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The Fifth Chapter Capacitance sensor
This chapter studies the electric capacity sensor's principle
and the application, but also involves the pressure, the fluid posi
tion and the current capacity measuring technique.
The First Section Capacitive Sensor Working
Principle and Structure Of The Form
In the above equation, which how many parameters a
re the variables? Which how many types can make th
e electric capacity sensor?
0 r AAC
d d
The electric capacity sensor's basic ideal formula is
The First Section Capacitive Sensor Working
Principle and Structure Of The Form
Changes A, d, in the three parameter random quantity, ma
y cause the plate electric capacity's electric capacity C change.
In fixed three parameter's two, may make three types the ele
ctric capacity sensors.
0 r AAC
d d
First, Variable area capacitive sensors
Chart a is the even shape of strip straight line displacement type structure, the p
ole plate 1 may control moves, is called moves the pole plate. Pole plate 2 fixed m
otionless, is called decides the pole plate. Chart b is the concentric cylinder defor
mation area type sensor. The outside cylinder is motionless, in the cylinder make
s the top and bottom translation in the outside cylinder. Chart c is an angular dis
placement's-like structure. The pole plate 2 axes by were measured that the obje
ct leads when revolves an angular displacement degree, two pole plate's cover
area A reduces, thus the electric capacity also reduces along with it.
Variable area capacitive sensor characteristics
Changes the area type electric capacity senso
r's input level is linear, the sensitivity is a cons
tant. This kind of sensor uses in examining pa
rameters and so on straight line displacement,
angular displacement, size.
Please draw Variable area capacitive sensor o
utput characteristic curve!
Second, Pole-changing from capacitive sensors
When moves the pole plate to measure when the object function causes the displacement, changed between two pole plate's distance d, thus causes the electric capacity to change.
When actual use, always causes initial distance b
etween electrodes d0 to be as far as possible smal
l, enhances the sensitivity, but this also has brou
ght changes the distance between electrodes type
capacitor's traveling schedule small shortcoming.
Pole-changing from capacitive sensor characteristic curves
From the chart may see that to enhance the sensitivity, should cause, when d0 small big? When changes distance between electrodes type electric capacity sensor's initial distance between electrodes d0 is small, its measuring range fill-out changes is small?
a) structure schematic drawing b) the electric capacity and the pole plate distance's relations 1- decide the pole plate 2- move the pole
plate
Third, variable dielectric constant
The comparative dielectric constant of each medium is
different, so the capacity will vary when different medium is
inserted into two plates of the capacity.
Several medium relative dielectric constants
The use of variable dielectric constant capacitance s
ensor
According to the above table, please analyze the influence
of different medium on variable dielectric constant
capacity. When dry paper and wet paper are inserted into
two plates of the capacity, in which situation will the
capacity increase? Which non-electrical quantity can be
measured?
The Second Section The measurement conversion circuit
(frequency modulation circuit) of capacity sensor
The frequency modulation circuit takes the capacity sensor
as a part of LC oscillator resonant circuit. When the capacit
y sensor works, the capacity Cx changes, and the frequency
f of oscillator will also correspondingly change.
The electric capacity type sensor's frequency modulation electric circuit and the electricity turbulent flow sensor has what difference? Are in the formula which quantities the variables?
0
1 5-2
2f
L C
Frequency modulation (FM) circuit
The high level and low level of TTL level
What is the voltage range respectively?
Online practice (integral) interface
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Registration interface
The interface after logging in
Complete the exercise Interface
Inductive Proximity Switches Product Index1.ECG Inductive Proximity Switches are precision, solid-state sensing devices th
at provide an attractive alternative to physically activated limit and control switches with their mechanical contacts, moving parts and attendant wear characteristics. ECG Proximity Switches are fully sealed against most hostile industrial environments. They are impervious to oils, organic cleaners, steam, water and dust as well as being immune to vibration. Usual positioning and operational constraints are virtually eliminated, while life span remains unaffected by problems related to mechanical wear.
