Sensors and Transducers

Sensors p.1MI 563, Dr. S.H. Upadhyay

SENSORS and TRANSDUCERSInterfacing to the Real World

Dr S.H. Upadhyay

Assistant Professor, MIED

Sensors p.2

• Sensors and actuators are two critical components of every closed loopcontrol system. Such a system is also called a mechatronics system . Atypical mechatronics system as shown in Figure consists of a sensing unit, acontroller, and an actuating unit.

• A sensing unit can be as simple as a single sensor or can consist ofadditional components such as filters, amplifiers, modulators, and othersignal conditioners.

• The controller accepts the information from the sensing unit, makesdecisions based on the control algorithm, and outputs commands to theactuating unit.

• The actuating unit consists of an actuator and optionally a power supply anda coupling mechanism

MI 563, Dr. S.H. Upadhyay

Sensors p.3

Sensors• Sensor is a device that when exposed to a physical phenomenon

(temperature, displacement, force, etc.) produces a proportionaloutput signal (electrical, mechanical, magnetic, etc.).

• The term transducer is often used synonymously with sensors.However, ideally, a sensor is a device that responds to a change inthe physical phenomenon.

• On the other hand, a transducer is a device that converts one formof energy into another form of energy.

• Sensors are transducers when they sense one form of energy inputand output in a different form of energy.

• For example, a thermocouple responds to a temperature change(thermal energy) and outputs a proportional change in electromotiveforce (electrical energy).

• Therefore, a thermocouple can be called a sensor and ortransducer.


Sensors p.4

Transducers

• Transducer– a device that converts a primary form of energy into a

corresponding signal with a different energy form• Primary Energy Forms: mechanical, thermal, electromagnetic,

optical, chemical, etc.

– take form of a sensor or an actuator

• Sensor (e.g., thermometer)

– a device that detects/measures a signal or stimulus

– acquires information from the “real world”

• Actuator (e.g., heater)

– a device that generates a signal or stimulus

real

world

sensor

actuator

intelligent

feedback

system


Sensors p.5

usable

values

Sensor Systems

Typically interested in electronic sensor– convert desired parameter into electrically measurable signal

• General Electronic Sensor– primary transducer: changes “real world” parameter into

electrical signal

– secondary transducer: converts electrical signal into analog or digital values

• Typical Electronic Sensor System

real

worldanalo

g

signal

primary

transducer

secondary

transducer

sensor

sensor

input

signal

(measurand)

microcontrollersignal processing

communication

sensor data

analog/digital

network

display


Sensors p.6

Example Electronic Sensor Systems

• Components vary with application– digital sensor within an instrument

• microcontroller– signal timing

– data storage

– analog sensor analyzed by a PC

– multiple sensors displayed over internet

µCsignal timing

memory

keypadsensor

sensor display

handheld instrument

PC

comm. card

sensor interfaceA/D, communication

signal processingsensor

e.g., RS232

PC

comm. card

internet

sensor

processor

comm.

sensor

processor

comm.

sensor bus sensor bus


Sensors p.7

Primary Transducers

• Conventional Transducerslarge, but generally reliable, based on older technology

– thermocouple: temperature difference

– compass (magnetic): direction

• Microelectronic Sensorsmillimeter sized, highly sensitive, less robust

– photodiode/phototransistor: photon energy (light)• infrared detectors, proximity/intrusion alarms

– piezoresisitve pressure sensor: air/fluid pressure

– microaccelerometers: vibration

– chemical senors: O2, CO2, Cl, Nitrates (explosives)

– DNA arrays: match DNA sequences


Sensors p.8

Performance terminology• Range- the range of a transducer define the limits between which the input

can vary• Span-maximum value of the input-min. value of the input

– For a load cell measurement of forces might have a range of 0 to 50 kN and a span of 50kN

• Error=measured value – true value of the quantity being measured– A sensor might give a resistance change of 10.2Ω when the true

change is 10.5 Ω . The error is thus -0.3 Ω.• Accuracy-extent to which the value indicated might be wrong.

– Accuracy of ±20C means reading of instrument may lie + or -20C.– Also expressed as % of full range output.– Range 0 to 2000C, accuracy ±5%, means result is expected to lie within

+ or -100C• Sensitivity: relation ship indicating how much output one gets per unit

input.– A resistance thermometer may have a sensitivity of 0.5 Ω/0C.– Many times sensitivity is expressed for input which is not being

measured.


