termpaper pressure sensor electrical
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LOVELY PROFESSI ONAL UNIVERSITY
PHAGWARA(PB.)
SUB. :- ELECTRICAL SCIENCE (ELE-101)
TOPIC :- PRESSURE SENSORS
Submitted to, Submitted by,
MR HARISH SIR, Omkar Kumar Jha
RH-4901-A12
10902923
MECHANICAL ENGG. (III rd TERM
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INDEX
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INTRODUCTION
A pressure sensor measures pressure, typically ofgases or liquids. Pressure
is an expression of the force required to stop a fluid from expanding, and is
usually stated in terms of force per unit area. A pressure sensor usually acts
as a transducer; it generates a signal as a function of the pressure imposed.
For the purposes of this article, such a signal is electrical.
Pressure sensors are used for control and monitoring in thousands of
everyday applications. Pressure sensors can also be used to indirectly
measure other variables such as fluid/gas flow, speed, water level, and
altitude. Pressure sensors can alternatively be called pressure transducers,
pressure transmitters, pressure senders, pressure indicators and piezometers,
manometers, among other names.
Pressure sensors can vary drastically in technology, design, performance,
application suitability and cost. A conservative estimate would be that there
may be over 50 technologies and at least 300 companies making pressure
sensors worldwide.
There is also a category of pressure sensors that are designed to measure in a
dynamic mode for capturing very high speed changes in pressure. Example
applications for this type of sensor would be in the measuring of combustion
pressure in an engine cylinder or in a gas turbine. These sensors are
commonly manufactured out ofpiezoelectric materials such as quartz.
Some pressure sensors, such as those found in some traffic enforcement
cameras, function in a binary (on/off) manner, i.e., when pressure is applied
to a pressure sensor, the sensor acts to complete or break an electrical circuit.
These types of sensors are also known as a pressure switch.
http://en.wikipedia.org/wiki/Pressurehttp://en.wikipedia.org/wiki/Gashttp://en.wikipedia.org/wiki/Liquidshttp://en.wikipedia.org/wiki/Transducerhttp://en.wikipedia.org/wiki/Function_%28mathematics%29http://en.wikipedia.org/wiki/Piezoelectrichttp://en.wikipedia.org/wiki/Traffic_enforcement_camerahttp://en.wikipedia.org/wiki/Traffic_enforcement_camerahttp://en.wikipedia.org/wiki/Pressure_switchhttp://en.wikipedia.org/wiki/Pressure_switchhttp://en.wikipedia.org/wiki/Traffic_enforcement_camerahttp://en.wikipedia.org/wiki/Traffic_enforcement_camerahttp://en.wikipedia.org/wiki/Traffic_enforcement_camerahttp://en.wikipedia.org/wiki/Piezoelectrichttp://en.wikipedia.org/wiki/Function_%28mathematics%29http://en.wikipedia.org/wiki/Transducerhttp://en.wikipedia.org/wiki/Liquidshttp://en.wikipedia.org/wiki/Gashttp://en.wikipedia.org/wiki/Pressure -
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TYPES
Pressure sensors can be classified in term of pressure ranges they measure,
temperature ranges of operation, and most importantly the type of pressure
they measure. In terms of pressure type, pressure sensors can be divided intofive categories:
Absolute pressure sensorThis sensor measures the pressure relative to perfect vacuum pressure (0 PSI
or no pressure). Atmospheric pressure is 101.325 kPa (14.7 PSI) at sea level
with reference to vacuum.
Gauge pressure sensorThis sensor is used in different applications because it can be calibrated to
measure the pressure relative to a given atmospheric pressure at a given
location. A tire pressure gauge is an example of gauge pressure indication.
When the tire pressure gauge reads 0 PSI, there is really 14.7 PSI
(atmospheric pressure) in the tire.
Vacuum pressure sensorThis sensor is used to measure pressure less than the atmospheric pressure ata given location. This has the potential to cause some confusion as industry
may refer to a vacuum sensor as one which is referenced to either
atmospheric pressure (ie measure Negative gauge pressure) or relative to
absolute vacuum.
