Physical%20Principles%20of%20Sensing

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Physical Principles of Sensing

• SOPHOMORE CLINIC I
• FALL 2004

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Definition

• A sensor is a device that receives a stimulus and
  responds with an electrical signal.

Fig 1.1
Level control system. A sight tube and the
operators eye form a sensor.
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What are some quantities that can be sensed?

• Motion, position, displacement
• Velocity and acceleration
• Force, strain
• Pressure
• Flow

• Sound
• Moisture
• Light
• Radiation
• Temperature
• Chemical presence

These quantities are the stimulus.
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The Response is an Electrical Signal

• When we say electrical we mean a signal which can
  be channeled, amplified and modified by
  electronic devices
• Voltage
• Current
• Charge

• The voltage, current or charge may be describe
  by
• Amplitude
• Frequency
• Phase
• Digital code

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Any sensor is an energy converter

• This conversion can be direct or it may require
  transducers.
• Example
• A chemical sensor may have a part which converts
  the energy of a chemical reaction into heat
  (transducer) and another part, a thermopile,
  which converts heat into an electrical signal.

Fig 1.2
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Physical Principles of Sensing

• Charges, fields potentials
• Capacitance
• Magnetism
• Induction
• Resistance
• Piezoelectric effect

• Seebeck and Peltier effects
• Thermal properties of materials
• Heat transfer
• Light

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Types of Sensor

• Direct
• A sensor that can convert a non-electrical
  stimulus into an electrical signal with
  intermediate stages.
• Thermocouple (temperature to voltage)
• Indirect
• A sensor that multiple conversion steps to
  transform the measured signal into an electrical
  signal.
• A fiber-optic displacement sensor
• Current ?photons ?current

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Todays TopicPhysical Principles that are Used
to Effect a Direct Conversion of Stimuli into
Electrical Signals
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Electric Charges, Fields, Potentials

• Any charged object is subject to a force when in
  the region of an electric field.
• A field can be used to detect the presence of
  charge or the opposite can be true and the force
  on a charge determined to detect a field.

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Other Geometries
The field is strongest at areas of highest
curvature
?charge/unit length
?charge/unit area
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Electric Dipole

• Dipoles are found in crystalline materials and
  form a foundation for piezoelectric and
  pyroelectric detectors.
• The dipole is a combination of 2 opposite charges
  placed 2a apart. The electric field is the
  vector sum to the two fields.

p represents the dipole moment
In the presence of an E field the dipole will
develop a torque
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Capacitance

• Two isolated conductive objects of arbitrary
  shape which can hold an electric charge is called
  a capacitor.
• An E field is developed between the two
  conductors.

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Capacitor as Displacement Sensor

• If the inner conductor can be moved in and out,
  the measured capacitance will be a function of l.

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Dielectric Constant

• The material between the plates of the capacitor
  can also be used to sense changes in the
  environment.
• When vacuum (or air) is replaced by another
  material, the capacitance increases by a factor
  of ?, known as the dielectric constant of the
  material
• The increase in C is due to the polarization of
  the molecules of the material used as an
  insulator.

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Example A Water Level Sensor

• The total capacitance of the coaxial sensor shown
  below is the capacitance of the water-free
  portion plus the capacitance of the water-filled
  portion. As the level of the water changes, the
  total capacitance changes.

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Magnetism

• There are two methods of generating a magnetic
  field
• Permanent magnets (magnetic materials).
• The magnetic field generated by a current.

Force is generated on a test magnet in the field
of magnetic materials.
A compass needle will respond to the magnetic
field generated by a current.
Magnetic field, B flux is the field density, ?B
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Sources of Magnetic Field
Electric current sets a circular magnetic field
around a conductor.
Moving electron sets a field, superposition of
field vectors results in a combined magnetic
field of a permanent magnet.
Magnets are useful for fabricating magnetic
sensors for the detection of motion,
displacement, and position.
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Induction

• A phenomenon related to magnetism is induction,
  the generation of voltage from a changing
  magnetic field.
• If the coil has no magnetic core, the flux is
  proportional to current and the voltage
  proportional to di/dt.

