THIYAGU. M AUTOMOTIVE SENSORS

Automotive Sensors

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Abstract: This paper will provide a review of past, present and future automotive

sensors. Today’s vehicles have become highly complex sophisticated electronic control systems and the majority of innovations have been solely achieved through electronics and the use of advanced sensors. The environment for these sensors continues to be increasingly challenging with respect to robustness, reliability, quality and cost.

Introduction:Today’s 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. Traditional sensors have been complemented by the addition of new sensors for new applications, for example, long range radar, optical steering torque sensors, tyre pressure monitoring systems and yaw rate sensors.

Sensor:A sensor is a device that measures a physical quantity and converts it into a

electrical signal which serve as inputs for control systems can be read by an observer or by an instrument.

Example: A thermocouple converts temperature to an output voltage which can be read by a voltmeter. For accuracy, all sensors need to be calibrated against known standards.

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Application of Sensors:Sensors are used in everyday objects such as touch-sensitive elevator buttons

(tactile sensor) and lamps which dim or brighten by touching the base. There are also innumerable applications for sensors of which most people are never aware. Applications include cars, machines, aerospace, medicine, manufacturing and robotics.A sensor is a device which receives and responds to a signal or stimulus. Here, the term "stimulus" means a property or a quantity that needs to be converted into electrical form.

Sensors in Automotive:

There are so many no of sensors in automotive field, here its few sensors are listed which are mainly used in the same.

Inlet Manifold Pressure Camshaft Position Air Temperature Fuel Temperature Fuel Pressure Knock Coolant Temperature EGR Valve Position Air Mass Flow Oxygen Adaptive Cruise Control Radar Parking Sensors Lane Departure Warning Driver Monitoring Hydraulic Pressure Lat/Long Acceleration Wheel Speed Yaw Rate Ride Height Position Steering Torque & Position Theft Prevention Occupant Airbag Systems Seat Belt Pre-tensioners Tyre Pressure Monitoring Occupant Detection Predestrian Detection Remote Keyless Entry

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Automatic Rain Wipers Automatic Headlamps Washer Fluid Level Headlamp Leveling

Sensor Market Trends (World):

Year Market Value

1991 $2.3b

1995 $3.8b

1997 $4.7b

2000 $7.0b

2006 $10.1b

2012 $15.8b

Air-fuel ratio sensor:

An air-fuel ratio sensor monitors the air-fuel ratio of an internal combustion engine. It is also called as air-fuel ratio gauge, air-fuel meter or air-fuel gauge. It reads the voltage output of an oxygen sensor, whether it be from a narrow band or wide band oxygen sensor.

Benefits of air-fuel ratio sensor: Determining the condition of the oxygen sensor: A malfunctioning oxygen

sensor will result in air-fuel ratios which respond more slowly to changing engine conditions. A damaged or defective sensor may lead to increased fuel consumption and increased pollutant emissions as well as decreased power, and throttle response.

Reducing emissions: Keeping the air-fuel mixture near the stoichometric ratio of 14.7:1 (for gasoline engines) allows the catalytic converter to operate at maximum efficiency.

Fuel economy : An air-fuel mixture leaner than the stoichometric ratio will result in near optimum fuel mileage, costing less per mile traveled and producing the least amount of CO2 emissions.

Engine performance : Carefully mapping out air-fuel ratios throughout the range of rpm and manifold pressure will maximize power output in addition to reducing the risk of detonation.

Example:

o High voltage: fuel mixture rich, little unburned oxygeno Low voltage: fuel mixture lean, excess oxygen

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Lean mixtures improve the fuel economy but also cause sharp rises in the amount of nitrogen oxides (NOX). Lean mixtures burn hotter and may cause rough idle, hard starting and stalling, and can even damage the catalytic converter, or burn valves in the engine. The risk of spark knock/engine knocking (detonation) is also increased when the engine is under load.

Mixtures that are richer than stoichometric allow for greater peak engine power when using gaseous fuels, due to the cooling effect of the evaporating fuel. This increases the intake oxygen density, allowing for more fuel to be combusted and more power developed. The ideal mixture in this type of operation depends on the individual engine.

