PRIMARY SENSOR
Characteristic and types
Definition of a sensor
• Def. 1. (Oxford dictionary)
– A device giving a signal for the detection or measurement
of a physical property to which it responds.
• Def. 2.
– A sensor is a device that receives a signal or stimulus and
response with an electrical signal.
Classification of sensors
Attributes which can be used to classify sensors:
. stimulus
. working principle
. properties (attributes of the characteristic)
. application
Measurements
Heisenberg (1927): ”The momentum and position of a particle can not both
be precisely determined at the same time.”
Measuring activity disturbs the physical process (loading effect).
Measurement error:
That is the difference between the measured value and the true value.
error = measured value - true value
Deterministic errors:
They are repeated at every measurement, e.g. reading offset or bias. Such
errors can be corrected by calibration.
Random errors:
They are caused by several parameters and change in time in an
unpredictable fashion. They can be quantified by mean errors, standard
deviation.
Precision:
Measurements with small deviation
Accuracy:
Measurements with small errors, i.e. small bias and high precision.
Sensor properties
output
factual
ideal
input
A sensor should represent a physical variable as fast and as
accurately as possible.
A sensor is represented by its characteristic.
Ideally, the sensor characteristic is a straight line
SENSOR CHARACTERISTIC
• Full scale input (input span)
– A range of stimuli that can be converted by
one sensor.
• Full scale output (output span)
– Full scale output is the algebraic difference
between the output signals measured with
maximum input stimulus and with minimum
input stimulus applied.
SENSOR CHARACTERISTIC
Accuracy : Error measurement
Sensitivity: change in output for unit change in
input
Resolution: the smallest change in the signal that
can be detected and accurately indicated by a
sensor.
Linearity: the closeness of the calibration curve to
a straight line.
Drift: the deviation from the null reading of the
sensor when the value is kept constant for a
long time.
SENSOR CHARACTERISTIC
Hysteresis: the indicated value depends on
direction of the test (increasing and decreasing)
Repeatability (precision): the maximum deviation
from the average of repeated measurements of
the same static variable.
Dynamic Characteristics: A sensor may have some
transient characteristic. The sensor can be
tested by a step response where the sensor
output is recorded for a sudden change of the
physical variable.
The rise time, delay time, peak time, settling time,
percentage overshoot should be as small as
possible.
Typical specification
Motion sensors
• These transducers measure the following
variables: displacement, velocity, acceleration,
force, and stress.
• Such measurements are used in mechanical
equipment such as servo-systems, robots, and
electrical drive systems.
• Motion sensors include the following types of
devices: potentiometers, resolvers, optical
encoders, variable inductance sensors
(displacement), tachometers (velocity), piezoresistive sensors (strain).
Resolver
• Resolvers are used in
accurate servo and robot
systems to measure angular
displacement. Their signal
can be differentiated to
obtain the velocity.
• The rotor is connected with
the rotating object and
contains a primary coil
supplied by an alternating
current from a source voltage
vref. The stator consists of
two windings separated by
90o, with induced voltages
V01= K vref sin θ
V02= K vref sin θ
Tachometer
• The permanent magnet
generates a steady and
uniform magnetic field.
Relative
• motion between the field and
the rotor induces voltages,
which is proportional
• to the speed of the rotor.
• The inductance gives the
tachometer a certain time
constant so that the
• tachometer cannot measure
fast transient accurately.
Optical encoders
• These are optical devices to
measure angular displacement
and angular velocity.
• A disk of an optical encoder is
connected to the rotating shaft.
• The disk has patterns (holes).
• On one side of the disk there is
a light source and on the other
photo-detectors. When the disk
rotates the light is going
through the holes and the
photo-detectors generate
series of pulses.
• There are two types of optical
encoders: incremental and
absolute.
Optical encoders
• The incremental encoder provides a pulse each
time the shaft has rotated a defined distance.
• The disc of an absolute encoder has several
concentric tracks, with each track having an
independent light source and photo detector.
• With this arrangement a unique binary or Gray
coded number can be produced for every shaft
position.
LVDT
• The two secondary coils are
connected in the opposite phase.
When the core is in the middle
there is no output voltage.
• Moving the core from the central
position unbalances the
secondaries, developing an
output.
Applications:
• To measure linear displacement,
e.g. for measuring tube lengths
in a steel plant,
• applied in linear
servomechanisms, etc.
Vout
displacement
LVDT
Strain gauge
Strain gauge
• When external forces
are applied to a
stationary object,
stress and strain are
the result.
• Stress is defined as
Strain gauge
• Strain is defined as the amount of
deformation per unit length of an object
when a load is applied.
Strain (ε) = ΔL/L
• Typical values for strain are less than
0.005 inch/inch and are often expressed
in micro-strain units:
1 μstrain = 106 strain
Strain gauge
• Strain may be compressive or tensile and
is typically measured by strain gages.
• It was Lord Kelvin who first reported in
1856 that metallic conductors subjected
to mechanical strain exhibit a change in
their electrical resistance.
• This phenomenon was first put to
practical use in the 1930s.
Strain gauge
• Fundamentally, all strain gages are
designed to convert mechanical motion
into an electronic signal.
