Seminar onPHYSICAL EFFECTS

FOR SENSORSBy

Sabeel Irshad B

Contents:IntroductionResistive effectsInductive effectsCapacitive effectsCharge Generating Effects

INTRODUCTION A sensor is a transducer whose purpose is to sense (that

is, to detect) some characteristic of its environment. Output of a sensor is generally as an electrical or optical

signal It is the effects which causes the changes that the sensor

is going to measure. Effects is nothing but the changes in the environment

which helps in measuring a quantity Property or quantity to be measured is called measurand.

Resistive Effects: The measurand directly or indirectly alters the

electrical resistance of a resistive element. Resistive sensors: A resistive sensor is a transducer or

electromechanical device that converts a mechanical change such as displacement into an electrical signal.

Resistivity is resistance stated in terms of length and cross-sectional area as shown in the equation

Resistance = (Resistivity * Length)/Area Changing the value of one of the above factor

changes the resistance

Influence of temperature• Thermistor is a type of resistor whose resistance varies

significantly with temperature.(Thermal + resistor).• There are two categories made depending on resistance will

either increase or decrease– NTC: Resistance will decrease with the increase in

temperature.• Rise in temperature increases the charge carriers promote the electrons

into the conduction band decreases the resistance.

– PTC: Resistance will increase with the increase in temperature.• Potential barriers are formed causing the resistance to increase .• At high temp material reverts to NTC behavior.

Thick and thin film resistors

Thin film: (0.1um or smaller)Resistive layer is sputtered on to the ceramic base.Laser trimming is used to create patterns to

increase the resistive path and calibrate the resistance values

Thick Film: (100um) Most used resistors Come usually as chip resistors(SMD) The resistance material is a special paste with a mixture of

binding, a carrier & metal oxide is deposited. Paste is deposited at 850 C, becomes glass like after cooling. Layers are added to increase the resistance value. All the above three are used to measure the engine temperature.

Length and angular proportionality of resistors:

Potentiometer Sensors: Most commonly used as position sensor It has a wiper contact linked to a mechanical shaft that can be either angular

(rotational) or linear (slider type) in its movement Which causes the resistance value between the wiper/slider and the two end

connections to change giving an electrical signal output. The output voltage signal is taken from the wiper terminal of the sliding contact Accelerator pedal, throttle valve, fuel tank level

PiezoresistivePiezoresistivity in metals and semiconductors Piezoresistive effect: The Piezoresistive effect is the

change in the electrical resistivity of a semiconductor or metal when mechanical strain is applied.

Strain changes the inner atomic spacing affecting the band gap.

Electrons are raised to conduction band, this results in change in the resistivity of material.

Piezoresistors are resistors made from a piezoresistive material and are usually used for measurement of mechanical stress.

Strain-gage resistors

A strain-gage resistor is a resistor in which the resistance changes with strain

It is literally glued on to the device where you want to measure strain.

When the device is under stress it may elongate or compress changing the resistance.

It is sensitive to smaller change in geometry.Pressure sensors(ABS) ,force sensors

Magnetic field dependency

Magnetoresistance Magnetoresistance is the property of a material to change the value of its

electrical resistance when an external magnetic field is applied to it. The magnetoresistive sensors are based on the magnetoresistive effect.

This effect changes the resistivity of a current carrying ferromagnetic material due to a magnetic field.

The resistance depends on the angle formed by the internal magnetization vector (M) of the ferromagnetic material and the direction of the current (I) flow

Speed or delivery angle measurement in injection pumps.

Light Dependency

Semiconductor Photoresistor A photoresistor or light-dependent resistor (LDR) or photocell is

a light-controlled variable resistor. The resistance of a photoresistor decreases with increasing

incident light intensity; in other words, it exhibits photoconductivity.

