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OPERATION OF SENSORS RELAY OPERATION PROCESS
Raxmonov Furqat Abduxakimovich.
Jizzax Politexnika instituti
Тел:
+998 91 566 06 64
A N N O T A T I O N:
Another type of sensors is determined by the nature of his physical
nature. The output amount, that is, resistance, indication, capacity, voltage, phase, phase,
frequency sensors are the most common.
A N N O T A S I YA:
Датчикларни бошқача тури,
чиқиш у миқдорини физикавий
табиатига
қараб ҳам белгиланади. Чиқиш миқдори–электр бўлган датчиклар, яьни
қаршилик, индуктивлик, сиғим, ток, кучланиш, фаза, частота датчиклари энг кўп
тарқалган.
А Н Н О Т А Ц И Я:
Другой тип датчиков определяется природой его физической
природы. Выходная сумма, то есть сопротивление, индикационная, емкость, напряжение,
фаза, фаза, датчики частоты являются наиболее распространенными.
The accurate and reliable operation of sensors determines the corresponding main indicators of
the entire system operation. Sensors must possess high sensitivity and accuracy, long service life
and reliability in operation, small dimensions and weight, as well as low cost.
Conditionally, sensors can be considered to consist of receiving, intermediate, and executive
parts. The
receiving part
, being affected by changes in the input quantity
x
, converts it into
some
intermediate
quantity. This quantity is compared with the reference (standard) value of a
similar physical quantity. Then, acting on the
executive part
of this sensor, it forms the output
signal
y
. According to the
physical composition of the input quantity x
—electrical, thermal,
mechanical, optical, acoustic, liquid and gas sensors are distinguished. Electrical sensors
measure current, voltage, power, frequency, magnetic flux; thermal sensors measure temperature
and heat quantity; mechanical sensors measure force, pressure, displacement, velocity,
acceleration; optical sensors measure light intensity, illumination; acoustic sensors measure
sound intensity, its frequency, power; liquid and gas sensors measure pressure and velocity.
Each type of sensor is, in turn, also classified according to
the operating principle of its
receiving part
, i.e., divided into groups. For example, optical sensors are divided into
photoelectric, photochemical, photothermal, and photomechanical groups. Another type of
sensors is also determined by
the physical nature of the output quantity y
. Sensors with
electrical output quantities, namely resistance, inductance, capacitance, current, voltage, phase,
frequency sensors are the most widespread.
Sensors are also divided into separate groups according to
their conversion of input signal x by
number and type
. Direct conversion sensors directly convert the input signal
x
into the output
signal
y
. Such sensors are convenient because they do not require intermediate conversion parts.
In sensors with intermediate conversion parts, multiple signal conversions lead to complications
and, to a certain extent, loss of accuracy. According to the form of
x-y
conversion, sensors are
divided into two groups: continuous and discrete (discontinuous) converters. Continuously
varying sensors are considered measuring devices. In them, continuous change of
x
corresponds
to continuous change of
y
. Often, discretely operating sensors monitor the state of discrete
objects, i.e., objects with finite states. Most monitored objects have two positions, namely
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"connected" and "disconnected" states. For this reason, discrete sensors are considered binary
information sensors with output quantities
y
=0 or
y
=1.
Due to the development of semiconductor technology and the widespread use of microprocessors
and computers in modern automatic systems, new ideas and directions have emerged in sensor
development. The characteristics of this development are marked by sensors working together
with microprocessors and computers. For this reason, an important quality of modern sensors is
their integrated design and small dimensions. Due to these characteristics, it became possible to
place several sensors in one housing and thereby create a combined sensor that simultaneously
measures several physical quantities.
Direct Conversion Sensors
An example of a direct conversion sensor is the
strain gauge
(Figure 3). They are used to
measure deformations and mechanical stresses on part surfaces. The strain gauge is made from
wire with high specific resistance and small diameter (0.006-0.020 mm) made of constantan in a
U-shape. The wire is placed densely and evenly between thin paper sheets and glued together.
The wire ends are welded to copper wires, through which the strain gauge is connected to the
measuring circuit. The strain gauge is firmly attached to the part surface and deforms together
with it. The relative change in resistance ΔR/R is proportional to the deformation Δl/l and the
stress on the part surface,
l
l
k
R
R
D
=
D
where
k
is a constant quantity.
Thus, in a strain gauge, a mechanical quantity (deformation) is directly converted to an electrical
(resistance) quantity.
Thermal sensors
are also simple in construction. In them, temperature is converted to voltage
(in thermocouples) or resistance change (in thermal resistors). Thermal resistors (Figure 3) are
made from steel, nickel, or platinum wires because their resistance depends on temperature. For
temperature measurement, ferrites and capacitors with magnetic and dielectric conductivity
sensitive to heat are used. In thermosensitive diodes and thyristors, the property of conductivity
dependence on temperature in the
p-n
junction in silicon crystal is utilized.
