25633083 sensors-and-transducers

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Chapter 2

Sensors and Transducers

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2.1 Sensors and Transducers Sensors are electronics devices that

measure the physical quantity or produces a signal relating to the quantity being measured.

Physical quantities can be temperature, pressure, light, current, weight etc.

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2.1 Sensors and Transducers Transducers are defined as

elements that when subject to some change experience a related change.

Thus we can say sensors are transducers, but a measurement system may use transducers in addition to the sensors.

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2.2 Performance Terminology The following terms are associated with

the performance of transducers and/or measurement system as a whole.

Range of a transducer is the limits with in which the input can vary.Thus a load cell having lower limit of 0 KN and higher limit of 50 KN has a range of 0 to 50 KN.

Span is the maximum value of the input minus the minimum value. For the above load cell the span is 50 KN.

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2.2 Performance Terminology Error is the difference between the

result of the measurement and the true value of the quantity.Error = Measured value - True valuee.g. if measurement system gives a temperature reading of 25ºC when actual is 24ºC, then the error is +1ºC. If actual temperature had been 26ºC then error would have been -1ºC.

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2.2 Performance Terminology Accuracy is the extent to which the

value indicated by a measurement system might be wrong.e.g. a temperature measuring instrument might be specified as having an accuracy of ±2 ºC. This would mean that the reading given by the instrument can be expected to lie with in + or -2ºC of the true value.

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2.2 Performance Terminology Sensitivity is the relationship indicating

how much output we will get per unit input i.e. output/input.e.g. a resistance thermometer may have a sensitivity of 0.5 Ω/ºC.

Hysteresis Error Transducers can give different outputs from the same value of quantity being measured according to whether that value has been reached by a continuously increasing change or a continuously decreasing change.

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2.2 Performance Terminology

Hysteresis Curve is shown in figure below.

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2.2 Performance Terminology Non-Linearity Error: For many

transducers a linear relationship between the input and output is assumed over the working range. But only few of transducers have a truly linear relationship and hence errors occur as a result of the assumption of linearity. The error is the maximum difference from straight line.

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2.2 Performance Terminology

Fig. Non-Linearity Errors usinga) End Range Valuesb) Best Straight Line for all valuesc) Best Straight Line through zero point

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2.2 Performance Terminology Repeatability/Reproducibility

This term is used to describe the ability of a transducer to give the same output for repeated applications of the same input value.Repeatability = (Max. – Min. Values given)/(Full Range) * 100

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2.2 Performance Terminology Stability

The stability is the ability of a transducer to give the same output when used to measure a constant input over a period of time.The term drift is often used to describe the change in output that occurs over time.

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2.2 Performance Terminology Dead band/time

The dead band or dead space of a transducer is the range of input values for which there is no output.The dead time is the length of time from the application of an input until the output begins to respond and change.

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2.2 Performance Terminology Resolution

The resolution is the smallest change in the input value that will produce an observable change in the output.

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2.2.1 Static and Dynamic Characteristics

The Static characteristics are the values given when steady-state conditions occur, i.e. the values given when the transducer has settled down after having received some input.

The Dynamic characteristics refer to the behaviour between the time that the input value changes and the time that the value given by the transducer settles down to the steady-state value.

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2.2.1 Static and Dynamic Characteristics

1-Response TimeThis is the time which elapses after a constant input is applied to the transducer up to the point at which the transducer gives an output corresponding to some specified percentage.e.g. if a mercury in glass thermometer is put in hot liquid there can be an appreciable time lapse, before the thermometer indicates the actual temperature of the liquid.

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2.2.1 Static and Dynamic Characteristics

Time ConstantThe time constant is the measure of the inertia of the sensor and so how fast it will react to changes in its input. The bigger the time constant the slower will be the reaction of a sensor to a changing input signal.

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2.2.1 Static and Dynamic Characteristics

Rise TimeThis is the time taken for the output to rise to some specified percentage of the steady state output. Often the rise time refers to the time taken for the output to rise from 10% of the steady-state value to 90 or 95% of the steady-state value.

Settling TimeThis is the time taken for the output to settle to with in some percentage, e.g. 2% of the steady-state value.

