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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
position and the current capacity measuring technique.
The First Section Capacitive Sensor Working
Principle and Structure Of The Form
The electric capacity sensor's basic ideal
formula is
C
A
d

 0 r A
d
In the above equation, which how many parameters
are the variables? Which how many types can make
the electric capacity sensor?
The First Section Capacitive Sensor Working
Principle and Structure Of The Form
C
A
d

 0 r A
d
Changes A, d, in the  three parameter random
quantity, may cause the plate electric capacity's
electric capacity C change.
In fixed three parameter's two, may make three types
the electric capacity sensors.
First, Variable area capacitive sensors
Chart a is the even shape of strip straight line displacement type structure,
the pole plate 1 may control moves, is called moves the pole plate. Pole plate
2 fixed motionless, is called decides the pole plate. Chart b is the concentric
cylinder deformation area type sensor. The outside cylinder is motionless, in
the cylinder makes the top and bottom translation in the outside cylinder.
Chart c is an angular displacement's-like structure. The pole plate 2 axes by
were measured that the object leads when revolves an angular displacement
 degree, two pole plate's cover area A reduces, thus the electric capacity also
Variable area capacitive sensor characteristics
Changes the area type electric capacity
sensor's input level is linear, the sensitivity is a
constant. This kind of sensor uses in
examining parameters and so on straight line
displacement, angular displacement, size.
Please draw Variable area capacitive sensor
output 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
between electrodes d0 to be as far as possible
small, enhances the sensitivity, but this also has
brought changes the distance between electrodes
type capacitor's traveling schedule small
shortcoming.
Pole-changing from capacitive sensor characteristic curves
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
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
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
sensor
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
capacity sensor works, the capacity Cx changes, and the
frequency f of oscillator will also correspondingly change.
1
f 
2 L0C
 5-2 
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?
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 Index
1.ECG Inductive Proximity Switches are precision, solid-state sensing devices
that 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
shown 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
and interelectrode dielectrical coefficient e. Make two
variables of them fixed, and the capacity C becomes
the simple function of another variable. So long as
the non-electrical quantity is transformed into the
change of electrode distance, area or dielectrical
coefficient, the non-electrical quantity can be
measured by measuring capacity.
First, capacitance
Level Meter
Teflon coat
Capacitance liquid level
restricting sensor
The difference between the
liquid level restricting sensor and the
liquid level transmitter is: the
former does not give the analog, but
the switching value. When the liquid
level arrives at the setting value, it
outputs the low level. But the output
can also be chosen as the high level
type.
The setting of the
liquid level
Establishes
the button
restricting sensor
The setting method of the intelligent
liquid level sensor is very simple:
Press the setting button by finger, and
loose it when the liquid level reaches
the setting value. Then the intelligent
instrument will remember this setting.
When using normally and the liquid
level exceeds this point, it will send out
alarming and control signals.
The setting button of the intelligent liquid level
restricting sensor
Normal work
indicating
lamp
Power
light
Ultra
limits the
lamp
Establishes
the button
Second, silicon micro-machining acceleration sensor
As shown in the picture, the
acceleration sensor is based on the
micro-machining technology,
measuring the alternating acceleration
(vibration) and inertia force or gravity
acceleration. Its working voltage is
2.7~5.25V, and the acceleration
measuring range is several g. The
voltage, whose output is proportional
to the acceleration, can output the
PWM pulse with its duty cycle in
proportion to the acceleration.
Micro-machining three-axis
acceleration sensor
Technical indexes:
Sensitivity: 500mV/g;
measuring range: 10g;
frequency range: 0.52000Hz; installment
resonance point: 8kHz;
resolution: 0.00004g;
weight: 200g; installation
screw thread: M5 mm;
linearity error: ≤1%
The working principle of silicon micro-machining
acceleration sensor