Operating Principle 2.An inductive proximity switch has three functional sections or stages, as show
n in Figure 1. They are: a radio frequency (RF) oscillator circuit that incorporates a coil with a ferrite core, a Schmidt trigger circuit, and a solid state output switching device (transistor in DC types, thyristor in AC types).
3.The oscillator circuit generates an electromagnetic field which is radiated from the active face of the switch. A metal object (target) introduced into this sensing field absorbs energy from the oscillator which, in turn, reduces the amplitude of oscillation. The trigger circuit detects the reduction and in response produces a signal that closes the output stage switching device. When the target leaves the sensing field the oscillator regenerates and the switch resets.
ECG Inductive Proximity Switches are responsive to all electrically conductive materials. They cannot, however, distinguish between different materials.
The Third Section The Application of Capacitance
Sensor
The capacity of the capacitance is influenced by
three factors: electrode distance x, relative area A an
d interelectrode dielectrical coefficient e. Make two v
ariables of them fixed, and the capacity C becomes th
e simple function of another variable. So long as the
non-electrical quantity is transformed into the chang
e of electrode distance, area or dielectrical coefficien
t, the non-electrical quantity can be measured by mea
suring capacity.
First, capacitance
Level Meter
Teflon coat
Capacitance liquid level
restricting sensor The difference between the liquid le
vel restricting sensor and the liquid l
evel transmitter is: the former does
not give the analog, but the switchin
g value. When the liquid level arrive
s at the setting value, it outputs the l
ow level. But the output can also be c
hosen as the high level type.
The setting of the
liquid level
restricting sensor
The setting method of the intelligent liq
uid level sensor is very simple:
Press the setting button by finger, an
d loose it when the liquid level reaches
the setting value. Then the intelligent i
nstrument will remember this setting.
When using normally and the liquid le
vel exceeds this point, it will send out a
larming and control signals.
Establishes the button
The setting button of the intelligent liquid level
restricting sensor
Ultra limits the lamp
Normal work indicating
lamp
Establishes the button
Power light
Second, silicon micro-machining acceleration sensor
As shown in the picture, the accelerati
on sensor is based on the micro-machi
ning technology, measuring the altern
ating acceleration (vibration) and inerti
a force or gravity acceleration. Its work
ing voltage is 2.7~5.25V, and the accel
eration measuring range is several g. T
he voltage, whose output is proportion
al to the acceleration, can output the P
WM pulse with its duty cycle in proport
ion to the acceleration.
Micro-machining three-axis
acceleration sensor
Technical indexes:
Sensitivity: 500mV/g; mea
suring range: 10g; frequen
cy range: 0.5-2000Hz; inst
allment resonance point: 8
kHz; resolution: 0.00004g;
weight: 200g; installation
screw thread: M5 mm; line
arity error: ≤1%
The working principle of silicon micro-machining
acceleration sensor
1- acceleration test unit
2- signal processing circuit
3- underlay 4- bottom polycryst
alline silicon (down-electrode)
5- polycrystalline silicon cantilever beam
6- top layer polycrystalline silicon (up-electrode)
Through microelectronic processing technology, the polycrystalline silicon
can be processed into multilayer structure products as three-layer polycrystalli
ne silicon plates, forming differential capacitance C1, C2. While the bottom and
top polycrystalline silicon stay motionless, the intermediate layer polycrystallin
e silicon can move slightly up and down. Its left side fixes on the substrate, ther
efore it is equal to the cantilever beam.
When it feels the up-down vibration, the differential change will happen t
o C1, C2. The signal processing circuit, sealed in the same capsule with the acce
leration test unit, will transform ΔC into the direct-current output voltage. Its e
xcitation source is also sealed in the same capsule. Since the elastic hysteresis of
silicon is very small, and cantilever beam is very light, the frequency response c
an reach above 1kHz, and the allowable acceleration scope can exceed 50g.
If three acceleration sensors are installed in three mutually perpendicular
direction in the capsule, the vibration or acceleration of three dimensional direc
tion can be measured.
