional types of accelerometers continue in
use today. These are capacitive-type sensors, where acceleration-induced movement of an electromachined thin-metal proof
mass (packaged in a ceramic body) is sensed. The sensors
feature integrally packaged custom IC circuits which provide
binary, high-level, pulse-width-modulated, output signals [40].
5) Angular Rate: Angular-rate9 inertial sensors have the
seventh greatest unit sales. Angular-rate sensors are used
as inputs for chassis suspension (vehicle roll and pitch)
and for vehicle stability (yaw); as well as inputs for body
rollover-crash-sensing (roll) and for vehicle-heading navigation
applications (yaw). Similar to acceleration sensors, automotive
angular-rate sensors also utilize MEMS technologies, 10 and
their operation is based on detection of the effects of Coriolis
forces acting on different types of vibrating mechanisms such
as: rings, tines, disks, or plates.
i) The vibrating-ring type of sensor incorporates a polysilicon suspended ring, where either electrostatic and magnetic fields have both been used to excite vibrations in the
ring. Either by capacitive or electromotive means (both
approaches are presently employed), electrodes detect
the effects of the Coriolis angular-rate force on the nodes
and anti-nodes in the ring’s vibration pattern with respect
to the sensor’s base [38, pages 1651–1652] and [41].
ii) The vibrating-tine sensor consists of a tuning-fork-like
tines, supported by a cantilever-like stem. The tines
are piezoelectrically driven into resonant vibration and
piezoresistive sense elements in the stem detect torsional
strain resulting from Coriolis angular-rate forces [42].
iii) Vibrating-plate and disk, sensors are electrostatically
driven/oscillated by comb electrodes, where Coriolis-force-induced lateral displacements of the plate, or
the tilt of the disk, are capacitively detected [38, pages
1648–1651] and [43], [44].
Another important automotive type of vibrating-tine angular-rate sensor is made using discrete electromechanical
construction [not the MEMS construction of sensor type (ii)
above]. This sensor is made from electro-formed quartz, with
vibrating tines ten times larger than the MEMS type sensor (10
mm tine length versus 1 mm in MEMS). In this sensor, drive
tines are piezoelectrically excited and piezoelectric elements
in a second set of pickup tines detect out-of-plane vibrations
(resulting from the Coriolis force). Although larger than MEMS
sensors, this type of sensor has a large share of the automotive
market because of its ruggedness and high performance [45].
6) Solar, Twilight and Glare Optical Detectors: Two types
of optical detectors are commonly used: (a) solar-heat-detecting
9The designation ’angular-rate’ sensor, rather than “gyro,” is appropriate because automotive sensors of this type employ vibrating mechanisms, rather than
gyroscopic spinning mechanisms, to detect angular rate.
10A major reason why MEMS technology is extensively used for both acceleration and angular-rate inertial sensors is that these sensors can be hermetically
sealed, without exposing the microelectronic circuitry to the outside environment, while still being able to detect their intended measurand (i.e., effects of
inertial).
FLEMING: OVERVIEW OF AUTOMOTIVE SENSORS
photodiodes which respond to near infrared wavelengths and (b)
twilight-detecting photodiodes which respond to visible wavelengths [46]. Solar and twilight sensors are typically mounted
atop the IP in automobiles. Solar sensors provide input signals for automatic temperature control systems, whereas twilight sensors are used to automatically turn on headlights. A
third application of optical detectors utilizes photosensitive microchips that detect visible-light/glare and which are used in automatic-dimming rearview mirrors [47].
7) Moisture/Rain: These sensors are usually mounted
facing the windshield, behind the rearview mirror. Typically,
moisture-detecting sensors emit IR (infrared) light beams
through the windshield. When rain droplets impinge on the
outside of the windshield, a higher refractive-index rain/liquid
layer is created. Depending on design (i.e., the angle of IR beam
incidence on the glass), the presence of rain on the windshield
makes IR light either refract away more, or reflect back more
[48]. These sensors provide feedback signals for automatic
windshield wiper control.
8) Fuel Level: Although other technologies have been
developed—e.g., optical, ultrasonic and capacitive—the potentiometer float-arm technology for fuel-level measurement
prevails because of its low cost, high reliability and durability
[49]. Thick-film resistive tracks are generally used in the
potentiometer. The float is designed to traverse a path near
the tank’s center for all fuel levels. An appropriate functional
relationship between sensor angle and fuel quantity for the
particular tank shape used in each vehicle is used. A running
average of fuel sensor output signals is utilized to compensate
for fuel slosh created due to vehicle motion.
