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Prof. Melvin P ManuelMEMS & Sensors Division

SENSEVIT University

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PHOTO RESISTOR

LDR Light Dependent ResistorTo measure the light intensity

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STRAIN GAUGE To measure the Strain

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PIEZO ELECTRIC SENSOR To measure the Force

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BOURDON TUBE & BELLOW To measure the Pressure

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HALL EFFECT SENSOR To measure magnetic field strength

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Sensor ClassificationPassive Sensors

Active Sensors

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Passive Sensors

A passive sensor does not need anyadditional energy source and directlygenerates an electric signal in response toan external stimulus.

ie. the input stimulus energy is converted by the sensor into the output signal .

The examples are a thermocouple, a photodiode, and a piezoelectric sensor.

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Active Sensors The active sensors require external power for

their operation, which is called an excitation signal . That signal is modified by the sensor to produce

the output signal. The active sensors sometimes are called

parametric because their own properties change in

response to an external effect and theseproperties can be subsequently converted intoelectric signals

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Example A thermistor is a temperature-sensitive resistor. It does not generate any electric signal, but by

passing an electric current through it (excitationsignal), its resistance can be measured by detecting variations in current and/or voltageacross the thermistor.

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TYPES OF SENSORS Thermal Sensors

Temperature sensors: Thermometers,Thermocouples, Temperature sensitiveresistors (Thermistors and resistancetemperature detectors) and bi-metalthermometers

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Mechanical

Pressure sensors: Pressure gauge, Elasticelements.Gas and liquid flow sensors:Orifice meter, Veturimeter, anemometer, flow meter, gas meter,water meter, mass flow sensorGas and liquid viscosity and density: viscometer,hydrometer, oscillating U-tube

Mechanical sensors: Acceleration sensor,position sensor, Strain gaugehumidity sensors: hygrometer

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ChemicalChemical proportion sensors:

oxygen sensors,ion-selective electrodes,pH glass electrodes,

Carbon monoxide detectors.

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Optical SensorsLight sensors, or photodetectors , including semiconductordevices such as photocells, photodiodes, phototransistors,and CCDs Infra-red sensor , especially used as occupancy sensor for

lighting and environmental controls. Proximity sensor- A type of distance sensor but lesssophisticated. Only detects a specific proximity. May beoptical - combination of a photocell and LED or laser.Fiber optic sensors.

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Acoustic SensorAcoustic : uses ultrasound time-of-flight echo return.Ultrasonic sensor: NDT applicationSound sensors : microphones,

hydrophones, seismometers.

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BioSensorArtificial sensors that mimic biologicalsensors by using a biological sensitivecomponent, are called biosensors.(Electronic nose)

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Other Sensors

Motion sensors: Speedometer,TachometerOrientation sensors: GyroscopeDistance sensor (noncontacting) Severaltechnologies can be applied to sensedistance: magnetostriction , Ultrasound

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Available Forms:

ThermocoupleThermistors

RTD (pt 100)

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LVDT (Linear Variable DifferentialTransformer)

To measure the linear Displacement

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The physical construction of a typical

LVDT consists of a movable core ofmagnetic material and three coilscomprising the static transformer. Oneof the three coils is the primary coil andthe other two are secondary coils.When the iron core slides through the

transformer, a certain number of coilwindings are affected by the proximityof the sliding core and thus generate aunique voltage output.

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Commercial LVDT:

LVDTs can be utilized in a wide variety ofapplications wherever position feedback isimportant

By courtesy of GW Lisk

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Proximity Sensors

Eddy Current Sensor

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An eddy current is a local electric current induced in aconductive material by the magnetic field produced bythe active coil.

This local electric current in turn induces a magnetic fieldopposite in sense to the one from the active coil and

reduces the inductance in the coil. When the distance between the target and the probe

changes, the impedance of the coil changescorrespondingly. This change in impedance can be

detected by a carefully arranged bridge circuit.

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Commercial Eddy Current Probes:

Application: Surface InspectionBy courtesy of Zetec

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Ultrasonic Proximity Sensor:

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A special sonic transducer is used for theultrasonic proximity sensors, which allows foralternate transmission and reception of soundwaves.

The ultrasonic transducer emits a number ofsonic waves which are reflected by an object,back to the ultrasonic transducer.

After emission of the sound waves, the

ultrasonic sensor will switch over to receivemode. The time elapsed between emitting and

receiving is proportional to the distance of theobject from the sensor.

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Commercial Sensor

Ultrasonic proximity sensor - detect thepresence of an object within 1 inch to 4feet. This sensor provides a linear outputso it can be used to detect approximatedistance also.

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IR Proximity Sensor:

Infrared proximity sensors work by

sending out a beam of IR light, and thencomputing the distance to any nearbyobjects from characteristics of the returned(reflected) signal

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OPTICAL SENSORS Electro optic sensor:

Photo Resistor A photo resistor is made of a high-resistance

semiconductor. If light falling on the device is of high enough

frequency, photons absorbed by thesemiconductor give bound electrons enoughenergy to jump into the conduction band.

The resulting free electron (and its holepartner) conduct electricity, thereby loweringresistance.

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Photodiode:

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A photodiode is a semiconductor diode thatfunctions as a photodetector. Photodiodes arepackaged with either a window or optical fibreconnection, to let in the light to the sensitive partof the device.

A phototransistor is in essence nothing morethan a bipolar transistor that is encased in atransparent case so that light can reach thebase-collector junction. The electrons that aregenerated by photons in the base-collector

junction are injected into the base, and thiscurrent is then amplified by the transistoroperation.

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Charge Coupled Device (CCD):

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Charge Coupled Device (CCD):A charge-coupled device (CCD) is an

analog shift register, enabling analog signals

(electric charges) to be transported throughsuccessive stages (capacitors) controlled bya clock signal. coupled device (CCD ) is ananalog shift register, enabling analog signals(electric charges) to be transported throughsuccessive stages (capacitors) controlled by aclock signal.

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MAGNETIC SENSOR HALL EFFECT

1. Electrons2. Hall element, or Hall sensor

3. Magnets4. Magnetic field5. Power source

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The Hall effect IC consists of Hall element andsignal conditioning circuitry to convert the Hallelement output into a digital output. The twodevices are mounted in a single package withthree active terminals( Power, GND and output).There are two choices of power consumption.Continuous powered devices with standardcurrent consumption and pulsed powereddevices at low duty cycles that significantlyreduce the average current consumption tomicroamps.

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Commercial Hall Effect IC

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Magnetic Pick Up

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Recent Trends in Sensors Micro ElectroMechanical System (MEMS)

SensorsMEMS is the technology of the very

smallThe sensor technology made significant

progress due to MEMS.

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MEMS Accelerometer