Download EKT314 - 3 - Tranducers

ELECTRONIC
INSTRUMENTATION
EKT314/4
3. Transducers
Contents

Electrical Transducers




Classification
Selection of Transducers
Photoelectric Transducers
Temperature Transducers
2
Transducers …definition

Device that converts energy from one
physical form into another.




Physical variable into signal variable
Sensor – input transducer
Actuator – output transducer
Two types of transducers


Mechanical
Electrical
3
Contents

Electrical Transducers




Classification
Selection of Transducers
Photoelectric Transducers
Temperature Transducers
4
Electrical Transducers

Sensing device that transform the
physical, optical or mechanical quantity
measurement directly to the electrical
voltage or current relative to the input
measurand.
5
Electrical Transducers:
Parameters

Sensitivity


Linearity


Electrical output per unit change in the input
measurand.
Linear relationship between physical parameter
and electrical signal.
Dynamic Range

Can be use under wide range of measurement
conditions.
6
Electrical Transducers:
Parameters …continued

Repeatability


Input output relationship should be
constant over a period of time.
Physical Size

Physically minimal in weight and volume.
7
Electrical Transducers:
Advantages




Electrical amplification and attenuation can be
easily done.
Output can be recorded and indicated
remotely.
Output can be modified to meet the
requirements of indicating or controlling
units.
Signals can be conditioned and mixed to
obtain any combinations.
8
Electrical Transducers:
Advantages …continued





Miniaturisation due to size and shape of
electrical transducers.
Contour design and dimensions can be chose
not to disturb the measurand phenomenon.
Low power required to control the system.
Effects of friction are minimised.
Mass-inertia effects are minimised.
9
Electrical Transducers:
Disadvantages



Low reliability due to ageing and drift of
the active components.
Can be expensive when associated with
the signal conditioner.
Less accuracy and resolution (in some
cases).
10
Transduction elements

Conversion of non-electrical quantity into
electrical signal by the transducers may need
it to be in two parts;




Sensing element – part that responds to the
changes in the physical quantity
Transduction element – this part transforms the
output from sensing element into electrical signals
Transduction elements sometime can be refer
as secondary transducer.
It occupy a different electrical phenomenon.
11
Transduction elements…
continued







Resistive
Inductive
Capacitive
Thermoelectric
Photo emissive
Piezoelectric
Electromagnetic



Photo resistive
Potentiometric
Frequency
generating
12
Contents

Electrical Transducers




Classification
Selection of Transducers
Photoelectric Transducers
Temperature Transducers
13
Classification of Transducers

Two categories:

Active




Self generating devices
Generates electrical signal directly in response
to the physical parameter.
Does not required external power source.
Passive


Operate under energy controlling principles.
Requires external electrical source.
14
Active Transducers
Type of Transducers
Electrical
Parameters
Principle of Operation
Thermocouple and
Thermopile
Voltage and
Current
EMF is generated across the junction of two
different metals or semiconductors when
that junction is heated.
Photovoltaic
Voltage and
Current
Voltage is generated across semiconductors
junction device when radiant energy
stimulated the cell.
Piezoelectric Pickup
Voltage and
Current
EMF is generated when external force is
applied to certain crystalline materials, such
as quartz.
Moving Coil Generator
Voltage and
Current
Voltage is generated from the moving of coil
in magnetic field.
15
Passive Transducers
Type of
Transducers
Electrical
Parameter
s
Principle of Operation
Photomultiplier Tube
Voltage and
Current
Secondary electron emission due to
incident radiation on photosensitive
cathode.
Photoemissive Cell
Voltage and
Current
Electron emission due to the incident
radiation upon photo emissive surface.
Hall effect Pickup
Voltage and
Current
Potential difference generated across a
semiconductor plate when magnetic flux
interacts with the applied current.
Ionisation Chamber
Voltage and
Current
Electron flow induced by ionisation of gas
due to the radio-active radiation.
16
Passive Transducers
…continued
Type of
Transducers
Electrical
Paramete
rs
Principle of Operation
Potentiometer
Resistance
Variation of resistance in a
potentiometer or bridge circuit due
to the positioning of the slider by an
external force.
Thermistor
Resistance
Resistance of certain metal oxide
with negative temperature
coefficient of resistance varies with
temperature.
Photoconductive
Cell
Resistance
Variation of resistance of a cell as a
circuit element with incident of light.
Resistance
Hygrometer
Resistance
Variation of resistance of a
conductive strip with moisture
content.
17
Passive Transducers
…continued
Type of Transducers
Electrical
Parameters
Principle of Operation
Dielectric Gauge
Capacitance
Variation of capacitance due to the changes
of dielectric.
Capacitor Microphone
Capacitance
Sound pressure varies the capacitances
between a fixed plate and a moveable
diaphragm.
Magnetic Circuit
Breaker
Inductance
Variation of self or mutual inductance of an
AC excited coil by changes in the magnetic
circuit.
Reluctance Pickup
Inductance
Reluctance of the magnetic circuits is
varied by changing the position of the iron
core of the coil.
18
Typical Applications of
Transducers