Cold engines also typically require more fuel and a richer mixture when first started, because fuel does not vaporize as well when cold and therefore requires more fuel to properly "saturate" the air. Rich mixtures also burn slower and decrease the risk of spark knock/engine knocking (detonation) when the engine is under load. However, rich mixtures sharply increase carbon monoxide (CO) emissions.

Oxygen sensors:

Oxygen sensors are installed in the exhaust system of the vehicle, attached to the engine's exhaust manifold, the sensor measures the ratio of the air-fuel mixture.

As mentioned above, there are two types of sensors available; narrow band and wide band. Narrow band sensors were the first to be introduced. The wide band sensor was introduced much later. A narrow band sensor has a non-linear output, and switches between the thresholds of lean (ca 100-200 mV) and rich (ca 650-800 mV) areas very steeply.

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rich lean

Engine output

Fuel consumption

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Also, narrow band sensors are temperature-dependent. If the exhaust gases become warmer, the output voltage in the lean area will rise, and in the rich area it will be lowered. Consequently, a sensor, without pre-heating has a lower lean-output and a higher rich-output, possibly even exceeding 1 Volt. The influence of temperature to voltage is smaller in the lean mode than in the rich mode.

The Engine Control Unit (ECU) tries to maintain a stoichiometric balance, where in the air-fuel mixture is approximately 14.7 times the mass of air to fuel for gasoline. This ratio is selected in order to maintain a neutral engine performance.

The average level of the sensor is defined as 450 mV. Since narrow band sensors cannot output a fixed voltage level between the lean and the rich areas, the ECU tries to control the engine by controlling the mixture between lean and rich in such a sufficiently fast manner, that the average level becomes ca 450 mV.

Crank sensor:

A crank sensor is a component used in an internal combustion engine to monitor the position or rotational speed of the crankshaft. This information is used by engine management systems to control ignition system timing and other engine parameters. Before electronic crank sensors were available, the distributor would have to be manually adjusted to a timing mark on the engine.

The crank sensor can be used in combination with a similar camshaft position sensor to monitor the relationship between the pistons and valves in the engine, which is particularly important in engines with variable valve timing. It is also commonly the primary source for the measurement of engine speed in revolutions per minute.

Crank sensors in engines usually consist of magnets and an inductive coil, or they may be based on magnetically triggered Hall effect semiconductor devices. Common mounting locations include the main crank pulley, the flywheel, and occasionally on the crankshaft itself. This sensor is the most important sensor in modern day engines. When it fails, there is a small chance the engine will start (engine will likely cut out after a few minutes of driving) but it mostly will not start.

Another type of crank sensor is used on bicycles to monitor the position of the crankset, usually for the cadence readout of a cyclocomputer.

Throttle position sensor:

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Throttle body showing "wide open throttle" sensor on the right

A throttle position sensor (TPS) is a sensor used to monitor the position of the throttle in an internal combustion engine. The sensor is usually located on the butterfly spindle so that it can directly monitor the position of the throttle valve butterfly.

The sensor is usually a potentiometer, and therefore provides a variable resistance dependent upon the position of the valve (and hence throttle position).

The sensor signal is used by the engine control unit (ECU) as an input to its control system. The ignition timing and fuel injection timing (and potentially other parameters) are altered depending upon the position of the throttle, and also depending on the rate of change of that position. For example, in fuel injected engines, in order to avoid stalling, extra fuel may be injected if the throttle is opened rapidly (mimicking the accelerator pump of carburetor systems).

More advanced forms of the sensor are also used, for example an extra closed throttle position sensor (CTPS) may be employed to indicate that the throttle is completely closed.

Some ECUs also control the throttle position and if that is done the position sensor is utilised in a feedback loop to enable that control.

Related to the TPS are accelerator pedal sensors, which often include a wide open throttle (WOT) sensor. The accelerator pedal sensors are used in "drive by wire" systems, and the most common use of a wide open throttle sensor is for the kickdown function on automatic transmissions.

