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Sensors
Submitted by
T.Naga Sasidhar
18E91A0428
CONTEN
TS
Defining sensors and transducers
Classifying sensors
Some important sensor characteristics
Some important physical principles used in sensing.
Advantages of electrical transducers. (sensors)
Sensors and transducers
Sensor is a device that receives a stimulus
and outputs an electrical signal
Upon a stimulus (the quantity, property or
the condition that is being sensed), sensors
generate electrical output.
Transducer is device that converts one form
of energy to another form of energy.
All sensors are transducers but all the
transducers are not necessarily sensors.
We want the stimulus to be converted to
electrical domain rather than any other
domain. (mechanical)
Sensors and transducers (Contd…)
A sensor can comprises of several transducers
•Example
A chemical sensor can comprise of a transducer that converts the
chemical energy to heat and another transducer that converts heat to
electrical signal.
Stimulus
Chemical reaction, Output
electrical signal
The direct sensor is a common part in every sensor and it make use
of different physical principals like hall effect, photoeffect etc.
Sensors and transducers (Contd…)
Thus a sensor can be a direct sensor (one element,
single transducer) or a complex sensor (several
elements, several transducers)
The stimulus for sensors can be any conceivable
physical or chemical characteristic such as
Light flux
Temperature
Pressure
Humidity
Vibration
Position
Displacement …
Classifying sensors
Different sensor classification criteria are available.
Passive
sensors
Generates the output by the applied stimulus itself. No
need additional energy source.
E.g. Thermocouple, piezo electric sensors
Sensors
Active
sensors
In addition to the stimulus there is a power source
applied to the sensor. The stimulus is modulated in the
excitation signal (external energy) and given as the
output.
E.g. Temperature sensitive resistors (Thermistors),
Classifying sensors (Contd…)
Depending on the reference selected
Absolute
sensors
Sense the stimulus with reference to an absolute
physical scale which is independent from the
measurement conditions
E.g. Thermistor
K scale
Sensors
Relative
sensors
Produce the output to the stimulus with respect to a
special case of the stimulus. Not directly connected
with an absolute physical scale.
E.g. Thermocouple
Sensor characteristics
We will look at the sensor as a black box and
consider some of its characteristics that characterize
the stimulus, output and the transition.
These sensor characteristics are significant when
selecting a sensor to a particular application.
Some may not be applicable for all type of sensors.
Frequency response and bandwidth
The range of frequencies within which thesensor
can be effectively used is called the
bandwidth. Usually defined as 3dB bandwidth.
That is the frequency range within which the
output lies between the (half of the maximumoutput)
and the (maximum output).
Sensor characteristics - Sensitivity
Sensitivity refers to the ability of the sensor to
produce a change in output for a unit change in
the stimulus.
Defined as Change of output/Change of input
Unit depends on the actual sensor which is
under interest.
Static error –The difference between the
measured and the actual values.
Error_Perc entage
Error Act
ual_Value
100 %
Sensor characteristics (Contd…)
Full Scale Input (Span) The dynamic range
of stimuli that the sensor can sense is called
the span. It gives the highest possible input
that can be sensed with an acceptable level
of accuracy. Sometimes the span is given by
decibels. (dB)
Eg: Thermistor from -900C to 1300C
Thermocouple from -1850C to 17000C
Capacitive Proximity Sensor - from 1mm
to 5cm
Sensor characteristics (Contd…)
Full Scale Output (FSO) This is the
algebraic difference between the
electrical output signals obtained with
the highest possible input and the
lowest possible input respectively.
Eg: 0V 5V for temperature sensor1
4-20mA for temperature sensor2
0-24V for load cell1
Sensor characteristics (Contd…)
Accuracy Tells how inaccurate the sensor is !!!
Measured as the highest deviation of a value
indicated by the sensor, from the actual (true) value
at that input.
The accuracy (inaccuracy) is expressed as,
Directly in terms of the measured output value
As a percentage of the input full scale (span)
In terms of the output signal
E.g. A piezo-resistive pressure sensor having 100 kPa span and
10Ω full scale output, has an accuracy of either +-0.5%, + 500Pa or +-0.05Ω
Sensor characteristics (Contd…)
Hysteresis Output of some sensors not only depend on the
present stimuli but also depend on the past inputs. (history).
Because of this the output can be different
for the same value of input when the
measurement is taken for increasing input
and decreasing input separately.
Physical and structural changes in
materials and friction can cause the
hysteresis
Sensor characteristics (Contd…)
Saturation – At some levels of the input stimuli the
output will no longer be responsive to the input.
Then further increase in stimuli will not produce
considerable output .
After the linear span the sensor
exhibits span end nonlinearity or
saturation.
Sensor characteristics (Contd…)
Repeatability
Ability of the sensor to produce the same
output in different measurement trials under the same
conditions.
Repeatability error is the inability to do so. Repeatability
error can be caused by build up charges, material plasticity,
thermal noise.
Dead band
Tells the insensitivity of a sensor over a given
range of input signal.
Sensor characteristics (Contd…)
Resolution The smallest change in the stimuli that
is capable of resulting a measurable output
difference.
Output impedance Plays an important role when
interfacing the sensors with other circuitry.
Sensor having voltage output
Sensor having current output
Sensor characteristics (Contd…)
Reliability Ability of a sensor to perform its function
under stated conditions for a given period of time.
Usually this figure is expressed as a probability that
the sensor will function properly with out a failure
over a given period of time.
But not commonly specified by the sensor
manufacturers.
Can be expressed as a MTBF (Mean Time Between
Failure) figure.
Some physical principles used for sensing
Capacitance
Inductance/Magnetism –
Resistance –
Temperature sensitivity
Strain sensitivity
Moisture sensitivity
Piezoelectric effect
Pyroelectric effect
Hall effect Seebeck effect
Sound waves Heat transfer -
Advantages of electrical transducers
(sensors)
Electrical amplification and attenuation is possible
Effect of friction is minimized.
Mass inertia effects are minimized.
Output can be indicated and recorded remotely.
Output can be modified or conditioned in order to
meet the requirement imposed by the other control
elements.
The electrical/electronics based systems can be
controlled with a small power.
Electrical signal can be easily transmitted for further
manipulation.
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