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2.3 Displacement, Position and Proximity

Displacement sensors are concerned with the measurement of the amount by which some object has been moved.

Position sensors are concerned with the determination of the position of some object with reference to some reference point.

Proximity sensors are a form of position sensor and are used to determine when an object has moved to within some particular critical distance of the sensor.

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2.3 Displacement, Position and Proximity

Following points should be considered in mind while selecting a displacement, position or proximity sensor.1-Size of Displacement.2-Type of Displacement (linear/angular).3-Required Resolution.4-Accuracy Required.5-Material of the measured object.6-The Cost.

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2.3 Displacement, Position and Proximity

Displacement and Position sensors are divided into two basic types;

Contact sensors in which the measured object comes into mechanical contact with the sensor.

Non-Contacting sensor in which there is no physical contact between the measured object and the sensor.

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2.3.1 Potentiometer Sensor A Potentiometer consists of a resistance

element with a sliding contact which can be moved over the length of the element.

Such element can be used for linear or rotary displacements, the displacement being converted into potential difference.

The rotary potentiometer consists of a circular wire wound track or a film of conductive plastic over which a rotatable sliding contact can be rotated.

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2.3.1 Potentiometer Sensor

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2.3.1 Potentiometer Sensor With the constant input voltage Vs, between

terminal 1 and 3, the output voltage Vo between terminal 2 and 3 is a fraction of the input voltage.

This fraction depends upon the ratio of the resistance R23 between terminal 2 and 3 compared with the total resistance R13 between terminal 1 and 3. i.e.Vo/Vs = R23/R13

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2.3.2 Strain-gauged Element The electrical resistance strain gauge is a metal

wire, metal foil strip, or a strip of semiconductor material which is wafer like and can be struck in to surfaces like a postage stamp.

When it is subjected to strain, its resistance R changes, the fractional change in resistance

ΔR/R = Gεwhere G, is the constant of proportionality and it is termed as gauge factor.

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2.3.2 Strain-gauged Element

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2.3.2 Strain-gauged Element Since strain is the ratio (change is length/

original length) then the resistance change of the strain gauge is a measurement of the change in length of the element to which the strain gauge is attached.

A problem with all strain gauges is that their resistance not only changes with strain but also with temperature. So to get an accurate result various ways of temperature elimination are used.

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2.3.3 Capacitive Element The capacitance C of a parallel plate

capacitor is given by;C = (εr.εo. A)/d

where, εr is the relative permittivity of the dielectric between the plates, εo is the permittivity of free space, A the area of overlap between the two plates and d the plate separation. Capacitive sensors used to measure linear displacements are shown in next slide.

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2.3.3 Capacitive Element Capacitor a) is used to measure

displacement by plate separation d. Capacitor b) is used to measure

displacement by overlap area A. Capacitor c) is used to measure

displacement by dielectric motion.

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2.3.3 Capacitive Element For the displacement changing the

plate separation, if the separation d is increased by displacement x then the capacitance becomes;C- ΔC = (εr.εo. A)/(d+x)Change in capacitance as a fraction of the initial capacitance is given by; ΔC/C = - (x/d)/[1+(x/d)]

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2.3.4 Differential Transformers The Linear Variable Differential Transformer

(LVDT) consists of three coils symmetrically spaced along an insulated tube.

The central coil is the primary coil and the other two are identical secondary coils which are connected in series in such away that their outputs oppose each other.

A magnetic core is moved through the central tube as a result of the displacement being monitored.

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2.3.4 Differential Transformers When there is an

alternating voltage inputto the primary coil, alternatinge.m.fs are induced in the secondary coil.

With the magnetic core central, the amount of magnetic material in each of the secondary coils is the same.

But when the core is displaced from the central position there is a greater amount of magnetic core in one coil than the other, e.g. more in secondary coil2 than coil 1.

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2.3.4 Differential Transformers The result is that a greater e.m.f is induced

in one coil than the other. There is then a net output from the two coils.

Since a greater displacement means even more core in one coil than the other, the output, the difference between the two e.m.fs increases the greater the displacement being monitored.