Pressure
Displacement
Force
Torque
Temperature
Sound
Power
Current







Magnetic Flux
Vibration
Velocity
Light
Position
Humidity
…
19
Contents

Electrical Transducers




Classification
Selection of Transducers
Photoelectric Transducers
Temperature Transducers
20
Selection of Transducers

Electrical Output Characteristics


Physical Environments


Compatibility of output impedance, frequency
response and the response time of the transducer
output signal with the recording devices or
measurement system.
Transducer selected should be able to endure the
environmental conditions.
Accuracy

Able to reproduce exact output signal when same
measurand is applied.
21
Selection of Transducers
…continued

Operating Range


Sensitivity


Range of transducer must be large enough to
cover all expected magnitudes of the measurand.
Provide sufficient output signal per unit of
measured input.
Errors

Should be as minimise as possible.
22
Selection of Transducers
…example
Source: RS Data Sheet – Thermistors - 1999
23
Contents

Electrical Transducers




Classification
Selection of Transducers
Photoelectric Transducers
Temperature Transducers
24
Photoelectric Transducers

Primary types




Photoemissive Cell
Photoconductive Cell
Photovoltaic Cell
Photojunctions
25
Photoemissive Cell



Electron emission due to the incident
radiation upon photo emissive surface.
Also known as phototube.
Three basic types of phototube:



Vacuum
Gas-filled
Photomultiplier
26
Photoemissive Cell …continued

Vacuum Phototube




Consists of rod anode and curvature
cathode in the vacuum glass.
Cathode is coated with emissive
materials that emit electrons when light
radiation occur on them.
Stable, consistent characteristics over
time when operate at low voltage and
protected against excessive light.
Moderate sensitivity due to small current
flow in the vacuum tube.
Source: http://cache.eb.com/eb/image?id=62961&rendTypeId=4
27
Photoemissive Cell …continued

Gas-filled Phototube




Same construction as vacuum phototube except
the presence of inert gas, usually argon into tube.
Emitted electrons are accelerated by the electric
field and cause ionisation. Anode current increases
due to high collision.
Provide gain over response up to 10 compared to
vacuum phototube.
Not stable as vacuum type, the characteristics are
not linear.
28
Photoemissive Cell …continued

Photomultiplier Phototube




Consists of evacuated glass envelope containing
photo cathode, anode and dynodes (additional
electrodes).
Each dynode is at higher voltage than the
previous dynode.
Electrons emitted by cathode are attracted to first
dynode and emitted again (secondary emission) to
the following dynodes.
High sensitivity and high frequency response but
large in size and expensive.
29
Photoemissive Cell …continued
Source: [Dally1993] pg. 148
30
Photomultiplier tube
31
Photoconductive Cell





Fabricated from semiconductor materials such as
Cadmium Sulphate (CdS) or Cadmium Selenide
(CdSe).
The increase of current with light intensity while the
voltage remain constant makes the resistance of
semiconductors decrease.
Also known as Photo Resistor, Light Dependent
Resistor (LDR).
Simple, high sensitivity and low cost.
Variations of temperature may affect resistance for a
particular light intensity.
32
Photoconductive Cell…
continued
a)
b)
Source: [Kalsi2005] pg. 421
Construction
Typical Curves of
Resistance vs.
Illumination
33
Photoconductive Cell…
example
The relay of (a) of figure above is to be controlled by a
photoconductive cell with the characteristics shown in (b) in figure
above. The potentiometer delivers 10mA at a 30V setting when the cell
is illuminated with 400 lm/m2 and is required to de-energized when cell
is dark. Calculate (i) the required series resistance, and the (ii) dark
current level. ([Kalsi2005] pg. 422 & 423).
34
Photoconductive Cell…
example
i.
From the characteristic in (b), cell resistance at
400 lm/m2 is 1kΩ.
Therefore,
So,
30V
I
R1  Rcell
30V
30
R1 
 Rcell 
 1000  2000 
I
0.01
35
Photoconductive Cell…
example
ii.
The cell dark resistance is 100kΩ
30
Dark current 
 0.3 mA
2000  100,000
36
Photovoltaic Cell