Sensors p.9

• Hysteresis error– Transducers can give different o/p for the same

value of the i/p-depending upon whether it has beengot for increasing value or for decreasing value.


Sensors p.10

Non Linearity Error

• For many transducers a linear relationship between theinput and output is assumed over the working range , i.e.a graph of output plotted against input is assumed togive a straight line.

• Few transducers, however have a truly relationship andthus errors occurs as a result of the assumption oflinearity.

• The error is define as the maximum difference from thestraight line.

• Various methods are used for the numerical expressionof the non-linearity error.

• The difference occur in determining the straight linerelationship which the error is specified.


Sensors p.11

• a- End range values

• b- best straight line for all values (using least square)

• C-best straight line through zero point


Sensors p.12

• Repeatability/reproducibility: The term repeatability andreproducibility of a transducer are used to describe itsability to give same o/p for repeated application of same i/pvalue.– The error resulting from the same output not being given with

repeated applications is usually expressed as a % of the full rangeoutput.

Repeatability=[(max-min value given)/full range]x100

• Stability: The stability of the transducer is its ability to givesame o/p when used to measure a constant input over aperiod of time.– Drift is used to define the change of output over time.– Zero drift is used for changes that occur in output when

there is zero input.


Sensors p.13

• Dead band/time: The dead band or dead space of atransducer is the range of input values for which there is nooutput.– For example, bearing friction in a flow meter using a rotor capacity

mean that there is no output until the input has reached a particularvelocity threshold.

– Dead time : length of time from the application of an inputuntil the output begins to respond the change.

• Resolution: when the input varies continuously over the range,the output signals for some sensors may change in smallsteps.– A wire-wound potentiometer is an example of such a sensor, the output

going up in steps as the potentiometer slider moves from one wire turnto the next.

– The resolution is the smallest change in i/p value that will produce anobservable change in o/p values.

• Output impedance: it is important to know this as sensor iseither connected in series or parallel in circuit.


Sensors p.14

Static & Dynamic Characteristics

• Static characteristic:– Values given when steady state condition occurs i.e.

the values given when the transducer has settle downafter having receiving some input.

• Dynamic Characteristic:– the behavior between the time that the input values

changes and the time that the values given bytransducers settle down to the steady-state value.

• Inputs

• Step input (0 to a constant value)

• Ramp i/p – i/p changed at steady rate.

• Sinusoidal input of a specified frequency.


Sensors p.15

• Response time:– this is the time which pass after a const. i/p (step input) is applied to

the transducer up to the point at which transducer gives valuescorresponding to some specific % of the value of the o/p. (say 95%)

• Time constant:– Time corresponding to 63.2% of o/p– The time constant is a measure of the inertia of the sensor and so how

fast it will react to changes in its i/p.

• Rise time:– Time taken for the o/p to rise to some specific % of steady state

o/p(10% to 90 or 95%).

• Settling time:– Time taken for the o/p to settle to within some % (2% of steady state

value)



Response to a step input

Sensors p.17

• Sensors can also be classified as passive or active .

– In passive sensors, the power required to produce the output is provided by thesensed physical phenomenon itself (such as a thermometer)

– whereas the active sensors require external power source (such as a straingage).

• Furthermore, sensors are classified as analog or digital based onthe type of output signal.

• Analog sensors produce continuous signals that are proportional to the sensedparameter and typically require analog-to-digital conversion before feeding tothe digital controller.

• Digital sensors on the other hand produce digital outputs that can be directlyinterfaced with the digital controller. Often, the digital outputs are produced byadding an analog-to-digital converter to the sensing unit.

• If many sensors are required, it is more economical to choose simple analogsensors and interface them to the digital controller equipped with a multi-channel analog-to-digital converter.


Sensors p.18

• We can further classify transducers according to their

function (displacement, temperature, force)

physical property (inductive, photo-voltaic, piezo-electric)

Sensor output


Sensor output is generally in the form of resistancechange or voltage change or capacitance change orcurrent change when input quantity is changed.Appropriate circuit is required to measure the abovechanges.