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2.3.4 Differential Transformers LVDTs have operating ranges from about ±2mm to

±400mm with non-linearity errors of about ±0.25%. LVDTs are very widely used as primary transducers

for monitoring displacements. The free end of the core may be spring loaded for contact with the surface being monitored, or threaded for mechanical connection.

They are also used as secondary transducers in the measurement of force, weight and pressure; these variables are transformed in to displacements which can be monitored by LVDT’s.

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2.3.5 Eddy Current Proximity Sensor If a coil is supplied with an alternating

current, an alternating magnetic field is produced. If there is a metal object in close proximity to this alternating magnetic field, then eddy currents are induced in it.

The eddy currents themselves produce a magnetic field. This distorts the magnetic field responsible for their production.

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2.3.5 Eddy Current Proximity Sensors As a result, the impedance of the coil

changes and so the amplitude of the alternating current. At some preset level, this change can be used to trigger a switch.

This type of sensor is used for detection of non-magnetic but conductive materials. They are inexpensive, small in size, highly reliable and are very sensitive to small displacements.

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2.3.6 Inductive proximity Switch

This consists of a coil wound round a core. When the end of the coil is close to a metal object its inductance changes. This change can be used to trigger a switch.

It is used for detection of metal objects and is best with ferrous metals.

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2.3.8 Pneumatic Sensors Pneumatic sensors involve the use of

compressed air, displacement or the proximity of an object being transformed in to a change in air pressure.

Low pressure air is allowed to escape through a port in the front of the sensor. This escaping air in the absence of any close by object, escapes and in doing so also reduces the pressure in the nearby sensor output port.

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2.3.8 Pneumatic Sensors

But if there is a close by object, the air cannot so readily escape and the result is that the pressure increases in the sensor output port. The output pressure from the sensor thus depends on the proximity of objects.

Typically 3-12mm displacements can be measured by this sensor.

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2.3.9 proximity Switches There are many forms of switches which are

activated by the presence of an object, to give an output to sensor which is either on or off.

Microswitch is a small electrical switch which requires physical contact and a small operating force to close the contacts.

On a conveyor belt presence of an item is determined by the weight on the belt.

Lever operated, Roller Operated and Cam Operated switches are examples of Proximity Microswitches.

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2.3.9 Proximity Switches Reed Switch consists of two magnetic switch

contacts sealed in a glass tube. When a magnet is brought close to the switch,

the magnetic reeds are attracted to each other and close the switch contacts.

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2.3.9 Proximity Switches Photosensitive devices can be used to detect

the presence of an opaque object by it breaking a beam of light, or infrared radiation, falling on such a device or by detectingthe light reflected back bythe object.

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2.3.10 Hall Effect Sensors

Home Work for Students

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2.4 Velocity and Motion These sensors are used to monitor

linear and angular velocities and detect motion.

The following are the main types of these sensors;

1- Incremental Encoders2- Tachogenerator3- Pyroelectric Sensors.

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2.4.1 Incremental Encoders The incremental encoders are used

to determine angular velocity by measuring number of pulses produced per second.

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2.4.1 Incremental Encoders A beam of light passes through slots in a disc

and is detected by a suitable light sensor. When the disc is rotated, a pulsed output is

produced by the sensor with the number of pulses being proportional to the angle through which the disc rotates.

Hence rotation of disc can be obtained by number of pulses produced.

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2.4.2 Tachogenerator A Tachogenerator is used to measure

angular velocity. Variable Reluctance Tachogenerator is most commonly used form of tachogenerator.

It consists of a toothed wheel of ferromagnetic material which is attached to the rotating shaft.

A pick-up coil is wound on a permanent magnet.

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2.4.2 TachogeneratorVariable Reluctance Tachogenerator:

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2.4.2 Tachogenerator As the wheel rotates, so the teeth move

past the coil and the air gap between the coil and ferromagnetic material changes.

Thus we have a magnetic circuit with an air gap which periodically changes.

As a result flux linked by a pick-up coil changes which in turn produces an alternating e.m.f. in the coil.