Semiconductor junction devices for
converting radiation energy into electrical
energy (voltage).
Also known as solar cell.
Conversion efficiency depends on the spectral
content and illumination intensity.
Main advantages is its ability to generate
voltage at fast response.
Can be used as energy converter (power
provider) directly.
37
Photovoltaic Cell …continued
Source: http://www.aurorasolarcar.com/solartech/celltech.html
38
Photojunctions

Photodiodes




Source: [Kalsi2005] pg. 424
Silicon diode with the
lens on its case to focus
the incident of the light
to the junction.
Without bias, operates
like photovoltaic devices
or voltage source.
When reverse bias
operates like
photoconductive device.
Important advantage is
fast response compared
to photoconductive
device. It is also small
and inexpensive.
39
Photojunctions …continued

Phototransistor



Source: [Kalsi2005] pg. 424
NPN device by addition
of junction to
photodiode.
Provide larger output
current compared to
photodiode for a given
amount of illumination on
a very small area.
More sensitive than a
photodiode up to the
factor of 100.
40
Contents

Electrical Transducers




Classification
Selection of Transducers
Photoelectric Transducers
Temperature Transducers
41
Temperature Transducers

Transducers that can be used to
measure temperature.




Resistance Temperature Detectors (RTD)
Thermocouples
Thermistors
Other temperature transducers
42
Resistance Temperature
Detectors (RTD)




Usually make use of platinum, nickel or
resistance wire elements.
Resistance varies with the change of
temperature.
Almost all metals give high resistance when
temperature increases.
High value of temperature coefficient is
required to sense a small changes in the
temperature.
43
Resistance Temperature
Detectors (RTD)… continued

The temperature ranges and coefficient of
resistance of various resistance wire can be
tabulated as in table below;
Material
Range (°C)
Coefficient of Resistance (Ω/Ω/°C)
Platinum
-200 ~ 850
0.0039
Copper
-200 ~ 260
0.0067
Nickel
-80 ~ 300
0.0043
44
Resistance Temperature
Detectors (RTD)… continued

Expression below relate the resistance of the
conductors and the temperature;
Rt  Rref 1  t 




Rt is resistance of the conductor at temperature
t°C
Rref is resistance of the reference temperature
(typically 0°C)
α is temperature coefficient of resistance
Δt is the difference between operating and
reference temperature
45
Resistance Temperature
Detectors (RTD)… continued


Platinum RTD is the most widely used.
Advantages:




Wide operating temperature range.
Stability at high temperature.
Linearity.
Disadvantages:



Low sensitivity.
Expensive.
Easily affected by contact resistance.
46
Resistance Temperature
Detectors (RTD)… continued

Two Lead Wire RTD


Uses Wheatstone’s
Bridge* to measure the
resistance.
Low cost but in order to
achieve high accuracy,
the circuit must be stable
and insensitive to the
variations of ambient
temperature.
*Refer to Kalsi2005 pg. 305 ~ 312
Source: [Kalsi2005] pg. 428
Es is supply voltage, Eo is
output voltage, R1, R2 and
R3 are fixed value resistors,
RL1 and RL2 are the
resistance of the two leads
and RT is resistance of RTD
47
Resistance Temperature
Detectors (RTD)… continued

Three Lead Wire RTD



Practical and accurate
method for most
industrial applications.
The bridge circuit
automatically balance
resistance change due to
ambient temperature
change.
Third lead is has no effect
on the bridge ratios and
balance.
Source: [Kalsi2005] pg. 428
48
Rt  Rref 1  t 
49
Rt  Rref 1  t 
50
Tutorial

Assume an RTD have an output of 90 ohm
at a particular temperature. Calculate what
the temperature is if the following RTD were
used to measure it:




100Ω Platinum RTD (0.0039).
10Ω Silver RTD.
50Ω Nickel RTD with 50 degrees reference.
250Ω Gold RTD @ 25°C.
51
52
Thermocouples


Consists of two different materials that are in thermal
and electrical contact.
Thermoelectric effect - Seebeck effect.