Sensors p.19

Displacement Measurements• Measurements of size, shape, and position utilize

displacement sensors

• Examples– diameter of part under stress (direct) – movement of a microphone diaphragm to quantify liquid

movement through the heart (indirect)

• Primary Transducer Types– Resistive Sensors (Potentiometers & Strain Gages)– Inductive Sensors– Capacitive Sensors– Piezoelectric Sensors

• Secondary Transducers– Wheatstone Bridge– Amplifiers


Sensors p.20

Resistance transducer

• Potentiometric principle


The object of whose motion is to be sensed isconnected to the wiper of potentiometer. Themovement changes voltage output. Voltage output willbe linear for a linear potentiometer. These type oftransducers have slow dynamic response, susceptibleto vibration and noise, wear etc.

Sensors p.21

Resistance transducer

Strain gauge principal

• When a wire is stretched, it gets thinner and longer andthe resistance changes. More the wire is strained morethe change in resistance.

• Gf is gage factor, which defines the sensitivity. It isdefined as change in resistance for unit strain. Gagefactor can vary from 2-6 for metallic strain gages.

• For semiconductor it varies from 40 to 200. Gage factorvalue is supplied by the manufacturer gauge principle


Sensors p.22

Wheatstone bridge for measuring resistance change

• Balanced bridge condition

• Potential difference between A & B is zero

• When strain is applied, bridge is in unbalanced condition. Potential difference between A & B is measured by external circuitry.


Sensors p.23

• A Wheatstone bridge is an electrical circuit invented by Samuel Hunter in 1833 andimproved and popularized by Sir Charles Wheatstone in 1843. It is used to measurean unknown electrical resistance by balancing two legs of a bridge circuit, one leg ofwhich includes the unknown component. Its operation is similar to the originalpotentiometer.

• In the figure, R4 is the unknown resistance to be measured; R1, R2 and R3 areresistors of known resistance and the resistance of R3 is adjustable. If the ratio ofthe two resistances in the known leg (R2 / R3) is equal to the ratio of the two in theunknown leg (R1 / R4), then the voltage between the two midpoints (B and A) will bezero and no current will flow through the galvanometer Vg. If the bridge isunbalanced, the direction of the current indicates whether R3 is too high or too low.R3 is varied until there is no current through the galvanometer, which then readszero.

• Detecting zero current with a galvanometer can be done to extremely high accuracy.Therefore, if R1, R2 and R3 are known to high precision, then R4 can be measured tohigh precision. Very small changes in R4 disturb the balance and are readily detected.

• At the point of balance, the ratio of R2 / R1 = R4 / R3

• This setup is frequently used in strain gauge and resistance thermometermeasurements, as it is usually faster to read a voltage level off a meter than toadjust a resistance to zero the voltage.


Sensors p.24

Inductance transducer• Based on Faraday’s law of induction in a coil. The induced voltage, or

electromotive force, is equal to the rate at which the magnetic fluxthrough the circuit changes.

• The inductance change can be caused by any of the following:

a.Variation in the geometry of the coil (change in number of turnsin a coil)

b.Change in the effective permeability of the medium in andaround coil

c.Change in the reluctance of the magnetic path or variation of theair gap

d.Change in mutual inductance (by a change in the coupling betweencoils 1 and 2 with aiding or opposing fields)


Sensors p.25

• The inductance type transducer consists of three parts: a coil, a movablemagnetic core and a pressure sensing element.

• The element is attached to the core and as pressure varies, the elementcauses the core to move inside the coil.

• An AC voltage is applied to the coil, and as the core moves the inductanceof the coil changes. The current through the coil will increase as theinductance decreases.

• For increased sensitivity the coil can be separated into two coils byutilizing a center tap, as shown in fig. As the core moves within the coilsthe inductance of one coil will increase, while other will decrease.


Sensors p.26

Linear Variable Differential Transducer

• LVDT most widely used for measurement of lineardisplacement. It is based on mutual inductance whichchanges with the position of central core.


Primary coil is excited by AC signal.Voltage is induced in secondary coil andamplitude depends on the position ofcore.

LVDTs are very stable, high resolution,high accuracy. Used for large as well asfor small displacements. 1 meter to a cmfull scale measurement.

Dynamic response is 1/10 of excitationfrequency and dependent on inertia ofthe core

Sensors p.27

Linear variable differential transducer• Two secondary coils are connected in series opposing configuration.

