Photo voltaic cell

• Generation of potential difference when lightstrikes a junction of two dissimilar metals is calledphotovoltaic effect.

• Photovoltaic cell works on principle ofphotovoltaic effect.

• Photovoltaic cell is an active transducer and neednot operate in vacuum or gas filled tube.

• Photodiode and photo transistor

Photo diode • The reverse bias is applied against the p-n junction that

results in a very high resistance.

• The light ray is made to fall on n-layer.

• This results in decrease in resistance of the diode,developing the reverse current due to sweep up of theelectron-hole pair.

• The reverse current is the measure of intensity of theincident radiation.

• The response of the photo diode is quick and they can beused as variable resistance devices.

Photo Transistor • A base collector is arranged in parallel to the photo diode, a

bipolar transistor is formed.

• In this p-n junction the collector base is photo sensitive.

• The incidence radiation on the base results in reversecurrent of photo diode, which is the base current of thetransistor.

• The base current is proportional to the intensity of light.

• If the base current is amplified by internal signalprocessing, then the higher sensitivity of the phototransistor can be achieved.

PROXIMITY SENSORS -Vinayak Kulkarni

PROXIMITY SENSORS

• A proximity sensor detects an objects when theobject approaches within the detection range andboundary of the sensor.

• Proximity sensors include all sensor that performnon contact detection in comparison to sensorssuch as limit switch, that detects the object byphysically contacting them.

• Proximity sensors are used in various facets ofmanufacturing for detecting the approach ofmetal and non metal objects.

Inductive proximity sensor

Inductive proximity sensors working principle

• Inductive proximity sensors detect the presenceof metallic objects.

• Their operating principle is based on a coil andhigh frequency oscillator that creates a field inthe close surroundings of the sensing surface.The presence of metal in the operating areacauses a change in the oscillation amplitude.This change is identified by a threshold circuit,which changes the output of the sensor. Theoperating distance of the sensor depends on thecoil’s size as well as the target’s shape, size andmaterial.

Inductive proximity sensors• An inductive sensor is an electronic proximity

sensor, which detects metallic objects withouttouching them.

• Electric current generates a magnetic field,which collapses generating a current that fallsasymptotically toward zero from its initial levelwhen the input electricity ceases.

Construction

• The proximity inductive sensor basicallyconsists of a wound coil located in front of apermanent magnet.

• The permanent magnet is encased in a ruggedhousing.

• The change in current in the coil is outputthrough the leads embedded in the resin.

• The leads connected to the display through aconnector gives signal for the presence of anobject in the vicinity.

Applications

• Common applications of inductive sensorsinclude metal detectors, car washes, and ahost of automated industrial processes.Because the sensor does not require physicalcontact it is particularly useful for applicationswhere access presents challenges or wheredirt is prevalent. The sensing range is rarelygreater than 6 cm, however, and it has nodirectionality.

Capacitive proximity sensors

Capacitive proximity sensors working principle• Capacitive sensors are used for non-contact

detection of metallic objects & nonmetallicobjects (liquid, plastic, wooden materials and soon). Capacitive proximity sensors use thevariation of capacitance between the sensor andthe object being detected. When the object is ata preset distance from the sensitive side of thesensor, an electronic circuit inside the sensorbegins to oscillate. The rise or fall of suchoscillation is identified by a threshold circuit thatdrives an amplifier for the operation of anexternal load.

Applications

• Capacitive touch sensors are used in manydevices such as laptop track pads, digital audioplayers, computer displays, mobile phones,mobile devices and others.

• More and more design engineers are selectingcapacitive sensors for their versatility,reliability and robustness and cost reductionover mechanical switches.

Ultrasonic sensors• Ultrasonic sensors are some times used in

place of optical sensors. Instead of using anlight beam, a high frequency sound wave isused.

• This sound wave is above normal hearingfrequencies and are called ultrasonic.Frequencies around 40 KHz are common.

What does an ultrasonic sensor do?

Sensor

• It emits a short Ultrasonic Pulse ...

What does an ultrasonic sensor do?

Sensor

• ...and waits for the echo.

What does an ultrasonic sensor do?

Sensor

distance =

waiting time x speed of sound / 2

• It calculates the distance to the object

– waiting time

– speed of sound

Optical sensor

Optical sensor• The proximity of the object is detected by the action of the

travelling light move.

• The light emitted by the transmitter focuses on the object whichreflects to be received by the receiver photo diode.

• The light from the emitting diode is focused by the transmitter lens,on to the object surface.

• The reflected waves travel back and received by the solid statephoto diode, through the receiver lens.

• The object within the range of the sensor can detect the presence.

• The focal length of the sensor lenses decide the range within whichthe proximity of the object is detected.

DESIRABLE FEATURES FOR SENSORS AND TRANSDUCER

FEATURES FUNCTIONS

PRECISION Should be as high as possible Deviation in measurement reading should be minimum.

ACCURACY Should be very highError between sensed and actual value should approach zero.

SPEED OF RESPONSE Time taken to respond should be minimumResponse should be instantaneous.

OPERATING RANGE Wide operating range Good accuracy over the range .

RELIABILITY Life to be high Frequent failure are not acceptable.

CALIBRATION Drift should be minimum Should be easy to calibrate

COST AND EASE The purchase cost should be lowThe installation and operation should be easy.