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Modern day sensors are Non Contact type, wherein a Magnet and a Hall Sensor is used. In the potentiometric type sensors, two metal parts are in contact with each other, while the butterfly valve is turned from zero to WOT, there is a change in the resistance and this change in resistance is given as the input to the ECU.

Non Contact type TPS work on the principle of Hall Effect, wherein the magnet is the dynamic part which mounted on the butterfly valve spindle and the hall sensor is mounted with the body and is stationary. When the magnet mounted on the spindle which is rotated from zero to WOT, there is a change in the magnetic field for the hall sensor. The change in the magnetic field is sensed by the hall sensor and the hall voltage generated is given as the input to the ECU. Normally a two pole magnet is used for TPS and the magnet may be of Diametrical type or Ring type or segment type, however the magnet is defined to have a certain magnetic field.

Manifold Absolute Pressure (MAP) sensor:

A manifold absolute pressure sensor (MAP) is one of the sensors used in an internal combustion engine's electronic control system. Engines that use a MAP sensor are typically fuel injected. The manifold absolute pressure sensor provides instantaneous manifold pressure information to the engine's electronic control unit (ECU). The data is used to calculate air density and determine the engine's air mass flow rate, which in turn determines the required fuel metering for optimum combustion (see stoichiometry). A fuel-injected engine may alternately use a MAF (mass air flow) sensor to detect the intake airflow. A typical configuration employs one or the other, but seldom both.

MAP sensor data can be converted to air mass data using the speed-density method. Engine speed (RPM) and air temperature are also necessary to complete the speed-density calculation. The MAP sensor can also be used in OBD II (on-board diagnostics) applications to test the EGR (exhaust gas recirculation) valve for functionality, an application typical in OBD II equipped General Motors engines.

Example:

The following example assumes the same engine speed and air temperature.

Condition 1:

An engine operating at WOT (wide open throttle) on top of a very high mountain has a MAP of about 15" Hg or 50 kPa (essentially equal to the barometer at that high altitude).

Condition 2:

The same engine at sea level will achieve 15" Hg of MAP at less than WOT due to the higher barometric pressure.

Vacuum comparison:

Vacuum is the difference between the absolute pressures of the intake manifold and atmosphere. Vacuum is a "gauge" pressure, since gauges by nature measure a pressure difference, not an absolute pressure. The engine fundamentally responds to air mass, not vacuum, and absolute pressure is necessary to calculate

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mass. The mass of air entering the engine is directly proportional to the air density, which is proportional to the absolute pressure, and inversely proportional to the absolute temperature.

EGR Testing:

With OBD II standards, vehicle manufacturers were required to test the EGR valve for functionality during driving. Some manufacturers use the MAP sensor to accomplish this. In these vehicles, they have a MAF sensor for their primary load sensor. The MAP sensor is then used for rationality checks and to test the EGR valve. The way they do this is during a deceleration of the vehicle when there is low absolute pressure in the intake manifold (i.e., a high vacuum present in the intake manifold relative to the outside air). During this low absolute pressure (i.e., high vacuum) the PCM will open the EGR valve and then monitor the MAP sensor's values. If the EGR is functioning properly, the manifold absolute pressure will increase as exhaust gases enter.

Curb sensor:

Curb sensor mounted behind the front wheel of a 1950s Rambler American.

Curb sensor on a 1973 VAZ-2103 Zsiguli (left).

Curb sensors or curb finders are springs or wires installed on a vehicle which act as "whiskers" to warn drivers that they are too close to the curb or other obstruction.

The devices are fitted low on the body, close to the wheels. As the vehicle approaches the curb, the protruding 'feelers' act as whiskers and scrape against the curb, making a noise and alerting the driver in time to avoid damaging the wheels or hubcaps. The feelers are manufactured to be flexible and do not easily break.

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Defect detector:

Overview of a wheel impact detector installation & Sensors for the wheel condition monitor.