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2.4.2 Tachogenerator If wheel contains ‘n’ teeth and rotates

with an angular velocity ω, then the flux change with time for the coil is given as;Ф= Ф ̥ + Фa cos nωtwhere Ф ̥ is the mean value of flux and Фa the amplitude of the flux variation. The induced e.m.f. e in the N turns of the pick-up coil is thus:

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2.4.2 Tachogeneratore= -N d(Ф)/dt =-N d/dt (Ф ̥ + Фa cos nωt) = N Фa n ω sin nωt

We can write;e= Emax sin ωtwhere the maximum value of the induced e.m.f. Emax is NФanω and so it is the measure of the angular velocity.

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2.4.3 Pyroelectric Sensor Pyroelectric Materials are crystalline

materials which produce charge in response to heat flow.

When such a material is heated to a temperature just below the Curie temperature in an electric field and material cooled while remaining in the field, electric dipoles with in the material line up and it becomes polarised.

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2.4.3 Pyroelectric SensorCurie Temperature:

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2.4.3 Pyroelectric Sensor When the field is removed the

material retains its polarisation. When the pyroelectric material is

exposed to infrared radiation, its temperature rises and this reduces the amount of polarisation in the material.

Hence the dipoles being shaken up more and losing their alignment.

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2.4.3 Pyroelectric Sensor Pyroelectric sensor consists of a

polarised pyroelectric crystal with thin metal film electrodes on opposite faces.

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2.4.3 Pyroelectric Sensor Because the crystal is polarised with

charged surfaces, ions are drawn from the surrounding air and electrons from any measurement circuit connected to the sensor to balance the surface charge.

If than infrared radiation is incident on the crystal and changes its temperature, the polarisation in the crystal is reduced thus results in charge reduction on the crystal surface.

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2.4.3 Pyroelectric Sensor The excess charge leaks away through

measurement circuit until the charge on the crystal once again is balanced by the charge on the electrodes.

The pyroelectric sensor thus behaves as a charge generator which generates charge when there is a change in its temperature. The relationship between change in charge Δq is proportional to the change in temperature Δt; Δq = kp Δt (Kp = sensitivity constant)

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2.5 Force A spring balance is an example of

force sensor in which a force, a weight, is applied to the scale pan.

This causes a displacement, i.e. the spring stretches. This displacement is then a measure of the force.

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2.5.1 Strain Gauge Load Cell The most widely used form of force-

measuring transducer is based on the use of electrical resistance strain gauges.

These are used to monitor the strain produced in some member when stretched, compressed or bent by the application of the force.

This arrangement is generally known as Load Cell.

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2.5.1 Strain Gauge Load CellLoad Cell:

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2.5.1 Strain Gauge Load Cell This is a cylindrical tube to which strain

gauges have been attached. When forces are applied to the cylinder to

compress it, then strain gauges give a resistance change.

This resistance is the measure of the strain and hence applied forces can be determined from it.

A signal conditioning circuit is used to eliminate the effect of temperature because temperature has an effect on resistance.

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2.6 Fluid Pressure Fluid pressure in industrial applications

can be measured by monitoring the elastic deformation of diaphragms, capsules, bellows and tubes.

The type of pressure measurements that can be required are, Absolute Pressure (where pressure is measured relative to zero-pressure), Differential Pressure (where a pressure difference is measured) and Gauge Pressure (where the pressure is measured relative to barometric pressure).

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2.6 Fluid Pressure In a diaphragm, when there is a

difference in pressure than the centre of diaphragm becomes displaced/bends.

This form of movement can be monitored by some form of displacement sensors e.g. strain gauge.

Corrugation in diaphragm results in greater sensitivity.

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2.6 Fluid Pressure Diaphragms a) Flat b) Corrugated

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2.6 Fluid Pressure Capsules can be considered to be just two

corrugated diaphragms combined and give greater sensitivity.

Bellow is a stake of capsules and even more sensitive.

A bellow can be combined with a LVDT to give a pressure sensor with an electrical output.

Capsules and Bellows are made of materials such as Stainless Steel, Phosphor Bronze, Nickel, with rubber and nylon.

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2.6 Fluid Pressure

Capsules, Bellows and LVDT with Bellows:

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2.6 Fluid Pressure A different form of deformation is obtained

using a tube with an elliptical cross-section. Increasing the pressure in the tube causes

it to tend to more circular cross-section. When such a tube is in the form of C-

shaped tube it is known as Bourdon Tube. The C opens to some extent when the pressure in the tube increases.