Direct conversion of temperature differences to electrical
voltage.
53
Thermocouples… continued





The junctions at different
temperature causes a circulation of
current.
Open circuit will generate voltage
that is relative to the Seebeck
current.
Electromagnetic force (Thomson
and Peltier) come from the
conductors where the density of
free charge carriers increases with
temperature.
Materials used for wire and sensing
junction temperature effect the
magnitude of the voltage
generated.
Reference temperature junction of
thermocouple also known as cold
junction.
54
Thermocouples… continued

Common thermocouple materials
combination:





Platinum/Platinum-Rhodium
Chromel/Alumel
Chromel/constantan
Copper/constantan
Iron/constantan
55
Thermocouples… continued
Thermocouple output voltage with respect to
temperature for various thermocouple material
Source: [Kalsi2005] pg. 434
56
Thermocouples… continued



Source: [Kalsi2005] pg. 436
If in thermocouple, the voltage
generated by the reference
junction is the same with the
one generated by the sensing
junction, this give null output
provided that both junctions is
at the same temperature.
This can be solve by a process
known as cold junctions
compensation.
The reference junction is now
at 0°C.
57
Thermocouples… continued



The isothermal block is
made of material that is
good conductor of heat but
poor in electricity.
Industries often use an
isothermal block that
contains a thermistor and
two reference junctions.
This setup known as
electronic ice point
reference.
Source: [Kalsi2005] pg. 436
58
Isothermal block
59
Thermocouples… continued

Advantages:




High speed.
Cheap.
Rugged.
Disadvantages:



Low accuracy.
Placed remote from measuring devices.
Reference junctions compensation.
60
Thermistors




Also called thermal resistors as the resistance
varies as a function of temperature.
Manufactured in the form of beads, discs and
rods.
Most thermistors have a negative coefficient
(NTC) of temperature resistance.
Three important characteristics:



Resistance-Temperature
Voltage-Current
Current-Time
61
Thermistors… continued

The resistance of thermistor, RT at a
temperature T can be formulated as
follows; 

R2  R1  e

1 1
 
 T2 T1 
 
From the equation above, b are
constants that can be determined by
the structure and material of thermistor
62
Thermistors… continued
Resistance versus temperature of a NTC Thermistor
Source: [Kalsi2005] pg. 396
63
Thermistors… continued


Major applications of thermistors are
measurement and control of temperature.
Other applications of thermistors:





Measurement of power at high frequencies.
Measurement of thermal conductivity.
Measurement of level, flow and pressure of
liquids.
Measurement of composition of gases.
Vacuum measurement.
64
Thermistors… continued

Advantages





High sensitivity especially
in NTC region.
Fast response over
narrow temperature
range.
Cold junction
compensation is not
required.
No problems on contact
and lead resistance.
Low cost and small size.

Limitations



Characteristics of
resistance and
temperature are nonlinear.
Not recommended for
wide temperature range
applications.
Need a shielded power
lines or filters and low
excitation current.
65
Thermistors… example
The circuit of figure below is used for temperature
measurement. The thermistor is 4kΩ type. The meter is 50mA
meter with a resistance of 3Ω, Rc is set to 17Ω, and supply
voltage Vt is 15V. What will be the meter reading at 77°F
(25°C) and at 150°F. ([Kalsi2005] pg. 398).
66
Thermistors… example
From the plot of temperature versus resistance,
the resistance at 25°C is 4kΩ. So, the current at
25°C is
Vt
15
I

 3.73 mA
Rt
4000  17  3
67
Thermistors… example
At 150°F, the resistance is approximately 950Ω.
The meter reading will be;
Vt
15
I 

 15 .5 mA
Rt
950  17  3
68
Other temperature
transducers

Bimetallic Strip




Two strips of different
metals welded together, in
the form of straight
cantilever beam with one
end fixed.
Due different thermal
coefficient, one of the
metals expands more than
the other when heated.
Commonly used as
thermostat.
Simple, cheap and robust.
69
Other temperature
transducers …continued

Integrated Circuit



Temperature sensing element
and signal conditioning
electronics in single monolithic
integrated circuit package.
Advantages; linearity, cheap
and high output.
Disadvantages; require power
supply, small range, selfheating, slow and limited
configuration.
70
Difference in performance
71
Other temperature
transducers …continued
IC Type Sensor
Source: [Kalsi2005] pg. 441
72
Other temperature
transducers …continued

Radiation Pyrometers




Operation based on StefanBoltzmann law – sense the
temperature from the energy
radiated from heated blackbody
optically.
Construct such that a heat is
focused onto the hot junction of
thermocouple.
Used for measuring a very high
temperature.
Two types of pyrometer that are
fixed focus and variable focus.
73