A defect detector is a device used on railroads to detect axle and signal problems in passing trains. The detectors are normally integrated into the tracks and often include sensors to detect several different kinds of problems that could occur. Defect detectors were one invention which enabled American Railroads to eliminate the caboose at the rear of the train, as well as various station agents stationed along the route to detect unsafe conditions. The use of defect detectors has since spread to other overseas railroads.

As early as the 1940s, automatic defect detectors were installed to improve what was normally done with the human eye by railroad workers. The detectors would transmit their data via wired links to remote read-outs in stations, offices or interlocking towers. If a defect was detected, an alarm would sound and the employee on duty would bring the train to a halt using hand or automatic signals.

Parking sensors:

Parking sensor detector

Parking sensors are proximity detectors for road vehicles which can alert the driver to unseen obstacles during parking manoeuvres. The ultrasonic sensors are currently available in several brands of luxury cars with a variety of brand names

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such as Parktronic and Parking Aid. Some systems are also available as additional upgrade kits for later installation.

Description:

Parking sensor systems use ultrasonic proximity sensors embedded in the front and/or rear bumpers, to measure the distances to nearby objects at low level. The sensors measure the time taken for each sound pulse to be reflected back to the receiver.

Depending on the speed of the vehicle and the distance to the obstacle, the system will warn the driver by visual and/or audible means about the risk of collision. The feedback to the driver will generally indicate the direction and proximity of the obstacle. Warnings are deactivated when the vehicle exceeds a certain speed, and can be switched off for situations such as stop-and-go traffic.

Radar gun:

U.S. Army soldier uses a radar gun to catch speeding violators at Tallil Air Base, Iraq.

A radar gun or speed gun is a small Doppler radar unit used to detect the speed of objects, especially trucks and automobiles for the purpose of regulating traffic, as well as pitched baseballs, runners or other moving objects in sports. A radar gun does not return information regarding the object's position. It relies on the Doppler effect applied to a radar beam to measure the speed of objects at which it is pointed. Radar guns may be hand-held or vehicle-mounted.

There are radar detectors on the market which can detect most police radar and laser systems. Conversely, in the spirit of electronic warfare, some police radars are equipped with detectors of operating detectors.

Working Principle:

Radar guns are, in their most simple form, radio transmitters and receivers. They send out a radio signal, then receive the same signal back as it bounces off the objects. However, the radar frequency is different when it comes back, and from that difference the radar gun can calculate object speed.

A radar beam is similar to a beam of light in that it spreads out as the distance from the signal origin increases. The signal then bounces off objects in the path of the beam and are reflected back to the gun. The gun uses the Doppler effect to calculate

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the speed of the object in the beam's path. Using a comparison of frequency shift between received images instead of the frequency shift between sent and received frequencies creates what is known as moving radar. Unless the radar system has a provision for converting own-vehicle-speed to an appropriate receiver frequency offset then the radar must be stationary to measure speed.

All bands of radar operate on different frequencies, work differently, and are very complicated. X band guns are becoming less common due to the fact the beam is strong and easily detectable. Also, most automatic doors utilize radio waves on X band and can possibly affect the readings of police radar. As a result K band and Ka band are most commonly used by police agencies.

Traffic radar comes in many models. There are hand held, stationary and moving radar instruments. Hand held units are mostly battery powered, and for the most part are used as stationary speed enforcement tools. Stationary radar is mounted in police vehicles, and may have one or two antennae. These are employed when the vehicle is parked. Moving radar is employed, as the name implies, when the police vehicle is in motion. These devices are very sophisticated, able to track vehicles approaching and receding both in front of and behind the patrol vehicle. They can also track the fastest vehicle in the selected radar beam, front or rear.

GPS:

GPS devices are capable of showing speed readings based on how far the receiver has moved since the last measurement (a second ago). As the GPS is an independent* system, its speed calculations are not subject to the same sources of error as the vehicle's speedometer. Instead, the GPS's positional accuracy, and therefore the accuracy of its calculated speed, is dependent on the satellite signal quality at the time. Speed calculations will be more accurate at higher speeds, when the ratio of positional error to positional change is lower. The GPS software may also use a moving average calculation to reduce error.