These are made up of materials as stainless steel and phosphor bronze.

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2.6 Fluid Pressure Tube pressure sensors:

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2.6.1 Piezoelectrical Sensors Piezoelectric materials are those which when

stretched or compressed generate electric charges with one face of the material becoming positively charged and the opposite face negatively charged.

As a result voltage is produced. During stretching or compressing charge

distribution in the crystal takes place so that there is a net displacement of charge.

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2.6.1 Piezoelectrical Sensors The net charge q on a surface is proportional

to the amount x by which the charges have been displaced, and since the displacement is proportional to the applied force F;q = kx = SFWhere k is a constant and S a constant termed the charge sensitivity and it depends upon the material and orientation of its crystals.

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2.6.1 Piezoelectrical Sensors Piezoelectric Sensors

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2.6.1 Piezoelectrical Sensors Metal electrodes are deposited on

opposite faces of the piezoelectric crystal. The capacitance C of the piezoelectric material between the plates is;C = (εo εr A)/twhere εr is the relative permittivity of the material, A is area and t its thickness.

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2.6.2 Tactile Sensor Tactile sensor is a particular form of pressure sensor and

used on the finger tips of robots to determine contact of hand with object.

They are also used for touch display screens where a physical contact has to be sensed.

One form of tactile sensor uses piezoelectric polyvinylidene fluoride (PVDF) film.

Two layers of film are used and are separated by a soft film which transmit vibrations.

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2.6.2 Tactile Sensors The lower PVDF film has an alternating

voltage applied to it and this results in mechanical oscillations of the film.

The intermediate film transmit these vibrations to the upper PVDF film.

These vibrations cause an alternating voltage to be produced across the upper film.

When pressure is applied to the upper PVDF film its vibrations are effected and the output alternating voltage is changed.

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2.6.2 Tactile SensorsTactile Sensor

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2.7 Liquid Flow The traditional methods of measuring the flow

rate of liquids involves devices based on the measurement of pressure drop occurring when a liquid flows through a constriction.

For a horizontal tube, where v1 is the fluid velocity, P1the pressure and A1 the cross-sectional area of the tube prior to the constriction.

v2 the velocity, P2 the pressure and A2 the cross-section area at the constriction, ρ the fluid density. Then Bernoulli’s equation gives;

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2.7 Liquid Flow

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2.7 Liquid Flow Since mass of liquid passing per second

through the tube prior to the constriction, we have A1v1ρ = A2v2ρ.

The quantity Q of liquid passing through the tube per second is A1v1=A2v2, hence

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2.7 Liquid Flow Thus it is seen that quantity of fluid

flowing through the pipe per second is proportional to √(pressure difference).

Measurements of pressure difference can thus be used to give a measure of the rate of flow.

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2.7.1 Orifice Plate The Orifice plate is simply a disc, with a

central hole, which is placed in the tube through which the fluid is flowing.

The pressure difference is measured between a point equal to the diameter of the tube upstream and a point equal to half the diameter downstream.

It is cheap, simple with no moving parts but does not work well with slurries.

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2.7.1 Orifice Plate

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2.7.2 Turbine Meter The turbine flow meter consists of a multi-

bladed rotor that is supported centrally in the pipe along which the flow occurs.

The fluid flow results in rotation of the rotor, the angular velocity being proportional to the flow rate.

The rate of revolution of the motor can be determined by using a magnetic pickup.

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2.7.2 Turbine Meter The pulses are counted and so the

number of revolutions of the rotor can be determined.

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2.8 Liquid Level The level of liquid in a vessel can be

measured directly by monitoring the position of the liquid surface or indirectly by measuring some variable related to the height.

Direct methods involve floats while indirect methods include the monitoring of the weight of the vessel by load cells.

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2.8.1 Floats A direct method of monitoring the level

of liquid in a vessel is by monitoring the movement of a float.

The displacement of the float causes a level arm to rotate and so move a slider across a potentiometer.

The result is an output of a voltage related to the height of liquid.

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2.8.1 Floats Other forms of this involve the

lever causing the core in a LVDT to displace, or stretch or compress a strain gauged element.