Differential pressure sensorThis sensor measures the difference between two or more pressures
introduced as inputs to the sensing unit, for example, measuring the pressuredrop across an oil filter. Differential pressure is also used to measure flow or
level in pressurized vessels.
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Sealed pressure sensorThis sensor is the same as the gauge pressure sensor except that it is
previously calibrated by manufacturers to measure pressure relative to sea
level pressure (14.7 PSI).
TECHNOLOGY
There are two basic categories of analog pressure sensors.
Force Collector Types These types of electronic pressure sensors generally
use a force collector (such a diaphragm, piston, bourdon tube, or bellows) to
measure strain (or deflection) due to applied force (pressure) over an area.
Piezoresistive Strain GageUses the piezoresistive effect of bonded or formed strain gages to
detect strain due to applied pressure. Common technology types are
Silicon (Monocrystalline), Polysilicon Thin Film, Bonded Metal Foil,
Thick Film, and Sputtered Thin Film. Generally, the strain gages are
connected to form a Wheatstone bridge circuit to maximize the output
of the sensor. This is the most commonly employed sensing
technology for general purpose pressure measurement. Generally,
these technologies are suited to measure absolute, gauge, vacuum, anddifferential pressures.
CapacitiveUses a diaghragm and pressure cavity to create a variable capacitor to
detect strain due to applied pressure. Common technologies use metal,
ceramic, and silicon diaphragms. Generally, these technologies are
most applied to low pressures (Absolute, Differential and Gauge)
ElectromagneticMeasures the displacement of a diaphragm by means of changes in
inductance (reluctance), LVDT, Hall Effect, or by eddy current
principal.
http://en.wikipedia.org/wiki/Piezoresistivehttp://en.wikipedia.org/wiki/Strain_gagehttp://en.wikipedia.org/wiki/Wheatstone_bridgehttp://en.wikipedia.org/wiki/Capacitorhttp://en.wikipedia.org/wiki/Inductancehttp://en.wikipedia.org/wiki/LVDThttp://en.wikipedia.org/wiki/Hall_Effecthttp://en.wikipedia.org/wiki/Eddy_currenthttp://en.wikipedia.org/wiki/Eddy_currenthttp://en.wikipedia.org/wiki/Hall_Effecthttp://en.wikipedia.org/wiki/LVDThttp://en.wikipedia.org/wiki/Inductancehttp://en.wikipedia.org/wiki/Capacitorhttp://en.wikipedia.org/wiki/Wheatstone_bridgehttp://en.wikipedia.org/wiki/Strain_gagehttp://en.wikipedia.org/wiki/Piezoresistive -
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PiezoelectricUses the piezoelectric effect in certain materials such as quartz to
measure the strain upon the sensing mechanism due to pressure. This
technology is commonly employed for the measurement of highlydynamic pressures.
OpticalUses the physical change of an optical fiber to detect strain due
applied pressure. A common example of this type utilizes Fiber Bragg
Gratings. This technology is employed in challenging applications
where the measurement may be highly remote, under high
temperature, or may benefit from the technologies inherent immunity
to eletromagnetic interference.
PotentiometricUses the motion of a wiper along a resistive mechanism to detect the
strain caused by applied pressure.
ResonantUses the changes in resonant frequency in a sensing mechanism tomeasure stress, or changes in gas density, caused by applied pressure.
This technology may be used in conjunction with a force collector,
such as those in the category above. Alternatively, resonant
technology may be employed by expose the resonating element itself
to the media, whereby the resonant frequency is dependent upon the
density of the media. Sensors have been made out of vibrating wire,
vibrating cylinders, quartz, and silicon MEMS. Generally, this
technology is considered to provide very stable readings over time.