At null position output will be zero. When away from null position,output will be in-phase or out of phase depending on the coremovement. Amplitude will be proportional to the position of core inlinear range.


Sensors p.28

Capacitance transducers

• Capacitance between two separated members is used forthe measurement of many physical phenomena. It is afunction of effective area of the conductors, separationbetween the conductors, the dielectric strength of thematerial. Change in capacitance can be brought about byvarying any of the above parameters.


Sensors p.29

• The capacitive transducer or sensor is nothing but the capacitorwith variable capacitance.

• The capacitive transducer comprises of two parallel metal platesthat are separated by the material such as air, which is called asthe dielectric material.

• In the typical capacitor the distance between the two plates isfixed, but in variable capacitance transducers the distance betweenthe two plates is variable.

• In the instruments using capacitance transducers the value of thecapacitance changes due to change in the value of the input quantitythat is to be measured.

• This change in capacitance can be measured easily and it iscalibrated against the input quantity, thus the value if the inputquantity can be measured directly.


Sensors p.30

Applications

• Precision positioning

• Disc drive industry

• Precision thickness measurements

• Non-conductive targets

• Machine tool metrology

• Assembly line testing


Sensors p.31

Piezoelectric transducers

• Piezoelectric material generate electric voltage whendeformed and vice versa. This is a reversible effect.This property is directional and the force to bemeasured is applied normal to the specific plane. Thevoltage across electrode is the charge generated due tomechanical action. The charge generated is proportionalto the magnitude of applied force. This also producessimilar effect in transverse direction.


Sensors p.32

• A piezoelectric transducer is a device whichtransforms one type of energy to another bytaking advantage of the piezoelectric propertiesof certain crystals or other materials.

• When a piezoelectric material is subjected tostress or force, it generates an electricalpotential or voltage proportional to themagnitude of the force. This makes thepiezoelectric transducer ideal as a converter ofmechanical energy or force into electricpotential.


Sensors p.33

• The high sensitivity of the piezoelectric transducermakes it useful in microphones, where it converts soundpressure into electric voltage; in precision balances; inaccelerometers and motion detectors; and as generatorsand detectors of ultrasound.

• Piezoelectric transducers are also used in non-destructive testing, in the generation of high voltages,and in many other applications requiring the precisesensing of motion or force.


Sensors p.34

• The piezoelectric effect also works in reverse,in that a voltage applied to a piezoelectricmaterial will cause that material to bend,stretch, or otherwise deform.

• This deformation is usually very slight andproportional to the voltage applied, and so thereverse piezoelectric effect offers a method ofprecision movement on the micro scale.

• A piezoelectric transducer may thus be used asan actuators for the exact adjustment of fineoptical instruments, lasers, and atomic forcemicroscopes.


Sensors p.35

• These piezoelectric devices can be used both assensors and actuators, so they're referred toas transducers, a term applied to any devicethat can convert one form of energy to another.

• Thus, both piezoelectric sensors andpiezoelectric actuators come under the headingof piezoelectric transducer. The sensor turnsmechanical energy into electric potential, andthe actuator converts electrical energy intomechanical force or motion.


Sensors p.36

Eddy Current Type• Probe contains two coils. One is active and the balance coil is

excited with high frequency 1 MHz. In normal condition the bridgeis balanced. When probe is close to a conducting surface, eddycurrents are formed and disturbs the magnetic field in active coil.Un-balance in the bridge is measure of distance. Eddy currents arestronger when target is closer to sensor. Range from 0.25 mm to 30mm. Target surface should be more than the probe diameter.


Sensors p.37

• Eddy current transducers (eddy current sensors) aredesigned for non-contact measurement of the vibration,display and rotation frequency of conducting objects.They are used for diagnostics of the industrial turbines,compressors, electric motors. Axial displacement andradial rotor shaft vibration are the main subjects tocontrol this way.

• Eddy current transducer (eddy current probe) consistsof non contact eddy probe, extension cable and probedriver. Eddy probe is a metal probe with dielectric tip atthe end and some piece of the coaxial cable at the otherend. The probe is connected to the driver by the coaxialextension cable.


