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?

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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

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