termpaper pressure sensor electrical

7/30/2019 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).
http://en.wikipedia.org/wiki/Altimeterhttp://en.wikipedia.org/wiki/Altimeter


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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.
http://en.wikipedia.org/wiki/Venturi_effecthttp://en.wikipedia.org/wiki/Venturi_effect


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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.

termpaper pressure sensor electrical

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