The application of acceleration sensors in
automobile
When the measured negative acceleration value surpasses the setting one, based on which the microprocessor judges as a collision, it will immediately inflate the foldable airbag in the front of the car and expand, support driver and front-row passenger's chest and head.
loaded with sensor's stuffed dummy
Aerocyst
The protective role of automobile airbag
The acceleration sensor can through controlling system make the airbag inflate immediately when cars collide.
Automobile aerocyst to pilot protective function
The protective role of airbag
for the driver
Through using the
acceleration sensor, the drill-
ground bomb can delay
explosion.
The sensor installs the position
Third, humidity capacitor
The moisture-absorbing insulation material can be e
mployed as the medium of the capacitance sensor, on bot
h sides of which the porous electrode is electroplated. Wh
en the relative humidity increases, the moisture-absorbin
g medium absorbs steam in the air, making the relative di
electric constant of the medium between two electrodes in
crease greatly (the relative dielectric constant of water is
80). Therefore the capacity increases.
The shape of
humidity capacitor
Absorbing water high polymer thin film
The shape of humidity
capacitor module and
sensor
The installation and application of humidity
sensor
The application in the field
Alarm apparatus' family
operational type
The shape of porous alumina humidity sensor
Fourth, capacity fuel content gauge
Mechanical fuel content
gauge
In the fuel tank, is loaded with in the similar bathroom water tank's float, drives the resistance wire type disc potentiometer through the release lever, instructs the oil mass by the ampere meter.
Capacity fuel content gauge When the fuel tank is filled with oil, the fluid position rises, and the indicator stays
at the angle place of qm. When the oil level lowers, the capacity Cx of the capacitance s
ensor reduces, the electric bridge loses balance, the servo motor reverses, the indicator
deflects anti-clockwise (the indicating value reduces), and simultaneously the sliding be
am of RP is driven to move. When the RP resistance reaches certain value, the electric
bridge achieves new equilibrium, the servo motor stops, and the indicator pauses in the
new position (q x).
This fuel gauge belongs to the open system or the closed-loop system?
Can the fuel content gauge shown on the above
page be used in the state of inclination? Why?
This fuel
content gau
ge can be u
sed for airc
raft fuel tan
k
Fifth, capacitance proximity switch
The tested object can be conductor, the insul
ator with much dielectric loss, the objects containi
ng water (e.g. feed, human body, and so on); it ca
n be grounding or non-grounding. Adjust the sens
itivity adjustment potentiometer at the end of the
proximity switch, and the action distance can be c
hanged according to the tested object.
The shape of the capacity proximity switch
Flush-type non-flush type
The installation of non-flush proximity switch
It is easy to damage the sensor if its height exceeds that o
f the installing brackets in installing the non-flush switch.
Fully sealed
waterproof
type
Long-distance type (broad range)
Electric capacity proximity switch's
specification
The application of capacitance proximity switch in measuring and controlling liquid level
The application of capacitance proximity switch in measuring
and controlling liquid and material position
Corresponding light beam demon
stration
The application of capacitance proximity switch in material position measurement
The application demonstration of capacitance
proximity switch in measuring and controlling
material position
The influence of different nonmetallic
testing objects on the action distance of
capacitance proximity switch
The Fourth Section Pressure and Flow
Measurement
First, the basic concepts of pressure
The pressure International unit is “Pascal”, abbreviation “handkerchief” (Pa).
In addition, the engineering uses many different pressure Measuring unit for a long time. Like “the technical atmosphere”, “the standard atmospheric pressure”, “the millimeter mercury column”, in the meteorology also uses “Pakistan” (bar) and “the request” for the Pressure unit. These units also will possibly see in some import measuring appliance instruction booklet.
