senzori capacitive pentru deplasari liniare

8/3/2019 Senzori Capacitive Pentru Deplasari Liniare

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SENZORI CAPACITIVE PENTRU DEPLASĂRI LINIARE

Principiul de funcţionare a senzorilor capacitive:

Principle of capacitive sensors

Design of capacitive sensors

Capacitive sensors basically consistof a capacitor with variablecapacitance. The diagram to the leftshows a simple plate capacitor andthe formula for determining itscapacitance C . A is the effectivesurface area of the plate, d is the

plate separation, ε 0 is the field

constant and ε r is the dielectricconstant of the medium betweenthe plates.

The principle of the sensor is basedon causing changes in the nonelectrical variable that interests usto be reflected in changes in thecapacitance of the capacitor so thatthey can then be measured using asuitable circuit. These changes incapacitance can either be due tochanges in the plate surface area,in the plate separation or in the

relative dielectric permittivity.



Changes in the plate separation

The capacitance of a platecapacitor is inversely proportional tothe distance d between the plates,

as shown in the above equation.Therefore, if external effects causethe separation to increase, thecapacitance decreases and viceversa (see animation right).Changes in separation can thusused to measured smalldisplacements. Due to the inverseproportionality, though, therelationship between the separationand the capacitance is non-linear.

The characteristic curve is ahyperbola.

Changes in the effective plate surface area

Instead of measuring displacement

by varying the plate separation, it ispossible to move the plates parallelto one another (as in the diagramright). This causes the effectivesurface area of the capacitor A todecrease since the plates onlypartially overlap, which results in alowering of the capacitance. Sincethe capacitance is directlyproportional to the overlappingsurface area, the characteristic in

this case is linear.



Changes in the relative dielectric permittivity

The capacitance of the capacitor isproportional to the relativepermittivity of the medium between

the two plates as shown in theabove equation. An increase in therelative permittivity due to thepresence of a different dielectricincreases the capacitance (seegraphic right). If the seconddielectric is only partially situatedwithin the capacitor, this can can berepresented as a parallelconnection of two differentcapacitors, each of which

exclusively has one of the twomaterials as its dielectric medium. If the original medium between the

plates is air (ε r =1), the capacitanceincreases linearly when the secondmedium is introduced.

The principle described here can, for example, be used to measure how full avessel is (see animation left). As the

vessel fills up, the liquid takes the placeof the air between the plates increasingthe relative dielectric permittivity and thusthe capacitance of the capacitor. If thismethod is used to measure a conducting liquid, one of the electrodes must beisolated, in which case the vessel itself acts as the second electrode. In a similar way, such a capacitive sensor can alsobe used to measure moisture levels or humidity.

Design of a tube capacitor



An alternative body design to the plate capacitor is as a tube capacitor (see right). The firstelectrode is formed by the enclosing tube, whilethe second electrode runs through the middle of the capacitor. With this design, the plate

separation is guaranteed to be constant and evenif the axis is rotated it does not effect thecapacitance. Displacement is measured in thiscase by moving the central electrode. This motionleads to a change in the effective surface areaand thus the capacitance as a whole.



Placa experimentală cu senzorul capacitiv:

'Capacitive displacement sensor' card SO4203-5W

General

The UNI-TRAIN Capacitive displacement sensor card contains a capacitivedisplacement sensor in the form of a plate capacitor with variable effective platearea. The detection of displacement is achieved when one of the plates ismoved. This leads to a change on the capacitor's effective surface area A, whichcan be measured when the capacitor is part of a measuring bridge supplied withAC current.

Move your mouse over the info icons on the

picture of the card to learn more details.



The following graphic shows the circuit for a capacitive displacement sensor.

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The effective plate surface area determines the capacitance. The capacitor ispart of a resonance or tuning circuit (1) and determines the resonant frequency f of the circuit. A frequency-voltage (f/V) converter(2) is connected after this circuitand it serves to convert the frequency into a proportional voltage output. Adifferential amplifier (3) with adjustable offset and gain modifies the output fromthe converter to provide the final output of the displacement sensor at the Out

socket.