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2.8.2 Differential Pressure Two basic types of instruments are used for

measurement of differential pressure. In figure 2.48 (a), the differential pressure cell

determines the pressure difference between the liquid at the base of the vessel and atmospheric pressure. The vessel is being open to the atmospheric pressure.

In figure 2.48 (b) the differential pressure cell monitors the difference in pressure between the base of the vessel and the air or gas above the surface of the liquid.

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2.8.2 Differential Pressure

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2.9 Temperature Temperature can be measured by

changes it causes in the form of expansion or contraction of gases, liquids or solids.

The change in electrical resistance of conductors, semiconductors and thermoelectric e.m.f.s.

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2.9.1 Bimetallic Strips Bimetallic Strips consists of two different

metal strips bonded together. The metals have different coefficients of

expansion and when temperature changes the composite strip bends into a curved strip, with the higher coefficient metal on the outside of the curve.

This deformation may be used as a temperature-controlled switch, meaning that the switch contacts close at a different temperature from that at which they open.

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2.9.1 Bimetallic Strips

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2.9.2 Resistance Temperature Detectors (RTDs) The resistance of most metals

increases, over a limited temperature range, in a reasonably linear way with temperature. The relationship is as;

Rt = R0 (1 + at)Where Rt is the resistance at temperature t °C, R0 is resistance at temperature 0 °C and a is temperature coefficient of resistance.

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2.9.2 Resistance Temperature Detectors (RTDs) Resistance Temperature Detectors are simple

resistive elements in the form of coils of wire of metals as platinum, nickel or nickel-copper alloys.

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2.9.3 Thermistors These are small pieces of material made from

mixtures of metal oxides, such as those of chromium, cobalt, iron, manganese and nickel.

These metal oxides are semiconductors. The material is formed into various forms of

elements such as beads, discs and rods. The resistance of conventional metal oxide

thermistors decreases in a very non-linear way with an increase in temperature.

These thermistors have negative temperature coefficients (NTC).

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2.9.3 Thermistors

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2.9.3 Thermistors The change in resistance per degree change in

temperature is considerably larger than that which occurs with metals. The equation for resistance-temperature for a thermistor can be written as; β/t

Rt = K (e)Where Rt is resistance at temperature t, with K and β being constants.

They are small in size and hence respond very rapidly to changes in temperature. They give very large changes in resistance per degree change in temperature.

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2.9.5 Thermocouples If two different metals are joined

together, a potential difference occurs across the junction.

The potential difference depends upon the metals used and the temperature of the junction.

A thermocouple is a complete circuit involving two such junctions. If both the junctions are at same temperature than there is no net e.m.f.

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2.9.5 Thermocouples

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2.9.5 Thermocouples The value of this e.m.f E depends on the two metals

concerned and the temperature t of both junctions. Usually one junction is held at 0°C and then to a

reasonable extent, the following relationship holds;E = at + b (t)²Where a and b are constants for metals concerned. Table 2.1 shows commonly used thermocouples with temperature ranges and sensitivities. Figure 2.55 shows thermoelectric e.m.f – temperature graphs of these thermocouples.

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2.9.5 Thermocouples

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2.10 Light Sensors Photodiodes are semiconductor junction

diodes which are connected in a circuit in reverse bias, giving a very high resistance.

So when light falls on the junction the diode resistance drops and current in the circuit rises appreciably.

A photodiode can thus be used as a variable resistance device controlled by the light incident on it. These have a very fast response to light.

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2.10 Light Sensors Phototransistors have a light-sensitive

collector-base p-n junction. When there is no incident light there is

a very small collector-to-emitter current.

When light is incident, a base current is produced that is directly proportional to the light intensity.

This leads to the production of a collector current which is then a measure of the light intensity.

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2.10 Light Sensors Photoresistor has a resistance

which depends on the intensity of the light falling on it, decreasing linearly as the intensity increases.

The cadmium sulphide photoresistor is most responsive to light.

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2.11 Selection of sensors Following factors needs to be considered

while selecting a sensor.1- The nature of the measurement

required.2- The nature of the output required.3- Then possible sensors can be identified

taking into account such factors as range, accuracy, speed of response, reliability, availability, cost.

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2.12 Inputting data by switches

Home Work for Students