# Transducers

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```					Transducers
ME 3251
Thermal Fluid Systems

1
Transducers
• Transform values of physical variables into
equivalent electrical signals
• Converts a signal from one form to
another form

2
Types of Transducers
• Variable-Resistance        • Photovoltaic Transducer
Transducer                 • Ionization Transducer
• Differential Transformer   • Magnetometer Search
(LVDT)                       Coil
• Capacitive Transducer      • Hall-Effect Transducer
• Piezoelectric Transducer   • Digital Displacement
• Photoelectric Transducer     Transducer
• Photoconductive
Transducer

3
Variable-Resistance Transducer
• Resistance Potentiometer, rheostat
• Converts linear or angular displacement into an electric
signal
• Can be used to convert force and pressure to a
displacement through mechanical methods
• Moving contact on a slide-wire or moving contact that
moves through an angular displacement on a solid
conductor (e.g. graphite)
• Fairly cheap – some can be expensive, though

4
Differential Transformer (LVDT)
•   Coverts displacement into electric signal
•   Three coils placed in a linear relationship with a magnetic core free
to move inside the coils
•   Alternating input voltage impressed in center coil, and output voltage
from two end coils depends on magnetic coupling between the core
and coils
•   The coupling is dependent on the position of the core
•   If the core remains near the center of the coils the output is very
nearly linear.
•   Most differential transformers are only operated in this linear range,
and these are called linear variable differential transducers (LVDT)

5
LVDT Continued

6
Capacitive Transducer
•   Measures a change in capacitance through a change in distance between 2 plates, d
•   Measures capacitance change through a change in overlapping area, A, resulting
from plate movement or a change in dielectric constant, ε
A
C = 0.225 ⋅ ε ⋅
2
•                          0.225 is   in 2   use 0.0885 for cm
d
ε = 1 for air
ε = 1 for plastics

7
Capacitive Transducer cont.
•   For liquid level measurements two electrodes are placed in the
liquid, and the dielectric constant varies between them according to
the liquid level
•   Thus, capacitance between electrodes is a direct indication of liquid
level

8
Piezoelectric Transducer
•   A force applied to a piezoelectric crystal
between two electrodes produces a stress
and corresponding deformation in the
crystal
•   Deformation produces potential difference
at crystal surface
•   The stress on the crystal produces an
output voltage:
E = gtp

where t is crystal thickness in m
p is impressed force in N
m2

g is voltage sensitivity        d
g=
ε
•   Piezoelectric crystals used as pressure
transducers for dynamic measurements

9
Photoelectric Transducer
•   Converts a light beam into a usable electric signal
•   Light strikes cathode and releases electrons
•   Electrons are attracted toward anode, thereby producing an electric
current in external circuit
•   Cathode and anode enclosed in glass or quartz, which is evacuated
or filled with inert gas
•   Photoelectric tubes useful for measuring light intensity

10
Photoconductive Transducers
• Voltage impressed on semiconductor material
• Light strikes semiconductor material and a decrease in
resistance occurs, thereby producing an increase in
current
• Useful for measurement of radiation at all wavelengths

11
Photovoltaic Transducers
• Sandwich design of a metal base plate and a thin
transparent metallic layer with a semiconductor material
in between
• Light strikes barrier between transparent metal layer and
semiconductor material, and a voltage is generated
• Most widely used application of photovoltaic cell is the
light exposure meter in photographic work

12
Ionization Transducer
• Converts displacement to a voltage through a
capacitance change
• As tube moves horizontally between the two plates, a dc
potential difference is created between electrodes 1 and
2

13
Magnetometer Search Coil
•   Transforms a magnetic
field signal into a voltage
•   A flat coil with a
specified number of
turns is placed in a
magnetic field, and a
voltage is generated
magnetic field, the
search coil must be
moving. Typically a
rotating coil is used in
this situation.

14
Hall-Effect Transducers
•   An external current passes
through a semiconductor
plate.
•   When a magnetic field is
impressed on a plate in a
direction perpendicular to
the plate surface, a voltage
is generated
•   This voltage is the Hall
voltage
• E = K ⋅ IB
H   H
t
where K H is the Hall
coefficient

15
Digital Displacement Transducers
•   Used to measure angular and linear displacements
•   For an angular measurement, a wheel rotates and light is alternately
transmitted and stopped through small cutouts
•   The signal is sent to the photodetector, amplified, then sent to a
counter.
•   The number of counts is proportional to angular displacement.
•   Sensitivity can be improved by increasing the number of cutouts

16
Digital Displacement Transducers
• For a linear measurement, small reflecting strips are
installed on a motion device.
• Light is then alternately reflected and absorbed with
linear motion presenting a signal to the photodetector
• Calibration with a known displacement standard must be
performed

17

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Description: Capacitive sensors are often known as the capacitance level meter, capacitance level meter capacitive sensing element is a cylindrical capacitor according to the principle of work, by the two capacitor plates insulated coaxial cylindrical inner and outer electrodes electrodes, between the two cylinders filled with a dielectric constant e of the electrolyte, the capacitance between two cylinders for the C = 2ΠeL/lnD/d, where L is the overlapping part of two tube length; D electrode diameter for the outer tube; d is the diameter of the inner cylinder electrode; e is the dielectric constant of the intermediate medium. In the actual measurement D, d, e is basically unchanged, so you can know the level measured at the level of C, which is easy to use with capacitive sensors, a simple structure and high sensitivity, low price and so one of the reasons .