ThermalUses the changes in thermal conductivity of a gas due to density
changes to measure pressure. A common example of this type is the
Pirani gage.
http://en.wikipedia.org/wiki/Piezoelectrichttp://en.wikipedia.org/wiki/Fiber_Bragg_Gratinghttp://en.wikipedia.org/wiki/Fiber_Bragg_Gratinghttp://en.wikipedia.org/wiki/Resonant_frequencyhttp://en.wikipedia.org/wiki/Thermal_conductivityhttp://en.wikipedia.org/wiki/Thermal_conductivityhttp://en.wikipedia.org/wiki/Resonant_frequencyhttp://en.wikipedia.org/wiki/Fiber_Bragg_Gratinghttp://en.wikipedia.org/wiki/Fiber_Bragg_Gratinghttp://en.wikipedia.org/wiki/Fiber_Bragg_Gratinghttp://en.wikipedia.org/wiki/Piezoelectric -
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IonizationMeasures the flow of charged gas particles (ions) which varies due to
density changes to measure pressure. Common examples are the Hot
and Cold Cathode gages.
OthersThere are numerous other ways to derive pressure from it's density
(speed of sound, mass, index of refraction) among others.
APPLICATION
Pressure sensingThis is the direct use of pressure sensors to measure pressure. This is useful
in weather instrumentation, aircraft, cars, and any other machinery that has
pressure functionality implemented.
Altitude sensingThis is useful in aircraft, rockets, satellites, weather balloons, and many
other applications. All these applications make use of the relationshipbetween changes in pressure relative to the altitude. This relationship is
governed by the following equation:
00198122.0
15.2881
19026.0
refp
p
h
This equation is calibrated for an altimeter, up to 36,090 feet (11,000 m).
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Outside that range, an error will be introduced which can be calculated
differently for each different pressure sensor. These error calculations will
factor in the error introduced by the change in temperature as we go up.
Barometric pressure sensors can have an altitude resolution of less than 1
meter, which is significantly better than GPS systems (about 20 meters
altitude resolution). In navigation applications altimeters are used to
distinguish between stacked road levels for car navigation and floor levels in
buildings for pedestrian navigation.
Flow sensingThis is the use of pressure sensors in conjunction with the venturi effect to
measure flow. Differential pressure is measured between two segments of a
venturi tube that have a different aperture. The pressure difference betweenthe two segments is directly proportional to the flow rate through the venturi
tube. A low pressure sensor is almost always required as the pressure
difference is relatively small.
Level / Depth sensingA pressure sensor may also be used to calculate the level of a fluid. This
technique is commonly employed to measure the depth of a submerged body
(such as a diver or submarine), or level of contents in a tank (such as in a
water tower). For most practical purposes, fluid level is directly proportional
to pressure. In the case of fresh water where the contents are under
atmospheric pressure, 1psi = 27.7 inH20 / 1Pa = 9.81 mmH20. The basic
equation for such a measurement is,
hgp
Where P = Pressure, p = Density of the Fluid, g = Standard Gravity, h =
Height of fluid column above pressure sensor.
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Leak TestingA pressure sensor may be used to sense the decay of pressure due to a
system leak. This is commonly done by either comparison to a known leak
using differential pressure, or by means of utilizing the pressure sensor tomeasure pressure change over time.
WORKING
With the steam age came the demand for pressure measuring instruments.
Bourdon tubes or bellows, where mechanical displacements were transferred
to an indicating pointer were the first pressure instruments, and are still in
use today. Pressure metrology is the technology of transducing pressure into
an electrical quantity. Normally, a diaphragm construction is used with
strain gauges either bonded to , or diffused into it, acting as resistive
elements. Under the pressure-induced strain, the resistive values change.
In capacitive technology, the pressure diaphragm is one plate of a capacitor
that changes its value under pressure-induced displacement.
Pressure sensing using diaphragm technology measures the difference in
pressure of the two sides of the diaphragm. Depending upon the relevant
pressure, we use the terms ABSOLUTE, where the reference is vacuum (1st
picture), GAUGE, where the reference is atmospheric pressure (2nd picture),
or DIFFERENTIAL, where the sensor has two ports for the measure of two
different pressure.
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This type of pressure sensor consists of a micro-machined silicon diaphragm
with piezo-resistive strain gauges diffused into it, fused to a silicon or glass
back plate. The resistors have a value of approx. 3.5 k Ohm. Pressure
induced strain increases the value of the radial resistors (r), and decreases the
value of the resistors (t) transverse to the radius. This resistance change can
be high as 30%. The resistors are connected as a Wheatstone Bridge, the
output of which is directly proportional to the pressure.