• A change in capacitance, inductance, or
resistance is proportional to the strain
experienced by the sensor.
Strain gauge
• If a wire is held under tension, it gets
slightly longer and its cross-sectional
area is reduced. This changes its
resistance (R) in proportion to the strain
sensitivity (S) of the wire's resistance.
When a strain is introduced, the strain
sensitivity, which is also called the gage
factor (GF), is given by:
GF = (ΔR/R)/(ΔL/L)
Strain gauge
• The ideal strain gage would change
resistance only due to the deformations
of the surface to which the sensor is
attached.
• However, in real applications,
temperature, material properties, the
adhesive that bonds the gage to the
surface, and the stability of the metal all
affect the detected resistance.
Strain gauge
• Because most materials do not have the
same properties in all directions, a
knowledge of the axial strain alone is
insufficient for a complete analysis.
Poisson, bending, and torsion strains
also need to be measured. Each requires
a different strain gage arrangement.
Strain gauge
• The deformation of an object can be
measured by mechanical, optical,
acoustical, pneumatic, and electrical
means.
• The earliest strain gages were
mechanical devices that measured strain
by measuring the change in length and
comparing it to the original length of the
object.
Strain gauge
• The most widely used characteristic that varies in
proportion to strain is electrical resistance. Although
capacitance and inductance-based strain gages have
been constructed, these devices' sensitivity to vibration,
their mounting requirements, and circuit complexity have
limited their application.
• The photoelectric gage uses a light beam, two fine
gratings, and a photocell detector to generate an
electrical current that is proportional to strain. The gage
length of these devices can be as short as 1/16 inch, but
they are costly and delicate.
Strain gauge
• The first bonded, metallic wire-type strain
gage was developed in 1938. The metallic
foil-type strain gage consists of a grid of
wire filament (a resistor) of approximately
0.001 in. (0.025 mm) thickness, bonded
directly to the strained surface by a thin
layer of epoxy resin
Strain gauge
Strain gauge
Application of Strain gauge
• Strain gages are used to measure displacement, force,
load, pressure, torque or weight. Modern strain-gage
transducers usually employ a grid of four strain elements
electrically connected to form a Wheatstone bridge
measuring circuit.
• The strain-gage sensor is one of the most widely used
means of load, weight, and force detection.
• As the force is applied, the support column experiences
elastic deformation and changes the electrical resistance
of each strain gage. By the use of a Wheatstone bridge,
the value of the load can be measured. Load cells are
popular weighing elements for tanks and silos and have
proven accurate in many other weighing applications.
Application of Strain gauge
• Strain gages may be bonded to cantilever
springs to measure the force of bending.
• The strain gages mounted on the top of the
beam experience tension, while the strain gages
on the bottom experience compression. The
transducers are wired in a Wheatstone circuit
and are used to determine the amount of force
applied to the beam.
Application of Strain gauge
• Strain-gage elements also are used widely in the
design of industrial pressure transmitters. Using
a bellows type pressure sensor in which the
reference pressure is sealed inside the bellows
on the right, while the other bellows is exposed
to the process pressure.
• When there is a difference between the two
pressures, the strain detector elements bonded
to the cantilever beam measure the resulting
compressive or tensile forces.
Application of Strain gauge
• A diaphragm-type pressure transducer is
created when four strain gages are attached to a
diaphragm.
• When the process pressure is applied to the
diaphragm, the two central gage elements are
subjected to tension, while the two gages at the
edges are subjected to compression.
• The corresponding changes in resistance are a
measure of the process pressure. When all of
the strain gages are subjected to the same
temperature, such as in this design, errors due
to operating temperature variations are reduced.
Piezoelectric Materials
• Many polymers, ceramics,
and molecules such as water
are permanently polarized:
some parts of the molecule
are positively charged, while
other parts of the molecule
are negatively charged.
Piezoelectric Materials
• When an electric field is applied
to these materials, these
polarized molecules will align
themselves with the electric
field, resulting in induced
dipoles within the molecular or
crystal structure of the material.
Piezoelectric Materials
Furthermore, a permanentlypolarized material such as
quartz (SiO2) or barium titanate
(BaTiO3) will produce an
electric field when the material
changes dimensions as a result
of an imposed mechanical
force.
These materials are
piezoelectric, and this
phenomenon is known as the
piezoelectric effect.
Piezoelectric Materials
• Conversely, an applied electric
field can cause a piezoelectric
material to change
dimensions.
• This phenomenon is known as
electrostriction, or the reverse
piezoelectric effect.
• Piezoelectric Effect Reverse
Piezoelectric Effect
Piezoelectric Materials
• Piezoelectric materials are
used in acoustic
transducers, which convert
acoustic (sound) waves into
electric fields, and electric
fields into acoustic waves.
Transducers are found in
telephones, stereo music
systems, and musical
instruments such as guitars
and drums.
Piezoelectric Materials
• Quartz, a piezoelectric material, is often found in
clocks and watches. An oscillating electric field
makes the quartz crystal resonate at its natural
frequency. The vibrations of this frequency are
counted and are used to keep the clock or watch
on time.
• A manufacturer has recently embedded
piezoelectric materials in skis in order to damp
out the vibrations of the skis and help keep the
ski edges in contact with the snow.