The Fifth Chapter Capacitance sensor

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# The Fifth Chapter Capacitance sensor - PowerPoint PPT Presentation

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.

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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

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

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 reduces along with it.

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

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 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.

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?

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.

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.

Establishes the button

The setting of the liquid level 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: 0.5-2000Hz; 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 (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 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

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.

Aerocyst

The protective role of automobile airbag

The acceleration sensor can through controlling system make the airbag inflate immediately when cars collide.

The protective role of airbag for the driverThrough 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 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 installation and application of humidity sensor

Alarm apparatus' family operational type

The application in the field

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 closed-loop 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 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.

Fully sealed waterproof type

The application of capacitance proximity switch in measuring and controlling liquid and material position

Corresponding light beam demonstration

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.

Second, differential pressure transmitter

Low pressure side air inlet

Electronic circuit position

High-tension side air inlet

Internal stainless steel-diaphragm position

• 1- air inlet on the high-pressure side
• 2- air inlet on the low-pressure 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

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 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 entrance

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

When fluid through throttle orifice, the speed of flow speeds up, latter takes pressure tube's place pressure to reduce.

Throttle orifice

Throttling gear contour

Latter takes the pressure tube

Throttle orifice

The pressure and flow speed change when the fluid flows through the throttling orifice

Throttle orifice

The pressure loss of venturi nozzle is relatively small.

Fluid entrance

Narrow spot

The working principle diagram of venturi nozzle

Venturi nozzle structure

Latter takes presses the mouth

First 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 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 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.
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