1- acceleration test
unit
2- signal processing
circuit
3- underlay
4- bottom
polycrystalline
silicon (downelectrode)
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
polycrystalline silicon plates, forming differential capacitance C1, C2. While the
bottom and top polycrystalline silicon stay motionless, the intermediate layer
polycrystalline silicon can move slightly up and down. Its left side fixes on the
substrate, therefore it is equal to the cantilever beam.
When it feels the up-down vibration, the differential change will happen to
C1, C2. The signal processing circuit, sealed in the same capsule with the
acceleration test unit, will transform ΔC into the direct-current output voltage.
Its excitation 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 can 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
direction can be measured.
The application of acceleration sensors in
automobile
loaded with
sensor's
stuffed
dummy
Aerocyst
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.
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 drillground bomb can delay
explosion.
The sensor installs
the position
Third, humidity capacitor
The moisture-absorbing insulation material can
be employed as the medium of the capacitance sensor,
on both sides of which the porous electrode is
electroplated. When the relative humidity increases,
the moisture-absorbing medium absorbs steam in the
air, making the relative dielectric constant of the
medium between two electrodes increase 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
Alarm apparatus' family
operational type
The application in the
field
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 sensor reduces, the electric bridge loses balance, the servo motor
reverses, the indicator deflects anti-clockwise (the indicating value reduces), and
simultaneously the sliding beam 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 closedloop system?
Can the fuel content gauge shown on the above
page be used in the state of inclination? Why?
This fuel
content
gauge can
be used for
aircraft fuel
tank
Fifth, capacitance proximity switch
The tested object can be conductor, the
insulator with much dielectric loss, the objects
containing water (e.g. feed, human body, and so
on); it can be grounding or non-grounding. Adjust
the sensitivity adjustment potentiometer at the
end of the proximity switch, and the action
distance can be changed 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 of the installing
brackets in installing the non-flush switch.
Long-distance type
(broad range)
Fully sealed
waterproof type
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
Correspon
ding light
beam
demonstrat
ion
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
Electro
nic
circuit
positio
n
Hightension
side air
inlet
Low
pressure
side air
inlet
Internal stainless steeldiaphragm position
The internal structure of capacitance
differential pressure transmitter











1- air inlet on the highpressure side
2- air inlet on the lowpressure side
3- filter leaf
4- cavity
5- flexible stainless steel
corrugated isolation
diaphragm
6- pressure transporting
silicone oil
7- concave glass wafer
8- concave gold-plated
electrode
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
Brings the pressure
and the fluid position
is proportional in the
high-tension side
housing:
p = g h
Differential
Pressure
Transmitter
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
capacitance differential pressure transmitter in flow
measurement
There are many methods in measuring flow, such as flow speed
measurement, the volume method, the quality method, the flume
method, and so on. The flow speed measurement includes methods like
the impeller type, the turbine type, the Carmen turbulent flow type
(another name is vortex type), the hot line type, the Doppler type, the
supersonic type, the electromagnetic type, the differential pressure
throttling type, and so on. According to the national manufacturing
standard, the discharge coefficient formula of standard throttling
equipment is perfect, therefore it is a reliable and standardized flow
sensor. Consequently the differential pressure flowmeter is widely used
in industry。
The wind speed of
impeller type, the
flowmeter
(reference materials
from Beijing
Beifang Dahe
Instrument and
Meter Limited
Company)
Fluid issuing
Rotameter
Rotor
Fluid
The differential pressure throttling type
flowmeter
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
pressure reduces.
The shortcoming of throttling flowmeter is that after the
fluid passes the throttling gear, the irreversible pressure loss
will occur. Moreover, when the fluid temperature t and
pressure p1 change, the fluid density will change with it.
Therefore the temperature and pressure must be modified.
The throttling
gear (the
pressure-taking
tube and internal
throttling orifice)
Latter takes
the pressure
tube
First takes the
pressure tube
Throttle
orifice
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 Throttle
orifice
orifice
The speed of
flow changes
quick, the
pressure changes
is small
Another form of throttling gear- - venturi
tube or venturi nozzle
The pressure loss of venturi nozzle is
relatively small.
Fluid
entrance
Narrow spot
The working principle diagram of venturi nozzle
Venturi nozzle structure
First takes
presses the
mouth
Latter
takes
presses the
mouth
Article hamlet
spray nozzle's in
pipeline position
The configuration of pressure guiding tube
The configuration of pressure guiding tube from
pressure-taking hole to the differential pressure
transmitter should be installed in accordance with the
stipulated standard. If the tested fluid is gas, the pressure
guiding tude should be installed on the top of the throttling
gear, in case that the liquid drop of the air blocks the
pressure-taking tube. If the tested fluid is liquid, the
pressure guiding tube should be installed at the bottom of
the throttling gear, in case that the air mixed in the liquid
affects pressure taking.
The configuration diagram of pressure guiding tube
The shape of throttling
flowmeter
The demonstration of Karmen vortex flowmeter
of Shanghai Kent Intelligence Limited Company
Another kind of new
speed of flow type
flowmeter
Gas or liquid
pipelines and eddy
current occurrence
cone
Homework
P117:2、5，



Supplements homework: Please surf the net and consult
materials on the specification of “technical service contract”
and the waterworks flow measurement. Draft a “technical
reformation project agreement”on behalf of your company
with 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.