Sensors p.38

Applications

• Electromagnetic braking

• Repulsive effects and levitation

• Attractive effects

• Identification of metals

• Vibration and position Sensing

• Structural testing


Sensors p.39

Hall effect transducers

• When a beam of charged particles passes through amagnetic field, forces act on the particles and the beamis deflected from its straight line path.

• A current flowing in a conductor is like a beam of movingcharges and thus can be deflected by a magnetic field.

• This effect is discovered by E.R.Hall in 1879 and iscalled the hall effect.

• A Hall effect sensor is a transducer that varies itsoutput voltage in response to changes in magnetic field.Hall sensors are used for proximity switching,positioning, speed detection, and current sensingapplications.


Sensors p.40

Hall effect transducers

• Hall effect occurs when a strip of conducting materialcarries current in the presence of a transverse magneticfield. The hall effect results in the production ofelectric field perpendicular to the directions of bothmagnetic filed and the current with the magnitudeproportional to the product of magnetic field strength,current and various properties of the conductor.


In the absence of magneticfield, potential between 3 & 4are same. When magnetic fluxpasses through the conductor asshown, potential V appearsbetween 3 &4.

Sensors p.41

Applications of Hall Sensor

• The Hall sensor is used in some automotive Fuel Level Indicators. A permanent magnet is mounted on the surface of a floating object. The current carrying conductor is fixed on the top of the tank lining up with the magnet. When the level of fuel rises, more amount of magnetic field is applied on the current resulting in higher Hall voltage. As the fuel level decreases, the Hall voltage will also decrease. The fuel level is indicated and displayed by proper signal condition of Hall voltage.

• The Hall sensor is also used in the brushless DC motor to sense the position of the rotor and to switch the transistor in the right sequence.


Sensors p.42

Linear and Rotational Sensors

• Linear and rotational position sensors are two of themost fundamental of all measurements used in a typicalmechatronics system.

• In general, the position sensors produce an electricaloutput that is proportional to the displacement theyexperience. There are contact type sensors such asstrain gage, LVDT, RVDT, tachometer, etc.

• The noncontact type includes encoders, hall effect,capacitance, inductance, and interferometer type. Theycan also be classified based on the range ofmeasurement.


Sensors p.43

• Usually the high-resolution type of sensors such as halleffect, fiber optic inductance, capacitance , and straingage are suitable for only very small range (typicallyfrom 0.1 mm to 5 mm).

• The differential transformers on the other hand, have amuch larger range with good resolution. Interferometertype sensors provide both very high resolution (in termsof microns) and large range of measurements (typicallyup to a meter).

• However, interferometer type sensors are bulky,expensive, and requires large set up time.


Sensors p.44

Acceleration Sensors• Measurement of acceleration is important for systems subject to

shock and vibration. Although acceleration can be derived from thetime history data obtainable from linear or rotary sensors, theaccelerometers whose output is directly proportional to theacceleration is preferred.

• Two common types include the seismic mass type and thepiezoelectric accelerometer.

• The seismic mass type accelerometer is based on the relativemotion between a mass and the supporting structure. The naturalfrequency of the seismic mass limits its use to low to mediumfrequency applications.

• The piezoelectric accelerometer, however, is compact and more

suitable for high frequency applications.


Sensors p.45

• Accelerometer is used to measure linear acceleration.The design is based on the inertial effects associatedwith a mass connected to a moving object through aspring, damper and displacement sensor. Characteristicsof a accelerometer are like spring mass system, secondorder system.


Sensors p.46

• Piezoelectric accelerometer : In this seismic mass isattached to Piezoelectric crystal, which producescharge when it is loaded. Here spring is only forpreloading the crystal. These accelerometers requiresno external power supply. These accelerometersproduces large output for its size.


Sensors p.47

Force, Torque, and Pressure Sensors

• Among many type of force/torque sensors, the straingage dynamometers and piezoelectric type are mostcommon. Both are available to measure force and/ortorque either in one axis or multiple axes.

• The dynamometers make use of mechanical membersthat experiences elastic deflection when loaded. Thesetypes of sensors are limited by their natural frequency.

• On the other hand, the piezoelectric sensors areparticularly suitable for dynamic loadings in a wide rangeof frequencies. They provide high stiffness, highresolution over a wide measurement range, and arecompact.


Sensors p.48

Force measurement

• Strain gage based : Strain gage is thin foil as shownbelow, with a polymer backing material. The resistanceof the foil changes when strained and this change inresistance is measured by Wheatstone bridge.