9) Near-Distance Obstacle Detection: Several technologies
exist–namely ultrasound, microwave radar, rf capacitance and
infrared multi beams; all are primarily used in reversing-aid
systems (“blind spot” monitoring systems have not yet reached
production status). The ultrasound technology is used in
widespread production because it offers wide-area, near-distance beam coverage and is low cost [50]. On the other hand,
wide-beamwidth microwave radar, although more costly, offers
advantages of greater range, better accuracy and ability to
operate in inclement weather. Ultrasound obstacle detection
is currently in production in reversing-aid systems on (obscured-rear-vision) minivan and SUV types of vehicles. Radar
types of obstacle reversing-aid detection are in production
on certain commercial vehicles, partly due to legislation in
some U.S. states that requires this feature for trucks. Hybrid
systems, which combine ultrasound (for wide-area close-proximity obstacle detection) with wide-beamwidth radar (for
extended-range, better accuracy, all-weather detection), are
expected to appear soon in production [51].
10) Far-Distance Obstacle Detection: Four main technologies are used–namely millimeter-wave radar, laser radar, IR
thermal imaging, and machine vision. Millimeter-wave and
laser radar are used primarily in vehicle ACC adaptive cruise
control systems (which control both speed and vehicle-to-vehicle spacing, rather than speed alone).
i) Millimeter-wave radar operates at specified government-regulated frequencies ranging between 24.7 GHz
to 77 GHz and it features the ability to accurately
305
operate in inclement weather. Some radars use FM/CW
frequency-modulated/continuous-wave signals which
allow measurement of range and range rate11 of as many
as 20 targets (including vehicles, roadway obstacles,
etc.). Other automotive radars use pulsed-dopplar and/or
monopulse operating principles. Most radars utilize
some form of beam scanning, with 10-Hz repetition
(update) sweep rates, to resolve, for example, whether
targets are in the same roadway lane, an adjacent lane,
or an oncoming lane. Some radars utilize all-electronic
(no moving parts) beam scanning [52], [53]. Other
radars utilize mechanically driven beam scanning [54].
Heavy-truck radars often use nonscanning, single-lane
coverage, fixed beams [55].
ii) Laser radar, or lidar (acronym derived from: light
radar), emits narrow, pulsed, IR beams at wavelengths
in the vicinity of 850 nm. Short-duration 25-ns pulses
are emitted sequentially over wide range of beam-scan
(both horizontal and vertical) directions. Transit times
of individual pulses determine distances to reflecting
targets. Beam scanning on automotive lidars is generally accomplished using electromechanically driven
mirror-scan mechanisms [56], [57]. Laser radar performance is diminished by inclement weather and/or dirty
lenses (actually, this limitation is promoted as a safety
benefit because it limits the use of ACC in poor weather
when driver visibility is also limited). On the other
hand, laser radar features high accuracy, wide angular
coverage and precise target location.12 13
iii) Passive IR, nonradiating, thermal imaging, night vision is available on production automobiles [61].
Development of two-dimensional, micromachined, IR
bolometric focal-plane arrays [62] was the key technical breakthrough most responsible for night vision
becoming low cost enough for automotive application.
For automotive night vision, a typical focal-plane array
consists of 240 lines, each 320 pixels wide, giving a
total of 76 800 pixel image elements. When infrared
thermal energy (from pedestrians, deer, other cars, etc.)
is incident on the array, each pixels alters its capacitance, which is electronically monitored and input to
a heads-up, real-time, small video display of a virtual
image viewed by the driver above the hood of the vehicle
[61].
iv) Machine vision is used to monitor a vehicle’s position
relative to roadway lane markings. When, for example, a
truck begins to stray outside its lane (possibly indicating
a drowsy driver problem), an audible lane-departure
“rumble strip” sound is sent to the speaker on whichever
side of the roadway the truck is departing [63]. The
11Range is derived from the transit time of the FM/CW return signal and range
rate is derived from the doppler frequency shift of the return signal.
12To take advantage of the respective advantages of millimeter-wave radar
and laser radar, these systems have been combined into one hybrid system,
which features premium performance derived from the best of both radar systems [58].
13To suppress mutual interference either among radar or laser beams, radiated
by multiple vehicles on the highway; spread-spectrum phase-modulated signalcoding techniques are used [59], [60].
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IEEE SENSORS JOURNAL, VOL. 1, NO. 4, DECEMBER 2001
vision sensor consists of a) a digital camera, with
typically 100 000 pixels and 120-dB dynamic range,
mounted on the windshield inside the truck cab; and b)
advanced image-recognition software that incorporate
lane-recognition/vehicle trajectory algorithms.