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Resistance

• If we apply a battery across two points of a
  piece of material, an E field will be set up
  where EV/l

The tendency of the material to resist the flow
of electrons is called its resistivity, ?, and we
say that the material has a particular electrical
resistance, R.
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Sensitivity of Resistance

• To Temperature

Specific resistivity of tungsten as a function of
temperature.
? is the temperature coefficient of resistivity.
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Application for Temperature Indication using a
Laminate of Materials with Two Different ?s.
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To Strain
Strain changes the geometry of a conductor and
its resistance.
Stress Youngs Modulus x strain
Since length is changing the factor of 2 in the
second equation becomes a variable which depends
on the material.
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To Moisture
For the hygristor, the resistance of the polymer
changes with the absorption of water molecules.
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The Piezoelectric Effect

• The piezoelectric effect is the generation of
  electric charge by a crystalline material upon
  subjecting it to stress.

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Piezoelectric Sensor
Because a crystal with deposited electrodes forms
a capacitor the voltage developed can be
expressed as
Where dx is the piezoelectric coefficient in the
x direction and Fx is the applied force in the x
direction.
Piezoelectric crystals are direct converters of
mechanical energy into electrical energy.
Laminated 2-layer piezoelectric sensor
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Pyroelectric Effect
PQ is the pyroelectric charge coefficient and Ps
is related to the charge developed on the
electrodes when the sensor is subjected to heat
flow.
If the sensor has the capacitor form
Pyroelectric materials are crystals capable of
generating an electrical charge in response to
heat flow.
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The material loses its usefulness at the Curie
Temp the point at which polarization disappears.
The electric charge reaches its peak nearly
instantaneously and then decays with a thermal
time constant, ?T
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Seebeck and Peltier Effects

• The Seebeck effect is a direct conversion of
  thermal energy into electric energy.

The varying temperature along the bar is a source
of electromotive force (voltage) and current will
flow. This is the principle behind the
thermocouple.
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Thermoelectric Loops
If a loop of conductor has points at 2 different
temperatures, current flows. But if there is a
single conductor no measurable net current flows.
If a loop of conductor has points at 2 different
temperatures, again current flows. If the loop
is composed of 2 different conductors, measurable
net current flows due to a difference in the
Seebeck coefficients.
?A and ?B are the Seebeck coefficients
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Output Voltage from Standard Thermocouples
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TYPES MATERIALS TEMP RANGES
Thermocouple Type Names of Materials Useful Application Range
B Platinum30 Rhodium () Platinum 6 Rhodium (-) 2500 -3100F 1370-1700C
C W5Re Tungsten 5 Rhenium () W26Re Tungsten 26 Rhenium (-) 3000-4200F 1650-2315C
E Chromel () Constantan (-) 200-1650F 95-900C
J Iron () Constantan (-) 200-1400F 95-760C
K Chromel () Alumel (-) 200-2300F 95-1260C
N Nicrosil () Nisil (-) 1200-2300F 650-1260C
R Platinum 13 Rhodium () Platinum (-) 1600-2640F 870-1450C
S Platinum 10 Rhodium () Platinum (-) 1800-2640F 980-1450C
T Copper () Constantan (-) -330-660F -200-350C
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The Peltier Effect
The Peltier effect concerns the reversible
absorption of heat which usually takes place when
an electric current crosses a junction between 2
dissimilar metals. It can produce heat or cold
depending on the direction of electric current
through the junction.
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Light(Electromagnetic Radiation)
E is the energy of the radiation c c x 108
m/s h 6.63 x 10-23 J-s ? is the wavelength of
the radiation
UV and visible photons have relatively high
energy levels and are easily detected. In the
far IR the energies become very small and thermal
detectors are used.
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Conclusions

• A sensor is a device that receives a stimulus and
  responds with an electrical signal.
• The final stage of any sensor is dependent upon
  the electrical properties of the sensor
  materials.
• The materials introduced today are used in the
  design and fabrication of many different types of
  sensors.