Wide end loops reduces thetransverse effect.Temperature compensationis required for hightemperature application. Itis skilled job to fix thestrain gage. Used indifferent configuration: Fullbridge, Half bridge andquarter bridge.

Sensors p.49

Force measurement

• Strain gage mounting : Strain gage are also available incombination of two or three at an angle of 450, 900 or1200


Bi-axial stress in a thin walled

pressure vessel

General state of stress on the

surface of a component

Sensors p.50

Pressure measurement

• Bellow type: The pressure change inside the bellowresults in mechanical movement. This can be connectedto LVDT and linear displacement can be measuredcorresponding to pressure change. Dynamic response ispoor.


Sensors p.51

Pressure measurement

• Strain gage based : This consist of a diaphragm, itdeforms when subjected to differential pressure. Thisdeformation is sensed by the strain gage mounted on it.Strain gages are directly etched on the silicondiaphragm along with the bridge and associated circuitryby modern microelectronics technology.


Diaphragm - flat and corrugated

Sensors p.52

Flow Sensors

• Flow sensing is relatively a difficult task. The fluid mediumcan be liquid, gas, or a mixture of the two. Furthermore, theflow could be laminar or turbulent and can be a time-varyingphenomenon.

• The venturimeter and orifice plate restrict the flow and usethe pressure difference to determine the flow rate. Thepitottube pressure probe is another popular method ofmeasuring flow rate. When positioned against the flow, theymeasure the total and static pressures. The flow velocity andin turn the flow rate can then be determined.

• The Rota meter and the turbine meters when placed in theflow path, rotate at a speed proportional to the flow rate.The electromagnetic flow meters use noncontact method.Magnetic field is applied in the transverse direction of theflow and the fluid acts as the conductor to induce voltageproportional to the flow rate.


Sensors p.53

Fluid flow measurement

• Based on Bernoulli’s principle. These are generally pre-calibrated and used. Pressure tappings are taken atprescribed location from the orifice on upstream anddown stream. Maintenance free and inexpensive, notvery accurate.


Sensors p.54

Fluid flow measurement

• Pitot tube : In this arrangement difference betweenstatic and dynamic pressure is the measure of flowvelocity. It is widely used for airspeed measurement.Usually it consist of two concentric tubes. Inner tubewhich measures dynamic pressure connects to one portof differential pressure transducer. Outer tube whichmeasures static pressure is connected to the other portof the pressure transducer.


Sensors p.55

• Rotameter : It consist of a tapered glass tube and afloat. Float rises due to buoyancy and fluid flow. Positionof the float gives the flow rate. It has to be vertical toobtain correct results. Not very accurate.


Sensors p.56

• Ultrasonic flow meters measure fluid velocity by passing high-frequencysound waves through fluid.

• A schematic diagram of the ultrasonic flow meter is as shown in FigureThe transmitters (T) provide the sound signal source. As the wave travelstowards the receivers (R), its velocity is influenced by the velocity of thefluid flow due to the doppler effect.

• The control circuit compares the time to interpret the flow rate. This canbe used for very high flow rates and can also be used for both upstreamand downstream flow.

• The other advantage is that it can be used for corrosive fluids, fluids withabrasive particles, as it is like a noncontact sensor.


Sensors p.57

Temperature Sensors

• A variety of devices are available to measure temperature,the most common of which are thermocouples, thermisters,resistance temperature detectors (RTD), and infrared types.

• Thermocouples are the most versatile, inexpensive, and havea wide range (up to 1200 ° C typical). A thermocouple simplyconsists of two dissimilar metal wires joined at the ends tocreate the sensing junction. When used in conjunction with areference junction, the temperature difference between thereference junction and the actual temperature shows up as avoltage potential.

• Thermisters are semiconductor devices whose resistancechanges as the temperature changes. They are good for veryhigh sensitivity measurements in a limited range of up to 100°C. The relationship between the temperature and theresistance is nonlinear.


Sensors p.58

Applications of Thermisters• Thermistors can be used as current-limiting devices for circuit protection,

as replacements for fuses.

• Thermistors are used as timers in the degaussing coil circuit of most CRTdisplays and televisions.