As mentioned in the satnav article, GPS data has been used to overturn a speeding ticket; the GPS logs showed the defendant traveling below the speed limit when they were ticketed. That the data came from a GPS device was likely less important than the fact that it was logged; logs from the vehicle's speedometer could likely have been used instead, had they existed. * some satnav devices may also use data from the car's systems to improve accuracy.

Speed sensor:

Speed sensors are machines used to detect the speed of an object, usually a transport vehicle. They include:

Wheel speed sensors Speedometers Pitometer logs Pitot tubes Piezo sensors (e.g. in a road surface) Doppler radar ANPR (where vehicles are timed over a fixed distance)

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Wheel speed sensor:

A wheel speed sensor or vehicle speed sensor (VSS) is a type of tachometer. It is a sender device used for reading the speed of a vehicle's wheel rotation. It usually consists of a toothed ring and pickup.

Special purpose speed sensors:

Road vehicles:

Wheel speed sensors are used in anti-lock braking systems.

Rotary speed sensors for rail vehicles:

Many of the subsystems in a rail vehicle, such as a locomotive or multiple unit, depend on a reliable and precise rotary speed signal, in some cases as a measure of the speed or changes in the speed. This applies in particular to traction control, but also to wheel slide protection, registration, train control, door control and so on. These tasks are performed by a number of rotary speed sensors that may be found in various parts of the vehicle.

In the past, sensors for this purpose often failed to function satisfactorily or were not reliable enough and gave rise to vehicle faults. This was particularly the case for the early mainly analogue sensors, but digital models were also affected.

This was mainly due to the extremely harsh operating conditions encountered in rail vehicles. The relevant standards specify detailed test criteria, but in practical operation the conditions encountered are often even more extreme (such as shock/vibration and especially electromagnetic compatibility (EMC)).

Speedometer:

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A speedometer is a device that measures the instantaneous speed of a land vehicle.

Inaccurate due to its mechanism, shape of fuel tank Gauge: resistance ↑, current ↓, bimetallic cools, straighten out, pull needle

form full to empty.

Newer car: resistor output into a microprocessor – compensate shape of tank

Damping needle movment up hill , down hill , turn

Speedometers for other vehicles have specific names and use other means of sensing speed. For a boat, this is a pit log. For an aircraft, this is an airspeed indicator.

The speedometer was invented by the Croatian Josip Belušić in 1888, and was originally called a velocimeter.

Operation:

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An eddy-current speedometer gauge on a car, showing the speed of the vehicle in kilometres per hour. Also shown is the tachometer, which displays the rate of rotation of the engine's crankshaft.

The eddy current speedometer has been used for over a century and is still in widespread use. Until the 1980s and the appearance of electronic speedometers it was the only type commonly used.

Originally patented by a German, Otto Schulze on 7 October 1902, it uses a rotating flexible cable usually driven by gearing linked to the tail shaft (output) of the vehicle's transmission. The early Volkswagen Beetle and many motorcycles, however, use a cable driven from a front wheel.

A small permanent magnet affixed to the rotating cable interacts with a small aluminum cup (called a speedcup) attached to the shaft of the pointer on the analogue instrument. As the magnet rotates near the cup, the changing magnetic field produces eddy currents in the cup, which themselves produce another magnetic field. The effect is that the magnet "drags" the cup, and thus the speedometer pointer, in the direction of its rotation with no mechanical connection between them.

The pointer shaft is held toward zero by a fine spring. The torque on the cup increases with the speed of rotation of the magnet (which is driven by the car's transmission.) Thus an increase in the speed of the car will twist the cup and speedometer pointer against the spring. When the torque due to the eddy currents in the cup equals that provided by the spring on the pointer shaft, the pointer will remain motionless and pointing to the appropriate number on the speedometer's dial.