Figure 1. Resistance sensors
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Resistance sensors
group includes the widely used rheostat sensor (Figure 1,c). They convert
the linear displacement of mechanisms into corresponding resistance
R
change. When slider
D
is
moved by distance
x
, the resistance
R
of the rheostat changes proportionally.
In
inductive sensors
, the measured quantity is converted to inductance change. For example: a
sensor (Figure 2) measures the thickness
h
of ferromagnetic material. If the value
h
increases,
then the air gap δ decreases, resulting in an increase in the inductance of coil O, which is
registered by the measuring circuit.
Figure 2. Thickness measurement circuits
In
capacitive sensors
, the relationship between capacitor capacitance, plate area, distance
between them, and dielectric constant is utilized. With the help of capacitive sensors, linear and
angular displacements, temperature, relative humidity of air, and other parameters can be
measured. Particularly, the capacitive sensor shown in Figure 3 measures the thickness
h
of a
sheet placed between plates A and B of a capacitor made of dielectric material.
Infrared radiation optical sensors
(Figure 3) measure the temperature of heated objects. It
consists of a lens 2 that registers infrared rays on the surface of sensitive element 3, and a heated
object 1 that emits rays. As a result of this measurement, the resistance of sensitive element 3
changes, and voltage appears at outputs 4 and 5. A similar sensor (bolometer) is used in
automatic detection devices for overheated bearings in trains.
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Figure 3. Scheme of the infrared light emitting sensor
Sensors with Intermediate Converters
These sensors consist of several direct conversion sensors operating in sequence. The output
quantity of one sensor serves as the input quantity for the next sensor.
The sensor shown in Figure 4 serves to convert angular velocity ω to capacitor
C
capacitance.
The receiving div of the sensor is a centrifugal regulator. It converts angular velocity to
centrifugal force that is compared with the compression force of spring П (intermediate part).
The force in the intermediate part leads to displacement δₓ of the regulator's lower sleeve covered
with the upper plate of capacitor
C
. The capacitor is the executive part of the sensor, whose
capacitance changes according to the distance δc between plates.
The sensor in Figure 4 converts voltage
U
to frequency
f
. Voltage
U
is measured by voltmeter
V
connected to variable capacitor
C
whose arrow is related to capacitance
C
. Capacitor
C
is
connected to the circuit of task-giving generator
G
whose output frequency
f
depends on
capacitance. Thus, the sensor performs the following conversions:
U→
angular displacement of
voltmeter
V
arrow
→ C → f
.
Discrete Conversion Sensors
These sensors monitor the state of objects and serve as sources of input information in railway
automation and telemechanics systems.
A
track circuit
(Figure 5) is used to monitor the freedom of track sections from rolling stock. A
track circuit is a part of a track section bounded by insulating joints IT. Power supply is
connected to the rails at one end of the track circuit, while at the other end a control device CD
operating on current in the rails used as conductors is connected. Usually, an electromagnetic or
induction relay is used as CD. If the section is free, a large current passes through CD (relay
Figure 4. Schemes of sensors with
intermediate converter
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armature is attracted). If the section is occupied by at least one wheelset (whose resistance is
0.06 Ohm and much smaller than CD resistance), the current in CD sharply decreases (relay
releases its armature). Thus, based on the CD state, one can judge whether the track section is
free
or
occupied.
The
magnetic sensor
shown in Figure 6 registers the passage of wagon wheels through a certain
point on the track. Such a sensor is called a contactless magnetic pedal and consists of permanent
magnet DM, coil Ch, and control apparatus CA. The pedal is placed near the rail. When a wheel
approaches the pedal, the magnetic field parameters in the DM magnetic apparatus change. As a
result, electromotive force (EMF) is generated in coil Ch, and current begins to flow, which is
registered by CA.
References
1.
Theory of Automatic Control. Course of lectures. Compiled by: Ph.D., Associate
Professor Tikhonov A.I. 2002. http://www.ispu.ru/library/lessons/Tihonov_2/index.htm
2.
N.V. Klinachev Theory of Automatic Regulation Systems (Educational-methodical
complex). http://model.exponenta.ru/lectures/index.htm
3.
Theory
of
Automatic
Control.
Course
of
lectures.
http://www.toehelp.ru/theory/tau/contents.html
4.
Mirakhmedov D.A. Theory of Automatic Control. -T.: "Uzbekistan", 1993.
Figure 6. Magnetic reaction (pedal)
Figure 5. Scheme of the rail chain
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