• Thermistors are used as resistance thermometers in low-temperaturemeasurements of the order of 10 K.

• Thermistors can be used as inrush-current limiting devices in power supplycircuits.

• Thermistors are regularly used in automotive applications. For example,they monitor things like coolant temperature and/or oil temperature insidethe engine and provide data to the ECU and, indirectly, to the dashboard.

• Thermistors can be also used to monitor the temperature of an incubator.

• Thermistors are also commonly used in modern digital thermometers andto monitor the temperature of battery packs while charging.


Sensors p.59

• The RTD s use the phenomenon that the resistance of ametal changes with temperature. They are, however,linear over a wide range and most stable.

• Applications:

• Resistance Thermometers can be used for a wide variety ofindustrial applications.

• A high electrical output can be obtained by using the RTD withmany types of simple resistance bridges. This high output can thenbe fed directly into recorders, temperature controllers,transmitters, or digital readouts which can be calibrated to readvery precise increments of temperature over wide dynamic ranges.RTD's can also be read out on precision laboratory bridges anddigital ohmmeters.


Sensors p.60

• Infrared type sensors use the radiation heat to sense the temperature from a distance. These noncontact sensors can also be used to sense a field of vision to generate a thermal map of a surface.

• Applications:• Night vision, Tracking ,Heating ,Communications ,Spectroscopy,

Meteorology, Thermography, Biological systems


Sensors p.61

Proximity Sensors• They are used to sense the proximity of an object relative to another

object. They usually provide a on or off signal indicating the presence orabsence of an object.

• Inductance, capacitance, photoelectric , and hall effect types are widelyused as proximity sensors.

• Inductance proximity sensors consist of a coil wound around a soft ironcore. The inductance of the sensor changes when a ferrous object is in itsproximity. This change is converted to a voltage-triggered switch.

• Capacitance types are similar to inductance except the proximity of anobject changes the gap and affects the capacitance.

• Photoelectric sensors are normally aligned with an infrared light source.The proximity of a moving object interrupts the light beam causing thevoltage level to change.

• Hall effect voltage is produced when a current-carrying conductor isexposed to a transverse magnetic field. The voltage is proportional totransverse distance between the hall effect sensor and an object in itsproximity.


Sensors p.62

Light Sensors• Light intensity and full field vision are two important measurements

used in many control applications. Phototransistors, photoresistors ,and photodiodes are some of the more common type of lightintensity sensors.

• A common photoresistor is made of cadmium sulphide whoseresistance is maximum when the sensor is in dark. When thephotoresistor is exposed to light, its resistance drops in proportionto the intensity of light. When interfaced with a circuit as shown inFigure and balanced, the change in light intensity will show up aschange in voltage. These sensors are simple, reliable, and cheap,used widely for measuring light intensity.


Sensors p.63

Smart Material Sensors• There are many new smart materials that are gaining more

applications as sensors, especially in distributed sensingcircumstances. Of these, optic fibers, piezoelectric, andmagnetostrictive materials have found applications. Within these,optic fibers are most used.

• Optic fibers can be used to sense strain, liquid level, force, andtemperature with very high resolution. Since they are economicalfor use as in situ distributed sensors on large areas, they havefound numerous applications in smart structure applications such asdamage sensors, vibration sensors, and cure-monitoring sensors.

• These sensors use the inherent material (glass and silica) propertyof optical fiber to sense the environment. Figure illustrates thebasic principle of operation of an embedded optic fiber used tosense displacement, force, or temperature. The relative change inthe transmitted intensity or spectrum is proportional to the changein the sensed parameter.


Sensors p.65

Micro- and Nanosensors• Microsensors (sometimes also called MEMS) are the miniaturized

version of the conventional macrosensors with improvedperformance and reduced cost. Silicon micromachining technologyhas helped the development of many microsensors and continues tobe one of the most active research and development topics in thisarea.

• Vision microsensors have found applications in medical technology. Afiberscope of approximately 0.2 mm in diameter has been developedto inspect flaws inside tubes.

• Another example is a microtactile sensor , which uses laser light todetect the contact between a catheter and the inner wall of bloodvessels during insertion that has sensitivity in the range of 1 mN.

• Similarly, the progress made in the area of nanotechnology hasfuelled the development of nanosensors. These are relatively newsensors that take one step further in the direction ofminiaturization and are expected to open new avenues for sensingapplications.