The return spring is calibrated such that a given revolution speed of the cable corresponds to a specific speed indication on the speedometer. This calibration must take into account several factors, including ratios of the tailshaft gears that drive the flexible cable, the final drive ratio in the differential, and the diameter of the driven tires.

Electronic Type Speedometer:

Many modern speedometers are electronic. A rotation sensor, usually mounted on the rear of the transmission, delivers a series of electronic pulses whose frequency corresponds to the rotational speed of the driveshaft. The sensor is typically a toothed metal disk positioned between a coil and a magnetic field sensor. As the disk turns, the teeth pass between the two, each time producing a pulse in the sensor as they affect the strength of the magnetic field it is measuring.[1] Alternatively, some manufactures rely on pulses coming from the ABS wheel sensors.

A computer converts the pulses to a speed and displays this speed on an electronically-controlled, analog-style needle or a digital display. Pulse counts may also be used to increment the odometer.

Another early form of electronic speedometer relies upon the interaction between a precision watch mechanism and a mechanical pulsator driven by the car's wheel or transmission. The watch mechanism endeavors to push the speedometer pointer toward zero, while the vehicle-driven pulsator tries to push it toward infinity. The

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position of the speedometer pointer reflects the relative magnitudes of the outputs of the two mechanisms.

Changing a car's tire size can throw off a speedometer's accuracy.

Pitometer log:

Figure 1: Photo of World War II US Navy submarine pitometer. This unit uses a mercury-based manometer to measure the difference in static and dynamic water pressure.

Pitometer logs (also known as pit logs) are devices used to measure a ship's speed relative to the water. They are used on both surface ships and submarines. Data from the pitometer log is usually fed directly into the ship's navigation system.

Principles of Operation:

Figure shows Illustration of a Mercury Manometer-Based Pitometer Log.

The basic technology of the pitometer log is similar to that of the pitot tube on an aircraft. Typically, the pitometer has a long tube that penetrates the ship's hull near the keel. The part of the pitometer protruding from the ship is sometimes called a pit sword or rodmeter. This tube usually has two openings: one facing the direction of seawater motion that is used to measure the dynamic pressure of the seawater and one at 90o to the direction of seawater motion that is used to measure the static seawater pressure. The dynamic pressure of the seawater is a function of the depth of the water and the speed of the vessel.

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In early realizations of the pitometer log, mercury manometers [6] were used to measure the pressure differences (see Figure 1).[4] Later realizations used approaches that would generate equalizing pressures within the pitometer that would balance out the dynamic pressure. This eliminated the need for mercury manometers. [7]

Analysis:

An expression can be derived for the velocity of water impacting the ship as a function of the difference in dynamic and static water pressure using Bernoulli's principle. The total pressure of the water in the tube with moving seawater can be described by Equation 1.

(Equation 1)

where pTotal is the total fluid pressure. pStatic is the static pressure, which strictly depends on depth. pDynamic is the fluid pressure caused by fluid motion.

Since water is an incompressible fluid, the dynamic pressure component of the total pressure can be expressed in terms of the water density and the water velocity as is shown in Equation 2.

(Equation 2)

where vWater is the speed of the fluid flow. ρ is the fluid density.

Equation 2 can be solved for the velocity of water in terms of the difference in pressure between the two legs of the manometer. Equation 3 shows that velocity is a function of the square root of the pressure difference.

(Equation 3)

Because the speed computed by the pitometer is a function of the difference between pressure readings, the pitometer does not produce an accurate result when the ship's velocity is low and the two pressure readings are nearly the same.

Optical sensor:

All the manufacturers previously active in this market used mainly optical sensors.

From one to four channels can be implemented, each channel having a photosensor that scans one of at most two signal tracks on a slotted disk. Experience shows that the possible number of channels achievable by this technique is still not enough. A number of subsystems therefore have to make do with looped-through signals from

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the wheel slide protection electronics and are therefore forced to accept, for instance, the available number of pulses, although a separate speed signal might well have some advantages.

The use of optical sensors has been familiar for many years and is widespread in industry. Unfortunately they do have two fundamental weaknesses that have always made it very difficult to get them to function reliably over a number of years, namely - the optical components are extremely susceptible to dirt, and - the light source ages too quickly.