Whetstone Bridge Circuit
1). Gold or aluminum wires are welded to the aluminum contacts on the chip
and to the glass feed-through, pins of the header.
2). TAB (Tape Automated Bonding). The contacts on the chip have a gold
dot.
A pretinned felxible printed circuit is directly soldered to these gold dots and
the other end to a PC-board, or the header. In the first method, the sensor
must be fixed on the header. The TAB printed circuit, however, holds the
sensor in place itself..
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Low cost sensors are devices where they are exposed to the media without
protection. The glass feed-through and the silicon cell is mounted in a plastic
housing with pressure ports for positive and negative pressure. (1st picture)
The silicon sensor with the TAB print is fixed between two plastic moldings
with pressure ports. (2nd picture)
The silicon sensor is bonded to a brass pressure port. The contacts are made
either by gold wires to soldering pins, or by TAB flexible printed circuit.
(3rd picture ).
The silicon sensor on the glass feed-through is mounted in stainless steel
housing, isolated by a thin stainless steel diaphragm and filled with silicone
oil. The pressure acts on the diaphragm and is transfered through the oil ontothe sensor. These transducers are fully tested for temperature and linearity
and the compensation resistor values given on the individual test sheets.
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Nickel diaphragms in brass housings brazed under high temperature or
brazed steel diaphragms in steel housings allow the fabrication of isolated
pressure sensors with low production costs, without substantially limiting
the area of application..
TERMINOLOGY
Pressure is sensed by mechanical elements such as plates, shells, and
tubes that are designed and constructed to deflect when pressure is applied.
This is the basic mechanism converting pressure to physical movement.
Next,this movement must be transduced to obtain an electrical or other output.
Finally, signal conditioning may be needed, depending on the type of sensor
and the application. Figure 8 illustrates the three functional
blocks.
Sensing Elements
The main types of sensing elements are Bourdon tubes, diaphragms,capsules,
and bellows. The Bourdon tube is a sealed tube that
deflects in response to applied pressure. All except diaphragms provide
a fairly large displacement that is useful in mechanical gauges and for
electrical sensors that require a significant movement.
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Mechanical Pressure Gauges.
In mechanical gauges, the motion created
by the sensing element is read directly by a dial or pointer. These devices
are typically seen in low-performance applications, including blood pressure
measurement and automotive pressure gauges. The mechanical approach
used
to couple the sensing element to the readout can introduce repeatability
errors, which will be discussed later. The mechanical mass of the gauges
also limits the frequency response and makes these sensors suitable only
for slowly changing measurements.
Electro-mechanical Pressure Sensors.
Electromechanical pressure sensors convert the applied pressure to an
electrical signal. A wide variety of materials and technologies has been used
in these devices, resulting
in performance vs. cost tradeoffs and suitability for applications. The
electrical output signal also provides a variety of choices for various
applications.
Sensor Effect.
A pressure sensor may be modeled as:
VOUT = kO + k1P
Where:
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kO = offset
k1= pressure sensitivity in V/pressure unit.
A sensor will typically exhibit temperature coefficients of offset
(also called null shift) and sensitivity. Linearity refers to deviations from the
ideal straight line described). One way to measure linearity is to use the least
squares
method, which gives a best fit straight line .
Repeatability refers to the sensor's ability to produce the same output with
consecutive applications of the same pressure.
Hysteresis refers to the ability of the sensor to give the same output when the
same increasing and then decreasing pressures are applied consecutively.
Temperature hysteresis refers to the ability of the sensor to give the same
output at a given temperature before and after a temperature cycle.
Repeatability and hysteresis effects are not easily compensated and are
indicators of the basic stability of the device.
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Gauge factor is a measure of the sensitivity of a sensor. It is defined as the
ratio of the change in an electrical transduction parameter over the full range
of pressure to the value of that parameter at zero pressure.
Pressure Sensor Technologies
Potentiometric Pressure Sensors.
Potentiometric pressure sensors use a Bourdon tube, capsule, or bellows to
drive a wiper arm on a resistiveelement. For reliable operation the wiper
must bear on the element with
some force, which leads to repeatability and hysteresis errors. These devices
are very low cost, however, and are used in low-performance applications
such as dashboard oil pressure gauges.