Sensors p.66

Switch

• A switch is a device that is used for making andbreaking electrical connections in a circuit.There are many types of these devices. Some ofthe more common ones that you may use areshown below. (microswitch, pushbutton, toggle,dip, slide, rotary).


Sensors p.67

• The switch shown in fig is called a SPDT switch, which is short for single-pole, double throw. A SPDT switch has three leads.

• The SPDT switch changes the pole between two different throw positions.

• The SPST switch is a single-pole (SP),single-throw (ST) device that opens or close a singleconnection.


Sensors p.68

Switch Terminology

• Beside the general type of switch (toggle, slide,pushbutton, etc) there are many configurationsof the contacts possible.

• Often you will see a switch in a schematicreferred to as a SPST or DPDT. These standfor Single Pole Single Throw and Double PoleDouble Throw.

• A switch with a single throw has it's lines eitherconnected or unconnected. In other words thereare two terminals with are electricallyconnected only when the switch is activated.


Sensors p.69

• A switch with a double throw has an extra terminalfor each pole so that there are two electrical pathspossible instead of just one.

• Another term used often with switches is NormallyOpen or Normally Closed.

• Most pushbutton-style switches are "normallyopen", meaning that the switch contacts are in theopen-circuit position when the switch is in the non-depressed state.

• Microswitches often have both normally open andnormally closed contacts and a common contact.

• When wiring a touch sensor with a microswitch, it iscustomary to use the normally open mode.


Sensors p.70

• Two common uses of switches in mechatronicsare power cutoff and as an input sensor.

• The microswitch is a type of touch sensors.• A microswitches is a small, momentary switches

that can be attached to bumpers to signal whenthe robot has run into an obstacle.

• A micro switch is housed in a rectangular bodyand has a very small button which is theexternal switching point.

• Usually, micro switches are also equipped withlever arms to reduce the force needed toactuate the switch.


Sensors p.71

Switch Circuitry

• The following figure shows how a single throwswitch can be wired to a sensor input port.

• When the switch is opened, the sensor input ispulled to the +5V supply by the pull up resistor.

• When the switch is closed, the input is tied toground, generating a zero voltage signal.


Sensors p.72

Switch Debouncing

• When mechanical switches are opened or closed, thereare brief current oscillations due to mechanical bouncingor electrical arcing. This phenomenon is called switchbounce.

• A problem that occurs with mechanical switches isswitch bounce.

• When a mechanical switch is switched to close thecontacts, we have one contact being moved towards theother. It hits the other and, because the contactingelements are elastic bounces.

• Similarly, when a mechanical switch is opened, bouncingcan occur. To overcome this problem either hardware orsoftware can be used.


Sensors p.73

• With software, the microprocessor is programmed todetect if the switch is closed and then wait, say 20 ms.After checking that bouncing has come to a close andswitch is in the same close position, the next part of theprogram can take place.

• The hardware solution to the bounce problem is basedon the use of a flip-flop.


Sensors p.74

Proximity Switches

• There are a number of forms of switch which can be activated by the presence of an object in order to give a proximity sensor with an output which is either on or off.


Sensors p.75

• The microswitch is a small electrical switchwhich requires physical contact and a smalloperating force to close the contacts.

• For example, in the case of determining thepresence of an item on a conveyor belt, thismight be actuated by the weight of the item onthe belt depressing the belt and hence a spring-loaded platform under it, with the movement ofthis platform then closing the switch.

• Above figures shows examples of ways suchswitches can be actuated.


Sensors p.76

Reed Switch

• Figure shows the basic form of a reed switch.

• It consists of two magnetic switch contactssealed in a glass tube.


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• When magnet is brought close to the switch,the magnetic reeds are attracted to each otherand close the switch contacts.

• It is a non-contact proximity switch.

• Such a switch is a very widely used for checkingthe closure of door.

• It is also used in tachometers


Sensors p.78

Keypads


Sensors p.79

• A keypad is an array of switches, perhaps thekeyboard of a computer or the touch inputmembrane pad for some device such as amicrowave oven.

• A contact type key of the form generally usedwith a keyboard is shown in fig., depressing thekey plunger forces the contact together withspring returning the key to the off positionwhen the key is released.


Sensors and Transducers

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