Even traces of dirt greatly reduce the amount of light that passes through the lens and can cause signal dropout. These encoders are therefore required to be very well sealed. Even sealing the encoder bearing to prevent it emitting grease is a problem that even the ingenuity of designers has been unable to fully resolve. Further problems are encountered when the pulse generators are used in environments in which the dew point is passed: the lenses fog and the signal is frequently interrupted.

The light sources used are light-emitting diodes (LEDs). But LEDs are always subject to ageing, which over a few years leads to a noticeably reduced beam. Attempts are made to compensate for this by using special regulators that gradually increase the current through the LED, but unfortunately this further accelerates the ageing process.

Variable reluctance sensor:

A variable reluctance sensor (VRS) is used to measure position and speed of moving metal components. This sensor consists of a permanent magnet, a ferromagnetic pole piece, a pickup coil, and a rotating toothed wheel.

As the gear teeth of the rotating wheel (or other target features) pass by the face of the magnet, the amount of magnetic flux passing through the magnet and consequently the coil varies. When the gear tooth is close to the sensor, the flux is at a maximum. When the tooth is further away, the flux drops off. The moving target results in a time-varying flux that induces a proportional voltage in the coil. Subsequent electronics are then used to process this signal to get a digital waveform that can be more readily counted and timed.

Although VR sensors are based on very mature technology, they still offer several significant advantages. The first is low cost - coil of wire and magnets are relatively inexpensive. Unfortunately, the low cost of the transducer is partially offset by the cost of the additional signal-processing circuitry needed to recover a useful signal. And because the magnitude of the signal developed by the VR sensor is proportional to target speed, it is difficult to design circuitry to accommodate very-low-speed signals. A given VR-sensing system has a definite limit as to how slow the target can move and still develop a usable signal. An alternative but more expensive technology is Hall effect sensor. Hall effect sensors are true zero-rpm sensors and actively supply information even when there's no transmission motion at all.

One area in which VR sensors excel, however, is in high-temperature applications. Because operating temperature is limited by the characteristics of the materials used in the device, with appropriate construction VR sensors can be made to operate at temperatures in excess of 300°C. An example of such an extreme application is

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sensing the turbine speed of a jet engine or engine cam shaft and crankshaft position control in an automobile.

VR sensor interface circuits VR sensors need waveform shaping for their output to be digitally readable. The normal output of a VR sensor is an analog signal, shaped much like a sine wave. The frequency and amplitude of the analog signal is proportional to the target's velocity. This waveform needs to be squared up, and flattened off by a comparator like electronic chip to be digitally readable. While discrete VR sensor interface circuits can be implemented, the semiconductor industry also offers integrated solutions. Examples are the MAX9924 to MAX9927 VR sensor interface IC from Maxim Integrated products, LM1815 VR sensor amplifier from National Semiconductor and NCV1124 from ON semiconductor. An integrated VR sensor interface circuit like the MAX9924 features a differential input stage to provide enhanced noise immunity, Precision Amplifier and Comparator with user enabled Internal Adaptive Peak Threshold or user programmed external threshold to provide a wide dynamic range and zero-crossing detection circuit to provide accurate phase Information.

Water sensor:

The Water in Fuel Sensor or WiF sensor indicates the presence of water in the fuel. It is installed in the fuel filter and when the water level in the water separator reachs the warning level the Wif send an electrical signal to the ECU or to dashboard (lamp). The Wif is used especially in the Common Rail engines to avoid the unit injector damage.

The Wif sensor uses the difference of electric conductivity through water and diesel fuel by 2 electrodes.

First generation Wif sensor uses a potting resin to isolate the electronic circuit, last generation of Wif sensor (the WS3 sensor in Surface-mount technology) is made totally without leakage using an innovative co-moulding process.

The last generation of water in fuel sensor has a high resistance to vibrations and to thermal excursion cycles.

The main Automotive WiF designer and producer is the SIGMAR.

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