Măsurători experimentale:

Experiments - Capacitive displacement measurement I

Zero-point calibration and adjusting the amplifier

The following experiment starts by investigating the functioning of a displacementsensor. Alongside this the zero point of the displacement sensor should becalibrated and the amplifier set up so that the sensor outputs a voltage in therange 0 - 10 V. The following diagram shows the appropriate measuring circuit.

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Assemble the following experiment circuit.

Open the oscilloscope and set the following parameters:

Oscilloscope settings

Operating mode X/T

Timebase 20 µs

Coupling channel A DC

Sensitivity channel A 200 mV

Coupling channel B OFF

Trigger Channel A

In order to set the offset and amplification, set the trimmer to its left-hand limitand move the capacitor plate to its lower limit (position 0). Copy the resultingoscilloscope trace into the placeholder below.

Se citeşte de pe osciloscop perioada semnalului, se calculează frecvenţa şi



rezultatele se trec în referat.

Now move the capacitor plate to its upper limit (position 60) and repeat theexperiment. Copy the resulting oscilloscope trace into the placeholder below.

Se citeşte de pe osciloscop perioada semnalului, se calculează frecvenţa şirezultatele se trec în referat.

What effect does the position of the displacement sensor have on thefrequency of the tuned circuit? Enter your answer into the text box below.

Întrebarea şi răspunsul se trec în referat.

Now close the oscilloscope. Open Voltmeter B and set the followingparameters:

Settings for Voltmeter BOperating mode DC

Display AV

Measuring range 20 V

Now move the displacement transducer to position 0 and adjust the offsettrimmer so that the sensor outputs a voltage of 0. Then move the sensor toposition 60 and set the trimmer for the gain so that output voltage reaches 10V.

Close the voltmeter again and open the oscilloscope again. Move thecapacitor plate to the bottom to the lower limit and determine thecorresponding frequency f of the tuned circuit. Move the capacitor plateupwards in steps of of 5 scale divisions and measure the frequencies at eachstep. Enter the value pairs into the table then switch it to Chart mode to displaythe characteristic.

Se desenează în referat un tabel în care se scriu rezultatelemăsurătorilor (poziţia senzorului, perioada semnalului, frecvenţa) şi pebaza rezultatului se trasează graficul poziţie-frecvenţă.

What is the shape of the resulting characteristic? How do you explain theresult? Enter your answer into the text box below.




Experiments - Capacitive displacement measurement II

Determining the sensor characteristic

The following experiment will determine the static characteristic of the sensor.The movable capacitor plates are moved in steps starting from the lower limit tothe upper limit. For each position x the corresponding output voltage uOut ismeasured via the measurement circuit. Voltmeter A is used to measure theoutput voltage. The following diagram shows the appropriate measurementcircuit.

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Assemble the following experiment circuit.

Open Voltmeter A and set the following parameters:

Settings for Voltmeter A

Operating mode DC

Display AV

Measuring range 10 V

Move the capacitor plate to its lower limit and measure the correspondingoutput voltage uOut. Move the core upwards in steps of 5 scale divisions andmeasure the output voltage on each occasion. Enter the pairs of values intothe table below then switch it into Chart mode to see the characteristic of thesensor.

Se desenează în referat un tabel în care se scriu rezultatele măsurătorilor (poziţia senzorului, tensiunea de ieşire al amplificatorului) şi pe bazarezultatului se trasează graficul poziţie-tensiune.

What is the shape of the characteristic? How do you explain the results? Enter your answer into the text box below.




Întrebări suplimentare:

1. În ce mărime electrică s-a transformat deplasarea liniară?

2. Ce fel de circuit este primul circuit şi ce rol îndeplineşte?3. Ce fel de circuit este al doilea circuit şi ce funcţie are?4. Ce fel de circuit este al treilea circuit şi cu ce scop se foloseşte?5. Prin ce metode se poate măsura o capacitate? Cum s-a măsurat în

cadrul acestei lucrări?6. Ce parametru al condensatorului s-a modificat cu deplasarea?7. Ce alt parametru al condensatorului se mai poate modifica pentru a

transforma deplasarea liniară în variaţie de capacitate?

Explica i cu cuvintele proprii ce ati în eles din aceast lucrare deţ ţ ă

laborator!La ce concluzii a i ajunsţ ?

senzori capacitive pentru deplasari liniare

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