Pressure unit conversion table
Second, differential pressure transmitter
High-tension side air inlet
Low pressure side air inlet
Electronic circuit position
Internal stainless steel-diaphragm position
The internal structure of capacitance
differential pressure transmitter 1- air inlet on the high-pr
essure side 2- air inlet on the low-pres
sure side 3- filter leaf 4- cavity 5- flexible stainless steel co
rrugated isolation diaphragm 6- pressure transporting si
licone oil 7- concave glass wafer 8- concave gold-plated elec
trode 9- elastic flat diaphragm 10- d cavity
The shape of all
kinds of capacitance
differential pressure
transmitter
The shape of all kinds of capacitance pressure
transmitter
(continued)
法兰
The shape of all kinds of capacitance pressure
transmitter (continued)
Third, the use of capacitance
differential pressure transmitter
Differential Pressure
Transmitter
Brings the pressure and the fluid position is proportional in the high-tension side housing:
p = g h
The use of capacitance differential
pressure transmitter in measuring
liquid level
Investment type fluviograph
Fourth, the basic concept of flow
Volume flow qV=Av, the unit is m3/h or L/s; Mass flow : qm=rAv , the unit is t /h or kg/s
Fifth, the application of throttling flowmeter and cap
acitance differential pressure transmitter in flow me
asurement
There are many methods in measuring flow, such as flow speed me
asurement, the volume method, the quality method, the flume method,
and so on. The flow speed measurement includes methods like the impe
ller type, the turbine type, the Carmen turbulent flow type (another na
me is vortex type), the hot line type, the Doppler type, the supersonic ty
pe, the electromagnetic type, the differential pressure throttling type, a
nd so on. According to the national manufacturing standard, the discha
rge coefficient formula of standard throttling equipment is perfect, ther
efore it is a reliable and standardized flow sensor. Consequently the dif
ferential pressure flowmeter is widely used in industry 。
The wind speed of i
mpeller type, the flo
wmeter (reference
materials from Beiji
ng Beifang Dahe Ins
trument and Meter
Limited Company)
Rotameter
Fluid entrance
Fluid issuing
Rotor
The differential pressure throttling type flow
meter
The so-called throttling gear is to establish an orifice with
its flow area narrower than the pipeline or the venturi nozzle.
When the fluid goes through this throttling gear, the jet area
contracts partially, the flow speed increases, and the pressur
e reduces.
The shortcoming of throttling flowmeter is that after the fl
uid passes the throttling gear, the irreversible pressure loss
will occur. Moreover, when the fluid temperature t and press
ure p1 change, the fluid density will change with it. Therefore
the temperature and pressure must be modified.
The throttling gea
r (the pressure-ta
king tube and inte
rnal throttling ori
fice)
Throttle orifice
Latter takes the pressure tube
First takes the
pressure tube When fluid through throttle orifice, the speed of flow speeds up, latter takes pressure tube's place pressure to reduce.
Throttling gear
contour
Throttle orifice
Latter takes the pressure tube
The pressure and flow
speed change when the fluid
flows through the throttling
orifice
Throttle orifice
The speed of flow changes quick, the pressure changes is small
Another form of throttling gear- - venturi t
ube or venturi nozzle
The pressure loss of venturi nozzle is relati
vely small.
Fluid entrance
Narrow spot
The working principle diagram of venturi nozzle
Venturi nozzle structure
Article hamlet spray nozzle's in pipeline position
First takes presses the mouth
Latter takes presses the mouth
The configuration of pressure guiding tube
The configuration of pressure guiding tube from press
ure-taking hole to the differential pressure transmitter sho
uld be installed in accordance with the stipulated standard.
If the tested fluid is gas, the pressure guiding tude should b
e installed on the top of the throttling gear, in case that the
liquid drop of the air blocks the pressure-taking tube. If th
e tested fluid is liquid, the pressure guiding tube should be
installed at the bottom of the throttling gear, in case that th
e air mixed in the liquid affects pressure taking.
The configuration diagram of pressure guiding tube
The shape of throttling flowm
eter
The demonstration of Karmen vortex flowmeter of Shanghai Kent Intelligence Limited Company
Gas or liquid pipelines and eddy current occurrence cone
Another kind of new speed of flow type flowmeter
Homework P117:2 、 5 , Supplements homework: Please surf the net and consult m
aterials on the specification of technical service contract an� �d the waterworks flow measurement. Draft a technical refo�rmation project agreement on behalf of your company wit�h the Waterworks. The agreement should include: the names of both sides, the rights and duties of both sides, the technical index, major technique plan, completion date, inspection method, payment method, and so on.
Title Standard format
Have a rest.