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Inductive Pressure Sensors.
Several configurations based on varying
inductance or inductive coupling are used in pressure sensors. They allrequire AC excitation of the coil(s) and, if a DC output is desired,
subsequent
demodulation and filtering. The linear variable differential transformer.
(LVDT) types have a fairly low frequency response due to the necessity of
driving the moving core of the differential transformer (see Figure 16).The LVDT uses the moving core to vary the inductive coupling between the
transformer primary and secondary.
Capacitive Pressure Sensors.
Capacitive pressure sensors typically use a thin diaphragm as one plate of a
capacitor. Applied pressure causes the diaphragm to deflect and the
capacitance to change. This change may or may not be linear and is typically
on the order of several picofarads
out of a total capacitance of 50-100 pF. The change in capacitance may beused to control the frequency of an oscillator or to vary the coupling of an
AC signal through a network. The electronics for signal conditioning should
be located close to the sensing element to prevent errors due to stray
capacitance.
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if the dielectric constant of the material between the plates isn't kept
constant, errors may result. Capacitive absolute pressure sensors with a
vacuum between the plates are ideal in this respect. Because the capacitance
of this sensor depends only on physical parameters, sensors with good
performance can be constructed using materials with low coefficients of
thermal expansion. Since the device has to be fairly large to obtain a usable
signal, frequency response may be a problem in some applications. Also,
low-pressure capacitive sensors exhibit acceleration and vibration sensitivity
due to the necessity
for a large, thin diaphragm. A basic capacitive sensor is shown and a more
complex differential pressure capsule is shown inFigure 18.
Piezoelectric Pressure Sensors.
Piezoelectric elements are bi-directional transducers capable ofconverting stress into an electric potential and vice versa. They consist ofmetallized quartz or ceramic materials. One important factor to remember is
that this is a dynamic effect, providing an output only when the input is
changing. This means that these sensors can be used only for varying
pressures. The piezoelectric element has a
high-impedance output and care must be taken to avoid loading the output
by the interface electronics. Some piezoelectric pressure sensors include
an internal amplifier to provide an easy electrical interface.
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Strain Gauge Pressure Sensors.
Strain gauge sensors originally used a metal diaphragm with strain gauges
bonded to it. A strain gauge measures the strain in a material subjected to
applied stress.
Automotive.
A wide variety of pressure applications exist in the modern electronically
controlled auto. Among the most important are:
Manifold absolute pressure (MAP). Many engine control systems use the
speed-density approach to intake air mass flow rate measurement. The mass
flow rate must be known so that the optimum amount of fuel can be injected.
MAP is used in conjunction with intake air temperature to compute the air
density. This requires a 15 psia range or higher (for supercharged or
turbocharged engines). It is also desirable to include an altitude correction in
the control system, and this requires measurement of barometric absolute
pressure (BAP). Some systems use a separate sensor, but it is more common
for the MAP sensor to do double duty since it reads atmospheric pressure fortwo conditions. One, before the engine begins cranking and two, whenever
the throttle is wide open.
Engine oil pressure. Engine lubrication requires pressures of 10-15 psig.
The oil pump is sized to achieve this pressure at idle and the pressure
increases with engine speed. A potentiometric gauge or pressure switch is
used for this function since precision isn't required.
Evaporative purge system leak detection. To reduce emissions, modern fuelsystems are not vented to the atmosphere. This means that fumes resulting
from temperature-induced pressure changes in the fuel tank are captured in a
carbon canister and later recycled through the engine. Government
regulations require that leaks in this system be detected by the onboard
diagnostics system. One approach is to pressurize the system and measure
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pressure decay over a fixed time interval. A 1 psig sensor is used for this
function.
Tire pressure. Recent development of the "run-flat"
tire has prompted development of a remote tire pressure measurementsystem. The reason is that a flat tire of this type is difficult to detect visually
and the distance over which it can be used without any pressure is limited.
PRESENT AND FUTURE
Pressure Sensor Opportunities (in automobile).