A photoresistor is made of a high resistance semiconductor. In the dark, resistance is as high as a few mega ohms (MΩ). While in the light, resistance is as low as a few hundred ohms. Rain sensors, dirt sensors

Inductive Effects

Effects of Faraday’s lawWhenever there is a relative motion between

conductor & magnetic field, flux linkage with the coil changes, this change in the flux induces voltage in the coil.

Induced voltage sensors

AlternatorAn alternator is an electrical generator that converts

mechanical energy to electrical energy in the form of alternating current

Rotating magnet and stationary armature either way can also be used.

Works on Faraday’s law.

Wiegand Effect It is a nonlinear magnetic effect Named after its discoverer John R. Wiegand Produced in specially annealed and hardened wire

called Wiegand wire. During manufacture, to give the wire its unique

magnetic properties, it is subjected to a series of twisting and untwisting operations to cold-work the outside shell of the wire while retaining a soft core within the wire, and then the wire is aged.

The result is that the magnetic coercivity of the outside shell is much larger than that of the inner core

Cont’d If a magnet is brought near the wire, the high coercivity

outer shell excludes the magnetic field from the inner soft core until the magnetic threshold is reached, whereupon the entire wire — both the outer shell and inner core — rapidly switches magnetization polarity.

This switchover occurs in a few microseconds, and is called the Wiegand effect.

Wiegand-wire core can increase the output voltage of a magnetic field sensor by several orders of magnitude as compared to a similar coil with a non-Wiegand core.

Used in Wheel speed sensors

Two Modes of Magnetic ExcitationSymmetric Switching Symmetric switching of

Wiegand wire occurs when alternating positive and negative magnetic fields of equal strength are used to magnetize and trigger the wire.

Asymmetric Switching Asymmetric switching

of Wiegand takes place when the wire is magnetized and triggered by magnetic fields of opposite polarity but unequal strength.

Capacitive Effects

The capacitance between two plates is determined by three things: Size of the plates: capacitance increases as the plate size

increases Gap Size: capacitance decreases as the gap increases Material between the plates (the dielectric): Dielectric material

will cause the capacitance to increase or decrease depending on the material

Acceleration sensor, yaw rate sensor, pressure sensor.

Charge Generating Effects

Piezoelectric Effects: Piezoelectric effect is the ability of certain materials to generate

an electric charge in response to applied mechanical stress. Imp characteristics of piezoelectric effect/material is it is

reversible. Knock sensor, airbag sensor, yaw-rate sensor.

Pyroelectric Effects:Pyroelectricity (from the Greek pyr, fire, and

electricity) is the ability of certain materials to generate a temporary voltage when they are heated or cooled.

IR sensor

Photoelectric charge generation

The photoelectric effect is the observation that many metals emit electrons when light shines upon them.

Electrons emitted in this manner can be called photoelectrons.

electrons are dislodged by the photoelectric effect if light reaches or exceeds a threshold frequency

CCD and CMOS Image sensors Found in Digital cameras Responsible for converting light into electrical signals

CCD( Charge coupled Device)Used in earlier digital cameras to turn images from analog

light signal into digital pixels.More expensive than its CMOS counterparts

CMOSUses transistors at each pixel to move charge through

traditional wire.Offers flexibility because each pixel is treated individually. cheaper

ConclusionAll the above effects are very important and are the

base of the sensor measuring technology. In most cases, a micro-sensor reaches a

significantly higher speed and sensitivity compared with the one with a macroscopic approach.

Sensor must be sensitive to the measured property Insensitive to any other property likely to be

encountered in its application

References: http://en.wikipedia.org/wiki/Capacitive_sensing http://en.wikipedia.org/wiki/Wiegand_effect http://www.nanomotion.com/piezo-ceramic-motor-technology/

piezoelectric-effect/ http://en.wikipedia.org/wiki/Piezoelectricity http://www.steves-digicams.com/knowledge-center/how-tos/digital-

camera-operation/ccd-vs-cmos-whats-the-difference.html#b http://electronics.howstuffworks.com/cameras-photography/digital/

question362.htm http://en.wikipedia.org/wiki/Photoelectric_effect