11) Additional Production Sensors: Additional automotive
sensors are in volume production, but are not covered in the
above paper. These sensors include
i) short-circuit-ring position sensor used in electronically
controlled diesel injection pumps;
ii) finger-type angular-position and angular-speed sensor;
iii) oil level/quality sensor using heated wires to detect
change in heat conductivity due to oil aging.
D. Multi-Axis Micromachined Inertial Sensors
1) Two-Axis Accelerometer/Tilt: Single-chip micromachined two-axis ( - , lateral-longitudinal, vehicle axes)
dual-function, sensors are used for vehicle-security systems
(e.g., towaway tilt detection) [81], [82].
2) Combined Angular-Rate/Acceleration: Micromachined
combined-function sensors are fabricated on the same substrate,
providing in one package dual independent measurements of
lateral vehicle acceleration and yaw angular rate for use in
chassis systems for input to vehicle stability systems and for
body systems for rollover-crash-sensing [83]. This sensor is in
major production in Europe.
VI. SUMMARY
V. EMERGING SENSOR TECHNOLOGIES
Emerging, state-of the-art, sensor technologies are defined
here as those which are currently in the R&D stage of development, or those which have been newly introduced and which
are expected to have a significant impact on automotive systems
development.
A. Engine Combustion Sensors
1) Spark Plug Ion-Current (Using Either dc or ac Applied
gap Voltage): Detects misfire and detonation/knock; and also
indicates in-cylinder peak pressure and air-fuel ratio [64], [65].
2) Fiber-Optic Diaphragm-Reflection: Detects in-cylinder
pressure waveform [66], [67].
3) Piezoresistive Silicon-Carbide-On-Insulator: Detects
in-cylinder pressure waveform [68].
B. Oil Quality/Deterioration Sensing
1) Stress-Based Predictive Method: Cumulative stress on
oil is calculated from combined effects of engine revolutions
and oil temperature [69].
2) Multisensor: Detects oil dielectric constant and oil level
(capacitively), plus oil temperature [70].
C. Engine/Transmission/Steering Torque Sensors
1) Twist-Angle Torsion-Bar: Twist angle due to applied
torque is detected potentiometrically (using sliding contacts
[71], [72]); and also via the following noncontact methods
i) optically, using variable apertures [73];
ii) optically, using displaceable bar codes [74];
iii) magnetically, using displaceable air gaps [75];
iv) electrically, using eddy current with variable shaded
poles [76].
2) Non-Torsion-Shaft Magnetoelastic Detection: In one
sensor, ac-excitation is used to detect changes in shaft permeability [77]. In the other type, permanently magnetized shafts
or sleeves self-generate a dc-magnetic flux signal [78], [79].
Both sensors operate without contacting the rotating shaft.
3) Engine-Crankshaft Speed Variation Due to Cylinder-Firings: Math algorithms are used to derive the engine torque from
measurements of engine-flywheel speed modulation due to individual cylinder firings—high-resolution rotational motion sensors are utilized [80].
A comprehensive review of current-production and emerging
state-of the-art automotive sensor technologies is made. This
paper covers nearly 50 different types of automotive sensors—all of which currently find widespread application, or
are expected to have a significant future impact on automotive
systems development. For automotive powertrain applications,
the predominant sensors in use today are rotational motion,
pressure and temperature. For chassis applications, predominant sensors include inertial acceleration and inertial angular
rate sensors. As opposed to powertrain and chassis, body
systems applications sensors are more diverse and no single
sensor types dominate. Ten types of emerging, state-of the-art,
sensors technologies are also identified.
ACKNOWLEDGMENT
The author would like to thank TRW for use of their resources
and facilities.
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William J. Fleming William Fleming received the
M.S.E. degree in electrical engineering and the Ph.D.
degree from the University of Michigan in 1966 and
1971, respectively.
He currently is a Senior Staff Technologist at
TRW Occupant Safety Systems in Washington, MI,
where he is developing new safety-restraint products
and doing related studies involving risk-analysis and
new technology. Before this, for five years, at TRW
Automotive Electronics Group, he developed new
types of sensors and actuators for use in automotive
control systems. Prior to TRW, for eleven years, he did sensor development for
automotive engine-control systems at General Motors Research Laboratories.
He developed a 3-day, Society of Automotive Engineers seminar on Sensor and
Actuator Technology and more than 800 persons from the automotive industry
have attended the seminar. For 27 years and continuing through present,
he serves as Automotive Electronics’ Senior Editor for the IEEE Vehicular
Technology Society Newsletter. He has published more than 30 papers and
holds three U.S. Patents.
Dr. Fleming was the recipient of the Vincent Bendix Award for best SAE
paper on automotive electronics. He also received the Avant Garde Award from
the IEEE Vehicular Technology Society.
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