Future development of the active and passive safety systems will enablethe vehicle to sense and interpret its surrounding environment,
recognize potentially dangerous situations and provide a pre-determined
level of support to the driver. Blind-spot detection, lane change
monitoring and roundabout shunt prevention are examples of systems that
could be enabled through the deployment of vision-based sensors and
multiple sensor data fusion techniques. Parking assistance systems are
already close to a production reality. Ultrasonic sensors mounted
On the sides of vehicles will measure the length and depth of a parking
space as the vehicle drives passes it. The system then calculates themaneuvers that are required to steer the vehicle safely into the space. The
driver is then provided with sound or visual instructions to complete the
task. A future enhancement would involve the use of electronically
controlled steering system to execute the maneuver automatically.
Sensor development engineers will need to work with vehicle stylists to
package and position sensors in the best location that allows sensor
functionality without obtrusive body features. Advanced electrical
architectures will also focus upon standardization and sharing of sensor
Outputs across a range of vehicle systems. The addition of sensor self-
diagnostic functions has been realized by many suppliers. The path to X-
by-Wire, that is the replacement of mechanical systems by electronically
Controlled electrically actuated systems (for example, by removal of the
steering column or hydraulic braking pipes), is a challenging one and will
only be successfully realized through electronics integration and the
continued innovation of new sensing technologies. Ultimately, the vehicle
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will become a cocoon of both inward facing and outward facing
electronic monitoring systems that will improve the safety and
survivability of its driver and passengers.
THIS CHART SHOWS THE ACTUAL ACCESSMENT OF THE PRESSURE SENSORS IN
THE AUTOMOBILE APPLICATION.The automobile equipment which usually operated by
sensors is:-
Engine Sensors Inlet Manifold Pressure Camshaft Position Air Temperature Fuel Temperature Fuel Pressure Knock Coolant Temperature EGR Valve Position
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Air Mass Flow Oxygen
Hybrid & Fuel Cell Vehicles Hydrogen Leak Detection Current Monitoring Pressure Flow Temperature Motor Speed & Position
Vehicle Control Hydraulic Pressure Lat/Long Acceleration
Wheel Speed Yaw Rate Ride Height Position Steering Torque & Position
Crash Avoidance Adaptive Cruise Control Radar Parking Sensors Lane Departure Warning Driver Monitoring
Passenger Comfort HVAC Temperature Humidity Air Vent Positions Seat Position Window Position
Passenger Convenience Remote Keyless Entry
Automatic Rain Wipers Automatic Headlamps Washer Fluid Level Headlamp Leveling
Safety & Security Theft Prevention
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Occupant Airbag Systems Seat Belt Pre-tensioner Tire Pressure Monitoring Occupant Detection Pedestrian Detection
SummaryTodays vehicles are pervaded with a diverse range of sensors providing
critical data for performance, safety, comfort and convenience functions.
The measurement of inlet manifold absolute pressure in early ignition and
fuelling control systems was one of the first and most successful
automotive applications of sensors, and continues to this day to be an
important parameter. Many other sensors including crankshaft position,
knock, air mass flow, exhaust gas and temperature sensors have beensubsequently used to enhance power train performance. The trend towards
ever increasing use of electronically controlled electrically actuated
systems on vehicles (for example, electrically powered steering, semi-
active ride control, slip control systems and adaptive cruise control) has
created new challenges and opportunities for sensor developers.
Traditional sensors have been complemented by the addition of new
sensors for new applications, for example, long range radar, optical
steering torque sensors, tire pressure monitoring systems and yaw rate
sensors. Sensor cost continues to be a significant factor in the selectioncriteria of automotive system designers, recognizing the reward of large
production volumes if successful. In addition, sensor suppliers must also
deliver the robustness and quality targets demanded of this automotive
market. Figure 1 provides some examples of sensors used on todays
vehicles. The past few decades have witnessed the development and
deployment of a range of sensing technologies that have both supported
and enabled the introduction of advanced electronic systems. The
environment for these sensors continues to be increasingly challenging
with respect to robustness, reliability, quality and cost. Sensor suppliersface the continuing mandate to deliver more capability at less cost.
Existing sensors will continue to find new applications, building upon
Their historical track record of performance and new sensors will emerge
to improve system functionality and enable future advanced systems.
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